Direct Load Monitoring in Rolling Element Bearing by Using Ultrasonic Time of Flight

Rolling element bearings find widespread use in numerous machines and they are one of key components in involved systems. Bearing failures can cause catastrophic events if they are not detected in time and result in increasing downtime and maintenance cost. The need for longer endurance life with less cost drives research on bearing condition monitoring. Abstract Load monitoring provides significant information for bearing design and residual service life prediction as load applied by each rolling element on a bearing raceway controls friction and wear. It is possible to infer bearing load from load cells or strain gauges on the shaft or bearing housing. However this is not always simply and uniquely related to the real load transmitted by rolling elements directly to the raceway. Firstly, the load sharing between rolling elements in the raceway is statically indeterminate. And secondly, in a machine with non-steady loading the load path is complex and highly transient being subject to dynamic behavior of the transmission. This project develops a non-invasive, safe and portable technique to measure the load that transmitted directly by a rolling element to the raceway by using ultrasound. Abstract The technique works by monitoring the time-of-flight (ToF) of ultrasound that travels in a raceway and reflects back from the contact face. A piezoelectric sensor was permanently bonded onto the external surface of the stationary raceway in a rolling element bearing. The ToF of an ultrasonic pulse from the sensor to the raceway-rolling element contact was measured which depends on the wave speed and the thickness of the raceway. Abstract The speed of an ultrasonic wave in a component changes with the state of the stress; known as the acoustoelastic effect. The thickness of the element varies when deflection occurs as the contacting surfaces are subjected to load. Therefore, the ultrasonic ToF in a raceway is load dependent. In practical measurements, it was found that the phase of the wave reflected from rolling contacts varied with contact conditions. The phase was determined by the contact stiffness and in simple peak to peak measurement, this appeared as a change in the ToF. For typical rolling contacts, the ToF changes caused by deflection and acoustoelastic effect are of the order of nanoseconds, while the apparent time shift from the phase change effect is in the same order. Abstract Despite the phase change having effect on reflected signals, it does not affect the envelope of these signals. In this work the Hilbert transform was used to calculate the envelope of the reflected pulses and thus this contact dependent phase shift was eliminated. Time difference between the envelope of reflected pulses in unloaded and loaded state was a result of load effect alone. Abstract Ultrasonic measurements have been carried out on a model line contact formed between a steel plate and a cylindrical bearing steel roller, and line contacts in a cylindrical roller bearing which was used for the planet gear of a wind turbine epicyclic gearbox, as well as on elliptical contacts in a radially loaded ball bearing (deep groove). The ToF changes under different contact loads were recorded and used to determine the deflection of the raceway. This was then related to load using a simple elastic contact model. Measured load from the ultrasonic reflection was compared with the applied load upon the contact and good agreement has been achieved. The ultrasonic ToF technique shows promise as an effective method for load monitoring in real bearing applications