There appears to have been a corporate loss of memory in the USA on how to build
space mechanisms (mechanically moving components) for long life and reliability.
A large number of satellite failures and anomalies have occurred recently (e.g.,
Galileo, Hubble, etc.). In addition, more demanding requirements have been
causing failures or anomalies to occur during the qualification testing of
future satellite and space platform mechanisms even before they are launched
(GOES-NEXT, CERES, Space Station Beta Joint Gimbal, etc.). For these reasons,
it is imperative to determine what worked in the past and what failed so that
the best selection of mechanical components can be made as well as to make
timely decisions on initiating research to develop any needed technology. The
purpose of this study was to capture and retrieve information relating to the
performance of mechanical moving equipment operating in space to determine what
components have operated successfully and what components have produced
Data were obtained through various sources, such as: (1) An extensive
literature review that included government contractor reports and technical
journals. (2) Communication and visits (when necessary) to the various NASA and
DOD centers and their designated contractors. This included contact with
project managers of current and prior NASA satellite programs as well as their
industry counterparts. (3) Requests for unpublished information were made to
NASA and industry. (4) A mail survey which was designed to establish specific
mechanism experience and also to solicit opinions of what should be included in
a future Space Mechanisms Design Guidelines Handbook.
The majority of the work was done at MTI under contract NAS3-27086. The
following acknowledgement section also lists some organizations and individuals
who contributed to the work.
The literature review required the assistance of knowledgeable technical
personnel. The assistance of Dr. Dantum Rao was helpful. Dr. E.M. Roberts
of the European Space Tribology Laboratory (ESTL) provided the European
literature review and a listing of experts; his efforts are acknowledged and
appreciated. Mr. Bobby McConnell of Tribotech Consultants also provided
valuable information from his knowledge of Air Force Space Mechanisms programs.
The authors appreciate those who responded to the Space Mechanism Survey.
Special recognition goes to Mr. Richard Fink and David Marks of the Honeywell
Electro Components Division, Durham, North Carolina, and to Mr. Bryan Workman of
the Honeywell Satellite Systems Operation who organized their many responses.
Recognition also goes to Laurence Bement of the National Aeronautics and Space
Administration Langley Research Center (NASA-LaRC) for his contribution on
Pyrotechnics; to Claudia Woods of NASA Goddard Space Flight Center (NASA-GFSC);
to Dennis Egan of Applied Innovation; and to Stuart Lowenthal of Lockheed
Missile & Space Company, Inc. We would also like to acknowledge the
reviewers of the manuscripts: Dr. Michael Khonsari of the University of
Pittsburgh, Mark Siebert of Toledo University and Ralph Jansen of the Ohio
Future National Aeronautics and Space Administration (NASA) space missions will
require advanced performance standards, increased life, and improved reliability
of mechanical systems and their components. Enhancements require learning from
past experience and transferring technology to newer generations. Accordingly,
NASA has embarked on a program to produce a Space Mechanisms Handbook that will
provide guidelines and recommendations to future mechanism designers. As part
of that program, a Lessons Learned study was performed to determine prior
anomalies and how to avoid them in the future. This report provides the
information obtained during the Lessons Learned study.
Three major categories of mechanisms were selected: deployable appendages,
rotating systems, and oscillating systems. Subsystems of these major categories
are as follows.
Retention and Release Mechanisms
Bearings, Lubrication, and Tribology Considerations
Antennas and Masts
Actuators, Transport Mechanisms, Switches
General and Miscellaneous
Control Moment Gyroscopes
Bearings and Lubrication
Slip Rings and Roll Rings
Information for the Lessons Learned study was retrieved from a
number of sources including:
Available Literature. The literature review proved to be
the most significant source of information. In particular, the 28 Annual
Proceedings of the Aerospace Mechanism Symposium was an extremely valuable
resource. Also, a NASA-Goddard publication on deployable appendages was very
informative. In compiling the literature review, a specific format was adhered
to. The ingredients of the format are described in Volume II, Literature
The constraints of the literature search limited publications to those that
described anomalies and/or lessons learned. Mechanism descriptions contained in
these publications were also summarized and documented for subsequent use in
generation of the handbook.
Industrial Survey. A survey form was created, which is
presented in the Survey Results section of this report. Over 600 surveys were
mailed with approximately 30 responses. Some significant information was
provided, especially by the Satellite Systems Operation and the Electro
Components Division of the Honeywell Corporation, who spent considerable time in
preparing information. Other responses provided additional reference material.
Subcontracts. The European Space Tribology Laboratory
(ESTL) contributed a review of the European Literature and provided a listing of
European experts. Also Bobby McConnell, of Tribotech Consultants, who has
considerable experience with military applications of space mechanisms
This report is organized into two volumes. Volume I provides a summary of
the lessons learned, the results of a needs analysis, the survey responses, a
listing of experts, a description of some available facilities, and a
compilation of references. The completed literature reviews comprise Volume II.
This section summarizes the lessons learned from the Survey Results and from the
Literature Review (Volume II) performed for the three main categories
(deployable appendages, rotating systems, and oscillating systems) and their
respective subsystems. Authors' names that appear in brackets, e.g., [Farley],
indicate that more detailed information on a topic is included in Volume II
under the same category, subsystem, and author/expert name.
One damper required replacement because of an air bubble (reason not
stated). However, it could be attributed to the selection of the viscous damper
fluid. McGhan-Nusil CY7300 silicone fluid is preferred because of stable
viscosity and low outgas characteristics.
A pin puller shaft fractured and rebounded into an unfired position.
Excessive hole drilling in this puller caused the failure. Pin pullers should
be x-ray inspected.
The array experienced inconsistent behavior of a microswitch; quality
assurance needed improvement.
Excessive bearing friction was experienced due to poor characteristics of
molybdenum disulfide (MoS2) lubricant at cold conditions. Under conditions of
cold temperature (-44 F) and the balling up phenomenon of MoS2, moisture
molecules could create frozen balls in the path of the rolling elements impeding
available driving torque. In fact, this is the major contributor believed to
have prevented the solar array from initially deploying while the spacecraft was
attached to the orbiter remote manipulator system. Application of
sputter-coated MoS2 with proper run-in may have avoided some of these problems.
Spring drives must use a minimum torque ratio of four. The deployment drive
systems each had a torque ratio of less than three. However, each drive had
redundant torsion springs and analysis indicated ample capacity for successful
deployment. In looking at the ERBS solar array deployment, it is conceivable
that a combination of cold temperatures, thermal gradients, MoS2 lubricant, and
insufficient torque margins are likely to result in deployment problems.
Torque increases and fluctuations can be caused by wire harnesses due to
cold temperatures and complex wiring paths. Testing is required at simulated
environments to resolve torque problems. [ESTL, Hostenkamp]
MILSTAR employed a flexible substrate solar array. A summary of lessons
learned during its development are as follows: [Gibb]
Minimize deployed mass at the deployed end of the array or mast (leave the
cover at the base).
Conduct analyses and test to ensure adequate blanket container preload.
Testing should include acoustic and shock testing.
Maximize spreader bar stiffness in the deployment plane, and perform
analysis to ensure acceptably low deflection to avoid panel warping or
Be aware that MoS2 coatings have a coefficient of friction dependent upon
humidity. Variations are on an order of magnitude between ambient and vacuum
conditions. Do not apply MoS2 coatings to both surfaces of a mating pair or
else a higher coefficient of friction will result than if only one surface is
To avoid panel sticking, insist on high-cleanliness standards during cell
bonding, especially when the process involves cutting film adhesives.
Do not rely on preload and friction to hold a blanket stack in place during
ascent; use a positive mechanical device, such as pins, skewers, or interlocking
Pip pins are used on many space mechanisms. Pip pins are most often used
when an astronaut will have direct interface with the mechanism. The main
reason for incorporating pip pins is convenience and their ability to provide
quick release of interfacing parts. [Skyles]
To prevent locking balls from vibrating out of their sockets, four balls
instead of two should be installed to provide redundancy if one ball falls out
of its socket.
Swaged tethers could create a tear hazard to an astronauts pressure suit.
Tether rings should be solid or have welded ends. Split rings can allow
disconnect and are also a tear hazard.
Dowel pins for handle attachments could loosen from vibration and thermal
effects. The handles should be welded to the pins to avoid loosening.
Liquid lubricants or grease could freeze and cause seizure of the pins. Dry
film lubricants should be used to lubricate all internal parts of the pip pin.
Double-acting pins should be provided that allow release capability when the
handle is either pushed or pulled to facilitate operation.
A Teflon sleeve should cover the tether swage fitting and cable termination,
providing a smooth surface and preventing the possibility of astronauts
contacting frayed or broken cable strands.
Hitch pins are recommended where the pip pin only has to be removed and not
Further development of ball retention is required. The present method is by
staking which is subject to error. Inspections have shown incomplete staking
that would allow the balls to fall out. Also, the staked material is relatively
thin and stress concentrations can be created at the tip of the staked material.
Vibrations can cause fracture and subsequent failure by ball loss.
Problems experienced with pyrotechnic pin pullers include:
Blow-by across O-ring seals caused by wearing of the MoS2 coating on the pin
that deposited on the O-ring preventing the seal. For pistons, an
electro-deposited, nickel/Teflon coating is recommended. Hard-anodized aluminum
uncoated housings were recommended, although in an earlier paper it was
indicated that steel housings would not distort as much as aluminum and combined
with a durable dry coating on the pin improved functional performance. [Bement]
For pyrotechnic devices, tests must be carried out to ensure that a device
has an acceptable functional margin where the functional margin is a comparison
of the energy that can be delivered to the device and the energy required to
operate the device. [Bement]
For pyrotechnic devices, material impact strength is usually a more critical
variable than inertia load capability especially when the temperature drops.
When the inertia load of a deployable system is low, a ductile material should
be selected, as opposed to a brittle material, for the actuator housing and
For a pin puller lot acceptance test, at least one puller should be exposed
to 125% explosive power in a cold environment. [Phan]
If the spacecraft requires a high level of cleanliness, then multiple
pyrotechnic seals must be used to prevent gas leakage. A hybrid sealing system
consists of Viton O-rings and silicon O-rings in tandem. [Phan]
A resettable binary latch mechanism was developed utilizing a paraffin
actuator as a motor. The polyamide rail failed due to unexpected high inertia
loads. Substitution of titanium resulted in galling. CDA 630 aluminum
nickel-bronze was finally selected because of its resistance to corrosion and
superior wear characteristics. After initial wear-in (approximately 50 cycles),
the toggle was burnished from contact with the rail and neither part
demonstrated significant wear during subsequent testing to 20,000 cycles.
For the same binary latch mechanism discussed above, thermal testing
revealed interference between the output shaft and its bushing at low
temperatures. The bushing, fabricated from polyamide, shrank into the output
shaft at -60o C and created friction. Enlarging the bushing bore corrected the
problem. Thermal binding must be considered not only for high-temperature
operation, but for low temperature as well.
Iteratively designing a complex mechanism in computer-aided design (CAD) and
using pasteboard mock-ups can be a more efficient process than detailed
mathematical analysis of component geometries (components can be visualized
throughout their range of motion, interferences can be identified and
eliminated, and kinematics and component shapes can be easily optimized by
simultaneously seeing the effect of changes in all positions).
The patented Lockheed Super*Zip spacecraft separation joint was planned for
use on the space shuttle to release the Centaur vehicle (120 in. diameter), the
inertial upper stage (91 in.) at the same separation plane as the Centaur, and
the Galileo spacecraft. Following functional failures of two out of a group of
five Lockheed Super*Zip spacecraft separation joints in a shuttle/Centaur
thermal development test series to quantify thermal effects, an evaluation
program was initiated on this and the related inertial upper stage and Galileo
systems to assist in preventing a recurrence of failure. The results of the
investigation revealed that thin ligaments are better than thick ones because
they are more difficult to sever under explosive conditions. Also, a
single-cord configuration indicated no trend toward tube rupture with increased
explosive load as did a double-cord configuration. [Bement]
A survey has been compiled on pyrotechnic devices for a 23-yr period that
included 84 serious component or system failures, 12 of which occurred in flight
with fully developed and qualified hardware. Table 1 lists anomalies.
The 1987 failure of two Magellan pin pullers has far greater potential
impact than is initially apparent. This pin puller was the same unit fully
qualified for the Viking Lander spacecraft for the 1976 landing on the surface
of Mars. After this experience and with its pedigree, two units from a
duplicate lot of pin pullers failed to function in a failure mode not recognized
in the original design, development, and qualification. First, the extremely
dynamic pressure impulse output of the NSI (as designed) when fired into a small
eccentrically vented cavity was severely attenuated, reducing the energy
available to stroke the piston. Furthermore, the bottom of the cavity was
deformed by the pressure into a groove in the piston, which had to stroke to
pull the pin. This device may have always been marginal when operated by a
Table 1: Anomalies of Pyrotechnic Devices
Firing pin assemblies corroded and
Pin puller failed during
system test (cartridge
Lack of understanding
Pin puller ruptured during
containment margin and variation in metal grain
Pin puller failed to stroke
against flight side load
output restricted, causing reduced output and housing deformation
Bad design; misapplication of
Pin puller failed to
function in LAT
produced insufficient pressure caused by coatings of pressurized
hardware; lack of understanding
Changed manufacturer and
Bolt cutter failed LAT (improper
For the source of failures, the shocking statistic is that 35 out of
84 (42%) of the failures were caused by lack of understanding; that is,
the personnel working the problem at the time, did not have the
technology needed to understand and correct the failure.
Unfortunately, 24 were mistakes caused by poor designs and
misapplication of hardware, which means that personnel did not apply the
known technology. The next 23 failures have to be categorized as
carelessness through manufacturer's poor procedures and quality control.
Since pyrotechnics are single-shot devices, past approaches for
demonstrating reliability have relied heavily on developing statistical
verification without a clear understanding of functional mechanisms and the
relative importance of system parameters. That is, once a successful
performance was achieved, emphasis was placed on accomplishing large numbers of
consecutive successes. (More than 2000 units are needed to establish a 99.99%
reliability at a 95% confidence level.) However, the current approach often is
to run full-scale systems tests on as few as six assemblies or less with no
statistical guarantee of reliability, and without an adequate understanding of
how the mechanisms function, which can be a prescription for disaster. [Bement]
The high-output paraffin actuator provides an alternative device to the
mechanism designer requiring significant mechanical work from a small, compact,
reliable component. The work can be generated from heat provided by internal
electrical resistance elements or from environmental temperature changes.
In internally heated configurations, the advantages over conventional
electrically powered actuators can be significant (low weight, resetability,
full verification before flight, high force, long stroke, gentle stroke, and
flexibility in materials of construction). [Tibbits]
Structural latches for modular assembly of spacecraft and space mechanisms
are exposed to a number of problems. Problems and suggestions are as follows.
Load control is a particular problem with hook systems where the load is
usually preset by rigging. Load changes due to thermal or dynamic fluctuations
cannot be compensated.
The selection methodology described by McCown, is an excellent guideline for
For roller screw latches, thread engagement is improved by providing
Rolling element latch interfaces reduce particle generation. The addition
of Teflon wiper seals control loose particles in the roller screw structural
latch and receptacle nut.
A run-in and clean-up procedure reduces particle generation from initial
actuations. The only reliable method to control the roller screw structural
latch preload was to limit motor power.
At launch, the near-infrared mapping spectrometer of the Galileo spacecraft
had two covers in place to protect the instrument from contamination. Two and a
half months after launch, initial attempts to eject the covers were
unsuccessful. It was subsequently determined that this was due to differential
expansion caused by the shield heater being energized. The shield heater was
turned off, the covers allowed to cool, and they were successfully ejected.
