Measuring the EHD Film Thickness in a Rotating Ball Bearing  Peter Ward*, Alan Leveille* and Peter Frantz*  Abstract  This paper presents two independent ways to directly measure the change in axial deflection of a bearing  that is proportional to the magnitude of the total elast ohydrodynamic (EHD) film developed. The first uses  multiple capacitance displacement gages as a direct measurement of axial displacement and the second  method uses the change in operating preload force to calculate the axial change in the test fixture and  bearing stack. Convergence of the results from two independent techniques adds confidence to the  experimental process. Either method can provide accurate EHD film thickness over extended time for a  complete bearing under varying load, speed and lubrication conditions. Additionally, the methods are  equally applicable to non-metallic materials, such as Si 3N4 balls, which prec lude the use of other  techniques. The method has some advantages to the practicing engineer over the classical ball on a flat quartz plate because the full range of bearing design parameters and kinematic interactions are  operating. The EHD film thicknesses of two oils and one grease were measured in an operating 304 sized angular  contact bearing for up to 600 hours of running time at 6000 rpm. EHD film thickness was determined by  simultaneously measuring the resultant displacement and axial force change of the outer rings due to collapse of the EHD film when the bearing pair was brought from full speed to rest. Because each  measurement of axial displacement was completed within 2 seconds, the results are not affected by a  contribution due to thermal expansion. We compared a common synthetic hydrocarbon base oil (Pennzane) with a common mineral oil (Coray). We found the synthetic oil film thickness to be  approximately one half that of the mineral oil. We also compared film thickness with both large and small  amounts of grease. We found that the large grease amount produced a prolonged run-in transient film that was thicker than the base oil steady state film. However, with a smaller amount of grease, the  thickness fell to a value that was comparable to the base oil. Initial assessment indicates that the degree  of lubricant starvation is greater than can be accounted for with conventional EHD theory.   Introduction  Ball bearings in numerous aerospace mechanisms are lubricated with a single charge of grease or oil that  is expected to last on the order of 10 years. Development and maintenance of an elastohydrodynamic (EHD) film that is sufficient to prevent wear is dependent upon application conditions, bearing design and  lubricant properties. As demands on lubricant performance grow, lubricant conditions are pushed towards  a deeply starved regime where conventional models of EHD film thickness break down [1]. Therefore, our experimental and theoretical tools for demonstrating lub ricant performance for a given application are  becoming increasingly compromised. For this reason, we devised an experimental approach to measure  EHD film thickness of an operating pair of bearings with two objectives. The first is to directly measure the film thickness in applications of immediate relevance to specific flight requirements to ensure an adequate  film is present. The second is to eval uate the validity of modern lubricant sta rvation models and to adapt  them for use in design of aerospace applications. This presentation is intended to be an introduction and demonstration of the experimental technique.  The experimental procedure presented here provides an accurate measure of the time dependent EHD  film thickness behavior. The magnitude of the EHD film in a bearing is proportional to the change in the  axial stick-out (end play) of the bearing between a condition of running at speed with an EHD film and a  condition of zero RPM with no EHD film. This change in end play was experimentally determined simultaneously with two independent techniques. The first was to measure it directly using air-gap  * The Aerospace Corporation, El Segundo, CA  Proceedings of the 39th Aerospace Mechanisms Symposium, NASA Marshall Space Flight Center, May 7-9, 2008  107