The History of Rotary Motion
by John H. Lienhard
THE WHEEL REVISITED
... It's fairly common knowledge that the wheel was invented around 5500 years ago. But what was really invented at that time?... the use of rotary motion.
... A primitive spinning spindle plays out wool or flax fibers while the operator keeps it moving with thumb and forefinger. Early doors, with a vertical shaft on one side, were anchored in sockets that let them swing open and shut.
The next step in sophistication was using bowstrings to drive back-and-forth rotary motions of drills and fire starters. These devices all ran one way to a limit, and then had to unwind. Continuous rotation was the conceptual hurdle. Two primary examples, the vehicle wheel and the potter's wheel, arose about the same time.
A potter's wheel is a horizontal turntable that holds a lump of clay and turns at least 100 rpm. Childe finds one potter's wheel from the region of the Tigris and Euphrates rivers (or present-day Iraq) from as early as 3300 BC. The earliest vehicle wheel turns up in a cuneiform document from same region in 3500 BC.
Those dates don't differ much, and examples are too rare to fix dates accurately. So, to the best of our knowledge, not just the wheel, but continuous rotation itself, dates from five and a half millennia ago in the Fertile Crescent of the ancient world. Another invention, closely kin to the wheel, was the compass for making circles. The first hinged compasses also trace to that same region, 5500 years ago.
Early wheels show a progression of understanding. The first wheels were cut from huge wooden slabs, built up of boards. In other words, the lay of the wood seems to fight the rotary motion it's meant to accomplish. It's not 'til 2000 BC that we find wheels with spokes. The spoke itself introduces a new subtlety since it's loaded in tension, not compression. A vehicle hangs on the wheel's upper spokes; it doesn't ride on the lower ones.
Other questions of rotation had to be answered: Wheels are best left free to rotate on a fixed axis. If they're anchored to a rotating axle, then they can't turn at different speeds going around a corner. The idea of a swiveled front axle, that can turn into a curve, is barely 2000 years old.
And so it is not the wheel itself, but the problem of rotation, that has dogged our minds for thousands of years. What the ancient Sumerians did was to recognize the problem. And we have (if I may) spun out the subtle ramifications ever since.
Please continue with Development of The Wheel
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