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O rings

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Wolfie06/11/2010 11:07:55
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OK I know what an O ring is, its one of those little rubber washer things thats circular in profile.
 
So if I want to use one to seal my cylinder around the piston rod gasket do I need to cut a V for it to sit in?  and do I use one that matches the outer diameter or the outer diameter plus a bit or what?
KWIL06/11/2010 11:56:44
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You obviously need The Model Engineers Handbook as well, more reading!
Andrew Johnston06/11/2010 12:20:31
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Try this handbook, it'll tell you more than you ever wanted to know about O-rings and seals.
 
 
Regards,
 
Andrew
Howard Jones06/11/2010 13:55:53
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an "o" ring  needs a 10% squeeze.
Stub Mandrel06/11/2010 14:41:45
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I have evolved my own method, which seems to work and does not depend on any critical machining of dimensions.
 
This is what I do for a piston rod gland, taking 1/8" diameter as an example, I use a ring that is nominally 1/8" bore and 1/16" cord size - so overall diameter 1/4"
 
The rod will be stainless steel, 1/8" nominal size, polished lightly to reduce wear on the o-ring. The hole for the rod in the end cover will be reamed 1/8" i.e. a good running fit for the rod, then counterbored  1/4" with a drill or d-bit about 3/16" deep. The gland itself will have a spigot about 3/16" long which will be a free fit in the end cover, and be drilled a very free fit on the piston rod.
 
It is essential that only one of the holes is a close fit on the rod (the one in the cover) as if they are both close fits and slightly mis-aligned the gland will bind. Using screws to hold down the gland, rather than screwing it in means the risk of misalignment is minimised compared to a screw-in gland.
 
I use one or three screws to hold down the gland cover and find that under these circumstances it is easy to adjust the pressure so the gland is pressure tight. The sealing surfaces are the ring against the bottom of the counterbore and on the surface of the rod, which means that only the surface finish of the counterbore and the size and surface finish of the rod are critical.
 
Neil
Wolfie06/11/2010 17:47:09
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Thats interesting. I have so far made 3 glands cos the piston rod binds when I screw it in to the end cover. The plans dictate a threaded spigot (if thats the bit that screws into the counterbore).
 
So if I drill or ream that out to say 5/32 then it will still seal in with an O ring and the rod won't bind any more?
Stub Mandrel06/11/2010 20:18:15
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Sure. The o-ring does the sealing and you only need on of the gland or cylinder cover to guide the piston rod - my logic says it should be the rigidly fixed piston cover that does the guiding.
 
Just make sure the hole in the gland doesn't allow the o-ring to distort or get pinched (both unlikely)
 
Neil
Dusty06/11/2010 21:35:23
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Wolfie
   If you are going to use 'O' rings you do not need a threaded gland. You are much better off using a plain gland. It appears that you have threaded the cylinder cover. This will need to be set up and bored concentric with the hole for the piston rod. If it is not concentric the 'O' ring will not work properly. New glands will need to be made with a spigot that fits the newly bored hole in the cylinder cover. The hole in the gland should be a good sliding fit on the piston rod, if you make it with clearance the 'O' ring will try to escape through the gap causing the 'O' ring to tear (yes even though it may only be a few thou gap)  The counterbore and the gland must be made so that the gap left between the bottom of the counterbore and the end of the gland is such that it allows the 'O' ring to roll a few thou. Charts can be obtained showing the nomenclature of 'O' ring groves and recesses. Your problem with the piston rod binding is probably caused by using a die to cut the thread on the gland. It is very difficult (if not impossible) to retain concentricity using a die. Either screwcut or make a new gland but do not drill it for the piston rod yet. When you have got your gland threaded, put a short length of brass in the chuck face it,drill and tap a hole the same size as the thread on you gland, make sure you can screw the gland right in. Now you can drill and ream for your piston rod. unscrew it and you will have a gland which may not be perfect but will be a lot better as far as concentricity is concerned.
Wolfie07/11/2010 01:00:05
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Okayyy so if I don't thread it, how does it stay in the cylinder cover? It'll just fall out right?
John Olsen07/11/2010 08:43:01
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Historically on full size engines there were two common ways of making a gland. One has a hole tapped into the cover. The inside end of the hole is smaller to give a running fit on the rod. The gland fitting also has a hole to fit the rod.  Soft packing is pushed into the hole and the gland fitting is screwed down to compress it enough to give a seal, but not so hard as to grip the rod too tight.
 
