Hydraulic Motor Working Principle

Hydraulic motor working principle, types, selection, and sizing

Hydraulic motors are among the most commonly used motors across various industries. Not only can they generate tremendous amounts of torque, but they are also rugged and come in compact sizes. These advantages make them ideal for construction, mining, and agricultural equipment applications.

However, these advantages will only be felt if engineers select the ideal type of hydraulic motor and size it correctly for the required application.

Hydraulic Motor

Figure 1: Hydraulic motors are among the most commonly used motors across a wide range of industries

Source: [PhotoBetulo/Adobe Stock]

Working principle of a hydraulic motor

A hydraulic motor converts hydraulic energy from oil (or any hydraulic fluid) into mechanical energy (in the form of rotary motion) to perform useful work. The basic hydraulic motor design typically comprises a reservoir (where the hydraulic fluid is stored), a pump, valves, pistons, and a rotating component.

During operation, the pump forces the hydraulic fluid from the reservoir, increasing its pressure and energy. This pressurized fluid passes through valves and strikes the cam and piston inside the hydraulic motor. As a result, this motion is transferred to the motor's rotating element (and shaft), causing it to start rotating. Finally, the hydraulic fluid is supplied back to the reservoir, and the process repeats.

Common types of hydraulic motors

Hydraulic motors come in different types, all of which have different features and suitability for different application needs. They are generally classified according to their rotating component as:

  1. Gear motors
  2. Piston motors
  3. Vane motors
#1 Gear motors

As its name implies, hydraulic gear motors use gear sets to transmit the rotational kinetic energy to mechanical energy. Gear motors usually have two interlocking gears enclosed in a housing, with hydraulic fluid inlet and outlet pipes located on opposite sides of the housing. When pressurized hydraulic fluid enters through the inlet pipes, it pushes the gears, causing them to rotate and transmit motion.

These gear motors are ideal in applications where high output speeds and low torque levels are essential requirements. For instance, they are ideal for mobile hydraulic applications and agricultural machinery (to drive conveyor belts).

Learn more about gear motors on GlobalSpec.

#2 Piston motors

Axial and radial piston motors are the two types of piston motors. They are quite similar in operation but differ slightly in configuration.

The axial piston motors feature axially-mounted through which pressurized fluid flows to generate mechanical energy. In contrast, a radial piston motor consists of several pistons in a cylindrical barrel that is free to rotate and mounted onto a drive shaft. Pressurized hydraulic fluid from a pump is fed to each piston through fluid paths in the barrel. This fluid motion forces the piston outward against the reaction ring, causing the barrel to rotate and generate torque.

Piston hydraulic motors operate at extremely high speeds while offering high efficiency. They can also withstand high mechanical and hydraulic shock loads, making them suitable for high-power applications.

#3 Vane motors

Vane motors feature a rotor with rectangular vanes mounted onto the drive shaft. When pressurized hydraulic fluid enters the motor through the inlet port, it strikes the vanes, causing it to rotate and generate torque. They have a simple design and offer low noise levels and versatility. For instance, they are ideal for injection molding and agricultural machinery applications.

Understanding some hydraulic motor rating terminologies
  • Rotational speed: is the speed at which the motors’ moving parts rotate in revolutions per minute (rpm).
  • Displacement per revolution: is the volume of fluid it takes to rotate the shaft connected to the motor through one complete revolution. It is measured in cubic centimeters per revolution.
  • Torque: is the rotational equivalent of a linear force. It describes the twisting force that the motor can deliver in Nm (Newton-meter) and can be expressed as starting torque or running torque. The motor starting torque is the torque that the motor must generate to turn a load from a standstill position. In contrast, running torque (or stall torque) describes the maximum torque the motor can generate when its output rotational speed is zero.
  • Volumetric efficiency: is the percentage of the theoretical pump flow available to do useful work in the hydraulic motor. It can be obtained by dividing the actual flow delivered by the pump (at a given pressure) by its theoretical flow.
  • Mechanical efficiency: is the percentage of the actual work done per revolution to the theoretical work done.
  • Overall efficiency: combines volumetric efficiency and mechanical efficiency. It is simply the product of mechanical efficiency and volumetric efficiency.

Learn more about hydraulic motor operating specifications on GlobalSpec.

Basics of selecting and sizing a hydraulic motor

When sizing a hydraulic motor for an application, it is recommended that engineers choose a motor type that matches the expected overall performance. For instance, hydraulic motors can generally be classified according to their performance as:

  • High Speed, Low Torque (HSLT) motors
  • Low Speed, High Torque (LSHT) motors

Gear and vane motors typically fall under the HSLT motor category. However, orbital-style gear motors, as well as vane motors with higher fluid displacement, typically fall under the LSHT category. Piston motors come in various designs with both the HSLT and LSHT classifications. For instance, in-line and bent-axis piston motors are classified as HSLT motors, while the radial piston motors fall under the LSHT category.

Basic formulas for sizing a hydraulic motor for an application

The basics for sizing hydraulic motors involve estimating the required speed, torque, and power for an application. The following equations provide engineers with basic formulas needed for sizing hydraulic motors.

  1. Speed, N (in rev/sec)
Speed Formula


            q = hydraulic fluid flow (in l/min)

            D = displacement (in cm3/rev)

            nv = volumetric efficiency

  1. Output torque, M (in Nm)
Output Formula


?p = differential pressure (bar)

            nm = mechanical efficiency.

  • Output power, P (in kW)
Output Power Formula


            nt = overall efficiency = nv * nm


After obtaining value for the speed, torque, and power of the hydraulic motor(s), engineers can make an initial selection based on the estimated load requirement of the system. Next, the pump can then be selected to match the motor’s flow and pressure.


While this article presents the basics of hydraulic motors, several other things must be considered when choosing and sizing a hydraulic motor for an application. For instance, engineers have to consider operating temperature, expected motor life, contamination potential, and type of control. Therefore, engineers are advised to reach out to hydraulic motor manufacturers to discuss their application requirements.