Mijares Tobias, Jorge Arturo (2018-04). Detection of Lubrication Starvation in Ball Bearings by Means of Lateral and Torsional Vibrations. Doctoral Dissertation.
Thesis
Considerable research has been conducted in fault detection and diagnosis for ball bearings but it has been focused on detecting the early stages of imminent faults due to fatigue. In reality, most bearings never reach the expected life or fatigue cycles due to problems related to maintenance or installation. This work studies lubrication starvation, which is one of the main causes of premature bearing failure. This work focuses on explaining the origin of the frequency indicators based on proposed bearing models. The validation of one of the models is achieved through a series of experiments measuring lateral vibrations and proving that the characteristic signal originates from the gap created by the absence of lubricant. Additionally, several lateral vibration indicators are compared for fault detection concluding that Fast-Kurtogram is the best technique for detecting lubrication starvation. A further diagnosis using envelope analysis verifies another model that proposes the ball pass frequency of the outer race (BPFO) as the main indicator of lubrication starvation in the frequency domain. An alternative method based on torsional vibrations at the shaft is additionally evaluated. A sensor based on the Time Interval Measurement System (TIMS) is developed using a field programmable gate array (FPGA) and a quadrature encoder to measure torsional vibrations. From the simulation of torque friction, the torque is found to not be significant compared to the driving torque of industrial motors. Hence the use of torsional vibrations as a mean to detect lubrication starvation is limited to applications in which bearing friction could impact the performance of the rotor drastically. The results are verified by the experiments in which the torsional vibrations are not able to detect changes in lubrication conditions. Finally, an energy analysis is presented to study the impact of lubrication starvation in the motor efficiency, which could be an economical motivator to encourage the research of condition monitoring of lubrication starvation. Lubrication starvation decreases the efficiency by dissipating power as heat and is dependent on the speed and static load rather than the load factor of the motor.
