When you specify a pneumatic pusher for a high-cycle packaging line, the cylinder bore influences four connected requirements: 1) force at the load, 2) stroke time, 3) compressed-air demand and 4) end-of-stroke deceleration. A larger piston area raises the available force and increases the volume that the valve and tubing must fill, so the cylinder, air path and cushioning need to be assessed against the same duty cycle.
Let’s work through the realistic example of a horizontal pusher with an 18 kg total moving mass, a 250 mm stroke, 40 complete cycles per minute and a required extension time of 0.35 seconds. In this case the Festo pneumatic cylinder will need a 50 mm bore and 20 mm rod. Here’s how we reached that figure.
1. Dynamic force at the cylinder ports
During extension, losses through the air-preparation unit, valve, fittings and tubing reduce the pressure reaching the cap-end port. At a measured 5.2 bar gauge, for instance, the 1,963 mm² piston area produces 1,021 N. Exhaust backpressure of 0.6 bar acting on the 1,649 mm² annular area opposes the stroke with about 99 N, leaving 922 N before seal friction.
A 650 N process load therefore leaves 272 N for acceleration, friction and normal variation. Accelerating the 18 kg assembly at 8 m/s² uses another 144 N, reducing the remaining allowance to 128 N. Use pressure measured at both cylinder ports during movement when you calculate this margin.
2. The 0.35-second extension flow requirement
Staying with our example, the cap-end swept volume is 0.491 litres. Filling that volume in 0.35 seconds requires an average 84 l/min at the pressure existing inside the Festo pneumatic cylinder:
Q = ALt
Expressed as normal litres (the equivalent air volume at approximately atmospheric pressure), the requirement is about 522 Nl/min on a simple pressure-ratio basis. Add the tubing dead volume and allow for temperature effects, then assess the valve, fittings and tubing at their actual upstream and downstream pressures – a catalogue flow figure taken under different test conditions cannot confirm the required stroke time on its own.
3. Air consumption across the full cycle
The annular volume on the return side of our pusher is 0.412 litres, so if both strokes operate at 5.2 bar gauge, the idealised air used by one complete cycle is:
(0.491 + 0.412) × 6.2 = 5.6 Nl
At 40 cycles per minute, average demand will be about 224 Nl/min, plus the air required by your connecting tubes and clearance volumes. This helps you assess machine supply and compressor capacity. The extension movement calls for approximately 522 Nl/min during its 0.35 seconds, which determines the required local flow capacity. A larger bore will increase both figures in proportion to its piston area.
4. Cushioning energy and deceleration distance
If the 18 kg assembly enters the cushioning zone at the average stroke speed of 0.714 m/s, it carries at least 4.6 J of kinetic energy:
Ek = 12mv2
That energy must be removed across the available deceleration distance. For the standard 50 mm DSBC Festo pneumatic cylinder, the maximum impact energy in the end position is listed as 1.0 J. This is the energy permitted when the piston reaches the end, rather than the total that pneumatic cushioning may dissipate beforehand. The 50 mm PPV version provides 22 mm of cushioning length, so verify its permitted mass-and-speed range at the actual working pressure, and if necessary, reduce the entry speed, extend the deceleration profile or add an external shock absorber.
Pneumatic axis design support from Hydrastar
At Hydrastar, we can help you match Festo pneumatic cylinders, directional valves, flow controls, fittings and tubing to your required force, stroke time and cushioning duty. Speak to one of our engineers today to discuss the complete pneumatic axis for your packaging application.
