The Perplexing ‘Shaft Passer’

Like jokes? Here is an engineer joke. It’s called the Shaft Passer and it’s a different kind of funny.

The Shaft Passer is a mythical mechanism that allows a spoked wheel to rotate despite having a shaft (such as the axle of another wheel) passing between its spokes.

Similar devices allow one cable to pass through another without breaking either. It works by using a spoked, rimless wheel that allows cables to pass through as it rotates. The ends of the spokes are widened, and the cable is held together by a short curved sleeve through which these spoke ends slide.

Here is a conceptual design of the Shaft Passer.

Shaft Passer diagram

There is even a 3D printable proof of concept you can download and create for yourself.

The famous reference to these devices was in Richard Feynman’s autobiographical “Surely you’re joking Mr Feynman“. The story goes like this:

One mechanical engineer at Frankfort was always trying to design things and could never get everything right. One time he designed a box full of gears, one of which was a big, eight-inch- diameter gear wheel that had six spokes.

The fella says excitedly “Well, boss, how is it? How is it?”

“Just fine!” the boss replies. “All you have to do is specify a shaft passer on each of the spokes, so the gear wheel can turn!” The guy had designed a shaft that went right between the spokes!

The boss went on to tell us that there was such a thing as a shaft passer (I thought he must have been joking). It was invented by the Germans during the war to keep the British minesweepers from catching the cables that held the German mines floating under water at a certain depth. With these shaft passers, the German cables could allow the British cables to pass through as if they were going through a revolving door.

So it was possible to put shaft passers on all the spokes, but the boss didn’t mean that the machinists should go to all that trouble; the guy should instead just redesign it and put the shaft somewhere else.

By the quote, one might gather that German engineers did invent the shaft passer. If that’s true, then it’s a mechanism not seen in operation today.

It’s certainly possible to build mind you, but the tolerances and tensions required would make it unlikely to function reliably under ‘real-world’ stresses.

Besides, there are simpler ways to route power around the wheel rather than through it. Needing such a device would indicate seriously poor choices early in the design process.

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