> Torque values are for the nuts and bolts, not the pieces holding them
> together. It's simply a matter of determining the size and grade of the
> bolt(s), and then looking up the torque values for them.
>
Whoa! I have to disagree with you on this one, Nolan. It's not that
simple. The tightening torque for a threaded fastener is only one, very
imprecise, way of measuring the clamping force of the fastener combination
(bolt/nut, nut/stud, bolt/internal threaded hole in component etc.). As
Randall pointed out, it is a somewhat imprecise measurement. In theory,
because of the "inclined plane" nature of the thread, the more force you
exert in the direction of the minor angle of the plane, the greater the
force exerted normal to the surface of the plane (= clamping force). The
force you exert on the bolt or nut by pulling on the wrench is multiplied by
the moment arm of the wrench you are using. Thus, a 45 lb pull on a 1 ft
wrench = 45 ft-lbs. If you are applying this torque to a 1/2 inch bolt, the
force at the thread is 45/(0.25/12) lbf, or 2,160 lbs. This circumferential
force around the diamter of the threaded fastener converts to clamping force
because the thread combination acts like an inclined plane, as I said
before. This is all well and good in theory, but the frictional force
beteen the mating parts is a significant part of the tightening force
required, and the frictional force can vary immensely, depending upon the
coefficiant of friction of the two mating surfaces, the presence or lack of
lubricant, the condition of the surfaces and the presence or lack of locking
devices. The torque you apply to a threaded fastener simply tells you that
you have applied a certain torque to the fastener - it is an indicator, but
nothing more than an indicator, of the clamping force that the fastener
applies. To truly measure clamping force, one must know the modulus of
elasticity for the material that the fastener is constructed from and the
degree of elongation (or compression) of the device producing the clamping
force. Torque does not give you this information. Also, there is no
"standard" torque for a given bolt. The torque recommended for a bolt in a
certain aplication is one that has been calculated, theoretically, to
produce the clamping force required for that application.
Michael Marr
1960 TR3A
Naperville, IL
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