Hydraulic and Pneumatic Systems and Their Maintenance
What this domain tests
Circuit components, valve functions, cylinder force and speed calculations, contamination control and safe fault-finding.
Core rules
- Pascal's law: pressure applied to a confined fluid is transmitted equally in all directions. Force F = p × A; cylinder speed v = Q/A.
- Pump types: gear (fixed displacement, cheap), vane (adjustable, balanced), piston (high pressure). A relief valve protects the system and sets maximum pressure; it is not a pressure regulator.
- Directional control valve notation: 4/3 means four ports, three positions. Centre conditions: closed centre holds load, tandem centre unloads the pump, open centre circulates oil freely.
- Flow control valves set actuator speed; pressure compensation keeps speed constant against load change. Accumulators store energy and damp shock – discharge before maintenance.
- Pneumatics: FRL unit = filter, regulator, lubricator. Air is compressible, so speed control is done with meter-out flow controls, and cylinders should never be used as structural stops.
- Contamination causes most hydraulic faults. Maintain filtration, keep reservoir breathers clean, and check oil temperature and viscosity.
Worked example
Bore 80 mm, rod 25 mm, pump pressure 50 bar, flow 20 L/min. Piston area A = π/4 × 0.08² = 5.027 × 10⁻³ m². Extending force = 5 MPa × 5.027 × 10⁻³ = 25.1 kN. Speed = (20/60 000) ÷ 5.027 × 10⁻³ = 0.066 m/s ≈ 66 mm/s. Retracting force uses the annulus area, which is smaller, so retraction is faster and weaker.
Traps that cost marks
- Using bore area for the retraction stroke instead of annulus area.
- Forgetting to convert bar to Pa (1 bar = 10⁵ Pa) or L/min to m³/s.
- Treating the relief valve as a speed control.