Observations of Spacecraft Bearing Lubr icant Redistribution Based on Thermal  Conductance Measurements Yoshimi R. Takeuchi*, Peter P. Frantz**and Michael R. Hilton*  Abstract The performance and life of precision ball bearings are critically dependent on maintaining a quantity of  oil at the ball/race interface that is sufficient to support a robust protective film. In space applications,  where parched conditions are intentionally the norm, harsh operating conditions can displace the small reserves of oil, resulting in reduced film thickness and premature wear. In the past, these effects have proven difficult to model or to measure experimentally. This paper describes a study addressing this  challenge, where bearing thermal conductance measurements are employed to infer changes in lubricant  quantity at the critical rolling interfaces. In the first part of the paper, we explain how the lubricant’s presence and its quantity impacts bearing  thermal conductance measurements. For a stationary bearing, we show that conductance is directly  related to the lubricant quantity in the ball/race contacts. Hence, aspects of bearing performance related  to oil quantity can be understood and insights improved with thermal conductance data. For a moving  bearing, a different mechanism of heat transfer dominates and is dependent on lubricant film thickness on the ball.  In the second part of the report, we discuss lubricant quantity observations based on bearing thermal  conductance measurements. Lubricant quantity, and thus bearing thermal conductance, depends on  various initial and operating conditions and is impacted further by the run-in process. A significant effect  of maximum run-in speed was also observed, with less oil remaining after obtaining higher speeds. Finally, we show that some of the lubricant that is displaced between the ball and race during run-in operation can be recovered during rest, and we measure the rate of recovery for one example. 1.0 Introduction Bearing life and performance is critically dependent on lubricant. Heat transfer is also dependent on  lubricant in space, therefore the two are linked. This paper will show bearing thermal properties depend on lubricant and its quantity, then, show how the conductance measurements can be used to infer lubricant behavior. The requirements for operation of space mechanisms present bearings a very different thermal  environment than mechanisms used in a terrestrial environment. In terrestrial applications, convection dominates the cooling mechanism. If air is not enough to cool it, the bearing is typically flooded with  lubricant for additional cooling. Thus, bearing thermal conductance tends to be a second or third order effect in most terrestrial applications. However, in the vacuum of space, essentially no air is present and flooding with lubricant is not feasible. Furthermore, in most cases, the bearing must operate with parched lubricant quantities and perform for  years under these conditions. In the absence of convection, bearing raceway temperatures are a productof bearing thermal conductance, heat generation, and the operational environmental temperature. In most  *The Aerospace Corporation, El Segundo, CA **The Aerospace Corporation, Colorado Springs, CO /g51/g85/g82/g70/g72/g72/g71/g76/g81/g74/g86/g3/g82/g73/g3/g87/g75/g72/g3/g23/g21/g81/g71/g3/g36/g72/g85/g82/g86/g83/g68/g70/g72/g3/g48/g72/g70/g75/g68/g81/g76/g86/g80/g86/g3/g54/g92/g80/g83/g82/g86/g76/g88/g80/g15/g3/g49/g36/g54/g36/g3/g42/g82/g71/g71/g68/g85 /g71/g3/g54/g83/g68/g70/g72/g3/g41/g79/g76/g74/g75/g87/g3/g38/g72/g81/g87/g72/g85/g15/g3/g48/g68/g92/g3/g20/g23/g16/g20/g25/g15/g3/g21/g19/g20/g23/g3 113