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Author: deajohn5556
Time: 12/01/10 18:27 PST
This is a reply to message no. 21278 by r. deloach
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Bolt load difference between col and hot

Sure, it is rather straightforward.

It can be simplified by assuming that the joint that is compressed by the bolt is very stiff compared to the bolt.  The bolt is initially in preload tension approximately as

P = T/.2D where T is torque in inch pounds and D bolt diameter in inches, and P is pounds

To calculate the bolt load increase or decrease going hot, you need to know its thermal expansion coefficient (CTE)  andf that of the parts being compressed.  If the expansions are the same there is no change in load over temperature.  If different then the bolt-joint combo wants to move delta CTE  x length x temp change  . The motion is resisted by the bolt stretch FL/AE where F is bolt load change and A its stress area and E its modulus.  Set equal to motion and we have

F = AE(delta CTE) x TEMP change where temp change is (500-70)F in your case.

If the bolt CTE is higher than joint it will loosenand if lower it will tighten.  The force F needs to be low enough to not overstress the bolt or loosen it and cause early failure.  Generally it should be no more thn 25% of the preload.  If a problem, you can always use Belleville washers and that will keep the preload fairly constant if sized properly.



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