A survey was completed in the United States of automobile service
managers which indicated that 23% of all service problems were
traced to loose fasteners, with even 12% of new cars being found
to have fasteners loose. Presented below is information and
techniques intended to assist an Engineer in ensuring that threaded
fasteners are tightened so that the desired preload is achieved.
The drive for improved quality throughout manufacturing industry
has had an impact on the assessment of the accurate measurement
of assembly line torques. It is no longer sufficient just to run
a nut down a bolt until it stops and hope that it is tight enough.
The critical weakness in many products is the region of joints
which exist in the design. Bolted joints in particular can be
a source of concern for the Engineer. A single bolt, inaccurately
or incorrectly tightened, can lead to the failure of the complete
product. Too high a tightening torque and the Engineer sustains
the risk of a bolt shank or thread stripping failure. Too low
a specified torque and the bolt tension can be inadequate to meet
functional requirements. Failure to meet the tightening specification
can have unfortunate consequences for the reliability of the product.
Such failures could occur either during production assembly or
during subsequent maintenance on the product after it had entered
service. Either is obviously undesirable.
The most prevalent controlled method of tightening threaded fasteners
is by tightening so that a specified torque is achieved. This
method is generally known as torque control. The major problem
related to this method is that the clamp force generated as the
result of an applied torque is dependent upon the design of the
fastener and the prevailing frictional conditions. Despite these
problems, it is still the most popular way of ensuring that an
assembled bolt complies with an engineering specification.
Distribution of the Tightening Torque within the Fastener
A major problem with using the torque control method to
indirectly control the clamp force in the fastener is that
only a small proportion of the torque is actually used to
extend the bolt. The majority is absorbed by friction between
the nut face and the joint and in the threads. Some fasteners
use a nylon insert or have a distorted thread so that a torque
is required to run the nut down an unclamped bolt thread.
Interestingly, for the same stress in the bolt, the required
tightening torque between the prevailing and non-prevailing
bolt types varies by less than 4%. The prevailing torque type
nut however results in significantly less preload (16%). An
example of the distribution of torque within the fastener
for a M10 Property Class 10.9 bolt is shown in the piechart.
(The chart is from the BOLTCALC program.)
Checking that a torque controlled tightened bolt complies
to a specified torque is known as torque auditing. Most major
manufacturing companies have in-house quality audits on their
products which include the checking that threaded fasteners
have been tightened so that they conform to the engineering
There are two approaches used to audit installation torque. The
first is dynamically, which, by the use of in-line transducers
attached to the tightening tool, the installation torque is measured
directly. The second approach is by an operator or inspector measuring
the torque after the installation has been completed.
Dynamic Torque Auditing
The dynamic method gives results which are independent of operator
reading accuracy. Generally powered torque tools are used in conjunction
with a computer based data collection system. Because this method
allows the automatic storage and retrieval of tightening data
it can be an important tool in statistical process control. The
disadvantage of the method is that it is generally expensive in
terms of capital cost and the technical support skills needed.
Torque Auditing after Assembly
There are three basic methods for the checking of torques
applied to bolts after their installation; namely, taking
the reading on a torque gauge when:
1. The socket begins to move away from the tightened position
in the tightening direction. This method is frequently referred
to as the "crack-on" method.
2. The socket begins to move away from the tightened position
in the un-tightening direction. This method is frequently referred
to as the "crack-off" method.
3. The fastener is re-tightened up to a marked position. With
the "marked fastener" method the socket approaches a
marked position in the tightening direction. Clear marks are first
scribed on the socket and onto the joint surface which will remain
stationary when the nut is rotated. (Avoid scribing on washers
since these can turn with the nut.) The nut is backed off by about
30 degrees, followed by re-tightening so that the scribed lines
The torque in all three methods should be applied in a slow and deliberate
manner in order that dynamic effects on the gauge reading
are minimised. It must always be ensured that the non-rotating
member, usually the bolt, is held secure when checking torques.
The torque reading should be checked as soon after the tightening
operation as possible and before any subsequent process such
as painting, heating etc. The torque readings are dependent
upon the coefficients of friction present under the nut face
and in the threads. If the fasteners are left too long, or
subjected to different environmental conditions before checking,
friction and consequently the torque values, can vary. Variation
can also be caused by embedding (plastic deformation) of the
threads and nut face/joint surface which does occur. This
embedding results in bolt tension reduction and affects the
tightening torque. The torque values can vary by as much as
20% if the bolts are left standing for two days.
"Crack-On" and "Crack-Off" Methods of Torque Auditing
With the "crack-on" and "crack-off"
methods there are two ways in which the breakaway torque can
be checked. The first is manually by having the operator "feel"
the point of breakaway. The second is by use of electronics
and a strain-gauged torque wrench and recording the breakaway
torque automatically. The automatic method removes the operator
variability which does occur.
Marked Fastener Method of Torque Auditing
The most consistent and accurate method for the checking of applied
torques after the installation has been completed is by the "marked
fastener" method. From tests carried out under laboratory
conditions, the maximum torque checking accuracy possible, if
the above guidelines are followed, is approximately ± 5%
of the value actually applied to the nut.
The specification of the tightening torque is of crucial importance
in determining the reliability of the joint. Different thread
and joint surface finishes all have an effect on tightening torque.
Special fasteners, such as those having a prevailing torque, flanged
heads, or reduced shanks, also require special consideration when
determining the tightening torque. Prevailing torque fasteners
use a nylon insert or have a distorted thread so that a torque
is required to run the nut down an unclamped bolt thread. For
free spinning nuts, the prevailing torque is zero.
Threads Locked by Adhesive
It has been shown by extensive test work that locking the male
to female thread by means of adhesive gives the fastened assembly
excellent resistance to vibration loosening. The adhesive may
be applied in liquid form at the assembly stage, or the threads
may have been coated previously with an anaerobic adhesive which
cures when the parts have been assembled.
Most thread locking adhesives tend to display a thread friction
coefficient which is higher than what is normally present in the
threads, also, a slight prevailing torque characteristic will
be present. By the correct specification of tightening torque
both these effects can be accounted for accurately. Full cure
of the adhesive is normally achieved within 24 hours but is dependent
upon the finish applied to the fastener, the bond gap and ambient
temperature. With the liquid applied adhesive an activator can
be used to improve curing time.
Measuring the assembly torque, dynamically, at the assembly stage does not
present any problem when using adhesive to lock the threads
together. Problems are present however when the assembly torque
is required to be checked after assembly. The use of any of
the three methods for torque auditing of threads which have
been bonded by adhesive does present problems. If the cured
bond is broken to check the torque, the vibration resistance
of the fastener assembly may be impaired. Secondly, the hardened
adhesive in the threads increases the thread's friction characteristics
so that on re-tightening less torque goes into achieving preload
and more in overcoming friction. Due to these reasons any
of the three methods for post assembly torque auditing are
unsuitable for threads which have been bonded together by
A common method of checking that the specified assembly has been
achieved is to re-tighten the joint up to the specified torque
whilst checking that the male relative to the female member does
not move. The effect of the adhesive is to augment the assembly
torque so that the breakloose torque is some 10% to 30% above
the value of the assembly torque. Hence if the torque specification
was correctly achieved, no rotation of the male relative to the
female member should occur at the assembly torque value.
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