Untested flight sequences frequently result in unexpected events. Because of
concern for contamination caused by spacecraft outgassing, a flight rule was
modified prior to launch requiring the shield heater to be energized before
cover deployment. The flight rule was in error by not requiring heater shutoff
before deployment. The lessons learned are that the cover should have been
tested in the flight environment and the consequences of flight rule changes
should be carefully evaluated. [JPL SSEF, Schaper]
During one of the solar array deployment tests in 1989, a pin puller in a
release mechanism was actuated, the pin was retracted inside its housing to
release the solar array, but then rebounded back out of its housing. The normal
function of a pin puller is to retract and stay flush inside the pin puller
housing. The malfunction of this pin puller did not stop the deployment of the
solar array because of the mechanical redundancy of the release mechanism. The
rebound of the pin outside the housing forced us to investigate our pin puller
design. The result of this investigation showed that an extra shear pin hole
was accidentally drilled at 120o away from the original shear pin hole on the
pin puller. From past pyroactuator design experience, it was determined that
material impact strength is usually a more critical variable than the ability of
the pin puller to withstand the high inertia load of a deployable system,
especially if the device is required to operate in a cold environment. Material
impact strength drastically drops when the temperature drops. It is recommended
that when the inertia load of the deployable system is low, a more ductile
material be selected over a brittle material for the actuator housing and
piston. It is also recommended that for the pin puller lot acceptance test,
that at least one test include subjecting the pin puller to zero inertia load in
the shear direction, and in a cold operating temperature, actuate the pin puller
with 125% explosive power. [Hinkle]
Some gas molecules will leak out of a pyrotechnic actuator assembly during
the actuation and some over a period of time after actuation, because most
pyrotechnic actuators have only a single Viton O-ring. If the spacecraft
requires an extremely high level of cleanliness, then a hybrid sealing system
must be designed for the actuator assembly. Hybrid sealing systems design
consists of a Viton O-ring and a silicon O-ring that are located side by side in
the leakage path. [Hinkle]
Microwelding of high-load contact areas can occur from induced random
vibrations that will prevent smooth transition of linear devices. [ESTL,
Thermal problems are prevalent with actuators and retention and release
mechanisms. Differences in coefficients of thermal expansion must be thoroughly
explored to avoid jamming and excessive torque. [ESTL, del Campo]
Vibrations can cause unwanted deployment of components. Additional
restraint is sometimes required. [ESTL, Henton-Jones]
Microswitches on the Magellan were mounted such that they would detect the
position of the solar panels (as opposed to the status of the latching
mechanism). As a result, they were just a position indicator and not a
"panels locked" indicator. Care should be taken in deciding where to
mount telemetry transducers to assure that the desired function is actually
being measured and not just a related function. [JPL SSEF, Wagoner]
The mechanical joint between the biaxial drive assembly and its mounting to
the spacecraft allowed the ACTS transmit antenna to shift locations during
launch. The four bolts did not maintain the proper preload and the joint
slipped during launch loading. This has reduced the available design adjustment
for the ACTS transmit antenna.
All mechanical joints that require precise alignments should not use
friction to maintain alignments during any type of loading. All types of
alignment joints should be matched drilled with body-bound bolts or be drilled
and pinned after assembly. [Survey, Collins]
A paraffin actuator used to open and close the main sensor cover on the
Clementine spacecraft experienced a heater failure during acceptance testing.
There were two causes of the problem: excessive temperature and stress from
driving the heater at high voltage (36 V), and mechanical stress on the heater
element from flowing wax within the actuator during heating. The problem was
resolved by remounting the heater and dropping voltage by incorporating a
resistor in series with the heater. In the future, a circuit with a zener diode
will be installed to keep the operating voltage from varying excessively.
A similar approach will be considered for other mechanisms sensitive to
variations in supply voltage and especially for other heat-actuated mechanisms.
Mechanisms that depend on frictional characteristics to restrain a load
during launch vibration may slip and relieve the applied load. Components such
as worm gears and lead screws are normally considered to be nonbackdrivable.
Certain conditions of vibration can cause backdriving to occur. [Survey, Fink]
Because of concerns about the nonconductive Tufram coating allowing a charge
buildup that might affect instruments for the Polar satellite, a change to a
conductive coating (NEDOX) was directed. Although, the coefficient of friction
of NEDOX was better than that of Tufram, the NEDOX-coated V-band failed to
release during acceptance testing. Previously, an engineering unit with a
Tufram-coated V-band was successfully released more than a dozen times. Extreme
care should be used when changing even the simplest process or procedure from
what had worked previously. Testing of the new coating prior to acceptance test
was bypassed due to budget and schedule constraints and the similarity of the
two coatings. In the end, neither schedule nor budget was saved and testing had
to be repeated with the final V-band design, which consisted of Tufram coating
only on the contacting surfaces of the band to minimize surface area for charge
buildup. [Survey, Osterberg]
The x-ray telescope covers for the Alexis spacecraft failed to operate
consistently during thermal vacuum testing. On orbit, three of the six covers
failed to open on the initial command. The lifting mechanism was not sufficient
to completely break the O-ring seal. Avoid covered O-rings if at all possible.
Spring-energized Teflon seals are a better solution. If O-ring seals are used,
ensure that the complete seal area is actively broken through by high-force
actuation. If it is not possible to actively break the seal, kick-off springs
opposite the hinge line should be used to supply the initial torque rather than
additional hinge line spring force. [Survey, Tibbits]
Thin-section, four-point contact ball bearings are increasingly employed in
spacecraft mechanisms because of the potential advantages they offer. Internal
preload, housing design and external axial clamping force on the bearing rings
all have a strong influence upon torque, conductance, and stiffness. The
thermal conductance of a dry or marginally lubricated bearing depends upon the
thermal strain and varies linearly with radial temperature difference between
the rings. Additionally, conductance is a function of type and quantity of
lubricant. The Coulomb torque exhibits almost a square relation with thermal
MoS2 solid lubricant films were prepared by radio frequency magnetron
sputtering on 440C steel, 52100 steel, and silicon substrates. [Hilton]
Multilayered films exhibited excellent endurance in sliding wear and thrust
bearing tests. Friction coefficients in ultra-high vacuum ranged between 0.05
With metal multilayer films, the optimum structural spacing is in the order
of 10 Nm.
Minimum metal layer thickness is best.
A thin surface overlay of pure MoS2 seems to facilitate transfer to an
Solid lubricant films are used in a variety of mechanisms on various
spacecraft and launch vehicles. Relative to liquid lubricants, solid lubricants
generally have lower vapor pressures, better boundary lubrication properties and
relative insensitivity to radiation effects, and operate in wider temperature
Lead coating has had good success as a solid lubricant in vacuum
Optimum performance of lead and other metals is achieved at approximately 1
um (micron) thickness.
Deposition of soft metals (Pb, Au, Ag, In) by ion plating provides excellent
adhesion. These films have been particularly effective in spacecraft bearings
found in solar array drive mechanisms in European satellites and on the Hubble
A particular disadvantage of lead is that it oxidizes rapidly and must be
stored in vacuum dry environments.
Gold and silver are used in situations requiring electrical conductivity.
Sputter-deposited MoS2 has a lower coefficient of friction than ion-plated
Pb (0.01 versus 0.1), which means that MoS2 components should develop less
General lubrication problems and lessons learned with spacecraft deployable
appendages include: [Devine]
Seizures of relative motion surfaces caused by excessive friction.
Vibration-induced fretting and adhesion due to excessive clearance in caging
Unlubricated surfaces exceeding bearing yield strength of substrate on
Seizures caused by dissimilar materials with high mutual solubility.
Maximum utilization of rolling surfaces as opposed to sliding motion should
Lubrication or separation of all moving surfaces either by suitable
aerospace grease or dry lubricant coating should be used. No exceptions are
allowed, even for lightly loaded friction-compatible surfaces.
On hard mating surfaces where hard coatings are used (such as Type III
anodizing on aluminum), loads must be kept below the bearing yield strength of
the substrate material (e.g., 60 ksi for 6061-T6 aluminum).
Smooth and polished surfaces are preferred.
Dissimilar material mating surfaces should have no mutual solid solubility
or at least one of the two should have a heavy dissimilar coating (e.g.,
nitride, carbide, or oxide).
Caging devices should be designed to positively preclude relative motion
between clamped surfaces when subjected to shipment or launch vibration.
Wet lubrication is generally preferred because friction is low and
predictable. The grease with the most heritage is the Braycote 600 series, a
synthetic-fluorinated oil-thickened grease with micron-size Teflon powder. The
grease has extremely low outgassing (TML <0.1% and CVCM <0.05% for the
standard 125 C 24-hr test) and concerns relative to contamination are negligible
for virtually all spacecraft applications. The wet lubricant usable temperature
range is -80 to 200 C.
For extreme low temperatures and cryogenic applications, solid lubricants
are preferred. Epoxy and polyamide-bonded films can be successfully employed
with proper application and burnishing to remove excess material.
Table 2 summarizes the technology shortfalls currently affecting Air Force
and Strategic Defense Initiative Organization (SDIO) mission requirements for
deployable components, together with suggested tribomaterials (or design)
solutions to overcome shortfalls. [Fehrenbacher]
Table 2: Tribomaterials for Deployment Mechanisms
Tribomaterials / Mechanical Solution
Solar Array Drive
Reversible fast stow and deploy (10-sec
rotation (0.3 to 15 deg/sec)
10- to 15-yr life, high torque
with very small
Reliability under quick transition from stowed to
Synthetic hydrocarbons (low vapor pressure and
(low friction and wear)
New polymeric retainers for ball
Launch load protection
Shape memory alloy, fatigue / reliability
Solid-lubricated mechanical release
Based upon a detailed study of the performance records of almost 400
satellites between 1958 and 1983, it was established that about 10% of the
successfully launched satellites had some type of deployment anomaly, the
majority of which were mechanical. [Feherenbacher]
Many of the problems experienced by satellites were due to thermal or
thermal gradient problems that reduced clearances or caused lubricants to fail.
It was demonstrated, on a test of a spacecraft oscillating scanner that the
polyalpholefin (PAO) oil provided excellent lubrication, consistent torque with
negligible torque noise, and good wear to 22,000 hr with the test still running.
The other oils (chloroaryalkylsiloxane (CAS), originally used in the
application, and a perfluoropolyalkylether (PFPE)) exhibited a reduction in
torque (loss of preload) and an increase in torque noise, as well as extensive
wear after a few thousand hours. [Feherenbacher]
The best options for solid lubricant films for space applications appear, at
present, to be ion-plated lead and ion-sputter deposition of, and/or
ion-assisted deposited, MoS2. These lamellar films have demonstrated very low
friction operation in sliding and/or high low-load rolling bearings and latch
and release mechanisms. They are under development for a variety of ball
bearing applications. For solid lubricants for satellite gears, lead films were
found to provide good lubrication after a breaking in period that produced a 100
Angstrom elastic film. Best results were obtained for a film thickness of 1
micron. Solid films of MoS2 and TiN in the same application resulted in
unacceptably short gear lives, demonstrating that tribomaterials must be
tailored to the system design. MoS2 has, in fact, been shown to be an excellent
lubricant for many space applications, although its reaction with atomic oxygen
requires further study. Recent sputtered-deposited films have shown a 10 to
100% increase in film life over previous MoS2-coated bearings. [Feherenbacher]
Both titanium carbide and titanium nitride have been demonstrated to be
effective wear coatings under appropriate conditions. Titanium carbide has been
applied to gyroscope ball bearings and has increased operational lifetime by an
order of magnitude in this application when used with an uncoated steel raceway
and superrefined mineral oil. To date, titanium nitride has been used only on
tool steels, but the Aerospace Corporation is currently testing both gimbal and
spin bearings with titanium-carbide and titanium-nitride-coated balls.
The development of new polymeric bearing retainer materials is a critical
need to achieve required bearing lifetime. The phenolic materials that are
commonly used have been demonstrated to absorb oil in a time-dependent and
nonreproducible manner. These retainer materials are unacceptable for the
missions under consideration unless an active lubricant supply system is used.
Avoid the use of wet lubricants where optical devices or sliding electrical
contacts are employed. [Rowntree]
PFPE fluids have produced high torque noise and excessive ball bearing wear.
The precise mechanism of degradation is still not established. In Europe, the
evidence points to chemical reaction between nascent wear particles and the
exposed oxygen in the Z-type molecule. The product is described as a metal
polymer, or "brown sugar", which is autophobic and thus repels the oil
from the ball/raceway contact region. [Rowntree]
Ion-plated lead films are extensively used in Europe. In solar array drives
alone, more than 2 million operational hours in orbit have been accumulated.
An important property of the lead film is its high load-carrying ability.
Under Hertzian contact, the as-deposited film flows plastically until a thin
film (10 Nm thickness or less) remains and then elastically deforms the
substrate. In this condition, the film can survive contact loads approaching
the static load capacity of a rolling element bearing. [Rowntree]
Thin films of gold have also been investigated as a bearing lubricant, but
the higher yield strength and ductile adhesional property results in
work-hardened debris and thus high torque noise. Silver and indium have been
investigated too, but actual usage in space is not reported. [Rowntree]
Sputtered application of MoS2 is the preferred and, perhaps, necessary
method. Sputtered films of MoS2 are currently applied to numerous space
components such as screw threads, ball bearings, sleeves, and bushes.
An investigation at ESTL of MoS2 films deposited by magnetron RF sputtering
in which the rate of evaporation of the MoS2 target is greatly increased by
magnetic field intensification of the plasma, led to significant gains in
triboproperties under vacuum. Not only is the friction coefficient remarkably
low under pure sliding motion (values of 0.005 to 0.04 are typical), but the
wear resistance of the film is greatly improved. [Rowntree]
Ion-beam application of MoS2 also appears promising, although it has not
been used in space mechanisms. A feature of this method is an increase in the
density of the film and it is possible that this compaction may help in
extending film lifetime. [Rowntree]
MoS2 works well with ceramics, such as titanium carbide and hot-pressed
silicon nitride. [Rowntree]
An extensive study restricted to air usage in the United Kingdom over 10
years ago, showed that the PTFE/glass fiber/MoS2 combination was preeminent in
terms of friction torque and endurance, provided that the maximum Hertzian
contact stress was kept below 1200 MPa at room temperature. [Rowntree]
Friction torque of ball bearings under thrust load and low temperature is
difficult to predict. Unexpected increases in friction torque have been
experienced. Testing at temperature is recommended to determine torque levels.
Linear rolling elements and cages can creep, which leads to high torque
spikes at the end of travel. These high forces were generated as the cages were
driven into contact with the bearing end stops, at which point any further
movement of the nonstationary races resulted in sliding motion between the races
and the sliding elements. In addition to causing higher friction forces, the
effect also resulted in more rapid wear of the MoS2 film. To prevent roller and
cage creep, and thus eradicate high end forces, a cage speed control device is
required to ensure the correct cage to ball speed ratio. [ESTL, Roberts]
Considerable care must be exercised when mounting close-clearance bearing
components into aluminum structures that must operate at cold temperatures.
Contraction of the aluminum must be accounted for. [Survey, Lowenthal]
The field of view of one of the single-axis antennas was restricted,
probably due to a pinched or snagged electrical cable that runs across one of
the single-axis antenna gimbal joints. The joint cable operation should have
been checked on the ground and the design modified accordingly. Cable circuitry
should avoid regions where the cable can get caught or snagged.
The single-axis antenna delayed deployment by nearly 3 hr when one of the
compartment attachment lugs came into contact with the compartment kick-off
spring mechanism. Interference between actuation devices and attachment lugs
should be avoided.
One of the single-axis antenna drive motors stalled because the biax service
loop harness became pinched between the boom and compartment. The motor was
reversed to relieve the pinch, and deployment proceeded normally. Reversible
motors can help correct deployment problems.
On the second flight of the INTELSAT V spacecraft, the time required for
successful deployment of the north solar array was longer than originally
predicted. The south polar array deployed as predicted. The difference in
deployment time was found to be due to a significant increase in hinge friction
at low temperatures and vacuum. The hinge friction problem was overcome by
increasing the bearing clearances to allow for greater temperature variations
and giving the hinges special lubrication.
The Galileo's high-gain antenna, which opens like an umbrella, never reached
the fully deployed condition. [Johnson]
The failure was caused by galling and excessive friction in the midpoint
restraint pins and V-groove socket of the struts, which required mechanical
drive torques in excess of motor capacity to free the pins and permit
Contact stress of any mating surfaces should not be great enough to cause
plastic deformation and/or destroy applied coatings.
Friction in vacuum can substantially exceed friction in atmosphere,
especially when coatings are destroyed and galling occurs.
Moments applied to ball screws severely degrade their capacity.
The use of a dry lubricant, specifically MoS2, on a mechanism that is going
to be operated in an atmosphere should be carefully evaluated. The wear rate of
the MoS2 in air is so much higher than in a vacuum, that any coatings could be
worn out by air testing and shipping lubrication, and not provide the desired
lubrication when needed. Replacing dry lubricated surfaces just prior to
launch, so that virgin lubricant surfaces are available is recommended, if
Shipping vibrations and ground testing can destroy coatings and dry
Vacuum deployment tests on the ground, should include simulated vibrations
prior to deployment.
