Figure 1 and 2 show two different types of radial piston motor, that of Figure 1 using an eccentric that gives one piston stroke/revolution whilst the cam unit of Figure 2 creates several piston strokes/revolution.

The eccentric motor shown is typical of several designs that are available for converting the piston force into output torque at the drive shaft. In the motor shown, the valve successively connects each cylinder to the supply and return for successive half revolutions of the shaft.

These motors generally operate at a maximum continuous pressure of 250/ 300bar with displacements up to 10,000cm3/rev and above.

The motor displacement can be altered between two levels by pressurising the pistons that control the position of the eccentric. In some motors the displacement can be controlled continuously in a closed loop control on inlet pressure or motor flow.

The particular type of radial piston cam motor shown in Figure 1 operates by transferring the pressure force on the piston, which is directed radially outwards, onto the cam by the use of rolling element bearings attached to the piston. Here the distributor valve, not shown in the figure, connects each piston to the high-pressure port when the piston is moving outwards thus creating a torque on the cylindrical cam. When the piston is moving inwards the valve connects the cylinder to the low-pressure port thus allowing fluid to be passed back to the return line.

Radial piston cam motors generally operate at a continuous pressure of 250/ 300bar with displacements up to, and beyond, 50,000cm3/rev and can be operated with two selectable displacements by short circuiting some of the cylinders to low pressure. Cam motors can be arranged to provide low levels of torque ripple and they have a high ratio of output torque to the motor mass

In the Gerotor, or Orbit type motor, as shown in Figures 3, the rotor centre rotates in an orbit as the rotor rolls in contact with the outer rim. This action causes the volumes trapped between the several contacts to vary with rotation of the shaft.