piston rings vs. silicone o-ring

Hi IAn,

Yes indeed Parker Seals actually make special sealing rings which are X shaped for some applications. But the important thing is to have no sharp corners and the ring must be allowed to move back and forth for correct operation.

Hydraulic cylinder manufacturers tend to favor conventional piston ring design with O-rings behind the actual rectangular section which rubs on the cylinder bore. Take a look at www.hitechseals.com for typical types.

For critical applications it's now not unusual to use a backing ring on one or both sides of the o-ring if the pressure exceeds what the plain o-ring can bear, A spiral PTFE ring is typical.

tss-static.com/remotemedia/media/globalformastercontent/downloadsautomaticlycreatedbyscript/catalogs/o_ring_gb_en.pdf

Edit - just a though but it would be VERY easy to machine your own non-spiral rings from PTFE. But how likely are we to need them?

Edited By Neil Wyatt on 26/03/2016 09:50:11

Have you actually studied Parkers' design book?? Like so many 'extracts' you've left out critical information and twisted others. O-rings that roll, fail in torsion. Pearls diagram does not show failure due to rolling but failure due to extremely high pressure extrusion. Thousands of psi. Those "recommended back-up pieces" are not there to allow the O-ring to roll. They are there to prevent extrusion and in fact are called that!

O-ring grooves work very well with straight sides and sharp inside corners. The surface finish inside the groove, expecially the bottom, is critical to sealing. Any scratches must be radial not axial. The tapered sides and inside radii are show as maximum conditions not to be exceeded.

Anyone following this is recommended to get the Parker Design Handbook and study it.

Ian, O-rings were used in reciprocating applications before model engineers even knew about them. The X-ring was invented by Parker. Rubber does compress!! That's how it seals! The shape distorts when it does compress.

Go get it right.

Pete@outta_here

Edited By Peter Krogh on 26/03/2016 09:57:03

With respect John, all that really reminds us is to break the corners of the groove don't cause surface damage to the ring.

They don't show a 'semi-circular groove' as in your first post, but they do show backing rings.

Extrusion should only happen at pressures above the o-ring's standard rating, and that's when a backup ring should be used.

Neil

Backup rings are used when the radial gap allows the pressure exerted to extrude the ring into the gap. I would not use an o ring in a piston that crossed a port or was used in any thing other than an oil system. There are many special shape and material seals for use in non oil based systems but need to be matched to both media and pressure. Trelleborg sealing systems have plenty of information on their web site if you need to get more confused!!

Howard

Rubber has similar compression to water!1

Ian S C

Ian, it depends on the type of rubber. Some are denser than others. And here's the biggie; water does compress! Not much but enough that the dielectric constant changes measureably. Back in the early '70s we did the experiments while investigating the use of pure water for high voltage insulation. Neat stuff.

Pete