Allchin slide valve help please

Hi Nigel,

Glad you've hopefully got it sorted. I'll second Neil's comment on your workmanship

Neil,

Even when Stephenson's gear is correctly designed, there will always be movement of the die block in mid gear. It's inherent in the design. With the eccentrics correctly set, the movement of the die block should be twice the lap + lead. Think of the eccentrics as doing the same job as the combination lever in Walschaert's gear. Stephenson's is a little complicated in that the lead varies over the range of the reverser travel whereas with Walschaert's the lead is constant . Normally with Stephenson's, the lead increases as you move from full gear to mid gear. To avoid too much lead towards mid gear, the valve gear is often designed to give perhaps zero or even negative lead in full gear.

The design of Stephenson's on traction engines is actually quite poor anyway. The suspension of the expansion link at the top is always going to give poor valve events compared to suspending it in the centre as done on most locos. Perhaps the designers of traction engines weren't too fussed about getting equal cut offs etc. !

John

Its a 4 shaft engine so rods should be crossed,

Not sure, I think it's an open rod valve gear as the rods are open when the eccentrics are towards the link. Crossed is when the rods are crossed when the eccentrics are towards the link. As Jason says though, four shaft TEs usually had crossed rods so I could be wrong.

John

Edit - Looking at Nigels photos, it is a crossed rod layout so the lead will decrease as you go from full gear to mid gear which ties in with what Nigel found.

Hmmm, wonder how you put crossed rods into a simulator - they seem to be set up for open but there must be a way.

Edited By John Baguley on 11/08/2013 21:12:55

Edited By John Baguley on 11/08/2013 21:15:34

Edited By John Baguley on 11/08/2013 21:21:28

Hi all,

I'm not at all into TEs but am intrigued with mechanical motions. I've just been looking at a simulation of Stevenson (not crossed) that shows the eccentric pins at 80 degrees either side of the crankpin. Am I mis-reading Nigel's pictures because to me the eccentric pins look as though they are at 100 degees or more eitherside of the crankpin.
Nigel, Where is your 82 degrees referencing to and from?
Is this something to do with the crossed eccentric links that is not yet apparent in Nigel's setup?
As previously noted by others, What can be seen in the photos looks exceptional

Bob D

Hi everyone

Thanks for all the interesting replies.

I am not sure what you mean by normal or crossed eccentric links but I hazard a guess that these are normal and crossed would have the eccentric arms connection to the reversing link swapped over.

For your reference I have attached some info from the Allchin book which I hope answers your questions.

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For now I have to leave it a few days. I feel confident it's OK and will run it on air a a couple of weeks.

Thanks

Nigel

As I said at the beginning the eccentric should lead the crank by 90deg plus 7-8deg , Hughes says 7.5.

So the angle between crank and eccentric is 90+7.5 = 97.5

The angle of the template is 82.5deg being the complementary angle of 97.5

This is the drawing I have given several people on how to get the initial timing of a TE, which is basically the same. based on a 4 shaft engine where the flywheel turns clockwise in forward gear. The template is held against the horizontal boiler top.

Edited By JasonB on 12/08/2013 07:50:18

Hi Jason

Thanks again for your info.

Something must be wrong on my engine, set at the 82.5 deg (97.5 deg the other way round) Hugh says that the slide valve should reciprocate with the reversing link in mid gear such that as the crank is rotated the inlet ports should just become visible via a thin black line appearing.

However, on my engine, in mid gear the reciprocation of the slide valve was only 1mm, significantly less than indicated by Hugh's book. Also in full gear the inlet ports did not open until the crank turned through about 40 deg.

So then I must have made some error somewhere, hopefully something I can identify and fix easily.

Thanks again

Nigel