Influence of Oil Lubrication on Spacecraft Bearing Thermal Conductance  Yoshimi R. Takeuch?, Matthew A. Eb;, Benjamin A. Blake*, Steven M. Demsk; and James T. Dickef  Abstract  Increasing demands on bearing performance and a lack of thermal conductance data for bearings in  space applications motivated The Aerospace Corporation to study heat transfer across angular-contact  ball bearings for space systems. Tests were conducted under controlled conditions including rotational  speed, temperature, axial load, and vacuum environment. Bearings with Nye Pennzane SHF2001  synthetic oil were compared with dry (non-lubricated) bearings. These comparisons show that dry and oil  lubricated bearings vary in thermal conductance by up to an order of magnitude. Experimental  measurements also indicated that sensitivity to other variables, such as axial load and temperature,  depends on whether the bearing is dry or oil lubricated, and whether it is in a static or dynamic (rotating)  state. Mechanisms of heat transfer are discussed for each of these states.  Introduction  In contrast to typical terrestrial applications, the absence of convection shifts the focus for thermal analysis  of rotational space hardware. In vacuum environments, conductance through the bearings often provides  the primary heat transfer path between the shaft and the housing. As such, temperature predictions for  rotating components, such as satellite instruments, or the bearing itself require knowledge of the bearing  thermal conductance. However, published literature provides little help on the subject, and thus bearing  thermal conductance is usually the significant unknown in the development of a thermal model of a  rotational system in space. When available, engineers often use heritage information for comparable  systems with similar bearings. Significant uncertainty arises with the advent of design changes, different  bearing geometry, different lubricant type or quantity, or dissimilar operational conditions. An absence of  thermal conductance data leads to challenges in using thermal models for guidance in the design process.  Existing literature yields limited thermal conductance information for static and low-speed bearings [l-91  and none for high-speeds. Yovansvich [I ,2] developed a mathematical model for the thermal conductance  of a non-lubricated, static (non-rotating) bearing. Experimental work followed, including studies on  spacecraft bearings performed by Stevens and Todd [3], from the European Space Tribology Laboratories  (ESTL). They measured thermal conductance across a bearing, up to a maximum speed of 2,500 RPM,  using an experimental setup designed by Deli1 et a1 [4-51. ESTL continued to study this subject over the  years, focusing on static or low-speed and large thin cross-section bearings [6-81.  Demand on bearing performance has grown and thermal concerns have increased as systems have  reached higher speeds. Current momentum wheels and control moment gyroscopes typically operate at  6,000-9,000 RPM [lo]. Future wheels, including energy storage flywheels, are envisioned to run at even  higher speeds, above 15,000 RPM. In addition to the need for high-speed data, low-speed applications  could benefit from a greater range of experimental data, and additional sources of counter verification  from other researchers.  To address these concerns, an experiment was designed to assess thermal conductance of bearings in  vacuum, at speeds ranging from 0 to 6,000 RPM. Controlled studies were conducted by varying variables  such as axial load, thermal boundary conditions, and rotational speed, individually for parametric studies.  The tests identified variables of importance for different bearing conditions, including dry, lubricated, static,  and dynamic states. Finally, qualitative theories for the mechanisms of heat transfer for each of these  conditions emerged.  * The Aerospace Corporation, Los Angeles, CA  Proceedings of the 38th Aerospace Mechanisms Symposium, Langley Research Center, May 17- 19,2006.  291