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Mechanical hardware flown on LDEF included the primary structure, fasteners, canisters, grapples, viscous damper, lubricants, seal, and composites. The LDEF deintegration team and several experimenters noted difficulties in removal of fastener and hardware during post-flight activities. The Systems SIG has investigated all reported instances, and in all cases the difficulties were attributed to galling during installation or post-flight removal. The results of the MIT Lincoln Laboratories test program validated LDEF findings that installation practices were responsible for the galling that led to high-removal torques and occasional fastener breakage. Correct selection of materials and lubricants, as well as proper mechanical procedures are essential to ensure successful on-orbit or post-flight installation and removal of hardware.

Both the rigidize-sensing grapple, used by the Shuttle's Remote Manipulator System (RMS) to activate the active experiments prior to deployment, and the flight-releasable grapple, used by the RMS to deploy and retrieve LDEF, worked as designed.

The viscous damper, used to provide stabilization of LDEF from deployment-induced oscillations, performed as designed and exhibited no signs of degradation. The damper has undergone extensive post-flight testing and has been returned to NASA LaRC in a flight-ready condition.

The majority of seals and lubricants used on LDEF were designed as functioning components of experiments and were, therefore, both shielded and hermetically sealed from exposure to the LEO environment. Post-flight testing has shown nominal behavior for these seals and lubricants. However, several lubricants were exposed to the LEO environment as experiment specimens. Post-flight analysis revealed a range of results for test specimens, that ranged from nominal behavior to complete loss of lubricant, depending on the particular lubricant and its location on LDEF.

• Previously reported loss of MoS₂ dry film lubricant exposed on trailing-edge tray D03, proved erroneous. Detailed examination of preflight documentation and photographs show that the lubricant was never applied to the stainless-steel substrates.
• Castrol Braycote 601 was used to lubricate the four drive shafts which opened and closed the AO178-01 canisters. The drive shafts, located on the exterior surface of tray A03, had minimal UV exposure due to canister shadowing. Initial analysis suggested changes in the 601, but further analysis found that residual solvent used to remove the Braycote from the drive shafts had been included in the initial analysis. Some slight differences had occurred between flight and non-flight materials but were likely caused by traces of moisture and/or contamination. Castrol reported no significant change in temperature at which decomposition begins or in the relative levels of base oil to thickness, indicating that Braycote 601 was unchanged.
• LDEF experiment AO138-10 (trailing-edge tray B03) was a static experiment consisting of 48 washer pairs machined from various metals representative of spacecraft mechanisms and loaded together with various contact pressures. Half of the washer pairs possessed one mating surface with Molykote Z (Dow Corning product), while the other half of the washer pairs possessed one mating surface coated with vapor deposited MoS₂. Ground-based specimens were kept loaded under vacuum conditions throughout LDEF's mission. The experimenter's objective was to evaluate the tendency of various metals to "coldweld" and to determine the effect of MoS₂ lubricants on coldwelding. No coldwelding was found to have taken place on any of the flight or ground specimens.
• Air-cured MoS₂ (per MIL-L-23398) was used on several components (Belleville washers, drive shafts, and linkages of each canister) of the five NASA-provided Environmental Exposure Control Canisters (EECCs). No visual changes were noted for the lubricant on the drive shafts and linkages. Sixteen of the washers were removed from a trailing-edge EECC for evaluation. Results showed burnished areas and areas of buildup. No abnormal wear was observed. Areas of lubricant thinning, often to bare metal, were present on some of the convex surfaces. However, none of these effects were attributed to the 69-month exposure to the LEO trailing-edge environment.
• Butyl seals were used to seal the drawers on the five EECCs. Except for the first nine (9) months when the drawers were open, the seals were shielded from the external spacecraft environment. Analysis consisted of comparing post-flight leak rates to pre-flight values. Increased rates were noted for two (2) of the three (3) canisters analyzed. However, it is unlikely that atomic oxygen was the cause for the increased rates for the two (2) leading-edge canisters as the atomic oxygen (AO) fluence for the first nine (9) months was only 2.3 x 10²⁰ atoms/cm². Results suggest that outgassing contaminants might have interfered with the on-orbit resealing of the drawers.

Two minor cases of metallic seizure have been reported by LDEF Principal Investigators. While neither of the two cases of adhesion were critical, they both point out the need to select the correct mating materials and surface treatments.

• One instance involves the indium washers of Experiment AO147 cold-flowing into adjoining aluminum and/or glass surfaces. This occurred on 16 flight assemblies but no assembled ground-control specimens existed for comparison. Because of indium's low melting temperature and solubility, it is expected that this adhesion would have occurred on ground assemblies subjected to the same temperatures and contact pressures as the assemblies flown on LDEF.
• The second case of adhesion involved seizure between steel springs and aluminum backing plates on the French Cooperative Passive Payload (FRECOPA) experiment. A total of 57 spring/backing plate assemblies were used on this trailing-edge experiment and four (4) or five (5) of these assemblies adhered to each other. Microscopy revealed transfer of aluminum to the spring. Both the steel and aluminum were untreated and neither alloy is known. No ground control assemblies exist. It is speculated that the adhesion was caused by fretting of the two mating surfaces which was induced by launch vibrations and/or thermal cycling. Fretting is a wear phenomenon that can occur between two tight-fitting surfaces subjected to a cyclic relative motion of extremely small amplitude. One stage of the fretting process is adhesion between fretting surfaces by the formation of bonded junctions between asperities of the mating surfaces. If the two mating surfaces are of dissimilar metals, the softer metal's oxide film will be disrupted resulting in transfer of the softer material to the harder material. This explains the transfer of the aluminum to the steel spring.

The results of this investigation showed that there have been no documented cases of a significant on-orbit coldweld event occurring on U.S. spacecraft. There have been a few documented cases of seizure occuring during on-orbit coldwelding experiments. However, the seized materials had been selected for the experiment because of their susceptibility to coldweld during vacuum testing on Earth. This susceptibility was enhanced by effective pre-flight cleanliness procedures.

With few exceptions, adhesives performed as expected. Several experimenters noted that the adhesives had darkened in areas that were exposed to UV. The most obvious adhesive failure was the loss of four solar cells bonded to an aluminum substrate using an unfilled epoxy. Two cells were on a leading-edge tray and the other two were bonded on a trailing-edge tray. No adhesive remained on the two leading-edge trays but some remained on the trailing-edge tray. This indicated that the bond failed at the cell/adhesive interface and then the adhesive was attacked by atomic oxygen. Posssible causes of failure included poor surface preparation and/or thermal expansion mismatch between the solar-cell substrate and the aluminum mounting plate.
BR<> The most significant finding for the fiber-reinforced organic composites was the atomic oxygen erosion of leading-edge specimens. While the measured erosion was not unexpected, the detailed comparison of ground-based predictions vs actual recession rates has not been completed. A thin protective coating of nickel and SiO₂ was used on leading-edge specimens to successfully prevent this erosion.

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