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. However, most hydraulic pumps cannot be used as hydraulic motors because they cannot be back driven. Also, a hydraulic motor is usually designed for the working pressure at both sides of the motor.Hydraulic motors and cylinders can be combined into hydraulic drive systems. One or more hydraulic pumps, coupled to one or more hydraulic motors, constitute a hydraulic transmission. Gear and vane motors are used in simple rotating systems. Their benefits include low initial cost and high rpm. A gear motor consists of two gears, the driven gear (attached to the output shaft by way of a key, etc) and the idler gear. High pressure oil is ported into one side of the gears, where it flows around the periphery of the gears, between the gear tips and the wall housings in which it resides, to the outlet port. The gears then mesh, not allowing the oil from the outlet side to flow back to the inlet side. A vane motor consists of housing with an eccentric bore, in which runs a rotor with vanes in it that slide in and out. The force differential created by the unbalanced force of the pressurized fluid on the vanes causes the rotor to spin in one direction. For high quality rotating drive systems plunger motors are generally used. Whereas the speed of hydraulic pumps ranges from 1200 to 1800 rpm, the machinery to be driven by the motor often requires a much lower speed. This means that when an axial plunger motor (swept volume maximum 2 liters) is used, a gearbox is usually needed. Radial piston motors are available in two basic types-crankshaft types (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 12 liters/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. Hydraulic motors usually have a drain connection for the internal leakage, which means that when the power unit is de-energized the hydraulic motor in the drive system will move if an external load is acting on it, such as a crane or winch with suspended load. In these cases there is always a need for a brake or a locking device. Hydraulic motors are used for many applications now such as winches and crane drives, wheel motors for military vehicles, self-driven cranes, and excavators. Conveyor and feeder drives, mixer and agitator drives, roll mills, drum drives for digesters, trommels and kilns, shredders for cars, tyres, cable and general garbage, drilling rigs and trench cutters.
