341 (a) Location during period of Dec ember 2014 to February 2015   (b) Locations of accelerator anomalies (4 day periods):  Black = Cooler Off 2002/084, 2003/359, 4 days  each; Blue= Cooler ON 2004/024 (indicated as year/day of year)  

Lessons Learned  & Conclusions  

The billions of rotations  achieved for 7 rotating mechanis ms demonstrates that the l ubricant screening  methods based on heritage with polyalphaolefin (PAO ) oils were effectively transferred to the Nye 2001T  oil. In the case of the LMC, there was a heightened concern for the presence of these  additives, which  exhibit much higher outgassing than the base oil. It was thought  that the additive could provide benefits  during the run- in process, so the screenings were all done similarly, but with the LMC bearing subassemblies baked prior to final i nstallation in the housings . This approach did not appear to have any  detrimental effects in terms of optics or lifetime for these units . Optical cleanliness continues to meet  specification after 16 years in- orbit, where the gain of the system has remained very stable. Despite one  cell having been gas -filled and the compensating cell being evacuated, there has been no significant  difference between the paired rotors . 

By current space mechanism standards, i t may be viewed  as fortunate that so much success has been  achieved for such customized rotating mechanisms  without pre- launch life testing. The schedule had  precluded life testing at representative speeds , and accelerated life tests would have misrepresent ed the  situation with an increased  film thickness  of lubricant . Without the desired life testing option, alternative   measures were taken to increase the probability of  long-life being achieved. The risk management  strategy included:   • Sourcing the experience of Draper Laboratories  involving design consultation,  lubricant  formulation,  and implementing a screening process  for the duplex bearings  (resulting in as many  as 5 iterations  of run- in, cleaning and re- lubrication)   • TiC coatings implemented for scan mirror motor bear ings  • Screening tests performed on the new lubricant formulation (Nye 2001T) for LMCs and  choppers, which was a worthwhile quality measure. (4-ball tribometer screening test successful ly  identif ied a degraded batch of t ricresyl phosphate additive prior to the new formulation being  applied in the flight mechanisms , saving a great deal of cost, time and potential investigation  efforts )  • Provision of power during launch, so as to run choppers and length modulators at 100 rpm  during launch, as well as the associ ated drive system provi sions to accommodate this mode 

Films were observed during screening of  LMC and chopper bearings, representing a suspected  ‘polymerization’ of the lubricant running in or near the elastohydrodynamic regime [4]. It is believed that  the TiC coating employed for the scan mirror motor bearings likely limit ed such phenomena, given that  the overall travel of three of the scan motors is equivalent to >107 rotations . The scan m otors clearly  operate in the boundary lubrication regime.    Significant achievement s were made for both types of linear drive mechanisms, with both pressure  modulated cells hav ing performed flawlessly . A small leak in  the PMC 2 system was managed using   standard in- orbit calibration so as to have minimal science impact.    The cooler failure, which was abrupt in nature, may have been due to electronics, harness or the mechanism . Loss of science return was minimized for the cooler system failure case due to the partial  redundancy of the optical channels of the instrument . The compressor from the impaired functional cooler  has been used to compensate the vibration of the active cooler for the majority of the mission, with a high degree of success . Some adjustments in compressor settings were required to deal with minor  drifts in  the amplitude of the i mpaired (free- piston) displacer  to avoid end- stop contact .