Hi Dave

I struggled with this on my first loco Rob Roy. If you get hold of a copy of the booklet on Walshaerts and Stevensons valve gear by the late Don Ashton it details a simple jig to set each pair of eccentrics correctly in relation to each other plus instructions for valve setting. The book is I believe still available and Don Ashton's website is still maintained and well worth a look, Camden used to stock it so try their website.

Dave

Edited By Dave Wootton on 21/04/2021 06:39:49

It is a very good question! And for all steam-engines with eccentric-driven valves.

Full-size engines were normally very carefully set out on the drawing-boards, and the eccentrics were located on keys. The only adjustment was on the valve-spindle, for setting the valve centrally on the ports in its mid-travel.

Many models rely on firm push-fits and grub-screws, and unless you provided a hole in the strap for the Allen key or screwdriver, adjusting them means dismounting the strap each time.

The drawings should give you at least the starting-point for locating the eccentrics, to save you having to draw it all out… or guess.

' So on general principles ….

The first principle is that the valve should move symmetrically across the ports in all positions and on a locomotive, the same travel both forwards and in reverse.

In mid-gear this travel = 2(lap + lead).

In full gear either direction = 2(lap + port opening + lead). Port opening to admission is not necessarily the full width of the port itself, according to the design.

Not all engines are designed with lead but it would show as a tiny opening of the port just as the piston reaches the true dead-centre point – that point is not easy to determine but there are geometrical ways to do that.

'

The second principle is that for Stephenson's Link Motion,

In mid-gear and at both dead-centres, the eccentric centre-lines (of maximum eccentricity) and that of their crank-web form a symmetrical Y diverging from the crank-axle axis.

That means the expansion-link and lifting-links should be perpendicular to a line parallel with the crank-web centre, joining its suspension-point and the axle centre; and the eccentric-rod pins should be equidistant from the crank-pin.

'

Really, it comes down to adjusting the eccentrics as far as possible to reach their symmetry at dead-centre and in mid-gear, then observing the valve's position. Centre it as necessary. Try again on the opposite dead-centre. If the results differ then probably one or both eccentrics need adjusting slightly.

Now repeat the tests in both full-gear positions, to ensure the port opens when it should, and the aperture is both equal at both ends, and at the same point in the stroke.

This does assume the reverser-stand's mid-gear position or reach-rod centres are not telling fibs, and some builders use a temporary, adjustable reach-rod to determine its true length for the individual engine.

This rather ideal approach may also be affected by the individual loco's design, introducing geometrical errors. I have never quite trusted for example, LBSC's trick of hanging the valve-rod clevis from a swinging-arm instead making it a small cross-head.

If effects like that become apparent, then adjusting the motion will be something of a compromise. I'd consider if my loco is going to romp round a continuous track or mainly amble back-and-forth along a short straight, and in the former case go for as good running forwards as possible even if at some cost to reverse efficiency.

Dave S, I've sent you a pm

but when using launch links it gets very difficult

I did of course mean loco links