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Lady Stephanie engine (drawing errors?)

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Del_61:
Hi everyone.

Just made a start on my Lady Stephanie engine. I bought the castings many years ago – they do look a bit like rough shapeless lumps at present. To be honest most of the “castings” could have been made from stock.

I also have the build articles from EIM by Tubal Cain and I have already spotted an important discrepancy between the drawings and the articles, namely the dimensions from the centre of the engine to the centre of the cylinder and centre of engine to the crankshaft centre.

In the EIM article this dimension is shown equally as 2 1/2″ either side of the engine centre line.

In the official Reeves drawings the dimension from the centre of the engine to the centre of the crankshaft is 2 1/2″, but the centre of engine to the centre of the cylinder is shown as 2 15/32″.

Its only 1/32″ but on an engine this small, it sounds significant. I think the official drawings are the correct dimensions.

Interesting that on the official drawings the corresponding dimensions on the beam are 2 1/2" equally spaced from the centre line...

So has anyone got any idea which is correct?, does it matter? For those who built this engine, which dimensions did you use? and are there any other dimensions that maybe incorrect or pitfalls I need to look out for?

All the best

Derek

Jo:
I built to the EIM article and had no problems.

Except possibly making/tapping the 12/14BA stuff, which was character building  ::)

Jo

pgp001:
It is the same on the Stuart Beam engine.
The beam is 7" centres, the crank is 3 1/2" offset from centre, but the cyclinder is ony 3 7/16" offset from centre.

There must be a good reason for this, but I have not got my head around it quite yet. I think it must be to do with the parallel motion geometry.
Phil

Charles Lamont:
Yes, it is to do with the geometry of the motion. On the LS this is not parallel motion, but something else which I think must involve some pretty fiendish geometry to deal with second-order effects. The crankshaft has been positioned so that the connecting rod will be at (roughly) the same angle to the vertical at 90° and 270° crank rotation. The offset at the cylinder means the link from the crosshead (if that is what it is called on a beam engine) to the beam will swing either side of vertical rather than only leaning towards the beam centre. This will reduce the forces in the motion, and I expect it will improve the linearity of the crosshead motion.   

pgp001:
I know everyone, (well maybe not everyone) seems to think that the parallel motion likeage will give a true linear connection from the beam to the piston rod.
But that is not the case, it looks more like a very slender figure of eight is plotted by the end of the piston rod.

When modelled up on Solidworks with all the usual constraints, the engine will not actually turn over due to this motion error locking it up.
So there needs to be a bit of play somewhere to get it to run in practice.

Phil

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