Pocket guide to Air motors

POCKET GUIDE TO AIR MOTORS

POCKET GUIDE TO AIR MOTORS

1. INTRODUCING THE AIR MOTOR The air motor is one of the toughest and most versatile power units available to the design engineer. The features and characteristics of the air motor make it the natural choice of power for industrial applications, present and future. Compact and lightweight An air motor weighs only 1/4 as much as an electric motor with the same output and occupies only 1/6 of the space. Air motors develop far more power in relation to their size and weight than most other motor types. Torque increases with load The output of an air motor is relatively constant within a wide speed...

Undamaged by overloads Air motors can be stalled indefinitely without overheating or sustaining any other type of damage. They can also be started and stopped repeatedly without limit. Ideal in hazardous and hostile environments Since air motors do not generate sparks they are ideal in areas where there are explosion and/or fire hazards. Moreover, their rugged design and construction make them ideal in saltladen and other corrosive atmospheres. Easily reversed Air motors work efficiently in either direction. They are easily reversed using a directional valve. Simple to install Air motors can...

There are several types of air motor. The most commonly used types are vane, piston and turbine motors. This pocket guide deals with vane motors only. Vane motors are produced with power ratings up to approximately 5 kW. - 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. - As a result of the...

Working principle 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...

LZL motors These motors are reversible but have only two air ports. One is the inlet and the other the outlet. LZL motors are designed to obtain excellent starting and low speed characteristics. This is achieved by 6 vanes that are pushed out to the cylinder wall by rods under the vanes. To support this function LZL motors also have compressed air fed under the vanes. Rotor speed LZL motors represent another type of vane motor. 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...

Motor Planetary gear Planetary gear (1), helical gear (2). Motor housing Output shaft Gears The rotor of a vane motor turns at quite high speed. The free speed of an LZB motor is typically around 20 000 rpm. LZL motors have free speeds from around 6 000 to 9 300 rpm. For most applications these speeds are too high and the rotor torque is also rather small. To convert a high speed and low torque to lower speed and higher torque, gears are used. Atlas Copco’s vane motors are supplied with different types of gears: Planetary gears and helical gears. (See appendix.) Lubrication free motors The traditional...

3. THE PERFORMANCE OF AN AIR MOTOR The performance of an air motor is dependent on the inlet pressure. 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 of 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...

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.” Torque [Nm] Power [kW] Max output Output formula P = (π x Mxn) / 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 performance curve for an air motor operating at a constant air pressure. Speed [rpm] The working point When selecting an air motor for an...

Starting torque The starting torque is the torque that a motor gives with blocked shaft when you feed full air pressure into it. It should be noted that all vane air motors produce a variable starting torque due to the position of the vanes in the motor. The lowest starting torque value is called the minimum starting torque and can be considered as a guaranteed value at start up. The variation differs between motor types and must be checked on an individual basis. It is notable that the torque variation is greater for reversible motors than for non-reversible motors and therefore the minimum...

4. THE USE OF GEAR UNITS Air motors operate at high speeds and although they can be controlled over a wide speed range, the output c haracteristics are not always suitable for the application. To achieve the required output an appropriate gear unit can be selected. The planetary and helical gear units used by Atlas Copco have a high level of efficiency that can be assumed to be 100%. Evidently, while the torque/speed relationship undergoes a considerable change, the power output remains virtually unchanged. The torque is increased and the speed is reduced proportional to the gear ratio. Speed...