Hydraulic motors are mechanical actuators that convert hydraulic pressure and flow into torque and angular displacement (rotation). The hydraulic motor is the rotary counterpart of the hydraulic cylinder. Conceptually, a hydraulic motor should be interchangeable with hydraulic pump, because it performs the opposite function -- much as the conceptual DC electric motor is interchangeable with a DC electrical generator. However, most hydraulic pumps cannot be used as hydraulic motors because they cannot be backdriven. Also, a hydraulic motor is usually designed for the working pressure at both sides of the motor. Hydraulic pumps, motors, and cylinders can be combined into hydraulic drive systems. One or more hydraulic pumps, coupled to one or more hydraulic motors, constitutes a hydraulic transmission.
Hydraulic motor types
Many designs are possible. The following types of hydraulic motors are available:
- Gear motor
- Vane motor
- Axial plunger motor bent axis
- Axial plunger motor Swash plate
- Radial plunger motor
Gear and vane motors are used in simple rotating systems that may be used occasionally. If high quality rotating drive systems are used in general plunger motors are used. Whereas the speed of hydraulic pumps is mostly 1200-1800 rpm, the machinery that should be driven by the motor asks for a much lower speed. This means that when an axial plunger motor (swept volume maximum 2 litres) is used, a gearbox is usually needed. Radial piston motors can be obtained with very large swept volumes. Sometimes because the total piston volume of the pump is large (sometimes up to 8-9 litres), sometimes because the pistons move more than one time each revolution (sometimes up to 250 (!) litres working swept volume). By decreasing the swept volume of the motor, the speed goes up and the maximum torque decreases. For a continuously adjustable swept volume, axial piston motors are used. Radial motors can get an adjustable swept volume by switching off some the plungers. Hydraulic motors usually have a leakage connection, which means that a hydraulic motor in a drive system can never hold the load, like a hydraulic cylinder can. There is always a need for a brake or a locking device device. Hydraulic motors are used for winch and crane drives and as wheel motors for military vehicles, self-driven cranes, and excavators.
