Matching an air motor to an application

08 July 2019

Franck Roussillon offers some advice on how to match the most suitable air motor to an application.

Torque, power and air consumption graph. P = power; Q = air consumption; M = torque; n = speed.
Torque, power and air consumption graph. P = power; Q = air consumption; M = torque; n = speed.

The popularity of air motors is growing - a trend driven by their small installation dimensions and their ability to be loaded until they stall, without damage. They can also well suited to use in harsh environments that are subject to effects such as dust (most are ATEX-certified), vibration and impact.

Air motors can be stopped and started continually without damage, and are reversible as standard. However, it is often their simple design principle that appeals most to specifiers: a factor that also makes them easy to service. The low number of moving parts can also enhance reliability. However, to make the best use of these benefits it is important to select the right air motor for an application.

To make the best selection, it is first advisable to consider the basic construction of an air motor. There are a number of different designs - including tooth gear and turbine types - but this article focusses on vane types as they are more suitable for regular operating cycles, where slower speeds are required.

The principle centres on a rotor with a number of vanes enclosed in a stator and in the cylinder body. Compressed air is supplied through one connection and air escapes from the other. For reliable starting, springs press the vanes against the rotor cylinder and the air pressure always bears at right angles against the surfaces. This function means that the motor torque generated is a result of the vane surfaces and air pressure.

Air motor performance is dependent on the inlet pressure. At a constant inlet pressure, air motors exhibit the characteristic linear output torque/speed relationship. However, by simply regulating the air supply, using the techniques of throttling or pressure regulation, the output, torque and speed of an air motor can easily be modified. 

A pneumatic motor achieves its maximum power when it is operating as close as possible to its rated nominal speed (50% of the rated idle speed). Energy balance and efficiency are best in this area because the compressed air is used efficiently and the power is at its maximum.

Across industry, oil and oil mist often need to be avoided to ensure a clean work environment. Many manufacturers are also keen to avoid using components that have to be lubricated. If unlubricated compressed air is used, it should comply with the relevant purity standards in order to guarantee the longest possible overall service life. Furthermore, if the unlubricated compressed air has a high water content, condensation can form inside the motor, causing the corrosion of internal components. A ball bearing can be destroyed in a remarkably short time if it comes into contact with a single water droplet. 

For food grade and other hygienic/high cleanliness applications, external components should be made from stainless steel. Take the Parker P1V-S range, for example. Here, the air motor and planetary reduction gear are built into a polished stainless-steel housing. The output shaft, which is also made of polished stainless steel, is sealed by a fluorocarbon (FKM) rubber seal. This design means the motors can also be deployed under water to a depth of around 8m.

Thanks to a cylindrical shape, there are no pockets that can accumulate dirt or bacteria. Additionally, the two halves of the motor body are sealed with a positive O-ring to prevent contamination. Operation is intended at intermittent intervals under non-lubrication conditions. For this reason, no particles of lubricant escape with the exhaust air and service costs are reduced.

Selection criteria
The first decision to be made is to identify which drive principle is best suited to the application. Air vane motors are well suited to regular operating cycles, where speeds are slow - say, less than 10,000rpm. In contrast, tooth gear air motors or turbines are suitable for continuous 24-hour operation, where speeds can be up to 140,000 rpm.

Environment choice is another major factor. To determine the optimum material, consider whether the air motor will operate in a normal production area, or one that is potentially explosive? Clearly, air motors have an advantage here, as electric motors typically cannot be used in ATEX-rated environments. The type of industry may also have a bearing on material selection - sectors such as paper, food processing, medical and pharmaceutical will all have an influence.

Many factors will come into play when it comes to calculating the required power of the air motor, including direction of rotation, air pressure working range, air class quality and mainly the expected torque and speed under load. 

Basic power can be calculated using a simple formula: P = M x n / 9550. Here, P is power output in kW, M is nominal torque in Nm, and n is nominal speed in rpm. Always select a motor that is slightly too fast and powerful, then regulate its speed and torque with a pressure regulator and/or throttle to achieve the optimum working point. Ensure that the pressure supplied to the inlet port of the motor is correct, so it can work at maximum capacity. If the valve supplying a large motor is too small or the supply line is underspecified, the pressure at the inlet port may be so low that the motor is unable to function. 

Other factors that will determine the selection of an air motor include the position in which it will be used. Also, consider whether standard or spring-loaded vanes be required. Spring-loaded vanes are selected to ensure they remain pressed against the cylinder when the motor stops and when working at low speeds. The spring-loaded vane option also prevents the vanes from sliding down in their track if vibration is introduced. They can, therefore, provide a higher starting torque, improved starting and low speed characteristics, because leakage over the vanes is minimised. 

Will an integrated brake be required? Integral spring-loaded disk brakes are typically released at a minimum air pressure of 5 bar. The brake is applied in the absence of pressure. As soon as the control port for the brake is placed under pressure, the piston is pressurised and the spring is compressed. The motor can now start and the torque is passed to the shaft. Ventilation air from the brake is connected to the atmosphere. 

In order to brake the motor, the control air to the brake is simply vented: the piston is pushed to the right by the spring, and the axle is jammed between the two brake disks. Brake motors must only ever be supplied with unlubricated air; otherwise, there is a risk of oil from the supply air getting into the brake unit, resulting in poor brake performance or no braking effect whatsoever.

Will a high-torque air motor be required? These drive solutions are particularly suitable for use in industrial agitators and mixers, as used in the paint, food and pharmaceutical industries. Such motors are also suitable for pneumatic drilling and grinding tools.

For intermittent, yet regular operating cycles, where speeds are low, vane-type air motors can be a good option. However, only by optimising the selection and installation process, users will be able to enjoy the potential benefits and maximise efficiency and longevity. 

Franck Roussillon is product manager BU Actuators at Parker Hannifin.

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