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Eccentrics

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Paul Lousick24/12/2019 20:46:46
1285 forum posts
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The valve eccentrics on my Ruston Proctor SD traction engine are set at 32 degrees (as shown on the supplied drawings) and the engine is running OK but I would be interested in knowing what angle other engines are using.

Paul

eccentric.jpg

JasonB24/12/2019 20:51:52
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The Fowler A7 has them at 38.2deg

John Olsen25/12/2019 04:23:21
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The ideal lead on the eccentrics will depend on the lap of the associated valve. If you are sufficiently curious, you could measure all the relevant parts and do a diagram or better yet, use the Dockstater software to see what the events are like.

John

Nigel Graham 219/02/2020 00:33:59
461 forum posts

The angle of advance is set by the designed lead (usually a tiny bit > 0 " as well as lap, so varies from engine to engine, but a bit over 30º seems quite typical for Stephenson's Link-Motion, full admission port openings and fairly late full-gear cut-off.

In fact your drawing gives what appears to be the lap (7/32 " and lead (that ' +1/64'. Which is quite possible as a traction-engine's engine itself, is designed to run fairly fast.

If I recall aright, the angle of advance on LBSC's Maid of Kent with Ken Harris' version of its Stephenson's Gear is around 35º. A loco is designed to be driven at speed on a very early cut-off, and the effect of that link motion is to increase the lead with notching-up, especially beyond what appears typical for a traction-engine.

The usual plots for eccentric / crank geometry are the Zeuner or the Reauleaux Diagrams, or simplified versions, both quite easy to draw. They simply relate valve-travel to crank and eccentric positions at dead-centre, but not the port openings themselves, nor the full effects of all the bits of steel in between.

Nigel Graham 219/02/2020 00:36:10
461 forum posts

Aaaarrrghh! Those blasted silly faces!

I thought I'd given the spaces necessary to prevent them, and I cannot edit them out!

Nigel Bennett19/02/2020 14:50:12
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Posted by Nigel Graham 2 on 19/02/2020 00:36:10:

Aaaarrrghh! Those blasted silly faces!

I thought I'd given the spaces necessary to prevent them, and I cannot edit them out!

Perhaps you should use millimetres instead... it's the inch symbols coupled with the brackets that causes the problem! (I'll get me coat).

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