

Video of a very inefficient concrete drill (read description) - joshwa
http://www.youtube.com/watch?v=5q-BH-tvxEg&fmt=18

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tjic
So, everyone gets that each stage in the reduction reduces the number of
rotations per second, and therefore the stage that's embedded in concrete is
moving very very slowly.

...but still, concrete being very brittle in tension, shouldn't it crack?

The answer is no, because of something called "backlash". In short, there's a
little bit of play in each of the gears, and one can "take up the play" in
each stage before force is transmitted through the stage. There's two kinds of
play: actual air gap, and also elasticity of the steel gear teeth.

Dealing with backlash is a key component in designing machine tools like metal
lathes and milling machines.

High end CNC tools use ball screws and other techniques to reduce the amount
of backlash experienced.

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regularfry
Given that everything's running in one direction, couldn't you manually take
up the play in each section during construction, working back from the
concrete block, so that the whole thing's at tension when you connect the
motor? I appreciate that wouldn't affect the elasticity much, but the air gap
would be taken care of at least...

