Please identify mystery linkage on this 0-6-0

[DKS]

David, YES. I think that must be how the SP&S link is. But I cannot understand how that works. There us a pivot in the connection to the center of the valve rod. So when the vertical link pivots, I don't see how that gets the valve rod to move. Where did you find that photo?

I found the image in a Google search for 'steam locomotive cylinder'. It's a commercial photo from a licensing service (Alamay, IIRC). Bear in mind I'm not an expert on this mechanism, but based on the photo, this is how I see it working:



The curved actuating lever swings forward and back, moving a bar under the cylinder (yellow arrows) via an intermediate linkage that appears to be adjustable, given the multiple holes in the bar on the left end of the linkage. It looks like there are inclined notches in the bar (red arrows) that raise the the bar up as it slides toward the drivers, presumably moving actuating rods in the cocks vertically (green arrows) to open them. The water/steam would appear to be released through the holes just to the left of the lower green arrowheads. The adjustable intermediate linkage does not appear to be present on all locos.

Based on the last photo Chris posted, it appears that one works a little differently; it looks like the bar just slides back and forth straight, and the inclined notches directly actuate the drain cocks (imagine the bar above flipped upside-down). There are probably many other variations on this theme.

Again, this is all speculation based on what I can see. I've done a fair bit of work on steam locos, but not on this particular mechnism.
 

[mmagliaro]

David,
What confuses me is that that "intermediate" linkage is not rigidly attached to the bar under the cylinder. So when the curved link swings back and forth, I can't imagine how that gets the intermediate link to move the valve bar. I would expect it to just pivot at the valve bar and flop back and forth.

[Chris333]

You are just push/pulling the bar. The bar is held inline by the drain cocks.

http://www.nelsonslocomotive.com/Shay/Engine/CylinderCocks/CylinderCocks.htm

http://ww3.tiki.ne.jp/~hwata/C53_201610e.htm

[DKS]

David,
What confuses me is that that "intermediate" linkage is not rigidly attached to the bar under the cylinder. So when the curved link swings back and forth, I can't imagine how that gets the intermediate link to move the valve bar. I would expect it to just pivot at the valve bar and flop back and forth.

It doesn't need to be rigidly attached; indeed, it shouldn't be, because it must accommodate the slight vertical arc of the end of the curved arm as it moves (the arm in Chris' photo is slotted to compensate for the arc). The intermediate linkage simply transfers the horizontal movement of the curved arm to the bar that slides back and forth under the cylinder. This bar slides through slots, so it remains in its proper orientation as it opens the drain cocks.

[mmagliaro]

It doesn't need to be rigidly attached; indeed, it shouldn't be, because it must accommodate the slight vertical arc of the end of the curved arm as it moves (the arm in Chris' photo is slotted to compensate for the arc). The intermediate linkage simply transfers the horizontal movement of the curved arm to the bar that slides back and forth under the cylinder. This bar slides through slots, so it remains in its proper orientation as it opens the drain cocks.

Yes, I see how there needs to be some accomodation for that arc.  But consider the two pivot points, P1, P2 I marked in your photo below:


Since that intermediate arm can pivot at both ends, how does it ever move the valve bar?  When the long curved arm swings toward the drivers, for example, why doesn't the intermediate arm just pivot at each end, leaving the valve bar right where it is?  I don't see how any actual physical force can get transferred from the curved arm to the valve bar by an intermediate link that can pivot at both ends.

[DKS]

Since that intermediate arm can pivot at both ends, how does it ever move the valve bar?  When the long curved arm swings toward the drivers, for example, why doesn't the intermediate arm just pivot at each end, leaving the valve bar right where it is?  I don't see how any actual physical force can get transferred from the curved arm to the valve bar by an intermediate link that can pivot at both ends.

Perhaps it might be helpful to think of the intermediate linkage as the tie rod in a steering system. Neither end of the tie rod is rigidly attached to anything; the rod simply transfers the motion of the steering mechanism to the steering knuckle on the front wheel via a ball joint (or socket) on each end.

