Rotary vane motors normally are used in applications requiring low- to medium-power outputs. Simple and compact vane motors most often drive portable power tools, but certainly are used in a host of mixing, driving, turning, and pulling applications as well.

Vane motors have axial vanes fitted into radial slots running the length of a rotor, which is mounted eccentric with the bore of the motor's body housing, Figure 2. The vanes are biased to seal against the housing interior wall by springs, cam action, or air pressure, depending on design. The centrifugal force that develops when the rotor turns aids this sealing action. Torque develops from pressure acting on one side of the vanes. Torque at the output shaft is proportional to the exposed vane area, the pressure, and the moment arm (radius from the rotor centerline to the center of the exposed vane) through which the pressure acts.

In a multi-vane motor, torque can be increased at a given speed by increasing the air pressure at the motor inlet to increase the pressure imbalance across the motor vanes. However, there are tradeoffs: increasing inlet air pressure increases air supply costs and generally leads to faster wear and shorter vane life.

Output power at a given speed determines air consumption. A small motor producing 1 hp and operating at 2,000 rpm using 80-psi air consumes the same volume of compressed air as a larger air motor producing 1 hp at 2,000 rpm using air at a lower, more economical pressure.

Rotary vane air motors are available with three to ten vanes. Increasing the number of vanes reduces internal leakage or blow-by and makes torque output more uniform and reliable at lower speeds. However, more vanes increase friction, cost of the motor, and decrease efficiency.

If, in a 3-vane design, one vane sticks in a retracted position, it can prevent the air motor from starting under load. Spring-biasing the vanes against the housing wall, porting pressure air to the base of the vanes, or camming the base of the vane prevents this problem, as does using a motor with four or more vanes.

Vane motors operate at speeds from 100 to 25,000 rpm at the rotor - depending on housing diameter - and deliver more power per pound than piston air motors. Because the vanes slide against the housing wall, many vane motors require lubricated air, particularly if short duty cycles are followed by long inactive periods. However, more and more motors continue to be designed to operate on non-lubricated air to serve critical applications and environmental concerns.

Operation of ungoverned vane air motors with no load at high speed should be avoided. When a multi-vane motor operates ungoverned under no load, its high speed can heat and char the vane tips as they rub against the cylinder wall. Abnormal wear and damage to other motor parts should also be expected.

Vane-type air motors are available in four basic mounting configurations: base, face, hub, and NEMA-flange. Base-mount models simply bolt onto a sub-base, and the load is belt-driven or directly coupled. Face and hub mounts are used when the motor must be mounted through a bulkhead or as an integral part of a driven device. NEMA-flange mounts enable air motors to directly replace NEMA-frame electric motors.

Gerotor air motors

Gerotor air motors, Figure 3, deliver high torque at low speed without additional gearing. When coupled with a 2-stage orbital planetary gear train, gerotor power elements provide torque at speeds down to 20 rpm. These motors are well suited to hazardous-environment applications where relatively high torque is needed in limited space.

Low-speed/high-torque gerotor air motors can deliver torque exceeding 250 lb-in. within a speed range of 20 to nearly 100 rpm from a 90-psi supply of compressed air. They are rated for continuous operation at supply pressures to 150 psi. Low rotating inertia of the gerotor design produces instant starting, stopping, or change in direction when the valve supplying the motor is shifted. Furthermore, the design prevents the motor from coasting or being backdriven, which can eliminate the need for external brakes. Like vane motors, they are much less sensitive to mounting orientation than piston motors are.

Turbine motors

Efficiency of an air motor is defined as the ratio of the actual power output to the theoretical power available from the compressed air for the expansion ratio at which the machine is operating. Turbines convert pneumatic power to mechanical power at about 65% to 75% efficiency. Turbine efficiency is higher than other air motors because sliding contact of parts does not occur to cause internal friction. As a result, there is no need for extensive lubrication. The absence of lubricating oil dramatically improves cold-weather performance.

Until recently, turbine air motors typically were used for applications requiring very high speed and very low starting torque - dental drills and jet aircraft engine starters being most typical. Now, however, turbine technology is being applied to starting small, medium, and large reciprocating engines. Turbine technology offers simple, highly efficient pneumatic starters that require no lubrication of their supply air, tolerate contaminants in the supply air, and need little maintenance. Turbine starters include a planetary gear reduction to bring the turbine's high rotor speed down to normal engine cranking speeds.

Turbine motors are relatively compact and light for their power-delivery capability. Higher gear ratios - from 9:1 through 20:1 - provide high stall torque and versatility for a variety of engines. Turbine horsepower is easily changed by limiting air flow through the motor.

Operation of a turbine air motor involves a nozzle that directs and meters air to a turbine wheel or rotor. It changes high-pressure, low-velocity air flow to low-pressure, high-velocity. The mass-flow rate of air passing through a turbine determines its horsepower. Changing the number of nozzles or nozzle passages changes power output proportionally. If a 16-nozzle starter is reduced to 8 nozzles, the altered starter will produce half the power of the original. Therefore, within the same basic starter configuration, many models can be designed that have a wide range of inlet pressures, cranking speeds, and cranking or stall torques. This capability, combined with various gearboxes, allows production of low-cost starters for a wide variety of applications. For example: turbine starters are available to crank engines with displacements from 305 to 23,800 in.3 at pressures from 40 through 435 psig.