Joy's Valve Simulation, by Simon Fraser, enhanced by Peter Lawrence

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Feb 22, 2020
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this is a simulation of Joy's Valve Gear, originally written by Simon Fraser, and slightly modified and enhanced by myself. its written in javascript, so runs in any browser window, and is easily modified. it currently simulates the original Muncaster Joy's Valve Engine, as well has a couple I added, the Van Vleck and the Davy Bros triple expansions, and the LNWR loco "Wessie" (pretty much every Joy's Valve Gear I could find on the internet)

download the tar-gzip file into a new clean directory and untar it with "tar -x -z <"
this will create the directory "latest" and all its subdirectories and files,
point your browser at "file://......./latest/index.html" and there you go.

Here's my take on designing Joy's Valve Gear, if you want symmetrical forward and reverse action then you want the "ellipse" being drawn in the lower right of the simulation to be nearly the same ellipse in both forward and reverse, or at least the points where the ellipse crosses the "valve events" and it seems this can be accomplished with
the radius rod and lower rod should be equal length,
and the trunnion pivot and lower link should be equally offset from the cylinder center
though Muncaster deviates a bit from this, so its not all that critical
the radius link should travel equally from vertical going from TDC to BDC, ditto for lower link
in my Muncaster the radius link goes approx +/- 25-deg, and lower link approx +/- 45-deg
I will add a drawing to show what I mean when I figure out a convenient way to do that (without CAD)


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the trunnion pivot and lower link should be equally offset from the cylinder center
Thank you Peter

My build is pretty far from attempting this linkage yet, but I found this ratio drawing of a Joy valve link. I have circled two dimensions with orange.

Is this the trunnion pivot and lower link dimension you refer to?

Could you please identify the lower rod and the radius rod that should be equal length?

which part is called the radius link?

Thank you for the help. Probably the most difficult part of the build for me, and that's what attracted me to this enigma of an engine.
Joy valve ratio_1.png

also found this diagram
Joy Valve Ratio_2.png

and the 1905 Muncaster plans have this one as reference
Muncaster Joy Links 1905 article.png

yes, it is the two dimensions you circled, both 1' 1-5/16", that being equal helps make the valve gear symmetrical in forward and reverse. note that in the Muncaster plans these two offsets are not equal,
you would have to use Simon Fraser's (or my modified) simulator to see how much effect this has.

the upper rod, connected to the valve spindle and to the trunnion, has a length that matches the radius of curvature of the trunnion, hence the name radius rod, this is common terminology in Walschaert's valve gear so I'm using it here too as they both have the same kind of curved trunnion for the same purpose.

for the most symmetrical valve action I believe the ends of the lower rod (at the bottom of all three drawings) should line up directly below the corresponding ends of the radius rod, you can see that this is not true in any of the above drawings, so its probably not that important, but again you can experiment using Simon Fraser's (or my modified) simulator to check.

I'll have to go back and inspect the Muncaster plans, if they specify the location of the fixed end of the lower rod, then the other end should be directly below the trunnion pivot point, and that would provide a way to infer its design length.

its mostly academic, you can always turn up the pressure to compensate for almost any error in a steam engine, but these things interest me :) !!!
If you like I can reference part dimensions on my engine; I assume they would scale.
Yes please. I was going to use a ratio of the above drawings and scale to my model. I haven't made the valve plate or valve yet so lead/lap is undefined.
thank you

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