Bill Freeman wrote:

Archie wrote:

As an addendum to this thread, in Drag racing
& Nascar type racing bolts, we use the Torque to yield
procedure of torque + degrees beyond, and the Stretch
method. The manufacturers of this hardware require
lubrication for attaining proper values, and assign
values depending on regular, or moly-based oils.
Archie

Yes.  Absolutely right.  The lube avoids excess torsion in
the bolt, which is not desired at all.  Tension is the goal.
Bolt stretch to reach yield is the actual goal, and if it can
be measured directly, it is the best method.  Other methods
depend on a lot of assumptions.  OTOH, they work well in
most cases.  A useful rule of thumb for devices where the
bolt is properly selected for the load and the clamped pieces
are capable of withstanding the bolt clamping force, "If the
bolt does not fail during torquing, it will never fail in service".
The point is that torquing will prevent fatigue failures.  Any
joint where it can't withstand full bolt torque may fail the
bolt in fatigue.

A fatigue failed bolt was caused by 1) too little torque at
assembly,  2) use of a substandard bolt,   or 3) bolted joint
design is wrong & bolt overstressed.    For machinery where
(3) is known to be false (airplane parts that rarely fail), you
only worry about getting a quality bolt and torquing it properly.
Any material which might ooze out and unload the bolt tension
(thick paint, soft gaskets, etc) will cause failure in a bolt loaded
with cyclic loading if the preload is lost.  A properly sized and
torqued bolt with a cyclic applied loading will "see" zero
fatigue cycles regardless of time in service.

Bolt stress area is much less than the area calculated from the
nominal bolt diameter, since failures are at the root of a
thread.  The helical nature of the threads makes this a
bit difficult to calculate, so I use Mark's Handbook tables.

**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

 
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