How do vibrations shake a screw loose?

[Johnleprekan]

Can anyone explain how vibrations actually shake a screw loose from a shelf? What is happening there?

 

[truesearch]

My suggestion !...screws are essentially 2 inclined planes wanting to slide past each other. It is only friction that prevents this happening. If there were no friction screws would have a tendency to unwind under any load at all.
I would imagine that constant vibration enables slippage to occur and this is bound to result in screws getting slacker. I have never come across vibration making screws tighten up !

 

[mkillman]

Expansion and contraction

as the sound waves pass through the object in question it expands and contracts this action is what's turning your screw

Wood expands screw loosens up some
Then then wood contracts but the screw is in a slightly diffrent place so it becomes a question of least resistance is it easier to sheer metal into wood or turn the screw?

I could be wrong but that's my take on it

 

[Johnleprekan]

Thank you.