OPERATING EFFICIENCIES OF PUMPS AND MOTORS
The leakage losses in pumps and motors are derived analytically using the method developed by Wilson  that is based on the laminar flow equations in Chapter 8, which produces a simple model for the volumetric efficiency. The mechanical losses are based on assuming that viscous forces between moving components are proportional to the viscosity and the relative velocity and that there is a coulomb friction force that is proportional to the pressure.
9.2 MECHANICAL AND VOLUMETRIC EFFICIENCY
The power loss mechanism is the same for both pumps and motors. However, the efficiency of pumps and motors is slightly different because of the way the losses affect the power flow ...