nuts and bolts

huangyhg

nuts and bolts....
hi
i'm currently in a debate with a co-worker about the hardness of nuts in bolts.  i was taught (and found the reference in my old machine design textbook) that nuts should be made from a softer material for a number of reasons:
first to yield - maintaining integrity of bolt
first to yield/compress - first threads deflect, transferring some of the load to the following threads, more evenly distributing forces through each, smaller chance of stripping/crushing threads
his side of the story is that you want the bolt to fail first, but doesn't really back up his reasoning.
if a bolt is softer when it is pretensioned, the bolt threads furthest from the end will begin to deflect and the nut loses contact with the bolt.   a much greater force is transferred to the remaining threads in the bolt, which inturn deflect and it is a domino effect.
any insight would be very helpful.
sf
~~bold are the ones who come over the line to fall~~
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hi staticfish
my understanding of bolts and nut joints are as follows:-
if a threaded joint is going to fail then it is preferable
that the bolt should break rather than the threads of the nut or bolt fail by stripping. this means that the length of threaded engagement should be sufficient to carry the full load necessary to break the screw without any threads stripping.my machinery's handbook gives various formula
for working out thread engagement of nuts and bolts made from the same and dissimlar materials which ensure that the
bolt would fail first. if you have ever had the need to tap aluminium and then insert steel screws it is common practice to tap the aluminium to a minimum depth of 3 x times bolt diameter.
hope this helps
regards
desertfox
bickford, an introduction to the design and behavior of bolted joints, ch 3 and 4 agrees with desertfox.