Hydraulic Systems
Core concepts
Understand open-center vs closed-center (load-sensing) systems, pumps (gear, piston, variable displacement), relief valves, flow dividers, accumulators, and remote valves. Know cylinder calculations: force = pressure x area, and extension vs retraction force differ because rod area reduces effective piston area. Understand flow rate vs speed: cylinder speed depends on flow and area, not pressure. Know contamination control, filtration, oil cleanliness, and the danger of hydraulic injection.
Traps
Questions often assume pressure determines cylinder speed; it is flow. Another trap is using full piston area for retraction force when the rod reduces area. Closed-center systems idle at low flow and respond to demand; do not diagnose low flow as a failed pump before checking the load-sensing signal or standby pressure. Air entrainment causes spongy operation and overheating.
Rule of thumb
Force = pressure x area; speed = flow / area. Retraction force uses (piston area - rod area). A 100 mm bore at 200 bar extends with roughly 15.7 kN per 100 bar of pressure times area; always convert units consistently.
Worked example
Bore 100 mm (area 78.5 cm²), rod 50 mm (rod area 19.6 cm²), pressure 200 bar (20 MPa). Extension force = 20e6 x 78.5e-4 = 157 kN. Retraction area = 78.5 - 19.6 = 58.9 cm², force = 20e6 x 58.9e-4 = 118 kN. Retraction is weaker, so size cylinders for the retract load.