Fluid Power Fundamentals
Fluid Power Fundamentals is the common language of the CFPS exam: pressure, force, flow, work, power, and energy applied to both hydraulic and pneumatic circuits. The exam tests whether you can move between ISO 1219 symbols, component behavior, and arithmetic without confusing pressure with flow or force with pressure.
Core rules
- Pascal's law: pressure in a confined fluid acts equally in all directions. Force multiplication comes from area difference: F = p × A.
- Pressure is created by resistance to flow; a pump or compressor does not create pressure by itself. Flow sets actuator speed: v = Q/A.
- Continuity for incompressible flow: Q = A1v1 = A2v2. Power = p × Q; hydraulic horsepower = (psi × gpm)/1714.
- Work = F × d = p × V. Useful output is less than input because of friction, pressure drop, and leakage.
- Viscosity, viscosity index, compressibility, air entrainment, and contamination determine fluid behavior. Symbols show function, not construction.
Worked example
A 4 in bore, 2 in rod cylinder at 2000 psi and 10 gpm extends. Piston area = π(2)² = 12.57 in², so extension force = 2000 × 12.57 = 25,130 lbf. Rod area = π(1)² = 3.14 in²; annulus = 9.42 in², so retraction force = 18,850 lbf. 10 gpm = 2310 in³/min; extension speed = 2310/12.57 = 184 in/min. Hydraulic power = (2000 × 10)/1714 = 11.7 hp. Force uses the area exposed to pressure; speed uses the same area receiving flow; retraction is not the bore area.
Traps
Using gauge pressure where absolute is required, using bore area on retraction, forgetting the 1714 constant, ignoring volumetric or mechanical efficiency, and treating pressure as a fixed pump output rather than a load-dependent result.