The performance of an air motor

Performance of an air motor is dependent on the inlet pressure.

The motor can operate over the complete torque curve


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 of an air motor can easily be modified. One of the features with air motors is that they can operate over the complete torque curve from free speed to standstill without any harm to the motor. The free speed* or idling speed is defined as the operating speed where there is no load on the output shaft.

*Free Speed = speed at which the outgoing shaft rotates when no load is applied.

          The Power Curve


“”The power that an air motor produces is simply the product of torque and speed. Air motors produce a characteristic power curve, with maximum power occurring at around 50 % of the free speed.

The torque produced at this point is often referred to as “torque at the maximum output.”

Output formula:
P = (π x M x n) / 30
M = (30 x P) / (π x n)
n = (30 x P) / (π x M)
P = power [kW]
M = torque [Nm]
n = speed [rpm]

                              The working point


When selecting an air motor for an application the first step is to establish the “working point”.

This is the combination of the desired operating speed for the motor and the torque required at that point.

Note: The point on the torque/ speed curve where the motor actually operates is called the working point.

The working point for an air motor
Air consumption

The air consumption for an air motor increases with the motor speed and thus is highest at free speed. Even at standstill condition (with full pressure applied) the motor consumes air. This depends on the internal leakage in the motor.

Note: Air consumption is measured in l/s. This is however not the actual volume that the compressed air occupies in the motor but it is measured as the volume it would occupy if allowed to expand to atmospheric pressure. This is a standard used for all pneumatic equipment.