Since vapor compressed with the air can condense and cause damage to tools, water traps are fitted and filters are provided to prevent any foreign matter from damaging or clogging the tool. Lubrication of the working parts is often achieved by the use of oil-fog lubricators fitted in the air line and supplying a finely atomized oil mist to the motor.
The main types of air motors are the vane, piston, and turbine designs. Turbines find applications where high speed and little torque are required, for instance in some of the small grinders or in a dentist’s drill. Heavier work, where massive power and good starting torque are required at fairly low speeds, generally involves the use of piston motors. Piston motors generally have pistons in a radial arrangement around a central crankshaft but in-line and V-shape configurations are also used.
The most common design is positive displacement motors of the vane type, used in power tools. With this type one can get the most power into a small space. It is a flexible power unit with a high-speed rotor which, when coupled to suitable gearing, is capable of producing high torque at the final drive.
These motors have multiple longitudinal vanes fitted into radial slots in a rotor which is mounted eccentrically in a cylinder, similar to the rotor of a rotary vane compressor. The compressed air enters the side of the cylinder at one end, and leaves through an exhaust port halfway along the cylinder and further around it, turning the rotor as it goes.