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Post by Deleted on Mar 26, 2019 11:42:31 GMT
Hi guys
In the near future, I'll need to order some Viton 'O' rings for my cylinders and I'd like to ask opinion on the best size to use. The bores will be 1.750 or 44.45 mm. Looking at those available at 45 mm OD (would this be the correct size?), I have a choice in inner ID, from 35 mm (5mm cross section) or up to 42 mm ID (1.5 mm cross section) with other sizes in between. I haven't machined to final size yet, so have some leeway if required.
Can anyone with experience with these things, advise which is best for a piston and steam, I'm thinking that 1.5mm cross section will wear out much quicker than something larger. Is 45mm OD too large for 44.45 mm OD bore (assuming I can get it that accurate), of course as things stand now, I can adjust the size of the bore (within reason) as I haven't machined up any pistons to match yet?
I have looked online for formula's but some seem a little above my paygrade, or is it that my mind isn't fully focussed today? I tried reading through the 'Parker' PDF file but decided it was a little confusing, too many figures and dimensions to take in...lol... a simple, you need this size ring to fit this size bore formula would make life much easier.
All thoughts welcome, BTW, I'm going the 'O' ring route as that's what Don recommends for gunmetal cylinders, plus it's easier to replace when required.
Pete
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Post by Roger on Mar 26, 2019 12:07:33 GMT
Hi Pete,
Obviously I don't have any practical experience on these, just general knowledge from other applications.
I wouldn't have thought that Viton was the right material for this application, it's too inflexible in my opinion. I'd use Silicone ones for piston rings. Bigger diameters are better for the reasons you suggest, so 3mm-4mm ought to be a good option. I think one of the suppliers like Polly or Kennions were mentioned before and that would be a guide.
As for the fit, I'd suggest no more than 10% compression on the diameter of the 'O' ring section, 5% is probably more than enough. The inside diameter of the 'O' ring is chosen so it's marginally smaller than the ID it's sitting on so it's not loose. The width is normally slightly more than the amount allowed for on the compression, so say 110% of the section for that.
So for your 44.45mm bore, let's use a 3.5mm O-ring as an example. For 5% compression, you need to allow a gap that's 0.95x3.5mm=3.325mm for the O-ring to squeeze into. That means the inner diameter is 44.45-3.325-3.325=37.8mm That would round down to an O-ring of 37mm ID, forget about the OD, that's not what you define.
The width could be 3.5x1.1=3.85mm
Just be aware that although all O-ring sizes are usually stipulated using the ID, there are some on eBay that specify the OD for some reason.
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rrmrd66
Part of the e-furniture
Posts: 339 
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Post by rrmrd66 on Mar 26, 2019 12:31:15 GMT
Hello Pete Roger is basically correct. I used the data below for 30 years whilst running Merkel-Freudenberg's largest UK technical distributor. It never let me down. www.powerparts.it/simrit_pdf/o-rings_static_seals.pdfSee page 558 onwards for dynamic applications. We were never keen on mixing Viton with steam btw. Silicone is a better bet. Good Luck Malcolm |
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Post by Deleted on Mar 26, 2019 15:50:15 GMT
Ok.. well I now have both outside cylinders the same at 1.743 or 44.27 mm, I can't recall what the middle cylinder measures but it will be close to this. I have a little polishing to do which may reduce this a thou or 2 further, not wanting to strain my old ticker anymore, I think I may leave it at that. I have found some 'silicon' O rings (pkt of 10 for £4.35 incl P&P) at 45mm OD, 38 ID and 3.5mm core, the 38mm ID is very close to what Don shows for the depth of groove on his pistons at 1.5 inch, the width of his groove is more at 3/16 or 4.7mm but I can adjust this if required. Would I be correct in thinking that the 45mm O ring will fit into the 44.27mm bore without causing too much resistance, seems ok in my mind?
