'O' rings

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Brucey
Posts: 32286
Joined: 4 Jan 2012, 6:25pm

'O' rings

Postby Brucey » 25 Jan 2019, 3:35pm

I've used O-rings for decades in both my hobbies and in my working life. I should know a fair bit about them by now.... even so I found myself caught out just yesterday, with a simple assembly that either would or wouldn't seal depending on selective assembly of just two choices of the three components concerned.

The assembly in question was a Zefal HP/HPX pump chuck; the 'sliding piston' part the lever cam bears against, inside the chuck, carries an 'O' ring and the oldest of my pumps had developed an intermittent leak when the cam was operated. The last time I used that particular pump it would work OK if the lever wasn't used (and the nut was adjusted to compress the seal slightly) but if the lever was used most often the thing would leak air.

I spent some time looking for debris in the mechanism (and found some) and also carefully examined the assembly to see if the seal was overlapping the air port in the bore perhaps; the thing has about a 2mm stroke and the 'land' inside the bore is only about 4mm wide; wear or bad tolerancing could easily be an issue. Eventually I had to conclude that these things were not causing a problem.

I've replaced the O ring in these pumps before and tbh I've just gone to my many boxes of 'O' rings and found one that looked about the right size, banged it in, and it has been OK. In over thirty years of using these pumps I have only had to change one or two O rings, and that was down to neglect or contamination with the wrong lubricant. [Quite recently I have taken to being sure to use silicone grease on nearly all O rings since this is unlikely to attack/degrade the O-ring regardless of the rubber grade used. In hindsight I have in this respect probably got away with murder previously.]

It turned out that the correct O ring is probably one that is 10mm ID, 2mm section (although it could be one that is 9.8mm ID and 1.9mm section), but that the closest size (in most of my boxes of O rings ) was closer to 1.8mm section and 3/8" ID. Close enough to work OK provided the other parts in the assembly were within a particular tolerance range. It also turned out that the lever cam also pushes the 'piston' part sideways in the bore (fairly obvious in hindsight) and one of the two pistons I tried was a very slightly smaller OD than the other, such that if the piston was pushed fully sideways in the bore, the side with the larger gap no longer always compressed the (slightly undersize) O ring section enough to allow a seal to be made.

One of the absolutely baffling manifestations of this was that the seal would work without grease, but wouldn't work with grease.... :shock: I eventually figured that for the piston to go fully off-centre, the O ring also had to slide within the groove, and did so more easily with grease present than not. I spent over an hour doing careful measurements and trying various combinations of parts before I figured the problem out; I found it hard to believe that differences I could barely measure were resulting in such a binary (work or not work, no middle ground) outcome.


Anyway fundamental problems are that

a) the world is infested with (very slightly different) standards for O rings and
b) measuring them is difficult.

On the latter point there are special tools for measuring O rings such as this cone tool;

Image

but with care, you can measure (small) new O rings with a set of Vernier calipers. If you measure a (clean, dry) O ring cross section between the flat parts of the jaws you will get close; however you may have squashed the O ring slightly (the more of the O ring you can get between the jaws the better) and it won't fall out of the jaws under its own weight until there is ~0 to 0.05mm clearance. With care you can determine the section width within ~0.1mm. Measuring the diameter is more difficult; O rings are always specified on the ID but with verniers measuring the OD is both easier and more accurate; again measuring between the flat parts of the Vernier jaws is the trick and there is a small range between 'obviously draggy' and 'loose/falling out easily'. If you think a new O ring might not be quite round then multiple measurements might be required.

Measuring used O rings is often a waste of time; they may be deformed, worn or swollen. If you can do, you are much better off measuring the grooves, diameters etc in the assembly and inferring what the correct size O ring might be. Groove design for O rings can vary with the application but the two main things are that

1) the O ring section has always to be compressed slightly in order to make a seal and
2) the O ring cross section shouldn't fill up more than 85% of the volume of the groove it sits in; this causes problems otherwise.

more useful groove design info here;

http://www.hitechseals.com/includes/pdf/o-ring_brochure.pdf

including tables which tell you how much the cross section of a 'stretched fit' O ring will reduce by.


Regarding standards, this pdf is dead handy;

https://o-ring.info/en/o-ring/Technical%20Handbook/16%20-%20ERIKS%20nv%20-%20O-ring%20Technical%20handbook%20-%20O-ring%20Size%20Chart.pdf

On the first page you can see

Standards in Different Countries Norm Cross Section (mm)
AS 568A, BS 1806; 1,78 2,62 3,53 5,33 6,99
DIN 3771 / ISO3601; 1,80 2,65 3,55 5,30 7,00
SMS 1586, BS 4518; 1,60 2,40 3,00 5,70 8,40
Japanese Norm JIS B 2401; 1,60 1,90 2,40 3,00 5,70 8,40
Metric; 1,00 1,50 2,00 2,50 3,00 5,00 10,00 12,00

i.e. that not all cross-section widths are present in all standards; this means if you have a random box full of O rings, you can narrow down the specifications to which they are liable to be manufactured. For example there is no point looking for a 2.5mm or 1.5mm section O ring in anything other than a kit made to metric standards. Note also that O rings have a tolerance on the section width; typically this is +/- ~0.08mm.

