Hydraulic motors differ from pumps because they can be designed to rotate in either direction, can have different seals to sustain high pressure at low rpm, or can have different bearings to withstand large transverse loads so they can drive sprockets, gears, or road wheels on vehicles. A rotating valve that distributes the pressure to the pistons in sequence causes the output shaft to rotate in the desired direction. The pistons are mounted in a block that holds the pistons perpendicular to the rotor. Each piston slides laterally on a flat surface inside the housing as it applies a force between the flat portion of the housing and the eccentric rotor. Figure 2.27 shows a radial piston pump with the pistons, 2, arranged radially around the rotor hub, 1. The rotor with the cylinders and the pistons are mounted with an eccentricity in the pump house 3. The pistons, which can slide within the cylinders with a special seal system, pull and then push the fluid (the arrows on the figure) through a central valve 4.
Orientation of the block relative to the housing is maintained by means of an Oldham coupling. The schematic principle of operation of an Oldham coupling is presented in Fig. 2.28. The main parts are the end plate 1, coupling plate 2, and block 3, which contains the pistons and the eccentric portion of the shaft. Slot a is cut into plate 1 and accepts track A, which is part of plate 2. Track B is perpendicular to track A and is located on the opposite side of plate 2 from track A. Slot b is cut into the block and accepts track B. Thus, any displacement of the block relative to plate 1, which is attached to the housing, can be decomposed into components parallel to tracks A and B. As the shaft turns the center of the eccentric and of the block, it will describe a circle about the center of the housing, but the block itself will not rotate. Pistons, block, and housing, therefore, will always maintain their proper orientation relative to one another.