During ground testing of the dynamics explorer, an end-of-travel shutoff
switch failed to activate during Astromast deployment. The microswitch failure
in space could have been catastrophic and points to the necessity for switch
redundancy for mission critical components. [Metzger]
The mechanism was to deploy/restow two large Hubble space telescope
deployable appendages in a varying but controlled manner. The initial predicted
aperture door mechanism temperatures could be well below -125 F. This proved to
be a problem for the grease plating of Braycote 3L-38RP on the angular contact
bearings. The solidification temperature of this lubricant is approximately
-120 F. The resulting stiffness of the lubricant caused unacceptably high
bearing torques even though the mechanism would operate to as low as -160 F.
Braycote 3L-38 RP grease works well in angular-contact bearings in a vacuum if
the temperature is kept above the grease solidification temperature. The range
of motion of the hinges is approximately 90. For the aperture door mechanism,
this motion takes place over 1 min, and for the high-gain antenna hinge, the
time is 7 min. [Greenfield]
During subsystem testing of the high-gain antenna configuration, the tests
were plagued by a problem that was finally diagnosed as lost motion. This
resulted in variable performance at the stowed position. In a mechanism, it is
important to eliminate all backlash to avoid lost motion. [Greenfield]
The Gamma Ray Observatory (GRO) had two solar array wings weighing
approximately 500 lb each and one high-gain antenna boom assembly weighing
approximately 525 lb. The high-gain antenna did not deploy when it was
initially commanded. A portion of the antenna release mechanism (close to the
antenna dish) was caught by a piece of thermal insulation blanket. The lessons
learned are: [Hinkle]
Deployment tests should be accomplished with the final configuration
including thermal blankets. Spacecraft attachments should be simulated
Interference that might be caused by thermal blankets should be evaluated
during the design process.
Mechanisms must be evaluated for vibration problems and appropriate damping
applied. Vibrations can cause unwanted deployment that must be constrained.
Stowage time can inhibit deployment mechanisms, especially at soft-contact
interfaces that require relative motion at deployment. Rubber contact could
cause separation problems by sucking or sticking. MoS2 coatings can alleviate
this problem. [ESTL, Barho]
Brush contact encoders are prone to a wear problem that renders them
unsuitable for use on high-accuracy, high-reliability space mechanisms. [ESTL,
After positioning GEOS in its final orbit, its eight booms and five
mechanisms were deployed. Two axial booms showed anomalies during deployment
and one of these, a long axial boom, extended to only about 80 to 90%. To
reduce the possibility of friction due to cold flow of guide rings, the
tightening torque was reduced and the Teflon guide modified. The release
mechanism modification mainly concerned the ball release piston and ball cage
area. The hard edge of the titanium sleeve was replaced by a soft aluminum
chamfer to prevent indentation of the balls. The ball cage holes that were
cylindrical in GEOS-1 are now conical to improve the ball release. [ESTL,
During the Hubble space telescope antenna pointing system testing, the
internal thermostats failed making the internal heaters inoperable. External
heaters were bonded to the outside surface of the gimbal housing with externally
mounted thermostats, completely bypassing the external circuit. Where possible,
the wiring for internal heaters and thermostats should be completely accessible
in case the internal components fail, during protoflight tests. [Ruebsamen]
During acceptance testing of space telescope antenna pointing system
gimbals, the external cover of the heater dislodged during thermal cycling test.
The cause was incompatibility of the coefficient of thermal expansion between
the cover materials with the housing materials and the fact that the cover and
housing were gold plated, which prevented a proper epoxy bond between two parts.
Three lessons learned were:
Heater covers should include a mechanical means of mounting along with the
epoxy bond (i.e., screws).
The area where the epoxy bond is to occur should be free of gold plating.
If external heaters are to be used, an epoxy designed for use as a thermal
conductor should be designated and its coefficient of thermal expansion should
match that of the major structure. [Survey, Ruebsamen]
A deployment actuator mechanism was developed for the Topex satellite.
Post-vibration testing showed that the dry lubricant film in the journal bearing
was flaking causing an increase in torque. The problem was determined to be
excessive lubricant film thickness. Applying the lubricant to one bearing
surface rather than both and burnishing the film to reduce thickness resolved
the problem. [Jones]
Thermal vacuum testing of the Topex deployment actuator viscous fluid rotary
damper revealed a region of undamped travel, immediately after deployment had
been initiated, followed by normal operation throughout the remainder of the
travel. The result was unacceptably high-impact loads in the damper input shaft
as damper operation returned to normal. Potential explanations included air
pockets and ineffective thermal compensation. The reason for the anomaly was
never confirmed and another damper was installed. Further investigation of
rotary dampers is required. [Jones]
For a mirror transport mechanism, loads were developed during vibration
testing that caused a rocking motion of the dihedral platform resulting in a
pivoting motion about the latch cone axis. This placed excessive loads on the
pivot flexures, causing them to fail. To limit the load, the pivots were
enclosed in sleeves that restricted radial movement to acceptable levels.
On April 11, 1991, a command to unfurl the Galileo spacecraft high-gain
antenna resulted in a stalled motor about 50 sec into deployment [Johnson],
which was considerably short of full antenna deployment. The prevalent theory
of cause has been that the undeployed ribs' locating pins were still locked or
stuck in their receptacles due to a misalignment taper plus a high-friction
condition. Tests revealed that the transportation environment resulted in a
classic fretting condition. Since the fretting condition of small oscillatory
translational movement coupled with high-frequency cycling was not duplicated in
friction testing, a coefficient of friction greater than 1.4 could have
resulted. The lesson learned was: [Glenn]
Packaging for shipment should not allow relative motion between components.
Redundant push-off springs should be incorporated for initial release of all
spacecraft deployable appendages. [Sharma]
An anomaly occurred with a plunger-activated, hermetically sealed switch
during ground testing of the Upper Atmosphere Research Satellite (UARS).
Gravity effects caused the plunger to deflect away from the switch preventing
motor cutoff at the desired position. The solution was to redesign the switch
activation device so it was not gravity sensitive. The lessons learned are:
Mechanisms must be designed for both ground test and space operation.
Plunger designs of switches could pose problems on ground test due to
gravity. Cam actuation may be preferable.
For harmonic drive support bearings, two oils were tested: NPT-4 (a
neopentylester spacecraft oil) and Pennzane SHF 2000 (a synthetic hydrocarbon
oil). The effects of antiwear additives tricresyl phosphate (TCP) and
naphthenate (PbNb) were also investigated. Pennzane with TCP gave the
longest life. The failure mechanisms with the two oils were
different. Bearings tested with oils plus TCP failed due to wear.
Bearings tested with oils plus PbNb failed due to a hard, lead-containing carbon
film, which resulted in high friction. From this it appeared that TCP
would be effective in light-load applications where low friction was important,
while PbNb would be more suitable for high-load applications where friction was
of secondary importance. [Kalegoras]
An optical actuator required accuracy better than conventional systems.
Table 3 summarizes the approaches taken by Lockheed to overcome the limitations
of conventional designs. [Lorell]
The use of a motor-driven screw or gear mechanism was rejected because of
mechanical inaccuracies. Piezoelectric devices require high voltages. The best
choice appears to be a voice coil-type actuator that can have high bandwidth
capabilities and is both simple and reliable.
The force unloading system proposed by Lockheed may be useful in other
satellite applications where it is necessary to maintain a continuous power
input. Eliminating the need for bearings and lubricants by the use of flex
pivots also has merit.
Dehydration of brake materials and accumulation of wear debris, trapped
between the opposing surfaces, can cause a marked reduction in friction of brake
materials. Problems have been encountered with the asbestos/phenolic friction
elements of the shuttle remote manipulator system. When slip tested under load,
the pads showed a greatly diminished friction in vacuum, which is fully
recovered on return to atmosphere. Polymers are also unacceptable because they
will not provide sufficient friction as a brake material in vacuum. Lessons
learned are as follows: [Hawthorne]
Some ceramics or cermets can provide stable and moderately high friction as
brake materials. This group includes Cr2O3 and Al2O3/SiC.
To ensure in-vacuo stable friction, run-in of opposed surfaces is
Table 3: Lockheed Solutions to Limitations of
Use of an electromagnet actuator in an
special low-noise sensor electronics
Use of electromagnetic actuator and
moderate equivalent gear
Stiction / friction
No bearings or lubricants;
exclusively flex pivots
High power consumption
Four-bar linkage (lever) and
Inability to cancel static friction
During development of robotic arms, the Europeans have found that
phenolic/asbestos brake materials present torque anomalies under thermal vacuum
A movable stop mechanism activated flaps to change telescope aperture on
command. The mechanism consists of a rotary solenoid that drives dual four-bar
linkages in synchronism to rotate butterfly flaps into position. During
testing, the mechanism jammed in the open position. Galling and scuffing of the
surfaces of a fixed stop and the mating stop surface on the actuating arm had
occurred. Some lessons learned are as follows: [Tweedt]
Ion-plated lead lubrication proved to be satisfactory for a lightly loaded,
low-speed, intermittent journal bearing type of application at cryogenic
temperature and in a vacuum.
Tungsten-carbide coating was effective in preventing galling and cold
welding of the contacting surfaces on the fixed stop and the mating surface of
the actuator arm when subjected to impact on contact.
The importance of exactly replicating the fits, geometry, and assembly
parameters of the engineering models in the subsequent production of flight
units has been very positively demonstrated.
Low temperature can cause reduction in clearance and consequent high
torques. Heaters may have to be applied to gear heads and other devices if
excessive friction results. [ESTL, Cawsey]
Panel hinges should have spherical bearings with axial clearance to avoid
Dampers are necessary to reduce kinetic energy at impact.
Tioxode-V provides an acceptable hard slippery coating. Avoid molydisulfide
Cone support angle <30 to avoid locking.
Joint actuators must have sufficient margin to overcome low-temperature wire
harness torque. Capability should be tested at temperature with wire harness.
Sensors should be applied to deployment devices to determine initial motion,
intermediate position, and latch-lock indication
For articulation motors, sensors should be applied for output shaft
position, null reference indication, speed, and current.
United States Air Force Mil Standard, MIL-A-83577B, sets forth general
requirements for the design, manufacture, quality control, and testing of moving
mechanical assemblies to be used on space launch vehicles. Many of the
requirements listed are based on anomalies in spacecraft. Deployables shall
(where practicable) be designed so that they are self supporting when placed in
any orientation relative to gravity while in either the stowed or deployed
configuration. Deployables shall be designed with sufficient motive force to
permit full operation during ground testing without depending upon the
assistance of gravity to demonstrate deployment.
Retention and Release Devices. Positive retention provisions shall be
provided for deployables in the stowed and in the deployed position. The
effects of deflections such as those induced by centrifugal forces or
differential thermal growth of any deployable with respect to its space vehicle
attachments shall be considered in the design of the attachments. Devices that
may be subject to binding due to misalignment, adverse tolerances, or
contamination shall not be used. Slip joints shall be avoided (where
Pin Pullers. Where pin pullers are used, such as cartridge-actuated or
nonexplosive pin pullers, they shall be designed to be in double shear. The
design, installation, and checkout procedures for pin pullers shall ensure that
loads due to misalignment of the pin are within design limits. A minimum
retraction force margin of safety of 100% at worst-case environmental conditions
and under worst-case tolerances shall be maintained for all nonexplosive pin
Bearings. For deployables, hinges, and linkages, self-aligning bearings
shall be used (where practicable) to preclude binding due to misalignments.
Bearings shall not be used for ground current return paths or to carry electric
current. All ferrous material bearings shall employ (where practicable) a
corrosion-resistant steel that is in accordance with QQ-S-763. Rolling element
bearings shall (where practicable) be of 440C stainless steel; however, 52100 or
M50 steels may be employed providing they are suitably protected from corrosion.
Dry Film Lubrication. Application of dry film lubricants to the surfaces of
bearings, V-band clamps, coil springs, leaf springs, clock springs, constant
force springs, gears, or other items shall be by an appropriate process.
Bonding, peening, sputtering, vacuum deposition, ion plating, or any other
process that provides a predictable, uniform, and repeatable lubricant film may
be appropriate. Composite materials containing dry film lubricant in their
composition may be used in appropriate applications. Where appropriate, dry
film lubricants should be burnished to provide a uniform film that reduces the
coefficient of friction from the as-applied condition and minimizes the
generation of lubricant powder. Corrosion-resistant materials shall be used in
bearings employing dry film lubricants. Consideration shall be given to
protection of MoS2 dry film lubricants from adverse affects due to exposure to
atmospheric humidity. Testing in a humid environment shall (where practicable)
either be avoided or minimized.
Hard Coatings. Hard coatings such as titanium carbide, titanium nitride,
and chromium may be used to extend life, reduce wear, prevent welding, reduce
friction, and prevent corrosion either with or without a liquid dry film
Cryogenic cooling is necessary for infrared detectors. There is a need for
a remotely controlled, motorized cryovalve that is simple, reliable, and compact
and can operate over extended periods of time in cryovac conditions. The
lessons learned are as follows: [Lorell]
In general, the mechanical problems that are encountered with cryomechanisms
are the result of: a) mismatches in the coefficients of thermal expansion, and
b) the friction and wear properties of moving parts.
Motors should be of the brushless or stepper design because brushes are
unreliable in vacuum. They must have adequate power to overcome friction even
with unlubricated surfaces. Motor leads to the outside are also thermal paths
along which heat can travel.
There are two sources of heat that can be of concern. One is the thermal
energy generated by the equipment inside the shell and the other is thermal
energy from the outside traveling along the wires.
Since the mechanisms are usually located inside the shell where they are
inaccessible, it is critical that they function with a high degree of
A 10-yr review of the major test observations at ESTL is given, during which
time some totally unexpected failure modes have been detected. Full confidence
now exists in many mechanisms and component designs, and much valuable data have
been obtained that are available to mechanism designers for improving
reliability. Lessons learned are as follows: [Parker]
Thermal vacuum testing has proved to be essential in providing a detailed
assessment of the reliability of complex mechanisms by subjecting them to
realistic simulations of the anticipated flight conditions, where lifetimes in
excess of 10 yr are now expected.
Thermal vacuum tests have been proved to be cost-effective in avoiding
delays and disturbances to a number of European projects, as several previously
unknown failure modes have been detected. There is now complete confidence in
many designs following independent, fully documented performance assessment.
Much valuable data have been obtained on many mechanisms and components
about their operational parameters, power dissipation, and wear processes.
There is frequent evidence of how important it is to implement comprehensive
inspection and product assurance systems at all stages of mechanism development
and construction, to avoid the human factors of accidents, errors, and poor
The solar maximum mission satellite was launched into orbit with experiments
to monitor solar activity. To obtain common object observations, experiments
must be coaligned within 90 sec of the spacecraft pointing vector. Lessons
learned are as follows: [Federline]
Using kinematic principles and good design practices, it is possible to
produce a stable support platform that is isolated mechanically and thermally
from its supporting structure and from experiments mounted on it.
Through the use of reference surfaces, gages, and optical measuring
techniques, it is possible to coalign experiments to a high degree of accuracy.
Several panels on the long-duration exposure facility were coated with
Everlube 620C, a common solid lubricant. It was completely degraded due to
ultraviolet exposure. Phenolic systems are susceptible to ultraviolet
degradation, a fact that should be transmitted to design engineers. [Survey,
Almost every deployment device related to a spacecraft on-orbit
configuration change is a mission-catastrophic single-point failure if it does
not function properly. The following are some ground rules from lessons learned
for designing such devices: [Hinkle]
All deployed appendage programs must have engineering test units.
All flight units and engineering test units must be testable to determine
Analyses must be verified by judicious hardware testing programs.
There must be adequate life testing early in the program.
There must be redundant backup systems in all critical areas.
Worst-case analyses and failure modes effects and critical analyses must be
performed and verified by actual hardware testing. Conditions that must be
considered include worst-case friction, misalignment, and excessive preload.
All devices should be designed to be as simple as possible to do an adequate
Consider the effects of mounting system redundancy and structure-induced
input forces not only on the devices but also on the internal components of the
Look for all possible hostile environmental effects and design to minimize
their impact. Pay particular attention to vacuum, thermal control, and g
effects that are not always intuitive to the designer.
Select devices that are directly testable and reusable to be qualified by
analysis rather than single-use devices that are statistically qualified to a
Use the largest possible margin of operation in all devices consistent with
consideration of undesirable effects on the surrounding hardware. These
undesirable effects include large forces developed by end-of-travel latch-up and
shock from pyrotechnic device firing.
