JonL
Elder Statesman
WWSME (Wiltshire)
Posts: 2,988
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Post by JonL on Feb 13, 2019 21:29:57 GMT
 Made the cylinder caps tonight for the open end (I'm still really bad with terminology). Tapped a thread for a gland but I'm using an o-ring in there. Next I need to drill the holes for the bolts, which I'll use my crude paper template technique for. I'm learning some things work for me and some things don't! I've also ordered the stainless for the piston rods. |
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JonL
Elder Statesman
WWSME (Wiltshire)
Posts: 2,988
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Post by JonL on Feb 16, 2019 19:16:28 GMT
After copious reading of the threads regarding piston rings, and offline chats with Roger, I decided to go with Silicone piston rings. As the piston is quite deep I had enough room to fit two. I worried that putting in two rings would badly increase the friction, but applying 25 psi to in steam chest resulted in me nearly shooting myself in the face with a piston rod.  I went with the technique discussed by a knowledgable chap on one of the other threads; The o-ring is sized for the bore, there is space under it, and a little space either side. The theory being that the sealing is carried out at the bore and on one side of the o-ring. It seems very effective but I won't know for sure until its got steam under it. I've used a cast iron piston in a cast iron bore for reasons of expansion. |
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Post by Cro on Feb 17, 2019 20:24:36 GMT
I was always taught to cut the groove for orings on pistons at least 1.5x the section of the orings so that they can roll and won't wear out as quick as they would stationary where you'll end up with D shaped rings pretty quick.
Just a thought and someone may prove me wrong.
Adam
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JonL
Elder Statesman
WWSME (Wiltshire)
Posts: 2,988
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Post by JonL on Feb 17, 2019 20:40:03 GMT
I was always taught to cut the groove for orings on pistons at least 1.5x the section of the orings so that they can roll and won't wear out as quick as they would stationary where you'll end up with D shaped rings pretty quick. Just a thought and someone may prove me wrong. Adam I did read some similar advice, with conflicting advice in the very next reply! I'll do a bit more research... In the meantime I've machined dummy pistons the full length of the bore with an extended piston rod coming out. The idea being I now have a very stable representation of the centre line of the cylinder to enable me to make up the slide bars with a high confidence of correct alignment. As mentioned earlier the slide bars on the 3.5 gauge Britannia don't actually contact the cylinder block. I also managed a brief spell to make up the piston gland nuts. This was during test fitting; I've since given them a going over with emery cloth.  |
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Post by Roger on Feb 17, 2019 20:49:29 GMT
I was always taught to cut the groove for orings on pistons at least 1.5x the section of the orings so that they can roll and won't wear out as quick as they would stationary where you'll end up with D shaped rings pretty quick. Just a thought and someone may prove me wrong. Adam I don't think you'll find that advice from any of the manufacturers. The only clearance you need is to deal with the bulging from compression in the other direction. That's not ever going to be 1.5x the section. Generally, you'll use no more than 20% compression on static seals and maybe 10% or less on dynamic ones. In that case you'll only make the width of the groove at say 110-115% of the cross section. In reality, it will always roll a little and there will always be a flattened area in the end. I understand that some people leave a gap under the 'O' ring, but that's not something that any manufacturer recommends. If you do it that way, it will pass a lot of steam when the piston reverses unless the 'O' ring is a snug fit on the width of the groove. |
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JonL
Elder Statesman
WWSME (Wiltshire)
Posts: 2,988
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Post by JonL on Feb 17, 2019 21:02:01 GMT
I didn't seem to get any blowby at 25 psi, but I realise thats a low figure and doesn't take into account the advanced wear Adam mentions.
Funnily enough my biggest conundrum was how many to fit. Most people seem to use one (or a cast iron ring) but I figured I've got a few more than that on my car, and it was a pack of 5 o-rings....
