Machine Design

Fluid Motors

Generally, hydraulic and pneumatic motors are organized along similar principles. The internal engineering of each is quite different, but the operating philosophies are very similar.

Axial-piston motors contain several pistons -- usually seven to nine -- that are extended by high-pressure fluid. The pistons are restrained at one end by an angled plate: As they bear against the plate, they generate a rotating force that may either twist the plate or the barrel in which the pistons are rotated. In most designs, the shaft is driven directly from either the barrel or the cam plate; in a few hydraulic motors, the shaft is driven through a differential-gear arrangement that permits low speed and high torque.

Axial-piston motors have a reputation for high volumetric efficiency, combined with excellent operation at both high and low speeds. In hydraulic designs, axial-piston motors produce maximum torques to 17,500 lb-in. from pressures to 5,000 psi, generating maximum speeds to around 4,500 rpm.

Pneumatic axial piston motors are available only in the smaller ranges, in sizes to 3 hp. Most axial piston air motors are grease lubricated and require provision for regular servicing.

Radial-piston motors have pistons that radiate out from the drive shaft, and are arrayed in a number of ways. They can produce torques over one million lb-in. at pressures exceeding 5,000 psi. Speeds can range from 0.1 to 2,000 rpm. Radial-piston air motors are generally limited to free speeds below 4,000 rpm, with design speeds below 2,000 rpm. They provide high starting torque, reliability, and built-in lubrication, but are not normally governed.

Gear-on-gear motors are the most common hydraulic units. They consist of a pair of matched spur or helical gears enclosed in a case. These units develop maximum torques of about 6,000 lb-in., accepting inlet pressures to 3,000 psi and operating to 3,000 rpm.

B>Gear-within-gear motors, often called gerotors, are very compact for their displacement. The inner gear seals against the outer to guard against fluid leakage. Tooth velocities and wear are low and power density is high. Gear-within-gear motors can accept pressures to 2,000 psi and deliver torques to 1,500 lb-in. at speed ranges to over 5,000 rpm. Gerotor air motors function similarly to their hydraulic counterparts, but are designed for low-speed operation, generally under 200 rpm.

Differential gear motors are a variation of the gerotor type, developed to produce high torque at low speed. The outer gear is held fixed, and the inner gear is allowed to orbit within it. A stub spline shaft, with eccentric action at both ends, connects the inner gear to the output shaft. Operating at pressures to 1,500 psi, these motors deliver torques to 3,700 lb-in. at speeds to 1,000 rpm.

Roller-gerotor motors are a variation on gerotor differential-gear motors. Lobes of the outer gear are replaced by rollers that reduce friction. Thus, the motors tend to have higher efficiencies, lower starting torques, and longer lives. These motors accept pressures up to 4,500 psi and produce torques to 16,400 lb-in., and speeds to 850 rpm.

Crescent gear motors have a small gear within a larger one, with a fixed crescent-shaped element between the gears. The gear teeth seal against the crescent. These motors produce speeds to 5,000 rpm, operating on maximum pressures of 500 psi.

Vane motors are used for both pneumatic and hydraulic operation. (In reality, the same motor is rarely suitable for operation on both hydraulic fluid and air, but the principle of vane operation is equally acceptable.) In both hydraulic and pneumatic versions, vane motors consist of a slotted rotor mounted eccentrically within a circuit cam ring. Vanes in the rotor slots are free to move in and out; they are often spring-loaded to the outward position. As air or fluid enters the motor, it applies force against the vane, turning the rotor and allowing the fluid to sweep from inlet to outlet ports.

Most hydraulic vane motors use a two or four-port configuration so that the fluid passes in one side of the vane and out the other, or passes in and out through two ports on each side of the vane. In the latter configuration, each vane provides torque to the rotor twice each revolution. The torque of a four-port motor is twice that of a similarly sized two-port motor, and speed is approximately half. A few vane motors for hydraulic applications use even more ports to create a high-torque, low-speed motor.

For normal units, with only two inlet ports, maximum torque is 4,000 lb-in. at maximum continuous pressures of about 2,500 psi. Top speed is 4,000 rpm.

Vane motors for pneumatic systems operate at free speeds to 13,000 rpm, with rated speed approximately 50% of that level. Torques well in excess of 2,000 lb-in. are available at rated operating pressures of 90 psi. These pneumatic vane motors must be provided with some method for feeding lubricant into the air stream because the outer ends of the vanes rub against the cylinder wall. They also typically require a governor to inhibit operation at free speed. An ungoverned vane motor, allowed to reach free speed, may damage itself or connected machinery.

Axial-vane motors use vanes that rotate instead of sweeping in and out to seal. These motors may have very small fixed clearances and low stalled friction for excellent low-speed capability. Unlike conventional vane motors, they cannot compensate for wear so they require good filtration for continued trouble-free service. They deliver torques to 3,200 lb-in., at continuous pressures to 2,000 psi, with continuous speeds in the range of 5 to 1,500 rpm.

Rotary abutment motors are hydraulic units that consist of a three-lobe rotor, with each lobe carrying a roller in a dovetail groove. The rollers provide a positive seal between the rotor and housing. The sealing arrangement is substantially frictionless and relatively insensitive to wear. The motor contains two rotary abutments, one of which rotates to pass a rotor lobe while the second seals the rotor hub. Timing gears between the output shaft and rotary abutments keep the rotor and abutments in proper phase. The motors provide continuous running torques as high as 3,200 lb-in. at continuous pressures of 2,000 psi. Maximum speed range is 650 to 1,400 rpm.

Turbine motors used exclusively in pneumatic systems, can be either single or multistage units. They can be driven by a wide variety of gases and vapors such as compressed air, natural gas, nitrogen, steam, and the Freons. Because turbine motors fully expand the compressed gas and have very low frictional losses, they are the most efficient type of pneumatic motor for integral-horsepower applications. Because the motors have no rubbing surfaces as in vane motors, no lubrication is required in the drive gas. Turbine motors are available with power ratings from fractional horsepower to 85 shaft hp at drive pressures to 90 psig. They also are available with or without gear reducers to provide a range of speeds from 0 to 25,000 rpm. Units without gear reducers typically weigh from 3 to 20 lb.

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