J&E wrote:
>
> Hi List!
> Question: If I put a torque wrench on a 'nut' would I get a different
> reading then if I put it on the head of the bolt?
> How would you be able to measure this acuretly? -Cosmo Kramer
Wouldn't want to confuse this issue (or not too much), but there are two
matters which may relate to your question. For most automotive
applications, there are three types of fasteners--bolts, nuts, studs and
capscrews--and in the case of bolts and capscrews, the distinction is
this: bolts capture pieces with through holes and have a nut installed
on the other end, while capscrews are used in blind holes. Studs, of
course, have the properties of both.
Each of these should be torqued in specific ways, because torque is a
inferred measurement. In the case of a capscrew going into a blind hole,
for example, it's impractical to measure the actual stretch of the bolt.
That's what torquing a fastener does--the more the advancement of the
threads, the more the fastener is stretched. The idea of this is to put
the fastener in tension by a load greater than the anticipated load
trying to pull the parts away from each other.
A good example of this would be the studs and nuts holding a cylinder
head onto an engine block. The job of the fasteners is to resist the
pressures developed in the cylinders and still maintain compression on
the gasket to prevent leakage, and to keep the surface of the head true
so that it doesn't crack from differential loads across its surface. Gas
pressure is trying to push the head away from the block. Because the
studs stretch by a predictable amount under load, if one were to just
tighten the nuts, say, finger tight, every time a cylinder fired, the
gas pressure would cause the studs to stretch a bit with the load and
the seal would be lost. Therefore, the torque applied to the nuts has to
create a load _greater_ than the force trying to pop the head off. In
simple terms, the studs are, by proper torquing, already carrying more
load than the gas pressure can apply, so the head doesn't move, or moves
very little.
Having said all that, the means of torquing a fastener or set of
fasteners (nut and bolt, or stud and bolt) is designed to minimize
errors in torquing. Because torquing a threaded fastener involves
turning it, torque applied can cause torsional loads in the longer stud
or bolt or capscrew. That's torque being applied which does not stretch
the bolt, but rather, twists it. When the turning force is relieved, the
long fastener just unwinds, so the actual load applied is less than what
one has read on the torque wrench. It's for this reason that the head of
the bolt is captured, and the nut is turned--there's somewhat less
likelihood (because of the way the face and threads of most nuts are
prepared) for the bolt or stud to twist. It's a more accurate
measurement of the actual stretch on the bolt.
In the case of a capscrew going into a blind hole, the amount of twist
generated in torquing a given size of capscrew has to be calculated or
measured empirically by the engineer to develop a table of torques which
can then be used reliably by others. In the case of the capscrew, the
torque in the manual reflects a torque which might be a small bit higher
than what is actually correct for the fastener size and grade, because
of the amount of twisting being done while torque is applied.
The general rule of thumb, unless the manual says otherwise, is to
capture a bolt head and turn the nut, for the reasons mentioned above.
That way, one can be sure that the fasteners are assembled properly.
Cheers.
--
Michael D. Porter
Roswell, NM
[mailto: mporter@zianet.com]
`70 GT6+ (being refurbished, slowly)
`71 GT6 Mk. III (organ donor)
`72 GT6 Mk. III (daily driver)
`64 TR4 (awaiting intensive care)
`80 TR7 (3.8 liter Buick-powered)
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