Hydraulic motors use a fluid--usually oil--to create motion. Hydraulic fluid is first pumped from a reservoir into pressurised tubes by a hydraulic pump, which is driven by an internal combustion engine. Those tubes take the fluid to the hydraulic motor or motors. The pressure turns the motors by flowing through them, then flows back into the reservoir where the whole process begins again.
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Axial Plunger Motors
Axial plunger motors use a hydraulic piston to spin a rotor. Hydraulic fluid is pumped into a hydraulic cylinder, where it pushes a plunger out. That plunger turns the rotor. Once the plunger reaches the end of its motion, the fluid flows back out of the cylinder and the plunger retracts as the rotor turns the rest of the way around.
Other Hydraulic Motors
Other types of hydraulic motors use the hydraulic fluid to directly turn the motor. The hydraulic fluid flows into a sealed casing, spins the motor assembly and flows back out. For example, in a gear motor, there are two meshed gears in the middle. The hydraulic fluid pushes against the gear teeth, turning them around, before flowing out through an outlet valve. In a vane motor, the hydraulic fluid pushes on a rotating vane, spinning it around a sealed case. These motors are typically designed to rotate at a set speed. They are often attached to gear boxes, which allow them to drive machinery at different speeds.