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Summary: Bolted joints
- How tight is tight? With
trilingual summary
By
Richard L. Nailen, EA Engineering Editor
Assembly, installation, and servicing of industrial
machinery all involve bolted joints. To ensure joint integrity,
bolts must be properly tightened.
That produces
an initial tension or reload in the bolt, as well as compressive or
clamping forces within joint members. Any external tensile load
applied to the joint will only act to reduce the clamping forces,
without significantly increasing the force within bolts themselves,
until that external load exceeds the initial reload.
Such external
loads are often cyclic, leading to premature fatigue failure of
bolls. Proper reload will prevent that, and is achieved by applying
the right amount of tightening torque. A torque wrench is often
used. Another way to tighten bolts, common in structural joints, is
the "turn-of-nut" method. After applying a small initial torque, the
nut is rotated a specified additional amount, such as 90 degrees, or
a half turn. Because this can easily result in over stressing the
boll, it must be done carefully with full knowledge of the materials
and stress limits involved.
However, the
exact relationship between torque and bolt tension will vary with
thread condition; the material (including plating); and lubrication.
The only true indication of reload, but the most difficult to
measure accurately, is the elongation of the tightened bolt.
Joint
integrity is also dependent upon several conditions in the joint
members themselves. Mating surfaces must be flat and at right angles
to the bolts. Otherwise, the joint may act as a spring, leading to
destructive resonant vibration. In electrical joints, poor contact
between the surfaces results in overheating.
Also, the bolt
head and nut must uniformly bear against surfaces with which they
are in contact. Otherwise, the bolt will undergo bending stress that
greatly reduces fatigue life (as much as 80% with only one degree of
surface angularity).
Regardless of how well the joint is tightened initially, vibration
or cyclic loading can cause fastener loosening. Tightening force
creates a small loosening torque within threads. Thai, plus the air
space always existing within the mating threads, inevitably leads to
loosening. Preventive measures include sealants that fill the voids;
locking washers; or special fastener designs that inhibit rotation.
Copyright 2003, Barks Publications, Inc., Chicago.
Reproduction by any means prohibited.
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