Wear on mechanical contacts - Issues and definitions

According to most glossaries and definitions in specialized technical literature, wear corresponds to "the progressive loss of material from the active surface of a body, as a result of the relative movement of another body on this surface".

From the mechanic's point of view, the notion of wear is much less restrictive, as it applies more generally to any event leading to a loss of function of components or systems. This point of view also corroborates more common definitions which associate wear with deterioration due to use.

Loss of function refers to malfunctions that may result from :

• dimensional and geometrical changes in parts, such as increased clearance due to loss of dimension (degradation of precision, etc.) or, on the contrary, clogging of interfaces due to accumulation of debris (blocking, jamming, etc.);

• surface degradation phenomena (scratches, grooves, cavities, outgrowths, etc.), which can have a damaging effect in a number of ways: impaired sealing, altered mechanical properties of components, deterioration in appearance, etc.

In addition to these possible manifestations of wear, which correspond to what can be observed on parts, there may be a number of direct or indirect effects, detrimental to the normal operation of a mechanism and/or its environment: vibrations, noise, particle emissions, increased power consumption, temperature levels, lubricant consumption...

In practice, the diversity of possible forms of deterioration poses the problem of defining wear criteria. Indeed, in cases where wear does not correspond to a quantifiable dimensional change (loss of dimension), it cannot be correlated with directly measurable quantities on machine components. In particular, the notion of wear rate, which expresses the loss of material as a function of the distance covered, is totally unsuitable for a large number of situations, so that it is often preferable to talk about durability, which refers to the time or number of operating cycles during which the function is maintained.

However, while wear is generally harmful, there is at least one case where the phenomenon, viewed exclusively from the angle of geometric evolution, is favorable. Thus, provided it is controlled, wear can be exploited to ensure the running-in of mechanisms.

In all the industrial sectors concerned by wear and tear, more than two-thirds of the problems encountered are mechanical (in this field, 30% of damage is attributable to wear and tear). Linked to the displacement of two bodies in relation to each other, wear is particularly common on all machines whose functions rely on contact: transmission...