O Ring Sizing

[Alan CharlestonParticipant @alancharleston78882]

[Alan CharlestonParticipant @alancharleston78882]

Hi,

I was mending a tap this morning which was leaking from the stem due to the failure of the O ring. I think I may have fixed it a long time ago and suspect I didn't use the correct O ring.

My question is:

Given the OD and width of the groove it sits in and the ID of the hole it slides into, is it possible to determine the correct size of the O ring?

Regards,

Alan C.

[Paul LousickParticipant @paullousick59116]

There are lots of tables on the internet for groove dimensions for O rings on the internet, such as this one from Parker Hydraulics **LINK**

Standard O rings for water taps are available from most hardware stores and are inexpensive.  Buy kit that has O rings, lubricant and a sealing washer if you are repairing a tap

Edited By Paul Lousick on 25/05/2023 07:37:49

[noel shelleyParticipant @noelshelley55608]

For what a assorted box costs and the messing about just buy a box set at about £15. You will have all the rings you will ever need . Noel

[Ches Green UKParticipant @chesgreenuk]

Paul,

Good link to the Parker design details.

I used BS 1806 and a Gaco O-Ring Design manual back in the day,. I believe BS 1806 has now been replaced by BS ISO 3601.

Modern tap O Ring sealing seems (to me) to be a law unto itself. I had to replace a leaking bathroom sink tap cartridge with the O Ring on it's bottom face, a few years ago. Problem was, the housing the cartridge mated/sealed with was a plastic moulding with an irregular sealing surface. I don't think these parts are that accurately made, at least from certain sources.

Ches

 

Edited By Ches Green UK on 25/05/2023 09:00:46

Edited By Ches Green UK on 25/05/2023 09:01:09

[HopperParticipant @hopper]

If you do want to work out the size, O rings are usually sized by the ID and the thickness of the ring.

So, you can measure the diameter of the groove to get the O ring ID, then measure the diameter of the hole the O-ring goes into and work out the thickness of the O-ring needed to make the difference, plus a little bit for interference/compresssion.

The groove is usually wider than the ring so not a reliable indicator of the ring's thickness.

I have found that some older taps seem to take different sized o-rings from what is supplied in the hardware store kits today and have had to measure them up and go to a good hardware store to buy the o ring by specific size.

[Ian PParticipant @ianp]

Posted by noel shelley on 25/05/2023 08:55:00:

For what a assorted box costs and the messing about just buy a box set at about £15. You will have all the rings you will ever need . Noel

I have have had two of the assorted O-ring boxes for over 10 years now and have used about five or six rings out of them as they never seem to contain the seal I need!

I buy rings online as as when needed

Ian P

[SillyOldDufferModerator @sillyoldduffer]

Posted by Hopper on 25/05/2023 09:04:54:

…

The groove is usually wider than the ring so not a reliable indicator of the ring's thickness.

…

Not quite right, if I've understood O-rings correctly.

O-rings are designed to flatten into a clean smooth groove by about 20%, and the groove allows space for the expansion – it's deliberately bigger than the O-ring by a calculated factor.

The direction of pressure decides if the O-ring fits snug to the inner or outer diameter of the groove. The rings are sized and positioned so the high-pressure squashes the O-ring firmly against the low pressure flank.

Have a look at the tap design. If the water pressure is going inside out, the O-ring should fit against the outer flank of the groove. If pressure is applied the other way round, the O-ring fits against the inner diameter.

Note the flattening by 20% part too. The O-ring should be thick enough to provide the required degree of flattening.

Before swotting up on O-rings I thought they were just another washer. They're actually quite high-tech, and it's important to use the right size and configuration. Although their squishy nature is fairly tolerant, it's probably unwise to substitute a metric O-ring for a similar Imperial size, or vice-versa.

The above advice covers O-rings as used to seal valves and pipes. Different guidance when an o-ring is used to seal sliding parts.

Dave

Edited By SillyOldDuffer on 25/05/2023 09:53:47

[JasonBModerator @jasonb]

Dave, I think Hopper is talking about when the ring seals a shaft against the SIDE of a hole not the end. As the OP says "stem" I would think it is that sort of seal rather than the end of say a ceramic cartridge that is being sealed against a flat seating.

[SillyOldDufferModerator @sillyoldduffer]

Posted by JasonB on 25/05/2023 11:33:40:

Dave, I think Hopper is talking about when the ring seals a shaft against the SIDE of a hole not the end. As the OP says "stem" I would think it is that sort of seal rather than the end of say a ceramic cartridge that is being sealed against a flat seating.

