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Today hydrostatic transmission is widely used in heavy-duty vehicles specially the ones used for agriculture and construction purposes. This study has focused on the design and improvement of the performance of these transmissions with significant attention to the operation and control of positive displacement pumps which is their most important component. These pumps are generally swash-plate type axial piston pumps. As
part of the study of control methods applied to the pump, it is necessary to extract and linearize the dynamic equations of the system if possible. Pumping action of the piston produces a torque on the swash plate. This torque is the most important term in the equations of motion in the swash plate. The describing equation of this torque is nonlinear and partially discontinous. The equation is a function of geometrical characteristics of the pump and operation conditions of the system. The model is obtained in general form, but it can be
liearized for commony used series of axial piston pumps and give the linear parameters of the equation of motion of
the swash plate. In addition, the in formations of a special kind of pump is
given for assessment and comparison of model parameters. Finally the model is
compared with Man ring and Johnson's model (1994).