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Post by Cro on Feb 17, 2019 21:05:52 GMT
I'm not saying the manufacturer recommends it but I'm going off experience of locos at the club, saw a pair come out of a loco done like that and after 12 months of use they are D shaped compared to a loco with rings of similar age and more use had much less wear.
Try and see is all you can do. I did Grandads Maid this way with the OD of the groove applying light pressure to the O-ring with them being a snug fit in the bore and they were perfect, had them out once since and no issues but not recently.
Adam
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Post by Roger on Feb 17, 2019 22:52:38 GMT
I'm not saying the manufacturer recommends it but I'm going off experience of locos at the club, saw a pair come out of a loco done like that and after 12 months of use they are D shaped compared to a loco with rings of similar age and more use had much less wear. Try and see is all you can do. I did Grandads Maid this way with the OD of the groove applying light pressure to the O-ring with them being a snug fit in the bore and they were perfect, had them out once since and no issues but not recently. Adam Fair comment, but there are so many variables, such as surface finish, 'O' ring material and oiling, I think that the sample size is way too small to come to any concrete conclusions without a lot more practical controlled experiments. You're in uncharted territory when you stray from the manufacturers recommended fitting arrangements which always compress the 'O' ring section between the parts to be sealed. |
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Post by Cro on Feb 18, 2019 6:32:54 GMT
I think in reality if we are looking at off the shelf products from engineering industry there are possibly much more suitable products for this application.
From memory I think I did the 9f in the same way as the maid but too long ago to remember, I'll have cylinders apart eventually before I finish it to do new valves so will check what a years running did.
Adam
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Post by Donald G on Feb 18, 2019 8:51:57 GMT
I and others in our club who use O rings for pistons always use the dimensions given on GLR Kennions website and have had no problems with wear.
Donald
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Post by doubletop on Feb 18, 2019 9:05:15 GMT
I've seen "Quad" O rings suggested in the past and plan to try them on my next piston ring replacement. If and when required allsealsinc.com/Piston_Seal_Application_Example.htmlI had thought they were some sort of special order but talking to my local supplier not so. They are equivalent to their similar sized brother. So for example a 218 O ring 1.25" I/D, 1.5" O/D, 0.125" Dia can be replaced by a "Quad" Q218 of similar dimensions. If you want it in Viton then its a Q218V. I don't know if that just their way of identifying them but it does make it simple. I say I've no real experience of using Quad O rings but they do look promising and worth a go. Pete |
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Post by Roger on Feb 18, 2019 9:14:25 GMT
I and others in our club who use O rings for pistons always use the dimensions given on GLR Kennions website and have had no problems with wear. Donald Hi Donald, That's interesting, because they are using 10% compression on the diameter and about 26% on the axial. That will certainly allow for some rolling though, and that might be why they do it in this application. The radial compression makes good sense, you don't want any more than that else it's going to wear too much and it's going to seal anyway, even with 5% compression. |
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Post by Roger on Feb 18, 2019 9:18:07 GMT
I've seen "Quad" O rings suggested in the past and plan to try them on my next piston ring replacement. If and when required allsealsinc.com/Piston_Seal_Application_Example.htmlI had thought they were some sort of special order but talking to my local supplier not so. They are equivalent to their similar sized brother. So for example a 218 O ring 1.25" I/D, 1.5" O/D, 0.125" Dia can be replaced by a "Quad" Q218 of similar dimensions. If you want it in Viton then its a Q218V. I don't know if that just their way of identifying them but it does make it simple. I say I've no real experience of using Quad O rings but they do look promising and worth a go. Pete Hi Pete, I also like the idea of Quad Rings, but do check the availability before designing them in. Just because they appear in a catalogue, doesn't mean they are on the shelf. If you're looking for Viton ones, I'd be very surprised if you didn't have to have them made to order and that's not cheap. This is what I had to do for the ones on the eccentric pump, and those were Nitrile, so ought to have been on the shelf really. I get the impression that industry is slow at adopting these, hence the lack of stock. It's a bit of a chicken and egg situation really. |
