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What is the working principle of an air motor?

The vane motor has a specific design and consists of only a few components

The vane motor has a specific design and consists of only a few components

There are several types of pneumatic motor. The most used types are vanes, piston, and turbine motors. This article deals with vane motors only. Vane motors are produced with power ratings up to approximately 5 kW.

  1. Front end plate
  2. Rotor
  3. Vane
  4. Cylinder
  5. Rear and plate
  6. Bearing

Design

A slotted rotor rotates eccentrically in the chamber formed by the cylinder and cylinder end plates. Since the rotor is off-center and its diameter smaller than that of the cylinder, a crescent-shaped chamber is created. The rotor slots are provided with vanes that move freely to divide the chamber into separate working chambers of different sizes. Due to the centrifugal force, often reinforced by the compressed air, the vanes are forced against the cylinder wall to seal the individual chambers. The actual efficiency of these seals is a function of what is called ”internal leakage”.
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What is working principle of the vane air motor?

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Thanks to this simple principle the energy of the compressed air is converted into rotational motion from chamber to chamber, and the motor turns.

  • A. The air enters the inlet chamber “a”. Vane 2 has just sealed off the chamber “b” between itself and vane 3. The pressure in chamber “b” is still the inlet pressure. This pressure acts on vane 3, moving it in a clockwise direction.

  • B. The vanes have rotated further and the expansion process in chamber “b” has started. The pressure in it is thereby reduced but there is still a net force moving the rotor forward as the area of vane 3 is larger than the area of vane 2 in chamber “b”. Furthermore, the inlet pressure acts on vane 2 in the inlet chamber “a”.

  • C.The vanes have moved further. Chamber “b” is now being emptied through the outlet and there is no more contribution from this chamber. The force moving the rotor forward now comes from the force on vane 1 and vane 2.

Clockwise / counterclockwise/ reversible

Working principle of an air motor | reversible motors have three air ports

Working principle of an air motor | reversible motors have three air ports

The motor turns in a clockwise direction as seen from the rear end. Besides this type of vane motor there are also counterclockwise and reversible motors. Counterclockwise motors are designed the same way as the clockwise motors, but the cross section is mirrored. For a reversible motor port “a” is the inlet at clockwise rotation. Port “c” is the main outlet and port “b” is the secondary outlet. At counterclockwise rotation port “b” becomes the inlet and port “a” the secondary outlet. Port “c” remains the main outlet. The Chicago Pneumatic motors are designed as described above.

Rotor speed

During starting and at slow speeds, some of the compressed air flows under the vanes to press them against the cylinder wall and seal the various working chambers. When the rotor rotates, the vanes are forced against the wall of the cylinder by centrifugal force. At high speeds, however, the pressure exerted against the wall by the vanes must not be too great, or excessive wear will result.

The amount of wear is a function of the third power of the sliding speed between the tip of the vane and the cylinder wall and, in actual practice, this determines the maximum rotational speed.

To keep the centrifugal force down, high-speed motors, or rather their rotors, are long and slim and equipped with only three or four vanes.

Number of vanes

During starting and at slow speeds, some of the compressed air flows under the vanes to press them against the cylinder wall and seal the various working chambers. When the rotor rotates, the vanes are forced against the wall of the cylinder by centrifugal force. At high speeds, however, the pressure exerted against the wall by the vanes must not be too great, or excessive wear will result.

The amount of wear is a function of the third power of the sliding speed between the tip of the vane and the cylinder wall and, in actual practice, this determines the maximum rotational speed. To keep the centrifugal force down, high-speed motors, or rather their rotors, are long and slim and equipped with only three or four vanes.

The number of vanes in the motor depends on which application the motor is designed for.

Gears

The rotor of a vane motor turns at quite high speed.

The free speed of a motor is typically around 20 000 rpm. For most applications, these speeds are too high, and the rotor torque is also small. To convert a high speed and low torque to lower speed and higher torque, gears are used.

Chicago Pneumatics' vane motors are supplied with several types of gears: planetary gears and helical gears.

Lubrication free motors

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The traditional vane motors are lubricated by the compressed air to which a small amount of oil is added. Lubrication free motors are specifically designed to last longer and deliver better performances without lubrication. These motors are equipped with vanes made of a special low friction material and have permanently lubricated bearings. When long service life is the priority lubricated motors should be chosen because their vanes last longer.

  • a. Motor
  • b. Motor housing
  • c. Planetary gear
  • d. Output shaft

On which markets are air motors used?