Paper: Effects of debris particles in sliding/rolling elastohydrodynamic contacts.
Authors: Nikas, G. K., Sayles, R. S., and Ioannides, E.
Where: Proceedings of the Institution of Mechanical Engineers (IMechE), Part J: Journal of Engineering Tribology, 1998, 212(5), 333-343. Available from the publisher.
A theoretical simulation of the behaviour of debris particles in elastohydrodynamic (EHD) contacts is an effective means for obtaining information regarding the life and performance of lubricated machine elements compared with costly experimentation. This paper indicates that debris particles are often responsible for two failure modes:
(a) scuffing caused by particle accumulation in the inlet zone of an EHD contact, and
(b) local melting due to high heat produced by the friction of debris in sliding contacts.
The predictions of this work are in agreement with experimental evidence in two ways: firstly, in that EHD contacts may fail because of scuffing if the lubricant becomes contaminated, where the failure due to inlet blockage by debris and eventually fluid starvation, and, secondly, in that sliding asperity contacts encounter high flash temperatures which may cause melting and thus plastic deformations.
Some figures from this work
This work deals with the thermomechanics and fluid mechanics aspects of the entrainment and entrapment of solid spherical particles with diameter typically in the range of 1-100 microns in rolling-sliding elastohydrodynamics contacts. From the thermomechanical part of the model, Fig. 1 below shows an example of the temperature on the counterface of a typical elastohydrodynamic contact during the entrapment, compression, shearing and eventual passage of a spherical solid ductile particle. The shearing and compression of the particle in the contact results in severe frictional heating which raises the surface temperature substantially as can be realized from Fig. 1.
Fig. 1. EHD counterface temperature rise from frictional heating during the passage of a soft ductile spherical particle.
From the fluid-mechanics part of the model, Fig. 2 below shows some possible trajectories of a 20 micron spherical solid particle left in front of a ball rolling-sliding on a flat surface. The film thickness far away from the ball is 100 microns. The particle is left on the flat and is affected by the fluid flow created by the moving ball. Depending on its initial position, the particle may collide with the ball, become entrapped, be rejected by the ball, or completely bypass it. The "contact semi-circle" is the line defining the imaginary boundary where a particle will come in first (undeformed) contact with both the ball and the flat (that is, the particle cannot enter that semi-circular area without being deformed first). Looking at the trajectories, it is easy to see some particles avoiding the ball while others run onto it and are either entrapped or rejected.
Fig. 2. Possible trajectories of a 20 micron spherical particle left in the upper half area of the graph.
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