Losses in hydrostatic displacement machines are comprised of the leakage losses in the displacement elements and the control surfaces (summarised in the volumetric efficiency η Vol) and moment losses, which are summarised in the hydromechanical efficiency η hm.
The effective delivery flow Q 1eff (flow rate at the pressure connection) of a pump is smaller than the theoretical drive rotation speed Q 1th by the leakage losses Q 1L (product of drive speed and displacement volume):
The effective drive torque M 1eff on the pump shaft is greater than the theoretical drive torque M 1th by the moment losses M 1V (Figure V 13 a):
In the case of the motor, the effective displacement flow Q 2eff is formed by the sum of the theoretical displacement flow Q 2th and the leaked oil Q 2L:
The moment created M 2eff is M 2v smaller than the theoretical (hydraulic) moment M 2th (b):
Figure V 13: Moment losses in hydraulic pumps (a) and (b) hydraulic motors