Rotary output devices such as air motors with built-in cycling valves and rotary actuators that make only a fraction of a turn are available to perform many functions. Because compressed air is the driving force, these devices are explosion-proof and can operate in dirty or wet atmospheres without the problems posed by electrical equipment. Carefully applied air-operated devices can be an improvement in many situations.

These and other air-operated components are explained and applied in the following chapters.

Typical hydraulic circuit

Figure 5-2 provides a pictorial representation and a schematic drawing of a typical hydraulic circuit. Notice that the hydraulic power unit is dedicated to this machine. Unlike pneumatic circuits, most hydraulic systems have a power unit that only operates one machine. (As mentioned before, some new installations are using a central hydraulic power source with piping throughout the plant to carry pressurized and return fluid.)

5-2. Schematic drawing of an air circuit with air-logic controls, and physical drawing of the components in the circuit.

Why a schematic drawing?

Schematic drawings make it possible to show circuit functions when using components from different manufacturers. A 4-way valve or other part from a different supplier may bear little resemblance to one from other suppliers. Using actual cutaways of a valve to show how a machine operates would be fine for one circuit using one supplier's valves. Nevertheless, another machine with different parts would have a completely different looking drawing. A person trying to work on these different machines would have to know each brand and how they affect operations. This means designing and trouble shooting every circuit would require special different knowledge. Using schematic symbols requires learning only one set of information for any component.

Schematic symbols also give more information than a picture of the part. It may be hard to impossible to tell if a 4-way valve is 3-position by looking at a pictorial representation while its symbol makes all features immediately clear. Another feature is by using ISO symbols the drawing can be read by persons from different languages. Any notes or the material list may be in a language foreign to you but following and understanding circuit function should not be a problem.