Nashville Engine's Piston Rods (Lincoln Funeral Locomotive)

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Nashville Engine's Piston Rods (Lincoln Funeral Locomotive)
Posted by DevinCamary on Monday, May 14, 2018 12:31 AM

Ok so i am trying to figure out how this Engine's piston rod's work. One is the drive shaft, the other is to the gab valve gear. the third one I believe is to cut off steam but I can;t see how that would be unless i an not being observant. Help please?

This is the Piston on question.

This is the best diagram i can see... but sadly that isn't what is on the Nashville. Help please?

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Posted by M636C on Monday, May 14, 2018 8:09 AM

Most locomotives of this period used a variation of Stephenson's valve gear.

In the photo, we can only see part of the mechanism so a proper comparison with the diagram is hard to make. But it is likely that the "Nashville" had the same arrangement, more or less.

The piston rod is guided by the crosshead which moves in the slide bars. THese are shown dotted in the drawing, and the engineman is leaning on the slide bars in the photo.

A lever is connected to the crosshead and this provides the movement of the slide valve, above the cylinder. The slide valve admits steam and allows steam to exhaust alternately at each end of the piston.

The movement of the valve is modified by the valve gear to vary the timing and the extent of the valve opening. This also allows the locomotive to reverse.

Reversing is obtained by lifting the eccentric rods so that the lower gab drives the valve rather than the upper gab (for forward gear).

The position of the gabs is controlled by the lever in the cab (usually called a "Johnson Bar".) The further forward the lever is placed, the more steam is admitted, used at starting. As the train moves faster, the lever is pulled back towards the centre.. To reverse, the lever is pulled back beyond the centre.

In this diagram, it appears that the lever is used only for reversing and a separate (wheel?) is used for varying the valve timing....

In later locomotives, a single control was used for both reversing and adjusting the valve timing, which could be either a lever or a wheel, and in modern locomotives was power assisted.

Does this help?


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Posted by Anonymous on Monday, May 14, 2018 8:23 AM

Model Railroader had an article about the funeral train in February 1995. According to this article the NASHVILLE had a Rogers cut-off valve gear. Here is MR's drawing:

From there you can go back and forth.

Here is a description of cut-off valve gears:

Scrolling down you find a reconstruction drawing on the next page. The valve gear wasn't patented and there are no historical descriptions.

Please read yourself. With my limited knowledge of valve gear I would mix too much up if I tried to recall the essence of the website.
Regards, Volker

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Posted by Overmod on Monday, May 14, 2018 1:18 PM

Most locomotives of this period used a variation of Stephenson's valve gear.

"Stephenson" gear (not invented by either George or Robert, BTW) is a reasonably perfected curved-link gear, likely an evolutionary refinement of slightly earlier straight-link gear.  This has little to do with gab gear other than that both systems actuate steam valves.  Its introduction is one of the great milestones in practical locomotive design, at least comparable to the introduction of Schmidt superheating.  But Nashville plainly has the older gab-gear setup, and has not been rebuilt to a link gear.  Instead, the engine has been provided with a riding cutoff arrangement, which I have partly described in a post to the similar topic in Steam and Preservation.

Johnson Bar reverse requires very substantial force to operate it, and any attempt at fine adjustment has to be made while the reciprocating mass of the gab system is walking back and forth -- this is NOT something easy to fine-tune, as a link motion would be, especially on a passenger engine to be worked at high speed over typical contemporary track. 

Now, even with the forms of link motion available after about 1849, cutoff would still involve lifting fairly substantial reach rods, and in my experience this was usually done with a tumbling shaft actuated by lever.  (We won't go into Winans VG with the cam and starting-bar arrangement!).  A riding cutoff on this type of valve gear would, as noted, allow quick and nearly effortless fine-tuning of admission independent of the stroke of the valve itself.  However, riding cutoff on a gab-gear engine ought to provide most of the advantages of fine adjustment, perhaps more than lever-operated link gear, with the pictured small wheel arrangement ... provided you didn't mind winding the wheel in and out fast (this is something characteristic of much later wheel reverses) ... at very likely MUCH less cost than would be involved in a conversion to link gear.

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Posted by DevinCamary on Tuesday, May 15, 2018 3:21 PM

Yeah both topics were made by me. I thought the first one was deleted by acvident so i made this one. But... turns out both uploaded. Haha....


Anyways yeah, seeing some of the diagrams I got a better understanding, sorta. Still don't know what the cut off plate looks like on the inside

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Posted by Overmod on Wednesday, May 16, 2018 12:13 AM

I thought the first [topic, in Steam and Preservation] was deleted by accident so I made this one.

Pick one to be the 'main' topic, and put a note at the end of the other one directing people to it.

Still don't know what the cut off plate looks like on the inside

There are a couple of versions, and they correspond to evolution of the general idea.  In the reference I provided from 1890 is part of the detail history and a drawing.

Remember that at this general period in steam-locomotive design the principles of valve operation are very different from what they would become with long-lap/long-travel piston valves and low back pressure at high exhaust mass.  The standard slide valve was arranged to have as short a mechanical travel as possible, and to run with minimal accelerations, to minimize the power needed to drive the valve train and the lubrication to keep the valve from wearing. 

The original form of the riding cutoff was to provide multiple ports just above the D valve that connected to the actual admission and exhaust tracts; the earlier versions of this ran on a 'partition plate' which could have multiple holes with multiple effective 'steam edges' (as depicted, although without comment, in the reference drawing, where there are three).  This cut off the flow of steam into the steam chest where the valve ran, with some of the aforementioned thermodynamic 'issues', and resulted in fine control of the cutoff without having to tinker with the Johnson bar, heavy gab-hooks running on the eccentrics, etc.

It was pretty clear that an easier and more elegant way to perform this would be to have the precise cutoff run, or ride, directly on the back of the D valve, with the enhanced steam porting (and some flow streamlining) within the valve itself.  Neither the contemporary valve materials nor the tribology were really up to the job, although some very interesting trick, slot, and multiple porting resulted.

The "Rogers" referred to is not the locomotive builder, or the Thomas Rogers interested in introducing the 'link gear' at this time; it's the Ethan Rogers who was the head of the Cuyahoga Steam Furnace Company for which the 'Cuyahoga Cut-Off' name applies  According to White (in his History of the American Locomotive 1830-1880), the cutoff gear was introduced in 1849 and was not patented.  Alas! his account will not show you the specific internals.  I find it strange that White considered the Cuyahoga company to be little involved in locomotives when more contemporary sources indicate it built the first successful locomotive in the then "West" and then provided power for the roads that became the Big Four and the LS&MS.

I believe the late Charlie Dockstader included a kinetic model of riding cutoff in his valve gear program (which I can no longer run even in emulation).  If you are in fact 3D modeling this arrangement in something like Inventor, looking at Charlie's program is likely to help you dramatically.

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