Pneumatic and Hydraulic Systems: Components, Circuits and Applications
Fluid power questions in the PAU combine a force or speed calculation with the reading of an ISO 1219 circuit symbol. You must be able to convert bar to pascal instantly, apply F = p·A to both strokes of a cylinder, and follow the air or oil path through a valve.
Physical basis
- Pascal's law: pressure applied to a confined fluid is transmitted equally in all directions, so F₁/A₁ = F₂/A₂ in a hydraulic press.
- Force: F = p·A. Speed: v = Q/A, with continuity A₁·v₁ = A₂·v₂.
- Units: 1 bar = 10⁵ Pa = 0.1 MPa ≈ 1 kgf/cm²; 1 L/min = 1.667×10⁻⁵ m³/s.
- Hydraulics uses incompressible oil at high pressure for large forces; pneumatics uses compressible air at 6–8 bar for fast, clean, low-cost motion.
Components
- Generation: compressor with receiver and dryer, or hydraulic pump with tank.
- Conditioning: FRL unit - filter, pressure regulator, lubricator - placed upstream of the circuit.
- Valves: relief valve limits maximum pressure; directional valves 3/2 (three ports, two positions) or 5/2 command double-acting cylinders; flow-control valves set speed; check valves enforce one direction.
- Actuators: single-acting cylinder (spring return, one port) and double-acting cylinder (two ports, both strokes powered).
- Accessories: accumulators store hydraulic energy and damp pulses; silencers and quick exhausts speed up air cylinders.
Speed control
- Meter-in controls the flow entering the cylinder; meter-out controls the exhaust and gives steadier movement against a resisting load. Bleed-off diverts part of the flow.
Worked example
A double-acting cylinder has a bore of 50 mm, a rod of 20 mm and works at 6 bar. Extension area A = π·50²/4 = 1 963 mm², so Fext = 0.6 N/mm² × 1 963 mm² = 1 178 N. Retraction area A = π(50² − 20²)/4 = 1 649 mm², so Fret = 0.6 × 1 649 = 990 N. With an oil flow of 10 L/min = 10 000 cm³/min, extension speed v = Q/A = 10 000/19.63 cm² ≈ 509 cm/min ≈ 5.1 m/min.
Traps that cost marks
- Entering 6 bar into F = p·A without converting to N/mm² (0.6) or Pa (6×10⁵).
- Using the full bore area for the retraction stroke and forgetting the rod.
- Mixing gauge pressure with absolute pressure.
- Reading a 5/2 valve as a 3/2 valve, or reversing which port is the pressure supply in the symbol.