Pneumatics
Pneumatics on the CFPS exam covers compressed-air production, treatment, FRL units, actuators, control valves, vacuum, and efficiency. Because air is compressible, the exam emphasizes absolute pressure, free-air consumption, and stable cylinder control.
Core rules
- Compressors are positive-displacement or dynamic. Flow is often stated in scfm (free air at standard conditions); absolute pressure = gauge pressure + 14.7 psi.
- Air treatment usually includes aftercooler, receiver, dryer, filter, regulator, and lubricator. In an FRL, the order is filter, regulator, lubricator; not every circuit needs a lubricator.
- Cylinder extension force = p × piston area. Retraction force = p × annulus area. Friction reduces output, often estimated at about 10%.
- Pneumatic speed control is usually meter-out because restricting exhaust gives more stable motion with compressible air. Cushions decelerate at stroke ends.
- Air consumption per stroke: free air volume = cylinder volume × (working psig + 14.7)/14.7. Include both strokes and cycles per minute. Vacuum force = vacuum level × cup area.
Worked example
A 2.5 in bore, 1 in rod cylinder at 80 psig, 4 in stroke, 10 cycles/min. Piston area = π(1.25)² = 4.91 in²; extension force = 80 × 4.91 = 393 lbf. Rod area = 0.785 in²; annulus = 4.12 in²; retraction force = 80 × 4.12 = 330 lbf. Extension volume = 4.91 × 4 = 19.6 in³; free air = 19.6 × (80 + 14.7)/14.7 = 126 in³. Retraction volume = 4.12 × 4 = 16.5 in³; free air = 106 in³. Total per cycle = 232 in³ = 0.134 ft³; at 10 cycles/min = 1.34 scfm. The key is using absolute pressure for consumption and the correct area for each direction.
Traps
Using gauge pressure instead of absolute pressure for air consumption, using bore area for retraction, ignoring friction, assuming a 5/2 valve locks a cylinder in mid-stroke, confusing scfm with acfm, and placing a lubricator upstream of the filter.