Walschaert Valve Gear Detail

[Runner]

Hi all,

I am building a 3.5" gauge locomotive based on LMS Black 5. The plans I am producing as I go along after studying pictures of the locomotive available on the internet. However, the Walschaert Valve gear has construction detail that is hidden on the completed locomotive. The area of concern that I am struggling with is in the area shown in the attached sketch. The die nut, I assume travels the length of the link, depending upon whether forward or reverse is selected. Forward the die nut is below the pivot on the link and reverse it is above it. Therefore it must travel past the position where the link is pivoted to enable the eccentric rod to impart motion to the link. I assume that the radius rod/die nut connection is as drawn. Please confirm. But how is the pivot constructed to enable the link to rotate but still allow the die nut to travel past the pivot point.

Thanks in advance,

Brian

 

[Charles Lamont]

In Martin Evans's opinion (and mine for what little that matters) a single sided expansion link is not really adequate for 3-1/2" gauge. Rather, for the Black Five arrangement, it should be a 5-layer sandwich fixed together by 2 through bolts at each end and supported in trunnions on both sides. The thin outer plates carry the trunnion pins on which the link pivots. Inside each of those is a plate with the curved slot for a die block, and in the middle is a spacer top and bottom with a gap between for the radius rod to pass through. The bottom spacer is extended to form the tail that is driven by the eccentric rod.

 

[kvom]

You are correct that the link must pivot about the center of the slot and the die must be able to travel the full length of the slot. Your drawing doesn't show how the pivot "nut" connects to the link. As drawn I don't see how it could work. On my A3 build the system is similar to what Charles states above. The radius rod is forked with the link between the forks and the die fastened to the forks with a cross pin. The two trunion on either side are fastened to the top and bottom of the link and have pivots. So the 5 layers are two trunions, two radius rod forks, and one link.

 

[Runner]

Thanks Charles, I intend to have a double sided expansion link, I was tempted to include a picture of the area of concern of a Black 5 locomotive, but that may have infringed copyright issues so I did a sketch which was simplified. Hopefully to show the problem.You have solved the problem that I had. In some locomotives the two outer plates are shaped the same as the inner plates, i.e shows the curve that the die nut runs in, in others it appeared to be a flat plate with holes. Your explanation solved that dichotomy.

Thanks kvom, your explanation supports what was previously stated. I will have to aquaint myself on the constuction details of a trunnion.

Brian

 

[chipenter]

LBSC did a black 5 called Doris , Have the articals in PDF format 27 of them , send me a pm with your email and I will forward them to you .

View attachment 2449-Doris-00.pdf

 

[kvom]

Here's the drawings for the link and trunions on the Kozo A3. The trunions are pocketed to allow the radius rod to go between them and the link, and the three are secured by screws (one trunion tapped).

 

[Runner]

Thanks chipenter PM sent.

Thanks kvom, most helpful.

Brian