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O rings

Failure by extrusion

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Kiwi Bloke08/04/2022 01:55:30
705 forum posts
1 photos

O rings, performing static sealing duty, on hydraulic rams on the front-end loader on my tractor fail slowly, apparently by extrusion. The design could have been better: seal back-up rings would probably have helped. Unfortunately, modification isn't practicable. The loader gets abused and treated badly. Unfortunately, my bad behaviour probably also can't be modified...

At present, common-or-garden nitrile rings (2.25 X 0.125 in) are in place. Does anyone know whether a different material might have greater resistance to extrusion?

jimmy b08/04/2022 03:40:03
791 forum posts
42 photos

Would "X" section rings be any use?


Hopper08/04/2022 04:09:01
6706 forum posts
347 photos

Viton O rings seem to seal better on old motorbike applications. But no idea under hydraulic pressure.

Edited instead of mine, JB

Edited By JasonB on 08/04/2022 07:03:21

JasonB08/04/2022 07:02:56
23076 forum posts
2771 photos
1 articles

Viton also resist some oils better and like nitrile come if different hardnesses the harder the higher the pressure they can take

HOWARDT08/04/2022 07:49:50
934 forum posts
39 photos

You could try a harder material. The extrusion gap between the two parts increases with harder material. Dependant on o-ring x section and groove form you may be able to use a different form of seal, contact a seal manufacturer with dimensions and see what they can offer.

mgnbuk08/04/2022 10:03:24
1210 forum posts
72 photos

Change your design to include PTFE Back-up rings on the side of the O-ring suffering from extrusion.

PTFE backup rings

Nigel B.

(just noticed your comment re-not feasible to re-design - sorry !)

Edited By mgnbuk on 08/04/2022 10:04:13

old mart08/04/2022 21:11:01
3912 forum posts
268 photos

The suggestion for a harder material from HOWARDT would be a good start. Either the clearances are too great, or the O ring material is being softened by the hydraulic fluid. Viton is likely to be harder than nitrile, and much more resistant to hydraulic fluid.

Edited By old mart on 08/04/2022 21:15:00

Edited By old mart on 08/04/2022 21:21:47

AdrianR09/04/2022 08:34:34
597 forum posts
36 photos

Does your ram have guide rings? are they worn? if they are it increases the gap on one side which makes extrusion worse.

You could also look at the SKF DPV seals


Kiwi Bloke10/04/2022 07:01:49
705 forum posts
1 photos

Thanks for the suggestions, folks. I'll see if it's possible to get more robust O rings, but we have to take what's available here in 3rd-world NZ - a seriously restricted range of engineering stuff in a country without an engineering heritage... I know the rest of the world is but a few clicks away, but I'm old-fashioned, and prefer to shop conventionally.

To clarify: the O ring seals the end of the cylinder to the body of the cylinder, so it's not a sliding seal. There is no true piston seal in this super-crude (but elegantly simple) design (Kiwi engineering at its best/worst). The dynamic seal is located in the removable end of the cylinder, acting on the ram's outer surface. There's also a wiper seal. The O ring sits in a groove in the seal-carrier (the removable end of the cylinder), and seals against the OD of a turned-down section of the cylinder, the seal-carrier being screwed onto the cylinder.

The designers should have given the O ring an easy life, but it's exposed to full hydraulic pressure, including the effects of a bucket-full of soil, etc., bouncing around when driving over uneven ground. Goodness knows what the peak pressures can be. I suppose it's doing quite well, all considered. Ideally, the O ring's groove should be widened, so a back-up ring can be fitted, but that takes time, whereas replacing the O ring takes minutes...

Edited By Kiwi Bloke on 10/04/2022 07:04:19

noel shelley10/04/2022 09:19:15
1447 forum posts
23 photos

Hi KIWI, For harder rings ask for shore 90, the nornal O ring is shore 70. It might be worth trying a metric ring of slightly larger dimension eg 3.5 mm section X 50mmID. For what they cost it's worth a try. Noel.

old mart10/04/2022 18:24:16
3912 forum posts
268 photos

Viton is most easily obtained in 70 shore hardness, but as it is likely to be more resistant to hydraulic fluid, it will not tend to soften and be extruded like the nitrile you are using.

You haven't mentioned the name of the hydraulic fluid you are using.

Neil Lickfold10/04/2022 19:50:31
892 forum posts
195 photos

If you go to Seal Imports, one in Hamilton NZ, they make seals on a special seal making lathe. They used to have a hydraulic engineer working there who designs seals and know all about the appropriate material to be using. Their prices are very reasonable as well for specially made seals too. Our work gets them made there.

There is also another seal making place in Auckland, not sure if it is connected to Seal Imports or not.


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