Threaded Fasteners: Smartify the Bolted Structure with MicroWire Sensors makes self-detection of fatigue, loosening and vibrations possible
The 3 primary types of Threaded Fasteners are Bolts, Screws,
In this article, we will focus on Bolts, which are threaded fasteners
used to join two or more unthreaded elements. The connection happens by relying
on a nut (which has internal threads): unlike screws, which often create their
own threads, however, usually are joining threaded objects.
The overview of the industry and the nature of the issues caused by the
The Bolts market alone has incredible volume, only in 2020 it has been
valued at USD 39.5 billion, and with the stated Compound
Annual Growth Rate (CAGR) of 5.7 %, it is expected to reach the valuation of
USD 46.854 Billion by
Depending on the industry the attention to the bolts, as well as their
possible failures – are subsequently different: for instance, the
aerospace industry requires close monitoring of fatigue of the
bolts while the petrochemical industry is primarily concerned
with the corrosion of the bolts. On the same topic – the automotive
industry which has one of the biggest usage volumes of bolts has
documented ongoing issues with loosening and fatigue of bolts.
Another industry that provides high (if not the highest) demand for bolts, in
regards to the volume, is the industry of industrial machinery,
where joint slip, fatigue, loosening, and corrosion alike can bring negative
Specifically in the industries mentioned above, despite the relatively low
price – bolts are there to join together and “protect” high-value
applications, which are integral for the final structure.
Bolt failure is not to be taken lightly, as it is a joining
structural piece of undeniable importance! Even if a structure has many bolts to
hold it together, the Zipper Effect is likely to eventually
damage the whole structure. An example of this effect is the damage of one bolt
on a Fixing Structure of turbine guide vanes. If one of the bolts is loose,
broken, or fails otherwise – the load on the next bolt is increased. As these
bolts are meant to bear a certain level of load, the additional weight makes
them fail, this comes in acceleration and the overall structure is exposed to
Especially when thinking about safety-critical applications such as aircraft
or on-ground passenger vehicles, it is important to have carefully
structured maintenance and monitoring strategy in place, which in most
cases is the responsibility of the manufacturer.
Types of Most Common Issues with Bolts: Classification
Surely cracks, fatigue, breakage, usual wear and tear are some of the more
obvious issues, however, looseness is essentially the beginning of the end
for the bolt, let’s explore the reasons for loosening further.
The common reasons for the looseness of bolts:
Vibrations: the small relative movement that bolts are
exposed to, makes them shift, turn, and get abnormally pressured in an opposite
direction relative to the nut, the same opposite small frictions can happen to
the objects that the bolt and the nut hold together – this causes the bolt to
unwind little-by-little, getting them loos
Over-tightening: additional amount of force on the bolt may
damage the bolt, the internal threads of the nut causing looseness or in fact,
other damages as well such as damaged screw heads, or micro embedding of the
bolt or the nut into the material it joints.
Under-tightening: In a bolted joint the bolt is essentially
stretched like a very stiff spring when tightened by the not, which creates
tension or an opposing clamp force. This force is essential to
keep the two objects between the bolt head and the nut together. When the bolt
loosened the clamping force is getting weaker or overall faded. This can result
in all kinds of issues depending on the application – from fluid leakage to
vibrations and fatigue of the
Thermal Expansion: The pre-load that a bolt can be exposed
to has a lot to do with its length. When a bolt is in a thermally unstable
environment, meaning it gets heated and cooled down a lot, it also expands and
shrinks accordingly, thus not being fit for the actual preload. This can loosen
the bolt consequently causing other issues
Design and R&D: The importance of some applications
that have to depend on the bolted joints, makes it integral for the engineers
and R&D professionals to design the bolted joints’ capabilities
accordingly. If a bolt in place is not fit to provide the necessary clamping
force which would keep the two objects together – the looseness may be
inevitable, unless the system is being closely and often examined. This is of
course not a preferred maintenance method.
Installation: perhaps the most obvious rule is to keep the
environment up to predefined standards during installation, as dirt, dust or any
foreign objects stuck in between the bolt and jointing unit will eventually
cause loosening, cracks or other fatigue-related issues.
