Pneumatic Motor Selection

This article, in the Hydraulic Pneumatics lessons section, is about pneumatic motor selection. We’ll share what to pay attention to when selecting a pneumatic motor, and which factors need to be calculated.

Factors Affecting Pneumatic Motor Selection

The pneumatic motor’s pressure efficiency
The pneumatic motor’s speed range
The pneumatic motor’s torque value
The pneumatic motor’s service life
The pneumatic motor’s cost
The pneumatic motor’s dimensions, weight, and mounting method

all affect motor selection.

Pneumatic motor selection and pressure efficiency

This is determined by the motor’s ability to withstand air pressure without causing any damage or an undesirable amount of internal leakage. Many motors are designed to operate within a certain pressure range. A motor’s pressure range varies between 2 bar and 10 bar.

Speed range in pneumatic motor selection

Depending on the motor type and design, this ranges from 500 rpm to 6000 rpm. Some small tools run at very high speeds, with speeds of around 1500 rpm being possible. The motor’s operating speed is determined based on the application.

The motor’s torque value in pneumatic motor selection

A motor’s torque value is the rotational force acting on a body. Motor torque is divided into three groups; these are:

Starting torque: this is the force the motor exerts to begin moving while fully at rest.

Running torque: this is a force acting while the motor is running, and it varies based on changes in air pressure.

Stall torque: this is the force required to stop a running motor.

Torque-speed relationship in pneumatic motors (above). In the torque-speed graph, torque is high while the motor is running at low speed. At high speed, torque is low.

By coupling motors together through a gear arrangement, a system combining low speed and high torque can be obtained.

A pneumatic motor’s output power, in kilowatts, is equal to the product of shaft speed and torque, and can be calculated using the equation below.

For example, suppose you want to calculate the power in kW of a motor with a torque value of 230 Nm, running at a speed of 2000 rpm. What would the output power be in this case?

Solution: 230 x 2000 / 60000 = 7.66 kW

Typical power curves of a pneumatic motor (above). Notice in the power curve graph shown that maximum output power is obtained at a moderate speed and, likewise, a moderate output torque. For this reason, it’s not necessary for the motor to run at maximum speed to achieve maximum power in any given application.

The pneumatic motor’s service life

In pneumatic motor selection, service life is directly related to operating hours, load cycles, and the quality of the compressed air powering the motor. Keep in mind that compressed air quality can be ensured through proper filtration and lubrication.

The pneumatic motor’s cost

The pneumatic motor’s cost includes its service life, operating cost, and part-replacement expenses, as well as the purchase cost related to installation, piping, coupling, and shaft mounting arrangements. Supplying compressed air for these motors requires a higher initial cost compared to hydraulic motors.

The pneumatic motor’s size, weight, and mounting method

Generally, motors with a one-piece construction that can be easily fitted into small spaces are preferred. The motor’s overall dimensions, shape, mounting type, shaft couplings, and air line pipe connections are generally provided in the manufacturer’s catalog.

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