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Model 983 - Watt's Sun and Planet Alternative to the Conventional Crankshaft

Most of us are familiar with the conventional crankshaft used in a car. Here is a concept drawing:

Because another gentleman has already patented the conventional crankshaft, James Watt had to come up with an alternative design.  He chose a sun and planet arrangement.

From the book 1800 Mechanical Movements, here is the concept of Watt's alternative crankshaft design:

I decided to make a wood model that included both the conventional crankshaft and Watt's alternative design.  Here is my Google Sketchup of my model:

I made an animation of the model to make sure everything worked ok.


Here are pictures of making the red piston and the brown cylinder:

Here is the blank for the flywheel:

Here is base piece glued up:

The only issue I had in making the walnut cylinder was the drilling for the cylinder.  I chose a 2-1/8" diameter bit...versus 2" to give some clearance for the 2" piston. I had to bore the cylinder with a 1-1/2" diamter bit first because my drill press does not have enough HP to run the 2-1/8" bit.  I also used my Forstner bit extension to drill that deep. When I got done, the bottom of the hole was about 2-1/4" and the top 2-1/8". I used the Dremel drum sander to increase the 2-1/8" to about 2-1/4" diameter.  I should have drilled from each end versus going all the way thru from one side.  I sort of knew I should have drilled from each end, but I forgot.

Here are photos of making the red piston.  I am still getting used to having red dust and not panicing.  To woodworkers, the color red usually means blood........and something very bad has happened.  With Padauk, red chips and dust are ok 


I used the drawknife to shave the 4 corners of the glued up blank to reduce lathe vibration. I used a small faceplate with a center screw plus used the tailstock point also. I used a horizontal 3/4x1 wood screw in the faceplate to avoid rotation. The padauk turned well with a gouge to get it to round.

I turned the OD to match the ID of the cylinder. When I got close to size, I moved the tailstock out of the way, and just supported with faceplate. I could take support bar on and off to slide the cylinder over the red piston to check the piston OD size. To cut it off to length, put on my 3-jaw chuck and grabbed on the finished end.  I band sawed most of the large OD off, then turned the rest to a smaller OD, so I could saw it to length using radial arm saw.

Here is photo of cutting the notches in the piston. I marked then sawed 2 slots with straight back small saw.  I band sawed out excess on one end.  On other end I drilled with 1/2" Forstner bit, but you have to watch and make sure Forstner bit is perpendicular when you drill.

On a previous model, the sides of the bearing assembly were not square after I glued them up. I used the black plastic squares from Rockler to help make sure these were square:

I glued a 3/4" thick hub ontothe flywheel, so I would have a place to dowel it to the shaft. I had to use my Forster extension to reach the hub without the drill press hitting the flywheel:

I used my square wood fixture to help me drill 3/8" dowel holes in the red connecting rods. The 3/8" dowel connects the round 3/4" portion of the connecting rod to the rectangular red portion.


I don't really have anything to mark the exact center of a 3/4" dowel. I decided to use the lathe to mark the center, then drill it using the wooden squaring fixture on the drill press. Here is picture of the lathe operation:


 Next I drilled the 3/8" hole using the drill press setup:

 After I made all the pieces, I tried out the model before gluing the components to the base. 


The first issue I ran into was the need for more flywheel intertia to keep the planet going around the sun at the end of the strokes.  I bought some lead pellets for a pellet gun (.177 caliber) and smashed each one with a ball peen hammer.  5 of the them fit into each 1/4" diameter hole I drilled into the flywheel. I glued a short piece of 1/4" dowel and glued it into the hole to secure the lead pellots.  I ground them off using the drum sander on the drill press. A picture of the pellets and flywheel are shown below:

I ground of the 1/4" dowels glued into each hole using the drum sander on the drill press. Then I chucked in the lathe to final sand. I chucked on the hub on one side, then used a dummy wood 3/4" shaft with 1/4" dowel to chuck it the other way:

The next problem was that I broke the 3/8" diameter dowel that the crank went into. I turned down a piece of 3/4x3/4" oak into a 2" long dowel with 3/8" inch diameter. The entire assembly would not turn easily.  I ran the sun and planet side by operating the conventional crankshaft connecting rod by hand.  It ran smoothly.  I then disconnected the sun and planet side and tried to run the piston alone using the conventional crankshaft.  This side of the assembly did not turn freely. The 3/8" oak dowel on the crank would bind in the 5/8" bearing piece.  It need more of an L/D ratio with respect to the 3/8" diameter dowel and the 5/8" thickness of the bearing. I made a round piece that was 5/8" thick with a 1.5" diameter.  I applied Johnsons wax to the oak dowel so it would not be glued, and I used the dowel to line up the new round piece with the existing 5/8" thick bearing side. This was still not successful, there is just too much force required to turn this model.

I went back to the drawing board and came up with a beefier crankshaft design. I made the main shaft 3/4" versus 3/8" and the crankshaft main shaft 1/2" versus 1/4". Here is the new beefier design:


Here is a close-up of the original crank design compared to the beefier crank design:

Here is a movie of just the conventional crankshaft side running:


And here is movie of both sides of the model running (before final sanding and finishing):

At this point, I decided I could go no further in terms of operating improvements, and headed for the finishing shop.  

Because I did some field engineering changes, the blank I had glued up for the base was a little too short.  I made a new base plate. When I assembled everything to the base plate, I Johnson waxed all moving parts..which made a noticable reduction in friction.

One challenge I had was holding the short red bar onto its shaft on the flywheel portion of the assembly:


By careful trial and error of placing the 3 main items, I got the unit to run smoothly before I glued these items to the base. Here are 2 pictures of the glued up model:

Here is a top view.  A guy can never have too many clamps!

And now for the finished model:

And here is a YouTube video of the model in action:


Closing Thoughts:

This model was a lot tougher to build than I originally anticipated. I had to add lead weight to the flywheel to get it to work.  The model also does not perform perfectly every that the gears on the planet and sun end sometimes get stuck or go in the wrong direction.  It would help if there was a ratchet pawl added to allow motion of the flywheel in one direction only.  It is fun to play with once  you get it going ok....and it illustrates Watt's alternative to the crankshaft very well.





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