Conventional Prevention methods:
Over the years, industrial engineers and designers have come up with great,
momentary solutions, or even solutions that helped last longer terms and to keep
the bolted joint in place, as designed.
Washers: As washers are designed to be wider than the bolt,
and generally also wider than the nut, they provide a wider surface area, which
helps to create or maintain better clamping force. Unfortunately, the common
simple washers have been proven to loosen bolts even faster when the environment
is exposed to high levels of vibrations. There are some options to overcome this
issue, such as locking washers that have teeth and digging into
the surface of the jointing material, however, this might not be acceptable for
a lot of critical applications such as aircraft. Additionally, the locking
washers negatively affect the possibility of re-tightening of the bolts. Another
clever solution is the wedge-locking washers which come in sets
of two and have opposing teeth. These wedge into each other somewhat preventing
or delaying the self re-tightening of the bolt.
Nut fixation: Variety of claver solutions have been made to
prevent the nut from falling off of the bolt, such as castellated nuts,
locking fastener systems, tab washers, etc. These usually add setup
time and as they are designed to help the bolt stay in, rather than to maintain
the clamp force – these do not solve the loosening issue and keep the
maintenance strategy the same, if not adding more hours to it
Lock nuts: These deserve special attention due to the wide
array of use within the industry. These include nylon or metal inserted within
the nut that are supposed to add friction, with this keeping the torque in
place. A similar connected idea is a spring within the bolt which can act as an
opposing tightening force when vibrations loosen the bolt. Unfortunately, Nylon
cannot withstand harsh chemical or high heat environments, additionally
embedding of the bolt or the nut into the jointed material can be an issue here
Double nuts: A variety of researchers have come to an
understanding that this simple solution of using thick and thin nuts can help
with the loosening. The different sizes of the nuts make them prone to different
rates of advancement on the threads of the bolt, thus a certain type of
vibration that would affect one of the nuts to unwind will be blocked by the
second nut. In fact this solution has been around for over 100 years.
Adhesives: Surely this makes it more of a chemical than
mechanical threading solution, however, the liquid adhesives, as well as heated
thermoplastic coatings have been successful in a variety of use cases. However,
some applications require re-winding and disassembling of the unit and adhesives
may lack in that department
Advanced Looseness Detection Methods for Bolted Structures: Sensor
Technologies of Today
For many years and even till today for a variety of applications, learning
about an error of a bolted structure happens too late. Usually, the looseness
affects arbitrary noise, vibrations or issues that are directly causing
mismanagement of the application.
The visual or vocal detection of loose bolts has been the way to go for many
years, and still is for some applications. Learning about the issue earlier on,
not only can save time and money but are of essential importance for
applications in automotive, aerospace civil engineering, and other applications
that carry safety-critical factors!
Visual methods are not bad or unnecessary. In fact, there is a good example
of log nut and wheel nut
indicators which are visualized options for frequent or random inspections
that drivers themselves can conduct every day, and see the loosening of a nut if
it appears on the wheel of their vehicle.
However, for the cases where the visual or vocal methods just aren’t
enough, what is the optimal tightening force for the specific bolt in the
How long can the bolt carry the load without unnecessary maintenance
What is the earliest possible time to know about the errors and how can these
errors potentially affect the overall system? In what time frames?
The aforementioned questions and the issues we have talked about earlier lack
one essential knowledge, to get to the answers, and it has a lot to do with the
axial force applied to the bolt!
The monitoring and especially the quality of the monitoring of the axial
force that a bolt is exposed to, is a significant factor in detecting the
majority of the errors that the bolt can cause (looseness being one of the most
important ones). There are variety of direct, indirect active, passive, computed
and other methods to monitor the axial force, and we have chosen some
exceptional ones to go through in this section of the article:
Tension indicator Washers (a Direct method)
The axial force is measured using washers with small arch-like protruding
features on them. The size of the “bumps” are custom designed per each joint
unit (specific per bolt, nut and the application in place). The “bumps” are
designed to be deformated in a controlled manner – when the axial force ends
up deforming these “bumps” it is an indicator that the axial force in place
has reached its limitations, and maintenance is needed. These are rather
straightforward methods in terms of installation and usage, however the users do
need to have specific washers designed per application, which comes with
complications. Its disadvantage is the need for washers that are specifically
designed for every particular joint.
