Page 62 - Tribology in Machine Design

Basic principles of tribology 49

Alternatively, regarding q as a shear stress, then, if <f> is the angle of shear in
an interval of time fit, shown by GEG' in Fig. 2.18

Hence for small rates of shear and thin layers of fluid, // may be defined as
the shear stress when the rate of shear is one radian per second. Thus the
physical dimensions of /i are

2.13.2. Fluid film in simple shear
The above considerations have been confined to the simple case of parallel
surfaces in relative tangential motion, and the only assumption made is that
the film is properly supplied with lubricant, so that it can maintain itself
between the surfaces. In Fig. 2.19 the sloping lines represent the velocity
distribution in the film so that the velocity at E is EF = v, and the velocity at
P is PQ = V. Thus

Figure 2.19

It will be shown later that, from considerations of equilibrium, the pressure
within a fluid film in simple shear must be uniform, i.e. there can be no
pressure gradient.
If the intensity of pressure per unit area of AB or CD is p and/is the
virtual coefficient of friction