Laboratory of Machine Elements and Machine Design




Author: Bakolas Vasileios
Title: Mixed friction and the development of micropitting on the flanks of spur gears.
Abstract: The goal of this work was the experimental and theoretical investigation of the mixed lubrica­tion regime, with special interest for the conditions relevant to micropitting as well as the in­vestigation of the existence of a critical parameter that could be used for the prediction of this particular gear failure.
The search for this critical parameter was concentrated on the stress field beneath the surface of the active gear flanks, since the nature of micropitting suggests that the flank fails mechanically.
For the theoretical investigation of the mixed lubrication contact a model was developed that is based on the superposition principle. The model assumes that the contact of two rough sur­faces in the presence of a lubricant can be sufficiently approximated be the superposition of a lubricated contact of smooth surfaces and a dry contact between rough surfaces. The major innovation of this model is that for the first time a mixed lubrication model can predict the real pressure and shear stress distribution on the surface of the contacting bodeis, thus allow­ing the exact calculation of the subsurface stress field.
For the practical application of this model, knowledge is necessary regarding various parame­ters that it is know that the influence the behavior of a linear contact under mixed lubricating conditions, such as:
· the bulk temperature field of the active flanks
· the roughness of the mating surfaces
· the physical and rheological properties of the lubricant
· the dry friction coefficient.
At the same time a number of experiments were carried out in order to collect data that were necessary for the execution and the verification of the calculations. Due to the fact that the experiments described in the literature are time-consuming and don't lead to safe conclusions regarding the existence of a critical parameter that is connected to the operating conditions, a new experimental procedure was developed which had the advantage of more rapid execution and the fact that it provides a clear link between operating conditions of a gear pair and the occurrence of micropitting.
The application of the model for the conditions of the experiments and the subsequent calcu­lation of the subsurface stress field along the mesh line allowed the identification of a critical shear stress, the mean maximum shear stress Γ25 , whose value is directly linked to the appearance and intensity of micropitting.
Thus the experimental procedure in connection with the mixed lubrication model are capable to determine a critical stress value for a gear pair system which is described among others by the material of the flanks, their roughness, the lubricant and the additives used. This stress can be compared to the stress resulting of the operation of the gear pair and can be determined with the use of the mixed lubrication model in order to establish the potential micropitting risk.


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