Fluid Power: Hydraulics and Pneumatics
What the exam tests
Pascal's law applied to cylinder force and speed, identification of circuit components from ISO 1219 symbols, and choosing between hydraulic and pneumatic drives.
Core rules
- Pascal's law: pressure applied to a confined fluid is transmitted equally in all directions. Force F = p × A.
- Hydraulics uses oil at roughly 20–200 bar for high force with accurate speed control; pneumatics uses air at 4–8 bar, is fast, clean and cheap, but cannot hold a load rigidly under a varying load.
- Pumps: gear (fixed displacement, cheap), vane (balanced, can be variable), piston (high pressure, high efficiency).
- A relief valve protects the circuit and is set above working pressure; a pressure-reducing valve sets a lower branch pressure; a flow control valve sets actuator speed.
- Directional control valves are named by ports and positions: a 4/2 valve has four ports and two positions; a 5/2 valve has separate exhausts for each side.
- Cylinder speed v = Q/A, and the retract speed is higher because the rod reduces the effective area.
- The FRL unit filters, regulates and lubricates air; an accumulator stores hydraulic energy and damps shock.
Worked example
A 63 mm bore cylinder at 50 bar extends against a load. A = π/4 × 63² = 3117 mm² and p = 0.5 N/mm², so F = 0.5 × 3117 ≈ 1558 N. With a 30 mm rod the retract area is 3117 − 707 = 2410 mm², giving about 1205 N - the reason a press is rated on its push stroke.
Traps
- Mixing bar with mm² without converting (1 bar = 0.1 N/mm²).
- Ignoring rod area on the retract stroke.
- Expecting a relief valve to control speed; it only limits pressure.