Page 49 - Tribology in Machine Design

36 Tribology in machine design

generalized melting point based on the liquid/vapour critical point will be
used

where T c is the critical temperature. Taking into account the expressions
discussed above, the final formula for the fractional film defect, /?, has the
form

Equation (2.67) is only valid for a simple lubricant without any additives.

(ii) Compounded lubricant
To remove the limitation imposed by eqn (2.67) and extending the concept
of the fractional film defect on compounded lubricants, it is necessary to
introduce the idea of temporary residence for both additive and base fluid
molecules on the lubricated metal surface in a dynamic equilibrium. For a
lubricant containing two components, additive (a) and base fluid (b), the
area A m arises from the spots originally occupied by both (a) and (b). Thus,

The fractional film defect for both (a) and (b) can be defined as

where A a and A b represent the original areas covered by (a) and (b),
respectively. The fraction of surface covered originally by the additive,
before contact, is

where A T = A a + A b is the real area of contact.
According to eqn (2.60), the fractional film defect of the compounded
lubricant can be expressed as

From eqn (2.69)

From eqn (2.70)

Taking the above into account, eqn (2.68) becomes

Reorganized, eqn (2.71) becomes