Seeking drawing source for a Simpson & Stockton model engine

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I suppose the crank at each end stops any risk of twisting and helps stop things binding up.

Tie rods give a solid location for the piston pivot, having it run without would put load on teh seals at the ends of teh piston's rod as it exits the side of the engien.
Seems like the piston could just move straight up and down, without rotating, and have the piston pin come out a vertical slot.

Then use a traditional connecting rod between the pin and the crank arm that is on the crankshaft.

This engine design seems overly complex for what it is doing.

There must be some logic behind the piston rotating.

As Paul Harvey (Famous radio broadcaster-1950-1980) use to say
"And now for the rest of the story "

But first , Thank you Pat and other followers for the information, and book references and all the comments . it is really appreciated !

I have the Live Steam issues from 1977 so was able to find the article - hot dog !
now I have the LS mag, but this time period occurred during a major life crisis sort of - changing Countries and jobs and moving shops and all- so never got read, only stored

But here is the story : Maybe 20 years ago i was asked by our local tech college if I could help their machine shop class get a steam engine running (class project)
They showed me the "Hedin" engine" which I had never seen before. As a Steam nut and a Manufacturing Engineer, I analyzed the model and solved the problems and the engines ran. So the instructor gave me one , which is what my friend saw , but I never saw any drawings even at the college, so had no idea where they came from. Was told they bought them but since the engine didn't work, I didn't follow up, after all these parts were made on CNC equipment ...sic

There were 3 major problems with Hedin's design as I recall that required remaking the parts for successful operation-so let me point out the 2 most important.
1. The "4" connecting rods must be DEAD NUTS accurate in length. and fabricating them individually is a fools game + having them different bores is
is asking for trouble- so - Make the four rods together and most important- line ream them as a stack with a 1/4" reamer with a 1/4" dowel at the other end. All will be the same length exactly ! now change the throw pins all to 1/4" not the 1/8" call out AND make the little crank throws all the same ( 1/4 and 1/4 )
2. The main valve design is for steam admission , not air !. Steam design has lead and lap ! So the spool valve has to be modified to run on air and eliminate the lap
3. The eccentric design has problems, but I do not recall what, but be careful .

The engines ran like mad after these changes

So my friend wants to replicate the build with my comments and I now have the drawings ,, thanks agian
So I looked at Julius De Waals prints and even though they are Metric, he pretty much followed the Hedin drawing except he doubled the sizes !
I noticed he made the rods all the same ( large bores) but inserted brass bushings for the two different crank throws. He also has the steam valve design with lap, so trying to run his engine on air will be difficult or very reduced power
Thanks again for the help !
Julius does tend to make his engine drawings for bigger sizes.

The actual source goes like this Anthony Mount found the patent darwings and made his model to those (polly engineering) that were published in ME or EiM magazine. Then Stuart Hart made his version based on Anthony's drawings (Potty eng) which were in ME magazine and on the net. Julius then made his drawings based on Stewarts.

I doubt any of them have seen the Hedin ones
One of the things I like about old steam engines, besides the majestic looks of them, is the variety that one can find as these engine designs transpired over the years.

I use to say that I pretty much understand how all steam engines work, back when I was not really that familiar with steam engines.

Now days, I realize that no matter how many steam engines I see, there will always be one more that will be confusing and difficult to understand how it operates.

Its a fun challenge game for me to try and figure out some of these more obscure designs.
And it is a good mental exercise too, which helps when I try to figure out work-related problems.

One of the engines that I had to study for a long time was the Ironclad Monitor engine, after I saw Rich's video of his build.
That one had me scratching my head for quite a while.

I have always said though that if humans designed it, then other humans can figure out how it works.

This thread is an excellent learning opportunity from many aspects.
Thanks for all who are contributing.
I love the learning that can be had here on this forum.
I like the knowledge that can be gained as much as I like engine building.

SO Rich, What are you talking about ? Air or Steam ?

so for those unfamiliar with the differences, let me show you
A big difference in steam engines is "cutoff" where the steam admission cycle is cut short (like 40 % ) which may be called " Lap" because the steam continues to expand and pressures the piston to continue movement. Air on the other hand , corresponds to "Boyles Law" that says that (if you shut off the air) doubling the volume reduces the pressure by 1/2.
SO force drops quickly when the valve closes ......So "steam " engines and "air" engines behave differently ...and for good reason...
so will a air engine run on steam ? Absolutely
So will a steam engine run on air ? all depends on the valve arrangement.
Lets look at Hedin's engine ( and De Waals )
i have copied the original drawing on the left ( modified by removing lines for clarity) and put a colored version on the right for explanation.
( please Note: the pistons position in the drawing is incorrect for the valve position so do NOT consider it as a part of this discussion)

The spool valve has red lands that cover the ports to the cylinder
The blue lines represent the steam/air admission (cavity)
The red arrows show the "Lap" in the valves operation

For normal "D" valve operation on air , The port hole size is the major determining factor, so lets say for discussion it is 1/8 . Now the stroke is 3 x the port size or 3/8 and the land of the valve must match the port size which is 1/8
Simple formula, but when you look at the drawing and the dims, you know it is wrong. Look at the Arrows and you can see sizable movement is required to move the valve before any air is allowed to enter the cylinder in either direction !

So there is some info to ponder if you wish to build any of these designs and run it on air pressure



  • Steam versus Air.jpg
    Steam versus Air.jpg
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Well, I just realized that Jason and Pat have posted a whole lot of information here and the technical
research and drawings are awesome . I have been on another mission ( and still am) and need to come back to study the data. I agree with Jason that They are reffering to the two cylinders being at 90 degrees so there is smoother power. But there may be another point ?? and that is counter-balance of the round pistons mass traveling in a oscillating motion ? . I would think those outside rods and their orientation may have an effect and understand Pat's concern and thoughts.
Yes, the main links, if i can call them that, travel in a arc, and like the Dake, the pistons thickness must be absolutely flat and parallel along with the cylinders' walls to minimize all leakage.

So much to do--so little time
My friend will go nuts when he rads this !