Have a look at Elmer's VR75 VR75 plans

Martin C

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.

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