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Modelling contact and friction between moving parts


Modelling contact and friction between moving parts is important to help understand the behaviour and lifetime of a mechanical system. This relates to tribology, and different surface coatings can enhance sliding or instead the resistance, depending on the application.

Stresses and strains will also change with the surface coating involved, leading to better – or worse – product lifetime. In transport industry, it directly relates to the engine consumption thus making it an economical and environmental challenge.

Objectives from modelling contact and friction

In this case, one wants to understand the effects of flaws on the steel shaft surface that will receive coating. Variation of performances and occasional failures occured on parts when microscopic cracks lay at the interface between steel and coating. The crack is geometrically represented in the model, and stresses are calculated for several lubrication and loading conditions.

The steel shaft slides against the fixed bronze connecting rod. Main assumption in the model is to investigate a planar contact where the two faces locally touch each other, rather than a cylinder / cylinder contact. The contact zone remains very small in the real life transmission system to make this assumption consistent.
One selected a non-linear Augmented Lagrangian algorithm to simulate contact and friction during the relative motion of parts.

Results from simulation and information extracted

One replicates the model for two cases : with and without flaw.

It reports much information on the stress distribution in the steel and in the coating. Tensile, shear and compressive stress line animations also provide visual data only available in modelling tools.

However, the most valuable information for the customer came from monitoring the stress at point located just beneath the flaw. It showed an expected stress growth during the horizontal motion, from contact entrance to contact exit. But it also showed a double stress peak at exit for the case with flaw, that could explain the decreased performance of real parts with flaw. Indeed, for parts rotating over millions of cycles, this rapid stress variation can cause more damage than a more monotonous stress increase in the case without flaw. It would cause more impact on the mechanical fatigue phenomena in the flawed part, even if the maximum stress value is lower.

Sketch of transmission system

Figure shows a sketch of the steel shaft with surface treatment of a few microns thickness, inside connecting rod.

Sketch of model representation

This figure shows the modelled contact zone with geometrical representation of the flaw.

Modelling contact and friction | Stress comparison

The stress data at point beneath the flaw shows influence of the flaw (orange curve) on the exit peak.

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