Proper installation should be verifiable. Knowledge of preloads, position
of parts, status of switches or other electrical interfaces should be known or
Bearing lubricant depletion between the ball race retainer causes cage
instability and subsequent pointing errors, increased bearing torque, and wheel
The practice of using steel bearings lubricated with mineral-oil-based
greases or superrefined mineral oils with porous phenolic cages is unacceptable.
Tests by Aerospace Corporation have shown that the phenolic cages continue to
absorb oil from the bearing ball contact regions during operation instead of
supplying oil, thereby hastening the onset of cage instability. [Feherenbacher]
An active oiling system can be incorporated to periodically lubricate the
bearings and avoid erratic torque behavior. The trade-off is added complexity.
The characteristic cage instability frequency is an inherent geometric mass
property of the cage/bearing system and is essentially invariant to external
vibration, bearing speed, or lubrication condition. [Lowenthal]
A biased cage has a different instability pattern and frequency than the
commercial unbiased cage. Both the cage motion pattern and the instability
frequencies are reasonably predictable. [Lowenthal]
Each cage-bearing design has a critical friction coefficient for
instability. The ball cage interface is much more critical than the cage land.
This was also predicted by computer simulation. [Lowenthal]
Increased lubricant viscosity enhances the chances and severity of
instability. Room temperature grease triggered instability, while room
temperature oil and warm grease did not. [Lowenthal]
Simulation computer codes can predict cage instabilities for steady-speed
conditions. They cannot predict stability onset as speed is ramped up.
Commercial codes are available from P.K. Gupta and Avcon Corporation. [Lowenthal]
Bearings shall meet ABEC 7, 7P, or 7T tolerance (or better) in accordance
with the AFBMA standards. [USAF MIL Standard, MIL-A-83577B; 1 February 1988]
Momentum wheel bearings shall operate in the elastohydrodynamic film regime
and confirmed by analysis or test. [USAF MIL Standard, MIL-A-83577B; 1 February
Magnetic bearings have the potential to provide a superior alternative to
ball bearings. [Yabu-uchi]
For a combined Earth scanner and momentum wheel, the bearing lubricant
condensation on the rotating scan mirror cannot interfere with the infrared
radiation reflection of the mirror. Pennzane X2000 was the preferred lubricant
over Bray 815Z because it has a much higher transmission in the region of the
horizons sensor's infrared bandpass. With the exception of its lower viscosity
index, the Pennzane X2000 was superior to the Bray 815Z. [Bialke-3]
Ball bearing lubrication remains the principal life-limiting problem on
momentum and reaction wheels. [Auer]
Means for lubricant replenishment can improve life characteristics. A
lubrication reservoir actuated by centrifugal force to relubricate the bearings
is described. The base oil is stored as a grease in a ring-type chamber which
is centrifuged out through orifices. The rate is limited by the thickener of
the grease that forms a microporous filter in the vicinity of the orifices.
Overlubrication, as measured by torque increase, does not appear to be a problem
for this system. [Auer]
Oil lubrication is favored over grease because of superior torque
characteristics and means of replenishment. [Auer]
The minimum amount of initial lubricant required is 2 mg. [Auer]
With the replenishment device, extended life appears promising. The test
results and flight operations are promising and not conclusive. Two ground test
wheels (3000 and 3800 rpm) had run for 16 yr after full qualification. These
wheels had been subjected to temperature cycling and showed minimal changes in
torque. The longest operational time in space was the OTS wheel, which had run
11.5 yr at the time this paper was presented. [Auer]
Passive oilers, which are programmed to release lubricant at a predetermined
rate, do not apply lubricant as needed. Overlubrication or underlubrication can
occur. Data from the digital signal processor indicate that after about 3 yr of
operation, the spacecraft again manifests instabilities associated with
lubricant depletion. A need exists for an active oiler, commandable from the
Bray 815Z lubricant, which has the positive qualities of low vapor pressure
and high viscosity index, is not a suitable lubricant for a reaction wheel.
Bray 815Z is a synthetic fluorocarbon which is unable to dissolve antiwear
additives. It performs well when the operating speed is sufficient to form an
elastohydrodynamic film, but it has poor performance in the boundary lubrication
regime. Thus, the Bray lubricant is not acceptable for a reaction wheel that
must go through zero speed. [Bialke]
An acceptable lubricant for reaction wheels available from Pennzoil is
Pennzane X2000 (a synthetic hydrocarbon) with 5% PbNp as an antiwear additive.
It has a very low vapor pressure, good viscosity index, and good boundary
lubrication qualities. [Bialke]
An ironless armature motor is ideal for the reaction wheel drive being both
power and weight efficient. [Bialke]
A Hall generator is the preferred tachometer. It has good accuracy, low
complexity, low power consumption, and zero speed measurement. [Bialke]
Stability of highly accurate pointing devices, such as those on the Hubble
space telescope, can be destroyed by reaction wheel assembly induced vibrations.
A Sperry damping device alleviated the problem. The central element is a
viscous fluid damped coil spring suspension system. Each reaction wheel
assembly is suspended on three units. Damping is provided by a low-volatility
silicone-based fluid (Dow Corning 200 series) confined by metal bellows to
internal cavities. [Hasna]
Conical Earth sensors on several satellites experienced lubricant failures
because the Bray 815Z lubricant is unstable at boundary lubrication conditions.
Changing to a chemically stable hydrocarbon lubricant (Pennzane 2000) with
extreme pressure additives, such as TCP or PbNp, resolved the problems.
The lifetime of oil-lubricated bearings is very temperature dependent.
Higher temperature results in higher evaporation rates and more surface
migration. Also, higher temperature lowers the viscosity which reduces the
elastohydrodynamic film. [Bialke-2]
The disturbing torques produced by reaction wheels at near-zero speed are
considerably greater than the normal torque noise level, with consequent
reduction in attitude control. For sensitive missions, adequate tests are
indispensable for the controller design. Bearing cage instability can be
catastrophic and is difficult to predict. This occurred on a reaction wheel.
ESTL and SNFA arranged a cure by using cages with loose-fit pockets and applying
10 ml of oil. Flight bearings will have nonequispaced pockets of the original
diameter. [ESTL, Stapf]
For reaction wheel support bearings, three oils were tested: SRG-40 (a
highly refined mineral oil), Nye 179 (a synthetic PAO oil), and Nye UC-7 (a
synthetic polyolester (POE) oil). All of the oils were formulated with TCP, and
tests were carried out in different atmospheres. The tests showed that the TCP
did not function as an antiwear additive in UC-7. Nye 179 was considered the
best choice for this application. Its performance in vacuum was far better than
that of SRG-40, and its performance in helium was the best of all oils tested.
The use of oxygen as a component of the fill gas of reaction wheels was
determined to be unnecessary and generally harmful to the life of the bearing
The major contributor to torque noise is the dc offset in the drive voltages
and the transmission gearing. [Cook]
A small amount of cross coupling between the inner and outer gimbal servo
loops causes variations in frequency response as a function of the inner gimbal
angle. These variations appear to be acceptable for current applications but
future improvements are needed. [Cook]
A single gimbal can provide greater torque capability for the same angular
momentum than a double gimbal. However, a double gimbal has less complex
control laws and greater flexibility to support a large variation in vehicle
The wheel material and dimensions should provide the necessary angular
momentum with a safety factor of 4 based on a yield stress at 105% of nominal
A two-stage parallel-path spur gear transmission with installed windup of
one gear with respect to the other will eliminate undesirable backlash. [Cook]
A control moment gyroscope bearing did not receive adequate lubrication from
a centrifugal lube nut. The following modifications were made to improve oil
Cage oil feed hole was reduced to overlap outer race groove under all
An additional set of feed holes was provided to centrifuge oil into the
center of the outer race contact area.
The retainer flange was widened and the ID slope changed to accommodate the
oil hole angle and increase lube nut overlap.
The retainer OD was increased to allow maximum extension into the race
groove of oil feed holes. [Survey, Dolan]
Worm gears can experience excessive torque under cold vacuum conditions due
to lubricant starvation. Soft extreme pressure greases that contain MoS2 are
effective in alleviating the problem (e.g., Braycote 608). [Purdy]
Careful worm gear run-in is essential to good operation for worm gears. The
break-in should be gradual with light loads and abundant lubrication. Purdy]
Techniques for replenishing the lubricant as it is wiped off the tooth
surfaces are beneficial to worm gear operation. On the Rexnord mechanism, a
wiper system was installed to force grease back onto the gear teeth. [Purdy]
In worm gears, unacceptable lubricant starvation can be caused by allowing
the gears to reach a stall condition. [Purdy]
General design guidelines for worm gears are given in Table 4.
Table 4: General Guidelines for Worm Gear Systems
Make the hob as nearly identical to the worm as possible.
Use slightly larger center distance for hobbing.
Optimize contact prior to break-in.
Make face width a maximum of 50% of worm diameter.
Avoid high-contact load on outer edges of gear teeth.
Avoid low-pressure angles on low-tooth-count gears.
Total count (worm gear) should be a minimum of 40.
Avoid geometric interference.
Avoid low speeds and stall.
Low speed promotes severe boundary lubrication.
Grease lubrication may require special techniques to maintain performance.
Oil film benefits from replenishment such as an oil bath.
Use fine surface finishes.
Improves lube and wear.
Set the gear setup so that initial contact pattern is on the
leaving side of the gear.
Provide oil reservoir on the entering side. Pattern will
grow to cover entire width over life.
Break in gradually with loads and abundant lubrication.
Break-in greatly increases life.
In a dual-wound dc brush motor gearhead, a shaft failure occurred by a
fracture in the cross section from the gear face to the bearing spigot. The
failure was attributed to excessive stress concentration and was ameliorated by
increasing the blend radius from 0.125 to 0.250 mm and thus reducing the stress
concentration factor from 4 to 1.5. The lesson is to examine the design for
stress concentrations carefully and ensure adequate safety margin. [Henson]
In a dual-wound dc brush motor gearhead, bearing failures were experienced.
Bearing loads should be carefully examined and a double bearing applied, if
necessary. A Tuftride process applied to the bearing spigots reduces wear
debris and avoids bearing contamination. The Tuftride process should be applied
after the bearing spigot is finished ground. The growth of the Tuftride process
is insignificant, and if applied prior to grinding, it could be removed by wear.
All gears used in moving mechanical assemblies shall be in accordance with
the standards of the American Gear Manufacturers Association (AGMA).
Hunting-tooth gear ratios shall be used, where the application is appropriate,
to distribute wear. For better protection of the gear teeth, the through
hardness or surface hardness (or both) may be increased, and the surface finish
of the teeth improved through grinding, honing, lapping, and prerun-in. The
through hardness may be increased by material or heat treatment changes. The
surface hardness may be increased by nitridng, carburizing, induction hardening,
or anodizing. Undercutting of spur gear pinions should be avoided. [MIL
Standard, MIL-A-83577B; 1 February 1988]
An harmonic drive flex spline galled severely at the bearing/flexcup
interface. The anomaly occurred due to poor material selection. Materials must
be carefully selected to avoid galling of sliding surfaces. Also, lubrication
helps to prevent galling. [Survey, Farley]
BRUSH-TYPE MOTORS SHOULD BE AVOIDED. CARBON BRUSHES WEAR EXCESSIVELY IN
VACUUM. Wear debris contaminates the bearings, increasing drag and reducing
life. Accumulated debris shorts the commutator, increasing current and
resulting in motor failure. Some organizations take necessary precautions in
material selection and coatings that permit brush motors to perform
satisfactorily (see Table 5). The use of these motors, however, require
substantiation by experience and/or test. [Sharma]
Table 5: Actuators Using Brush Motors
High-torque gear motor
150 ft-lb torque driver
Solar Maximum Repair
Latch gear motor
Solar Maximum Repair
Linear actuator (1000 lb)
Linear acutator (15 lb)
Gamma Ray Observatory
Umbilical disconnect mechanism
Rocket nozzle extension
Atlas Centaur II
dc common drive unit
dc gear motor
Gear motors (various sizes)
Solar boom deployment
Atmosphere Research Satellite
Redundant drive motor
Worm gear drive
Payload spin motor (integral hp)
Titan Launch Vehicle
Brushless dc, permanent-magnet dc, and brushless stepper motors are the
preferred motors. [Sharma]
Stepper motors can have positioning errors due to:
The drive electronics
The attitude control electronics
A power supply interruption
A single-event upset in the electronics
A mechanical failure of the unit. [Sharma]
The torque margin for any motor > 3, where TM = Ta/Tr - 1
Ta = available torque
Tr = resistance torque
TM = torque margin. [Sharma]
The design torque margins should be verified during flight testing. [Sharma]
In rotary actuators, a hard-stop collision can cause the rotor to continue
to turn, which elastically winds up the harmonic drive. The spring energy of
the harmonic drive can catapult the motor backward three steps as it unloads.
The motor then picks up the pulse signals as if it were starting from standstill
and drives into the hard stop repeating the same series of events over and over.
A hard stop must be avoided by proper application of end-of-travel limit
Motor drives for rotary actuators should be a dc torque motor or stepper
motor. The dc motor should be of the brushless permanent-magnet type with
position and velocity sensors. [Sharma-2]
The output shafts of rotary actuators should be supported by a pair of
back-to-back duplex bearings (for high-moment resistance) preloaded for a
desired stiffness and life span. [Sharma-2]
In a brush motor gearhead, brush debris caused problems and reinforces the
view that brushless motors should be applied if possible. The brush material
was Boeing Compound 046-45 because of good wear resistance in vacuum. It
consists primarily of MoS2, which requires purging for operation in normal
atmosphere. Post-test examination revealed brush debris that had blown around
the gaseous purge applied during air operation. During the initial phase of the
ambient life test, the winding current trace became noisy and the shaft speed
reduced. It was concluded that the temporary anomalous performance was caused
by a brush fragment. [Henson]
Motor winding redundancy is recommended in the event of winding or
connection failure. A clever scheme is described by Henson. [Henson]
Stepper motor stability is very dependent on friction and damping and it is
important to ensure that adequate friction and damping are present over the
range of operating conditions. [Kackley]
Superimposing rotordynamic behavior and separatrices on the phase plane
technique is a valuable tool for analyzing stepper motor stability. [Kackley]
Simulation analyses is valuable to determine problems and solutions prior to
building and testing expensive hardware. [Kackley]
High bearing torque was encountered on the despin drive assembly of two
flight models of the HELIOS solar science satellite. Improving the
perpendicularity of the lower bearing inner race seat from 0.8 to 0.3 arc-min
dropped bearing drag at -50 C about 40%. [Phinney]
Bearing distortions can increase bearing torque. On the HELIOS despin drive
assembly, a favorable indexed rotation of the end plate reduced torque
significantly. The end plate and aluminum housing were distorted. [Phinney]
Small, fractional horsepower motors are likely to experience
cold-temperature performance problems if Pennzane or Rheolube bearing lubricant
is used. The lubricants become very stiff at cold temperatures and start-up
torque becomes appreciable. A lubricant-channeling phenomenon was observed,
which interfered with cold motor start-up and running capability. Braycote
Micronic 601 bearing lubricant did not interfere with motor performance. Small
rotary components may be sensitive to lubricant effects not seen in larger
hardware. Special testing should be planned to evaluate cold-temperature
lubricant start-up as well as running torque in small components. [Survey,
Retainer instability is a major cause of bearing failure in control moment
gyroscopes, momentum wheels, and reaction wheels. [Boesiger]
There is a critical friction level associated with each retainer design,
beyond which the retainer is unstable. [Boesiger]
Ball pocket friction is more critical to stability than retainer land
friction for the bearings investigated by Boesiger. [Boesiger]
Optimization of the retainer design was accomplished by computer simulations
coupled with experimentation. Computer codes were useful tools and
qualitatively compared with experiment. [Boesiger]
Operating ball bearings lubricated with Z25 (a perfluoroether) in a vacuum
results in an interaction between the lubricant and the races. This resulted in
race wear and oil degradation. [Baxter]
When ball bearings lubricated with YVAC 40/11 (a perfluoroether oil
recommended for instrument bearings) are operated in a vacuum, there is no oil
degradation and no wear. However, the higher viscosity of the YVAC 40/11 leads
to undesirable running torque. [Baxter]
Modification of the bearing surfaces with inert coatings, that results in
decoupling of the lubricant from the bearing surfaces offers the best course to
ensure maximum lubricant life. [Baxter]
Most mechanical problems with momentum/reaction wheels, control moment
gyroscopes, and gyroscopes are lubrication problems. Table 6 provides a sample
of experience. [Fleischauer]
There are two primary types of lubrication problems:
Supply or loss of lubricant
Chemical reaction (oxidation, polymerization) of lubricant. [Fleischauer]
Synthetic oils can increase life by a factor of 10. [Fleischauer]
Sputter-deposited solid lubricant thin films provide low friction and long
Hard coatings and ceramic parts are used for low torque noise.
