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Upgrading to Stephenson Gear

Better to do it now....

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Iain Downs22/02/2020 20:57:39
562 forum posts
454 photos

Regular readers know that I'm in the (slow) process of building a (mainly) self designed vertical engine of about 2 inch bore.

Originally my idea was to do the least possible and then upgrade bits once it works. So I have a simple eccentric in my design with the idea of upgrading to Stephenson linkage later.

However as I approach the point where I can see eccentrics on the horizon (and not just in this forum!), I'm starting to think it would be easier to design and build it in now.

As I understand it, I need to run 2 eccentrics which are 'out of phase' each attached to a bar with the slide valve attached to a slot which it can be moved along to go from full forward to full reverse (with a bit in the middle where I guess it does nothing at all).

I have an assumption I want to check and a question to ask.

The assumption is that the lead of each eccentric must be set individually, that is they two are not (for example) 90 degrees apart but 90 + 2* lead or something like that. At some point I will be asking for help in understanding how to optimise the lead angle, but not today! So the two eccentrics need to be fixed individually to the crank and not fixed together. It would be easier if I can fix them.

The second question is if the attachment to the expansion link affects the throw of the eccentric. In fact I think I've answered this. In my book (Valves and Valve Gear for Steam Locomotives by Lake and Reidinger) the first pictures I saw (47) had the slot to one side of the pivots for the eccentric links. This would mean that the spindle would move a little less than the eccentric.

However Fig 58 shows the slot above the pivots so the range of motion will be the same.

Any Internet resources (or plans I can copy find 'inspiration' in) would be most welcome

Iain

Martin Connelly22/02/2020 22:59:51
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1026 forum posts
122 photos

Have a look at Elmer's VR75 VR75 plans

Martin C

duncan webster23/02/2020 01:07:22
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2439 forum posts
39 photos

Have a look at this **LINK**

What Don Ashton doesn't know about Stephenson's isn't worth knowing

JasonB23/02/2020 07:07:58
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Moderator
17322 forum posts
1865 photos
1 articles

You don't have to have separate eccentrics they can both be turned as one part but you are then stuck with the angles you machined them at, many Stuart engines are like this as a similar example and in full size the eccentrics are often keyed to the shaft so have no adjustment . For now I would suggest you allow for some adjustment so separate would be best or at the very least one eccentric that mounts onto the others boss.

Movement of valve is not reduced at full travel of the expansion link but by moving it part way savings in steam (or air) can be had.

Iain Downs23/02/2020 16:56:30
562 forum posts
454 photos

Thanks to all for your feedback. I shall have a think!

Iain

John Olsen24/02/2020 03:14:03
1017 forum posts
86 photos
1 articles

If you were not using any lead the two eccentrics would be 180 degrees apart. They both have lead applies, but since one is for reverse the lead is in the opposite direction. For a first approximation, you can regard each one as if it was for a single eccentric gear for that direction of rotation. I suspect that the way it got invented was when someone realised that by connecting the two parts of a gab gear with a curved slot you could avoid a lot of mechanical clunkiness, and that the subtleties of early cutoff came later.

The two types of expansion link are launch type, with the eccentric straps connected below the slot, and locomotive type, with the eccentric straps connected to the ends of the slot. There are also differences with the way the weigh shaft attaches, it may be attached at one end of the link, or in the middle. If it attaches at one end, that should ideally be the reverse gear end.

There are some subtleties to the whole thing. When in mid gear, the link kind of sums the motion from the two eccentrics, and the result is that the valve moves by about the amount of the lap, not doing much. A bit towards one or the other direction, It will actually admit steam, but for a shorter proportion of the stroke, giving early cutoff. This is a useful feature, although not all that important on a model engine not working for its living.

Whether the eccentric rods are crossed or uncrossed is another subtlety. If they are uncrossed when the valve is at maximum upwards travel, the total valve travel (when in mid gear) will be more than if they are crossed when the valve is at maximum upwards travel. This can also be used to change the characteristics when early cutoff is being used. Again, probably not a great concern on a model engine.

If you can get hold of the Dockstadter valve gear design programme, it is a great help in figuring out what is happening.

John

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