ABSTRACT
This chapter considers some of the other equipment to drive the control device used to manipulate the process variable to bring it into line with the desired value. Hydraulics are used to increase the driving force available with a smaller cylinder bore, and outwardly there is very little difference in design between the pneumatic and hydraulic cylinders; the differences, however, lie in the wall thickness, piston construction, and piston rod diameters and in the mounting arrangements of these two types of cylinders. There are three main types of actuators manufactured: the rack and pinion, the Scotch yoke, and the rolling diaphragm, each differentiated by the means used to transmit the drive to the control device. The name control drive is usually given to an assembly of a pneumatic power cylinder with positioner and linkage mechanism in a protective housing supplied as a complete piece of equipment.
