Pneumatics and hydraulics: circuits, components and symbols
The exam expects you to size a cylinder, read and draw ISO 1219 symbols, and explain how speed and force are controlled in a fluid-power circuit.
Fundamentals
- Pressure p = F/A. Useful conversions: 1 bar = 10⁵ Pa = 0.1 N/mm²; 1 MPa = 10 bar.
- Flow continuity Q = v·A; fluid power P = p·Q (Pa × m³/s = W).
- Pneumatics uses compressible air at 6–10 bar, is fast and clean but imprecise; hydraulics uses incompressible oil at 100–300 bar for high forces and needs a pressure relief valve for safety.
- Boyle's law for isothermal air: p1·V1 = p2·V2.
Components and symbols
- Compressor, cooler/dryer, filter-regulator-lubricator unit, accumulator (hydraulics).
- Valves are named positions/ports: 3/2 normally closed for a single-acting cylinder, 5/2 for a double-acting cylinder, 2/2 for on-off, plus non-return, throttle, quick-exhaust and silencer.
- In ISO 1219 symbols the squares are positions: box connections show ports, arrows show the flow path, springs return the spool.
Cylinders
- Extension: F = p·A_piston·η. Retraction: F = p·(A_piston − A_rod)·η - the rod reduces the effective area.
- Speed control by throttling: meter-out for resistive (pulling) loads, meter-in for pushing loads where stick-slip must be avoided.
Worked example
Bore D = 50 mm, rod d = 20 mm, p = 6 bar = 0.6 N/mm², η = 0.9. A_piston = π·25² = 1963.5 mm², so F_ext = 0.6 × 1963.5 × 0.9 = 1060 N. Annular area = 1963.5 − π·10² = 1649.3 mm², so F_ret = 0.6 × 1649.3 × 0.9 = 891 N. With Q = 10 L/min during extension (166 667 mm³/s), v = Q/A = 166 667/1963.5 ≈ 85 mm/s.
Traps that cost marks
- Using the full bore area for retraction.
- Mixing L/min with m³/s in P = p·Q (10 L/min = 1.667 × 10⁻⁴ m³/s).
- Reading a valve as 2/5 instead of 5/2.
- Confusing gauge and absolute pressure in Boyle's law.
- Forgetting efficiency in force calculations.