Could be, that's why I said 'have a look at the tap design'.

Dave

[Grindstone CowboyParticipant @grindstonecowboy]

Posted by Ian P on 25/05/2023 09:30:23:

I have have had two of the assorted O-ring boxes for over 10 years now and have used about five or six rings out of them as they never seem to contain the seal I need!

I buy rings online as as when needed

Ian P

Me too, they are never quite right, despite having a box of imperial and one of metric.

I do recall reading somewhere that, in certain applications, there should be enough room in the groove to allow the o-ring to roll. This might have been for something specialised, though, not taps and rotating shafts.

Rob

[noel shelleyParticipant @noelshelley55608]

Tubal Cains Model engineers Handbook gives some details on using O rings. I recently had a case where the BS o rings seemed wrong . The kit I suspect a cheap one, was in a BS box with the numbers but inside the lid was a key that gave the BS size number and the nearest Metric O ring, these were NOT BS size rings. Then there is the material,and the hardness, standard is 70 but 90 is often available. One can, certainly for larger sizes, buy O ring cord, A in a kit or B loose and make ones own. making a joining block is easy and the glue is super glue. YES it does work ! Noel.

[not done it yetParticipant @notdoneityet]

Posted by noel shelley on 25/05/2023 08:55:00:

For what a assorted box costs and the messing about just buy a box set at about £15. You will have all the rings you will ever need . Noel

Yes! That ^. Occasionally one might need rings in exotic materials, but my boxes of imperial and metric have kept me out of trouble for the last approx 20 years.

Mine will long out-last me, but the rings already used have paid for the boxes multiple times I expect. My wife bought me a box of tiny rings for little money. The ones used in a very high pressure pump would have cost considerably more than the whole box of rings.

[VicParticipant @vic]

Posted by Ian P on 25/05/2023 09:30:23:

Posted by noel shelley on 25/05/2023 08:55:00:

For what a assorted box costs and the messing about just buy a box set at about £15. You will have all the rings you will ever need . Noel

I have have had two of the assorted O-ring boxes for over 10 years now and have used about five or six rings out of them as they never seem to contain the seal I need!

I buy rings online as as when needed

Ian P

Agreed Ian. I’ve got three boxes of assorted O rings and they never seem to contain the size needed. I too buy o rings as required from a specialist supplier on eBay.

[Alan CharlestonParticipant @alancharleston78882]

Thanks for all the comments. If I understand it correctly, the ID of the O ring should match the OD of the bottom of the groove and the cross section of the ring should be the radius of the receiving hole minus the radius of the bottom of the groove times 1.2 to give the 20% flattening as described by Dave.

Regards,

Alan C.

[HopperParticipant @hopper]

Posted by Alan Charleston on 26/05/2023 06:58:04:

Thanks for all the comments. If I understand it correctly, the ID of the O ring should match the OD of the bottom of the groove and the cross section of the ring should be the radius of the receiving hole minus the radius of the bottom of the groove times 1.2 to give the 20% flattening as described by Dave.

Regards,

Alan C.

That's about it. I would not hold the 20 per cent as gospel. It can vary with application or the manufacturer's whim. Same with the ID. Sometimes they make the groove diameter slightly different to the ID of the o-ring, depending on application, whim etc, but not by much.

[HopperParticipant @hopper]

Posted by SillyOldDuffer on 25/05/2023 09:53:12:

Posted by Hopper on 25/05/2023 09:04:54:

…

The groove is usually wider than the ring so not a reliable indicator of the ring's thickness.

…

Not quite right, if I've understood O-rings correctly.

O-rings are designed to flatten into a clean smooth groove by about 20%, and the groove allows space for the expansion – it's deliberately bigger than the O-ring by a calculated factor.

That's pretty much what I was saying. EG the groove for a 2mm thick o-ring will be more than 2mm. Exactly how much depends on application, pressures, manufacturer's whim or whatever. Just don't measure the width of the groove then try to find an o-ring exactly that thick.

[Martin Johnson 1Participant @martinjohnson1]

Posted by Alan Charleston on 26/05/2023 06:58:04:

Thanks for all the comments. If I understand it correctly, the ID of the O ring should match the OD of the bottom of the groove and the cross section of the ring should be the radius of the receiving hole minus the radius of the bottom of the groove times 1.2 to give the 20% flattening as described by Dave.

Regards,

Alan C.

Not really. You can use upt to about 10% stretch on the ID (from memory). You will also find different allowances for "nip". Nip that is right for a high pressure hydraulic system is going to make for a very stiff domestic tap, for example.

Martin

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