Pneumatic and hydraulic systems: components, schematics and circuit design
What the exam tests
Symbol recognition (ISO 1219), circuit design for cylinders and motors, pressure/flow control, force and speed calculations, and safety.
Fundamentals
Pascal's principle: pressure is transmitted equally, p = F/A. Hydraulic oil is nearly incompressible, so cylinders are stiff and precise; pneumatics uses compressed air, is compressible, fast and clean but less precise. Flow Q = A·v, so extension speed v = Q/A. Force F = p·A (effective area; subtract rod area on retraction). Power = p·Q.
Components and symbols
- Compressor, air receiver, filter–regulator–lubricator (FRL), dryer.
- Directional valves: 2/2, 3/2, 4/2, 5/2; first number = ports, second = positions. A 5/2 valve has two exhaust ports and two actuator lines.
- Pressure relief valve protects the circuit; pressure-reducing valve sets branch pressure; flow-control valve sets speed; check valve allows one direction.
- Accumulator stores energy and damps pulses; intensifier raises pressure.
- Cylinders: single-acting (spring return), double-acting; cushioning at end of stroke.
- Symbols are drawn in the unactuated state; arrows show flow paths.
Circuit design
Typical sequence: a double-acting cylinder extends when a 3/2 or 5/2 valve is actuated, and retracts on spring or pilot return. Meter-in or meter-out flow control sets speed. For two cylinders, use sequence valves or a PLC with solenoid valves. Include a relief valve on the pump and a safety interlock.
Worked example
A hydraulic cylinder has bore d = 80 mm and rod d_r = 40 mm. Pump pressure p = 100 bar = 10 MPa. Extension force = p·(πd²/4) = 10 N/mm² × 5027 mm² = 50.3 kN. Retraction force = p·π(d² − d_r²)/4 = 10 × (5027 − 1257) = 37.7 kN. With flow Q = 20 L/min = 333,333 mm³/s, extension speed v = Q/A = 333,333/5027 = 66 mm/s.
Traps
- Using bore area for retraction.
- Forgetting to convert bar to Pa or N/mm².
- Drawing valve symbols in the actuated position.
- Omitting the relief valve or misplacing the flow control.
- Ignoring air compressibility in positioning calculations.