Air motors offer clear benefits

Written by: Paul Fanning | Published:
Air motors offer clear benefits

As with any technology, the advantages of air motors are more obvious in some applications than others. Even so, uses for them range from the food industry to medical and even nuclear power generation.

Air motors occupy a niche as far as power sources are concerned, being well-suited to those applications that require a compact, lightweight source of smooth, vibration-less power. Air motors stop and start almost instantly and provide extremely variable torque and speed without complicated controls. They can operate in hot, corrosive, and wet environments without damage, and are unaffected by continuous stalling or overload. In addition, they are instantly reversible and, unlike electric motors, run cool and start without shock, meaning there is no heat buildup and no electric sparks to damage the motor.

Because they are cooled constantly by injected air, air motors will generally not overheat even at high speed. This fact, in conjunction with the fact that they cannot burn out and do not produce electric sparks, means that air motors have long been a popular choice in environments where there is a danger of explosion. And, while explosion-proof electric motors are available, they are both considerably larger and more expensive than their air motor equivalents.

Air motors develop more horsepower per cubic centimetre than do most electric motors of standard design. An electric motor that develops the same horsepower as a typical rotary vane air motor (one of the two most widely used types of air motors) rated 2.5hp at 1,800rpm and 90psi, might be two to three times heavier. The low weight of air motors when compared to their electric equivalents makes them ideal for use in hand-held power tools or in robot end or arm applications, as well as any applications where easy changeover of the motor is necessary.

However, to compare air motors against electric motors purely on a power/size relationship does not do justice to one of the their key advantages: the relationship between torque and speed.

An air motor slows down when load increases. Its torque increases at the same time until it matches the load. The air motor continues to provide increased torque until it stalls, then maintains the stalled condition without harming the motor. As the load is reduced, an air motor increases speed and the torque decreases to match the reduced load. When the load is either increased or decreased, speed can be controlled by increasing or decreasing air pressure.

Equally, it is unhelpful to talk in terms purely of 'air motors', since there are different types suited to particular applications. Probably the best-known are rotary motion vane-type air motors. These are used to start large industrial diesel or natural gas engines.

Stored energy in the form of compressed air, enters the sealed motor chamber and exerts pressure against the vanes of a rotor. The rotating element is a slotted rotor mounted on a drive shaft. Each slot of the rotor is fitted with a freely sliding rectangular vane. The vanes are extended to the housing walls using springs, cam action, or air pressure, depending on the motor design. Air is pumped through the motor input, which pushes on the vanes creating the rotational motion of the central shaft. Rotation speeds can vary between 100 and 25,000 rpm depending on several factors which include the amount of air pressure at the motor inlet and the diameter of the housing.

Piston-type air motors are designed withbetween two and six cylinders connected to the device's shaft. As air pressure acts upon the pistons, they move in alternating directions supplying rotating power to the motor shaft.
Torque specifications of piston air motors depend on the amount of air pressure applied to the device, the number of cylinders, the size of each piston, the distance each piston moves within the motor body and the speed of the device. These kinds of air motors are useful in situations requiring high power and starting torque, and low speed accuracy.

Piston motors break down between radial and axial types. Radial air piston motors have a robust construction and are well-suited to continuous operation. They have the highest starting torque of any air motor and are particularly beneficial for applications such mining machinery and mine transport. Overlapping power impulses provide smooth torque. Sizes range from fractions of a kW to about 25kW at 4,500rpm.

Axial air piston motors are more compact than radial-piston motors, making them ideal for mounting as starter motors for industrial diesel generators. Their design is more complex than vane motors, and they are grease lubricated. However, axial air piston motors are smoother in operation and can deliver maximum power at much lower speeds. Smaller and lighter than electrical gear motors of the same power rating, axial-piston motors also tolerate higher ambient temperatures. Maximum size is about 4.5kW.

An instance where a piston motor was found to be more suitable than a rotary vane motor was with international oil and gas well services company Blue Manta. A bad experience with the reliability of a vane air motor prompted the company to look for a better-performing alternative. This type of motor is ideal for operation in ATEX environments, but the existing model was proving inefficient in extreme conditions, a problem exacerbated by its inaccessibility for repair and maintenance. Both were resolved when the company switched to a Huco Dynatork piston air motor.

Air motors are used by Blue Manta to tension cable as it is lowered into the well which can be offshore or in the desert. In both environments the efficiency of the vane motor was compromised by the extreme conditions. Its design makes it very difficult to seal, exposing the unit to the ingress of salt water and sand. By the same token air is lost to the atmosphere rather than being converted into motion.

The Huco Dynatork piston air motor, however, was immune to such problems. The operating principle of the product is simple. Via an integral rotary valve, air up to 100psi is supplied to each of three pistons in turn. The free floating pistons transmit torque on start-up that can be adjusted via a pressure regulator. This results in high torque at variable low speed.

As the motor traps the compressed air within the cylinder, maximum energy conversion is assured. The design is also easily sealable so not only does it consume up to 80% less air than its vane counterpart, it is also far more reliable in hostile environments. Equally, Huco Dynatork's piston motors' internal pneumatic porting makes them much less sensitive to temperature extremes and less prone to damage.

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