On the Source of Vibrations in Damaged Rolling Bearings

Rolling element bearings are the most widely used machine elements, so a great effort has been devoted to the development of effective procedures for their monitoring and diagnostics. Today, several signal processing techniques provide useful methodologies to extract diagnostic information from the vibrations of the machines, on the basis of estimated relations between cause and effect. One can state that vibration occurs due to the interaction between a damaged surface and a rolling element during machine running. However, the actual source of the vibration measured on the case is not completely clear. Usually, it is described as an impact, but the way of this hypothetic impact is difficult to explain. As a matter of fact, a rolling element in a bearing is highly loaded; when it strikes a surface defect, a brusque stiffness decrease takes place, caused by the lack of material corresponding to the damage. This sudden variation of load distribution between rolling elements seems a realistic hypothesis as a source of vibration. In this paper, a multi-degree of freedom non-linear model is implemented to determine the characteristics of the ball-race interaction in the loaded zone of a ball bearing. It simulates the dynamic behaviour of each rolling element in the presence of surface pitting. The variation of load distribution when the bearing runs is investigated both for integer and damaged bearing. A bearing without any surface damage but machined to introduce a stiffness variation is then studied and similarity between its behaviour and that of a damaged bearing is demonstrated. Experimental results confirm this outcome