Explicit dynamics finite element modelling of defective rolling element bearings

Abstract

Rolling element bearings are widely used in rotating machinery across various industries and their failure is a dominant factor that contributes to machinery breakdown, consequently causing significant economic losses. Numerous experimental and analytical studies have been conducted in the past to understand the vibration response of non-defective and defective rolling element bearings, which have localised, extended, and distributed defects. Previous models have focused on simulating the defect-related impulses, which are generally observed in practice in measured vibration signals, and they implement envelope analysis to predict the significant defect-related frequency components. The work presented in this thesis is focused on developing an understanding of the underlying physical mechanism by which defect-related impulses are generated in defective rolling element bearings. A novel explicit dynamics finite element (FE) model of a rolling element bearing having a localised outer raceway defect, line spall, was developed and solved using a commercially available FE software package, LSDYNA. In addition to simulating the vibration response of the bearing, the dynamic contact interaction between the rolling elements and raceways of the bearing were modelled. An in-depth investigation of the rolling element-to-raceway contact forces was undertaken and variations in the forces, as the rolling elements traverse through the defect, were analysed. The contact force analysis has also led to the development of an understanding of the physics behind the low- and high-frequency characteristic vibration signatures generated by the rolling elements as they enter and exit a defect. It was found that no impulse-like signals are generated during the gradual de-stressing or unloading of the rolling elements as they enter into a defect, which explains the low-frequency characteristics of the de-stressing event. In contrast, a burst of multiple, short-duration, force impulses is generated as the rolling elements re-stress between the raceways in the vicinity of the end of a defect, which explains the high-frequency impulsive characteristics of the re-stressing event. Based on the results of the FE analysis of the rolling element bearing, a mathematical model was developed to predict the gradual de-stressing of the rolling elements as they enter into a raceway defect. Experimental testing on a rolling element bearing, commonly used in the railway industry, and having a line spall machined on its outer raceway was undertaken. The numerically modelled vibration response obtained using the FE model of the rolling element bearing was compared with the experimentally measured data, and a favourable agreement between the modelled and measured results was achieved. Numerical rolling element-to-raceway contact forces were compared with corresponding analytical results calculated using a quasi-static load distribution analytical model presented in this thesis. A parametric study to investigate the effects of varying radial load and rotational speed on the vibration response of the bearing and rolling element-to-raceway contact forces was undertaken. It was found that the magnitude of the defect-related vibration impulses and contact forces generated during the re-stressing of the rolling elements increases with increasing load and speed. The modelled contact forces were correlated with bearing vibration signals, and it was found that the amplitude of the contact forces and acceleration produced during the re-stressing of the rolling elements is much greater than when the rolling elements strike the defective surface. In other words, although a rolling element can impact the surface of a defect and generate a low amplitude acceleration signal, a much higher acceleration signal is generated when the rolling elements are re-stressed between the raceways as they exit from the defect. These higher acceleration signals, generated during the re-stressing phase, are the ones that are generally observed in practice, and subsequently used for bearing diagnosis. The work presented in this thesis has provided definitive physical and quantitative explanations for the impulsive acceleration signals measured when a bearing element passes through a defect.