Pneumatic Clamp: Force, Air Consumption and Slow Retract
A pick-and-place station uses a double-acting pneumatic cylinder to clamp a 40 kg plate. The cylinder has a 63 mm bore, a 16 mm rod and a 150 mm stroke, and is supplied at 6 bar. The extend port is controlled by a 5/2 solenoid valve and a one-way flow-control valve; the retract port has a quick-exhaust valve. The clamp must exert at least 1.5 kN with a mechanical efficiency of 0.85 and complete the 150 mm extend stroke in 0.8 s. The compressor is rated at 10 L/min of free air (ANR) with a 6 bar receiver. The retract stroke is slow and jerky when the plate is vertical, while the extend stroke is acceptable. The cylinder is connected with 2 m of 6 mm inside-diameter hose. The filter-regulator is set to 6 bar, but the gauge at the cylinder drops to 4.8 bar during retract. The 5/2 valve has a nominal flow of 700 L/min (ANR) at 6 bar. The station has no pressure switch or check valve on the vertical load.
1. Calculate the extend and retract forces, the required extend speed and the free-air consumption. Is the compressor adequate?
Read the worked answer
Bore area A = π/4 × 0.063² = 3.117×10⁻³ m². At 6 bar gauge (6×10⁵ Pa), theoretical extend force is 6×10⁵ × 3.117×10⁻³ = 1870 N. With η = 0.85, usable force is 1590 N, so the 1.5 kN requirement is just met.
Rod area = π/4 × 0.016² = 2.01×10⁻⁴ m². Annulus area = 3.117×10⁻³ − 2.01×10⁻⁴ = 2.916×10⁻³ m². Retract force = 6×10⁵ × 2.916×10⁻³ × 0.85 = 1487 N, slightly below the extend force but still close to 1.5 kN.
Required speed = 0.150/0.8 = 0.1875 m/s. Extend flow at 6 bar gauge = A × v = 3.117×10⁻³ × 0.1875 = 5.84×10⁻⁴ m³/s = 35.1 L/min. Expanding to free air at 7 bar absolute gives 35.1 × 7 ≈ 246 L/min ANR. The compressor supplies only 10 L/min, about 25 times too little, so it cannot sustain the required cycle. A receiver can cover a single short stroke, but the average consumption and recovery time must still be checked; a much larger compressor or a lower cycle rate is required.
2. Explain the slow, jerky retract. Include the pressure drop, hose size, quick-exhaust placement and flow-control method, and propose a corrected circuit.
Read the worked answer
The gauge drop from 6 bar to 4.8 bar shows a 1.2 bar loss during retract. With a 6 mm hose, the retract flow of about 33 L/min at 6 bar gives a high air velocity and a significant hose pressure drop, especially over 2 m plus fittings. A quick-exhaust valve is only effective if it is mounted at or very near the cylinder port; if it is at the valve, exhaust air must travel back down the hose, and if it is installed backwards it can restrict flow. The one-way flow-control valve may also be set as meter-in on the retract side, which makes a vertical load slow and jerky because the exhaust is not controlled.
Correct the circuit by using meter-out flow control on the retract port, mounting the quick-exhaust valve directly at the cylinder port with the correct arrow direction, and increasing the hose to 8–10 mm ID or shortening the run. Check the 5/2 valve port size and the filter-regulator flow. A vertical load should also have a pilot-operated check valve or mechanical lock to prevent dropping.
3. Propose the safety changes required for the vertical load and for loss of air pressure.
Read the worked answer
The vertical plate can fall if air pressure is lost or if the retract line is vented. Fit a pilot-operated check valve or a mechanical safety lock on the cylinder so the load is held when pressure is removed; a simple check valve in the supply is not sufficient because it does not lock the cylinder ports. Add a pressure switch that detects low air pressure and removes the enable signal from the valve, and use a fail-safe valve arrangement so the clamp remains closed or the load is supported on loss of power or air.
Provide a guarded enclosure or light curtain around the vertical stroke, and ensure the exhaust air is directed away from operators. Add a manual lockout for maintenance and a controlled exhaust-and-verify procedure before anyone reaches into the clamp area. The 40 kg plate and the 1.5 kN clamp force make trapping and dropping hazards significant; the safety function should be independent of the normal PLC output.