394. C. Parsons's patent device for converting reciprocating motion into rotary,
an endless rack provided with grooves on its side gearing with a pinion
having two concentric flanges of different diameters. A substitute
for crank in oscillating cylinder engines.
Looking at the concept and the Description of Operation from the 1868 book, it was
not intuitively obvious to me how this device works. How does the two diameter
cam work? It is rotationally attached to the small gear, and how does it automatically
flip over at the end of the stroke? I consulted with several fellow engineers, and no one was certain for sure. In fact one
engineer was convinced it would not work because the two diameter cam piece
would flip over when you don't want it to........in mid-stroke. I searched for Parson's patent on this device in both the US and England archives. I
found some of his patents, most notably for the steam turbine engine........which is
probably his most valuable invention. I was not able to find a Parson's patent covering
model 394!! I went ahead and laid out the mechanism in Google Sketchup. From the 3D view, it
looks to me like the two diameter piece (the point of it) hits the curved section at the
end of the travel, and this forces it to flip.
Here is the Sketchup 3D model of my version of Model 394: 
I decide to add a simulated piston.......like a steam engine would have had back
in 1868...which is the red piece. This also helps to guide the endless rack on the gear
from an in and out of page viewpoint. Then to show and compare a real crankshaft, I
added the crank on the right hand side to drive it. One can compare a conventional
crankshaft and Parson's alternative method on the same model.
I used the same gear generator program to print a paper copy of the gears. I chose the following gear design: 
I had to make the endless gear rack from 3 different gears: an internal gear, a small
external gear, and a straight rack. I decided to combine them using paper and glue
versus doing it on Sketchup. The only way to get the gear design from the gear
generator program into Sketchup is to do the following process: 1. Print paper copy of gear
2. Scan paper copy into JPG file
3. Use free program "Rastor to Vector" to convert JPG to DXF
4. Import DXF into Sketchup
5. Manually rescale gear to correct scale Using the paper gears, plus Elmer's glue, I joined all the elements to make the endless
rack. Then I used the 5-step process noted above to get it into Sketchup.
I first made the small pinion gear and the endless rack. When I first tried to run the
pinion gear on the endless rack, the pinion ran smooth on the straight rack section.
However, when I got to the curved ends, the pinion would not run at all I thought I made an error on the inputs to the gear generator, but the endless rack
matched my paper cut-out. I began sanding both the curved ends and the pinion gear.
After about an hour of sanding using the Dremel drum sander plus several files, I got
the pinion to run smooth all the way around the endless rack In hindsight, maybe I should have increased the pressure angle of the gears from 25 to
35. This makes the gears more pointed and they might have required less hand
sanding. This is the 1st time I have made internal gears versus external gears.
 
Boy, after struggling for many hours, and making 3 different cam profiles, I am about
to conclude that this concept does not work in reality. I got the two diameter cam to
shift around at the end of the strokes, but it flips the wrong way when the endless rack
is in the up position. Parson's model 394 might be patented, but it is a problem
because I can not make it work  If I add a pawl to the two diameter cam and hold the shaft fixed, I could probably
make it work. Although Parson's might be the inventor of the modern steam
turbine engine, this idea appears to be a dud!! For now, I have thrown in the towel on making this model work. If you are contenplating building it, email me first.
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