Post by steamer5 on Apr 9, 2019 9:09:57 GMT
Hi All,
Following on from the discussion in Greenglades (Pete's) thread, Building Don Young's Doncaster, I thought I would create a thread about piston valves & there ring.
I mentioned in Pete's thread the info I had been given on making Teflon rings for piston valves & had posted an abbreviated version of the following.....he asked if it would be possible to post more info, so here's the full article & drawing to go with it.
Please feel free to add any other info on making piston valves & rings, so that finding information on these will hopefully be a little easier to find in the future
Teflon Piston rings for Piston Valves
Since the successful introduction of using Teflon for piston valve rings by Maurie Turner of the Adelaide Miniature Steam Railway Society in South Australia, several other model engineers have followed suit with excellent results.
Like Maurie they too have found Teflon rings to be superior to conventional rings, claiming there is no blow-by and negligible wear using virgin grade P.T.F.E. (Teflon), provided certain criteria are adhered to in their manufacture and fitting.
The rings (one on each end of the valve bobbin) are quite simple to make, there being no split to worry about. However, as there is probably no other place on a steam locomotive requiring more care to fits and surface finish than in the piston valve region, good workmanship is essential for their successful operation. One of the main problems in working with Teflon is the difficulty in obtaining accurate measurement due to its lack of rigidity.
Micrometer measurement can be unreliable for final sizes therefore the required fits are best arrived at by 'feel' which may require some experimentation at first.
Before starting to make the rings it is paramount to make sure the bore of the valve liner is parallel and lapped to a smooth polished finish, as any roughness will soon wear the Teflon and lead to leakage. Another important point for success is to keep the ring wall thickness on the thin side due to the greater thermal expansion of Teflon. To avoid binding in the liner at operating temperature, experience has shown that the ring wall thickness should be no more than 0.093" (2.4mm, 3/32") for a 1" bore (scale accordingly for other bore sizes).
To make the rings the following procedure has proved successful.
1. Turn up a mandrel about 3 thou larger in diameter than the ring landing on the bobbins, and stepped to a finish length (width) of the ring.
2 Chuck a piece of Teflon rod and turn the outside diameter 5 to 10 thou over the diameter of the finished bore of the valve liner and then bore out the center to a tight push on the mandrel.
3 Next face and part off each ring 5 to 10 thou over the width of the ring gap on the bobbin.
4 Push a ring on the mandrel and carefully reduce the diameter by taking fine cuts with a sharp tool until the ring is a very neat fit in the liner -a fit which needs to be pushed in with the thumbs with little resistance. Using the same tool setting, machine the outer rings to give the same push fit.
5 Finally replace each ring in turn on the mandrel and up against the shoulder and face the width to a tight fit in the gap on the bobbin.
6 The rings are fitted to the piston valve bobbin which is illustrated in the accompanying sketch and have no side clearance but because they have about 3 thou clearance on the diameter of the bobbin they can expand inwards
as they attain operating temperature, while at the same time maintain a perfect seal against the bore of the liner.
The rings are plain and have no grooves and should be lubricated with light engine oil on assembly in the liner.
One other important point for long trouble-free running with Teflon rings is to make sure the snifter or drifting valve is of ample size to avoid ashes being drawn into the valve space when coasting with the regulator closed.
As for results, those members who have fitted Teflon rings in this way are more than pleased with their performance. One member who has been running regularly 3 times a month for the previous 2½ years removed the piston valves for inspection and on measurement found no discernible wear. He uses 20% less coal and less water top-ups not to mention a crisp sharp exhaust beat as there is no more leakage past the rings. A good test for blow-by is to put the reversing wheel in mid-gear and open the regulator - if nothing comes out of the blast nozzle you have a perfect seal.
Piston Valve with Teflon Rings by Kerrin Galvin, on Flickr
Cheers Kerrin
Following on from the discussion in Greenglades (Pete's) thread, Building Don Young's Doncaster, I thought I would create a thread about piston valves & there ring.
I mentioned in Pete's thread the info I had been given on making Teflon rings for piston valves & had posted an abbreviated version of the following.....he asked if it would be possible to post more info, so here's the full article & drawing to go with it.
Please feel free to add any other info on making piston valves & rings, so that finding information on these will hopefully be a little easier to find in the future
Teflon Piston rings for Piston Valves
Since the successful introduction of using Teflon for piston valve rings by Maurie Turner of the Adelaide Miniature Steam Railway Society in South Australia, several other model engineers have followed suit with excellent results.
Like Maurie they too have found Teflon rings to be superior to conventional rings, claiming there is no blow-by and negligible wear using virgin grade P.T.F.E. (Teflon), provided certain criteria are adhered to in their manufacture and fitting.
The rings (one on each end of the valve bobbin) are quite simple to make, there being no split to worry about. However, as there is probably no other place on a steam locomotive requiring more care to fits and surface finish than in the piston valve region, good workmanship is essential for their successful operation. One of the main problems in working with Teflon is the difficulty in obtaining accurate measurement due to its lack of rigidity.
Micrometer measurement can be unreliable for final sizes therefore the required fits are best arrived at by 'feel' which may require some experimentation at first.
Before starting to make the rings it is paramount to make sure the bore of the valve liner is parallel and lapped to a smooth polished finish, as any roughness will soon wear the Teflon and lead to leakage. Another important point for success is to keep the ring wall thickness on the thin side due to the greater thermal expansion of Teflon. To avoid binding in the liner at operating temperature, experience has shown that the ring wall thickness should be no more than 0.093" (2.4mm, 3/32") for a 1" bore (scale accordingly for other bore sizes).
To make the rings the following procedure has proved successful.
1. Turn up a mandrel about 3 thou larger in diameter than the ring landing on the bobbins, and stepped to a finish length (width) of the ring.
2 Chuck a piece of Teflon rod and turn the outside diameter 5 to 10 thou over the diameter of the finished bore of the valve liner and then bore out the center to a tight push on the mandrel.
3 Next face and part off each ring 5 to 10 thou over the width of the ring gap on the bobbin.
4 Push a ring on the mandrel and carefully reduce the diameter by taking fine cuts with a sharp tool until the ring is a very neat fit in the liner -a fit which needs to be pushed in with the thumbs with little resistance. Using the same tool setting, machine the outer rings to give the same push fit.
5 Finally replace each ring in turn on the mandrel and up against the shoulder and face the width to a tight fit in the gap on the bobbin.
6 The rings are fitted to the piston valve bobbin which is illustrated in the accompanying sketch and have no side clearance but because they have about 3 thou clearance on the diameter of the bobbin they can expand inwards
as they attain operating temperature, while at the same time maintain a perfect seal against the bore of the liner.
The rings are plain and have no grooves and should be lubricated with light engine oil on assembly in the liner.
One other important point for long trouble-free running with Teflon rings is to make sure the snifter or drifting valve is of ample size to avoid ashes being drawn into the valve space when coasting with the regulator closed.
As for results, those members who have fitted Teflon rings in this way are more than pleased with their performance. One member who has been running regularly 3 times a month for the previous 2½ years removed the piston valves for inspection and on measurement found no discernible wear. He uses 20% less coal and less water top-ups not to mention a crisp sharp exhaust beat as there is no more leakage past the rings. A good test for blow-by is to put the reversing wheel in mid-gear and open the regulator - if nothing comes out of the blast nozzle you have a perfect seal.
Piston Valve with Teflon Rings by Kerrin Galvin, on Flickr
Cheers Kerrin