| Strength of Threaded Fasteners
Fastener failure on a product can
have potentially disastrous consequences. In an attempt to
ensure that such consequences do not occur, rigorous and extensive
testing of a product is frequently completed. However in many
applications, extensive testing is neither practical nor economic.
In such instances, the Engineer usually relies upon analytical
analysis together with his experience and judgment to ensure
that failure does not occur.
Failure of a threaded fastener generally
occurs in one of three modes. Failure through the shank or
threaded section of the fastener, thread stripping of the
external thread, or thirdly, thread stripping of the internally
threaded member. Considering each in turn:
Failure through the male thread or thread
The majority of fastener failures occur with fracture through
the male thread. Under static loads, the strength of the thread
is determined by the stress area. This is based upon the mean
of the minor and pitch thread diameters. Engineering handbooks
have, typically, tables of stress areas for various thread sizes.
When a bolt is tightened the shank
sustains a direct stress, due to the elongation strain, together
with a torsional stress, due to the torque acting on the threads.
Most tables of bolt tightening torques ignore the torsional
stress and assume a direct stress in the threads of some proportion
of the bolts yield stress, usually 75%. For high frictional
conditions the magnitude of the torsional stress can be such
that when combined with the direct stress, an equivalent stress
over yield can result, leading to failure. A more consistent
approach is to determine the magnitude of the direct stress
which, when combined with the torsional, this will give an
equivalent stress of some proportion of yield. The proportion
commonly used with this approach is 90%. The computer program
TORQUE provides state of the art
analysis for the torque tightening of threaded fasteners.
High performance bolts are often
designed so that the plain shank is smaller than the stress
diameter of the thread. This is done so that the stretch that
occurs under the preload induced from the tightening process
is maximised. With this type of bolt, failure, if overtightened,
will occur in the plain shank region as shown in the photograph.
Thread stripping can be a problem
in many designs where tapped holes are required in low tensile
material. In general terms, thread stripping of both the internal
and external threads must be avoided if a reliable design
is to be achieved. If the bolt breaks on tightening, it is
obvious that a replacement is required. Thread stripping tends
to be gradual in nature. If the thread stripping mode can
occur, assemblies may enter into service which are partially
failed, this may have disastrous consequences.
The photograph above is from a scanning
electron microscope showing a bolt thread about to strip.
The joint surface interface was at the right hand side, you
can see from the image that the first thread has the greatest
distortion. The thread stripping mechanism is complex and
involves thread bending (that occurs under the high loads)
and nut dilation (which results in the shear plane moving).
To precisely predict the force and
mode of failure of a threaded assembly demands consideration
of a large number of factors. Thread stripping is a complex
phenomenon. The following factors all have an important effect
on the stripping strength of a thread:
1. The variation in the dimensions
of the thread, (such as major, pitch and minor diameters)
has a significant effect on both internal and external threads
2. Tensile and shear strength variations
in the material for both the internal and external threads.
3. The effect of radial displacement
of the nut or tapped component (generally known as nut dilation)
in reducing the shear strength of the threads. The tensile
force in the fastener acts on the threads and a wedging action
generates a radial displacement which reduces thread strength.
4. The effect of bending of the threads,
caused by the action of the fastener's tensile force acting
on the vee threads, resulting in a wedging action that decreases
the shear area of the threads.
5. The effect which production variations
in the threaded assembly, such as slight hole taper or bellmouthing,
can have on thread strength.
The strength of a nut or bolt thread
cannot be viewed in isolation without considering the inter-dependence
which both elements have on the strength of the assembly.
One of the problems in predicting thread stripping strength
is that, without considering such effects as thread bending,
nut dilation or bellmouthing, an optimistic result occurs.
The actual stripping strength being lower than the calculated.
The program FASTENER allows a state
of the art analysis to be performed to determine the stripping
strength of a threaded assembly.
A complicating factor which can occur
when a drilled hole is tapped, is bellmouthing. This is a
slight taper on the hole which is usually encountered on most
drilled holes to some degree. This taper extends normally
for about half the diameter from the start of the hole. The
cause of this tapering is torsional and transverse flexibility
of the drill together with instability of the drill point
during entry into the material. Bellmouthing can be minimised
by the use of close fitting, well aligned and rigid drill
bushes together with accurate drill sharpening.
Holes exhibiting bell mouthing will,
when tapped, experience a variable thread height along the
length of the hole. This variation can be significant on short
lengths of engagement and fine pitches. The net effect of
bellmouthing is to reduce the shear area of the external threads.
The finer the thread the more pronounced is the effect of
Influence of tap-drill size on thread
In tapped holes, the thread height
is dictated by the diameter of the tapping drill. To reduce
the risk of failure, the Design Engineer is often cautious
and specifies high percentages of thread height in tapped
holes. From a production standpoint these higher percentages
of thread height result in higher tapping torques, increased
tap breakages and, as such, are not favored. For short lengths
of thread engagement, the minor diameter size - resulting
from the tapping drill - has a significant effect on assembly
strength. Studies have shown that for threaded assemblies
of usual proportions, tap-drill size is relatively unimportant
so long as the percentage of thread height is greater than
60%. Tapping costs are likely to be lower if the lowest possible
thread height is used.
The effect of a low proportion of
thread height is to reduce the shear area of the external
thread, this is illustrated in figure 1. For very low thread
heights, the shear plane through the threads need not be parallel
to the thread axis, this is illustrated in figure 2. Such
failure modes are difficult to predict and can be easily eliminated
by maintaining a reasonable percentage thread height.
The tensile force present in the fastener
during tightening acts on the vee threads to produce a wedging
action which results in a radial displacement. This radial
displacement is generally known as nut dilation and occurs
in threaded bosses as well as conventional nuts. Theoretical
and practical studies of this phenomenon indicate that the
top face of the nut contracts in a radial direction while
its bearing surface expands. The net effect of this dilation
is to reduce the shear area of both the internal and external
The stripping strength of an assembly
can be improved by increasing the width across flats of the
nut, or boss diameter, up to about 1.9 times the nominal thread
diameter. This increases the stiffness locally around the
internal thread and reduces radial expansion.
The tensile force present in the fastener
during the tightening process results in a degree of thread
bending between internal and external threads. Thread bending
reduces the shear area of both internal and external threads.
A dominating factor controlling the degree of thread bending
is the ratio between the strength of internal and external
threads. The strength ratio is the ratio between the force
necessary to cause the nut thread to strip, divided by the
force required to cause the bolt thread to strip.