Abstract
The paper presents theoretical and mathematical models of the torque of mechanical losses in a hydraulic rotational motor with constant capacity qMt per one shaft revolution (with constant theoretical working volume VMt) and with variable capacity qMgv = bM qMt per one shaft revolution (with variable geometrical working volume VMgv). The models are to be used in the laboratory and simulation investigations of motor energy losses aimed at evaluation of the motor energy efficiency and hydrostatic drive efficiency.