A hydraulic motor is a mechanical actuator that converts hydraulic pressure and flow into torque and angular displacement (rotation). The hydraulic motor is the rotary counterpart of the hydraulic cylinder.
Radial piston motors are available in two basic types. Crankshaft type (e.g. Staffa or SAI motors) with a single cam and pistons pushing inwards. This type of motor is basically an old design but is one which has extremely high starting torque characteristics. They are available in displacements from 40cc/rev up to about 50 litres/rev but can sometimes be limited in power output. Crankshaft type Radial Piston Motors are capable of running at “creep” speeds and some can run seamlessly up to 1500 rpm whilst offering virtually constant output torque chacteristics. This makes them still the most versatile design.
Multilobe cam ring type (e.g. Hägglunds or Poclain type) have a cam ring with multiple lobes and the piston rollers push outwardly against the cam ring. This produces a very smooth output with high starting torque but they are often limited in the upper speed range. This type of motor is available in a very wide range from about 1 litre/rev to 250 litres/rev. These motors are particularly good on low speed applications and can develop very high power.