Pete
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Post by Roger on Mar 26, 2019 16:14:35 GMT
Ok.. well I now have both outside cylinders the same at 1.743 or 44.27 mm, I can't recall what the middle cylinder measures but it will be close to this. I have a little polishing to do which may reduce this a thou or 2 further, not wanting to strain my old ticker anymore, I think I may leave it at that. I have found some 'silicon' O rings (pkt of 10 for £4.35 incl P&P) at 45mm OD, 38 ID and 3.5mm core, the 38mm ID is very close to what Don shows for the depth of groove on his pistons at 1.5 inch, the width of his groove is more at 3/16 or 4.7mm but I can adjust this if required. Would I be correct in thinking that the 45mm O ring will fit into the 44.27mm bore without causing too much resistance, seems ok in my mind? Pete Hi Pete, You would be better off with 'O' rings 1mm smaller, ideally you want them to touch the inside of the groove rather than have to be forced down into it. I'm sure you can find every 1mm increment if you look. I wouldn't go too wide on the groove because you'll get excessive rolling which will happen at the end of every stroke. The width is not usually much wider than the 'O' ring. |
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Post by Deleted on Mar 26, 2019 16:27:40 GMT
Ok, to play safe and allow me to experiment I have now ordered 44 and 45 mm o rings, both at 3.5mm core. I'm thinking groove width of 4mm, depth of the groove, of course, will have to be different for the 44 mm size.
Cheers
Pete
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Post by Roger on Mar 26, 2019 17:03:57 GMT
Ok, to play safe and allow me to experiment I have now ordered 44 and 45 mm o rings, both at 3.5mm core. I'm thinking groove width of 4mm, depth of the groove, of course, will have to be different for the 44 mm size. Cheers Pete I'm not sure why you are using such a wide groove. Take a look at the chart that Malcolm posted a link to, I think 4mm is way too wide. I don't see any special reason why this application would require different groove dimensions from the norm. You can always make the groove wider, but you can't go the other way. |
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Post by atgordon on Mar 26, 2019 17:05:22 GMT
It's weird how sometimes you find someone asking the very question you have been pondering ... recently, I saw a reference to gas valve regulators, and thought it might be an interesting exercise to draw one to see if it can be fitted in the dome of a s/simplex (it can easily). To operate the valve, I needed to look at a backhead stuffing box (and I learned a lot from Julian's Boxhill build which really shows the Stroudley stuffing box very well). A more modern take might use O rings .... and like you, I quickly realized that this is really a very complex question! I did find that Parker has a very good O Ring Handbook (more comprehensive than the O ring catalog), that really covers the various materials that can be used for what application ... going through that reveals that high temperature steam is a challenge, with few elastomers being suitable. And as you found, once you think you have a materials that might work, the various forms of seal and use adds another fiddle factor. Helpfully, Parker offers an online O ring calculator. Once I have finished the PhD needed to understand it, I'm sure it will be helpful ... Google gives a wealth of info .... much of it conflicting, which only adds to the confusion! (see this link to steam compatible seal materials - they don't recommend viton either!), but they do recommend AFLAS (tradename) which is a TFE/P compound. It seems to be ideal for high pressure steam, including superheated steam. The Parker handbook data confirms the usage too. I'm still no closer to working out my stuffing box dimensions, but I guess it will use TFE/P O rings, well, once I work out the face seal and rotary seal groove sizes ... graphite coated string seems a very attractive option for my usage. Good luck on working out what to use for your pistons! |
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Post by Deleted on Mar 26, 2019 17:28:41 GMT
yes, Tony, that Parker chart is a bit daunting, well it is for me, a case of too much info puts me into overload. I'm sure if it was the only source that I would be forced to spend more time taking it all in, but life's too short and after all, it's not rocket science. I can have a play with the two sizes that I've ordered, they are coming from the far east so will take a few weeks. In the meantime, I have ordered some very fine 'Timesaver' lapping compound for Bronze which should keep me busy polishing the bores, I'll need to make up some type of 'mop' to do the job, probably something to cover the brake cylinder honing kit blades that I have borrowed from my son. all good fun...  Pete |
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Post by ettingtonliam on Mar 26, 2019 19:32:53 GMT
I got mine from Barnwell. They supply imperial as well as metric O rings, so no fretting about the nearest metric size to an imperial dimension, and yes they do a nominal 1 3/4" OD by 1/8" ring, and give a simple table for groove dimensions. It didn't seem difficult at all, unless I've really missed something.
