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# Dynamic O-ring Design

## Dynamic O-ring Design

(OP)
I am trying to size o-rings for a piston in a cylinder.  The cylinder ID is 12.8" +- 0.030".  The OD of the cylinder, along with the width,depth, and type of gland are all design variables that I can control.  I have room for two sets of o-rings, and plan to proceed with two o-rings.  The piston will see a pressure that may range from 2000-6000 psig, and it is imperative that no air leak past the o-rings; however, the piston is dynamic and should be moved by the pressure (i.e., o-ring compression can not be terribly high).  I plan on starting by selecting a nominal o-ring cross-section size that can be expected to provide a seal in these conditions, and then designing the gland and piston around that o-ring.

Can anyone suggest how I can go about initially gauging this size?  Any equations or internet-based calculators?  Any help would be greatly appreciated.

Thanks,

Haf

### RE: Dynamic O-ring Design

As a general rule you want between 3% and 20% squeeze. You want less for dynamic seals. More for lower pressure seals. your groove width should be at least 40% larger in area than your o-ring cross section. But, +/- 0.03 is a lot of slop for an o-ring seal. Unless you can reduce the tolerance are going to have to go with at least a 1/2" cross section to compensate and that isn't off the shelf and probably won't work well in a dynamic situation (it'll probably roll on you). You'll also have to add in backup rings to keep the o-ring from extruding through the piston gap required by your cylinder tolerance. You're probably better off trying an energized seal like a polypack. Try Parker Hannifins (sp)web site for sizing and selection or go to a vendor and get an O-ring handbook off them.

### RE: Dynamic O-ring Design

(OP)
Hush,

Thanks for the reply.  I realize that +-0.03 is a lot of slop, but, unfortunately, I have to work with that.  This system is being retrofitted with o-rings and was originally designed by someone else (o-ring design was not considered at the time).

What exactly is a polypack?  Could you give me a few details?  I am willing to consider any sealing option.

Thanks,

Haf

### RE: Dynamic O-ring Design

Its basically a v-ring made out of teflon with an o-ring stuck in the middle. The o-ring acts as a spring to keep the lip of the v-ring energized. Try www.parker.com for a look at one, they also have a variety of other seal styles that may suit you even better. Almost any seal manufacturer will have something similar. For a good basic text on sealing technologies try 'Industrial Sealing Technology' by Buchter.

Hugh

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