297 Film thickness measurements were performed using a device that has been described elsewhere  [13-16].  Two angular contact bearings are mounted in a spring preloaded DF arrangement. As the EHD film  thickness increases with speed, the resulting deflection of the outer rings is measured with capacitance  proximity sensors. This displacement is simultaneously determined from changes in the dynamic preload  using a load cell placed in series with the bearing preload. Figure 14 shows a schematic representation of  the bearing deflection resulting from film thickness growth. At low speed, the balls are in intimate contact  with the raceway surfaces. At high speed, the gr owth of the EHD film alters the bearing geometry, displacing  the outer race to the right, and reducing the contact angle from b 1 to b 2. The change in film thickness is  inferred from the outer ring displacement using a geometric model that represents the fi lm thickness as an  effective change in the ball diameter from d 1 to d 2. The change in contact angle is determined by measuring  changes in the ball group frequency with respect to the shaft speed.  

Figure 14. Cross section of an angular contact bearing at  low and high speeds, showing effects of  increasing EHD film thickness, modeled as an effective increase in ball diameter  

Tests were conducted using poly alpha olefin samples of various viscosity. These  lubricants  were chosen  due to their relevance to aerospace applications, their availability with different viscosities, and their simple  composition compared to refined mineral oils.  The oil samples were not formulated with additives. A small  amount of oil was appli ed to bearings by first impregnating the retainers, and then centrifuging the retainer  to remove bulk oil from the surface. The metal parts were coated with a thin film by solvent casting from a  10% solution of oil in heptane solvent.   

After running- in the lubricant at 3000 rpm for 500 hours at room temperature, film thickness vs speed  profiles were collected. These experiments were conducted by operating the bearings at a chosen speed,  and then quickly stopping the shaft to collapse the film. The resulti ng rapid change in outer ring position  was recorded and converted to oil film thickness using our geometric relationship. By measuring an  instantaneous change in position, we avoid effects of thermal relaxation on the bearing structure that would  otherwise overwhelm and obscure changes in the oil film thickness.  This process was repeated after  sequential increases in shaft speed from 60 to 3000 rpm.  

An example of our results is shown in Figure 15, where EHD film thickness of two different PAO oils is  plotted against speed from 60 to 3000 rpm.  Orange circles are results from tests with a low viscosity PAO  (10 cS at 100° C), and blue circles are from a more viscous oil (40 cS at 100° C). Tests were done with the  bearings at 30° C. We find that the PAO40 fil m thickness increases much more rapidly with speed than the  PAO10, as expected by theoretical estimates based on the Hamrock Dowson film thickness model. [17, 18]   The more interesting observation, however, is the deviation from this model at high speeds due to kinematic  starvation of the rolling contact. [19] We find that in this  case the onset of starvation occurs at an   approximately 600 rpm for the PAO40 and 900 for the PAO10. The difference i s likely due to the reduced