Pneumatic Pick-and-Place Pressure Droop
A pick-and-place station uses a double-acting pneumatic cylinder with 50 mm bore, 20 mm rod diameter and 250 mm stroke. It extends and retracts 12 times per minute at 6.0 bar gauge; the compressor delivers 250 L/min free air delivery (FAD) and the receiver is 100 L. During a shift the header pressure falls from 6.5 bar to 5.2 bar over 40 minutes, and the cylinder becomes sluggish near the end of stroke. The FRL unit is set to 6 bar, but the filter bowl contains visible water and the drain is never opened. The cylinder exhaust is throttled by two one-way flow control valves set about 1.5 turns open. A leak test on the cylinder shows no rod-seal leakage, but with the supply isolated the header pressure drops from 6.5 to 5.8 bar in 10 minutes with no machines running. The station is the only consumer on that branch. Production reports inconsistent gripping height, especially on the first cycle after a pause.
1. Calculate the free-air consumption per cycle and the average demand. Compare it with the compressor capacity and state what the result implies.
Read the worked answer
Bore area = π/4 × 0.050² = 0.0019635 m². Rod area = π/4 × 0.020² = 0.0003142 m². Annulus area = 0.0016493 m². Extend volume = 0.0019635 × 0.250 = 0.0004909 m³ = 0.4909 L. Retract volume = 0.0016493 × 0.250 = 0.0004123 m³ = 0.4123 L. At 6 bar gauge, absolute pressure is about 7.013 bar, so free air per extend = 0.4909 × 7.013 = 3.44 L and per retract = 0.4123 × 7.013 = 2.89 L. Total per cycle = 6.33 L. At 12 cycles/min the average demand is about 76 L/min FAD. That is only about 30% of the 250 L/min compressor rating. Therefore normal cylinder consumption does not explain the pressure falling from 6.5 to 5.2 bar. The fault is likely a leak, a restricted FRL or branch line, or incorrect compressor loading/unloader operation.
2. Interpret the isolated pressure-decay test and explain why the first cycle after a pause is inconsistent.
Read the worked answer
A 100 L receiver dropping 0.7 bar in 10 minutes represents 100 × 0.7 = 70 bar·L, or about 7 L/min of free-air equivalent. That is a real leak, although small compared with the compressor capacity. Because the branch has only this station, the leak can drain the local line and receiver when the compressor is unloaded or the station is idle. After a pause, the first cycle starts from a lower stored pressure, so the cylinder moves slowly or stops short; once the compressor reloads, later cycles appear better. The water in the filter bowl is also significant: a saturated or partially blocked filter element causes a pressure drop that increases with flow, and water can carry downstream to valves and flow controls. Check the FRL differential pressure during a cycle, inspect the filter element, and repair the leak before adjusting flow-control valves.
3. Recommend corrective actions and the checks needed to confirm the station is reliable.
Read the worked answer
Isolate and repair the leak, then drain the filter bowl and replace the element. Fit a proper air dryer or coalescing filter if water is chronic, and establish daily draining. Measure pressure at the cylinder ports during extend and retract; a large difference between the FRL gauge and the cylinder gauge indicates restriction. Verify compressor cut-in/cut-out pressures and unloader operation, and check that the branch pipe and fittings are not undersized. Set the flow-control valves by observing actual cycle time, not by turns open, and confirm cushioning is effective. After repair, repeat the isolated decay test; a sound system should hold pressure much longer. Run at least 50 cycles and record cycle time, header pressure and first-cycle behaviour after a 10-minute pause. Only then release the station to production.