Page 47 - Tribology in Machine Design

34 Tribology in machine design

Under elastohydrodynamic conditions, the minimum film thickness for
cylindrical contacts of smooth surfaces can be calculated from

In the case of point contacts on smooth surfaces the minimum film
thickness can be calculated from the expression

When operating sliding contacts with thin films, it is necessary to ascertain
that they are not in the boundary lubrication regime. This can be done by
calculating the specific film thickness or the lambda ratio

It is usual that S = (Ri + R 2)/2 = K sk, where K sk = 1.1 l# a is the r.m.s. height
of surface roughness.
If the lambda ratio is larger than 3 it is usual to assume that the
probability of the metal-metal asperity contact is insignificant and
therefore no adhesive wear is possible. Similarly, the lubricating film is thick
enough to prevent fatigue failure of the rubbing surfaces. However, if /I is
less than 1.0, the operating regime is boundary lubrication and some
adhesive and fatigue wear would be likely. Thus, the change in the
operating conditions of the contact should be seriously considered. If this is
not possible for practical reasons, the mode of asperity contact should be
determined by examining the plasticity index, \\i.
However, in the mixed lubrication regime in which /I is in the range
1.0-3.0, where most machine sliding contacts or sliding/rolling contacts
operate, the total load is shared between the asperity load W and the film
load W s, and only the load supported by the contacting asperities should
contribute to wear. When \l/ is less than 0.6 the contact between asperities
will be considered to be elastic under all practical loads, and when it is
greater than 1.0 the contact will be regarded as being partially plastic even
under the lightest load. When the range is between 0.6 and 1.0, the mode of
contact is mixed and an increase in load can change the contact of some
asperities from elastic to plastic. When \j/ < 0.6, seizure is rather unlikely but
metal-metal asperity contact is probable because of the fluctuation of the
adsorbed lubricant molecules, and therefore the idea of fractional film
defects should be introduced and examined.

2.11.3. Fractional film defects

(i) Simple lubricant
A property of some measurable influence, which has a critical effect on wear
in the lubricated contact, is the heat of adsorption of the lubricant. This is
particularly true in the case of the adhesive wear resulting from direct
metal-metal asperity contacts. If lubricant molecules remain attached to