Measurement of Thermally Induced Loads in Back-to-Back Angular Contact Spindle Bearings Using Strain Gauges

Thermally induced bearing loads can potentially create serious problems for metal cutting spindles when used at high speeds. Proper spindle bearing set-up can minimize, but cannot eliminate this problem. Measuring the thermally induced load can alert the user to a potential problem or can be used to control the load directly. The purpose of this paper is to describe the design of a thermally induced bearing load sensor using strain gauges placed around the outer race. A box spindle with strain gauges on a pair of angular contact ball bearings located in the front of the spindle is used in this analysis. In order to calculate the thermally induced bearing load, the outputs of the strain gauges were recorded over a one second interval, sampled at 7500 Hz from each strain gauge and the root mean square of the deviation of this data from its mean is calculated. This value is a measure of the ball load. This calculated output is calibrated by a quadratic regression of these data to applied axial loads over a range of 0 to 2800 N. Values for each bearing were averaged to yield a front and rear value. The repeatability error for the front bearing sensor is 1.36%, and its accuracy is 98.9%. The repeatability error for the rear bearing sensor is 2.17% and its accuracy is 98.3%. These relatively low repeatability errors are attributable in part to filtering that does reduce the sensors’ bandwidth, but not significantly for measuring the relatively slowly changing thermally induced loads. Sensor design improvements and potential avenues of future research are discussed.