all  steel bearing. However , one data point at 222 N ( 50 lb ) suggests a deviation from this trend as the  load is increased .  

Figure 11. LQF vs. axial load for a range of average temperature but constant temperature gradient   of 12°C for 305- size all -Steel Bearing  

Discussion    The results show that LQF generally increases with T ave, DT, and preload . However, we do not directly  conclude from this that the quantity of lubricant in the rolling and sliding interfaces is also increasing with  each variable . Instead, it is likely that the full complexity of bearing drag is not fully realized in the model  described above, and that there are other factors that might influence the calculated values of LQF . For this  reason, we assert that the measured values of LQF are to be used cautiously for purposes of thermal modeling, and to demonstrate the degree to which LQF may fluctuate with changes in test conditions . It  may also be used as a guide to deeper analysis of bearing function as it depends on the interplay between temperature, mechanics, and oil quantity . In the discussion below, we explore some possible causes and  implications of the observed behavior . 

Dependence of LQF on T ave  For both test bearings, the data show that the normalized LQF increases in direct proportion with the  average temperature of the test bearing (Figures 2, 3, 7, and 8) . We surmise that this is primarily due to an  increase in the actual quantity of lubricant in the rolling and sliding interfaces . This may be partly due to  reduced viscosity of the oil, resulting in improved oil mobility . Lower viscosity may also augment  the  influence of dynamic centripetal forces driving oil  on the surface of the balls towards the rolling contact at  the expense of static surface forces that drive the oil towards spreading more uniformly on the surfaces .  When the environment is warmer, there may also be some reduction in the significance of localized heating at the ball path, which creates a thermocapillary differential to drive oil away from the contact .    A similar dependence on temperature has been observed with aging gyroscopes on satellites  [10]. With  this application, the magnitude of the dependence of viscous bearing drag on temperature is used to  estimate oil quantity . When gyroscopes become starved of oil, the sensitivity of viscous drag to brief  temperature fluctuations becomes diminished. However, during periods of elevated time-average  temperature, a robust viscous response has been observed to recover . During these periods of recovery,  bearing noise due to retainer instability is often quieted. This has been interpreted as a reversible increase  in the oil quantity, caused by increased bearing temperature.    LQF comparisons between the hybrid and all- steel bearing generally indicate that the hybrid bearing has a  lower LQF than the equivalent all- steel bearing under similar condit ions. One example is shown in   Figure 12. This indicates that under similar thermal and mechanical conditions, the hybrid bearing will have  lower viscous torque. While there seemed to be some degree of variances between the all -steel and hybrid  NASA/CP—2018-219887 554