Page 46 - Tribology in Machine Design

Basic principles of tribology 33

2.11.1. Rheological lubrication regime

As a first step in a calculating procedure the operating rheological
lubrication regime must be determined. It can be examined by evaluating
the viscosity parameter g v and the elasticity parameter g e

where w is the normal load per unit width of the contact, R is the relative
radius of curvature of the contacting surfaces, E' is the effective elastic
modulus, ^ 0 is the lubricant viscosity at inlet conditions and V is the
relative surface velocity.
The range of hydrodynamic lubrication is expressed by eqns (2.52) and
(2.53) for the g v and 0 e inequalities as follows:

Operating conditions outside the limitations for g v and g e are defined as
elastohydrodynamic lubrication. The range of the speed parameter g s and
the load parameter #, for practical elastohydrodynamic lubrication must be
limited to within the following range of inequalities:

where a is the pressure-viscosity coefficient. Equations (2.52), (2.53), (2.54)
and (2.55) help to establish whether or not the lubricated contact is in the
hydrodynamic or elastohydrodynamic lubrication regime.

2.11.2. Functional lubrication regime
In the hydrodynamic lubrication regime, the minimum film thickness for
smooth surfaces can be calculated from the following formula:

where 4.9 is a constant referring to a rigid solid with an isoviscous lubricant.