Seeking drawing source for a Simpson & Stockton model engine

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Richard Carlstedt

Well-Known Member
Jul 9, 2007
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Green Bay ,Wisconsin
A good friend of mine wants to make a model of the Simpson and Stockton rotary piston engine.
This is not the drawings or model offered by Polly Engineering in the UK ( called "short stroke" )
and covered is a MEW issue years ago.
Recently Juilius De Waal did a fabulous reproduction of the drawings and they are available free on his website IJulius mentions that he was given prints from a old magazine as a source before he redrew it .

My friend wants those original prints which I thought may have been in Modeltec in the 80's but I do not know.
The model was made by a Mr Robert s. Hedin but I find nothing when searching for him.

So does anyone have a index of Modeltec that could answer that or know where it was done originally
The one Julius based his on is by Stewart Heart and part of his "potty" range of engines and was in model engineer magazine and is a barstock engine.

The one from Polly (bruce) engineeris is from castings.

Stew's build here Potty Simpson and Shipton Short Stroke Engine
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"The Steam Engine", Robert Scott Burn.



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I think if I were going to build this engine (I am not, but others may), I would just redraw and/or put into 3D the original dimensions from the Robert Burn book.

Why use other's dimensions when you can just use the originals from the 1800's ?

It is a bit like a Dake in some respects, although Dake took it to another level with his piston inside a piston design.

Julius's drawing shows the cranks at 90 degrees, but the original engine clearly does not have the cranks at 90 degrees, as shown in Fig. 95 on page 106 of the Burn book.
But reading the text of the Burn book, it says the cranks are at right angles to each other, so the drawing in the Burn book is suspect, or I am just not understanding the section.

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It appears that Julius's drawings adhere to the Live Steam drawings, with the typical DeWaal flywheel bling added.
Old steam engines had spoked flywheels almost always, not round holes, if you are looking for authenticity.

The Live Steam Magazine drawings do not adhere to the original drawings in the Burn book very well, but have instead been simplified quite a bit, and converted into a barstock build.

Interesting in that the wear is only on the side walls of the oval cylinder, and not on the top or bottom.
The original engine had a spring loaded movable side to take up wear.

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Pat, I can't see any dimensions in those pages from the book so that would be one reason I would use my own, second is I like to work in metric. The other reason the majority of builders will alter sizes is to suit the machine they will be building the model on - not much point in drawing it all up based on a 12" flywheel if you only have a 7x 16 Minilathe or Shereline to work with so best to change the sizes.

Good find of the Live Steam source.
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Those Imperial Journal 1853 engravings are really nice.

I guess it depends on how deep someone wants to get into the build, whether they start from the old engravings and make drawings to some scale, or save time and use the Live Steam or other drawings.

Its a very interesting engine for sure.

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That is an excellent engraving with lots of accurate detail.

Wear plates on the left and right sides of the cylinder, with take-up adjustment on the right side.

It does appear to suggest a twin arrangement.

I will have to go back and look at the animation that is at one of the links above, and study the motion again.

I don't really understand the dual cranks at 90 degrees yet.

Some of the drawings seem to show the pin in the piston moving in an arc, but the simulation shows a straight-line motion.

The engraving that Jason just posted shows sort of a flower pedal opening in the ends of the cylinder, so I am quite confused.

I am watching a slow motion simulation, but I still don't really follow the need for the two cranks at 90 degrees.

Scratching my head on this one.

Two cranks at 90deg is as I said easily explained by the twin which is common practice both with twins and compounds so there is no dead ctr and as the twin is described as a marine engine even more important to have easy starting in any position.

The description in one of your links says the slot is actually curved but shown straight. That rof "F" on my engraving which links the conrod to the piston rod will only allow it to move in an arc
It would seem that the engine shown on pages 106 and 107 above is a single-cylinder design.

Looking at Stew's build, it is also a single-cylinder engine.

Here is a nice translucent simulation:

I will try to analyze this engine, and make some assumptions.

See if anyone agrees with my assumptions, since they are basically my best guesses as to how this engine works.

1. The two rods between the crankshaft and the piston pin are not necessary for the engine to run, as long as the slot on either side of the engine is accurately machined.
I assume the rods give a simple means of adjustment, but the piston pin could just as easily be constrained in the slot, but without easy adjustment for wear.

2. The engine does not need two crank arms.
One crank arm would be sufficient for this engine to run.

3. The piston itself could be thought of as a crank arm, and one could envision the piston as flat, acting like a torque wrench, which I think I saw in another design.

4. The maximum torque would be achieved when the piston is at mid-stroke ?

5. Little or no torque would be produced when the piston is at TDC and BDC (top-dead-center and bottom-dead-center), since any force on the piston pushes straight down on the piston pin, which is at the center of the curved slot.
Torque equals force times distance, and in this case there is force, but no distance.

6. The wear is on the cylinder sidewalls only, and not on the rounded ends of the cylinder.

7. Simplifying the engine, and removing the two connecting rods between the crankshaft and the piston crank pins, and using only one connecting rod on one side of the engine.......Lets look at the relationship between the crank arm that is attached to the piston pin, and the crank arm attached to the crankshaft.
The top and bottom crank arms appear to be close or exactly aligned, ie: when the top crank arm is facing down, the bottom crank arm is also facing down.

8. When the piston is centered in the cylinder vertically (top-to-bottom), the piston pin is offset to one side.
In this position, the two crank arms appear to be pointing straight down, but the rod connecting the two crank arms is angled away from straight vertical due to the offset of the piston pin.
This would appear to be a dead-center for the engine, since the force on the piston will not act on the crankshaft when the lower crank arm is vertical.

9. When the piston is at BDC, the crank arms are approximately 45 degrees from vertical.

10. The piston for this engine is basically a large contrained eccentric, ie: an eccentric that can move up and down only, when it rotates.
The piston has no side-to-side motion, however the piston pin does move side-to-side.
Instead of an eccentric contrained on a shaft, which produces a linear motion due to the offset center of the eccentric, the eccentric is constrained vertically, and this produces a rotating motion on the offset eccentric center.
So instead of the typical eccentric arrangement where rotational motion is converted into linear motion, in the case of this engine, linear motion is converted into rotational motion.

11. It appears that the crank arms that are on either side of the piston pin are at 90 degrees to each other, and the crank arms at the crankshaft also appear to be 90 degrees to each other.

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12. Even with two crank arms on the piston pin, and two crank arms on the crankshaft, and both sets of crank arms at 90 degrees, the engine would still seem to have dead centers (not self starting) when the piston is either at TDC or BDC ?

13. If I were designing this engine, and it had one crank arm only on the piston, I would place the crank arm so that it faced either up or down when the piston was at TDC or BDC.
It would seem that the twin crank arms at 90 degrees is the equivalent of one crank arm at zero degrees.
So why two crank arms at 90 degrees ?
I am not convinced the designer was following any particular logic or scientific method when designing this engine.

14. The two crank arms at 90 degrees would do nothing to alleviate the engine dead centers, and a single cylinder version of this engine would not be self starting ?

15. It would be pretty easy to mock up a single cylinder, with a single crank arm top and bottom, and no tie rods on either side, and the crank arm on the piston directly opposite the large lobe.
I believe this configuration would run in simulation (in Solidworks, an engine will not simulate if there is a constraint problem), and would prove that many of the moving parts on this engine could be eliminated without affecting the engine function.

All you would need is a crankshaft with one crank arm, one connecting rod, and one piston, one piston pin, and one crank arm on the piston pin.
The piston pin would have to fit the arced slot that it rides in.
The upper and lower crank arms would face down (exactly vertical) when the piston was at TDC.
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