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I had a typical bending drinking straw and when I twisted the outside tip, I expected the straw to rotate in an L-shape, so that the other end of the straw would be rotating sideways. Instead, I found that the straw remained perfectly in place, and the other end twisted in place regardless of the bent angle.

So I was just curious: Do they ever use this particular mechanism in engineering, rather than, say, gears?

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  • $\begingroup$ Early table-top hockey sets used this exact method : attached a spring, bent 90 degrees from handle/rod to the "player" on the rink. $\endgroup$ – Carl Witthoft Nov 9 '17 at 15:56
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Yes, on some ratchet & socket sets there is a flexible drive : two hardened steel springs, one wound one way one the other. But it won't change the direction unless it points 180 degrees the other way.

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  • $\begingroup$ Also, some old fashioned dentist drills had very long flex drives. They were often treadle driven, just to add to the horror. My scroll saw has an rotary tool attachment that runs directly off the main motor shaft. The flex drive (if you remove it from it's sheath) looks like a very long but tightly wound spring that is highly flexible in bending but strongly resists twisting along its length, so you can bend it through 180 degrees while still transmitting a fairly strong torque from one end to the other at high RPMs. $\endgroup$ – user6335 Nov 9 '17 at 9:08
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    $\begingroup$ @Wossname both good examples, also a speedo cable used on cars before it was all electronic - which is where I got from a lorry a broken speedo cable as a "pull-thru" as it does not get cut by metal panels and it is long...... $\endgroup$ – Solar Mike Nov 9 '17 at 11:01
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There are a few types of joint which fall broadly into this category as well as the spring type arrangement used in flexible drives for pendant drills etc there is a joint called a rag joint which uses reinforced rubber of leather to transmit rotation through an angle.

Equally any sort of flexible tubing like fuel hose can be an effective way to improvise a flexible shaft coupling for prototyping etc.

While things like paired universal joints and constant velocity joints are now more common for power applications an elastic joint has some advantages in that it is simple, easy to miniaturise and can help to isolate vibration and shock loading.

Equally CV joints are often protected by convoluted rubber gaiters (boots) which work on the same principal, although these are there to provide an external seal to contain grease and keep out contamination rather than transmit power.

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