Lubricant additives, such as TCP, provide protection of contacting surfaces
to reduce wear and minimize torque. [Fleischauer]
The relative wear life of PAO lubricants is significantly greater than PFPE
Lubricant replenishment is a major problem with spacecraft bearings. A
centrifugal bearing cartridge design, invented at the Draper Laboratories,
provides a method for a continuous and controlled supply of lubricant without
incurring excessive torque. [Singer]
Table 6: Partial Listing of Momentum / Reaction Wheel,
Control Moment Gyroscope, and Gyroscope Experience
Navstar / GPS
Reaction wheel; four per satellite
On-orbit and test failures; high torque
New lubrication qualification
High-speed cage instability
Force, mass resonance
Force, mass; biased cages
Bearings / lubricant could not be delivered
Extensive bearing run-in and screening
Large momentum wheel
Torque / temperature anomalies
Drive rate / torque
lubrication, cage processing
Large control moment gyroscopes; > one per satellite
Lube loss, cage instability
Active oiler system, new oil
Retainerless instrument bearings avoid cage instability and are advantageous
if ball impact is not a problem. [Singer]
Screening tests provide an accurate and expeditious approach to predict
bearing cartridge performance. [Singer]
Accuracy, mobility, and lifetime requirements require tribological
interaction early in the design phase. A significant number of spacecraft
anomalies are attributable to tribology problems as outlined in Volume II, page
329, Table 1. [Fleischauer]
Problems in rotating assemblies are often caused by ineffective lubricant
supply, caused by inadequate initial amount, loss via transport processes,
normal consumption without resupply, and chemical degradation. [Fleischauer]
Perfluorinated lubricants cannot dissolve antiwear additives, and thus are
not suitable for boundary-lubricated applications (reaction wheels, gimbals).
PAO lubricants significantly outlast a silicone oil for oscillatory motion.
It is expected that these synthetic oils can be applied to high-speed
applications to provide added protection against retainer instability and wear.
Titanium-carbide-coated balls have been used in gyroscope bearings with
uncoated steel raceways and superrefined mineral oil to produce operational
lifetimes a factor of ten or more longer than for uncoated balls. The
commercial process for coating bearing parts with TiC is available in the United
States but has only been used for instrument bearings of the type used in
gyroscopes. Titanium-nitride coatings are used for tool steels to provide much
longer service lives and considerable research and development is underway to
use TiN for bearings and gears. Tests are currently under way to test the
performance of both gimbal and spin bearings with TiC- and TiN-coated balls.
Analysis of lubricants from laboratory tests in control moment gyroscope
bearings show depletion of oil from grease samples taken from control moment
gyroscope spin bearing cage surfaces and no depletion from samples of bulk
grease. Analysis of metal parts show little evidence of wear although some
metal is found in degraded lubricant. Analytical simulations and measurements
of bearing motions suggest cage instability may be involved in retainer wear and
ultimate bearing torque increases. [Fleischauer]
PAO and multiple-alkylated cyclic compounds (Pennzane) are excellent
candidates for use in spin bearings of reaction/momentum wheels, in solar array
drives, and in rate gyroscopes. The PAO lubricant did well in both hard vacuum
and in atmosphere of 380-torr helium gas. [Fleischauer]
The formulation of Pennzane with TCP in solution outperformed the other
systems tested, however, considerable wear was noted. [Fleischauer]
Pennzane plus Naphthalene had less torque life than with TCP additive, but
there was no evidence of wear. The increased torque failure was caused by the
accumulation of excessive protective additive film containing metallic lead and
carbonaceous material. The conclusion is that varying the amount of Napthenate
could lead to superior friction and wear performance. Future tests are planned.
A run-in period is recommended for lubricated ball bearings to transfer
lubricant from the ball pockets of the sacrificial retainer to the balls and
then to the raceways. Approximately 1.3 106 cycles of run-in were accomplished.
An optical chopper assembly had power beyond specifications. The bearing
power loss is very sensitive to the amount of lubricant present and, from
observations during testing, this was the probable cause of the anomaly.
For low-speed operation (<3000 rpm) a fixed lubricant oil quantity in a
shielded bearing is adequate for a 10-yr life. Bendix applies shielded contact
bearings: R4A, R6, R8, and R10. Lubricants include Winsor lube (MIL-L-6085A)
plus 5% TCP. [Allied Signal Aerospace (Bendix)]
For high-speed operation (>3000 rpm), a make-up mechanism must be
provided to overcome the oil loss due to centrifugal force. Angular-contact
bearings (104H, 106H, 107H, and 305H) are applied. Bendix uses proprietary
design phenolic retainers and an active continuous lubrication system. KG-80
(MIL-L-83176A) superrefined mineral oil is utilized. [Allied Signal Aerospace
Grease lubrication for momentum/reaction wheels is not recommended because
of high running torque, torque variations, uncertain lubricant supply, and
retainer instability. [Allied Signal Aerospace (Bendix)]
To produce a boundary lubricating film using a liquid lubricant, a run-in
must be performed. [Vest]
For high-load boundary-lubricated contacts, a bonded solid film lubricant,
such as MoS2, is recommended. [Vest]
For slow-speed ball bearing rotation, a Teflon or MoS2 filled grease, or a
transfer film lubricating polymeric cage is recommended. [Vest]
Sliding motion applications are mostly boundary lubricated. A high
load-carrying grease with an extreme pressure gradient additive must be used.
The grease produces a high load-carrying solid film, such as Teflon, MoS2,
graphite, or TCP between the rubbing surfaces. [Vest]
Despin bearings lubricated with ion-plated lead were capable of meeting the
GIOTTO mission life with an insignificant noise spectrum. [Todd]
Tests of the complete energized despin mechanism on GIOTTO showed that the
stepper motor harmonics excited a strong undamped torsional resonance of the
antenna at certain speeds. [Todd]
After approximately three months of life testing, the lubricant in the
Galileo slip ring bearing (KG80 oil) had a thick, black, gooey appearance and
the bearing friction torque was higher than expected. Even careful cleaning of
spacecraft components can leave residues that may eventually react with adjacent
materials. Cleaning processes must be followed by an outgassing vacuum-bake
treatment. This is particularly important for porous materials that may have
absorbed various fluids, including the cleaning medium itself. [JPL SSEF,
Phenolic retainers must be carefully and thoroughly dried to remove any
absorbed moisture before they are impregnated with oil. Otherwise, the retainer
will not saturate and can absorb and remove oil from the bearing it is intended
to lubricate. Thus, the retainer becomes a liability rather than an asset.
Current telemetry can detect spin motor current, which is proportional to
the drag torque, and an increase in bearing temperature, which is also
indicative of the increase in drag torque. These measurements provide an
indication of the need to supply fresh oil to the system. The authors propose
to use Coray 100 (an uninhibited napthenic-base machine and engine oil with a
viscosity of about 110 cs at 40 C) to resupply the Andock C grease that
lubricates the control moment gyroscope bearings. The system is essentially a
pressurized reservoir with a solenoid activated by the loss of lubricant sensor.
[Smith] In the reviewer's opinion, in a weightless environment there are
questions about how a drop of oil will behave. Even at a distance of 0.004 in.,
the drop may not transfer smoothly. It may touch and then be slung off the
moving surfaces to create a number of finer droplets that will float around the
bearing housing. Any lubrication scheme should be evaluated in a weightless
X-rays can be used to provide images of balls in a bearing that are not
directly visible. Using the x-ray technique, it is possible to measure the
contact angle in assembled bearings. [Fowler]
Advanced elastohydrodynamic computer simulation techniques can provide
benefit in design of space lubrication systems. [Benzing]
Failure modes of despin mechanical assemblies operation include the following:
Insufficient bearing lubricant film thickness
Lubricant incompatibility with system materials
Inadequate lubricant quantity
Improper lubricant transfer
Bearing and cage instability
Slip ring and brush wear
Cage wear. [Benzing]
Bearings operating in the boundary lubrication regime (i.e., contact
asperities) shall be avoided where practicable. If bearings must be operated in
the boundary lubrication regime, a boundary lubricant with good antiwear
characteristics shall be used. Perflourinated polyether and silicone lubricants
should be avoided in this regime except where light loads and limited travel are
expected. Where bearing lubricant reservoirs are used, the reservoir shall be
attached, where practicable, to an area of relatively high temperature to
enhance molecular and surface flow into the bearing. Barrier films or shielding
or both may be used to separate the bearing from reservoirs to minimize surface
migration such as that caused by loss of lubricant due to wicking action of the
reservoir. Incorporation of the above techniques dictates that the lubricant
transfer mechanism be primarily by molecular flow. The preferred approach to
lubrication of bearings involves placing the reservoirs in intimate contact with
the bearing races and adding a larger amount of lubricant than would be
ordinarily required to provide acceptable lubricant films. The above method of
lubrication is preferred providing that any increase in churning torques can be
tolerated. [MIL Standard, MIL-A-83577B; 1 February 1988]
Bearing lubrication tests and supporting analyses shall be used to show that
the chosen lubricant transport mechanisms, such as surface migration,
vaporization, and wick action provide effective lubricant films over the
expected operating temperatures, thermal gradients, and internal environments.
If providing adequate life of bearings depends on their operating in an
elastohydrodynamic lubrication regime and not in the boundary lubrication
regime, and it cannot be clearly shown by analysis that the bearing operating
range as well is well within the regime, then a test method (such as contact
resistant measurements) shall be used to establish that an elastohydrodynamic
film is being generated. In general, the lubrication system variables that
should be substantiated by component development tests include (as appropriate)
amount of lubricant, retainer design, reservoir design, and the reservoir
proximity to the areas requiring lubrication. When liquid lubrication is used,
the design shall ensure that migration of the lubricant through the seals is not
excessive or detrimental to the space vehicle. [MIL Standard MIL-A-83577B; 1
Roll rings are effective rotary joint electrical transfer devices and avoid
the deficiencies of slip rings and flex capsules. Flex capsules are limited
with respect to rotation and fatigue life. Slip rings wear due to sliding
electrical contacts, generate debris, and require lubrication. [Batista]
Roll rings have had considerable development for both high- and low-power
applications. They are reliable, low-noise, drag-torque devices and should
receive primary consideration for rotary joint electrical transfer applications.
To accomplish noise reduction, plating processes, plating purity, and
cleaning processes must be carefully controlled. [Batista]
High-purity plating and elimination of metallic oxides from surfaces by
stringent reduction of low-nobility metals in the gold-plating process enhances
noise reduction. [Batista]
Software that models geometric tolerances and maximizes rolling efficiency
is very helpful to roll ring design. [Batista]
Flexure fatigue of the roll rings must be considered and the rings designed
to accommodate the specified life cycling. Fatigue tests carried out on
beryllium-copper roll rings made from bar stock showed that the endurance limit
was approximately 20% lower than published data. The published data were
generated from test samples that had grains oriented in the most advantageous
direction. Thus, the design of the roll ring should be based on a lower
endurance limit with adequate safety margin. [Smith]
Contamination of the flexures and ring surfaces can cause high noise. A
principal source of the noise is copper and lead oxides on the surface.
Contamination can come from plating, migration of substrate materials through
the plating, or migration from adjacent components. Great care must be taken to
ensure that contamination is not introduced during plating and is not allowed to
take place after plating. [Smith]
Corona effects can be prevented by avoiding line of sight between conductors
of different potential and by using appropriate insulation. [Smith]
A high correlation was found between the presence of silicones in the system
and resultant electrical noise. The primary source was silicone grease used to
lubricate other components. Silicone sources should be eliminated around roll
Primary sources of outside contamination include: organic films, silicone,
and metal oxides. Migration of metallic oxides can come from solder used to
attach the lead wires. In the Holloman roll ring design, solder was separated
from critical surfaces by plastic rings with good results. [Smith]
Particularly important in a signal roll ring application is the isolation of
adjacent circuits. [Smith]
For high power transfer, a multiple-flexure design in which the flexures are
separated by rolling idlers is required. [Smith]
Slip ring assemblies were constructed of gold- or silver-plated rings and
wire wipers lubricated with the same fluid lubricants used in the bearings of
the despin mechanical assemblies. However, extreme care is required to prevent
excessive oxidation of the MoS2 lubricant and simultaneous tarnish formation
that results in unacceptable electrical noise and even measurable torque
increases. Many such problems with electrical noise can be traced to the
fabrication and assembly practices during construction of the slip ring
mechanisms, but even after incorporation into satellites it is still necessary
to protect the brushes from atmospheric exposure. [Fleischauer]
Anomalous values of contact resistance was found in both pyrotechnic and
power/signal slip ring assemblies. Contamination of slip rings can occur if
they are exposed to atmosphere for any length of time. The material chosen was
Ag/C/MoS2 (12% MoS2). Subsequent high resistance was due to surface
contamination (possibly Ag2O or Ag2S). [Atlas]
For slip ring assemblies, provide adequate and proper lubrication of the
rings and brushes when self-lubricating contacts are not employed. A liquid
lubricant is necessary if significant rotation is involved. Ball Aerospace
Systems Division's (BASD) most widely used lubricant consists of a highly
refined mineral oil with extreme pressure additive. Recently, a synthetic oil
with improved characteristics has come into use. With the mineral oil,
reservoirs are placed along the brush access slots in the housing. Vapor
pressure of the new oil is so low that surface films are sufficient for
multiyear missions and reservoirs are not required. [Phinney]
Measure brush forces and correct, if required. Brush force must be set
carefully. BASD rings have used 3 to 5 gm of force and lubricants that produce
a friction coefficient of 0.3 to 0.5. [Phinney]
Brush wear particles remain under the brush pad and provide an additional
lubricating medium that prevents further wear. [Phinney]
The precaution of side-by-side brushes is unnecessary because of the 5-yr
demonstrated life with single-groove bearings. [Phinney]
For self-lubricating brushes: [Phinney]
Coat the brush springs with thin films of polyurethane.
To minimize vibration problems on brush assemblies of this type, maintain
brush height <0.090 in.
The Ag/MoS2 brush on silver is outstanding in vacuum but it is not good in
air. To eliminate electrical noise, slip rings with this brush material should
only be operated in dry nitrogen or vacuum.
Remove all humidity before starting.
For space applications, power brushes are operated at current densities in
the 100- to 150-A/in.2 range and contact pressures of 6 psi. Signal brushes
commonly have pad face areas in the 0.007 in.2 range (0.060 0.12 in.) or less
and brush force is set about 20 gm.
Friction coefficients are in the 0.25 to 0.50 range.
Conduct hard vacuum run-in tests, followed by disassembly, run-in wear
debris removal, reassembly, and checkout.
Evaluate slip ring performance during drive acceptance tests, which should
always include thermal vacuum operation.
Maintain coordination with suppliers. They have developed significant
experience and knowledge. Sources include: Electro-Minatures Corp, Moonachie,
New Jersey; KDI Electro-Tec, Blacksburg, Virginia; and Poly-Scientific Division,
Litton Industries, Inc., Blacksburg, Virginia.
Prepare definitive specifications.
Conduct detailed review of suppliers design, materials selection, and
Inspect critical manufacturing and test operations at the supplier's
Some experiences have indicated the need to improve the dynamic noise
performance of dry film lubricated, silver bearing slip ring/brush combinations.
