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Bilgram's valve diagram a graphical method to design a steam valve

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Dan Rowe:
I struck up an offline conversation with Chris about the valve gear for the Sabino because he made a comment that he wanted to make this engine with a lot of detail. The only valve gear I have really studied is Stephenson link motion. This is mainly because of my interest in Shay locomotives which are essentially a marine engine on a locomotive. The very first Shay built by Ephraim Shay was a marine engine on a flatcar. A Shay locomotive is really just a marine engine that got lost in the woods.

When I discovered Bilgram's diagram I was thrilled as I thought my struggle to learn valve gears was ended. Bilgram's diagram is a powerful tool to design a steam valve but it does not really help in the design of the rest of Stephenson link motion or any other type of reverse gear.

It was easy for Chris to get accurate measurements of the link motion but when I asked about the angle of advance he gave me an approximate number because it is not really possible to say looking at a built engine where the center of the eccentrics are in relationship to the crank pin for the cylinder. 

If the eccentrics were keyed to the shaft and fixed like a Shay and this was an IMPORTANT detail to the builder then the angle of advance would be a key piece of information. Lucky for me the valve travel and the angle of advance are listed on the drawing card index for every Shay.   

With those two numbers and knowing that Shays used 1/16" of lead, I can calculate the cutoff for that Shay. I do not think it is a coincidence that this is true.

I was not sure what would be a good number for the cutoff for this engine but in the book "Machine Drawing and Design" by William Ripper, there is a design for a two cylinder launch with all the parts and a Zeuner diagram. I thought I could simply look at the Zeuner diagram and state the angle of advance but it is really not quite that easy even if you have drawn that diagram as a learning example. I took the easy way out and drew a Bilgram diagram using ONLY the valve travel, the outside lap, and the lead. This worked to 74.92 degrees.

I also told Chris that for model engines it is customary to set the lead to zero. This is a match to the advice given by Dave on the Sabino thread.
                                                                                             

--- Quote from: steamer on April 10, 2022, 03:56:39 PM ---The last indicator card for Sabino that I saw was 75% cut off and just a smidge of lead.     I would dispense with the lead on an engine this small.   
Dave

--- End quote ---

More on how to use a Bilgram diagram to design a steam valve in the next post.
Cheers Dan

crueby:
This should be an intersting journey, looking forward to the diagrams and methods!
Chris

Admiral_dk:

--- Quote ---A Shay locomotive is really just a marine engine that got lost in the woods.
--- End quote ---

 :lolb:   :lolb:   Thank you for that laugh Dan  :cheers:

Per

crueby:

--- Quote from: Admiral_dk on April 11, 2022, 10:33:29 PM ---
--- Quote ---A Shay locomotive is really just a marine engine that got lost in the woods.
--- End quote ---

 :lolb:   :lolb:   Thank you for that laugh Dan  :cheers:

Per

--- End quote ---
Then a Lombard is just a train on portable tracks!

Dan Rowe:
Per, I just really wish i was the one who came up with that phrase but It was Dave Latrope circa 2005 where I got it from.

Chris some Lombards like the Phoenix have vertical cylinders so a Shay on endless tracks.

As I mentioned I only used three numbers to calculate the cutoff for the Ripper launch engine.
1) Valve Travel
2) Steam or outside lap
3) Lead or the amount the valve is open at TDC

Most readers will know that TDC stands for top dead center and BDC stands for bottom dead center. This gets rid of the clumsy naming found in a lot of old steam books where the ends of the cylinder are called crank end and head end. A steam engine had a head at both ends. I am mostly interested in vertical engines so I draw valve diagrams with TDC at the top and BDC at the bottom.

Here is the Bilgram I drew to find the angle of advance of the Ripper launch engine.



Cheers Dan

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