(Again we quote from other websites with excellent technical information
regarding bolted joints, and recommend that the reader visit and study these
sites.)
"Torque and How to Use it Correctly
The most common term used when installing fasteners is
"torque". Under normal circumstances, a person tightens bolts by using
a suitable wrench and if a tightness
specification is given in ft-lbs., a torque wrench is used to indicate that the
specified torque is reached. In reality, all
that a pointer on a torque wrench dial indicates is the resistance to
turning when mating two threaded surfaces. Approximately 50% of the
applied torque is wasted in overcoming the mating friction under the
head, 40% is friction resistance in the threads, and only 10% of
the total ft-lbs. exerted produces
the tension in the bolt. As tightening proceeds, a maximum torque value
will be attained, followed by a sharp
decrease in torque as additional turning is attempted.
The decrease in the torque is an indication of loss in
tightness; the bolt was overloaded and the maximum bolt strength
exceeded.
So far, it has been explained that steel
acts elastically within a wide range of
applied loads. The elasticity property which allows a screw to
return to its initial length upon unloading, ceases at the yield point.
As the bolt is being tightened above the
yield point, a permanent elongation
starts to set in prior to reaching the maximum strength which
the screw can sustain. As the result of such behavior the clamping force
which is produced from the tension as a torque is applied increases
proportionally faster in the elastic range. Above the yield point, the
rate of increase of the clamping force
diminishes with increased loading, since
the tightening energy is wasted in the permanent stretching of a
screw. It is recommended that a screw be tightened, either to a load just
below a yield point, or slightly above, but not to limits too close to
the maximum strength of a screw since a
tightness might easily be lost due to the
rapid stress affects on the internal microscopic structure of the
steel. The load levels at which yield occurs is different for each grade
of bolt. As the strength of steel
increases, the yield point increases correspondingly,
meaning that the bolt's ability to carry higher loads is
enhanced. Therefore, Grade 5 steel capscrews have a higher yield point
than Grade 2 capscrews, and can be tightened to carry greater service
loads. The same applies to Grade 8, where much higher loads can be
applied than to Grade 5.
The next thing to be concerned with is the
assembly load which is to be sustained by
the mating members. The fastening members must be
tightened to loads which exceed the carrying load of the
assembly, otherwise the fastening parts may either fail during the
installation or subsequently during the performance in service. Usually,
a load is either static (not moving) or dynamic (joint is moving in
service) where the joint load is acting either in shear or in tension.
Whatever the case may be, a designing
engineer expresses a joint load either in
lbs. or psi (pounds per square inch). When a bolt is
tightened, the resulting load exerted by the bolt should exceed the
expected assembly load and have the bolting members acting on the expected
assembly load and have the bolting members acting on the
joint assembly rather than the joint assembly acting on the fastening
components. This is a basic concept similar to a person attempting to
lift or carry a load. Unless an individual is strong enough to overcome
the weight of an object, he will either drop it or not be able to lift it
up at all. The same principle
applies to an assembly joint which is to be tightened.
A failure to properly secure the components will occur and
the purpose of fastening will not be achieved if the bolts are not
capable of tightening to loads higher
than demanded by the assembly joint. This is
where the accurate stress state analysis of the assembly members and
the selection of the proper grade of bolts becomes critically important.
The real problem in tightening arises when suggested torque
values are indiscriminately applied
without due consideration of the elements discussed
above. Reaching such a torque on a dial gauge means
absolutely nothing unless the corresponding tension is measured
with respect to the requirements of the
assembly joint. It has been established above
that a bolt during continuous tightening experiences a maximum
torque and then decreases rapidly in value until a failure occurs.
Torque measures resistance only. As one
continues to turn a nut, resistance
increases indicating rising torque values on a dial gauge. The
resistance comes about by the tension forces which are created as a bolt
stretches under an applied load. The magnitude of the resistance forces
which produce the clamping force are equal and opposite in direction to
the stretching force (tensile load). Therefore, we are interested in the
resulting clamping load which is produced as we begin to tighten at
various torque values.
To summarize, once a bolt is snugged, every
turn of a nut produces an increase in
torque resulting in a longitudinal stretch of a bolt which
creates the pulling back effect (similar to a stretched rubber band), and
clamps on the component members of an assembly. One should not
exceed tightening beyond the allowable limits because the pulling back
effect is lost and the clamping load on the assembled joint is
deteriorated.
The surface finish of a bolt plays a determinant role on the clamping
load. For purposes of fully appreciating the correlation between a
torque and a clamping force (tension), keep in mind that torque
measures resistance to turning. If the resistance is expressed as
friction, then all one needs to do is lubricate
the surface and the friction-resistance
is decreased. But, this is of no help unless the
installing mechanic can make an association between the torque applied
and the clamping force produced so that the fastening components are
put within the allowable tension ranges for the optimum performance in
service. Therefore, one must by pre-testing determine for
each existing surface condition, the
torque range which will produce the appropriate clamping
force - tension. There are available instruments on the market
which will correctly correlate the TORQUE-TENSION relationship and
prevent costly repairs and unnecessary downtime.
Let us take a bolt of an arbitrary size and decide to make one
to nearly perfect dimensional tolerances
and smooth thread surfaces; one with poor
dimensional tolerances and irregularly rough threads; and one with
a plating (for example cadmium) applied subsequent to manufacturing.
If we apply 40 ft.-lbs. of torque to each sample, the
resulting clamping loads will be in the
following order: Rough Sample: Low Smooth Sample:
Higher than rough Plated Sample: Relatively the highest
The above results illustrate how premature failures come about if one
strictly relies on a torque reading as the final measure of tightness. To
get equivalent clamping force - tension on the above samples, one must
determine the torque values for each condition, so that the clamping
forces are uniform in each case.
Torque to be applied can be calculated by using the empirical
equation:
KDW Where:
T=12
T=Torque K=Friction
Factor K=0.20 for Non-Lubricated surfaces
K=0.15 for Lubricated Surfaces
Note: Other values for K are possible where different surface
conditions or additional lubricants are
used.
D= Nominal bolt diameter (in.) W= Bolt Tension (lbs.) (Clampload
or Preload)
where W= 70% of the proof load (lbs.) (Proof load value listed in SAE
J429)
When calculating for a plated condition, substitute 0.15 for K
and T becomes 263 ft -lbs. It is,
therefore, shown empirically that lubricated fasteners
(plated or otherwise) are to be torqued at a lower value, otherwise
excessive clamploads and failure during the installation will
result.
The empirical equation can be put to practical use only where
K conditions are properly determined by
the user, and the assembly conditions
evaluated.
Spiralock
Reprinted with permission of Lake Erie Screw Corporation"
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