Hydraulic Motor Guide: What It Is, How It Works, and Different Types

Hydraulic Motor Guide: What It Is, How It Works, and Different Types

By on May 28th, 2019 in Hydraulics

hydraulic motor

Did your conveyor belt stop? How about the screw-drive of the auger? Backhoe, bulldozer or forklift? What do these machines all have in common? A hydraulic motor.

Any machinery that needs rotational force or torque probably uses a hydraulic motor. Hydraulic motors convert the energy of liquid displacement into mechanical energy. They direct the energy of the displaced fluid to rotate pistons, vanes or gears.

It sounds complicated, but if you’ve ever squeezed a water balloon you’ve seen water displacement. Displacement and the speed of rotation determine the power capacity of a hydraulic motor.

Read on to learn more.

More About the Hydraulic Motor

A hydraulic pump converts mechanical power into fluid energy. From there, the fluid pushes against vanes or pistons attached to a crankshaft. This converts the fluid energy back into a smooth force to move an external load.

You’re already familiar with the concept of displacement thanks to the water balloon squish. You can’t really compress the water. It takes up the same amount of room. Once you tie a knot in the balloon, the liquid just squishes to a different part of the balloon when you press on it.

It takes up the same amount of space as it did without you pressing on it. Now picture water in the balloon and poke a small pinhole in the end.  If you press down on the balloon, you’ll release the water at high speed through the hole, even if you don’t apply much force.

When you push down on the balloon you apply pressure to the water, which tries to escape any way it can. Now visualize a hydraulic motor. This application shows how pressure on the liquid multiplies force, which then powers more complex machinery.

What Determines the Power of a Hydraulic Motor?

The flow and pressure drop of the motor determines the power generated. The pressure drop determines the torque the motor generates. The flow (determined by speed) is proportional to the power output. Hydraulic motors can range from 10,000 rpm (high speed)to low-speed hydraulic motors that rotate 0.5 per minute.

The volume of fluid to turn the motor output shaft through one revolution is called the motor displacement. The most common units of motor displacement are cubed inches or cubed centimeters per rotation. Hydraulic motor displacement may be fixed or variable.

Torque output is expressed in Newton meters, inch-pounds or foot-pounds. Motor torque ratings are for a specific pressure drop across the motor. They are theoretical figures to indicate the torque available at the motor shaft. They assume no mechanical losses. Running torque is a percentage of its theoretical torque. It considers some loss and inefficiency.

Hydraulic Motor Types

There are several types of hydraulic motors. The following are some of the most common. In all cases, a pump pressurizes a liquid (usually an oil) to move a vane, gear or piston, which then moves a load.

Bent-axis Piston 

A piston motor offers high flow, high pressure, and high efficiency. Bent-axis delivers power in a compact footprint. Pistons move up and down within the cylinder block. Motion is converted into rotary movement to power machinery.

Radial Piston 

A radial piston motor is used in cranes, winches, and drilling equipment. It is also known as a low-speed high torque motor. It is very efficient and has a large capacity. Piston motors drive heavy-duty construction equipment, winches, ship-cranes, and all kinds of heavy hydraulic offshore drilling equipment.


Also know as an epicyclic gear motor, gear ring motor or gerotor motor. These motors can produce large torques at very slow speeds. They do not leak oil at higher pressures and form closed chambers.

Typically they have six teeth and seven lobes. The axis of the inner rotor is offset from the axis of the outer rotor. Both rotate on their individual axes. Typically these are found in the oil and gas industry.


Simple and inexpensive, a gear motor is medium to high speed. To reduce the output speed of the shaft, additional gears reduce the rotation. Gear motors are noisy and are most efficient at lower pressures.

A gear motor consists of two gears. A gear attached to the output shaft and idler gear. The high-pressure liquid is forced on one side of the gears towards an outlet on the other side.

The gears mesh and the liquid flows along the gear tips and the housing. A small amount of oil lubricates the gears, then drains.  As the gears, housing or bushings wear down, the efficiency of the motor is lost.


Hydraulic vane motors leak less internally than gear motors. They are better for lower speed, lower pressure applications. You find them in screw-drives, injection molding, and agricultural applications.

A vane motor is easy to maintain due to its modular design. It’s low noise and efficient.

Part-Turn Actuator

Also called a rotary actuator. They can only rotate left or right over an angle.  The maximum angle is 300 degrees.

Part-turn actuators do not have any external moving parts. They are smaller than cylinders. They are used in bunker slides, push, pull, lift and mix operations and to open butterfly valves.

Sourcing the Right Hydraulic Motor

What are your system requirements? Consider the load and speed, range of load, etc.

Are serviceability and access a concern? RG Group provides expertise with hydraulic motors from Parker. As a premium distributor, we can provide hydraulic motors to your precise specs with a very short delivery time, as well as hydraulic motor spare parts. We also provide service and installation.

Are you rebuilding or replacing an older component? The manufacturer, make and model number can help identify an exact match or help find a suitable upgrade. There are several types of hydraulic motors available. There are many more than the typical types listed above.

For expert help, RG Group offers online resources and consultation services.

Contact us today.