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- the material on this site provides additional information on this topic.
One of the
major problems with the use of bolted joints is the precision, with regard
to achieving an accurate preload, of the bolt tightening method selected.
Insufficient preload, caused by an inaccurate tightening method, is a
frequent cause of bolted joint failure. It is important for the Designer
to appreciate the features and characteristics of the main methods employed
to tighten bolts. Presented below is a brief summary of the major bolt
tightening methods. Note however that whatever method is used to tighten
a bolt, a degree of bolt preload scatter is to be expected.
There are six main methods used to control the preload
of a threaded fastener. Specifically:
1. Torque control tightening.
2. Angle control tightening.
3. Yield controlled tightening.
4. Bolt stretch method.
5. Heat tightening.
6. Use of tension indicating methods.
Torque Control Tightening
Controlling the torque which a fastener is tightened to is the
most popular means of controlling preload. The nominal torque
necessary to tighten the bolt to a given preload can be determined
either from tables, or, by calculation using a relationship between
torque and the resulting bolt tension.
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, will give
an equivalent stress of some proportion of yield. The proportion
commonly used with this approach is 90%.
Torque prevailing fasteners (such as Nyloc, Cleveloc nuts etc.)
are often used where there exists a risk of vibration loosening.
The prevailing torque has the effect of increasing the torsional
stress in the bolt shank during tightening. This affects the conversion
of the tightening torque into bolt preload and should be allowed
for when determining the correct torque value for this type of
As can been seen by study of the above chart, a fundamental problem with torque tightening is that because the
majority of the torque is used to overcome friction (usually between 85% and
95% of the applied torque), slight variations in the frictional
conditions can lead to large changes in the bolt preload. This
effect can be reduced by the use of so called friction stabilisers. These
are substances which are coated onto the fasteners to reduce the
frictional scatter. Other ways to improve the accuracy of the
1. Do not use plain washers; their use can result in relative
motion to change from the nut to washer, to washer to joint surface,
during tightening. This as the effect of changing the friction
radius and hence affects the torque-tension relationship. If,
because of excessive bearing pressure, a larger bearing face is
required, thought should be given to the use of flanged nuts and
2. Determine the correct tightening torque by the completion of
tests. Strain gauges can be attached to the bolt shank and tightening
completed on the actual joint. A load cell under the bolt head
can be used, however it is not as accurate as strain gauging,
since the joint characteristics have been changed.
3. If it is not feasible to establish by testwork the actual tightening
torque, determine the tightening torque using the best information
available i.e. fastener finish, nut head bearing surface size
and prevailing torque characteristics, if applicable. (The computer
program TORQUE developed by Bolt Science can allow for all these effects.)
4. Ensure that the tightening torque value is specified on the
assembly drawing. Quotation of a plus or minus 5% tolerance is
good practice. More unusually, quote that a calibrated torque
wrench is to be used to check the torque after installation. The
method used to tighten the bolt has a significant influence on
the preload scatter (see below).
Angle Controlled Tightening
This method, also known as turn of the nut method, was introduced
for manual assembly shortly after the second World War when a
certain tightening angle was specified. The method has been applied
for use with power wrenches, the bolt being tightened to a predetermined
angle beyond the elastic range and results in a small variation
in the preload due, in part, to the yield stress tolerance. The
main disadvantages of this method lie in the necessity for precise,
and, if possible, experimental determination of the angle; also
the fastener can only sustain a limited number of re-applications
before it fails.
Yield Controlled Tightening
This method, developed by the SPS organisation, is also known
under the proprietary name "Joint Control Method". Very
accurate preloads can be achieved by this method by minimising
the influence of friction and its scatter. The method has its
roots in a craftsman's "sense of feel" on the wrench
which allowed him to detect the yield point of the fastener with
reasonable precision. With the electronic equivalent of this method,
a control system is used which is sensitive to the torque gradient
of the bolt being tightened. Rapid detection of the change in
slope of this gradient indicates the yield point has been reached
and stops the tightening process. This is achieved by incorporating
sensors to read torque and angle during the tightening process.
