In brief
A hydraulic motor converts the pressure and flow of a hydraulic fluid into a rotary motion with high torque, often at low speed.
Contents
What is Hydraulic motor?
The hydraulic motor is the rotating counterpart to the hydraulic cylinder. Its design resembles a hydraulic pump, but it works in the opposite direction: the pressurised incoming oil drives the displacement elements and generates torque at the shaft. Hydraulic motors are compact, overload-proof and infinitely variable in speed.
Depending on the design, a distinction is made between gear, vane, axial piston and radial piston motors as well as orbital motors based on the gerotor principle. High-speed motors operate at high speeds and are often combined with a gearbox, while low-speed motors deliver high torque directly, for example for screw conveyors, agitators, winding or roller drives. The central parameter is displacement per revolution; torque is proportional to displacement and pressure differential, and speed is proportional to flow rate.
Wear on running surfaces and seals leads to increasing leakage flow and falling torque. Contaminated oil, inadequate case drain and pressure peaks are among the most common causes of damage. In procurement, it should be noted that motors of the same size can differ considerably in shaft, flange and ports. The complete type designation from the nameplate is therefore more important than the size alone.
In practice: maintenance and procurement
For a replacement, the design, displacement, maximum pressure, speed range, shaft end, flange type, port sizes and direction of rotation are needed. Whether a case drain port is present and how it is routed should be clarified before replacement.
Synonyms and abbreviations: hydromotor · orbital motor · radial piston motor · oil motor · hydraulic drive motor
Frequently asked questions
What is the difference between a hydraulic motor and a hydraulic pump?
The pump converts mechanical energy from a drive into hydraulic energy, while the motor converts hydraulic energy back into rotary motion. Many designs are similar in construction but are optimised for their function, for example in terms of bearings, seals and direction of rotation. Not every pump can simply be operated as a motor.
How do you calculate the torque of a hydraulic motor?
The theoretical torque is the displacement multiplied by the pressure differential between inlet and outlet, divided by two pi. With the displacement in cubic centimetres and the pressure in bar, conversion gives the torque in newton metres. The mechanical efficiency reduces the actual value.
What information is needed to replace a hydraulic motor?
Important are the design, displacement, maximum operating pressure, speed range, direction of rotation or reversibility, shaft form and diameter, mounting flange and port type and size. Information on the case drain port and, where applicable, integrated brakes or valves is also needed. Nameplate data and the manufacturer part number are the quickest route to identification.
Why is a hydraulic motor losing power?
The most common cause is internal wear, which increases the leakage flow so that less oil does useful work. Low system pressure, an incorrectly set pressure relief valve, clogged filters or oil that is too hot and thin also reduce torque and speed. Measuring the case drain flow helps to identify the motor as the cause.