In  preparation for these experiments, bearings were flushed with heptane and relubricated by depositing  130 mg of Pennzane 2001 with a syringe. Portions of the oil charge were applied to each ball, the raceways,  and the phenolic retainer . The bearing was then rotated to distribute the oil .  

Each bearing was run- in in vacuum at room temperature at 6000 rpm . In each case, the measured value  of heat  generation decreased erratically until stabilizing after approximately 3 hours . Stability of heat  generation, conductance, and temperature were all used to establish that the lubricant had been run- in.  After this run- in period, the bearings were weighed to determine the quantity of oil remaining in the bearing.  These values are reported in Table 2 . The remainder of the oil was believed to be thrown from the bearings  during run- in, and no longer able to contribute to ongoing lubrication of the critical inte rfaces . Similar tests  of other bearings in this test configuration have been done to determine the quantity of oil that is lost after continued operation at 6000 rpm . These have shown that after completing the initial run- in, no additional  oil (within the gravimetric measurement uncertainty of approximately 10 mg) is lost from the bearing due to prolonged operation at 6000 rpm . This observation was not detectably affected by variations in the thermal  conditions of the tests . Therefore, we deduce that changes in the viscous drag reported below are due to  changes in distribution of oil  within the bearing, and not due to changes in the total quantity of oil in the  bearing.  

Table 2. Lubricant Pre and Post Run- In Weights   Bearing  type Dry bearing  weight (g)  Initial oil  weight (mg)  Post run -in oil  weight (mg)  Post run -in  oil/bearing %   Steel  228.54  130 100 0.0438%   Hybrid  188.05  130 90 0.0479%  

Results from the All-Steel Bearing    This section shows  the results of tests done on the 305-size all- steel angular contact ball bearing. In this  series of tests, the applied axial load ranged from 89 to 222 N ( 20 to 50 lb) . The exact bearing temperature  was not controlled to a specific predetermined temperature, but rather was varied and recorded while  testing under a wide set of heat sinks and heat lamp output . This resulted in a range of average  temperatures from approximately 12 to 35°C , and a temperature differential that ranged from approximately  8 to 16 °C. The LQF  observations  throughout this range of test  conditions  will be presented in the next  sections.  

Effect of Average Temperature on LQF   Figures 2 represent  the normalized LQF as a function of average temperature (Tave) for a 305- size all -steel  bearing, where each graph displays  a different axial load. In these graphs , each curve represents a  particular fixed temperature differential  value (D T). The trends show that LQF generally increases in direct  proportion with the average temperature. The rate of increase is least at 89 N ( 20 lb) preload, but nearly  equivalent through the range from 133 to 222 N (30 to 50 lb ). As the preload is increased, there appears to  be a bias offset that increases with the DT, but there is no detectable change in the slope throughout the  range of DT.   In Figure 3, the DT=12°C data from each preload were grouped together to show the relative effect of preload on the temperature sensitivity. This plot shows the relative insensitivity to preload in the   133 to 222 N  (30 to 50  lb) range, with the moderately less effect of T ave in the 89 N ( 20-lb) case.      NASA/CP—2018-219887 548