Page 44 - Tribology in Machine Design

Basic principles of tribology 31

If A/i' = A/i + <5 p, then the probability density function is

The probability that A/i' is negative, i.e. the probability of asperity contact,
is given by

2.11 The wear in Wear occurs as a result of interaction between two contacting surfaces.
lubricated contacts Although understanding of the various mechanisms of wear, as discussed
earlier, is improving, no reliable and simple quantitative law comparable
with that for friction has been evolved. An innovative and rational design of
sliding contacts for wear prevention can, therefore, only be achieved if a
basic theoretical description of the wear phenomenon exists.
In lubricated contacts, wear can only take place when the lambda ratio is
less than 1. The predominant wear mechanism depends strongly on the
environmental and operating conditions. Usually, more than one mechan-
ism may be operating simultaneously in a given situation, but often the
wear rate is controlled by a single dominating process. It is reasonable to
assume, therefore, that any analytical model of wear for partially lubricated
contacts should contain adequate expressions for calculating the volume of
worn material resulting from the various modes of wear. Furthermore, it is
essential, in the case of lubricated contacts, to realize that both the
contacting asperities and the lubricating film contribute to supporting the
load. Thus, only the component of the total load, on the contact supported
directly by the contacting asperities, contributes to the wear on the
interacting surfaces.
First, let us consider the wear of partially lubricated contacts as a
complex process consisting of various wear mechanisms. This involves
setting up a compound equation of the type

where V denotes the volume of worn material and the subscripts f, a, c and d
refer to fatigue, adhesion, corrosion and abrasion, respectively. This not
only recognizes the prevalence of mixed modes but also permits compen-
sation for their interactions. In eqn (2.50), abrasion has a unique role.
Because all the available mathematical models for primary wear assume
clean components and a clean lubricating medium, there will therefore be
no abrasion until wear particles have accumulated in the contact zone.
Thus V d becomes a function of the total wear V of uncertain form, but is
probably a step function. It appears that if V A is dominant in the wear
process, it must overshadow all other terms in eqn (2.50).
When V d does not dominate eqn (2.50) it is possible to make some
predictions about the interaction terms. Thus it is known that corrosion