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 Joel's Blog

Plumier, Crankshaft Design, And How We Waste Time at TFWW

11/02/2010

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 drawing on the whiteboard to explain what we were talking about.

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.

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.

Join the conversation
11/02/2010 Zach
I have wondered about this as well, and have no answers...

One possibility is that when the crank is loose and left to hang by gravity, the bend could be used to put the handle or pedal in the best position...

Z
11/02/2010 John Vernier
One other possibility is that the U shape is a much more forgiving way to determine the precise sweep of the crank during forging: once the ends are punched for handle and axle, one final heat allows the sweep to be set by bending the curve one way or the other.
11/02/2010 Matthew Groves http://www.rationalartisan.com
The cross section of these arms don't really lend themselves to flexing, but perhaps there's another reason.

If I was hand forging an arm like this, I can easily see why it's easier to put a curve in the arm....adjustability.

Having a curve allows the maker to adjust the distance between the two pivot points to exactly where he wants it, or exactly as the application requires.

If it were a straight arm, it wouldn't be impossible to lengthen or shorten the arm if it were "off", but it would be a whole lot easier to ease or tighten the curve to achieve the same effect.
11/02/2010 Bill Dalton http://Www.docwks.com
If they are wood it would break before stripping the threads and keep one from using to much torque.
11/02/2010 Wesley Tanner
Nineteenth-century iron handpresses (for printing) often had this type of handle used to run the bed (the coffin) of the press in and out under the platen (printing surface). My own press, an 1863 Albion from London, has such a handle, and I've never noticed any difference in turning it (the rounce) either way. The only thing that has ever come to mind is that the shape might be more forgiving of the considerable amount of torque being applied at the beginning of each use. In normal use this would be many thousands of times over the lifetime of the press. And indeed many of these presses still have them working well into the current century.

Wesley
11/02/2010 Wesley Tanner
Here's a link to a picture of a rounce handle on a replica of an eighteenth-century wooden handpress at Texas A&M.
http://www.flickr.com/photos/7864355@N06/514857796/in/set-72157600269118170/

Wesley
11/02/2010 Christian Groves
Its probably a 17th century attempt to get around the top-dead-centre problem. By utilising a curved handle they probably thought when the crank was top dead centre the curved handle gave a sideways force to get over it. Once flywheels came along to get over the problem and after many design iterations they probably realised you don't need it and it died out.
11/03/2010 Fred Krow
The mass on the curve acts as a counter weight and gives a flywheel effect.

Regards,
FK
11/03/2010 Stephen Shepherd http://www.fullchisel.com/blog
John got it right, the curved crank could easily be adjusted to the proper distance between the shaft and the knob or pitman. And it looked good.

Stephen
11/07/2010 Harlow Chandler
Curved cranks are seen in countless applications. My mother had a spinning wheel with a curved crank on the big wheel (no need for flywheel effect there). They were common on handcrank telephones (also spinning reels and many other hand cranks (no adjustability advantage there). Spring action? Really? The ones in the drawings look pretty stiff to me, and I can't picture the whole rig needing any such dampening effect, and again, if there were what advantage would there be on a fisherman's spinning reel for example? There's apparently some obviously incorrect folk wisdom that the "longer" crank gives more leverage. Over the decades every now and then a bicycle crank is made this way to overcome the dead center problem--or so they say. It doesn't, of course. I don't have the answer either; even the aesthetic notion--that it just looks good--isn't convincing in all cases. All I can say is that if there is any mechanical advantage you'd see curved cranks on the unbelievably expensive racing bicycles in the Tour de France. These guys care about seconds in an all day race, and their cranks are straight. Thanks a lot guys; how am I supposed to get to sleep tonight?
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