Re: “Son, It was your idea to be a railroader…” (the inside the cab story of #168’s incident)

Earl Wrote:
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> Not mentioned, is when the reverse lever is moved
> back toward center, both forward and reverse
> eccentrics combine to make the motion for the
> valve timing, shortening the valve stroke and
> cutting off of the steam admission. Also when in
> full forward or reverse motion, the opposite
> direction eccentric does provide a small amount of
> motion to the valve events, which further
> complicates valve setting.
>

This was the major improvement in stephenson valve gear over the hook motion which preceded it. In the older type of motion the engine had no ability to adjust cutoff. You had full forward or full reverse and that was it. Hook motion resembles stephenson gear at a glance--it has those double eccentrics--but without the curved sliding link for the block.

During the 19th century when these parts were often made out of (relatively brittle) cast iron, the inside placement was regarded as something of a safety feature. If an eccentric disintegrated it would simply bang around under the boiler between the frames and not usually cause much of an issue. An outside mounted gear doing the same could (and on rare occasion, did) fling metal parts back up into the cab or into people too close trackside. That was one of several reasons why stephenson valve gear hung on so long after the development of walschaerts. In addition, when it did fail, the engine could quite often limp home due to the partial redundancy of the arrangement. 168's recent experience demonstrates these traits. Such contingencies are justifiably annoying for the folks who have to operate and repair it, but as a (unintended) demonstration of the era's technology I appreciate it on that level.