Space Mechanisms Lessons Learned Study

FOREWORD

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  (3) Requests for unpublished information were made to NASA and industry.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.

ACKNOWLEDGEMENTS

LIST OF FIGURES

1. Space Mechanisms Survey Form
2. ESTL Vacuum Chamber
3. Boundary Lubrication Accelerated Screening Tester
4. Honeywell Environmental Test Facility
5. Honeywell Environmental Thermal Chamber
6. Honeywell Shaker Table Facility and Control Room
7. Pyrotechnic Test Facility at NASA - Langley
8. Pyrotechnic Test Cells at NASA - Langley
9. Pyrotechnic Test Cells Outside at NASA - Langley

LIST OF TABLES

1. Anomalies of Pyrotechnic Devices
2. Tribomaterials for Deployment Mechanisms
3. Lockheed Solutions to Limitations of Conventional Designs
4. General Guidelines for Worm Gear Systems
5. Actuators Using Brush Motors
6. Partial Listing of Momentum/Reaction Wheel, Control Moment Gyroscope, and Gyroscope Experience
7. Factors Tending to Increase Blocking

INTRODUCTION

Three major categories of mechanisms were selected: deployable appendages, rotating systems, and oscillating systems.  Subsystems of these major categories are as follows.

Deployable Appendages

• Solar Arrays
• Retention and Release Mechanisms
• Bearings, Lubrication, and Tribology Considerations
• Antennas and Masts
• Actuators, Transport Mechanisms, Switches
• General and Miscellaneous

Rotating Systems

• Momentum Wheels
• Reaction Wheels
• Control Moment Gyroscopes
• Gears
• Motors
• Bearings and Lubrication
• Slip Rings and Roll Rings
• Miscellaneous

Oscillating Systems

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 Review.

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 contributed information.

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.

SUMMARY OF LESSONS LEARNED

Deployable Appendages

Solar Arrays

• Cosmic background explorer [Farley]
  • 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.
• Earth Radiation Budget Satellite (ERBS) [Mollerick]
  • 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.  [Fleischauer, Hilton]
  • 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 wrinkling.
  • 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 coated.
  • 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 sections.

Retention and Release Mechanisms

• 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 reinstalled.
  • 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 piston.  [Phan]
  • 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.  [Maus]
  • 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 single-cartridge input.

• 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.  [Tibbits]
• 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.  [McCown]
  • 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 designers.
  • For roller screw latches, thread engagement is improved by providing lead-ins.
  • 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, Coquelet]
• 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.  [Survey, Purdy]
• 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]

Bearings, Lubrication, and Tribology Considerations

• 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 strain.  [Rowntree]
• 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 and 0.08.
  • 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 uncoated surface.
• 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 ranges.  [Hilton]
  • Lead coating has had good success as a solid lubricant in vacuum applications.
  • 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 space telescope.
  • 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 torque.
• 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 devices.
  • Unlubricated surfaces exceeding bearing yield strength of substrate on hard-coated materials.
  • Seizures caused by dissimilar materials with high mutual solubility.
  • Maximum utilization of rolling surfaces as opposed to sliding motion should be employed.
  • 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]

• 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.  [Feherenbacher]
• 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.  [Feherenbacher]
• 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.  [Feherenbacher]
• 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.  [Rowntree]
• 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.  [Rowntree]
• 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.  [ESTL, Kaese]
• 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]

Antennas and Masts

• Tracking and data relay satellite (TDRS) [Luce]
  • 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 deployment.
  • 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 feasible.
  • Shipping vibrations and ground testing can destroy coatings and dry lubricants.
  • 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 accurately.
  • 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.  [ESTL, Abarrategui]
• 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, Gallagher]
• 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, Schmidt]
• 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]

Actuators, Transport Mechanisms, and Switches

• 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.  [Stark]
• 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: [Leary]
  • 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 recommended.

• During development of robotic arms, the Europeans have found that phenolic/asbestos brake materials present torque anomalies under thermal vacuum conditions.  [Priesett]
• 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]

General and Miscellaneous

• Anomalies
  • Nonredundancy of the motion-producing elements.
  • Insufficient torque margin.
  • Snagging.
  • Stiction.
  • Binding of panel hinges.
  • Excessive impact loads from deployment.
  • Poor selection of solid lubricants. Molydisulfide solid lubricants absorb water, which can freeze and jam hinges and V-cone guides.
  • Improper cone angle for cone supports.
  • Excessive bending stiffness of wire harness at low temperatures.
• Lessons Learned
  • Torque Ratio: Tr = available torque/resisting torque > 4
    Torque margin: Tm = (available torque -resisting torque)/resisting torque > 3
    Tm = Tr -1.
  • Panel hinges should have spherical bearings with axial clearance to avoid binding.
  • Dampers are necessary to reduce kinetic energy at impact.
  • Tioxode-V provides an acceptable hard slippery coating.  Avoid molydisulfide solid lubricants.
  • 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 practicable).
  • 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 pullers.
  • 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 lubricant.
• 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 reliability.
• 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 judgment.
• 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, Gresham]
• 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 deployment margins.
  • 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 job.
  • 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 devices.
  • 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 pass/fail criterion.
  • 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 testable.

Rotating Systems

Momentum Wheels

• Bearing lubricant depletion between the ball race retainer causes cage instability and subsequent pointing errors, increased bearing torque, and wheel vibration.  [Feherenbacher]
• 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.  [Feherenbacher]
• 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 1988]
• 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]

Reaction Wheels

• 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 ground.  [McConnell]
• 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.  [Bialke-2]
• 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.  [Kalegoras]
• 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 lubricant.  [Kalegoras]

Control Moment Gyroscopes

• 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 inertia.  [Cook]
• 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 speed.  [Cook]
• 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 supply:
  • Cage oil feed hole was reduced to overlap outer race groove under all conditions.
  • 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]

Gears

• 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.

• 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.  [Henson]
• 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]

Motors

• 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]

• Brushless dc, permanent-magnet dc, and brushless stepper motors are the preferred motors.  [Sharma]
• Stepper motors can have positioning errors due to:
  • The encoder
  • 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 switches.  [Sharma-2]
• 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, Marks]

Bearings and Lubrication

• 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 life.  [Fleischauer]
• Hard coatings and ceramic parts are used for low torque noise.  [Fleischauer]
• 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 lubricants.  [Fleischauer]
• 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]

• 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).  [Fleischauer]
• 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.  [Fleischauer]
• 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.  [Fleischauer]
• 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.  [Fleischauer]
• 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.  [Fleischauer-3]
• 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.  [Allen]
• 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 (Bendix)]
• 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, Langmaier]
• 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.  [Bertrand]
• 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 environment.  [Murray]
• 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
  • Lubricant creep
  • Lubricant dewetting
  • Lubricant degradation
  • Bearing and cage instability
  • Torque variations
  • Slip ring and brush wear
  • Lubricant volatility
  • 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 February 1988]

Slip Rings and Roll Rings

• 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.  [Batista]
• 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 rings.  [Smith]
• 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 processes.
  • Inspect critical manufacturing and test operations at the supplier's facility.
• Some experiences have indicated the need to improve the dynamic noise performance of dry film lubricated, silver bearing slip ring/brush combinations.  [Matteo]
• 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 contamination.  [Matteo]
• 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]

Miscellaneous

Potentiometers

• 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.  [Dubitschek]

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 motors.  [Robinson]
• 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]

Vapor Compressor

• 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]

Oscillating Systems

• 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.  [Fleischauer]
• 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.  [Fleischauer]
• 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 t