Determination of Speed-Dependent Thermal Resistance of Ball Bearings

Abstract:
With the trend towards smaller and high powerdensity machines, the thermal behavior of the machine is becoming increasingly important. Besides the windings and the magnets in Permanent Magnet Synchronous Machines (PMSMs), also the bearings are one of the critical components, especially in the absence of a rotor cooling. The rotation of the balls inside a ball bearing significantly influences the thermal behavior of the bearing itself. There exist two approaches in literature, where on the one hand the ball resistance is kept constant over the whole speed range and on the other hand it is neglected. Both approaches lead to incorrect temperature predictions of the bearing in case of a speed-variant drive system. Therefore, we present a novel modeling approach with a speed-dependent and speed-independent thermal ball resistance. Contact resistances, convection phenomena and the power loss inside the bearing are calculated as proposed in literature. A brief summary of the contact resistances in a ball bearing is presented. A test bench is set up to validate the predicted thermal resistance of a bearing for a varying speed, where it can be seen that the thermal resistance is decreasing for an increasing rotational speed. In addition, the influence of the radial load and the mean temperature of the bearing is analyzed. The radial load has a minor influence on the thermal resistance of the bearing in the observed range, whereas an increasing mean temperature of the bearing leads to a decreasing thermal resistance due to a reduced viscosity of the lubricant. The simulation results show a good accordance to the measurement results and therefore the approach can be assumed to be valid for the observed ball bearing.