The working principle of a hydraulic motor is as follows

Here is the detailed working principle of a hydraulic motor, illustrated with the example of a small marine radial piston hydraulic motor (similar to a radial piston pump in design but functioning as a motor), explaining how it converts hydraulic power into rotational mechanical kinetic energy output:

The working principle of a radial piston marine hydraulic motor is shown in Figure 4.23 (animation). The swashplate 1 and the oil deflector plate 4 are fixed, while the pistons 3 can move within the bores of the cylinder block 2. The axis of the swashplate intersects the central axis of the cylinder block at an angle δ. When high-pressure oil enters the piston bores of the cylinder block through the oil passage in the oil deflector plate, it pushes the pistons in the high-pressure chambers against the swashplate. The reaction force F exerted by the swashplate on the pistons is resolved into a radial component fx and a vertical component fy. The radial component fx is balanced by the hydraulic force on the pistons, while the vertical component fy generates a rotational torque in the cylinder block, causing the shaft 5 to rotate. The torque produced by the hydraulic motor's gear reducer coupling is the sum of the torques generated by all the pistons in the high-pressure chambers, where r is the radius of the circle on which the pistons are arranged in the cylinder block, and θ is the angle between the piston and the vertical axis of the cylinder block.

It can be seen that the torque generated by each piston varies with the angle θ, resulting in a pulsating total torque output from the hydraulic motor.

In principle, hydraulic pumps and hydraulic motors of the same type can be interchanged. However, generally speaking, an unmodified hydraulic pump cannot be used as a hydraulic motor. This is because, considering factors such as pressure balance and automatic compensation for clearance sealing, the structures of the suction and discharge chambers in hydraulic pumps are mostly asymmetrical and designed for unidirectional rotation only. In contrast, as a hydraulic motor, it usually requires bidirectional rotation and structural symmetry.