Fast Air Pump Advice

[falcon14110]

I'm in need of a way to seal a pneumatic piston. The piston needs a bore ID over 7.5 inches with a stroke between 2 to 3 inches. The piston will stroke an entire cycle in 400ms. The pressure within the cylinder will not exceed 6psi. The volume of air which the piston will move is enclosed so I can allow no leakage otherwise the system will fail. I am trying to find a way to seal the piston to the bore which will not leak yet be able to survive the speeds over millions of cycles. I would love to use U-cup or other mechanical seal however with the speeds concern me. I've also looked at alternatives such as diaphragms however finding them in bore IDs over 4 inches becomes hit or miss and I would like off the shelf components.

Any ideas or inputs?

 

[Compositepro]

Define "no-leakage". Even diaphragms have permeability. A bellows or rolling diaphragm are options.

 

[falcon14110]

As I said the entire system is enclosed. when the piston is at top dead center the system is pressurized to 6psi. Before the At bottom dead center the system will be at .25psi. Before the system starts the piston is brought to bottom dead center and pre-charged to the .25psi. The system will be cycled at speed for two hours. While it's cycling there can not be more then a .1psi drop in the entire system.

Hope that helps.

 

[btrueblood]

Contact RPP corporation. They make rolling diaphragms for us at approx. 6.5 and 10 inch diameters. Also Marsh Bellofram, around 8 inch diameter. You will probably need to spend around $250 to $500 for a small quantity order.

At 6 psi peak pressure, you could darn near mold your own diaphragm from urethane casting resins. Or buy sheet butyl rubber (low permeability) and make a flat diaphragm, but it would have to be a fairly large diameter to allow for the stroke you want.

Another alternative would be to dis-assemble a small hot water expansion tank, bladder-type, and extract the bladder, then adapt it for your use. Or just buy a replacement bladder from the tank mfgr.