Pneumatics and hydraulics: components, symbols, and circuit behavior
This domain tests reading fluid-power symbols and predicting actuator force, speed, and control behaviour. Expect compressors, FRL units, directional valves, flow controls, cylinders, accumulators, and pressure relief.
Core rules
- Symbols show function, not construction. A 5/2 valve has five ports and two positions; boxes show flow paths, and the spring or solenoid symbol shows the normal position.
- An FRL unit filters, regulates, and lubricates air; the regulator sets downstream pressure, and the gauge reads that pressure.
- Cylinder extend force is F = p × A_bore. Retract force uses the annulus area: A = π/4(D² − d²), where d is rod diameter.
- Meter-in and meter-out flow controls set speed; meter-out is common on pneumatic cylinders for better load control.
- Hydraulic systems need pressure relief, filtration, and safe dissipation of stored energy; accumulators can retain pressure after pump shutdown.
Worked example
A cylinder has a 50 mm bore, a 20 mm rod, and 6 bar supply. Extend area = π/4 × 0.050² = 0.001963 m²; extend force = 600,000 × 0.001963 = 1,178 N. Retract area = π/4 × (0.050² − 0.020²) = 0.001649 m²; retract force = 600,000 × 0.001649 = 989 N. The reasoning: gauge pressure is used in pascals, and the rod reduces the retract area, so retract force is lower.
Traps
Using bore area for retract force; mixing gauge and absolute pressure; ignoring pressure loss in lines; assuming a valve centre condition blocks all ports; and treating air as incompressible. On hydraulics, failing to relieve or bleed stored pressure is both a calculation error and a safety failure.