capacitance displacement gages. The second way was to deduce it by combining the measured spring  rate of the test rig in series with the calculated spring rate of the two bearings and then calculating what  the axial change in deflection had to be to account for a measured axial force change between running  with an EHD film and zero RPM with zero EHD film. The concurrence of these two methods provides a way of validating that the test set-up is correct fo r any given test. The measure ment of axial change is  similar in principle to the method of Tyler, et al [2]. However, our approach introduces a nearly isothermal  measurement that eliminates the very significant effect of temperature on the measured change in axial stickout. To illustrate the importance of temperature, for steel parts and a 15-degree operating contact  angle, a 1-degree F uncertainty in temperature across the bearing results in a measured stickout change  of 34 micro-inches for the 304 size bearing reported herein. This uncertainty is much larger than the actual EHD film thickness.  Several other techniques for measuring film thickness in an operating rolling contact bearing have been  described in the literature. Measurements of acoustic emission have been used to record the noise  created by asperity contacts between the balls and races [3]. This technique is well suited for in-situ monitoring of real bearing applications, but does not provide a direct quantitative measure of the film  thickness and loses resolution when the film thickness grows beyond 3 /g3times the composite surface  roughness. The electrical capacitance and resistance across the bearing has been used to infer the EHD  film thickness [4,5]. However, the inferred film thickness is often dep endent on uncertain estimates of  several properties, such as the oil volume outside the contact, the size and shape of the contact, and the dielectric properties of the material between the surfaces. Ultrasonic reflection [6] is a promising new  technique, but it is challenging to produce sufficient resolution under the relevant speed conditions.  Furthermore, it measure s only the film between the ball and outer race and not the inner race, which is  often more deeply starved.    Analytical Method  As the film between the ball and races grows, the inner ring is forced to deflect with respect to the outer  ring, resulting in a change in stick-out. To analyze the geometric situation, the increased film thickness is  modeled as an increase in the overall ball diameter. Figure 1 shows the change in inner ring position  caused by the hypothetical increase in the ball diameter. Figures 1A and 1B each show the cross section of an outer ring, ball, and inner ring, from top to bottom. Figure 1A represents the bearing with no  lubricant film between the ball and race, such as would occur when the bearing is at rest. Figure 1B   Figure 1.  Cross section of a bearing raceway/ball contact at rest (A) and with a hypothetical  lubricant film surrounding the ball (B).  shows the high-speed bearing, with the appearance of a lubricant film as a ring surrounding the ball. This  film is shown with uniform thickness around the ball only for the purpose of illustration. The inner ring  deflects to the left and the contact angle is reduced. The inner ring deflection is equal to one half of the  change in end play of the bearing resulting from a change in the effective diameter of the ball. Additional  deflection due to elastic deformation of the balls and races has also been accounted in our model. Details  of the technique and results of our film thickness measurements are described below.   108