Example 2

The shaft diameter and bearing internal diameter in a journal bearing system are 100mm and 105mm respectively. The bearing length is 140mm and the viscosity of lubricating oil is 0.09 kg/m.s. Calculate the power required to overcome friction when the shaft is rotated at 1500rpm and is concentric with the bearing.

The flow of lubricating oil in a journal bearing can be treated as "Couette flow" because the film thickness is very small (Fig. 2). In this case the gap width .
Thus the shear stress may be approximated as,

Frictional force,
Frictional power,