Hydraulic motors

Hydraulic motors convert fluidpower into mechanical energy. Highpressurefluid flow in a circuit is used topush vanes, gears or pistons attached toan output shaft, with power capacity ofa 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, fixeddisplacement 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.

IMAGE COURTESY OF LIEBHERR

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 around the gears and housing to the outlet port. It is a constant-displacement motor.

A second type of gear motor is the gerotor, or internal-gear, motor. The internal gear has one less tooth than the outer gear, and it rotates and seals against the outer component to minimize bypass leakage. The inner gear connects to the output shaft, and speeds and power density of the unit can be quite high.

Another variation is the roller-gerotor motor, where rollers replace the lobes of the outer gear to minimize friction. They tend to provide smooth, low-speed operation and have higher efficiencies and longer lives.

One concern with gear motors is leakage from the inlet to outlet, which reduces motor efficiency and generates heat. In addition to their low cost, gear motors do not fail as quickly or as easily as other styles, because the gears wear down the housing and bushings before a catastrophic failure can occur.

Vane motors operate in the mediumpressure and cost range. Torque develops by pressure acting on exposed surfaces of vanes that slide in and out of slots in the rotor, which connects to the output shaft. As the rotor turns, vanes follow the surface of a cam ring and carry fluid from inlet to outlet. Vane motors are fixed-displacement types.

Piston motors are also available in a variety of styles, including radial, axial and other less common designs. Radialpiston motors feature pistons arranged perpendicularly to the crankshaft’s axis in barrels that radiate out from the drive shaft. Fluid pressure moves the pistons linearly and causes the crankshaft to rotate. This reciprocating action against a lobed cam ring can produce extremely high torques with very low to moderate speeds.

Axial-piston designs feature a number of pistons arranged in a circular pattern inside a housing (cylinder block, rotor or barrel). This housing rotates about its axis by a shaft that is aligned with the pumping pistons.

There are two designs of axial-piston motors. The first is the swashplate design where the pistons and drive shaft are parallel. The second is the bent-axis design, where the pistons are arranged at some angle to the main drive shaft. In this design, the up-and-down motion of the pistons is converted to rotary motion through a ball joint.

Axial-piston motors are noted for high volumetric efficiency as well as good low-and high-speed performance. These motors can be fixed-displacement or variable-displacement, depending on the design. For instance, the piston stroke can be varied in the latter type by changing the angle at which the swash plate is inclined.

IMAGE COURTESY OF EATON

SPECIFYING MOTORS

There are several important factors to consider when selecting a hydraulic motor. You must know the maximum operating pressure, speed and torque that the motor will need to accommodate. Knowing its displacement and flow requirements within a system is equally important. The type of operating fluid and tolerance for contamination are other considerations.

In broad terms, gear motors tend to be suited for medium flows and pressures, and are the most economical. Vane motors offer medium pressure ratings and high flows, with a mid-range cost. At the most expensive, piston motors offer the highest flow, pressure and efficiency ratings.

Cost is clearly a major factor in any component selection, but initial cost and expected life are just one part of the equation. Users must also know the motor’s efficiency rating, as this will factor in whether it runs costeffectively or not. In addition, a component that is easy to repair and maintain or is easily changed out with other brands will reduce overall system costs in the end. Finally, consider the motor’s size and weight, as this will impact the size and weight of the system or machine with which it is being used.