Tell me more ×
Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It's 100% free, no registration required.
up vote 7 down vote favorite

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.

share|improve this question

1 Answer

up vote 5 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.

share|improve this answer
Okay, but what if I screw it in from the other side? :) – user404153 Jul 20 '12 at 21:39
...and now chop the head off the screw so you don't remember which way it came in. – user404153 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? – user404153 Jul 20 '12 at 21:47
If you can get it into a place where it is evenly balance at maximum compression it can get started in either direction, but then there is a gradient and it will tend to continue in the direction initially selected. Getting it to a balanced state is likely to be very hard in practice. – dmckee♦ Jul 20 '12 at 21:49
show 4 more comments

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.