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Post by Deleted on Mar 27, 2019 8:03:51 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. Don's figures are basically the same Jason although he gives his in imperial at 3/16 width and 1.500 ID. |
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jasonb
Elder Statesman
Posts: 1,239
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Post by jasonb on Mar 27, 2019 13:53:01 GMT
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Post by Roger on Mar 27, 2019 18:17:35 GMT
I think it's very easy to over think this O-ring business where in fact it's very simple for our purposes. All you have to do is decide the section and then choose the percentage compression. Dynamic applications like this use between 5 and 10 percent, I'd use 5 to keep the friction low.
As long as there's enough side clearance to accommodate the expanded width, that's all you need. Charts just present this data in an expanded form and muddy the water in my opinion, I never use them.
If it's a static application, I use 10-15 percent. It only gets more complicated for extreme pressures, speeds and temperatures that don't need to concern us here.
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Post by andyhigham on Mar 27, 2019 20:01:54 GMT
The side clearance also helps sealing.
The pressure pushes the ring to the opposite side of the groove then goes down the clearance and behind the ring pushing it against the bore. Just like a piston ring!
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Post by Deleted on Sept 19, 2019 17:42:08 GMT
Hi chaps
re-visiting this discussion re-'O' rings and taking on board what's been said I thought I'd share where I currently am with 4472's cylinders and see what you guys think.
After a marathon hand machining exercise, I have now got to the final stages and will share what I have so far.
This post is mainly for your views and how easy the piston should advance in the bore using silicon O rings. Details are as follows
Cylinders:
Left and right: Bore 1.746, piston groove depth 1.517
Middle: Bore 1.734, piston groove depth 1.502
ring section is 0.138
The grooves are currently less than the app I'm using would suggest which states 1.506 and 1.494 respectively, I have deliberately left these oversize, preferring to work my way down to what feels right.
At these dimensions and using regulated air to judge how they are working, I can move the pistons with a little less than 20PSI (air escapes too quickly to move them the full stroke) and blow them clean out of the cylinder at 35 PSI. Please note that the covers are off and air escapes very easily through the steam chambers which are open to atmosphere so no pressure is able to build which doesn't help in being able to judge what's what.
Looking at what the app suggests for the depth of the grooves and allowing a 10% compression for the rings this would mean each cylinder would work out with approx an extra 5 thou RAD above the 10% compression max recommended thus increasing said compression if that makes sense? IE: groove 1.506. ring when compressed 0.249 giving 1.755 overall dia which is 9 thou over the bore of 1.746, middle cylinder works out at 11 thou.
Currently, the pistons to me do feel a little tight when operating by hand and I will probably increase the groove depths to match what the app shows. This brings me to the question of how freely should the pistons move within their bore? Since they are silicon 'O' rings I believe they are not going to bed in and loosen up like proper cast iron piston rings would, or do silicon rings bed in a little too? Considering the compression seems to work out higher than that recommended using the above figures should I look at increasing the groove depth further still?
This probably makes no sense and hoping that some can work out my ramble, over to those of you who use 'o' rings, am I on the right track?
Cheers
Pete
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Post by Roger on Sept 19, 2019 18:31:14 GMT
Hi Pete,
10% compression on the section is about as low as I'd go on any application, although they will seal with less. I'm sure 10% will be fine on the pistons so I'd aim for precisely that. If it goes a shade under then so be it.
I make your current compressions 17.03% on the left and right and 15.94% on the middle which is why they feel tight.
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Post by Deleted on Sept 19, 2019 18:54:38 GMT
Thanks for the input Roger, I hadn't actually worked out the current %, just knew I was on the safe side for now, so thanks for taking the time to do that. Tomorrow I'll increase the groove depth to that suggested on the app. This will still be above the 10% compression going by the fact of the overall dia being greater than the bores. They are getting close, I spent an hour or so today with more honing using the timesaver compound plus some thin machine oil, I'm pleased with the result. Hopefully, I'll get them finished tomorrow, I've re-worked the glands a little as they were a little tight when fully tightened, much better now...  Cheers Pete |
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Post by Roger on Sept 19, 2019 20:18:46 GMT
Hi Pete,
You'll find that a small change in compression makes a big difference in the force it takes to move the pistons. Don't worry if you end up slightly under the intended compression, it will still seal.
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Post by delaplume on Sept 20, 2019 7:57:29 GMT
Hi, Ettingtonliam, Try this}---- ISBN 0 85242 715 8 ----- Hopefully you can read from these}-----    |
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Post by ettingtonliam on Sept 20, 2019 11:12:46 GMT
I did ask back in March, but splashed the cash and bought one myself! Useful for lots of things isn't it?
