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Air motors are used to produce continuous rotary power from a compressed air system. They boast a number of advantages over electric motors:

• Because they do not require electrical power, air motors can be used in volatile atmospheres.
• They generally have a higher power density, so a smaller air motor can deliver the same power as its electric counterpart.
• Unlike electric motors, many air motors can operate without the need for auxiliary speed reducers.
• Overloads that exceed stall torque generally cause no harm to air motors. With electric motors, overloads can trip circuit breakers, so an operator must reset them before restarting equipment.
• Air motor speed can be regulated through simple flow-control valves instead of expensive and complicated electronic speed controls.
• Air motor torque can be varied simply by regulating pressure.
• Air motors do not need magnetic starters, overload protection, or the host of other support components required by electric motors.
• Air motors generate much less heat than electric motors.

As one would expect, electric motors do possess some advantages over air motors:

• If no convenient source of compressed air exists for an application, the cost of an air motor and its associated support equipment (motor-driven compressor, controls, filters, valves, etc.) will exceed that of an electric motor and its support equipment.
• Air motors consume relatively expensive compressed air, so the cost of operating them will probably be greater than that of operating electric motors.
• Even though electronic speed controls escalate the cost of electric motor drives, they control speed more accurately (within ±1% of desired speed) than air motor controls do.
• Air motors operated directly from a plant air system are susceptible to speed and torque variations if system flow and pressure fluctuate.

Common designs of air motors include rotary vane, axial piston, radial piston, gerotor, turbine, V-type, and diaphragm. Rotary vane, axial- and radial-piston, and gerotor air motors are most commonly used for industrial applications. These designs operate with highest efficiency and longevity from lubricated air. Of course, specific designs are available for applications where lubricated air proves undesirable. Turbine motors are used where very high speed but low starting torque are required. V-type and diaphragm air motors are used primarily for special applications and will not be covered here.

Piston motors

Piston air motors are used in applications requiring high power, high starting torque, and accurate speed control at low speeds. They have either two, three, four, five, or six cylinders arranged either axially or radially within a housing. Output torque is developed by pressure acting on pistons that reciprocate within the cylinders.

Motors with four or more cylinders provide relatively smooth torque at a given operating speed because power pulses overlap: two or more pistons undergo a power stroke at any time within a revolution. Motors designed with overlapping power strokes and accurate balancing are vibration-free at all speeds.

Power developed by a piston motor depends on the inlet pressure, the number of pistons, and piston area, stroke, and speed. At any given inlet pressure, more power can be obtained from a motor that runs at a higher speed, has a larger piston diameter, more pistons, or longer stroke. Speed-limiting factors are the inertia of the moving parts (which has a greater effect in radial- than in axial-piston motors) and the design of the valve that controls inlet and exhaust to the pistons.

Radial- and axial-piston motors have one significant limitation: they are internally lubricated, so oil and grease supplies must be checked periodically and replenished. They must be mounted in a horizontal position to provide proper lubrication to bearing areas. However, at least one manufacturer offers a radial-piston motor with the shaft vertically-down as a standard configuration. Other mounting positions from any manufacturer require special lubrication configurations.

Radial-piston motors feature robust, oil-lubricated construction and are well-suited to continuous operation. They have the highest starting torque of any air motor and are particularly beneficial for applications involving high starting loads. Overlapping power impulses provide smooth torque in both forward and reverse directions. Sizes range to about 35 hp for speeds to 4,500 rpm.

Axial-piston motors, Figure 1, are more compact than radial-piston motors, making them ideal for mounting in close quarters. Their design is more complex and costly than vane motors, and they are grease lubricated. However, axial-piston motors run smoother and deliver maximum power at much lower speeds than vane motors can. Smaller and lighter than electrical gear motors of the same power rating, axial-piston motors also tolerate higher ambient temperatures. Maximum size is about 3½ hp.