Loosening of Bolts and Threaded Fasteners
A significant advantage of a bolted joint over other joint
types, such as welded and riveted joints, is that they are
capable of being dismantled. This feature however, can cause
problems if it unintentionally occurs as a result of operational
conditions. Such unintentional loosening, frequently called
vibrational loosening in much of the published literature,
is an important phenomenon and is widely mis-understood by
Engineers. It is important for the Designer to be aware of
the bolt loosening mechanisms which can operate in order to
design reliable joints. The information presented below is
key information for the Designer on the theory of vibration
loosening of threaded fasteners and how such loosening can
Study of most Engineering magazines
will reveal the multitude of proprietary locking mechanisms
available for fasteners. For the Designer without the theoretical
knowledge of why fasteners self loosen, this represents a
bewildering choice. Presented below is key information, for
the Designer, on why fasteners self loosen, and, how it can
It is widely believed that vibration
causes bolt loosening. By far the most frequent cause of loosening
is side sliding of the nut or bolt head relative to the joint,
resulting in relative motion occurring in the threads. If
this does not occur, then the bolts will not loosen, even
if the joint is subjected to severe vibration. By a detailed
analysis of the joint it is possible to determine the clamp
force required to be provided by the bolts to prevent joint
Often fatigue failure is a result
of the bolt self-loosening which reduces the clamp force acting
on the joint. Joint slip then occurs which leads the the bolt
being subjected to bending loads and subsequently failing
Pre-loaded bolts (or nuts) rotate
loose, as soon as relative motion between the male and female
threads takes place. This motion cancels the friction grip
and originates an off torque which is proportional to the
thread pitch and to the preload. The off torque rotates the
screw loose, if the friction under the nut or bolt head bearing
surface is overcome, by this torque.
There are three common causes of the
relative motion occurring in the threads:
1. Bending of parts which results in forces being induced
at the friction surface. If slip occurs, the head and threads
will slip which can lead to loosening.
2. Differential thermal effects caused as a result of either
differences in temperature or differences in clamped materials.
3. Applied forces on the joint can lead to shifting of the
joint surfaces leading to bolt loosening.
Work completed during the 1960's in
Germany indicated that transversely applied alternating forces
generate the most severe conditions for self loosening. The
result of these studies led to the design of a testing machine
which allowed quantitative information to be obtained on the
locking performance of self locking fasteners. Such machines,
often called Junkers machines (a video of such a machine can
be seen - see the bottom of this article) in the literature
- after it's inventor, have been used over the last twenty
years by the major automotive and aerospace manufacturers
to assess the performance of proprietary self locking fasteners.
As a result, a rationalisation of the variety of locking devices
used by such major companies has occurred. For example, conventional
spring lock washers are no longer specified, because it has
been shown that they actually aid self loosening rather than
prevent it. There are a multitude of thread locking devices
available. Through the efforts of the American National Standards
Subcommittee B18:20 on locking fasteners, three basic locking
fastener categories have been established. They are: free
spinning, friction locking, and chemical locking.
The free spinning type are plain bolts
with a circumferential row of teeth under the washer head.
These are ramped, allowing the bolt to rotate in the clamping
direction, but lock into the bearing surface when rotated
in the loosening direction. The "Whizlock" is in
Friction locking categories can be
sub-divided into two groupings, metallic and non-metallic.
The metallic friction locking fastener usually has a distorted
thread which provides a prevailing torque; an example of this
category is the "Philidas" nut. Non-metallic friction
locking devices have plastic inserts which provides a thread
locking function; an example being the "Nyloc" nut.
The chemical locking category are
adhesives which fill the gaps between the male and female
threads and bond them together; "Loctite" is an
example. Such adhesives are now available in micro-encapsulated
form and can be pre-applied to the thread.
To identify which category is the
most suitable for an application, requires a careful consideration
of the application. In brief, the chemical locking category
provides the greatest resistance to vibration loosening, followed
by the free spinning locking fastener. However each category
has dis-advantages as well as advantages, the most suitable
method being dependent upon the application.
In general terms, the key to preventing
self loosening of fasteners is to ensure that:
1. There is sufficient clamp force present on the joint interface
to prevent relative motion between the bolt head or nut and
2. The joint is designed to allow for the effects of embedding
and stress relaxation.
