The effect of static and dynamic misalignment on ball bearing radial stiffness for variable axial preloads

Abstract

The primary objective of the research project was to determine through rotordynamic computer modeling and experimental testing, the radial stiffness of a symmetric angular contact ball bearing for static and dynamic misalignment under various axial loads. A bearing test rig was specifically designed and manufactured to test the angular contact ball bearing. The test rig maximum speed was 20,000 rpm with the ability to apply 1000 lb axially. Testing was conducted for thrust loads of 0, 156 N, 356 N, 1,148 N and 1,962 N. Static angular misalignment between the inner and outer race ranged between 0.75 mil/in. and 3.0 -mil/in. misalignment. The dynamic misalignment testing was performed with a negative and a positive taper in the bearing support system. There were two different testing methods utilized to determine the radial stiffness of the ball bearing: 1) eigenvalue frequency analysis and 2) critical speed transition tests. Each method used experimental measurements in conjunction with computer simulation to determine the radial ball bearing spring rate. The eigenvalue frequency analysis was only implemented to the 0.75 mil/in. misalignment case due to the poor vibration transmissibility through the ball bearing for the larger cases of angular misalignment. Experimental measurements of the static pull tests, forward eigenvalues, backward eigenvalues, backward critical speeds, and forward critical speeds were matched with XLTRC model simulations to determine the stiffness values for each of the cases. Eigenvalue data was collected with a Hewlett Packard Spectrum Dynamic Signal Analyzer and critical speed transitions were monitored with ADRE, a condition monitoring data acquisitions system. Results include critical speed maps, Campbell Diagrams, Bode response plots, waterfall plots, and spectrum plots. Both forward and backward critical speeds were measured during the experiments. The effect of increasing static angular misalignment increased the radial ball bearing spring rate. Both the positive taper and negative taper cases indicated a softening effect when compared to the aligned (no taper) stiffness values. Significant nonlinearity was observed in the deflection and vibration of the test rig. The rotating system also revealed signs of bifurcation for higher degrees of static misalignment. The negative taper configuration generated significant sub-synchronous vibrations excited at the forward and backward eigenvalues.