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I am trying to figure out why vibrations (say, from an engine) loosen screws. It seems to me that there is evident symmetry between loosening and tightening a screw. I am wondering what breaks this symmetry.

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up vote 7 down vote accepted

The forces on the screw are not symmetric. Once the screw is no longer turning loosely in the hole tightening the screw compresses the two materials held together (i.e. increases the stress on the material, i.e. stores energy in the material), while loosening reduced the compression (i.e. releases the stress).

So a random dislocation will be more likely to occur in the "loose" direction than the "tight" direction.

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Okay, but what if I screw it in from the other side? :) –  Ryan Thorngren Jul 20 '12 at 21:39
...and now chop the head off the screw so you don't remember which way it came in. –  Ryan Thorngren Jul 20 '12 at 21:40
It is the energy stored in the distortion of the screw and/or the material in which it is turned that provides the gradient. As long as the screw is under tension or the medium is under compression vibration will tended to prefer the loosening direction. –  dmckee Jul 20 '12 at 21:44
My point is that you can remove the screw by screwing it all the way through. Why doesn't the vibration push it out that way? –  Ryan Thorngren Jul 20 '12 at 21:47
An infinitely long screw in a infinitely long uniform medium is balanced. The asymmetry always arises from the energy requirement to strain the screw and the medium. If there is no strain there is no preference, but then the screw never gets to be "tight" in the first place. This really is just the physics of a spring: small strains obey Hooke's law. –  dmckee Jul 20 '12 at 21:55
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Regardless of whether the "local" situation is symmetric or not with respect to loosening and tightening, what you essentially have is a random walk. At any point in time, the screw can stay where it is, get a little looser or get a little tighter. There is, in practical terms, a limit as to how tight the screw can get but no limit on how loose. For any degree of looseness, there's a positive probability that you'll eventually reach that point, assuming the vibration is strong enough to move the screw at all.

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