Mid-life scuffing failure in automotive cam-follower contacts

Abstract

There are several practically important engineering systems that fail by scuffing long after the initial running-in has been completed. A common feature of these systems is repeated contact between the same points on the mating surfaces. The processes leading up to such mid-life scuffing failures have been examined by monitoring the development of cam follower surface roughness and wear at regular intervals during a series of valve train wear tests in a fired engine. By the application of both statistical and numerical models for the elastic contact of rough surfaces, the following stages in the scuffing process have been identified: (a) surface roughening in the mild-wear regime, which progressively increased maximum asperity contact pressures until (b) the elastic shakedown limit was exceeded, causing plastic deformation wear, accelerated roughening and enlargement of valleys, leading eventually to (c) a transition to high rates of wear, probably resulting from hydrodynamic pressure loss and oil-film collapse.

Wear in boundary lubrication

“…Bell and Colgan [29] and Bell and Willemse [30] used component cam± follower wear data obtained from bench tests to correlate with oil-®lm thickness and to calculate the oil-®lm thickness using a ®nite element program. The wear contours and approximate amount of wear of the cam±follower contact in an engine was successfully simu lated.…”

“…Once the rheology is de®ned, then¯uid ®lm thickness can be estimated from the equatio n given by D owson and H igginson [27]: hˆ1:63 Z 0:7 0 s 0:54 0 V 0:7 R 0:43 L 0:13 E 00:03 where hˆfilm thickness Z 0ˆv iscosit y aˆpressure ¡ ¡ viscosity coefficient Vˆvelocity Rˆeffective radius of contacting bodies Lˆload E 0ˆe lastic modulus There are several computer programs available to calculate the contact stresses,¯uid¯ow, temperatures and elastohydrodynamic lifts [28], but to link these calculations to wear is a large step. Bell and Colgan [29] and Bell and Willemse [30] used component cam± follower wear data obtained from bench tests to correlate with oil-®lm thickness and to calculate the oil-®lm thickness using a ®nite element program. The wear contours and approximate amount of wear of the cam±follower contact in an engine was successfully simu lated.…”

In situ stylus profilometer for a high frequency reciprocating tribometer

“…Wear-in or run-in is a particular type of wear transition caused by contact evolution that occurs early during the contact lifecycle. During this regime, surface asperities are worn away removing fine geometric intolerances to make a conformal contact and at the same time reducing the contact pressure [10][11][12]. When an equilibrium is established the contact enters a period of steady state or linear wear, figure 1.…”