Effect of Cracks and Dynamic Harmonic Excitation Loading on Tribological Performance


Student thesis: Doctoral Thesis

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  • Wei YUAN


Awarding Institution
Award date4 Oct 2016


The friction pairs of a differential gear end-face sliding on a thrusting washer, often have defects on the rubbing surfaces and suffer dynamic loading when the vehicles are running on the uneven road. The factors of cracks and dynamic harmonic excitation (DHE) loading play crucial roles in tribological performance of sliding pairs during the vehicle running. However, the evaluation of the effect of macro cracks and DHE loading on tribological performance of sliding pairs, remains elusive thus far, and a fast wear particle classification method is required to recognize wear modes and quantify wear levels. Therefore, a new method of wear particle shape classification, which is expected to apply to online wear monitoring techniques, should be proposed to determine the wear condition of sliding pairs. In this study, the following aspects are described.
First, a new shape classification method of wear particles is proposed based on the proposed parameters of wear particle shape. Debris features and types are identified through the classification and regression tree (CART) method. By the fusion of the proposed parameters of wear particle shape and CART method, the results of the wear debris analysis enable the maintenance team to diagnose faults appropriately. This method will be used to determine the wear modes of differential gear end-face sliding on washer later.
Second, the wear properties of surfaces with macro cracks were assessed through experimental investigation. Macro cracks perpendicular to the sliding direction were fabricated on specimens. The duration of running-in periods and root mean square of the particle coverage area values, a parameter relating to wear rate, correlate approximately linearly with the number of macro cracks. The macro cracks tend to promote plastic deformation on the contact disc surfaces, and material plastic deformation of the crack edges vary with the magnitude of applied normal loads.
Third, the effects of various spring-connecting loads and excitation intervals of DHE loading on tribological performance of sliding conditions were investigated. The results indicate that adequate spring-connecting loads tend to result in a low coefficient of friction. High-frequent excitation facilitates the acceleration of the process of the running-in period. Main surface damage or wear modes, such as fatigue, material side flow, and plowing grooves are caused by DHE loading.
Fourth, the independent action and interaction of macro crack and DHE loading on the tribological performance of the differential gear end-face sliding on the washer, were experimentally investigated. The results suggest that high DHE loading tends to increase friction force and result in plowing grooves, material side flow and plastic deformation on washer surfaces. However, a macro crack on the washer specimen can avoid friction failure and severe wear even under DHE loading. Unstable friction state and the degenerative tendency of the friction process can be detected using the decomposed low-frequency band signals of DWT.
Fifth, the wear characteristics of a differential gear end-face sliding on a washer under insufficient lubrication were determined under the effect of macro crack and DHE loading. Results suggest that (i) abrasive wear occurs with shallow and narrow plowing grooves with a crack on the thrusting washer, (ii) complex wear modes, such as fatigue, plowing and plastic deformation, take place under DHE loading, and (iii) the interaction of macro crack and DHE loading will lead to fatal damage and should be avoided completely.
In summary, the tribology performance of sliding pairs was evaluated with the influence of the factors of macro cracks and DHE loading, mainly using (i) shape classification of wear particles, (ii) experimental analysis on a ball-on-disc rig, and (iii) bench tests with wear mode analysis on a self-designed rig with the mating surfaces of a differential gear end-face and a thrusting washer. This study is expected to facilitate the improvement of frictional surface design and identification of the working condition of the friction pairs.