What is a hydraulic motor?

I’ve asked similar questions, “what is a hydraulic cylinder,” and “what is a hydraulic pump,” so I thought I’d add another query to the list, and ask “what is a hydraulic motor?”

Just as with the previous two articles, this may seem like an elementary question, especially if you’re a fluid power professional. For me the answer is obvious, but I can never forget that there was a time I didn’t know, so if just one person Googles the question and gets this response, I’ll consider it successful.

The hydraulic energy is transmitted into the system via the pump, where it itself had used mechanical force as its source of input energy. The pump pushed on the hydraulic fluid, which in turn pushes on the gears, vanes or pistons of the hydraulic motor. It is really that simple; the pump pushes the fluid and the fluid pushes the motor.

Two qualities of a hydraulic motor are its torque and speed capability. Hydraulic motor torque is a function of its displacement and the pressure drop across the motor, both of which deserve explanation. Displacement is defined as the theoretical volume of fluid the motor will require to turn one revolution, and this is a product of the size of the gear/vane/piston and how far the gear/vane/piston is away from its center of rotation (i.e., the radius). A 100 cc motor will theoretically spin one revolution if you stuff 100 cc (100 ml) of fluid into it. I say “theoretically” because some fluid is lost to leakage within internal clearances, so it could actually take 110-200 cc of fluid to create one revolution, depending on the type of motor.

Pressure drop is the second factor of hydraulic motor torque. You may question why I say “pressure drop” rather than just pressure; it’s because pressure can and often does exist at the exhaust port of the motor, and it’s the difference between the inlet and exhaust ports that creates a net pressure for work. So hydraulic motor torque is calculated by:

pressure drop x displacement / 6.28.

The last number is the 2pi constant required because we’re talking about a rotational device.

The second quality of a hydraulic motor is its speed capability. The function of steady-state speed, measured in revolutions per minute, is a combination of the factors of displacement and input flow. A larger displacement motor will take more flow to spin as quickly as a smaller motor, and vice versa. So if you want more torque for your motor application, you will need to provide it with more flow if you want it to spin as quickly as before. I should be clear that using displacement and flow to calculate speed is a steady-state calculation only. When acceleration and changes in angular velocity are required for dynamic applications, the math gets very advanced, so I recommend you contact your local motion control expert to help with your application.

Hydraulic motors come in varying degrees of quality, precision and torque. Low-end orbital motors are quiet and reliable, but are inefficient, especially at higher pressure and speed. Gear motors are simple and inexpensive, and although great for medium-speed applications, they can be noisy. Vane motors are fairly efficient and available in very large displacement options, but they are definitely not available in bargain shop pricing. High-end piston motors are available in various configurations, with radial and bent-axis being the two most common. Radial piston motors are extremely efficient and capable of astounding torque capacity, but are expensive. Bent-axis piston motors are the model of compact, high-speed efficiency, and few other devices on the planet can provide more power for their size, this side of a nuclear reactor.