Thanks for the info anyway.
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Post by Deleted on Sept 20, 2019 11:37:55 GMT
Hi guys with today's modern ways may I suggest the app that was kindly suggested to me by Tony (atgordon) of this parish. It's an excellent program which will calculate all the dimensions for you be it for piston, rod or surface sealing applications. All you need to do for 'piston' is select 'dynamic, enter the bore and planned ring cross-section (metric or imperial) and it will do the rest for you. Here's the link to the android app, I'm sure there will be one for the iphone out there too. download.cnet.com/O-Ring-Master/3000-20432_4-78177328.htmlCheers Pete |
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Midland
Elder Statesman
Posts: 1,875
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Post by Midland on Sept 23, 2019 9:54:47 GMT
Pete, Hope you do nnt mind my using your thread here but I have to ask the assembled experteese. My piston valves on the 990 have cast iron rings but I am not sure they are optimum. As a matter of principle, would it be better to convert to silicon rings? The problem here is that the Midland used outside admission piston valves on this loco (Deeley), rather that inside, so loss of pressure is an issue.
Thanks guys and gals,
David
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Post by Roger on Sept 23, 2019 12:41:31 GMT
Pete, Hope you do nnt mind my using your thread here but I have to ask the assembled experteese. My piston valves on the 990 have cast iron rings but I am not sure they are optimum. As a matter of principle, would it be better to convert to silicon rings? The problem here is that the Midland used outside admission piston valves on this loco (Deeley), rather that inside, so loss of pressure is an issue. Thanks guys and gals, David Hi David, You can't use any sort of 'O' rings on piston valves, they won't pass the ports without geting chewed up. The main drive pistons are a different matter because they don't have to pass any ports. |
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Post by delaplume on Sept 23, 2019 14:55:25 GMT
Hello all,
David, what Roger says re}-- getting chewed up is quite correct but, for optimum iron rings in an outside admission piston valve loco then why not have a chat with anyone involved with a Southern Bullied Pacific as these were also outside admission locos...
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Post by Deleted on Sept 23, 2019 14:59:14 GMT
David, for my piston valves I plan to use (when I can afford it) Fluorosint which is a glass filled PTFE, IIRC Roger is going this route too.
Pete
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Post by Roger on Sept 23, 2019 15:02:43 GMT
David, for my piston valves I plan to use (when I can afford it) Fluorosint which is a glass filled PTFE, IIRC Roger is going this route too. Pete Hi Pete, I'm currently using PTFE, but I'll probably switch to Fluorosint since I've got some big enough to do that. Fluorosint is finer when cutting and less 'chewy' than PTFE. The biggest advantage is that it's coefficient of expansion is about 3 times better than PTFE, similar to that of Aluminium. |
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Post by delaplume on Sept 23, 2019 15:03:36 GMT
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Post by Deleted on Sept 23, 2019 15:04:45 GMT
David, for my piston valves I plan to use (when I can afford it) Fluorosint which is a glass filled PTFE, IIRC Roger is going this route too. Pete Hi Pete, I'm currently using PTFE, but I'll probably switch to Fluorosint since I've got some big enough to do that. Fluorosint is finer when cutting and less 'chewy' than PTFE. The biggest advantage is that it's coefficient of expansion is about 3 times better than PTFE, similar to that of Aluminium. Yes, that's what I have read, looks ideal, just so damn expensive and I haven't found anyone who sells tube (which should save some money) in small quantities.. well, not yet... |
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Post by steamer5 on Sept 23, 2019 20:32:57 GMT
Hi guys with today's modern ways may I suggest the app that was kindly suggested to me by Tony (atgordon) of this parish. It's an excellent program which will calculate all the dimensions for you be it for piston, rod or surface sealing applications. All you need to do for 'piston' is select 'dynamic, enter the bore and planned ring cross-section (metric or imperial) and it will do the rest for you. Here's the link to the android app, I'm sure there will be one for the iphone out there too. download.cnet.com/O-Ring-Master/3000-20432_4-78177328.htmlCheers Pete Hi Pete, I just went & had a look in the App Store & they do the same app for iPhone! Downloaded it, so thanks for the pointer! Oh it also covers X-rings! Cheers Kerrin |
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Post by Deleted on Sept 23, 2019 20:42:45 GMT
It's good isn't it.... |
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