A hydraulic motor is a type of motor that converts the flow of a liquid under pressure into rotational motion. In essence, a hydraulic motor is simply a hydraulic pump working backward: Instead of mechanical parts moving fluid by pushing on it, high-pressure fluid pushes on the mechanical parts, causing them to move. Broadly speaking, there are three kinds of hydraulic motor: gear, vane and piston.
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The gear type of hydraulic motor consists of two interlocking gears in an oblong housing, with inlet and outlet pipes on opposite sides. High pressure oil is injected into the housing, where it flows around the periphery, between the tips of the gear teeth and the housing wall. This pushes on the gears and causes them to rotate. The interlocking teeth of the gears prevent the oil from flowing back around from the outlet to the inlet side, resulting in continuous rotation, according to the University of Dayton.
Vane motors have a single rotor with vanes sticking out from it inside a housing. Two inlet ports are located on opposite sides of the motor, with two outlet ports between them. The vanes on the rotor are mounted on rocker arms; when the vane is near an inlet port, it moves against the housing wall, creating a tight seal, which moves the vane near the outlet port away from the wall, loosening the seal and letting the oil flow from the cylinder. Because the oil is always pushing against the vanes tangentially, the rotor is rotated by the fluid pressure.
Piston motors consist of a cylinder barrel containing a number of pistons. High-pressure oil is forced into the centre of the barrel, where something like a distributor cap directs it to different pistons, forcing the pistons outward against an angled plate. By pushing on the plate at an angle, the pistons rotate the plate and generate torque.
A number of different parameters define hydraulic motors. The Hydraulics & Pneumatics website defines the most common terms in the industry, including motor displacement, torque output, breakaway torque, mechanical efficiency and slippage. “Motor displacement” refers to how much fluid is needed to turn the motor through one revolution; the lower the volume needed for a given torque, the better. “Torque output” refers to how much torque the motor generates; the higher the torque, the more powerful the motor. “Breakaway torque” measures how much torque is needed to start the load turning, which is always more than is needed to keep it in motion. “Mechanical efficiency” is the ratio of the actual torque produced to the torque the motor should theoretically produce. “Slippage” measures how much fluid passes through the motor without performing work.
Strengths and Weaknesses
Gear motors operate best at high speeds but are inefficient at low speeds. They tend to be noisy but are generally the least expensive. Vane motors tend to be less efficient than piston motors and do not tolerate low speeds well, but they also tend to be cheaper than piston motors. Piston motors have high efficiency and are durable. They tolerate high pressures and low speeds much better than other motors, though they are usually more expensive.
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