Hydraulic Systems: Pumps, Valves, Cylinders, and Circuits
What this domain tests
Pump types and controls, pressure and flow diagnosis, load-holding valves, cylinder and seal failures, hose and fitting selection, and contamination control on mobile equipment.
Core rules
- Pumps: gear (fixed displacement, simple), vane (fixed or variable), and axial piston (variable, with pressure compensator or load-sensing control). A compensator holds pressure by destroking, so a deadheaded variable pump should fall to low flow, not stay at full stroke.
- Relief valve: sets maximum system pressure; it should not pass oil in normal work. A relief stuck open gives heat and slow functions; stuck closed gives blown hoses and stalled engines.
- Load holding: counterbalance valves and pilot-operated check valves hold a load when the spool is centred. Pilot-operated checks need pilot pressure to open - testing must account for that.
- Cylinders: drift with the engine off and the valve neutral points to the load-holding valve; drift with the engine running and oil warm points to internal bypass past the piston seal. A bent rod or scored barrel scores the seal in turn.
- Contamination: ISO cleanliness codes describe particle counts. Most failures start with dirt, water, or heat. Sample oil from a turbulent, mid-tank point, not the bottom drain.
- Hoses: match pressure rating, temperature, and bend radius. A hose with a pinhole is replaced, never repaired at the crimp.
Worked example
A boom settles overnight with the engine off. Isolate the cylinder: plug the line ports at the cylinder and see if the rod still moves. If it holds, the fault is in the valve or counterbalance; if it still drifts, the piston seal is bypassing. This isolates the circuit half by half instead of changing parts.
Traps
- Adjusting relief pressure to make a slow function faster - it only raises system heating and stress.
- Diagnosing pump failure when the suction strainer is plugged (cavitation sounds like marbles).
- Ignoring aeration: foamy oil from a suction leak causes noisy, jerky operation and oxidation.