To initiate noise, some form of dielectric contamination must exist at the
slip ring/brush interface. [Matteo]
A reduction in brush spring force to significantly less than the design
minimum force must occur to render the slip ring assembly susceptible to
The fewer the number of brush/ring sets in contact with each signal circuit,
the more statistically susceptible the circuit is to contamination-induced
resistance variations. [Matteo]
Critical sensor signals should be carried by two parallel slip rings, thus,
placing four brushes in parallel. [Matteo]
Synchronization of sensor sampling times and drive pulses must be maintained
at all times. [Matteo]
Slip ring assemblies of the dry lubricant type must be purged with clean,
dry nitrogen at all times (except when precluded by other tests) up to as close
to launch time as possible. [Matteo]
Margin above normal brush force (approximately 80%) should be provided to
account for in-process or in-service degradation. [Matteo]
The individual brushes of a brush pair should be electrically separated to
enable in-process measurement of individual brush/ring contact resistances
during testing. [Matteo]
Whenever possible, critical signals should be amplified before passing
across the slip rings. [Matteo]
Long storage times can result in slip ring contamination. They must be
examined and cleaned prior to installation. Also, investigations should be
conducted to provide nonpollutant materials. [ESTL, Atlas]
During flight assembly, an open shield and a shield shorted to a conductor
were discovered on a flight slip ring assembly. X-rays of the unit revealed
that the assembly had been improperly reworked at the vendor. Inspectors should
look for obvious signs of rework, such as a different color of epoxy, and the
paperwork should be checked if the rework was recorded. The data sheet should
have a line for each measurement and require that the actual meter reading be
recorded. The specification should have then been listed and a check mark
placed in either a pass or fail column. A quick scan would then tell if any
failures were present and still allow the detailed information to be recorded.
The test equipment, calibration, and temperature should also be required on the
data sheet. [Survey, Osterberg]
After accelerating to 30 rpm, the Teflon toroid ball separators on the GGS
slip rings shredded. The failure was traced to exceeding the pressure-velocity
limits of the toroid material. Toroids should be used with only lightly loaded
bearings due to the stress on the nonconforming outer diameter of the toroid to
the adjacent ball, which was three times higher than the pocket stress for GGS.
Toroids allow balls to bunch up, making it difficult to predict dynamic
performance. Bearing drag torques are less predictable with toroids.
Accelerated testing of bearings is not recommended because even at
subelastohydrodynamic speeds, the wear mechanisms can be very nonlinear.
Pressure-velocity curves should be determined and used to check all new retainer
designs. These curves need to be established for the various materials used and
the method of analysis made consistent for all programs. [Survey, Osterberg]
A potentiometer for a position sensor experienced excessive electrical
noise, unacceptable wear of beryllium copper contacts, and beryllium-copper
contact breakage. When lubricated with Bray 815Z, the lubricant beaded and
contained wear debris. Wiper contacts of Paliney-7 (a precious metal alloy
primarily comprised of Palladium), silver, gold, and platinum proved effective.
To assure adequate contact, the contact force was increased to 204 cN. The
material combinations must be compatible for rubbing contact and have low
coefficient of friction, and the mating surfaces must have sufficient preload to
assure contact. [Iskenderian]
Hub connections should not loosen during vibrations. In this instance, each
steel hub was first mechanically fastened to the shaft with two set screw joints
(one cone point, one cup point) at 90o to each other, then bonded with a bead of
epoxy at the shaft hub interface. The set screws themselves were blocked from
backing out by a drop of epoxy. [Iskenderian]
The potentiometers should not be contaminated by shipping packages.
Individual nylon bags were employed. [Iskenderian]
Cryogenic Grating Drive Mechanism
To avoid undesirable temperature gradients and barreling of the bearing,
flexible copper thermal strapping (shunts) were added to both rotating and
stationary components. [Dubitschek]
To ensure precise accuracy control of bearing preload over the temperature
range, a flexible diaphragm of similar thermal characteristics was incorporated.
Payload Spin Assembly
The dc drive motor assemblies failed insulation testing because of brush
wear debris and insulation cracking. The problem was resolved by applying a
coating of chemglaze to the windings. Anomaly reinforces use of brushless
The spin bearing drag torque, especially at -23 , was too high. The problem
was due to a mismatch in the coefficient of thermal expansions of aluminum
housings and steel bearing races. This was resolved by interference fitting the
steel races into the aluminum housings and then final grinding the steel
raceways in place. Thermal mismatch of bearing components and housing can cause
serious torque and cage problems. The potential problem should be recognized
and analyzed prior to build. [Robinson]
During run-up for a room-temperature operational test on the flight payload
spin assembly, the unit shut down after achieving an approximate speed of 30
rpm. All of the power FETS were blown due to a runaway oscillating condition.
No problems were experienced during testing of the engineering model. S-level
FETS were used in the flight unit, while low-quality FETS were used in the
engineering unit. Investigation revealed that the S-level FETS were too fast
for the snubber circuits and created instability, leading to a major failure.
The low-quality engineering FETS were slow enough for the snubber circuits to
handle. The lesson learned is that any changes made to a successful engineering
model design should be analyzed before incorporation into qualification/flight
hardware. [Survey, Robinson]
Multichannel Chopper System
Special floating mounts had to be developed for the slit plate and chopper
disk to maintain their dimensional accuracy and alignment. [Krueger]
To maintain dimensional tolerances under varying environmental conditions
and in the presence of thermal gradients, the slit plate and chopper disk were
made from INVAR 36, a low expansion metal. [Krueger]
To achieve the desired accuracy and minimize possible distortion from
internal machining-induced stresses, electrical discharge machining was used for
the final machining of the slit plate and the chopper disk and for machining the
apertures for these components. [Krueger]
For a close sliding fit of two shafts with limited motion, the dry
lubrication approach was unsatisfactory and increased the friction between the
two parts to an unacceptable level. A very small amount of Krytox oil applied
to the inner shaft was the solution. [Krueger]
Oil lubrication is not feasible for reciprocating machines in space because
of zero gravity. Grease-packed rolling elements are generally used. Cam
interfaces are critical life-limiting elements because of the large radius of
curvature of the cam's surface compared with the roller radius. Cams made of
nitrided steel or coated with tungsten carbide or titanium carbide deteriorated
rapidly. Excellent results were obtained with through-hardened steels for cam
and roller and also with boronized cam surfaces. [ESTL, Berner]
The original design of the ISTP despin platform employed 8-in. thin-section
bearings, a one-piece phenolic retainer, a hollow steel shaft, and an aluminum
scalloped housing with a band of titanium around the bearings. The bearings ran
at 10 rpm. The one-piece phenolic cage warped causing high torques. Teflon
toroids were installed and a full titanium scalloped housing was incorporated.
Lobing in the bearing due to mounting caused ball speed variations, high
retainer loads, and badly damaged toroids. Finally, a four-piece segmented
phenolic retainer was installed and life tests did not result in torque
anomalies. [Survey, Woods]
General information on anomalies and lessons learned for gimbal systems from
NASA-Goddard is as follows: [Sharma]
Oil replenishment for small oscillatory motions as experienced by gimbal
bearings is difficult. Torque magnitudes increase significantly due to oil
breakdown or bearing starvation.
In a test conducted at Hughes Aircraft Company, Bray 815Z had half the
initial torque of Apiezon C (with an extreme pressure additive) in a 4 gimbal
bearing test. However, after 7 104 cycles, the Bray lubricant turned to brown
sugar and the bearing torque quickly increased by a factor of 10. The Apiezon Z
continued without torque increase to the end of the test (8 106 cycles).
Lessons learned are: 1) to prevent destructive chemistry; surfaces in
contact must be passivated in some manner; and 2) ceramic hard coatings, such as
TiN or TiC, will eliminate catalytic action; replacing stainless steel 440C
balls with ceramic SiN4 balls eliminates lubricant breakdown; and, to prevent
oil starvation, it is good practice to use a porous ball retainer, which
functions like a reservoir of oil and dilutes breakdown products.
For typical gimbal mechanisms, ball bearings are forced to oscillate over
very small arcs (dither) and then turn to a new position and continue to dither.
The gimbal system combines the severity of boundary lubrication with fretting
motion of contacting surfaces. Gimbals are critical elements of most pointing
mechanisms, antennas, sensors (telescopes) and weapons platforms, and of control
moment gyroscopes. Usually, performance levels are met when systems are first
tested, but with time, lubricant degradation, bearing wear (or both) degrade
performance levels so that mission requirements no longer can be satisfied.
Gimbals and electrical contacts have consistently been a source of anomalies
and failures. Gimbal bearings that operate in an oscillatory (dithering) mode
and rarely make a full revolution are troublesome. [Fleischauer]
Ultra-low friction-durable films of MoS2 are deposited by various
ion-sputter deposition processes. They can be used for some sliding
applications, very low load bearings, or for latching and release mechanisms.
There is very encouraging evidence that ion-assisted, sputter-deposited MoS2
films can provide ultra-low friction operation even in air applications.
For oscillating gimbal applications, sputter-coated MoS2 films were
recommended. Benign ball retainers (those that do not transfer films) were
tested with good results. [Fleischauer]
During the thermal vacuum test phase of the GOES-7 spacecraft, the primary
scan mirror system exhibited unacceptably high drive friction. The observed
friction was found to correlate with small misalignments of the mirror structure
and unavoidable loads induced by the vehicle spin. The friction became very
high at the end of the oscillation. During the spacecraft spin tests, the
torque was found to be sensitive to spin speed and load. The solution to these
problems was to reduce the moment loads by using larger race curvature to reduce
alignment sensitivity. Also, the frame limit was shifted 5 whenever the torque
became too high, so the balls could roll over the torque bumps at the end of
CAS and PFPE oils were seriously degraded under oscillating load and vacuum
testing and produced excessive bearing wear and torque noise for <2500 hr of
operation. A PAO lubricant with a TCP wear additive did not cause bearing
failure and had run 11,000 hr with no indication of a problem. Its success has
been (attributed by the authors) to the TCP additive. This type oil should be
used for oscillating applications. [Carre]
When properly made and installed, lightly preloaded duplex bearings having
phenolic laminate separators and lubricated with thin films of BASD 36234 liquid
lubricant can withstand more than 16 million low-angle oscillating cycles
without any signs of degradation and without significant torque variation.
Blocking can occur in oscillating duplex bearings even at extremely narrow
angles of motion. Blocking is a condition where some of the bearing balls jam
into ends of the separator pockets as a result of creeping away from their
centered position. [Phinney]
Table 7 shows the effects of various factors on blocking. [Lowenthal]
Races should be slip fits, if possible, to assure proper performance and prevent blocking. [Phinney]
Soft (spring) preloading is better than hard preloading if bearing torque is critical. [Phinney]
Thrust bearing has all balls loaded; no
opportunity for ball spacing to readjust
Torque spikes are the result of buildup of debris at the ends of the ball
travel. The debris was primarily aluminum particles with small amount of
titanium debris and dry lubricant from the inner race retaining nut threads.
Using aluminum tools during assembly can produce aluminum
debris that contaminates
the bearings; use titanium tools instead. [Phinney]
Race conformity and separator type can have a dramatic effect on bearing
torque. Slightly opening the race conformity and switching to alternating ball
toroid separators reduced the excessive torque problem to an acceptable level.
Particular attention must be paid to gimbal bearings to avoid blocking.
Blocking torque phenomena for gimbal bearings is very much dependent on friction
levels between the ball and race as well as the retainer ball pocket. The
bearing should be well aligned, the race conformity should be increased as much
as possible without incurring a contact stress problem, and the ball retainers
should have either generous pocket clearance, slots or alternating ball toroids.
Bearing computer codes are useful in determining appropriate race
Excessive thermal gradients across the races can have a significant effect
on internal preload and contact stress and can cause torque problems in gimbal
bearings. Bulk temperature effects are much less severe. If tight control over
preload is necessary, heaters are recommended to maintain proper temperature.
Bearing computer codes are useful in establishing an acceptable operating
temperature envelope. [Lowenthal]
Large, thin-sectioned bearings in stiff mounts are particularly vulnerable
to torque excursions from rolled over debris and degraded lubricant. Prolonged
dither gimbal cycles should be minimized and periodic, have a longer stroke, and
maintenance cycles should be included to maximize bearing life. [Lowenthal]
Conformity ratio has a dramatic effect on torque levels. A buildup of
compacted debris in the contact zone, reduces the ball/race conformity ratio and
can cause a torque increase of a factor of five above normal torque levels.
Liquid lubricant torque levels are less than cage lubricants or solid
Of many lubrication systems tested, Pennzane SHF 2000 lubricant was the best
for all conditions of operation. [Gill]
Torque levels dramatically increase when direction changes. [Gill]
Variable angle of oscillations are preferred, particularly for solid
For oscillating scanner bearings, three oils were tested: G.E. Versilube
F-50 (a CAS oil); Brayco 815Z (PFPE oil); and Nye 188B (a synthetic hydrocarbon
oil, PAO). The PAO oil outperformed the other oils by a wide margin. The
primary reason for this was the presence of the antiwear additive, TCP, in the
PAO oil. The other two oils suffered rapid degradation. [Kalegoras]
For the angular-contact ball bearings in the shutter and filter wheel
mechanisms of the Michelson Doppler Imager (MDI), lubrication with Bray 815Z oil
met and exceeded the design life goals. [Akin]
For the thin-section ball bearings in the tuning motors of the MDI,
lubrication with Bray 815Z oil did not meet the design life goals, but
lubrication with Braycote 600 grease did meet the goals. [Akin]
Using bearings for small rocking motion applications has its problems. Even
with a porous retainer, there is no fresh supply of oil to replenish the
contacting surfaces when the motions are small oscillatory. Torque can
skyrocket as either the oil breaks down or the bearing starves. In a test
conducted at Hughes, Bray 815Z had half the initial torque of Apiezon C (with an
extreme pressure additive) in a 4 gimbal bearing test. But after 7 104 cycles,
the Bray turned to brown sugar and the bearing torque quickly increased by a
factor of 10. [Hinkle]
To prevent destructive chemistry, the surfaces in contact need to be
passivated in some manner. Ceramic hard coatings, such as TiN or TiC, will
eliminate catalytic reaction. Conventional nitride hard coatings are also
effective. In the case of ball bearings, replacing the stainless steel 440C
balls with ceramic SiN3 balls eliminates breakdown. To prevent starvation, it
is always good practice to use a porous ball retainer that functions like a
reservoir of oil and dilutes any breakdown products.
Each of the beta gimbals on space station Freedom have four 18-in. diameter
ball bearings with a specified 30-yr life. The original bearings failed after
one week. The cause of failure was incompatible bearing materials and
lubricant. Using the SEM/AES/XPS Tribometer, a substantial number of
accelerated tribological tests were run in simulated low Earth orbit environment
on a variety of bearing materials and solid lubricated composites. Based on
these tests, improved materials were recommended and subsequent testing for an
equivalent 35-yr life was successfully completed. Simple material tests should
be run before selecting materials and building full-scale hardware. With
suitable equipment, it is also possible to accelerate the testing while
controlling the critical parameters. [Survey, Naerheim]
Many gimbal systems have travel limited by physical features and protection
against contact of these features in the form of stops, both mechanical and
electrical. With the very high forces available due to large gear ratios,
significant damage can be done if the motor is driven past the normal stopping
range. Equipment should be designed with a foolproof means of stopping the
motor drive when approaching the limit of travel to prevent damage to the
equipment or operator injury. Even though the position can be easily monitored,
it is likely that during initial checkout the unit will be driven beyond its
normal range of travel. [Survey, Sutter]
Maximum torque was exceeded during gimbal checkout of the Hubble space
telescope. The problem was harness interference. Where possible, preflight
testing of the gimbal over its whole gimbal travel must be performed to
determine if the wire harness or any other obstruction, such as thermal
blankets, will prevent gimbal travel. The wiring harness must be designed to
eliminate service loops where they are not necessary to prevent harness
obstruction. [Survey, Ruebsamen]
A gimbal torque anomaly occurred during space telescope antenna pointing
system testing for the Hubble telescope. The cause was a tight curvature ratio
of the balls to the race. The curvature ratios were changed to 53% on the inner
race and 54% on the outer race. Also the rigid phenolic cage was replaced by a
set of Teflon toroids. [Survey, Ruebsame]
A gimbal Kapton strip heater burned out during testing of the high-gain
antenna pointing system. The failure was due to excessive input power coupled
with epoxy vaporization. The epoxy was changed from a standard structural epoxy
to a thermally conductive material. Also, bonding voids were eliminated when
bonding the heaters to the shaft. Lessons learned were:
Minimize power density below 9 W/in.2.
Eliminate voids when bonding heaters to the shaft.
The epoxy must be a highly filled, thermally conductive material and must be able to handle high power densities. [Survey, Ruebsamen]
A review of the information compiled for the Lessons Learned study reveals that
bearing and lubrication problems are the most prevalent and, thus, improved
technologies are most needed in these areas. This was further substantiated by
a survey conducted by Fusaro, where the number one need was for liquid
lubricants. There are other areas of importance. The principal needs derived
from the study are given below.