I hope folk find this post useful.

Two final points are that

1) (if needs must) you can adjust the fit of O rings (for non-critical applications) that are slightly too wide in cross-section. The method is to fit the O ring on an arbor, then spin it up in an electric drill. You can use fine wet and dry paper (with water as a lubricant) to gently abrade away material until the seal is a better fit.
2) for anything that is safety critical as well as the size being correct, you need to be 100% sure that the material is the correct sort for the application. A seal in the wrong material can be worse than no seal at all; it can fail at any time.

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

Steve O'C
Posts: 38
Joined: 3 Mar 2013, 1:32pm

Re: 'O' rings

Postby Steve O'C » 25 Jan 2019, 9:46pm

for anything that is safety critical as well as the size being correct, you need to be 100% sure that the material is the correct sort for the application. A seal in the wrong material can be worse than no seal at all; it can fail at any time.


Indeed

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fastpedaller
Posts: 1764
Joined: 10 Jul 2014, 1:12pm
Location: Norfolk

Re: 'O' rings

Postby fastpedaller » 25 Jan 2019, 11:20pm

Brucey wrote: for anything that is safety critical as well as the size being correct, you need to be 100% sure that the material is the correct sort for the application. A seal in the wrong material can be worse than no seal at all; it can fail at any time.

cheers


Absolutely... A neighbour (who should know better because he works for a car manufacturer!) arrived at my door one day asking if I had any O rings.
It turned out he needed one for the fuel injector on his Range Rover - I think It was correct to give the advice to get the manufacturer's part, as my O ring may not be compatible.

Brucey
Posts: 32286
Joined: 4 Jan 2012, 6:25pm

Re: 'O' rings

Postby Brucey » 26 Jan 2019, 1:30am

fuel injectors (petrol, port injection type) often look like this

Image

with a seal at each end. The seals on this injector appear to be the same part (and indeed might be in this case) and don't look 'special'. Yet they are special and furthermore they have a very different job to do at each end. The inlet manifold seal (left end near the pintle cap) seals the inlet manifold and stops air leaks. It occasionally sees fuel, and the maximum pressure it needs to seal is about 10psi. If it fails there is an air leak and the engine won't run well. Worst case is probably on an open loop engine where the air leak may cause lean running and hole a piston. But it is unlikely to kill you.

This is in contrast to the other seal, which seals to the fuel rail. This holds petrol (at about 50psi). If this fails or leaks there is a pretty fair chance of a major fire. The fuel pump will happily push over 1 litre a minute of fuel out and this makes for a pretty good blaze.


At one time the manufacturers were concerned enough to make fuel injector seals obviously different to one another and obviously different from other O rings that you might find in a workshop;

Image

this scheme worked very well until suddenly you could buy O rings that were not at all suitable in similar colours.

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

francovendee
Posts: 724
Joined: 5 May 2009, 6:32am

Re: 'O' rings

Postby francovendee » 26 Jan 2019, 8:04am

Hi Brucey,
Have you ever come across 'O' rings in kit form? I seem to remember them at one company I worked. They had them in the R&D workshop.
The rubber came in a variety of diameters and the kit had a way of cutting the rubber square. It was then joined with some sort of glue. I got them to make me one for a 'homer' and it worked OK.

Greystoke
Posts: 134
Joined: 8 May 2018, 7:41am
Location: Lincolnshire

Re: 'O' rings

Postby Greystoke » 26 Jan 2019, 8:50am

Air rifles are another piece of kit where o ring sizes are critical.....

Airsporter1st
Posts: 561
Joined: 8 Oct 2016, 3:14pm

Re: 'O' rings

Postby Airsporter1st » 26 Jan 2019, 8:59am

francovendee wrote:Hi Brucey,
Have you ever come across 'O' rings in kit form? I seem to remember them at one company I worked. They had them in the R&D workshop.
The rubber came in a variety of diameters and the kit had a way of cutting the rubber square. It was then joined with some sort of glue. I got them to make me one for a 'homer' and it worked OK.


The 'kit form' 'O' rings using rubber cord and glue are regularly used in industry, but almost exclusively on lower pressure, non-critical, static applications. The glued joint is a definite weak point. It has to be accurately cut and the very act of cutting, even using a purpose-made tool, deforms the rubber such that it is very difficult to achieve a plane face to the cuts. The cut ends have to be perfectly aligned and the correct primer and glue for the grade of elastomer must be used. The resistance of the glue itself to the sealed medium has also to be considered.

As the diameter (as opposed to cross section*) gets smaller, it is practically a physical impossibility to fabricate an 'O' ring from cord.