The other way is much the same except instead of the hole being tapped, there are two or three studs arranged around the hole. The gland has a flange with matching holes, and there are nuts that pull the gland fitting down onto the packing. The down side with this type is that if you do not tighten the nuts evenly it will pull the fitting onto an angle, tending to jam the rod. They were possibly the most common in full size, partly because you really don't want to have to put a two foot diameter cover onto a lathe and then screw cut the gland fitting.
 
Either can be adapted for  use with O rings, although it is possibly easier with the stud type since you can make it so that it actually has a shoulder and so will only go in so far leaving just the right shaped space for the O ring. In the smaller engines the studs need to be quite small, eg 7 or 8 BA, or M2 if you prefer. With teh stud type adapted for O rings, you don't get the problem with it trying to angle, since it goes up against the shoulder and no further. This maybe calls for a sketch...
 
regards
John
John Olsen07/11/2010 09:26:12
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Ok, here is a picture of the type with the studs,although I have not actually shown the studs themselves.
 
That little space where the O ring sits should actually have a little clearance top and bottom. The bore in the cover is such as to give a little nip on the outside size...I don't have the book to hand, but for a sliding seal it is not very much. As you will see, the little shoulder stops you clamping the gland follower down too far so you will get the right clearance.
 
regards
John

Edited By John Olsen on 07/11/2010 09:27:19

Wolfie07/11/2010 10:22:15
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OK got it, thanks.
 
The design I have actually has the bore in the cover projecting from the cover so this version is out this time round as the gland would have nothing to screw into!
 
OK as I understand it from these posts you don't actually squish the O ring down hard, just enough to make a seal. Which implies that the screw in version will be better as you can adjust it.

Edited By Wolfie on 07/11/2010 10:23:34

John Olsen07/11/2010 21:41:07
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Well, so long as you do not adjust it too tight...there actually should be a little play up and down along the rod, which means the screw is not in particularly tight.
 
Yes, usually if the design is meant for the type with studs there will be a diamond shape cast in the cover to provide a place for them. That can be machined onto a cover made from bar stock, but it is a bit fiddly.
 
regards
John
Stub Mandrel11/11/2010 22:13:40
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I do it a different way to John. My O-ring is not able to roll, but it doesn't need to be a light fit in the bore of the cover. it works for me.
 
Neil
John Olsen11/11/2010 23:45:06
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Well, if it works, keep doing it. I just follow the proportions given by Tubal cain in the handbook.
 
Incidently I normally use bright stainless rod in as supplied condition, this is not a ground rod, it is smooth surface (rolled?) and seems kind to O rings. Although it is hard to know about life since these are stationary engines, they run at most a few hours per year for demo purposes.
 
regards
John
Stub Mandrel14/11/2010 20:52:29
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Funnily the advice to polish stainless came from Tubal Cain, but teh stuff I use always appears to be ground.
 
Its a tough task finding it in 1/16"
 
Neil
Wolfie15/11/2010 12:47:58
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Well I've bought a box of O rings so I'll check it out when I get back to my workshop.
 
I'm pretty busy at the moment with the SMW show in Telford just finished (plastic modellings version of the annual Model Engineering show)  and of course Christmas coming up so I get busy around October/November with customer's Christmas builds.
Wolfie23/11/2010 21:13:32
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OK I still haven't got back to the workbench. However I have a huge box of Model Engineer mags, about 200 of them dating all the way back to the 1940s and 50s some of them and I'm slowly ploughing my way through them and I read a small article on O rings thats left me with the impression that they rotate in situ??
 
Is this correct?
Gordon W24/11/2010 09:54:03
2011 forum posts
Hi Wolfie, those old mags are great, see how they did things with next to nothing, except a lot of skill. O rings do rotate in some designs, also can roll along the shaft in other designs. Generally only at low speeds, and / or special materials. If your O rings are of unknown material it's worth testing before use , leave in the required liquid for some time and check for swelling, softening etc.

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