Solid lubricant hard coatings, that will not produce wear debris are
desirable to improve actuator reliability. Relative to liquid or grease
lubricants, solid lubricants generally have lower vapor pressures, better
boundary lubrication properties and relative insensitivity to radiation effects,
and operate in wider temperature ranges. Investigation of ion-plated lead
coatings, which have enjoyed good success in Europe should be undertaken, as
well as ion-sputtered MoS2. In European solar array drives alone, more than 2
million operational hours have been accumulated with ion-plated lead films. An
important property of the lead film is its high load-carrying ability. Under
Hertzian contact, the as-deposited film flows plastically until a thin film (10
Nm or less thickness) remains and then elastically deforms the substrate. In
this condition, the film can survive contact loads approaching the static load
capacity of a rolling element bearing. Burnishing of epoxy and polyamide films
to remove excess material may be acceptable and should receive further
Thermal problems (binding) are prevalent with actuators and retention and
release mechanisms. Differences in coefficients of thermal expansion must be
thoroughly explored to avoid jamming and excessive torque. Most problems occur
at low temperatures. Considerable care must be exercised when mounting close
clearance bearing components into aluminum structures that must operate at cold
temperatures. More detailed finite-element analyses to establish clearances and
tolerances is needed.
The functional margin of pyrotechnic devices must be determined by test to
assure actuation. The functional margin is a comparison of the energy that can
be delivered to the device and the energy required to operate the device.
Consultation with Bement at NASA-Langley is recommended.
Increased use of the latest CAD software is needed. Iteratively designing a
complex mechanism in CAD and using pasteboard mockups can be a more efficient
process than detailed mathematical analysis of component geometries.
Improved quality control of microswitches is required.
Harnesses and cables have caused torque problems because of snagging and
stiffness at low temperatures. These problems should be further addressed
through analytical and empirical methods.
The development of new polymeric bearing retainer materials is critical to
achieve bearing lifetime. The phenolic materials that are commonly used have
been demonstrated to absorb oil in a time-dependent and nonreproducible manner.
These materials are unacceptable for the missions under consideration unless an
active lubricant supply system is used. Retainerless bearings should be further
studied and developed.
Bearing lubricant depletion between the ball race retainer causes cage
instability and subsequent pointing errors, increased bearing torque and wheel
vibration. Phenolic cages continuously absorb oil from the contact regions
instead of supplying oil, thereby hastening the onset of cage instability. A
concentrated effort to develop active oil systems that periodically or
continuously lubricate the bearings should be undertaken [Singer, Auer].
Lubricant degradation is a common failure mode and is not amenable to
accelerated testing. Continued development of vacuum tribometers such as those
at NASA- Glenn is needed (see "Facilities" section), and extensive
experimentation conducted to better understand and combat lubricant degradation.
Both titanium carbide and titanium nitride have demonstrated to be effective
wear coatings under appropriate conditions. Titanium carbide has been applied
to gyroscope ball bearings and has increased operational lifetime by an order of
magnitude in this application when used with an uncoated steel raceway and
superrefined mineral oil. To date, titanium nitride has been used only on tool
steels, but the Aerospace Corporation is currently testing both gimbal and spin
bearings with Titanium-carbide and titanium-nitride-coated balls. These
investigations should continue and effectiveness quantified.
In a weightless environment, there are questions about how a drop of oil
will behave. Even at a distance of 0.004 in., the drop may not transfer
smoothly. It may touch and be slung off the moving surfaces to create a number
of finer droplets that will float around the bearing housing. Lubrication
phenomenon should be examined in a weightless environment by, for example,
Bearing simulation computer codes can predict cage instabilities for
steady-speed conditions, bearing performance parameters, elastohydrodynamic
lubricant thickness, etc. More extensive use and continued development of
computer codes is recommended and should include nonsymmetric cages, lubricant
starvation, thermal effects, retainerless bearings, and acceleration and
Roll rings have demonstrated excellent performance; continued development is
Small, fractional horsepower motors have experienced cold-temperature
performance problems, primarily due to high lubricant viscosity. Development
should be planned to evaluate and improve cold-temperature lubricant start-up as
well as running torque in small components.
Development of screening tests for various components can improve
reliability in an expeditious and accurate manner. Screening tests are
presently being used, with good success, for bearing cartridges, and extension
to other components is recommended.
Gimbal bearings, which operate in an oscillatory (dithering) mode and rarely
make a full revolution, are the primary problem area.
For oscillating gimbal applications, solid dry lubricants, such as
sputter-coated MoS2, with benign ball retainers (those that do not transfer
films) are recommended. Continued development of solid lubricants for gimbal
races, with objectives of low torque and no debris formation is suggested.
Race conformity and separator type can have a dramatic effect on bearing
torque. Blocking phenomenon should be avoided. Computer code development for
oscillating bearings, that can determine effects of race conformity, predict
blocking, and establish consequences of debris formation on driving torque would
be a useful design tool and is recommended.
MTI and NASA developed a survey form and solicited various industries for
information. The information requested is indicated on the sample form shown in
Figure 1. Figure 1 and the rest of the Survey Results are in the Space
Mechanisms Lessons Learned Study. The study is available in the
NASA Space Mechanisms Handbook and Reference
Over 600 survey forms were transmitted and approximately 30 replies were
received. The replies varied in quality from scribbles to detailed amounts of
lessons learned. Honeywell Electro Components and the Honeywell Satellite
Systems Operation were particularly responsive. The significant responses
ESTL was the first laboratory outside the main European Space Agency (ESA)
establishments to become fully compliant with ESA's standards for test houses
(ESA PSS-01-203), contamination and cleanliness control (ESA PSS-021-201),
thermal vacuum tests for screening space materials (ESA PSS-01-702), and also
There are some 25 vacuum chambers, 4 physical vapor deposition chambers for
lubricant application, and 3 tribometers.
ESTL's facilities are designed for versatility and flexibility. A wide range
of mechanisms and components can be accommodated. Laboratory systems can be
adapted and modified to suit customers' requirements.
ESTL has a total laboratory floor area of 550 m2; approximately 300 m2 of this
area is better than Class 10,000 (U.S. Federal Standard 109E), with Class 100
areas maintained for component inspection, and mechanism and bearing assembly.
ESTL has 11 chambers from 150 to 1000 mm in diameter (up to 0.95 m3 in working
volume), with an achievable vacuum pressure down to 10-9 mbar, and typical
temperature range of -150 C to +150 C (see Figure 2).
Emphasis is laid on vacuum chamber cleanliness and pump reliability.
Turbomolecular pumps are used for initial pumping; the lowest pressures are
achieved with ion pumps or cryogenic pumps. With such systems, there is no risk
of chamber contamination. Long-term performance (more than eight years with ion
pumps) can be guaranteed.
Each large chamber is equipped with two or more thermal-radiation shrouds and
electric heaters. With these, a wide range of thermal conditions (including
rapid changes of temperature can be achieved.
Dedicated facilities include those for testing the torque disturbance and
qualification of solar array drives, measuring directional accuracy of antenna
pointing mechanisms, gearbox performance evaluations (from 1- to 500-Nm output
torque), scanner simulation, and motor/gearhead evaluation.
Experimental rigs that have been used in the above chambers include those to
study separable electrical and fluid connectors, slip rings, motor commutation,
oscillatory bearing behavior, gears (four-square arrangement) and thermal
conductance measurements of Hertzian contacts.
Baseplate temperatures down to 4.2 K can be achieved within a 6000-cm3 (6-liter)
working volume (will accept mechanisms to 20 cm in diameter) and at pressures
below 10-8 mbar. Rolling element bearing evaluations can be performed with in
situ cryocompatible torque and force transducers. Pin-on-disk friction and wear
evaluations are also possible in this facility.
Loads to 150 N and speeds to 500 rpm can be achieved. These were designed by
ESTL and used to evaluate the basic tribological properties of materials and dry
lubricants. Specific applications cover both vacuum and air environments.
Ball Bearing Test Facilities.
Fourteen vacuum chambers (nominal size 150 mm diameter) are available to measure
bearing torque behavior (dc and noise level) continuously under thermal vacuum
conditions. Radial thermal conductance of ball bearings can also be measured.
The test chambers are also used for PVD bearing lubrication characterization.
Vacuum-compatible piezo and RVDT transducers are used to measure transmitted
torques directly. Typically speeds are to 1500 rpm, bearing preload as
required. An air bearing rig is available for cage stability studies (speeds to
Gearbox Test Facility.
This facility is used for invacuo testing of high-torque gearboxes (up to 500 Nm
output torque) for space usage (e.g., robotic actuators).
The tester is used to perform accelerated screening of the boundary lubrication
(and degradation) behavior of liquid lubricants, bearing materials, and surface
treatments. Loads to 500 N and speeds to 3000 rpm can be achieved (see Figure
The following conditions can be achieved: up to 800 C; stroke length: 1 to 15
mm; load range: 20 to 250 N; stroke frequency: -2 strokes/sec. Friction and
wear can be measured. The gaseous environment can be controlled to -15 ppm of
water vapor and oxygen.
Data logging facilities vary from standard pen and UV recorders, programmable
DVMs, etc., to stand-alone 386/486-based data loggers, with logging rates up to
50,000 readings/sec. Appropriate data handling, analysis, and graphical display
can be performed.
Honeywell Electromagnetic Control's (HEC's) fully equipped and well-staffed
testing department performs a wide variety of electromechanical,
electromagnetic, and electronic tests. The 3200-ft^2 facility is routinely used
to conduct standard functional, environmental, and life tests. Additionally,
HEC custom designs and fabricates consoles and fixtures to test dynamic and
functional characteristics. All technicians are ESD trained in parts handling.
Measuring equipment is calibrated regularly in accordance with the Bureau of
Standards requirements. Major equipment includes:
Thermal chambers (temperature range from -73 to +125 C)
Thermal vacuum chambers (cryogenic and diffusion: vacuum to 1*10^-6 torr)
Thermal vibration chambers
Vibration equipment (sine and random vibration capability)
Thermal humidity chamber
Computerized automatic test equipment.
Functional Test Capabilities.
Equipment for functional capabilities tests include computerized automatic test
equipment, which uses a laser interferometer to track movement to 0.000001
in./step; a Spectral Dynamics SD380 signal analyzer, which checks harmonic
content or distortion of a signal and plots distortion against frequency; and
shaft torque sensors, which have a load range of 10 oz-in. to over 2000 in.-lb
to check output torque of motors.
Augmenting the functional test laboratory equipment are direct-drive rate
tables for testing drag torque and detent torque, various phase angle voltmeters
that determine the phase relationship of ac voltage at various frequencies,
digital oscilloscopes for troubleshooting and monitoring rotor calibration, and
a Digasine AA gage for checking rotational accuracy to within 0.1 min.
Environmental Test Capabilities.
The environmental test lab (Figure 4) utilizes eight thermal chambers (Figure 5)
that perform a variety of tests, including thermal cycling and thermal shock in
a temperature range of -73 to +125 C. Three of the eight thermal chambers are
also vacuum chambers capable of producing 1 x 10^-6 torr of vacuum. Vibration
capability (sine and random) for subassemblies and end item products is
generated by four vibrators, three of which are thermal, and range in size from
2 ft^3 to approximately 50 ft^3 (Figure 6). Mechanical shock and thermal
humidity tests are also performed.
Life Test Capabilities.
Life tests encompass a number of tests performed in the thermal chambers
(temperature range from -73 to +125 C), including burn-in tests whereby a unit
is powered and left in the temperature-controlled chamber for 96 hr, and cycling
unit tests, whereby the temperature is varied along with powering and unpowering
the unit. This test is often done for 300 hr. Thermal humidity tests are also
This facility is utilized for material property testing of metallic and
nonmetallic materials at ambient, thermal, and/or vacuum conditions. Industrial
load test frames and test systems can test specimens with tensile loading as
well as comprehensive loading. The STL has a wide variety of test systems
available for structural testing, including servohydraulic test systems,
electromechanical test systems, a computer-controlled load system, and various
The RHTF and SRHTF use electrically powered radiant heater arrays that utilize
graphite-resistance elements and water-cooled reflectors for reliable and
efficient operation. The radiant heaters are operated in test chambers that
contain vacuum pumps to allow simulation of temperature and pressure conditions.
Cryogenic cooling panels are employed to allow preconditioning of TPS material
samples to simulate on-orbit cold soak. The RHTF has a 10-ft diameter vacuum
chamber (R-1) that can accommodate test articles as large as 6 x 8 ft and can
simulate temperature gradients through use of multizone temperature control.
The SRHTF can accommodate test panels up to 2 x 2 ft and can simulate uniform
temperature conditions. The heater in the SRHTF was transferred to the RHTF and
was used with the R-1 vacuum chamber for radiant heat test programs during the
past year. The SRH will be deactivated when an 8-ft diameter chamber (R-2)
The vibration and acoustic laboratories (contained in the VATF) are capable of
performing the wide range of tests needed to evaluate all aspects of acoustic,
vibration, structural dynamic, and shock problems. This facility has the
capability for development, qualification, and acceptance testing, not only of
aerospace vehicles and equipment, but also nonaerospace equipment that is to be
subjected to high-intensity acoustic noise, vibration, and shock environments.
The VATF test team works in cooperation with visiting users during every phase
of a test program to optimize test support and assure accomplishment of test
objectives. This facility provided extensive dynamic structural test support
for shuttle orbiter certification. State-of-the-art techniques are incorporated
in all facility laboratories; the facility has unsurpassed low-frequency
acoustic test capabilities, and provides unparalleled features for accomplishing
acoustic testing, mechanically induced vibration testing, and empirical modal
analysis within one building. Laboratory arrangements and test support systems
are equally suited for readily and efficiently dynamic testing of small
components or large assemblies.
The MTL provides the NASA-Johnson Space Flight Center with the capability for
supporting experimental investigations and evaluations of materials for current
and advanced programs. The following tasks are typical of those conducted with
Failure investigations of both metallic and nonmetallic materials
Chemical analyses of contaminants and material samples.
Evaluation of mechanical properties of materials
Nondestructive testing of materials
Evaluation of the compatibility of materials with the space environment
Preparation and evaluation of special elastomeric compounds
Preparation of TPS tiles and test articles.
Facilities available in the laboratory include:
Scanning electron microscopes
Fourier transform infrared spectrometer.
Nondestructive test equipment
Materials evaluation equipment
Instron mechanical test device
X-ray fluorescence and diffraction, metallograph, atomic oxygen testing apparatus
Tile coating furnace
Rubber mill used for the preparation of special formulations
A new experimental apparatus is being developed for the lubrication and wear of
materials. The experimental setup consists of a shoe-on-drum arrangement, which
allows a maximum of five material couples to be evaluated simultaneously under
sliding wear conditions. The material specimens can also be exposed to a
flowing atomic oxygen discharge and UV radiation in order to provide for the
accelerated testing of spacecraft materials. Coefficient of friction and
surface chemistry and morphology data may be collected using this system.
The pyrotechnic test facility contains the Langley Research Center aerospace
environmental and functional simulation equipment used for the handling and
testing of small-scale potentially hazardous materials, including explosive and
pyrotechnic materials, devices, and systems (see Figure 7). The facility
contains three 12- by 18-ft test cells, which are used for assembly and
checkout, environmental testing, and test firing, respectively (see Figure 8).
A 30- by 60-ft general-purpose, high-bay, open work area is used for system
testing and contains control systems for test capabilities for small items,
including remotely operated vibration (2000 lbf); mechanical shock (30,000 g for
0.2 ms); constant acceleration (200 g); thermal (-320 to +600 F); thermal vacuum
(-320 to +200 F at vacuums to 1 10-7 mm Hg); electrostatic discharge (25,000 V
with 500 pf capacitor); electrical and mechanical firing systems; and high-speed
measurements (40-kHz response analog) of acceleration; force; pressure;
temperature; and explosive performance monitoring systems. Adjacent facilities,
containing larger test cells, provide an expanded capability of testing pounds
of high-explosive materials (see Figure 9).
Langley Research Center has the capability to test materials and systems that
are potentially hazardous to both personnel and equipment. Specially designed
facilities on a land area, measuring 1000 by 1400 ft, are located in a remote
area of the center to allow dissipation of noise and venting, as well as
capturing fragments and debris produced by pressurized systems and explosive
devices. The facilities were originally created to assemble and test rocket
motors and explosive devices. The facilities are surrounded by fences to
control traffic. Earth berms around the large test facility and storage sites
provide protection, meeting all military requirements for all but
mass-detonating explosives, such as bombs. Testing is accomplished remotely,
providing high-speed electronic and photographic recording of a variety of
parameters, such as force, pressure, temperature, velocity, ignition, and
energy. The following facilities are in this test area.
