Compressed air as an energy source has a lot of advantages over electric power, especially when it comes to hand tools. It is often the first choice for such tools as impact wrenches, pulse tools, screwdrivers, nut runners, drills, grinders, sanders and so on.

There are three main operating types of air motors: vane, piston and turbine. Regardless of the design, there are a few characteristics of air motors that are universally valued and make them the motor of choice for certain applications.

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Air motor advantages

• Compressed air, the source of power, is readily available and inexpensive
• Since air is the working fluid and torque is produced mechanically rather than electrically, air motors are non-sparking and are inherently safe to use in hazardous environments (e.g., oil rigs, mining, process control).
• As the compressed air power source moves through the motor, it expands and naturally cools. As a result, air motors are not subject to overheating. They can work to ambient temperatures as high as 300° F.
• Unlike electric motors, if the torque load exceeds the motor capability it will not damage the motor. Whereas an electrical motor may trip a breaker or cause insulation damage through a surge in current, air motors are easily depowered, cleared and restarted.
• Most air motors can be configured so that they are inherently reversible. Just by switching the inlet and the outlet ports, the airflow is reversed, and the motor turns the opposite direction.
• Air motors, due to the fact that they are not full of heavy copper windings like electric motors, are inherently lower in weight for the same power output. At the same size, an air motor may be able to generate two to three times the power of an electric equivalent.
• Torque and speed for air motors can be controlled either through pressure or flow regulation.

Types of air motors

Each of the three main types of air motors operates a little differently so it is worth spending a little time reviewing each type and where it is best used.

Vane motor

This is the simplest and most common type of air motor that you would likely encounter. The working element is a cylinder with several radial slots running the length of the cylinder. The rotating axis of the cylinder is offset from the center of the housing and each of the radial slots has a sliding vane in it. Since the axis of rotation of the working cylinder is offset from the center of the motor, as the cylinder turns, the enclosed volume at the inlet side increases, while the enclosed volume at the outlet side decreases. This differential pressure across each of the vanes, as the cylinder rotates, creates a net torque on the cylinder. This is a high flow and moderately high torque motor.

Piston motor

If you’ve ever seen an airplane engine with radial pistons, then you’ll have an idea of how a piston air motor is designed. As compressed air enters the inlet side it will fill up the first piston cavity, causing the piston arm to rotate the crankshaft and opening up the next cavity. This action continues until the piston bottoms out in its cavity at which point further rotation exposes the piston cavity to the outlet side of the motor. Typical piston motors have anywhere from three to six pistons.

Turbine motor

Whereas the vane and piston-type of motors work on the principal of pressurizing then depressurizing successive cavities, the turbine air motor uses a very different principle. Compressed air entering the inlet side pressurizes a circular area which contains angled vanes around the periphery to create a rotary flow. This flow impinges on a set of “buckets” on the edge of a rotary wheel which captures the kinetic energy of the flow to spin the rotary wheel at very high speeds. This action is similar to gas or steam turbine designs. Speeds up to 400,000 RPM are not unusual.

The table below gives a summary of the attributes of each of these motor types and common uses.

Table 1: Comparison of air motor types

Air motor construction and control

Electric motors rely on copper wire to generate an electromagnetic field which provides the motive force to the rotor. Using steel to help shape and concentrate these fields is an integral part of the design. By contrast, air motors use pressurized cavities to create the motive force through differential pressure. Much of an air motor, therefore, consist of space to provide room for the air to move. This has the advantage that the power/weight ratio can vary by changing the material choices according to the needs of the application.

If an air motor needs to work in heavy duty washdown applications (chemical exposure), then the primary material of the body could be made of stainless steel. For more every day commercial/industrial duty it would not be unusual to find air motors made of aluminum. Some aluminum alloys are as strong as steel but at one third the weight.

Finally, in light duty applications (and also in applications that may need some chemical resistance) certain plastics are suitable. These may require metallic inserts at potential wear points but are light and can be more flexible in their configurations due to injection molding and/or 3D printing options for manufacture.

Regardless of the material or type of air motor the main methods of control are to regulate either the pressure or the flow rate. Usually, speed is a function of the flow rate. A faster flow rate means a higher volume per minute which is a higher rotary speed. High pressure operation creates a higher differential pressure between the intake and the outlet, which translates to a higher torque. A typical compressed air system found in a commercial metalworking shop might run an air tank at a pressure between 90 psi to 150 psi. Since one tank may serve several air motors which are run at different times, it’s not unusual for the supply pressure to vary at different times. These fluctuations are normal and do not usually impact operations. However, some manufacturers offer constant pressure regulators, for example if maintaining a certain minimum torque is required. Compared to electronic control, pressure and flow regulators are fairly simple.

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Air vs. electric

Both air-powered and electric powered motors have been around for a long time. The first viable electric motor was sold in 1886 and the first pneumatic drill was invented in 1871. Both technologies have evolved side-by-side in the intervening roughly 150 years and each has its preferred uses. Although most people often think “electric” when they envision a motor, it is good to be aware of the compressed air alternatives, their flexibility and the options.