Pneumatics and Hydraulics
Pneumatics and Hydraulics
This domain tests fluid-power principles, component symbols and circuit reasoning: force and speed of cylinders, pressure and flow relationships, valves, compressors, FRL units and contamination control. Numeric force and speed questions are frequent, so keep the units consistent.
Core rules
- Pascal's law: pressure applied to a confined fluid is transmitted equally in all directions. Force F = p × A. Use 1 bar = 0.1 MPa = 100 kPa = 0.1 N/mm².
- Cylinder extending force uses the full bore area; retracting force uses the annulus area (bore area minus rod area).
- Speed v = Q/A, so flow controls speed and pressure controls force. A meter-in circuit controls extension speed; meter-out controls a runaway load.
- A relief valve limits system pressure and protects the pump; a pressure-reducing valve sets a lower branch pressure; a check valve allows one-way flow; a 5/2 valve has five ports and two positions.
- Air is compressible and gives fast, clean, cushioned action but poor positional stiffness; hydraulic oil is incompressible and gives high force with precise positioning. Viscosity grade and filtration matter: most hydraulic faults are contamination faults.
Worked example
A cylinder of 80 mm bore and 25 mm rod operates at 6 bar. Bore area = π/4 × 80² = 5026 mm²; force out = 5026 × 0.6 = 3016 N, about 3.0 kN. Rod area = 491 mm²; annulus = 4535 mm²; force in = 4535 × 0.6 = 2721 N. Extending at 100 mm/s needs Q = 5026 × 100 = 502,600 mm³/s, about 30 L/min.
Traps
Using bore area for retraction; confusing gauge pressure with absolute pressure; forgetting the rod reduces return force; assuming air gives rigid positioning.