Hydraulic Systems
Hydraulic Systems tests your grasp of pressure, flow, and contamination control on equipment that uses hydraulics for steering, lifting, transmission drive, and implement control. Expect Pascal's law, pump types, relief and directional valves, load-sensing systems, cylinder force and speed calculations, accumulators, hose ratings, filtration, and systematic pressure and flow testing.
Pressure is load; flow is speed
- Pressure is created by resistance to flow. The pump does not make pressure; the load does. Flow, set by pump displacement and engine speed, determines actuator speed.
- Cylinder force equals pressure times area; area equals pi times radius squared. The rod reduces the annulus area, so retract force is lower than extend force at the same pressure.
- The main relief valve protects the system and sets maximum pressure. A load-sensing or closed-centre system holds low standby pressure until a valve signals demand; pressure rises only to what the load requires.
- Pump types: external and internal gear, vane, and axial piston. Piston pumps tolerate higher pressure and can be variable displacement for hydrostatic drives.
- Contamination causes most hydraulic failures. Keep oil clean, use the specified micron-rated filters, and cap and plug every opened line and coupler.
- Hose working pressure is far below burst pressure; assemblies are rated at roughly a 4:1 safety factor. Match pressure rating, temperature, and bend radius, and relieve pressure before coupling or uncoupling.
- Accumulators store pressurized nitrogen and hold deadly stored energy; discharge them before any service.
Worked example
A lift cylinder has a 100 mm bore, a 50 mm rod, and sees 200 bar. Extend area is pi x 0.05 squared = 0.00785 square metres, so extend force is 200 bar (20 MPa) x 0.00785 = about 157 kN. Annulus area is 0.00785 minus 0.00196 = 0.00589 square metres, giving about 118 kN of retract force. If the pump supplies 40 L/min, extend speed is 40,000 cubic centimetres per minute divided by 78.5 square centimetres, about 5.1 metres per minute. The reasoning: force comes from area, speed comes from flow, and the rod area explains why the cylinder lifts more than it pulls.
Traps that cost marks
- Raising relief valve pressure to fix a slow cylinder when the real fault is a worn pump, restricted filter, or internal leakage.
- Believing pressure rises with engine speed; flow does, pressure follows load.
- Mixing hydraulic oils or using engine oil in a system that specifies a different grade.
- Cracking a fitting without relieving pressure at the couplers or accumulator first.