torque values for fasteners General Introduction
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General Introduction
A guide to specifying
Torque Techinacal
Information 
torque values for
fasteners
The following notes are given
as a guide only. It is recommended that torque values derived from
formulae should not be used without comparison to figures obtained using
practical tests.
Generally, in the majority of applications, the
reliability of the joint is dependent upon the bolt's ability to clamp the
parts together. Adequate clamping prevents relative motion between parts
of the joint and leakage from joints containing gaskets. Measuring a
bolt's clamp force is difficult, especially under production assembly
conditions. The clamp force generated by a bolt can be indirectly
controlled by regulating the applied torque. This method, known as
torque control, is by far the most popular method of
controlling a bolt's clamp force. The initial clamp force generated by the
bolt is frequently called preload.
There is a
link between the torque applied to a bolt and the resulting preload. A
problem exists in that friction has a large influence on how much torque
is converted into preload. Besides the torque required to stretch the
bolt, torque is also required to overcome friction in
the threads and under the nut face. Typically, only 10% to 15% of the
torque is used to stretch the bolt. Of the remaining torque, typically 30%
is dissipated in the threads and 50% to 55% under the nut face. Because
friction is such an important factor in the relationship between torque
and preload, variations in friction have a significant influence on the
bolt's preload. Different bolt surface finishes generally have different
friction values.
The torque required for a socket headed screw
will not be the same as that required for the same size standard
hexagon bolt. The larger bearing face of the standard bolt will
result in an increased torque being required compared to a socket headed
screw. This is because more torque is being dissipated between the nut
face and the joint surface.
Stresses induced into a bolt
When a bolt is tightened the shank and thread
sustain a direct (tensile) stress due to it being stretched. In addition,
a torsional stress is induced due to the torque acting on the threads.
These two stresses are combined into a single equivalent stress to allow a
comparison to be made to the bolt's yield strength. In order to
effectively utilise the strength of the bolt, yet leave some margin for
any loading the bolt would sustain in service, an equivalent stress of 90%
of yield is commonly used.
This approach has a
number of advantages over the method where a direct stress, and hence
preload value, is assumed in the bolt. For high thread friction values, a
high torsional stress results in the bolt. Less of the available strength
of the bolt is being utilised in such a case to generate preload. In the
extreme case when a nut has seized on the bolt thread, all the applied
torque is sustained as torsional stess with no preload being available. In
the other extreme, low thread friction results in higher
preloads.
Backgrownd
The following information is provided to assist
Engineers wishing to establish the theoretical torque value for a
particular fastener. Caution should be exercised when using
theoretical values because the preload and torque is dependent upon the
friction values selected
Calculation Procedure
The formulae used
are applicable to metric and uinified thread forms which have a thread
flank angle of 60°. The calculation procedure distinguishes between thread
and underhead friction as well as differences which can be caused by
bearing face diameter variations. The procedure comprises the following
steps:
1.
Fastener Details.
2. Determination of the
tensile stress in the threded section.
3.Establish the
preload
4.Determine the
tightening torque
