While reading the thread about hurdy-gurdies I opened this link which reminded me of a question I’ve long wanted to ask.
The b&w illustration of the girl with the long hair shows the crank on her hurdy-gurdy. It is similar in basic design to a lot of large and small pre-mid 20th century handcranks that I’ve seen. Instead of the crankarm radiating straight out of the hub like a spoke on a wagon wheel, it curves in the shape of a “C”.
This design is also be found in machine wheels of the period. Instead of having radial spokes, again like a wagon wheel, they have these curved “spokes” going from the hub to the rim.
Now, I’m no rocket scientist but I was pretty good at basic HS and college physics, and I can’t figure out any advantage to the curved-crank (or curved-spoke) design. At first I thought it would add an element of springiness to the transfer of power from the crank to the axle that might spare the hub from overstress, but I think if you “do the math” that theory is ultimately an illusion. Or maybe not.
Maybe just for decoration; on a curvaceous instrument like that, a straight crank might look quite out of place.
Possibly related are the curved or s-shaped spokes found on some old cast iron wheels - not sure if this too was done for decoration, or maybe to deal with stresses formed by the cooling process after casting.
As Mangetout said, for cast metal objects, the metal tends to shrink as it cools. Thinner portions cool and shrink faster and can ‘hot tear’ away from their connection point to thicker places, which tend to remain molten longer. If you cast a wheel rim, straight spokes, and hub as a single unit, the shrinking can cause the spokes to snap. If you change the spoke to a curved shape, the shrinking spoke will straighten out slightly but not be as likely to snap or warp the rim.
As for the sort of S-shaped crank handles seen on hurdy-gurdies, I suspect this is an aesthetic choice rather than an engineering one. I have seen spiral cranks made such that the weight of the crank and handle is symmetrical about the axle, so that the handle has no ‘preferred’ position when it is released; but this can be achieved with a straight handle and a balancing counterweight too.
But that would also be true of a straight bar welded off center or even tangent to the hub.
For spokes that are welded to wheels, like on water main handles, I can see that the curve gives some protection from metal expansion and contraction in extreme temperatures. As the ring expands, it would tend to pull away from the spokes, but bent spokes will straighten slightly before they break the weld.
But I’m sure there must be another good reason, since besides wheels and spokes the curve is also on things like pump handles.
FTR, I remember from my engineroom days that the only valve handwheels that had curved braces were cast iron. Most of the handwheels were aluminum and had straight spokes, regardless of the size (e.g. a 3-foot monster on a steam valve).
Might be a cast-iron thing.
It’s probably mostly for looks. Although, I notice that the straight cranks stick out more. Almost all of the curved ones have a lower profile. That may not be a necessary feature, but it seems to hold true for many of them. There might be some advantages to the curved form, but there are quite a few examples of straight-crank hurdy gurdys, most of them on the older examples from c. 500–600 years ago.
When it comes to cast iron, things that meet at right angles are prone to cracking. Since you’re trying to avoid machining (at least, if you want to keep the main virtues of cast iron intact: it’s cheap and easy) it makes sense to cast in a shape that is optimal for your material. Aesthetics don’t hurt here, and in this case a functional shape also happens to look nice. Round metal bars are prone to bending at odd angles if subjected to stress. Any bends in just about any direction would adversely affect the function of the z-shape, which seems to be used most for the cranks made from round bars. Anyone who has tried to straighten wire or bar stock can tell you that round stuff is a pain to straighten out once bent.
The flatter cross-section of the curved cranks probably resists bending better under most conditions and could easily be fixed if it did bend “up” or “down” (toward or away from the instrument body) by taking it off, finding a flat surface, and pounding on it to flatten it out again. A bend along the curve would have little to no effect on its function, if you could even bend it that way; it’s significantly stronger to stress in that direction. On the other hand, if you treat your instruments well, the resistance to damage and ease of repair would make no difference at all.