Buildings for receiving, packaging unpackaging, and storage of pyrotechnic and
explosive devices are completely surrounded by an earth berm.
Control Room and Test Cells
The control room provides automatic programming and monitoring of remote tests
conducted in the three test cells. Two cells measure 15 by 19 ft and the third
is 20 by 19 ft; all three have vertical clearances to 18 ft with overhead cranes
to 15 ft. The cells have full-access, up doors, as well as roll-back ceilings.
A fragment-containing net covers the entire width of the building. Capabilities
include 250,000 lb of thrust and testing 5 lb of high explosives. Emergency
containment of up to 6000 lb of double-base rocket motor propellant is provided
by thick, reinforced concrete covered by earth, as well as earth berms on two
The majority of equipment in MSFC labs is for general tribological testing. This
equipment can be used to evaluate various types of lubricants such as oils,
greases, dry film, and deposited thin film. Below is a list of equipment:
Block-on-ring friction and wear testing machine
Four ball wear testers for measurement of extreme pressure properties of lubricating fluids
Falex multipurpose friction and wear tester
Cryogenic traction tester
Rolling contact fatigue tester
Pin and Vee-block wear tester
Vacuum manifold with 12 independent stations with feed-throughs for electrical
power and thermocouples. Some stations have water feed-throughs and
there are several blanked off ports available for other feed-throughs.
This facility consists of a 12.7-mm (0.5-in.) diameter cylinder that rolls
against a 50.8-mm (2-in.) diameter crowned disk, corresponding to the ball and
raceway of a rolling element bearing, respectively. The rotational speed of the
two samples is controlled by two continuous variable-speed electric motors
capable of up to 23,000 rpm and 5,000 rpm, respectively. Thus, the slip/roll
ratio is controllable over the whole range from pure sliding to pure rolling.
The ball retainer function is simulated by pressing a pin of retainer material
against the cylinder. Pure sliding can be studied by keeping the cylinder
spindle stationary while rotating the disc spindle or vice versa. The contact
stress between the two rotating samples is continuously adjustable in real time
up to about 4000 MPa mean Hertz stress or higher depending on contact geometry
and elastic modulus of the materials. The pin/cylinder contact stress is
adjustable independent of the cylinder/disk loading. Both forces are monitored
by separate load cells, as is the traction force between the disk and cylinder.
Experiments can be performed either in ultra-high vacuum or up to one
atmosphere of oxygen, hydrogen, and a variety of other gases. The disk and
cylinder samples can be internally cooled to liquid nitrogen temperature.
Design provisions have been made to accommodate high-temperature testing using
laser heating. The temperature is monitored at the cylinder/disk contact
separation point with an infrared thermometer with a focal point smaller than
the Hertz contact ellipse. The total wear is monitored with a capacitance
displacement probe. A 500-kHz acoustic emission detector monitors sample
contact and film breakdown.
The facility is equipped with an 8-channel computer data acquisition system
that displays in real time on a color monitor and stores on a hard disk drive
the disk/cylinder and pin/cylinder normal forces and the disk/cylinder friction
force, the motor speeds, the wear, and the temperature. The data retrieval,
reduction, calculations, and graphing have been automated using a dedicated
The test chamber is equipped with an SEM, a scanning Auger electron
spectroscope (AES) and a small-area multichannel x-ray photoelectron
spectroscope (XPS) operated via an Apollo 3500 computer and PHI surface analysis
software to examine the topography and chemically analyze the wear track as the
test is running. The chemical composition of the test environment is monitored
by a VG triple-filter quadropole mass spectrometer via a 386/20 computer using
VG software. Chemical composition depth profiling of the wear track can be done
using the ion sputter gun. The facility is also equipped with a high-speed
video camera operated as a strobe to obtain real-time freeze-frame images of the
wear track using a dedicated 486/66 computer image analysis system.
The University of Maryland's space environment simulation is limited. There is
a small vacuum chamber on campus. The proximity of the University to Goddard
Space Flight Center is an asset.
The University of Maryland offers one of the four neutral buoyancy facilities
in the country. The Neutral Buoyancy Research Facility houses a 50-ft diameter,
25-ft deep neutral buoyancy tank. This is used to simulate the weightlessness
of space while performing various operations.
There are numerous publications on space mechanisms available through various
societies and publishing firms. The preponderance of material is contained in
the 28 volumes of the Aerospace Mechanisms Symposia (AMS) sponsored by NASA.
Following is a listing of all the AMS papers sorted by topic.
Some additional references were supplied via the survey responses. These are
included below. A strong list of Pyrotechnic publications was provided by
Bement. The publications listed below by Fusaro provide an informative
prospective of space mechanism technology.
Most of the Fusaro reports below are available for download.
Fusaro, R.L. "Tribology Needs for Future Space and Aeronautical Systems.",
NASA Technical Memorandum 104525, December 1991.
Fusaro, R.L. "Government/Industry Response to Questionnaire on Space
Mechanisms/ Tribology Technology Needs.", NASA Technical Memorandum 104358, May 1991.
Fusaro, R.L. "Lubrication of Space Systems - Challenges and Potential
Solutions.", NASA Technical Memorandum 105560, April 1992.
Fusaro, R.L. and M.M. Khonsari. "Liquid Lubrication for Space Applications.",
NASA Technical Memorandum 105198, July 1992.
Index of Papers Sorted by Topic
Aerospace Mechanism Symposia Proceedings (Volumes 1 through 28)
Center for Aerospace Structures
Department of Aerospace Engineering Sciences
University of Colorado
Murphy, T.J., K.E. David, and H.W. Babel. "Solid-Film Lubricants and
Thermal Control Coatings Flown Aboard the E01M-3 MDA Sub-Experiment." McDonnell
Douglas Aerospace Report MOC 93H1394, Presented at 32nd Aerospace Sciences
Meeting and Exhibit, Reno, Nevada, January 1994.
Freeman, Michael. "On-Orbit Deployment Anomalies: What Can Be Done?" 17th
Space Simulation Conference (N93-15603), pp. 113-136.
Heard, Walter L., Watson, Judith J. "Results of the Access Construction
Shuttle Flight Experiment." AIAA Space Systems Technology Conference, San
Diego, California, AIAA #86-1186, June 1992.
Masri, S.F., Miller, R.K., Traina, M.I. "Development of Bearing Friction
Models from Experimental Measurements." Journal of Sound and Vibration, Vol.
148, pp. 455-475, 1991.
Misawa, M., Yasaka, T., Miyake, S. "Analytical and Experimental
Investigations for Satellite Antenna Deployment Mechanisms." Journal of
Spacecraft and Rockets, Vol. 26, No. 3, AIAA Paper No. 88-2225, pp. 181-187,
Misawa, Masayoshi. "Deployment Reliability Prediction for Large Satellite
Antennas Driven by Spring Mechanisms." AIA Paper No. 93-1621.
Nuss, H.E. "Space Simulation Facilities and Recent Experience in Satellite
Thermal Testing." Vacuum, Vol. 37, No. 3, 4, pp. 297-302, 1987.
Parker, K. "Some Experiences of Thermal Vacuum Testing of Spacecraft
Mechanisms." Vacuum, Vol. 37, No. 3, 4, pp. 303-307, 1987.
Rhodes, Marvin D. "Design Considerations for Joints in Deployable Space
Truss Structures." First NASA/DOD CSI Technology Conference, Norfolk, Virginia,
NASA Contract No. NAS8-31352. "Solar Array Flight Experiment Final
Report." April 1986.
Tzou, H.S., Rong, Y. "Contact Dynamics of a Spherical Joint and a Jointed
Truss-Cell System." AIAA Journal, pp. 81-88, January 1991.
Young, Leighton E., Pack, Homer C. "Solar Array Flight Experiment/Dynamic
Augmentation Experiment." NASA Technical Paper 2690, 1987.
Adams, L.R. "Hing Specification for a Square-Faceted Tetrahedral Truss."
NASA Contractor Report 172272, January 1984.
Bowden, M. Dugundji, J. "Effects of Joint Damping and Joint Nonlinearity on
the Dynamics of Space Structures." AIAA SDM Issues of the International Space
Station Conference, Williamsbury, Virginia, AIAA No. 88-2480, April 1988.
Freudenstein, F., Maki, E.R. "The Creation of Mechanisms According to
Kinematic Structure and Function." Journal of Environment and Planning B, pp.
Heimerdinger, H. "An Antenna Pointing Mechanism for Large Reflector
Antennas." 15th Aerospace Mechanisms Symposium, NASA Conference Publication
Kellemaier, H. Vorbrugg, H. Pontoppidan, K. "The MBB Unfurlable Mesh Antenna
(UMA) Design and Development." AIAA 11th Communication Satellite Systems
Conference, San Diego, California, March 1986.
Marks, G., Anders, C., Draisey, S., Elzeki, M. "The Olympus Solar Array
Development and Test Program." Proceedings of the 4th European Symposium,
"Photovoltaic Generators in Space," Cannes, September 1984, ESA SP-210, November
Rowntree, R.A., Roberts, E.W., Todd, M.J. "Tribological Design - The
Spacecraft Industry." 15th Leeds-Lyons Symposium, September 1988.
Satter, C.M., Kuo, C. "Thermal/Mechanical Analysis of a Panel Attachment
System for the PSR." SPIE Conference, Orlando, Florida, SPIE Proceedings, Vol.
1114, pp. 1114-51, March 1989.
Wada, B.K., Kuo, C.P., Glaser, R.J. "Extension of Ground-Based Testing for
Large Space Systems." AIAA 26th SDM, AIAA Paper No. 85-0757, pp. 477-483, 1985.
References Provided Through Survey
Masuko, M., W.R. Jones Jr., and L.S. Hemlick. "Tribological
Characteristics of Perfluoropolyether Liquid Lubricants under Sliding
Conditions." NASA TM 106257, July 1993.
Masuko, W.R. Jones Jr., R. Jansen, B. Ebihara, and S.V. Pepper. "A Vacuum
Four-Ball Tribometer to Evaluate Liquid Lubricants for Space Applications."
NASA TM 106264, July 1993.
Jones, W.R. Jr.The Properties of Perfluoropolyethers Used for Space
Applications." NASA TM 106275, July 1993.
Jones, W.R. Jr., et. al. "The Preliminary Evaluation of Liquid Lubricants
for Space Application by Vacuum Tribology." Preprint from 28th Aerospace
Mechanisms Symposium, NASA Lewis Research Center, May 1994.
References Provided Through Survey
Pyrotechnic Test Facility
Lake, E.R., Thompson, S.J., and Drexelius, V.W. "A Study of the Role of
Pyrotechnics on the Space Shuttle Program." NASA CR-2292, September 1973.
Bement, Laurence J., and Schimmel, Morry L. "Integration of Pyrotechnics
into Aerospace Systems." Presented at the 27th Aerospace Mechanisms Symposium,
NASA Ames Research Center, May 1993.
Bement, Laurence J. "Pyrotechnic System Failures: Causes and Prevention."
NASA TM 100633, June 1988.
Bement, Laurence J., and Schimmel, Morry L. "Determination of Pyrotechnic
Functional Margin." Presented at the 1991 SAFE Symposium, Las Vegas, Nevada,
Elern, Herbert. "Modern Pyrotechnics." Chemical Publishing Company, 1961.
MIL-P-46994/B, Amendment 3. "General Specification for Boron/Potassium
Drexelius, V.W., and Schimmel, M.L. "A Simplified Approach to Parachute
Mortar Design." Presented at the Seventh Symposium on Explosives and
Pyrotechnics, Philadelphia, Pennsylvania, September 1971.
SKB26100066. "Design and Performance Specification for NASA Standard
Initiator-1 (NSI-1)." January 1990.
Meyer, Rudolph. "Explosives." Printed by Verlag Chemie, 1977.
"Properties of Explosives of Military Interest." AMCP 706-177, AD 764340,
U.S. Army Materiel Command, January 1971.
Rouch, L.L., and Maycock, J.N. "Explosive and Pyrotechnic Aging
Demonstration." NASA CR-2622, February 1976.
WS5003J. "Material Specification for HNS Explosive." Naval Surface Weapons
Center, February 1981.
Kilmer, E.E. "Heat-Resistant Explosives for Space Applications." Journal
of Spacecraft and Rockets, Vol. 5, No. 10, October 1968.
MIL-STD-1576 (USAF). "Electroexplosive Subsystem Safety Requirements and
Test Methods for Space Systems, July 1984.
DOD-E-83578A (USAF). "Explosive Ordnance for Space Vehicles (Metric),
General Specification for." October 1987.
NSTS 08060, Revision G. "Space Shuttle System Pyrotechnic Specification."
Lake, E.R. "Percussion Primers, Design Requirements." McDonnell Douglas
Corporation Report MDC A0514, Revision B, April 1982.
Schimmel, Morry L. "The F-111 Crew Module: Major Challenge for Thermally
Stable Explosives." U.S. Naval Ordnance Laboratory, White Oak, Silver Spring,
Maryland, June 1970.
Schimmel, Morry L., and Kirk, Bruce. "Study of Explosive Propagation Across
Air Gaps." McDonnell Aircraft Corporation Report B331, December 1964.
Schimmel, Morry L. "Quantitative Understanding of Explosive Stimulus
Transfer." NASA CR-2341, December 1973.
Bement, Laurence J. "Helicopter (RSRA) In-Flight Escape System Component
Qualification." Presented at the Tenth Symposium on Explosives and
Pyrotechnics, San Francisco, California, February 1979.
Persson, Per-Anders. "Fuse." U.S. Patent 3,590,739, July 1971.
Chenault, Clarence F., McCrae, Jack E. Jr., Bryson, Robert R., and Yang,
Lien C. "The Small ICBM Laser Ordnance Firing System." AIAA 92-1328.
Ankeney, D.P., Marrs, D.M., Mason, B.E., Smith, R.L., and Faith, W.N.
"Laser Initiation of Propellants and Explosives." Selected Papers, SP93-09 on
Laser Ignition, Published by the Chemical Propulsion Information Agency,
Drexelius, V.W., and Berger, Harold. "Neutron Radiographic Inspection of
Ordnance Components." Presented at the Fifth Symposium on Electroexplosive
Devices, Philadelphia, Pennsylvania, June 1967.
Bement, Laurence J. "Monitoring of Explosive/Pyrotechnic Performance."
Presented at the Seventh Symposium on Explosives and Pyrotechnics, Philadelphia,
Pennsylvania, September 1971.
Schimmel, Morry L., and Drexelius, Victor W. "Measurement of Explosive
Output." Presented at the Fifth Symposium of Electroexplosive Devices, the
Franklin Institute, June 1967.
Bement, Laurence J., and Schimmel, Morry L. "Cartridge Output Testing:
Methods to Overcome Closed-Bomb Shortcomings." Presented at the 1990 SAFE
Symposium, San Antonio, Texas, December 1990.
Bement, Laurence J., Schimmel, Morry L., Karp, Harold, and Magenot,
Michael C. "Development and Demonstration of an NSI-Derived Gas Generating
Cartridge (NGGC)." Presented at the 1994 NASA Pyrotechnic Systems Workshop,
Albuquerque, New Mexico, February 1994.
Bement, Laurence J., and Schimmel, Morry L. "Ignitability Test Method."
Presented at the 1988 SAFE Symposium, Las Vegas, Nevada, December 1988.
Bement, Laurence J., and Schimmel, Morry L. "Ignitability Test Method, Part
2." Presented at the 1989 SAFE Symposium, New Orleans, Louisiana, December
Bement, Laurence J., Doris, Thomas A., and Schimmel, Morry L. "Output
Testing of Small-Arms Primers." Presented at the 1990 SAFE Symposium, San
Antonio, Texas, December 1990.
Bement, Laurence J., and Schimmel, Morry L. "Approach for Service Life of
Explosive Devices for Aircraft Escape Systems." NASA TM 86323, February 1985.
Bement, Laurence J., Kayser, Eleonore G., and Schimmel, Morry L. "Service
Life Evaluation of Rigid Explosive Transfer Lines." NASA TP2143, August 1983.
Bement, Laurence J., and Schimmel, Morry L. "Investigation of Super*Zip
Separation Joint." NASA TM 4031, May 1988.
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