393 Plain Bearing Bushings 

Sputtered MoS 2 is well known as a solid lubricant for ball bearings where the predominant relative  movement is a combination of sliding and rolling fric tion. However, this experience cannot be found in a  gearbox lubrication application. The planetary wheels are rotating around the pins pressed into the planet carriers. This journal bearing was one  of the critical areas where the Dicronite coat ing wore off. Therefore  a redundancy was requested for this function. Plain bearing bushings made of plastics have been found to fulfill this requirement for increased safety against abrasion. In case of local lubrication failure these  bushings fulfill journal bearing functions and pr event direct metal to metal contact.    The bushings were tolerance such that they shrink onto the planet axis in cryo in order to gain a defined sliding surface. That requires a small low-temperat ure embrittlement at temperatures down to -246°C,  relatively low CTE and good sliding properties in cryo, all not self-evident for organic plastics.   Different materials have been considered to be implemented in this application:    Vespel SP1 / Vespel SP3. Vespel is a well know n plastic material used in space applications;  type SP3 is self-lubricating by containing small particles of MoS 2 while SP1 represents the  unfilled material.   Copper Metal Matrix Composite (Cu-MMC). The self-lubricated Cu-MMC has been developed  by Austrian Research Centers for use in tribol ogical sliding contacts under vacuum. It is based  on copper matrix with inclusions of solid-lubr icant particles and one of its highest advantages  compared to plastics is its low CTE.   Victrex PEEK 450G is supposed to be applicabl e in particular for cryogenic applications   Torlon 4203 PAI stands out due to its high ductility – even at cryogenic temperatures   Sintimid 15M / 30M is a sintered polyimide designed for cryogenic applications and filled with  either 15% MoS 2 or 30%, respectively.   PGM-HT contains of PTFE filled with glass fibers (15%) and MoS 2 particles (5%) and is often  used for ball bearing cages   Bushings have been manufactured of all the materials as well as sliding samples for friction coefficient comparison. Friction tests under ambient in combination with MoS 2 powder should give a first impression  of the tribological behavior. While Vespel SP1, Vi ctrex and Torlon showed a good machinability, Sintimid,  Cu-MMC and Vespel SP3 were very brittle during ma nufacturing (the higher the filling percentage or the  bigger the particles, the wo rse was the machinability) and some of the bushings broke during machining.  It turned out that filled material is not suitable for this application as the marginal wall thickness and the  particle size are in the same range.  The bushings were assembled onto the planet axes and exposed to a liquid nitrogen dip test. Sintimid bushings cracked or indeed broke during the test and therefore failed, Victrex material was undamaged in principle but showed slight white marks after remova l of the bushings that are suspected to be material  degradations.  Based on the experiences with Vespel in an Astrium space application, this material has been selected  for the FM1 model but Torlon has been considered to be implemented in one of the breadboard models in order to allocate a backup solution.   Development Concept and Success-Oriented Approach   A success-oriented approach has been chosen due to  programmatic reasons:  the FM1 was produced  and tested with the design described above without t he qualification program having started yet. In order  to gain confidence, two breadboard models, so call ed Pre-Qualification Models, have been set up. As  every stage of the planetary gear has three (1 st stage four, respectively) planet axes that are equally  loaded, different bushings could be assembled and their behavior during life test can be compared.  NASA/CP-2010-216272