Air Actuators Selection Factors
Air actuators have a minimum height Hmin (compressed state) and a maximum height Hmax (no end stop).
The maximum stroke attainable with an air actuator is the difference between the maximum height and the minimum height. This stroke length or any part thereof can be used.
Greater stroke lengths can be achieved by using a scissors design or by stacking two or more actuators (figure right). Mounting plates must be fitted between each pair of actuators used in a multiple unit design. The plates must be equipped with lateral guides.
In contrast to conventional pneumatic cylinders, the effective area of an air actuator Aw (effective diameter dw) is not the same as its outer diameter. In addition, the effective diameter can change with the operating height. Therefore the force exerted by an air actuator decreases progressively with stroke, and this decrease varies according to the size and type of the unit (figure below).
At the design stage, check the force/stroke data to see if the actuator exerts the force required at the top end of its stroke and determine the pressure required for this.
Air actuators function like single-acting cylinders; a return force is needed to restore the actuator to its compressed height.
Convolution air springs can function when completely depressurized. Sleeve type and rolling lobe actuators require a minimum pressure to permit the bellows to roll down over the piston.