O ring groove profile

[Peter Simpson 1Participant @petersimpson1]

Looking to machine the piston for a steam brake cylinder, The bore 13/16th The groove could be machined to suite a suitable O ring. Does the O ring need to be a snug fit in the machined groove ?

[Peter Simpson 1Participant @petersimpson1]

[Chris Evans 6Participant @chrisevans6]

No need for a snug fit. The "O" ring needs somewhere to expand into, charts are available showing all the dimensions to use. From memory a 0.070" section seal needs a groove width of around 0.090" and depth of 0.051"

Check the above sizes before committing, it's been several years since I used "O" seals.

[David Marks 2Participant @davidmarks2]

Normally the manufacturer/supplier of the O ring will have a data sheet which defines the shape/size of the housing into which the O ring is to be fitted. Try (for example) the data available from a company such as James Walker who manufacture O rings.

[Chris Evans 6Participant @chrisevans6]

Posted by David Marks 2 on 18/02/2021 09:12:14:

Normally the manufacturer/supplier of the O ring will have a data sheet which defines the shape/size of the housing into which the O ring is to be fitted. Try (for example) the data available from a company such as James Walker who manufacture O rings.

That has roused my memory. It is the James Walker charts I have in the workshop somewhere……..

[Clive Brown 1Participant @clivebrown1]

Tubal Cain's "Model Engineers Handbook" covers O-rings and groove sizes. Quite a useful reference for data that's otherwise not too easy to find in suitable form fot models.

[JasonBModerator @jasonb]

As Clive says the charts in TC book or a Reeves printed catalogue give groove sizes for our type of use which compress the ring far less than commercial charts which reduces drag yet still gives a perfectly adequate seal for the relatively low pressures.

[peter smith 5Participant @petersmith5]

I have a tobacco tin full of specially ground 1/4” sq tool bits to create internal grooves to suit o rings. All were professionally ground by time served toolmaker at Fairly Aviation from 1936 till the company merged with Westlands and moved from Hayes to the west of England. They came in very useful to create small boring bars. My father in law worked for FA all through the 2 nd world war, 12 hour shifts followed by 6 hour “ fire watch”. MIL also worked there and they shared fire watch duties. Tin hats were the order of the day (night) and he could not understand where the rattling noise was coming from.. She always put her hair in metal curlers ( rollers ) and she was shaking in cold and fear.

He made parts for the famous Swordfish aircraft that crippled a big bit of German maritime engineering. After the war it was helicopters for the French and UK governments. Great confusion between inch, cm and mm caused by the French who insisted on using both cm and mm on the same drawing. BS ???

When my son was about 5 we all went to Keith Fordice ‘s aircraft museum in Torbay. Every part he made had his and other code numbers stamped on them. There was the body of old Whirlwind for the kids to play in/on. He said what his number was and his face was a picture when I told my son proudly “ granddad made this”.

The conversation was overheard by a member of staff and within minutes the former Tomorrow’s World and Farnborough commentator was shaking his hand. What an amazing afternoon and full guided tour. Just writing about it gives moist eyes.

Keep taking the tablets

Pete

[Nigel Graham 2Participant @nigelgraham2]

A fine point about O-rings and their grooves, is that they do differ by class of application – static seal, rotating and reciprocating.

I would not like to say how important this is for our relatively modest needs, but we may as well go by what the manufacturers recommend.

[JasonBModerator @jasonb]

As I said above what the makers say usually gives far too much friction for our needs, the sources I mentioned give details for the various situations that you mention but with far less compression of the rings, ideal for smooth low speed running and not needing excess pressure to overcome stiction..

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