Pneumatics and hydraulics

Almost every machine uses compressed air: cylinders that push, grippers that pick up parts, vacuum cups that lift them. And where very large forces are needed, such as presses, press brakes or mobile machinery, there is hydraulics. A maintenance technician has to be able to read a pneumatic circuit, recognise each valve by its name and, above all, work without leaving trapped pressure where their hands are going.

Pneumatics and hydraulics: the differences

Where the air comes from

The compressor fills a receiver to about 7–10 bar. The air goes through a dryer (to remove the water that condenses) and reaches each machine through the distribution network. At the machine inlet there is always an air preparation unit (FRL):

• Lockable shut-off valve: to remove the air and fit a padlock while working on the machine.
• Filter: traps dirt and water. Its bowl is drained, manually or automatically.
• Regulator with gauge: sets the working pressure, usually about 6 bar.
• Lubricator: less and less common, because modern components come greased for life.
• Soft-start valve: lets the air in gradually when switched on, so cylinders do not slam into position.

Cylinders

• Single-acting: air pushes one way and a spring returns it. It uses less air, but has less stroke and force.
• Double-acting: air pushes both ways. The most common.

The theoretical force is pressure times piston area: F = p × A. At 6 bar (0.6 N/mm²) a 50 mm bore cylinder (1963 mm²) extends with about 1180 N. It pushes less when retracting, because the rod takes up area: with a 20 mm rod, about 990 N. In practice it is somewhat less because of friction.

To know where the cylinder is, magnetic sensors are clamped in the barrel slot (they detect the magnet in the piston), or mechanical limit switches are used.

Directional control valves

They are named with two numbers: ports / positions. A 5/2 valve has five ports and two positions.

• 3/2: for single-acting cylinders, vacuum cups or switching air on and off.
• 5/2: for double-acting cylinders. The most common.
• 5/3: three positions; in the centre, depending on the type, both chambers are blocked, exhausted or pressurised. It is used to stop the cylinder in mid-stroke.

Ports are numbered (ISO 5599): 1 supply, 2 and 4 working ports, 3 and 5 exhausts, and 12 and 14 pilots (14 connects 1 to 4). Older North American drawings use letters instead: P, A, B, EA and EB.

Depending on how it returns, a valve is monostable (a spring returns it to rest when the signal is removed) or bistable (it stays in its last position: it has memory). Valves are operated by hand, by a roller, by air (pilot) or by a solenoid, which is the usual choice when a PLC is in control.

Speed control

Cylinder speed is set with one-way flow control valves, and the right way is to restrict the air coming out (meter-out). The cylinder then moves pushed on one side and held back on the other, smoothly. If you restrict the inlet (meter-in), the piston moves in jerks.

Sequences and the displacement-step diagram

A sequence is written with letters and signs: A+ B+ A− B− means "A extends, B extends, A retracts, B retracts". The displacement-step diagram draws the position of each cylinder step by step and is the clearest way to see which sensor triggers each movement. Some sequences, such as A+ B+ B− A−, have overlapping signals (an order is still active when the opposite one should take over) and need memories or a PLC to work. You can try a sequence in the pneumatics simulator.

How much air it uses

A cylinder’s consumption per stroke is its volume times the absolute pressure ratio: Q ≈ A × stroke × (p + 1), with p in bar. A 50 mm bore, 200 mm stroke cylinder at 6 bar uses about 2.7 litres of free air each time it extends. Leaks in the network often waste more than many machines put together: finding and fixing them is one of the biggest savings.

Hydraulics: the basics

• Pump: gear, vane or piston. It delivers flow; pressure builds when something resists it.
• Pressure relief valve: mandatory. If pressure rises above the setting, it returns oil to the tank and protects the system.
• 4/3 directional valves: the most used. Their centre position decides what happens at rest: closed centre (the cylinder is locked), tandem centre (the pump unloads to tank at no pressure) or open centre.
• Accumulators: store oil under pressure with gas. They still hold pressure with the pump stopped.
• Oil: works best between about 40 and 60 °C, and contamination is the main cause of failures: filters are essential maintenance.

Safety

• Stopping is not depressurising. Before loosening a fitting or reaching in: close and lock the shut-off valve, bleed the air or oil and check the gauge reads zero. Lockout procedures (OSHA 1910.147 in the US) require stored energy to be released.
• Pressure gets trapped in cylinders with closed-centre valves, pilot-operated check valves and accumulators. A part can move on its own when a hose is loosened.
• A jet of oil under pressure goes through the skin. An oil injection injury is a medical emergency even if it looks like a pinprick. Never look for a leak by running your hand along a hose.
• Compressed air is not for cleaning clothes or skin. It can force air into the blood or the eyes.

Typical mistakes

• Loosening a hose with the system pressurised because "the machine is stopped".
• Controlling speed by restricting the inlet, so the cylinder moves in jerks.
• Swapping a monostable valve for a bistable one (or the other way round) without thinking about power loss.
• Working out the retract force with the full piston area.
• Turning up the regulator to "fix" a slow cylinder, instead of looking for the leak or the closed flow control.

Summary

• Pneumatics: air at about 6 bar, fast and clean. Hydraulics: oil at 100–300 bar, force and precision.
• A valve is named by ports/positions; a 5/2 drives a double-acting cylinder.
• Monostable returns to rest; bistable remembers its last position.
• F = p × A; when retracting, the rod area does not count.
• Speed is controlled by restricting the exhaust (meter-out).
• Before working: shut off, lock out, bleed and check the pressure is zero.