abby
Statesman
Posts: 927
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Post by abby on Jun 22, 2007 8:57:32 GMT
Hi all. Although I have been active in model making and workshop practice for many years I have never built a loco , apart from a small oscillating model of a colliery underground loco. I decided to build a gauge 1 "DOT" after being given drawings by an friend. Progress has been good , bit of a learning curve , but I would be pleased if someone could answer this question; How do you correctly adjust the valve timing ? The loco has simple stevenson gear with slip eccentric for reversing, and it appears to me that the only adjustment is via the lead angle on the eccentric , if this is adjusted to give any cut-off at one end it alters the other end to its detriment. It appears to me that the thing can only be set symetrical at one point and the smallest adjustment to anything makes it asymetrical. I have a couple of books on loco building - one by LBSC - but can find no satisfactory explanation and a web search has also turned up nothing to illuminate me. Am I missing something? Abby
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Post by chris vine on Jun 22, 2007 9:30:14 GMT
Hi Abby,
If it is a slip eccentric valve gear there will only be one eccentric per cylinder. If you set the eccentric on the axle for forward running so that the steam is admitted say at top dead centre, then the timing for reverse running depends on the angle the eccentric can slip. Maybe it is this "slip" angle which is wrong?
The only other adjustment is to centre the valve so that the opening and timing is the same at each end.
Chris.
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Post by Shawki Shlemon on Jun 22, 2007 9:40:40 GMT
Basically the valve travel ( [valve port + lap] x 2 ) = 2 x the offset of the eccentric . Is this wright? Is the engine designed with lap or not ? Is the size of the valve = all port + gaps between ports + two laps if applicable ? What I mean here is the valve correct size ? If all the above information is correct then eccentric will decide how much the valve will travel . The length of the rod will decide when it will start . All the rest should fall in place .The valve should start to open when the piston is at the end of the stroke .In fwd the the eccentric will be pushed against fwd stop and I assume the slip angle is correct .
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Post by Tel on Jun 22, 2007 10:55:04 GMT
Have you had a look at the 'Tich' book Abby? From memory there's some good info there but regrettably someone must have needed my copy more than me
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Post by baggo on Jun 22, 2007 11:14:38 GMT
Hi Abby,
I have the original ME series on building Dot. I can send you the part dealing with the valve timing if it will help. As you suggest, the main timing is done by setting the eccentric but to get port openings equal may involve lengthening (by making a new one!) or shortening the valve. The valve travel may also need adjusting by filing a bit off the shoulders of the stop collar or soldering a bit of shim on to them.
John
(scans on their way)
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abby
Statesman
Posts: 927
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Post by abby on Jun 22, 2007 20:20:02 GMT
Many thanks to all , and Baggo for the scans , problem is now solved - I hope !
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