Pneumatics

Why it matters

Compressed air runs grippers, clamps, sorters, packaging lines and air tools in almost every plant. Pneumatic systems are cleaner and simpler than hydraulics, but they leak, they need dry, clean air, and they store energy too. A mechanic who can trace a pneumatic circuit finds the stuck valve or the cut tube quickly.

Pneumatics vs hydraulics

Air is compressible; oil is nearly not. That difference drives everything:

• Pneumatic systems run at lower pressure, typically around 80 to 100 psi (about 5.5 to 7 bar), so forces are smaller.
• Air cushions motion, so cylinders are fast but less precise, and they can bounce or creep under load.
• Exhaust air vents to atmosphere, so there is no return line to a tank.
• Leaks waste energy (compressed air is expensive to make) but do not make a mess.

Force still follows force = pressure × area. A 2 in bore cylinder (area 3.14 in²) at 90 psi pushes about 280 lbf.

The supply side

1. Compressor makes the air, usually a rotary screw or piston machine.
2. Receiver tank stores air and smooths demand.
3. Dryer removes moisture. Compressed air holds a lot of water, which condenses in pipes and ruins valves and tools.
4. Distribution piping with drops taken from the top of the main so water stays in the main.
5. FRL unit at each machine: Filter (removes water and particles, with a drain), Regulator (sets the machine's working pressure, with a gauge) and Lubricator (adds a fine oil mist, though many modern components are pre-lubricated and must not be lubricated).
6. A lockout valve (shut-off and dump valve) that isolates and exhausts the machine's air.

Actuators

• Single-acting cylinder: air extends it, a spring returns it (or the reverse).
• Double-acting cylinder: air drives it both ways through two ports.
• Rodless cylinders, rotary actuators and grippers for other motions.
• Vacuum generators (venturi) and suction cups for picking parts.

Cylinders often carry reed switches or magnetic sensors that tell the PLC whether the cylinder is extended or retracted.

Valves and symbols

Pneumatic symbols follow the same ISO 1219 style as hydraulics:

• Each square on a valve is one position. A 5/2 valve has 5 ports and 2 positions, the standard way to drive a double-acting cylinder. A 3/2 valve drives a single-acting cylinder.
• Port numbers: 1 = supply, 2 and 4 = outlets to the actuator, 3 and 5 = exhausts. Pilot or control ports are 12 and 14.
• An exhaust is a triangle (open) at the port, often with a silencer.
• The ends of the valve show how it is operated: solenoid, spring return, push button, roller lever or air pilot.
• Flow control valves (a throttle with a bypass check) set cylinder speed. They are usually fitted to meter out, restricting exhaust air, which gives smoother motion.
• Quick exhaust valves, shuttle valves (OR) and two-pressure valves (AND) build logic in pure-air circuits.

Tracing a circuit: worked example

A pick-and-place arm has a double-acting cylinder driven by a 5/2 single-solenoid, spring-return valve, with meter-out flow controls on both cylinder ports.

1. Start at the FRL: regulator at 80 psi. Air goes to port 1 of the valve.
2. Solenoid off (spring position): port 1 connects to port 2, which feeds the rod end. The cylinder is retracted. The cap end exhausts from port 4 to port 5.
3. Solenoid on: port 1 connects to port 4, which feeds the cap end. The cylinder extends. Rod-end air exhausts through its flow control, then from 2 to 3.
4. The flow control on the rod-end line therefore sets extend speed; the one on the cap-end line sets retract speed.

If the cylinder extends but will not retract, suspect a stuck valve spool, a failed spring, a solenoid that is still energized (check the PLC output), or a kinked rod-end line.

Troubleshooting tips

• Listen for leaks with the machine idle; use soapy water or an ultrasonic leak detector.
• Check the regulator gauge first. Low supply pressure causes many "weak" or "slow" faults.
• Drain filter bowls; water in the lines causes sticky valves.
• Most solenoid valves have a manual override button, but pressing it moves the machine. Use it only when the area is clear and your procedure allows it.

Common mistakes

• Using compressed air to blow off clothes or skin. Air can enter the body through the skin and cause serious injury. OSHA limits air used for cleaning to under 30 psi with chip guarding and PPE.
• Disconnecting a line without dumping pressure. Hoses whip and cylinders move.
• Forgetting that a cylinder can still move after the supply is shut off if air is trapped between valve and cylinder.

Safety

Compressed air is stored energy: lockout includes closing and locking the dump valve and verifying zero pressure at the gauge, plus blocking anything that could fall. Get your employer's training before working on air systems. OSHA's compressed gas and equipment page: https://www.osha.gov/compressed-gas-equipment (Checked on 2026-10-23.)

Recap

• Air is compressible: lower force, faster, cushioned motion, exhaust to atmosphere.
• Supply: compressor, receiver, dryer, FRL, lockout dump valve.
• 5/2 valve ports: 1 supply, 2 and 4 outlets, 3 and 5 exhausts.
• Meter-out flow controls set cylinder speed.

Try it

Open the Do tab and take the quiz on tracing pneumatic circuits.