A Look At The Piston Air Motor

Published: 17th August 2011
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If one has been to the ubiquitous tyre shop or watched the slick Formula Teams changing tyres in double quick time on the TV , you will have seen the air hammer tools with their staccato hammering action, being used. The tools are driven by compressed air and at their heart are piston air motors. Wikipedia defines the piston air motor (sometimes also known as an air drive, or compressed air engine) as a machine that is powered by compressed air and it converts reciprocating motion into rotary motion.

How they work is that compressed air is fed continuously into the top of a cylinder and this pushes down a spring-loaded piston until the top of the piston passes the exhaust port(s) and the cylinder then goes back up and the two-stroke cycle begins again. This is the hammering noise that you hear in the tyre workshop. For most applications, the piston air motor comprises between two and six cylinders to smooth out the power generated by overlapping the power pulses. The reciprocating action is converted to rotary motion in a similar fashion to a normal combustion engine. This type is called the radial type and it is the simplest type with the least moving parts. They are commercially available in sizes from fractions of a kilowatt to about 25 kW at 4,500 rpm

Alternatively, the axial type arranges the cylinders around the shaft and the power is transmitted from the piston con rods using spur gears. This arrangement is often used where a low axial profile is required say as a hand-held drill. The efficiencies of the two types are similar but with the axial type requiring more maintenance on high torque-applications. They do also produce slightly less torque than the equivalent axial type.

The piston air motor has a number of characteristics that make it ideal where an electric motor would not be a good choice. The air motor does not spark and therefore it can be used in potentially explosive atmospheres as found in petro-chemical installations, automotive workshops, mines and fume laden paint spray booths. They can be more compact with a smaller axial cross-section than the equivalent electric motor. They can be run at low speeds (by throttling the compressed air supply) and retain their high efficiency; typically 70%.

The air piston motor drive develops its maximum torque at start up or when stalled and this characteristic makes it eminently suitable for the air spanners used in the tyre shop. If you have ever had to take off a wheel nut after the car has been in the type shop, you can attest to the difficulty of undoing the wheel nut by your car spanner. This high torque at zero rpm would damage an electric motor or cause power circuit trips to operate without some protective clutch arrangement.

Another advantage of the air motor over the electric motor is that adiabatic expansion of the compressed air causes it to cool thus takes heat out of the motor. This is entirely opposite to the electric motor where waste energy is converted to heat and thus electric motors need specific cooling measures like internal axial fans, spur gears and over-temperature cut-off devices. Piston air motors have short-stoke piston action and low-weight parts; thus making then have a low-interia. This characteristic makes the piston air motor start and stop very quickly and as a result, applications of a stepper motor are simple, effective and accurate in positioning. The electronic control of compressed air pulses makes this an ideal industrial machine tool application.

These days, the smaller piston air motors are produced in robust plastics and stainless steel such as the Huco Dynatork motors, which run on oil-free compressed air and these have opened the applications to clean environments such as found in food production and medical applications.

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