Transient Analysis of Isothermal Elastohydrodynamic Point Contacts

In machine elements such as gears, ball and rolling bearings and cams and followers, it is very important to understand the behaviour of elastohydrodynamic films, formed between contacting surfaces in order to improve working performance and to prevent surface failure by wear or fatigue. As applied load, speed and the conjunction geometry vary with time under realistic operating conditions, the generated lubricant films exhibit a transitory nature. This occurs with normal approach or separation of contacting bodies. This thesis aims to provide a general solution for the problem of isothermal elastohydrodynamic lubrication (EHL) of point contacts under transient conditions. Transient lubrication is particularly concerned with the effect of squeeze film motion on the lubricant film thickness and pressure distribution. There has been a dearth of investigation to determine the effect of flow direction, particularly under high loads on film thickness and pressure distribution. The Reynolds equation has been derived and extended to accommodate combined rolling and normal approach of contacting bodies. An initial approach to the solution of the problem has been based upon quasi-static solutions, where a constant squeeze film motion has been assumed. A multi-level/multigrid numerical method has been employed to obtain solutions for pressure distribution and film thickness for a wide range of loads, speeds, squeeze film velocity and different contacting materials. Rapid convergence has been achieved with accurate results with a combination of central and backward differences, introducing a weight factor in the right hand side of the Reynolds’ equation. Under transient dynamic conditions the squeeze film velocity is a time and spatial dependent parameter. In these cases, the solution method adopted is the modified Newton-Raphson, with some appropriate numerical procedures. The contact of a steel ball on a flat glass race has been considered, when subjected to a sinusoidal load. The effect of squeeze film motion on film thickness and pressure distribution has been investigated. The numerical results have been compared with those of other studies for the circular point contact of a ball against a flat glass disc under oscillating conditions. The comparison with the experimental results shows good qualitative agreement. The effect of varying the direction of lubricant entrainment upon pressure distribution and oil film thickness for elliptical point contact conjunctions has been examined under combined rolling and sliding motions. An Effective Influence Newton-Raphson method is employed in local point distributed or global line distributed low relaxation iterations. The method employed enables determination of pressure distribution and film shape at high loads, which are encountered in most practical applications. The numerical predictions have been validated against experimental results. The results obtained in this study show that the developed numerical methods have been quite successful in prediction of realistic and practical contact conditions, where the applied load is usually quite high and the entraining velocity can be reasonably low. By making use of a weight factor in multi-grid/multi-level methods, the convergence difficulties can be overcome without scarifying accuracy under high loads and low speeds. The numerical predictions for the steady state and transient film shapes conform closely to the experimental results for the same conditions up to 100 (nm) minimum film thickness.

URI

https://bradscholars.brad.ac.uk/handle/10454/20599

Type

Thesis

Qualification name

PhD