Effects of Speed on Scuffing in Mixed Lubrication

Abstract

Theories of failure of elastohydrodynamic lubrication are briefly reviewed, but none that relate to scuffing per se and no general criterion that accounts for the sensitivity of scuffing to rolling as well as sliding speed are found. A theoretical investigation of micro-EHL by Baglin, for surface finishes with a lay parallel to the sliding direction, predicts boundaries in the operating condition domain to a regime of mixed lubrication in which little elastic deformation of asperities by micro-EHL is expected. A new thermal model incorporating salient features of scuffing in mixed lubrication conditions is described. It is shown to give the form of a boundary in the sliding/rolling speed domain above which localized temperatures close to melting may be expected and below which lower temperatures suggest running-in without scuffing may be expected. Results of scuffing tests on circumferentially ground discs, at sliding and rolling speeds in the range 3-10 m/s, are reported and shown for surfaces with a distinguishable mainscale wavelength in their topography, (a) to provide further support for the location of the boundaries to the mixed lubrication regime in the operating domain predicted by Baglin and (b) to match the form of the thermal model in the speed domain. Implications for engineering practice are briefly discussed.

A review of scuffing models

“…At a critical combination of speed and load, the temperature rise increases the contact area and friction resulting in an unstable temperature condition. More recently, Kelly and Barnes [67] have described a thermal model, where a boundary is formed in the sliding/rolling speed domain for disks operating under mixed lubrication conditions. Above the boundary, scuffing is expected when the localised asperity tip temperature approaches melting and below the boundary deformation without scuffing is expected.…”

Aspects of thrust cone tribology: Part 1: Effects of slide to roll ratio on surface failure mechanisms in twin-disc tests

“…To do so, values of 3 for both P and Q in equation (1) were adopted. Some justification for the use of these values is given in the previous work (7). At higher sliding speeds, above the upper boundary, relatively high rises in disc bulk temperature per load increment, which to some extent must reflect asperity tip temperature rises, were observed.…”

“…For example, it might be thought that the lower hardness of the through-hardened EN24, approximately half that of EN36, requires a corresponding increase in Q, via H in equation (7). However, this material ran-in to a far greater extent in the central regime on the speed domain and underwent relatively gross plastic deformation, perhaps even ratcheting, in the lower regime (8).…”

“…Such finish, of course, characterizes thrust cone surfaces, and since for the purposes of comparison the conjunction in such bearings can often be considered as a line or point contact, the use of circumferentially ground discs usefully simulates many aspects of their performance. Using EN36A discs, LUED had identified a boundary in the sliding=rolling speed domain between scuffing with and without running-in (7) and were on a convergent course with DRA in wishing to extend the range of surface speeds investigated (8,9). This led to the support acknowledged below for a collaborative 12 month project between DRA and LUED.…”

Mid-life scuffing failure in automotive cam-follower contacts

“…The vast majority of experimental investigations of scuffing are of a short-term nature, focusing on failures that occur during the running-in process, in tests of systematically increasing severity [1,2]. However, in several practically important engineering systems, such as automotive valve trains, hydraulic pumps and certain gear systems, scuffing can occur after an extended period of steady wear, without any increase in the severity of the externally applied conditions.…”