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Tools & Craft # 96: Plumier, Crankshaft Design, And How We Waste Time at TFWW

By Joel Moskowitz - May 23, 2018

in Tools & Craft

I got called to the back of the shop a few days ago where some of my colleagues were having a heated discussion about crankshafts. They were perusing a copy of Charles Plumier's 1701 L'Art De Tourner, Ou De Faire En Perfection Toutes Sortes d'Ouvrages Au Tour and one wag noticed that all the crankshafts in the pictures weren't connected to either a flywheel or a screw by a straight rod, like in a steam engine, but a "U" shaped arm instead. The question that they were asking me was why, I had no idea, and we spent most of the afternoon tossing theories around and making cryptic drawings on the whiteboard to explain what we were talking about.

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The following theories were proposed:

- It looks more elegant - maybe.

- It makes for a stronger casting - not in 1703 - the arms were forged - "S" casting spokes came later.

- It was easier to turn because the U protected your arm - maybe but isn't a wood disk easier to make and cheaper?

- The spring action made the handle more forgiving - I'll buy that.

- The spring action in the "U" shape acts like a mini transmission making for smoother and easier starts and stops and in the case of a screw allows for smoother action if there are variable amounts of resistance when turning the screw. This is the reason I like the best - solid engineering.

Tim said it best, "It's a sweeter action," and he's right. That a vast number of foot and hand powered crankshafts are shaped this way means that the "U" design was an accepted standard form of construction. Individual craftsman might not have understood the engineering of springs and inertia, or the nuances of ergonomics, but taken as a package, as a group, the design must have been thought "sweeter" or they would have not wasted the extra material.

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I am not aware of any historical information on why cranks should look like this, but I haven't looked yet either - it might be out there staring me in the face. I am curious if you can think of any other reasons to make a crankshaft in this form, or if you know of any documentation. But for now at least - fun is over - it's time to get back to work.

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This "Tools & Craft" section is provided courtesy of Joel Moskowitz, founder of Tools for Working Wood, the Brooklyn-based catalog retailer of everything from hand tools to Festool; check out their online shop here. Joel also founded Gramercy Tools, the award-winning boutique manufacturer of hand tools made the old-fashioned way: Built to work and built to last.

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Paul Rogers

6 years ago

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Unless perfectly balanced, a flywheel has a propensity to always come to rest with its heaviest point at bottom dead centre: in the case of a treadle powered piece of equipment, a straight connecting rod will simply try to pull straight downwards, thus not translating into rotary motion, without a push on the flywheel to start it off. A 'U' shaped con-rod should ensure that downward pressure on the treadle will translate to rotary motion, without the need for a push on the flywheel.

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- Jon Plummer
  
  Paul Rogers
  
  6 years ago
  
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  This is the right answer. For treadles and other vertical linear--to-rotary motion linkages, you don't want the linkage to stop bottom dead center or you'll have to get a hand down there to push it away from center to get it started again. So the cranks curved to put its center of balance away from the line between its ends. Make a handful of cranks this way and you'll find them repurposed for other things, or representing the definition of a good crank...
  
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Z Hulett

6 years ago

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It gives you a space to bang on if the crank gets in a bind somehow.

Also, it might help in keeping the female threads from binding onto the shaft. If the crank were straight and the shaft was hard to turn, there might be some torsional forces transferred to the shaft, making the threads bind up. The curved handle means the transferred force is not in the same axis as the force applied to the handle, which could prevent binding.
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