Pneumatics and hydraulics: components, circuit design and simulation
The exam asks for cylinder forces and speeds, correct interpretation of ISO 1219 symbols and the design or reading of a circuit with directional, flow and pressure valves. Hydraulics and pneumatics share the same formulas but differ in pressure level, compressibility and achievable force.
Magnitudes and components
- p = F/A (pascal); 1 bar = 10⁵ Pa; 1 MPa = 10 bar.
- Flow Q = A·v = V/t; continuity in series circuits.
- Supply: compressor with filter-regulator-lubricator (pneumatics); pump, tank, filter, pressure relief valve and gauge (hydraulics).
- Cylinders single or double acting; directional valves 2/2, 3/2, 4/2, 5/2; flow-control and non-return valves; pressure-limiting and pressure-reducing valves.
- Actuation: manual, lever, cam, spring, solenoid, pneumatic or hydraulic pilot.
Circuit design
- Single-acting cylinder: pressure extends, spring returns, normally with a 3/2 valve.
- Double-acting cylinder: 4/2 or 5/2 valve, one line per chamber.
- Speed control by throttling at the inlet (meter-in) or at the outlet (meter-out); meter-out is more stable with pulling loads.
- Hydraulics works at 100–300 bar with incompressible oil: high force, precise; pneumatics at 6–8 bar, fast and clean but compressible and with lower efficiency.
Worked example
A double-acting cylinder with 50 mm bore and 20 mm rod at p = 6 bar. Extension: A = π·0.025² = 1.963·10⁻³ m², so F = 6·10⁵ · 1.963·10⁻³ = 1178 N. Retraction uses the annulus A' = π(0.025² − 0.010²) = 1.649·10⁻³ m², giving F = 989 N. With Q = 10 L/min = 1.667·10⁻⁴ m³/s entering the full bore, v = Q/A = 0.085 m/s ≈ 8.5 cm/s.
Marks lost
- Using the diameter instead of the radius.
- Forgetting to subtract the rod area on retraction.
- Converting bar to pascal incorrectly.
- Mixing up 3/2 and 5/2 valves, or misplacing the connections.
- Treating air as incompressible and ignoring efficiency losses.