Hydraulics
Hydraulics on the CFPS exam tests positive-displacement pumps, pressure and flow controls, actuators, accumulators, filtration, and troubleshooting. You must predict circuit behavior from component settings and calculate force, speed, flow, and heat.
Core rules
- A positive-displacement pump delivers nearly constant flow per revolution. Theoretical flow = displacement × rpm; actual flow = theoretical × volumetric efficiency.
- Pump pressure rises to whatever the load requires, limited by the relief valve. The relief valve is a safety/limit device, not a normal flow regulator; a pressure-compensated pump reduces flow at its set pressure.
- Directional valve center conditions matter: closed, open, tandem, and float. Flow controls include meter-in, meter-out, and bleed-off; meter-out is preferred for overrunning loads because it restricts exhaust and prevents runaway.
- Cylinder force = p × effective area; speed = Q/effective area. Accumulators store energy; precharge is set with the oil side empty.
- Pressure drop × flow generates heat. Cavitation is vapor formation; aeration is entrained air. Filters and coolers must be sized for the circuit.
Worked example
A pump of 2.5 in³/rev runs at 1800 rpm with 92% volumetric efficiency. Theoretical flow = 2.5 × 1800 = 4500 in³/min; 4500/231 = 19.5 gpm; actual = 19.5 × 0.92 = 17.9 gpm. At 2000 psi, hydraulic power = (2000 × 17.9)/1714 = 20.9 hp. A 3 in bore cylinder has 7.07 in²; at 8 gpm (1848 in³/min), extension speed = 1848/7.07 = 261 in/min. The reasoning: displacement and rpm set theoretical flow, efficiency reduces it, and speed depends on flow divided by the area being pressurized.
Traps
Treating the relief valve as a flow control, assuming pump pressure is constant regardless of load, using bore area for retraction, ignoring volumetric efficiency, applying meter-in to an overrunning load, and setting accumulator precharge with oil pressure on the gas side.