Basics of EHL for practical application

Abstract

SUMMARYElastohydrodynamic lubrication (EHL) is present in all lubricated components whose elements roll together, including gears, rolling bearing, cams and constant velocity joints. These are characterised by having very localised and thus very high pressure contact, of order 1 to 3 GPa, between the elements.Two important practical properties of EHL contacts are the lubricant film thickness and the friction, and lubricant and machine designers and users need to be able to predict both of these. In principle they can be determined from full numerical solution of the elastohydrodynamic problem. However this is quite difficult and time-consuming and requires detailed knowledge of the rheology of the lubricant film at high pressure and shear rate. This paper is aimed at practising engineers and describes alternative approaches, i.e. how EHL can be applied to predict film thickness and friction in practical applications.

Evaluation of Wear Phenomena of Journal Bearings by Close to Component Testing and Application of a Numerical Wear Assessment

“…Besides the dependency on temperature and pressure, lubricants are also dependent on the shear-rate [33,[35][36][37]. This especially holds for conventional multi-grade oils.…”

The Influence of Base Oil Properties on the Friction Behaviour of Lithium Greases in Rolling/Sliding Concentrated Contacts

“…Such behaviour is expected at high speeds, where grease friction is determined by its base oil properties as discussed above. Indeed, EHL friction with oil is known to be strongly dependant on the molecular structure of the oil, including molecular shape and flexibility, since these characteristics determine the 'fluid' friction arising from shearing the film molecular layers at high pressure [35][36][37]. Low shear strength, and thus low friction, is favoured by oils containing linear, aligned and flexible molecules.…”

Multi-body modelling of rolling element bearings and performance evaluation with localised damage

“…The integration of this shear stress in the contact area gives the friction force f T . In many cases, the friction force is obtained as the modulus of the normal force multiplied by a friction coefficient μ, i.e., as the shear stress τ, a function of η and γ  [40]. Thus,…”

“…where τ e is the value of the shear stress at which shear thinning starts to become significant, p is the mean pressure in the contact, η 0 is the dynamic viscosity of the lubricant at atmospheric pressure, α is an average pressure-viscosity coefficient, U is the mean of the rolling speeds of the two surfaces with respect to the contact, SRR is the slide-roll-ratio [40], and h c is the central film thickness. Values for τ e and α depending on the mean film temperature and the mean contact pressure are given by Spikes [40].…”

“…Values for τ e and α depending on the mean film temperature and the mean contact pressure are given by Spikes [40]. Hamrock and Dowson [11] offer an expression for the cental film thickness for elliptical contacts.…”