Bill Babcock wrote:
> Hmmm. Makes good sense. I'm trying to overcome the problem that it's a
> blind hole and you can't get the stud to bottom because there's always
> some residual oil. You'd think the stud would only be in compression until
> you torqued the nut on top of the pedestal, then it would transition to
> tension. But I can see how it would be hard to say what was really going
> on from top to bottom of the thread engagement. Might be compression at
> the bottom, slack in the middle, and tension at the top, with perhaps even
> less threads taking the load than if the bolt was slack in the hole.
I think residual oil is what's steering you astray. Oil is liquid (and slightly
compressible), so it's going to be
forced up through the threads, anyway, as the fastener is drawn down into the
bore. But, the bottom of the boss is not
so compressible (in the case of cast-iron engines).
You're right that the stud would be in compression (when forced down into the
boss) until the nut is torqued and tension
is applied. But, what do you have to indicate either compression or tension? A
torque reading from a torque wrench.
Let's say you double-nut the stud and bury the end in the boss, then torque the
stud down until it seats hard in the
boss and begins to compress. That requires torque in the same direction as when
tightening the stud nut. Then, you
loosen the double-nutting (which likely doesn't change the initial torque
compressing the stud), and you install the
head (or, perhaps the rocker pedestal, as case may be), and then begin to
tighten the nut. Some of the torque you
_previously exerted_, indicated on the wrench, is _being undone_ when then
tightening the nut, because it's inducing a
tensile force pulling in the opposite direction of the compressive force.
It's harder to visualize with a blind hole than with, say, a rod bolt cinching
up a rod cap, because the rod bolt is
free to stretch. In that example, it's unimpeded in the tensile direction. The
more the force applied in tightening in
one direction, the more it stretches, until it yields. But with a bolt or stud
bottoming, the force applied in torque,
because the fastener is not free to move in tension, compresses the bolt. Some
amount of the force tightening the nut on
the other end first relieves that compression, and only after that relief is
made does the bolt or stud begin to be
loaded in tension. If one tightens a stud against a blind bore to, say, 30
lb-ft, and when the head is installed, and a
stud nut is torqued on the stud to 30 lb-ft, the tension on the stud is 0, even
though one thinks 30 lb-ft has been
applied to load the stud in tension.
The whole point of using bolts and studs is to provide clamping force, and that
clamping force should exceed the cycling
loads imposed on the bolt or stud. If the stud or bolt is loaded in excess of
the peak load imposed on it, it doesn't
change its linear dimension. In theory, nothing moves (in reality, the math
says that the bolt does stretch a minutely
small amount, but not enough to matter).
If one starts out with an insufficient torque because of first having to
relieve compressive loads, the cycling loads
may exceed the clamp load, and things then move, and something will eventually
fail. ARP bolts have helped minimize this
problem for you because their elongation is so small compared to the load.
Whenever the hell I get the race car together, I have no doubt whatsoever that
you'd be able to drive rings around me,
but I continue to think of racing (or, more accurately, the prospect of racing)
in large part as a wonderful mental
exercise in materials science. I'm a bit weird that way, but time on the
Bridgeport is almost as much fun as time behind
the wheel. Maybe I'll feel differently when I get some track time, though.
Cheers, Bill. Hope that keeps it together.
--
Michael D. Porter
Roswell, NM
[mailto:mporter@zianet.com]
Never let anyone drive you crazy when you know it's within walking distance.
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