In a message dated 10/17/2000 4:44:22 PM Eastern Daylight Time,
ccrobins@ktc.com writes:
<< Now, in the example you saw, was the "correct" torque value at the
edge of elasticity for the bolt, or below it? >>
The torque wrench was set at the correct torque value for the bolt being
installed. Here is a more technical explanation of what happens, from Bowman
Technical Series, Fastener Facts, a truly interesting treatise on the subject
of fasteners. It illustrates what I was talking about, not being a technical
engineer:
"The basic function of a nut is to help produce and maintain the tension load
on the fastener when it is tightened agaist the pieces being held together.
As the fastener elongates, it starts to apply a compressive load to the nut.
The first contacted threead of the bolt experiences a tremendous amount of
tension from its entire body. Some of this load is transferred to the nut
causing less tension in the bolt at the second thread. Then
the second thread transfers part of this load to the third thread, and so
on. The threads of the bolt will actually expand, much like the coils of a
spring, as it is placed in tension. At the same time, the compressive forces,
acting on the bearing surfaces of the nut, squeeze the bottom threads in the
nut together. Because this deformation causes an uneven load distribution in
the nut, the first few threads of the nut will almost always yield
plastically. It has been generally accepted that the first thread of a UNC
nut, the one against the work, will have to support 38% of the total load on
that bolt. The second thread will have to support an additional 25% of the
bolt load, and the third thread supports 18%. Therefore, on a coarse threaded
fastener, SI 0b of the total load of the bolt is supported by the first three
threads. The outermost threads are hardly loaded. Beyond
the sixth thread, the flanks of the threads won’t even touch.
To allow the load to he distributed to the mating threads, the nut must
be slightly softer than the bolt, so its threads can plastically shift and
flow for the slight amount required to match the expanding contours of the
threads on the bolt. For example, Bowmalloy® nuts are heat treated to a
Rockwell hardness of 26-36C while Bowmalloy® cap screws have a hardness of
38- 42C.
If a nut was reused on the same bolt, or even a new bolt, there would
no longer he a perfect thread match because of the slight, permanent thread
distortion of the nut. This will create more friction between the flanks of
the threads on the nut and bolt as it is being tightened. Friction is very
important as it is a function of torque.
The first time the nut was applied, 40% of the mechanical work energy
required to rotate the nut onto the bolt was consumed in rotational (torsion)
friction between the mating flanks of their threads. Additionally, 50%
friction is created at the interface of either the nut and the work surface
or at the washer face under the head of the bolt and work, depending upon
which one is being turned to produce bolt stretch. Therefore, there is a
total of 90% work energy being consumed by friction before any energy is
applied to
even start stretching the bolt. Since the threads of the nut have been
distorted by the first use, these threads will create more galling friction
between the mating threads as torque is applied during reuse. Assuming this
increase is only 3%, total nut friction is now at 43%. This leaves only 7%
work energy available to tighten the fastener instead of the expected 10%.
Since a higher percentage of torque is consumed by this increased friction,
less twisting force is left to apply tension or to stretch the bolt.
Therefore, the bolt is not stretched as far as it had been the first time,
and a loss of clamping force is experienced.
[Unable to display image]
Original clamping force can be regained only if a new nut is assembled
and tightened onto the same bolt, or if a lubricant is used that makes up for
all of the added friction in the threads. In a critical application, it is
strongly recommended to replace both the bolt and the nut. The reuse of a cap
screw is not as critical as a nut, PROVIDING the user is certain the cap
screw was properly installed and maintained during its previous service and
that the bolt did not experience yield."
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