Strain Gauges (a Direct method)
These are essentially pressure sensors and
quite good ones for a variety of use cases. These are, i.e. piezoelectric
sensors and can be used both as washers or attached onto the bolt. They often
provide proper accuracy and help with testing or research purposes, however,
these are rather expensive and not quite fit for the harsh environments where
the measurements are necessary. They have the need for power wiring connections
which brings additional headaches for the manufacturers of final units such as
electric motors, wind turbines, etc.
Torque Control (a Direct method)
This usually happens in a rather manual manner, using the torque wrench
technique where an inspector checkers the expected torque level with a torque
meter in specifically identified time intervals. Even though an old-fashioned
one, this method is still widely used in a variety of industries.
Impedance Method (an Indirect method)
This is an indirect axial force detection method for bolted structures,
specifically to detect looseness. Other than a piezoelectric component, this
method also requires a processor and of course an impedance analyzer. The method
is based on the fact that an impedance of a system is a fixed value (in case it
stays in the same condition or state), and any variation of the impedance
occurring due to some flaw such as looseness corrosion or cracks – would
affect the impedance as well.
Vibration Method (an Indirect method)
Noisy and vibrating environment comes with specific effects on the bolting
parts of the structure, thus monitoring the vibrations helps to detect or
estimate the looseness of the bolts. And vice versa, as looseness of bolts
“causes” the system to vibrate, thus vibration monitoring comes with an
estimation of bolt looseness. Here it has to be taken into consideration that a
structure with looseness or with loose bolted joints creates non-linear
vibrations. A piezoelectric accelerometer or laser vibrometer can be used to
mo.nitor vibration signature, and if there are any errors in the signature.
A big advantage of the method is that it can be rather
cost-effective and simple to perform, i.e an impact test carried out
with a hammer can show the vibration signature like we mentioned above. The big
disadvantage of the method is that it usually provides proper information in a
test set while analysing performing research activities, however in such
geometrically inaccessible locations such as aircraft turbines – this may not
be the first choice of the engineer.
Maintaining Proper Tension Ensures the Bolts Stay as Tight as
A good Bolted joint means a combination of design, appropriate clamp force /
the axial force, monitoring, and maintenance strategy to keep the bolted
structure in check and react whenever necessary.
Unique use cases: What else is out there?
Fastener Technology International magazine has presented an
interesting case where a certain-size bolt (M24) was used to join the engine to
the chassis. After a while, the bolts started to come loose and even fail. Some
semi-successful temporary attempts were made to keep the nuts in place, at which
time the investigations found that the torque level specified during design was
not implemented due to the harsh accessibility of the tooling. The solution was
later reached by using smaller bolts with higher strength, Luckily the failed
bolts here were not crucial for the timely operation of the bus, however, this
example still comes to show the necessity of finding out the problems
To continue the thought above maintenance strategies are acquiring a key role
nowadays with structural health
monitoring (SHM) of composite, jointed, automotive, and other structures and
industries. An in situ system designed to provide data from the bolted joint can
help access the kinds of preventive and predictive maintenance that will save
time and financial resources spent on the maintenance or replacement. More
importantly, an in situ system can help detect potentially dangerous defects
early on, via a quick assessment of the detected real-time anomaly.
An additional possibility comes with newer technologies, sometimes the novel
factor of these SHM technologies are fundamental to the monitoring methods such
as the RVmagnetics MicroWire sensors. Unlike the above-mentioned
methods, these sensors can provide a possibility of contactless monitoring of
torque and axial force on the bolt WHILE tightening, as well as during the
lifetime of the bolted structure.
The MicroWire sensor has magnetic properties, which allows them to receive
real-time data without direct power wiring – making it possible to have them
placed into applications for real-life use, in addition to research purposes.
Another feature of microwire making the aforementioned possible is the
resistance of the wire to harsh chemical and thermal conditions!
Be it detecting vibration, pressure, or torque – one wire can be enough to
receive very local data directly from the bolt, without direct connection as the
knowledge you receive in real-time is acquired thanks to the magnetic field
created by the sensing head.