Hydraulics and Pneumatics
What the exam tests
This domain tests pressure, flow, force, speed, valves, filtration, air treatment, and stored-energy safety in hydraulic and pneumatic equipment. Expect calculations on cylinders and practical questions on contamination, leakage, and safe depressurisation.
Core rules
- Pascal's law gives force F = p × A. In a cylinder, extension uses the full bore area; retraction uses the annulus area, bore area minus rod area.
- Flow determines speed: v = Q / A. Pressure is set by load and resistance, not by pump displacement alone; a relief valve limits maximum pressure.
- Hydraulic oil cleanliness is critical. Most failures start with contamination, overheating, or air ingress. Filters must meet the specified micron rating and be changed without introducing dirt.
- Pneumatic systems need dry, filtered air and correct lubricator operation where fitted. Water and oil carry-over damage seals and spool valves.
- Before opening a circuit, isolate, lock, release stored pressure, and account for gravity-loaded loads and accumulators.
Worked example
A cylinder has an 80 mm bore and a 45 mm rod. Pressure is 120 bar = 12 N/mm². Bore area = π × 80² / 4 = 5027 mm², so extension force = 12 × 5027 = 60.3 kN. Annulus area = π × (80² − 45²) / 4 = 3436 mm², so retraction force = 12 × 3436 = 41.2 kN. At 20 L/min, extension speed = 333333 mm³/s / 5027 mm² = 66.3 mm/s.
Traps
Using bore area for retraction; confusing gauge pressure with absolute pressure; treating a closed valve as zero energy when an accumulator is charged; and adding oil without checking the specified grade and cleanliness.