Characterization of Periodic Disturbances In Rolling Element Bearings

The objective of this research is to observe and characterize periodic fluctuations in friction force of ball-element bearings that occur during velocity tracking motion. It is proposed that this periodic fluctuation in friction force is caused by the motion of the balls. We hope to show this relation by demonstrating that the frequency of the periodic fluctuation is equal to the frequency of the balls passing a position in the race.\ud \ud To illustrate the relation between the fluctuating friction force and the motion of the balls, a testbed has been built to measure friction force, ball passage rate, and velocity error during velocity tracking motion. The velocity error will be calculated from the measurement of position, and may show how the periodic fluctuations in friction force act like a periodic disturbance to the velocity. This paper will discuss the design and fabrication of the testbed, and the resulting measured signals that will be processed to determine their periodic content and to show how they are correlated. However, before inspecting the test results, some qualitative analysis of the system and models of the measured signals will be discussed to give insight into what we may expect from the results of the velocity tracking tests. \ud \ud An optical sensor has been designed and built to detect the motion of the balls in the race. The optical sensor measures the light reflected off the surface of a ball as it passes the sensing fiber. It was necessary to make some adjustments to the initial design of the sensor to correct for an instability in the signal. These adjustments, and the cause of the instability, will be discussed in this paper.\ud \ud Some ball bearings display an odd sticking behavior, where the friction force greatly increases beyond the approximated static friction force. This sticking behavior will be discussed, and how it relates to the motion of the balls in the race will be illustrated. It will also be discussed how the spectral density of friction force can be used to evaluate the performance of a bearing