Sizing a pneumatic cylinder to a load and system 1

[slmechanis]

So, just an exercise really, but something ive been mulling over.

Lets stick with round numbers, I have a pneumatic cylinder which needs to move a 10 lb load at the cylinder, and 4" stroke is required, the system has a flow rate of 40cv, and 200psi so there's your supply.

Now the question is: how do you size the cylinder to actuate as quickly as possible with those limitations? Logic dictates you want the smallest cylinder possible to make the most of your flow rate which dictates speed, but there must be a way to mathematically figure out how much a load would slow down a cylinder and discover where more force isn't giving you more acceleration, its just slowing you down because of the limited flow.

 

[EdStainless]

What does the Bimba catalog say about sizing?

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P.E. Metallurgy, Plymouth Tube

 

[Occupant]

Let's say you are using a 1/2" cylinder. That would require an air pressure of ~50psi to move the load. The choking point for air would be ~0.58, meaning 50/0.58=86psi. There is no advantage to supplying a higher input pressure because the flow would be choked off. A C_v of 40 would be meaningless for this type of application.

 

[slmechanis]

@occupant, thanks!

so force with an extra 58% pressure, plus a little bit of slush for friction and such gives you the ideal pressure to use for a given load.

Logically I was thinking that max accepted CV for a cylinder would be equal to the max a tube of the same size could hold (so for a 1/2" tube it would be ~13.84cv) right? Any additional CV beyond that 13.8 would be "wasted" but that is the figure to go with?

1/2" cylinder, @86psi thats balanced to the load, in my mind it seems like I wouldn't want "just enough" force to move the load, I'd want a little more than that to achieve a higher starting acceleration and reduce total cycle time, in this case. I'm a little surprised that higher pressure air doesn't expand measurably faster.

I mean it was a bit of a silly question to begin with, it started when I looked at one of those truck mounted pumpkin launcher cannons that run hilarious amounts of air at really high PSI, and it all seemed wasteful with what I knew about pneumatics.

 

[Occupant]

That's not what I meant. You said, you have 200psi available, but more that 86psi supply in the given situation doesn't do anything for you because you have reached sonic conditions, no extra "oomph" possible. Normally you'd use a pressure regulator to get the pressure down to the required load level anyway. A valve to drive a load/cylinder like that would have ~1/4" hose and a valve with a C_v of ~0.8. Don't forget: large valves have high response times to shift - you lose the advantage of higher air flow and, given your data, you'd have to fill the excessively large supply line/valve chambers as well before anything happens.

 

[slmechanis]

Yeah that makes sense, It was all hypothetical to begin with.