Hydraulic Motors


  • Discuss the function of a hydraulic motor
  • List some common uses for a hydraulic motor
  • Explain common points of failure and maintenance needs

What is a Rotary Actuator?

Let's start with some definitions.

An actuator is the technical term for a hydraulic component that converts energy from pressurized hydraulic fluid into motion. This component is often the piece of equipment that actually performs the work of the hydraulic system.

In practical terms, hydraulic actuators are usually motors, or cylinders. Cylinders are also called linear actuators, since their motion is in a straight line. And hydraulic motors, of course, can also be referred to as rotary actuators.

Even though it's more to say, sometimes the term "rotary actuator" can be less confusing than "motor", especially when electric motors and heat engines start to enter the conversation.

The Rotary Actuators family is a little bigger than just hydraulic motors. It also includes semi-rotary actuators, like helical, rack and pinion, and vane actuators.


Rack and Pinion


Why Use a Hydraulic Motor?

Let's look at a simple hydraulic system that uses hydraulic motors.

For the sake of this example, only the right side driving circuit is shown.

This Generic Brand™ skid steer loader relies on a diesel engine to turn the hydraulic pump. The pump pushes hydraulic fluid through the propulsion and steering system, into the hydraulic motor.


Energy Conversions

Diesel engine converts chemical energy to rotational energy

Pump converts rotational energy to a combination of kinetic and potential energy

Motor converts the kinetic/potential energy combination back to rotational energy

Energy starts out at a prime mover (engine, or electric motor) and is converted multiple times, only to end at the shaft of a hydraulic motor. Many hydraulic systems begin with rotational energy, and after several energy-losing conversions, arrive back at rotational energy once more. Why bother with the whole hydraulic system and going to a rotary actuator, anyway? Isn't that inefficient?

Advantages of a Hydraulic Motor

The conversion to the hydraulic motors is very necessary. If the wheels were driven directly by the engine, the operator would be deeply unhappy with the skid steer loader’s performance! Both sets of wheels could only go in the same direction, and reverse would not be an option.

Many rotary actuators are engineered to be bi-directional, providing reverse capability to the loader. Flow can be diverted away from the actuators, allowing the loader to stand still, without turning the engine off.

The skidsteer loader can drive, stop, and reverse thanks to the hydraulic motor.

Schematic Symbol

The schematic symbol for a motor is a circle, with an arrow (or arrows) indicating the direction of flow. The arrow(s) will always point inward, representing flow into the motor.

Does the motor symbol look familiar? It should; it's almost the same as the symbol for a pump. The only difference is in the direction that the arrow points. This isn't an accident; functionally, a motor is very similar to a pump. It just takes in fluid flow, to turn a shaft, rather than producing flow from the power of a turning shaft. In fact, a motor can become a pump in the right circumstances!




Just like other schematic symbols, extra symbols can be "stacked" onto the basic motor symbol to indicate extra functionality.

Bi-directional Motor

Bi-directional Variable Displacement Motor

Bi-directional Variable Displacement Motor With Case Drain

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Vacuum Pressure
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