The most common are pneumatically-operated actuators. These consist of a cylindrical tu.be with pistons, which transmit the linear piston movement, according to the principle of ''rack-and-pinion'' or ''scotch-yoke'' , to a rotary movement of the sl1aft (Fig. 35). By adding integrated spring units (single-acting actuators), it is possible to drive the actuator to a specific safety position in the case of air loss or control signal loss. Due to the torque characteristics of the butterfly vaIve, it is more efficient to use scotchyoke operated actuators. This combination can be dimensionally smaller, so the air consumption is 20% to 30% lower than rack-and-pinion actuators.

The benefits include, if required, short actuating times, the long lifetime (more than 500,000 cycles for actuators with torques of <1,000 Nm) and the low purchase costs compared to electrical actuators. A disadvantage are the higher operating costs caused by the compressed air consumption.

The functional enhancement to pneumatic actuators with pneumatic auxiliary valves, various position feedback devices for positioning monitoring and ·valve positioners for flow control, etc. Makes it possible to adapt these devices optimally to suit the different operating conditions.