I've seen many images of that valve gear but nobody's referred to it as -reversed-.

I might have a model or 2 with that.


I see it now, my NKP 4-6-4 is hooked on the bottom, not the top,making it normal.

Another interesting twist on this were Garratt locomotives. The early ones used direct gear on both engine units. This resulted in the radius rods being up on one engine and down on the other depending on the direction of travel. Later Garratts had direct gear on one engine unit and indirect on the other so that both sets of radius rods were either up or down depending on the direction of travel.

Actually, that's a great idea, Hugh - no balancing spring necessary since the weight of one radius rod would balance the weight of the other on the opposite engine.

dave

Yea, that definitely makes sense, especially on the earlier, smaller Garratts with manual reversers. I don't know why they changed the arrangement on later locomotives; once they had power reversers I guess it was less of an issue. It may have been stress-related as Gary intimates in his post above. If the Garratt was chiefly operated in one direction (as many Garratts were despite their dual-direction capability) indirect Walschaerts on the "rear" engine unit would reduce the stress in the expansion links on the rear engine unit with the locomotive running "forward" since the indirect gear would actually be working "directly" in "reverse" (!).

A friend asked me to post this Question:

A response to this very old topic can be found in a recent Youtube video which includes an explanation of direct and indirect Walschaerts valve gear.

The explanation of how the gear works and the purpose of each of the components are exceptional.

https://www.youtube.com/watch?v=O6rlfXCsoPk&t=19s

I too have been wondering about this for years. CN seemingly had some affinity for the indirect motion, as most if not all of their H class 10 wheelers were built that way (others as well.)These were not switchers that went backwards as much as forwards. Most branches had turntables at the end so these engines likely spent the vast majority of their operating time in forward gear. With almost no advantage that I can see and numerous disadvantages, it’s always been a head scratcher for me. I wonder if there’s anyone who actually knows the answer.
Mark

If indirect valve gear would have some advantages for locomotives running mostly in reverse, this brings up the question, did SP AC class locomotives, or the rear engines of Garratts, have it?

USATC Consolidation Alaska RR #557 and her sisters have indirect Walschaerts valve gear. I have heard that some of the S-160s used a direct version of the valve gear. There was a modification in the very late S-160 production by Lima in which a screw type reverser was used instead of a Johnson bar (export) or power reverse (U.S. version). Maybe there was a change to direct gear made at the same time?

I don’t know about the AC’s, but see my old post about Garratts above.

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Thanks for that answer Kelly. Seems plausible. I don’t know but I suspect many of the H class ten wheelers we’re probably built with Johnson bars

I've read that the main concern with the indirect valve gear, was the possibility of the radius rod dropping down into full reverse while running forward. Because, of the rods or levers coming apart, that ran from the bell crank/ tumbling shaft to the Johnson bar or power reverse. I don't know how bad it would be to go into full reverse, all at once, at track speed. I could imagine any number of bad consequences. Or maybe, just going into reverse and spinning the drivers backwards.

The same thing happening to direct valve gear, would be to just drop down into full forward gear.

I'm wondering how many instances of the control linkage coming apart in service with either direct or indirect motion were recorded? If it were common, we'd see chunks of rods strewn along the ROW. Are we overthinking a situation that almost never occurred?

But there is a gear on each side of the locomotive. If one side of the linkage were to break, it seems the pressure from the other side would work against dropping suddenly into reverse.