This was interesting, from the Shop-Talk list.
"In order for a bolt to hold reliable against vibration, it needs to be taken
high enough up its stress/strain curve - to permenantly stretch it slightly.
So a grade 8 bolt will need more load to remain tight than a grade 5 of the
same size. Sometimes going up to a grade 8 will introduce loosening problem,
if the components being fixed cannot support the higher yield stress of the
grade 8 bolt. There are such applications that I have heard of where the
solution was to return to lower grade bolts, or drop the diameter of the
higher grade bolt. The lesson is that going up a grade should be done where
really neccessary, and not just on the basis of 'Grade 8 are all we stock'
or 'higher grade must be better than lower'"
It makes sense.
Allen Hefner
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