Hydraulic motors are one of many mechanical components that make your machinery work the way it’s supposed to. Here, we look at what hydraulic motors are, and why they’re important.

What are Hydraulic Motors? 

A hydraulic motor converts hydraulic energy into mechanical energy; a rotating shaft. It uses hydraulic pressure and flows to generate the required torque and rotation. You can use hydraulic motors for many applications, such as winches, crane drives, self-propelled farming equipment, excavators, mixer and agitator drives, roll mills, etc.


The power produced by a hydraulic motor is determined by the flow and pressure drop of the motor. The displacement and pressure drop of the motor determines the torque it generates. The power output is thus directly proportional to the speed. The hydraulic motors range from high-speed motors of up to 10,000 rpm to low-speed hydraulic motors with a minimum of 0.5 rpm. Low-speed hydraulic motors can generate large torques at low speeds.


The hydraulic motor must be geared to hydraulic system requirements; issues such as load, operating pressure, speed, serviceability, etc. must be taken into account. There are different types of hydraulic motors, such as hydraulic gear motors (internal and external), piston motors and hydraulic vane motors, to name a few.


What Does Motor Displacement Mean?

Motor displacement refers to the volume of fluid required to turn the motor output shaft through one revolution. The most common units of motor displacement are in.3 or cm3 per revolution. Hydraulic motor displacement may be fixed or variable. A fixed-displacement motor provides constant torque. Controlling the amount of input flow into the motor varies the speed. A variable-displacement motor provides variable torque and variable speed. With input flow and pressure constant, varying the displacement can vary the torque-speed ratio to meet load requirements.


Torque Output and Hydraulic Motors

Torque output is expressed in inch-pounds or foot-pounds or in Newton meters. It is a function of system pressure and motor displacement. Motor torque ratings usually are given for a specific pressure drop across the motor. Theoretical figures indicate the torque available at the motor shaft, assuming no mechanical losses.


Breakaway torque is the torque required to get a stationary load turning. More torque is required to start a load moving than to keep it moving.


Running torque can refer to a motor’s load or to the motor. When it refers to a load, it indicates the torque required to keep the load turning. When it refers to the motor, it indicates the actual torque that a motor can develop to keep a load turning. Running torque considers a motor’s inefficiency and is a percentage of its theoretical torque. The running torque of common gear, vane, and piston motors is approximately 90% of theoretical.