pneumatic rotary actuators (or air motors)

PNEUMATIC ROTARY ACTUATORS (OR AIR MOTORS)

1. What are Air Motors ?

• The function of an air motor (or a pneumatic rotary actuator) is to convert the pressure energy of the compressed air into rotary mechanical energy.

• Types: Like hydraulic motors, air motors can also be classified based on their degree of angular movement as :

(i) Continuous rotary air motors, and

(a) Gear motors, (b) Vane motors, and (c) Piston motors.

(ii) Limited rotation air motors.

(a) Vane type, and (b) Piston type.

2. Continuous Rotary Air Motors

These air motors with unlimited angle of rotation (even upto 10,000 revolutions per minute) are most widely used to provide very high rotational speeds. The construction and operation of continuous rotary air motors are very much similar to that of the hydraulic motors discussed in Sections 5.10-5.12.

1. Gear motors: In the gear motors, rotary motion is produced by the unbalanced pneumatic forces on the gear teeth. Thus the torque is generated by the pressure of the air against the teeth profiles of the two meshed gear wheels.

2. Vane motors: In the vane motors, the pressurised air acting on the vanes cause them to rotate and thus developing the torque output. They are suitable for low speed applications than gear motors.

3. Piston motors: Piston motors are positive displacement motors which can develop an output torque at the shaft by allowing pressurised air to act on the pistons. The two types of piston motors are axial- and radial-piston motors.

3. Limited Rotation Air Motors

The limited rotation air motors provide rotary output motion over a finite angle. The standard rotations of the shaft of these motors are 94°, 184°, and 364°. The construction and operation of limited rotation air motors are very much similar to that of the hydraulic motors discussed in Section 5.13.

4. Advantages of Air Motors (over Electrical and Hydraulic Motors)

The advantages of air motors compared to that of electrical and hydraulic motors are as follows:

 (i) Air motors do not require electrical power.

 (ii) Air motors have a higher power to weight ratio (i.e., a smaller air motor can deliver the same power as its electric motor). In fact, the air motor is one-fourth weight of electric motor for same power rating.

(iii) Torque of air motor can be varied easily by simply varying pressure.

(iv) Air motors are safe to be used even in volatile atmospheres (hot, corrosive and wet environments) without damage as air is not explosive.

(v) The direction of the motion of air motor can be reversed easily.

5. Applications of Air Motors

The important applications of air motors include conveyor belts, bench grinders, agitators and mixers, feeding devices, hoists, machine feeders, tool devices, vibrators, and pipe threaders.

6. Comparison between Air Motors and Air Cylinders

Table 12.3 presents the differences between air motors and air cylinders.