I have installed cut 'O' rings in split mechanical seals and they have lasted for years (although at least by using a cut 'O' ring, deformation during cutting is not such an issue, because the cut ends match) and I have also installed kit 'O' rings which have had to be re-installed several times due to leakage. They are a bit hit and miss no matter how much care is taken.

* 'O' rings are sized by I.D. and cross section.

See here for a very useful handbook, which I recommended in an earlier thread, "Replacement piston seals for Shimano hydros": https://o-ring.info/en/technical-info/technical-o-ring-handbook/. I think one of Brucey's links above is an extract from that same book.
Last edited by Airsporter1st on 26 Jan 2019, 9:17am, edited 2 times in total.

Airsporter1st
Posts: 561
Joined: 8 Oct 2016, 3:14pm

Re: 'O' rings

Postby Airsporter1st » 26 Jan 2019, 9:14am

Greystoke wrote:Air rifles are another piece of kit where o ring sizes are critical.....


Can't beat a good old leather piston washer lubed with Neatsfoot oil on a springer - such as the BSA Airsporter :wink:

Brucey
Posts: 32286
Joined: 4 Jan 2012, 6:25pm

Re: 'O' rings

Postby Brucey » 26 Jan 2019, 11:53am

re kit O-rings. Yes I have used them extensively, mainly as vacuum seals. Making up the seals is a fiddly job to do well and one of my former colleagues had the best knack for making them up. Other folk would ask him to make up seals because they were usually better more reliable seals than they would manage. The bond line was not only a position where leaks were more likely, it was also noticeably stiffer than other parts of the seal (because of the glue) and this caused problems too. Often these seals were not fully static ones, i.e. the sealed joint would be repeatedly opened and closed. Where possible it was always a better idea to use a moulded seal not a made-up one.

Note that with vacuum seals the acceptable leak rates are often orders of magnitude less than is tolerable in other applications; a seal that might (say) leak one drip of a fluid per day might be a complete failure in a vacuum system.

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

pete75
Posts: 10482
Joined: 24 Jul 2007, 2:37pm

Re: 'O' rings

Postby pete75 » 26 Jan 2019, 11:59am

francovendee wrote:Hi Brucey,
Have you ever come across 'O' rings in kit form? I seem to remember them at one company I worked. They had them in the R&D workshop.
The rubber came in a variety of diameters and the kit had a way of cutting the rubber square. It was then joined with some sort of glue. I got them to make me one for a 'homer' and it worked OK.


When I was working as a refrigeration engineer years ago - late seventies early eighties we had these kits. Made by Loctite and I think super glue was invented by them for use in these kits. Remember the firts adverts for superglue - they had a chap sticking the ends of a rubber ring together and swinging on it. They did that because it was the most effective use of the glue as it was what it was developed for.
You're right the kit had a special cutter which ensured the ends were square.

Airsporter1st
Posts: 561
Joined: 8 Oct 2016, 3:14pm

Re: 'O' rings

Postby Airsporter1st » 26 Jan 2019, 4:21pm

pete75 wrote:
francovendee wrote:Hi Brucey,
Have you ever come across 'O' rings in kit form? I seem to remember them at one company I worked. They had them in the R&D workshop.
The rubber came in a variety of diameters and the kit had a way of cutting the rubber square. It was then joined with some sort of glue. I got them to make me one for a 'homer' and it worked OK.


When I was working as a refrigeration engineer years ago - late seventies early eighties we had these kits. Made by Loctite and I think super glue was invented by them for use in these kits. Remember the firts adverts for superglue - they had a chap sticking the ends of a rubber ring together and swinging on it. They did that because it was the most effective use of the glue as it was what it was developed for.
You're right the kit had a special cutter which ensured the ends were square.


The cutter I had - until I sold it following my retirement - actually cut the cord with a 'V' shape. This made for a more torturous leakage path, provided a bigger surface area for gluing and also made it slightly easier to align the ends when gluing.

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Cunobelin
Posts: 8755
Joined: 6 Feb 2007, 7:22pm

Re: 'O' rings

Postby Cunobelin » 26 Jan 2019, 4:38pm

My father worked for a number of companies designing and manufacturing pipes (the big yellow gas mains)

They found that sometimes the "O" ring seals slipped and looked at square or octagonal options to stop this.

His idea was not to clean them up and remove flashing when manufacturing. It provided a reasonable grip and stopped them rolling during fitting

Brucey
Posts: 32286
Joined: 4 Jan 2012, 6:25pm

Re: 'O' rings

Postby Brucey » 26 Jan 2019, 4:48pm

in some cases vacuum seals which were subject to hard use would wear their sealing faces (because of unavoidable contamination), so that even with a new O ring fitted, sealing would be imperfect. Annoyingly the corners of the O-ring groove would usually be just fine; the damage would be confined to the middle of the groove and the middle of the mating face. In a few cases we used seals with this cross-section

Image

as a replacement. Even though there is a theoretical trapped volume between the seal lips, and the way the seal is pressurised into place by the working pressure is different, in practice these were usually a great improvement over the continued use of a worn part with any round section seal.

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~