An investigation into vibrations excited by discrete faults in rolling element bearings

Although the field of vibration monitoring is more than thirty years old a review of the literature revealed a lack of understanding concerning the form of the source of vibrations excited by discrete faults in rolling element bearings. As a rolling element passes through a discrete fault a periodic forcing function is produced that excites oscillations of the bearing and the structure that surrounds it. This forcing function is the source, and the characteristic shape of one period describes the form of the source. The assumption that the form of the source is an impulse is universal throughout the literature; however no studies have been reported that have investigated the validity of this assumption. What is more many of the observed properties of the oscillations excited by discrete faults are not explained by this assumption. The project reported in this dissertation is an experimental study of the form of the source and the form of the transient response excited by discrete race faults in a ball bearing. Fundamental differences between the observed transient responses and those suggested by the literature were identified. These differences result from the significant effect the motion of the ball entering the fault has on the observed transient response. The properties of the observed response are described and used to explain some results from vibration monitoring techniques not explained by the assumption that the source is an impulse. Central to these results is the discovery that the form of the source is influenced by the width and location of the fault, the rotational speed and the magnitude of the applied radial load. The results of the experimental investigation are further explained by two original kinematic models. These models describe the motion of the outer race, relative to the inner race, of the radially loaded ball bearing with a discrete fault on the outer race. The models demonstrate the influence of the impulse response of the system on the generation of the source in addition to the behaviour of the system in response to the source. They provide a mechanism to explain the modulating influence of the load zone, and demonstrate how the form of the source is influenced by the fault width, the rotational speed and the magnitude of the applied radial load.