FLUID POWER HANDBOOK
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HYDRAULIC
MOTORS
HYDRAULIC
motors convert fluid power into mechanical energy. High-pressure fluid flow in a circuit is used to push vanes, gears or pistons attached to an output shaft, with power capacity of a hydraulic motor dictated by its design, size and speed, among other factors. Much like electric motors, hydraulic motors generate rotational motion and torque. However, hydraulic motors require no electricity and can withstand dusty and dirty environments, extreme heat, and even submersion. Perhaps most significantly, hydraulic motors have exceptional power-to-weight ratios. In terms of power capacity, an electric motor can weigh 20 times more than an equivalent-rated hydraulic motor. Some hydraulic motors offer high speed capabilities, such as in fan drives. Others, for instance winches, move heavy loads at low speeds, sometimes less than one rpm. They are used in industrial applications such as augers, conveyors and mixers, as well as in rolling mills, where they are preferred thanks to their robust nature and resistance to heat. Likewise, hydraulic motors are especially suited to mobile machinery, where they are often the primary drive in off-highway equipment. Hydrostatic drive systems transmit engine power to the drive wheels with exceptional versatility and reliability. Hydraulic wheel motor’s speed control and smooth reversibility make them perfect for use on backhoes, skid-steers and wheeled loaders. Motors are also used in tracked vehicles such as excavators and bulldozers, where the high power density of hydraulic motors let them achieve substantial torque in a relatively small package. Hydraulic motors are rated according to several parameters, including torque capacity, speed range, pressure limitations, efficiency and displacement. Displacement is the amount of fluid needed to turn the output shaft one revolution, and it is usually rated in terms of cc/rev or cu.in./rev. The units can be either fixed- or variable-displacement and operate either bidirectionally or unidirectionally. With input flow and operating pressure constant, fixed40
FLUID POWER WORLD
7 • 2020
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displacement designs provide constant torque and speed. In contrast, under constant flow and pressure conditions, a variable motor can vary torque and speed. Thus, variable motors have a wider speed range capacity. In general, valves control direction and speed of a hydraulic motor. With proper relief-valve settings, motors can be stalled without damage. And some can be used for dynamic braking.
MOTOR DESIGNS There are several types of hydraulic motor, including gear, vane and piston units. They are usually similar in construction to the analogous hydraulic pumps. Gear motors are probably the most popular designs, and they come in several versions. External gear motors feature a matched pair of spur or helical gears enclosed in a housing. One is the driven gear — which is attached to the output shaft — and the other an idler gear. Their function is simple: high-pressure oil is ported into one side of the meshing gears and forces them to rotate. Oil flows www.fluidpowerworld.com
