Hydraulic motors are powered by pressurized hydraulic fluid and transfer rotational kinetic energy to mechanical devices. Hydraulic motors, when powered by a mechanical source, can rotate in reverse direction and act as a pump. Operating specifications and features are the most important parameters to consider when searching for hydraulic motors.

Types of Hydraulic Motors

The most important attribute to consider when searching for hydraulic motors is the motor type. Choices for motor type include:

Axial piston motors use an axially-mounted piston to generate mechanical energy. High pressure flow into the motor forces the piston to move in the chamber, generating output torque.

Radial piston hydraulic motors use pistons mounted radially about a central axis to generate energy. An alternate-form radial piston motor uses multiple interconnected pistons, usually in a star pattern, to generate energy. Oil supply enters the piston chambers, moving each individual piston and generating torque. Multiple pistons increase the displacement per revolution through the motor, increasing the output torque.

Internal gear motors use internal gears to produce mechanical energy. Pressurized fluid turns the internal gears, producing output torque.

External gear motors use externally-mounted gears to produce mechanical energy. Pressurized fluid forces the external gears to turn, producing output torque.

Vane motors use a vane to generate mechanical energy. Pressurized fluid strikes the blades in the vane, causing it to rotate and produce output torque.

Basics of Charlynn Hydraulic MotorsVideo Credit: AgriSupplyHowTo

Operating Specifications

Operating specifications to consider for hydraulic motors include:

Operating torque is the torque the motor is capable of delivering. Operating torque depends directly on the pressure of the working fluid delivered to the motor.

Operating pressure is the pressure of the working fluid delivered to the hydraulic motor. Working fluid is pressurized by an outside source before it is delivered to the motor. Working pressure affects operating torque, speed, flow and horsepower of the motor.

Operating speed is the speed at which the hydraulic motors’ moving parts rotate. Operating speed is expressed in revolutions per minute, or similar terms.

Operating temperature is the fluid temperature range the motor can accommodate. Minimum and maximum operating temperatures are dependent on motor internal component materials, and can vary greatly between products.

Power that the motor is capable of delivering is dependent on the pressure and flow of the fluid through the motor.

Fluid flow through the motor is expressed in terms of gallons per minute, or similar units.

Fluid viscosity is a measure of the fluid's resistance to shear, and is measured in centipoise. Centipoise is a common metric unit of dynamic viscosity equal to 0.01 poise or 1 millipascal second. The dynamic viscosity of water at 20 degrees C is about 1 centipoise. The correct unit is cP, but cPs and cPo are sometimes used.

Displacement per revolution is the fluid volume displaced per revolution of the motor and is measured in cubic centimeters (cc) per revolution, or similar units.

Motor weight is measured in pounds or similar units.

Additional features to consider when searching for hydraulic motors include the ability to be mounted in any position, rating for continuous duty, and quiet operation.

Standards

ISO 4392-1 - Hydraulic fluid power

ISO 9206 - Aerospace - constant displacement hydraulic motors - general specifications

SAE AS7997 - Motors, aircraft hydraulic, constant displacement general specifications

GOST 20719 - Hydraulic motors - acceptance and methods of tests

Image credits:

Bosch Rexroth | Western Integrated Technologies | Viking Pump, Inc. - A Unit of IDEX Corporation