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Post by doubletop on Feb 18, 2019 9:40:30 GMT
I've seen "Quad" O rings suggested in the past and plan to try them on my next piston ring replacement. If and when required allsealsinc.com/Piston_Seal_Application_Example.htmlI had thought they were some sort of special order but talking to my local supplier not so. They are equivalent to their similar sized brother. So for example a 218 O ring 1.25" I/D, 1.5" O/D, 0.125" Dia can be replaced by a "Quad" Q218 of similar dimensions. If you want it in Viton then its a Q218V. I don't know if that just their way of identifying them but it does make it simple. I say I've no real experience of using Quad O rings but they do look promising and worth a go. Pete Hi Pete, I also like the idea of Quad Rings, but do check the availability before designing them in. Just because they appear in a catalogue, doesn't mean they are on the shelf. If you're looking for Viton ones, I'd be very surprised if you didn't have to have them made to order and that's not cheap. This is what I had to do for the ones on the eccentric pump, and those were Nitrile, so ought to have been on the shelf really. I get the impression that industry is slow at adopting these, hence the lack of stock. It's a bit of a chicken and egg situation really. I thought I'd try them on some brake cylinders I was making so went to our local supplier to discuss. There was no tutting or "we'll need to order". We discussed a size and he went to the store and got them for me. I was surprised how easy it was. Decide what normal O ring you would use, tag a 'Q' on the front of the number, call the supplier and see what reaction you get. Pete |
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Post by terrier060 on Feb 18, 2019 14:41:13 GMT
I was always taught to cut the groove for orings on pistons at least 1.5x the section of the orings so that they can roll and won't wear out as quick as they would stationary where you'll end up with D shaped rings pretty quick. Just a thought and someone may prove me wrong. Adam Hi Adam When I looked at mine after many years of running they did have flats on them, but still did not blow by. I think the secret with these rings is that the pressure behind them automatically pushes the ring tight on the far face of the piston groove and the bore, so they will always seal unless the rubber goes off. That is why in my opinion only one ring is necessary and I have always used only one ring, even in the 'Terriers'. But I must add I am not an expert on 'O' ring technology only by personal experience. It was about 50 years ago I used rings on my tank engine, so I think I used Viton rings though I can't be sure. I have used 'O'rings in my major beam engine as well and that runs very sweetly. Ed |
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Post by Deleted on Feb 18, 2019 14:53:16 GMT
Just a thought but if using two rings with the wider spacing that this instils are we not risking messing up the timing. I'm thinking of lead/lap across the ports? BTW, Don specifies one 'O' ring for 'Doncaster'.
I'm just curious about others more experienced than I thoughts?
Pete
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JonL
Elder Statesman
WWSME (Wiltshire)
Posts: 2,988
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Post by JonL on Feb 18, 2019 16:33:06 GMT
This is for the piston, the valve has glands in the normal fashion. Steam is admitted at each end so it shouldn't be a worry. Two is probably overkill I admit!  |
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Post by Deleted on Feb 18, 2019 17:04:12 GMT
This is for the piston, the valve has glands in the normal fashion. Steam is admitted at each end so it shouldn't be a worry. Two is probably overkill I admit! ah.. good point Nobby, sorry I'm clearly not firing on all pistons today...  |
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Post by simplyloco on Feb 18, 2019 17:16:14 GMT
This is for the piston, the valve has glands in the normal fashion. Steam is admitted at each end so it shouldn't be a worry. Two is probably overkill I admit! ah.. good point Nobby, sorry I'm clearly not firing on all pistons today... Piston broke?😊 |
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JonL
Elder Statesman
WWSME (Wiltshire)
Posts: 2,988
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Post by JonL on Feb 18, 2019 19:21:01 GMT
ah.. good point Nobby, sorry I'm clearly not firing on all pistons today... You are normally a very good source of quality hints and info so I think a day off is allowed! |
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