A pneumatic actuator converts energy, in the form of compressed air, into mechanical motion. They can be linear or rotary. In a linear pneumatic actuator, the air drives a piston in a shaft. Pneumatic rotary actuators convert compressed air pressure into an oscillating rotary motion. In HVAC, rotary actuators can be used on VAV terminals, dampers and valves. For more on HVAC and actuators, see radiator actuators and valves and motorized fire and smoke dampers.