vacuum situations, thermal conductance through the bearing and housing becomes a primary driver in maintaining stable temperatures.  Bearing thermal conductance and heat generation are dependent on a complex interaction of secondary  factors, including the bearing geometry, internal loads, materials properties, operating and environmental conditions, and lubricant distribution. Heat generation within the bearing is the thermal energy gained andis equal to the mechanical energy lost based on the energy conservation concept. Thus, heat generation  is the product of torque and rotational bearing speed. Since torque and speed are commonly measured  values, bearing heat generation tends to be a commonly known quantity. Thermal conductance represents the bulk effective heat transfer between inner and outer race; the inverse of thermal  resistance. The value of this parameter tends to be poorly known. Bearing thermal conductance and heat  generation are dissimilar and respond to thermal and mechanical environments in very different ways. By  measuring the lesser known parameter, thermal conductance, a different set of observations on bearinglubricant behavior can be made. This paper will show that for static bearings, thermal conductance depends on lubricant distribution in the  ball to race contacts. For example, a meniscus of oil collected at the interface between a ball and race  provides for a larger effective area of contact and heat transfer between the two stationary bodies. This  paper will also show that for a bearing in motion, the mechanism of heat transfer differs. The lubricant distribution on the ball dominates the thermal conductance measurement. In the first part of the paper, we describe the use of thermal conductance measurements to establish the  mechanisms of heat transfer, which are sensitive to lubricant distribution. Once the relation between bearing thermal conductance and the lubricant is demonstrated, we show that thermal conductance  measurements can be used as a qualitative indicator of lubricant quantity, to monitor behavior during the run-in process, and to deduce the impact of lubricant recovery during bearing rest times. 1.1 Nomenclature G – (W/°C) bulk effective conductance across the bearing Ai – (m2) ball to inner race Hertzian contact area Ao – (m2) ball to outer race Hertzian contact area A–  ( m2) average inner and outer ball to race Hertzian contact area A0i – (m2) area due to lubricant in ball to inner race contact area meniscus A0o – (m2) area due to lubricant in ball to outer race contact area A0 – (m2) average inner and outer lubricant areas P – (N) axial load Ri – (°C/W) thermal resistance from the inner race to the ball in a bearing Ro – (°C/W) thermal resistance from the outer race through the ball of a bearing Rb – (°C/W) total thermal resistance across a single bearing ball ai – (m) major axis of the Hertzian contact ellipse between the ball to inner race bi – (m) minor axis of the Hertzian contact ellipse between the ball to inner race ao – (m) major axis of the Hertzian contact ellipse between the ball to outer race bo – (m) minor axis of the Hertzian contact ellipse between the ball to outer race a – (m) average of the inner and outer race major axis of the Hertzian contact ellipse b – (m) average of the inner and outer race minor axis of the Hertzian contact ellipse k1 – (W/m-°C) thermal conductivity of the race materials  k2 – (W/m-°C) thermal conductivity of the ball material 114