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Author: tdAlpha
Time: 10/20/04 15:09 PST
This is a reply to message no. 14768 by samingale
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Current Topic:
Why loosening torque is lesser than the tightening torque for bolts/screws
There are two main forces at work here: Axial tension along the bolt, and the friction between the threads.

When you tighten the bolt, it is stretched out (creating an axial force), which will make the bolt want to pull out of the hole.
Also, the tighter it is, the more friction will be present.

When tightening a bolt you are working against friction, and you are also working to stretch out the bolt.  When loosening the bolt, you are working against friction, but the axial tension in the bolt is actually helping pull the bolt loose.

Therefore, even without any fatigue, the loosening torque will be less than tightening torque even immediately after the bolt has been tightened
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