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A number of mechanism (mechanical moving component) failures and
anomalies have recently occurred on satellites. In addition, more
demanding operating and life requirements have caused mechanism
failures or anomalies to occur even before some satellites were
launched (e.g., during the qualification testing of GOES-NEXT,
CERES, and the Space Station Freedom Beta Joint Gimbal). For these
reasons, it is imperative to determine which mechanisms worked
in the past and which have failed so that the best selection of
mechanically moving components can be made for future satellites.
It is also important to know where the problem areas are so that
timely decisions can be made on the initiation of research to
develop future needed technology.
To chronicle the life and performance characteristics of mechanisms
operating in a space environment, a Space Mechanisms Lessons Learned
Study was conducted. The work was conducted by the NASA Lewis
Research Center and by Mechanical Technologies Inc. (MTI) under
contract NAS3-27086. The expectation of the study was to capture
and retrieve information relating to the life and performance
of mechanisms operating in the space environment to determine
what components had operated successfully and what components
had produced anomalies. The table lists some mechanism anomalies
found in spacecraft that are discussed in two publications on
this subject (refs. 1 and 2).
| Conditions | Problem | Impact | |
| Momentum wheel spin bearings | 3600-rpm, grease-packed bearings; room temperature to 100 °F | Torque and temperature anomalies | Single-point mission failure; possible indication of failure |
| Sensor support bearing | Preloaded ball bearings oscillatory motion | Failure in test | >$500,000 testing |
| Sensor launch clamp | Clamp located inside thermal blanketed craft | Seizure on launch pad | Single-point failure prohibited launch or mission failure |
| Harmonic drives | Very low speed; temperature <150 °F; fluorocarbon lubricant; boundary condition | Excessive wear; lube failure in test | Failure will degrade mission or possible mission failure; changed lubricant |
| Slip rings; brush contacts | MoS2/Ag/C brushes on Ag rings; numerous recurrences | Excessive electrical noise due to moisture and corrosion | Inability to point communications antennas; reduced mission objective |
| Potentiometer for ATP control | Low temperature; light-load fluid lubricant | Electrical noise lube thickening open circuit | Loss of pointing reduced mission ~$500,000 testing |
| Control moment gyroscope | Oil injection on bearing land | Bearing failure lube design wrong | Premature mission failure |
| Control moment gyroscope | Very high torque for slewing | Bearing failure | Loss of mission; >$1 million test and anneal |
| Momentum wheel | Grease lubricated | Torque and temperature anomalies | Possible mission failure |
| Propellent pump gearbox | High speed | Contractor switching lubricants | Possible launch failure with new lube |
| Slip rings; brush contacts | MoS2/Ag/C brushes on Ag rings | Excessive noise due to oxidation of MoS2 | Rework brushes and rings; delivery delay |
| Gear mechanism | High loads; fluorocarbon grease; boundary conditions | Lube degradation | System failure |
| Synchronous motor assembly | Mineral oil grease-packed bearings | Motor failure due to increased bearing drag | Failure would degrade mission |
| Momentum wheel spin bearings | High speed; mineral oil grease | Possible lubricant degradation in testing | Single-point mission failure |
| Inertial guidance synchronous motor bearing | High speed; mineral oil grease | Possible chemical reaction between grease and iron surface during storage | Guidance failure |
| Harmonic drive | Low-temperature operation; fluorocarbon grease | Low-temperature viscosity of grease causes excessive torque | Failure will degrade mission |
| Momentum wheel; active lubrication system | High-speed; long-life requirement | Inability to deliver adequate lubricant quantity | System will not meet lifetime requirement |
| SADM | Large launch loads on MoS2- lubricated bearings | Test of static loads | Possible single-point failure; passed test |
| Gimbal bearings on test; telescope | Low-temperature; dry (MoS2) lubricant | Tested in air friction increase | Modified specification to do inert gas test; passed |
| Spin bearing | Large diameter, thin cross section bearing | Humidity-induced dimensional instability of cotton-phenolic retainer | Possible target acquisition failure; changed to metal ball separator |
| Gas bearing; gyroscope | Alumina surfaces; stearate lubricant | Erratic friction on startup; uneven lube during test | Reliability problem for flight units; major rework if failure |
| Foil bearings for turbomachinery | High-strength alloy; CFx-polyamide lubricant; temperature extremes | High friction startup after standing | Potential system failure; inability to start turbine |
The goal of building longer-life unmanned satellites and space
probes has created a demand for meaningful accelerated test methods
to simulate long-term service in space. This is particularly true
for tribological components used in space--such as bearings, seals,
and gears. In addition, there is an urgent need for lightweight,
low-torque, durable mechanisms that can operate efficiently in
a hard vacuum environment.
In response to this need, a study was conducted by Lewis and MTI
(under contract NAS3-27086) to determine if any mechanisms (which
operate in the space environment) would benefit from accelerated
testing techniques (ref. 3). The study investigated the current
types of accelerated testing techniques, their shortfalls, and
the need to develop new techniques. An accelerated testing technology
"roadmap" was developed for assessing the life and reliability
of spacecraft mechanical systems by accelerated testing methods.
The "roadmap" suggested that system components testing,
analytical modeling, computer codes, and computer smart systems
could be integrated into a methodology that could be used to predict
or verify the life and reliability of a mechanical system. The
study team suggested that a space mechanism mechanical system
be tested to demonstrate that the methods developed could adequately
predict the life and/or performance of a mechanism. Included in
the "roadmap" are the experimental equipment needed,
the test procedures, the time guidelines, and cost analysis.
Previous articleLast updated April 30, 1997
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