3. Proven thread locking devices are specified. Specifically,
thread locking compounds - such as "Loctite", flanged
fasteners such as "Whizlok" or torque prevailing
fasteners such as "Nyloc". In general, loose washers,
of the plain or spring variety, are not generally advisable.
The self loosening of fasteners is
just one aspect of bolted joint design the Designer must consider
during the design process. As can be seen in the photo at
the side, even if threads are completely locked together by
adhesive, problems cannot be prevented if the bolt preload
is insufficient to prevent joint movement. The photo shows
a M12 bolt that has been partially worn away by movement.
details about the vibration loosening of threaded fasteners
Applying state of the art analytical
analysis to prevent vibrational loosening of threaded fasteners
can be complicated. 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.
to see our poster on why nuts and bolts can self-loosen
of our Test Services on Fastener Vibration Testing
our Test and Consultancy Services Brochure
details of tests completed on the double nut locking system
details of tests on helical spring washers
More details on this topic are available
on our online training course on Bolting Technology.
You may also be interested in:
of threaded fasteners
Published in the Fastener and
Fixing magazine in July 2011, this article looks into
the causes of self-loosening of threaded fasteners and
what steps can be taken to prevent loosening. A key
advantage of threaded fasteners over the majority of
other joining methods is that they can be dis-assembled
and re-used. This feature is often the reason why threaded
fasteners are used in preference to other joining methods
and they often play a vital role in maintaining a product's
structural integrity. However, they are also a significant
source of problems in machinery and other assemblies.
The reasons for such problems are due, in part, to them
unintentionally self-loosening. Such self-loosening
has been a problem since the start of the industrial
revolution and for the last 150 years inventors have
been devising ways in which it can be prevented.
Many of the common types of
locking methods for threaded fasteners were invented
over 100 years ago, however it is only relatively recently
that the main mechanism that is considered to cause
self loosening has become understood.
of Prevailing Torque Nuts
Published in the November 2009
issue of Fastener and Fixing magazine, this article
looks into the causes of why prevailing torque type
nuts can sometimes experience complete self-loosening.
The work is based upon some original work on the subject.
Prevailing torque nuts, often referred to as stiff nuts,
are one of the commonest methods of providing resistance
to self-loosening. Patents for this type of nut began
to appear in the 1860's and many of the principal types
can be traced back over 100 years. One advantage of
this type of nut is that the locking feature can be
checked at the time of assembly by measuring the prevailing
torque. The present standard test code (ISO 2230) specifies
performance requirements to ensure that nuts meet a
certain minimum standard. Key requirements are that
the first prevailing torque in the tightening direction
must not exceed a maximum value and that the first and
fifth prevailing torque in the untightening direction
must achieve certain minimum values.
There have been a number of
cases of prevailing torque nuts becoming detached from
bolts leading to a catastrophic failure of the joint.
The cause of such detachments has not been understood
and one reason for this is that such complete loosening
has not been able to be reproduced in a standard Junker
test. The article reports on how a Junker vibration
machine was adapeted so that axial loading as well as
transverse joint movement could be induced into a joint.
The experiments conducted using the modified Junkers
machine have demonstrated that the combination of axial
and transverse load have a profound effect on the loosening
of prevailing torque nuts.
The use of two
nuts to prevent self-loosening
Published in the November 2008
issue of Fastener and Fixing magazine, this article
looks at the use of two nuts to prevent self-loosening.
Many types of old machinery have two nuts on the bolts.
A thin nut is frequently used in these applications.
Sometimes the thin nut can be observed on top of a standard
thickness nut and on other installations, it’s
next to the joint, under the thick nut. Although it
may seem counter-intuitive, the thin nut should go next
to the joint and not be put on last. In other applications,
for example on column attachments, two standard thickness
nuts are frequently used.
A series of tests were conducted
to investigate the effectiveness of the two-nut method
in terms of resistance to self-loosening. A Junker transverse
vibration test machine was used with M10 nuts and bolts.
The results are illustrated in figure 6. With the small
nut on top, both nuts can be observed to rotate together
and can subsequently come completely loose. The results
are slightly better than is normally observed with a
single plain nut. With the small nut next to the joint,
some relaxation occurs but not a significant amount
of self-loosening . The performance of the two-nut method,
when properly applied, provides a superior locking capability
when compared to many so-called lock nuts. The proper
application of the two-nut method is time intensive
and requires a degree of skill and is hence unlikely
to make a major comeback on new machinery any time soon.