Now, imagine P1 and P2 are the ball joints of the tie rod.


 

[mmagliaro]

Yes, but the tie rod only works because it is moving in the same direction as the exerted force.
In the picture below, the red arrow is the direction the tie rod moves back and forth to turn the wheel.

But in the locomotive assembly, it would be moving more like the yellow arrow.  And in the case of a tie rod assembly,
if the inboard end of it moves up and down, the wheel doesn't move.  The tie rod can only move the wheel when
it moves back and forth horizontally.

The analogous case for the locomotive linkage would be that that intermediate link would have to be pushed in and out away from the frame.  But it doesn't move that way.  The curved swinging arm can only move one end of the intermediate link fore and aft.

I still don't see how that direction of motion would make the other end move horizontally to make the valve bar move.

[Doug G.]

I don't think there is any pivot on the bottom bar. I think it just moves linearly right along with the curved bar and the valve bar moves in turn.

Oops, I misread your post, Max. I thought you meant there was another pivot in addition to the ones at the curved rod and valve bar. However, I think there is enough linear motion with the curved bar moving back and forth there to move the valve bar. There has to be a limit to the pivoting action.

Doug

[DKS]

I think part of the problem may be the lack of a whole picture. So, I've rendered a simplfied version of the entire linkage assembly.



In the illustration, red arrows indicate linear motion; blue arrows indicate rotary motion. Parts names are not "official" terms for those parts, but terms I use to hopefully convey a better sense of what is happening.

The control rod from the cab moves forward and back (with respect to the locomotive) and moves the top end of the bell crank. The bell crank pivots on the pivot rod, which also transfers rotary motion to a "half crank" on the fireman's side of the locomotive pilot (not shown) to actuate the cocks on that side.

The bottom of the bell crank transfers forward and back motion (now reversed) to the cylinder cock actuating bar, which slides forward and back to open the drain cocks, via the intermediate link.

The intermediate link is rendered as a rod instead of a flat bar for the sake of simplicity. It can freely pivot on both ends, but there is nowhere else for it to move because the motion of the parts to which it is attached is constrained: the bell crank and actuating bar can only move forward and back, so the intermediate link can only move forward and back.

I hope this clarifies how the system works. It's not defying any laws of motion.
 

[peteski]

I'm with Max on this. Even if this illustration is correct, what type of a valve would be controlled by a bar which slides fore and aft?

[DKS]

I'm with Max on this. Even if this illustration is correct, what type of a valve would be controlled by a bar which slides fore and aft?

Look closely at the photo at the top of the page. The bar has a pair of sloped edges. As the bar slides, it also raises and lowers just a little--enough to open the cocks. The photo that Chris posted shows an alternate design: the sloped edges face upwards, so the bar only slides, but the slopes engage the valves.

Here's an illustration showing a simplified schematic of the parts and their movements.


 

[svedblen]

Great schematics DKS!
As with many of the best engineering solutions, simple when explaind but tougher to come up with.

[DKS]

Great schematics DKS!
As with many of the best engineering solutions, simple when explaind but tougher to come up with.

Thanks, but in this case I think it's a very simple mechanism that has proven to be very difficult to explain, for reasons I can't quite fathom.

[DKS]

Here's another close-up photo. In this case, the locomotive has three drain cocks per cylinder, instead of the usual two (the third is out of the image to the left).

At any rate, you can more clearly see the sliding bar, and the slopes that actuate the cocks. You can also see the intermediate linkage connecting the sliding bar to a bell crank--one that's configured differently from the others, but operates in the same manner. In this case, the crank changes vertical motion into horizontal; there is another crank above, out of frame, that converts horizontal motion to vertical.



 

[DKS]

Here's yet another cylinder cock linkage system. It has all of the same parts as the rest we've been discussing, simply arranged differently. Hopefully this one is clear enough to illustrate its operating principles. In this case, the bell crank is located under the cylinder, but it works exactly the same way as the others.