When air motors fit the bill

Aug. 6, 2007
When safe, reliable power is necessary, air motors can easily generate torque and rotational motion from compressed air.

A mention of motors in an industrial setting may bring to mind the typical electric motor. But system designers should also be aware of the many advantages of air motors over their more famous brethren. These power-dense components provide the engineer some interesting benefits.

Air motors have higher power density than electric motors, so they can transmit more power from the same envelope or the same power from a smaller envelope. This is especially true when loads must be driven at less than the nominal speed of the electric motor — which necessitates using an electric gearmotor or electric motor with separate gearbox. In general, the lower the nominal speed of an electric motor, the larger and heavier it is — a 1,200-rpm electric motor is larger, heavier, and more expensive than a 3600-rpm electric motor. An air motor designed to operate at 1200 rpm may be smaller, lighter, and less expensive than even a 3,600-rpm electric motor.

Air motors generate much less heat, if any. This is especially important in applications with frequent starts and stops, because electric motors must be greatly oversized to dissipate heat generated from high starting torque. Electric motors can be fitted with a clutch to enable rapid starting, but this adds complexity to the system and causes the motor to consume electrical power even when it is disengaged from the drive by the clutch. Enclosures that enable them to tolerate moisture — and especially explosion-proof enclosures — can add substantial size and weight to an electric motor.

To control air motors, no electronic speed controls are required as in the case of electric motor. Air motors can easily be controlled by simple flow control by means of valves. By regulating the pressure, the torque produced by the air motors can easily be varied. Also, air motors do not need any magnetic starters like electric motors.

Air motors are used in a wide range of applications, such as: powering production machinery, raising foundry copes and drags, rotating turntables, mixing solutions, powering cranes, valve actuation, and as a power source for a host of industrial and even consumer applications.

More advantages

When subjected to a torque overload, air motors simply stall, and consume no power unless they are rotating. Subject an electric motor to a excessive torque, and it will burn out.

Applications where the motor must operate in high heat, high dust, wet, or potentially explosive conditions will require special TEFC electric motors with NEMA enclosures designed for these environments. This drives the overall cost of the motor up. However, pneumatic motors do not need electrical power and can operate in volatile environments with simple options such as epoxy coatings or high temperature grease. In fact, a pneumatic motor can operate in temperatures in excess of 200° F.

Figure 1A — Standard rotary vane air motors are well-suited for applications that offer limited space but demand high speeds.

One last issue to consider: electric motors typically cannot be repaired by the average maintenance shop in a manufacturing facility. They must be sent to a facility that specializes in electric motor repair to be re-wound and insulated. Air motors can be repaired by any technician that is experienced in repairing tools, pumps, compressors, gear boxes, or other rotating equipment. Therefore, downtime is reduced, turnaround time is reduced, and the overall cost of operating a pneumatic motor is reduced as well.

Therefore, although the initial cost of an electric motor may appear to be less than that of an air motor, the overall cost of ownership will be greater in many applications.

Rotary vane design

Figure 1B — This vane motor is intended for applications having limited radial space. Because of its small OD, this motor transmits low torque at high speed. It has an integral planetary gearbox to increase torque and reduce speed.

Rotary vane air motors are typically small in size and deliver high speeds and low torques. In an application where a high-speed/lowtorque power source is required in a limited space envelope, the rotary vane motor is an excellent option. These motors are well suited to machines used in printing, packaging, food processing, canning, and bottling. Because of their design, rotary vane motors can be fitted with various abrasive wheels, cutting tools and used in machining, grinding, and drilling applications.

Axial-piston design

Figure 2 — Axial piston motors are generally used for applications requiring high torque. One common use is for start-stop appkications.

Axial-piston motors are relatively compact for the amount of torque and horsepower delivered. They are also very controllable — inherent in the design is the ability to precisely manage the rotating speed of the motor.

High torque is a characteristic of piston-type air motors, making them especially desirable for applications involving heavy starting loads. The overlap of power impulses in axialpiston air motors provides even torque and full power in either direction of rotation. They should be operated under load and in a horizontal manner.

Axial-piston motors are also ideal for start/stop operations. It is not recommended that they be operated at speeds greater than 75% of free speed. Their enclosed-type construction allows operation in corrosive or dusty atmospheres. These motors can perform well in ambient temperatures up to 200° F.

Radial-piston design

Figure 3 — Radial-piston motors can deliver high torque and power at low speeds.

Heavy-duty construction makes radial-piston air motors suitable for continuous operation. A counterbalanced crankshaft is supported by rolling-element bearings. A slinger distributes oil to all moving parts of the motor. Oil level and drain plugs are easily accessible.

Radial-piston motors deliver high torque and horsepower at relatively slow speeds — under 3000 rpm. Like the axial-piston motor, the radial-piston motor provides excellent control of motor speed. Both radial- and axial-piston motors are great for applications where the motors must start under a load. Due to these features, radial-piston motors are commonly used for driving conveyors, rotating large drums and tumblers, and for rotating positive-displacement pumps.

Lubrication concerns

Lubricators are recommend in the air lines feeding air motor and should be located as close to the motor as possible to ensure adequate lubrication. The only type of air motor designed to run on non-lubricated air is the radial-piston model. Oil-less models have PTFE piston rings and an oil-lubricated crankcase. Vane type air motors come in an oil-less version, but all types work better with lubricated air. Any pneumatic device using lubricated air should have a coalescing muffler to remove atomized or residual oil from the air exhausted to atmosphere.

Air motor selection

To assist in the selection of an air motor, a manufacturer will need any two of the following items:
1. Power required in hp or kW.
2. Speed required against load in rpm.
3. Work load in lb-ft, lb-in., N-m, etc., of dynamic torque (moving torque load).

Because air motors depend on input air pressure for performance, these factors must also be considered:
1. Air pressure (psig or bar) at motor location.
2. Pipe size of air supply system to assure adequate volume (cfm or m3/min) so as to minimize line loss (air pressure drop during motor operation).
3. Plant air system pressure may vary during the day, due to use by other equipment; therefore, base the selection of the air motor on the lowest psig or bars that can be expected. Then, by using a pressure regulator in the line at the motor, a uniform air supply is assured.

In applications of intermittent short service, some air motors can be operated over the complete range indicated for each without harm or significant wear. In applications involving sustained continuous operation for long periods, however, some guidelines are suggested. Do not operate piston-type motors at more than 75% of free speed. Consult the manufacturer for applications requiring continuous operation of rotary vane motors at maximum power or for any applications requiring vertical operation of these motors. Review service manuals carefully.

For two known and one unknown, the following formulas can be used:

Torque (lb-ft) = (1,000 × hp) ÷ (0.19 × rpm)
Horsepower = (0.19 × torque × rpm) ÷ 1,000
Speed (rpm) = (1,000 × hp) ÷ (0.19 × torque)

Cooper Power Tools, Lexington, S. C., is now part of Apex Tool Group. Click here for information on ATG's air motors.

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