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Post by Roger on Mar 26, 2019 19:44:35 GMT
I got mine from Barnwell. They supply imperial as well as metric O rings, so no fretting about the nearest metric size to an imperial dimension, and yes they do a nominal 1 3/4" OD by 1/8" ring, and give a simple table for groove dimensions. It didn't seem difficult at all, unless I've really missed something. Whatever you go for, whether it's imperial or metric sizes, you're not aiming for a perfect fit on the OD. The ID is what you calculate based on the compression of the cross section. That's almost certainly going to result in a size that's unavailable. It's of no consequence though, you just select the nearest one that's smaller, whether that's 1/32" or 1mm. On the diameter we're talking about, you could be 2mm under size on the ID and it wouldn't matter at all. Personally, I'd stick with Metric since it's never going to become obsolete. |
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Post by ettingtonliam on Mar 26, 2019 20:00:05 GMT
Its not going to result in a size thats unavailable if you are making the parts, just machine the groove to suit the rings.
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Post by Deleted on Mar 26, 2019 20:17:19 GMT
Ok, to play safe and allow me to experiment I have now ordered 44 and 45 mm o rings, both at 3.5mm core. I'm thinking groove width of 4mm, depth of the groove, of course, will have to be different for the 44 mm size. Cheers Pete I'm not sure why you are using such a wide groove. Take a look at the chart that Malcolm posted a link to, I think 4mm is way too wide. I don't see any special reason why this application would require different groove dimensions from the norm. You can always make the groove wider, but you can't go the other way. Well, it didn't seem that wide to me when looking at the charts, it's only 1/2 mm wider that the ring itself. I can make the groove any size that suits as nothing is made yet.. |
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jasonb
Elder Statesman
Posts: 1,239
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Post by jasonb on Mar 26, 2019 20:54:28 GMT
I usually go by the charts in Model Engineers Handbook which are the same as in the old Reeves printed catalogues. For 3.5mm section I would go 4.06mm wide and groove depth 3.35mm less any piston to cylinder clearance. Works on my engines including IC ones running Vitron rings.
For your size cylinders a BS222 would be what I would use.
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Post by Deleted on Mar 26, 2019 21:08:35 GMT
Thanks Jason...I'll look into that too...am I correct in saying this is Nitrile?
Cheers
Pete
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Post by Deleted on Mar 26, 2019 22:54:59 GMT
A big thank you to Tony (atgordon) for pointing me in the direction of a free phone app ( O-RING MASTER) that works out all the dimensions for you. Don must have used the same formular as his dimensions are very close. His groove width is correct at 4.7 for a 3.5 cs ring, his depth ID is very close too at 38.1 vs 38.67 for the app, Don gives imperial so perhaps it's closer than I give him credit. I recommend the app to everyone, give it a try... Pete |
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jasonb
Elder Statesman
Posts: 1,239
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Post by jasonb on Mar 27, 2019 7:21:13 GMT
The Charts I mention will work for all the usual ring materials.
One thing that I will point out is that they give two different sets of figures, one is the usual commercial groove sizes which I suspect the phone app will also give and a second set of sizes for Model Engineering use based on extensive tests by Arnold Thorp (Southworth engines etc) which give a good seal but far less friction and therefore drag than the more highly compressed commercial groove sizes will produce.
The commercial figures will seal upto 1500psi which is a lot tighter than any steam model is likely to see.
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Post by Deleted on Mar 27, 2019 7:39:31 GMT
Thank's Jason, perhaps that explains Don's groove depth being 0.5mm deeper? All very interesting stuff... Pete |
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jasonb
Elder Statesman
Posts: 1,239
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Post by jasonb on Mar 27, 2019 7:54:43 GMT
Just looked in the book again at the other set of figures and they come out very similar to the ones from the app at 4.78mm wide and 38.15 base dia of groove so you may well get more stiction with those figures.
The PDF posted earlier would also seem to be aimed more for higher pressures too, they give hardness of rings with the change over point between 70 and 90 being 6.3Mpa or about 900psi.
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Post by ettingtonliam on Mar 27, 2019 7:59:19 GMT
I usually go by the charts in Model Engineers Handbook which are the same as in the old Reeves printed catalogues. For 3.5mm section I would go 4.06mm wide and groove depth 3.35mm less any piston to cylinder clearance. Works on my engines including IC ones running Vitron rings. For your size cylinders a BS222 would be what I would use. Where do I find the 'Model Engineer's Handbook' you speak of, or at least the O ring charts from them? |
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