Can anyone help with pneumatics?

Question

A friend is creating a new reject system for a inspection machine, he need to know what size piston to use to 'punch' a faulty container off a production line, the container will be 500 g, the piston will operate at 4 bar, and it will need to extend say 50mm in 0.5 sec, can anybody work out the piston size showing all working out and relevant formula, the conveyor is made of stainless steel and is 100mm wide. show one calculation for sprung loaded return piston and one for double acting

notes: The container is glass. Cylinder must extend 25mm not 50 mm

Answer 1

There would be a lot to be said for empirical design, since friction will be a very important consideration.

Are you sure about that 0.5seconds? the force needed for acceleration will be quite small, since a newton is the force need to accelerate 1kg at 1m/s^2 , we can see that it won't take much to accelerate 1/2kg  at 0.2m/s^2  (25mm in 1/2s = 0.1m/s,  0.1 / 0.5 = 0.2m/s)

The force need to slide the container across the conveyor will depend on the coefficient of friction, which would best be determined by experiment, but for a first approximation, suppose about 1 to be on the safe side (some sources say 0.5 to 0.7 for glass on metal), for a 500gm mass on the Earth, that would only be about 5N.

4 bar is about 4N /cm^2, a cylinder with an effective area of say, 2cm^2 might be a reasonable starting point. I work in the USA,  where inches are a common unit for manufactured items, I know   3/4" diameter cylinder are readily available (about 19mm dia. and 2.9cm^2)  That would probably be where I would start.

2.9cm^2 area * 2.5cmstroke  = 7.3cm^3 of air per stroke (1/2 second) or a flow rate of about 15cm^3 per second, a rather low rate, so a small valve and fairly small hose (depending on length) will probably work.  double acting cylinder would require the air flow for both extend and retract, the spring return would require air only for extend, but you would need to consider the return spring when calculating the force required, one would have to check a cylinder catalog for spring forces, perhaps a slightly larger cylinder might be considered for single acting maybe 25mm dia.  though I have estimated high and rounded up in the preceding calculations, so a 3/4" (20mm) dia. cyl. might still be OK.