Jack W. Drews wrote:
> result in a higher tension in the bolt. Furthermore, good practice on
> setting a torque value is to stress the bolt to 80% of its yield.
Most everyone has answered the question about lubed vs. unlubed torques, but I
think I need to clarify this business of
torquing to 80% of yield. I'm sure there are exceptions, both in specific
instances and by manufacturer, but the GM
engineering standard for fastener preload, as I recall, is 80% of _proof load_.
Proof load is defined by GM as 75% of
yield. That means that GM sized and graded fasteners to nominally 60% of yield
(80% x 75%).
Which means, simply, that once the maximum load is determined, the fastener
must be able to provide a clamping preload
to exceed the load by a suitable safety factor (about 1.5) _and_ the fastener
itself had to be sized/graded to reach
that figure at nominally 60% of yield strength.
That may sound like overkill, but it resulted in very few fastener failures, if
good procedures were followed (I'm not
saying that this engineering procedure protected the product from assembly line
errors).
Structural and civil engineering standards are to preload to 2/3rds of yield. I
think the difference is that the
accelerations of dynamic loads upon fixed structures is relatively smaller than
in automotive applications.
Preload is preload, whether it's provided by a big fastener or a small
fastener--the difference is in the preload safety
factor. Although I can't come up with a definitive source for this feeling of
mine, I think fasteners subject to
cyclical loads are more likely to fail from fatigue the closer they're
preloaded to their yield point. That's the
distinct advantage of ARP fasteners--their yield point is so high that they can
provide a superior clamp load at a
relatively low percentage of yield strength. As well, because of Young's
modulus, they provide that higher clamping
force with small elongation--an important consideration, I think, for fatigue
strength.
All that said, I have no idea about what engineering standards were used by
Standard-Triumph to calculate the clamp
loads and torques for the wet-sleeve engine. Without a good idea of the grade
of fastener being used, it's not easy to
determine. Would be fun to take the standard torque/clamp load formula and work
backwards from the torques provided in
the Triumph manual. For what it's worth, the factory manual doesn't mention
lubricating the stock cylinder head studs or
nuts, or the rod bolts, before tightening.
Cheers.
--
Michael D. Porter
Roswell, NM
Never let anyone drive you crazy when you know it's within walking distance.
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