Thermally induced bearing loads have long been recognized as a key factor impacting the reliability and performance of machine tool spindle systems. This is particularly true for reconfigurable machine tool spindles which may experience a wide range of external loads, processes and spindle speeds. The models of thermally induced bearing load which have been developed thus far have calculated the thermal expansion of the spindle’s components using a classical solution that assumes the spindle is a circular cylinder of infinite length and that the temperature within the cylinder only varies in the radial direction. While this approach to calculating the thermal expansion provides reasonably accurate predictions of the bearing load if the thermal gradients in the axial and radial directions are small, it can result in large errors in the calculated bearing load if the thermal gradients within the spindle become large. The purpose of this paper is to present a new model of thermally induced spindle bearing load that uses a finite element model to calculate the thermal expansion of the spindle components. The model includes the thermally and mechanically induced spindle bearing loads in a back to back angular contact bearing pair that are due to radial and axial thermal expansion as well as the centrifugal forces and moments within the bearings. Simulation results are used to compare and contrast bearing load predictions that are based upon both a finite element and a classical thermal expansion calculation. The results demonstrate that the bearing load predictions based upon the classical thermal expansion calculation substantially under predict the bearing load as the heat load, due to increasing spindle speeds, is increased. As these errors in the predicted bearing load may be high enough to alter important design decisions, it is concluded that a finite element, or equivalent, thermal expansion calculation be used in future thermally induced bearing load models unless the thermal gradients within the spindle are known to be small.

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