Pneumatics, Hydraulics and Fluid Power
Pneumatics, Hydraulics and Fluid Power tests compressed-air and hydraulic components, actuators, valves, and safe troubleshooting. Expect FRL units, dryers, compressors, cylinders, directional valves, relief valves, accumulators, contamination control, and force calculations.
Core rules
- Pressure is force per area: F = P × A. For a cylinder, extension uses full bore area; retraction uses bore area minus rod area.
- Hydraulic systems are incompressible and stiff; air systems are compressible and springy. Both need filtration, pressure regulation, and a relief or safety device.
- A hydraulic relief valve protects the circuit and should not be used as a routine flow control. Speed control is by flow control, not by deadheading the pump.
- Clean air requires a dryer and filter; lubricators are used only where air tools or actuators need lubrication and must not contaminate instruments.
- Contamination is the main hydraulic failure cause. Maintain fluid cleanliness, change filters, and check for air ingress and overheating.
Worked example
A hydraulic cylinder has a 4 in bore and a 2 in rod; supply pressure is 1000 psi. Extension area = π × 2² = 12.57 in², so force = 1000 × 12.57 = 12,570 lbf. Retraction area = π × (2² − 1²) = π × 3 = 9.42 in², so force = 9,420 lbf. At 80% mechanical efficiency, useful forces are about 10,050 lbf and 7,540 lbf. The reasoning is to use the correct effective area; using the bore for retraction overstates retract force.
Traps
Using bore area for retraction, deadheading a pump against a closed valve, omitting a relief valve, and assuming air pressure gives instant force regardless of compressibility are common. Also do not install a lubricator upstream of instruments or analyzers. Verify actuator cushioning and hose ratings before pressurizing.
Key points
- F = P × A; rod reduces retract area.
- Relief valves protect, not meter flow.
- Air is compressible; hydraulics are stiff.
- Control speed with flow controls.
- Contamination causes most hydraulic faults.