I wanted to build a tall, wood gear pendulum clock that would meet my wife's requirement of no tick-tock. The only movement I know of that does that is the Grasshopper movement developed by John Harrison in 1722. He didn't develop it because it was silent; he did it because by indexing the pallets on the tip of the tooth it eliminates sliding friction, the enemy of accuracy with mechanical clocks. But that action also makes it almost silent. As a side note, Harrison was the guy who invented a clock that could keep time accurately enough for navigation at sea, which allowed sailors to accurately know longitude any time of day, in any weather. The guy was a dogged genius who started out as a carpenter, so there's hope for the rest of us.
There are a few sources on Harrison's grasshopper which explains how the geometry works, and even gives critical dimensions for some versions of it, but nowhere could I find an accurate drawing of one. I finally found a rendering of one online, copied that, pulled it into my CNC software, and scaled it so the dimensions matched those I needed. I didn't know if this would really work, so I built a test rig just to evaluate the grasshopper on the escape wheel that I designed go with it. Harrison was very adamant about many aspects of clock building. He claimed that a 200 tooth escape wheel to be best, though I don't think he offered any evidence why, that the pendulum should swing in an arc of over 10 degrees, and it should weigh 3 lbs. Most clocks at that time had pendulums that only swung about 3 degrees and often weighed close to 50 lbs. I went through the calculations to see if Harrison was correct when he said both would give the same force. He was right. I couldn't imagine what the structure would have to be to be stable with a 50 lb weight swinging back and forth, which made Harrison's design even more attractive.
To my delight, it worked immediately, and it was, indeed, almost silent, with the only noise coming when the pallets release and strike the brass composer that guides it back to the tip of the tooth on the next half cycle. With confidence it would work, I constructed the going train (wheels and pinions) to go with it. The wooden escape wheel above looks very nice but many of the teeth chipped away during machining. Since the pallets rely on catching precisely on the tip of the tooth, unlike other mechanisms which ride on the sides, this wouldn't do, so I machined one from 1/4" cast acrylic. It was about as perfect as I could expect, so I decided to make all the wheels out of acrylic, and just the pinions and connectors out of wood.
Clock wheel and pinion gear ratios is pretty straight forward. You can do them by hand, but I created a simple spreadsheet to do it. Once you have the number of teeth (ratios) for each wheel and pinion figured out, you need to determine the size of the wheels and the center to center distances. You can do that manually, too, or with a spreadsheet which I did, but then you have to draw the wheels in order to machine them. Doing that manually, as I had to for the escape wheel, is very tedious, but I found a calculator by Matthias Wandal at reasonable cost that works very well and draws the wheels and pinion sets for you after you input a range of variables. Best of all, it can output files in several formats, including those who would be cutting the wheels by hand, or a dxf and other formats for CAD use, which is the format I used. Once I had all the wheels and pinions into my CAD/CAM software (VCarve Pro) I could machine them on my CNC.
To those who think cutting the wheels and other parts on a CNC is cheating, I'm fine with that. My hat's off to anyone who can do this level of precision by hand. I'm a very patient guy, but I do have limits. Even after the parts are cut, there is a lot of hand work needed to fit the arbors, bushings, connectors, what not, together to assemble a workable clock. Hours upon hours of work.
I assembled the clock and soon found it wouldn't run with the typical gravity weight drive system, even with 11 lbs of weight, so I decided to use a form of remontoire drive/rewind system instead, using the outside of the center wheel to drive the clock. That reduces the weight dramatically, only about 1/3 lb was required in a test I did.
More to follow.
John
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Originally Posted by Patrick McCarthy