Since angle of rotation and torque are both measured by the control
system, permissible values can be used to detect fasteners which
lie outside their specification (having too low a yield for example).
A small degree of preload scatter still results from this method
due to the influence of friction. The method detects the yield
point of the fastener under the action of combined tension and
torsion. The higher the thread friction, the higher the torsional
stress, which, for a given yield value, results in a lower preload
due to a lower direct stress.
The method has been used in critical applications, such as cylinder
head and conn-rod bolts, in order that consistently high preloads
can be achieved (which can allow smaller bolts to be used). However,
because of the cost of the tools necessary to use this method
(a hand wrench incorporating the control circuitry costs many
times more than a conventional torque wrench), widespread adoption
of this method is unlikely. (Although manufacturers may be able
to invest in the equipment, unless service staff have similar
equipment, the Designer cannot depend upon high preloads being
maintained in the field.)
Bolt Stretch Method
A problem relating to the tightening of large bolts is that very
high tightening torques are required. Although this can be partly
overcome by the use of hydraulic torque wrenches (the reaction
of the torque however can be a problem), the use of hydraulic
tensioning devices is commonplace for bolts over 20mm in diameter.
The method uses a small hydraulic ram which fits over the nut,
the threaded portion of the bolt/stud protrudes well past the
nut and a threaded puller is attached. Hydraulic oil from a small
pump acts upon the hydraulic ram which in turn acts upon the puller.
This is transmitted to the bolt resulting in extension occurring.
The nut can then be rotated by hand with the aid of an integral
socket aided by a tommy bar.
Control of the hydraulic pressure effectively controls the preload
in the bolt. A small amount of preload reduction however does
occur when the pressure is removed as the nut elastically deforms
under the load. Removal of nuts corroded to the bolts can be a
problem with this method.
Tension Indicating Methods
Heat tightening utilises the thermal expansion characteristics
of the bolt. The bolt is heated and expands: the nut is indexed
(using the angle of turn method) and the system allowed to cool.
As the bolt attempts to contract it is constrained longitudinally
by the clamped material and a preload results. Methods of heating
include direct flame, sheathed heating coil and carbon resistance
elements. The process is slow, especially if the strain in the
bolt is to be measured, since the system must return to ambient
temperature for each measurement. This is not a widely used method
and is generally used only on very large bolts.
This category includes the use of special load indicating bolts,
load indicating washers and the use of methods which determine
the length change of the fastener. There are a wide number of
ways bolt tension can be indirectly measured and the discussion
presented here is not exhaustive.
Special bolts have been designed which will give an indication
of the force in the bolt. One such fastener is the Rotabolt which
measures bolt extension by the use of a central gauge pin which
passes down a centrally drilled hole in the bolt. Underneath the
head of the gauge pin, a rota is retained which is free to spin
in a very accurately set gap. The fastener stretches elastically,
whereas the gauge pin does not move since it experiences no load.
As tightening continues, the bolt will stretch sufficiently to
eliminate the gap and prevent the rota from being able to be rotated.
This is the indication that the bolt is correctly loaded. Another
proprietary fastener uses a similar method. The HiBolt uses a
pin located centrally down the bolt as does the Rotabolt except
the pin is gripped by the slight contraction of the bolt diameter;
the pin being locked when the correct preload is reached.
The use of load indicating washers is widespread in structural
engineering. Such washers have small raised pips on their surface
which plastically deform under load. The correct preload is achieved
when a predetermined gap is present between the washer and the
underhead of the bolt. This is measured using feeler gauges. Generally
they are not used in mechanical engineering, but are, extensively,
in civil engineering.
The extension which a bolt experiences can be measured either
using a micrometer or by a more sophisticated means such as using
ultrasonics. The extension can be related to preload either directly,
by calibration, or indirectly, by calculation. If ultrasonic measurement
is used then the end of the bolt shank and the head may require
surface grinding to give a good acoustic reflector.
To assist the Engineer in overcoming the problems associated with the use of threaded fasteners and bolted joints, Bolt Science has developed a number of computer programs. These programs are designed to be easy to use so that an engineer without detailed knowledge in this field can solve problems related to this subject.