Peter Cowan/Lexi Cameron wrote:

**Many** engineers do not understand threaded fasteners
very well.  I have had to take one to the lab and break a
bunch of bolts to prove a lot of std bolt application engineering
facts.  Some of it is quite surprising to the uninitiated.

Bill
Long EZ builder & pilot
mechanical engineer

Well I'm not an engineer nor apparently am I initiated. This discussion has
opened up a whole world of parameters that I didn't know existed!  I do
appreciate the info so far and I don't expect you guys to give me
engineering courses but I really need to go back to basics on how a fastener
in tension works (I assume that the shear situation is much simpler. True?)
Can you recommend a basic text on the subject?

Mark's Mechanical Engineers Standard Handbook is good to have for all sorts
of things.

For instance why is "bolt stretch to yield" the goal? This whole discussion
of fatigue and torque load is fuzzy. I'm having difficulty relating this to
the top motor mount bolts for example. I'd thought that in many cases torque
values were given to produce even tension over a surface.

My guess is that there are lots more "uninitiated" out there. But, again,
pointing me at a text is fine.

It is a long story on bolt stretch and fatigue. Every material stretches
and the stress is directly proportional to stretch (more or less).
Yield is when the material finally gives up stretching and returning
to its original length and takes a new set or length. This is also called 
the elastic limit. Steel still has some resistance left however. Ultimate 
is when the material finally breaks.

The study  of all this is called "strength of materials".

As Bill pointed out the tension a bolt will take is less than the shear
because the max stress is at the root of the threads while shear stress
is usually on the shank of the bolt.
 
Paul Lamar
 
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