Figure 17. Type 7004 Bearing mean torque and PGM-HT film thickness vs. revolutions in vacuum  In future work, the facility will be used to characterize not only self-lubricating bearings, but also solid and  fluid lubricated bearings using oils and greases. For appropriately characterized fluid lubricants, it is hoped  that the lubricating film thickness measured will be correlated to predictions. For solid and self-lubricating  bearings, it is hoped that the speed effects on cage wear and transfer film formation (including perhaps its  viscoelasticity), will be understood so as to substantiate or not the accelerated test approach with increased  speed and/or load.  Discussion  Accelerated testing of oil or grease lubricated bearings by increasing speed alone or speed and temperature  remains risky (high probability of a misleading result). In addition to the main uncertainties regarding the  rheological behavior of the fluid (especially at the high pressures within the contacts) and its flow, especially  where grease is used, the local quantity of lubricant available seems difficult to predict. The conventional  Langmuir theory seems to be predict higher evaporation rates for space oils than are measured in practice,  yet fully saturated cages seem capable of absorbing a high proportion of the free oils within a bearing within  their phenolic matrix. These competing factors and others mentioned above result in relatively high  uncertainty concerning the lubrication regime in any space bearing either under nominal or accelerated test  conditions. However, a series of carefully controlled tests in which the film thickness is monitored throughout  test using the ABTR could improve understanding and even remove some of the more significant test  uncertainties.  For solid and self-lubricating bearings, since the friction and wear behavior of both lubricant, transfer film  and cage material are relatively well known and correlated to contact stress, then the possibility of  accelerated test by speed, load or both seems feasible and indeed this method has already been used with  some success. Nevertheless, the data newly available from the ABTR will allow the substantiation of claims  concerning the validity of accelerated test methods and a ranking of the concerns listed above as of primary  or secondary significance. This facility will also provide information of dynamic and static film thickness and  may prove or dis-prove the assertions that behavior is substantially speed independent.   Conclusions  Despite considerable developments in understanding and characterization of fluid lubricants in recent  years, accelerated test of fluid lubricated bearings remains highly likely to produce misleading results for  the reasons discussed.  Life tests not carried out under an appropriately representative environment must be avoided as they will  inevitably result in a tribological under-test. For example, results from Braycote 601EF suggest that tribolifetime of the grease performance is 1-3 orders of magnitude slower in air or nitrogen than in vacuum and  similar results are documented for oils (and of course for solid lubricants).  NASA/CP—2018-219887 525