Acceptable Clearance with Backup Ring 1

[bryansonnier]

TLDR: Is there a standard or literature that covers what the limit is on clearance to seal WITH a backup ring.

Okay, so there is tons of literature about o-ring extrusion gap clearances and the durometer needed for each (could be parker did it and everyone copied but its widely published). In ALL and i mean !!ALL!! the literature they say "if you're extrusion gap is to the right of this curve then you need to use a backup ring". So okay i have a high torque 1 rpm application that i have to seal to 5000 psi. The torque is driven by a bevel gear and i have bronze bushings (CD955) to keep the rotating part inline. My current extrusion gap is a little on the high side but within standard o-ring tolerances however after only a couple thousand cycles the bushings have worn to the point where the rotating part is rubbing metal to metal at the area where the o-rings are (because of the close tolerance).

Here is the question: Is there a standard or literature of any sort (i'll even take disreputable at this point) that covers what the limit is on clearance to seal WITH a backup ring.

I would like to push my clearance which is currently 0.004" to about 0.010" to give more room for part wear and extend cycle life.

 

[Pud]

Shouldn't you be using a lip seal for rotating shaft?


Politicians like to panic, they need activity. It is their substitute for achievement.

 

[3DDave]

Seems like the problem is wear in the bronze bushings.

Sure you can open that gap; go for it. Be prepared for the o-ring to be blown out like chewing gum at some point.

What I can suggest, if you really want that, is to polish the contacting surface to a 3 - 5 microinch finish. Going from 20-30 microinch down to 3-5 can get a 10X improvement. I would also suggest some other bearing ring to be put in place to prevent side loads from getting to the o-ring.

 

[hydtools]

Backup rings are generally 90 durometer. So clearances for that hardness would apply.

Ted

 

[bryansonnier]

3DDave
Do you have an idea of bearing material that would be well suited for high loads? Also you mentioned a more polished surface, is that for the o-ring seal face or is that for the surface area the bushings are riding on?

Update my seal vendor sent a data sheet on Hytrel backup rings that have a 55 Shore D hardness. Maybe this will help others. We have also tried PEEK in the past but may move away from that for issues not covered here.

For current design i have settled on a 0.010 extrusion gap with 90 durometer o-rings and a hytrel backup ring. We'll see how it goes.

 

[hydtools]

Do not polish sealing surfaces. Too smooth surface will not retain lubricant and the seal will wear against a wiped dry surface.

Perhaps your bushing wear is due too insufficient lubrication between the load bearing surfaces and is not a material fault.

Ted

 

[3DDave]

That's what a vendor told us; has to be rough to retain lubricant. The seals were failing after about 10 hours. We polished the surface - I never heard of another seal failure after that.

Turns out if the mating surface isn't abrasive there isn't a source of wear.

The problem in rotating contact is that the motion doesn't have the ability to transfer lubricant under the seal. All that happens is the high spots get the lube forced out, leaving them dry and in contact with the soft seal, which then gets gouged, starts excluding more lubricant and cascading to failure.

In linear sliding, that's a different story.

 

[Tuckabag]

If you can't improve the wear in the bearings, can you increase the groove width to accomodate a PEEK/similar bush?
It will provide the tight extrusion gap for the o-ring/backup and then allow you to increase the clearance on the metal/metal parts?
See attached sketch.