Air compressors, pneumatics and hydraulics are the machines and parts that move things using air or oil under pressure. A compressor squeezes air or another gas into a smaller space, which raises its pressure. Pneumatics uses compressed air to move parts. Hydraulics uses oil under pressure to move heavy loads.
Consider a bottling line in which a small cylinder pushes each cap onto a bottle. The factory has one compressor in a separate room. It compresses air to about 7 bar above the pressure of the air around us. So the air in the pipes is at about eight times the outside pressure. Pipes carry this air to the line. A valve, switched by the PLC, lets air into the cylinder. The air pushes a piston, the piston pushes the cap, and the air is let out so that the piston returns.
Compressed air is simple and clean, and good for fast, light movements. It is not strong enough for heavy work. A press that shapes sheet metal needs hydraulics. A pump pushes oil at pressures of hundreds of bar into a cylinder. Oil can hardly be squeezed into a smaller space, so the press moves with great force and steadily.
The air must be prepared before use. Compressing air also compresses the water vapour in it, which condenses into drops in the pipes. A dryer and a filter remove the water and the oil droplets, since water in the cylinders rusts them and spoils products. A pipe network that is not maintained develops leaks at its joints. Plants often find that a large part of the compressed air they make escapes through such leaks, and they hire a specialist to find them with an ultrasonic leak detector.
Making compressed air costs a lot of electricity, and leaks waste it. A variable frequency drive on the compressor motor saves power when the demand is low. Compressors are also used in HVAC systems, where a gas is compressed to remove heat from a building.
A compressor moves the parts of an assembly line. Because it wears, its readings go to asset performance management. The makers sell spare parts and repairs for years, under aftermarket services.