The 3 primary types of Threaded Fasteners are Bolts, Screws, and Nuts.

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 Bolts’ Errors

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 2030.

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 consequences.

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 damage.

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 structure.
  • 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 as well.
  • 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 specific application?

How long can the bolt carry the load without unnecessary maintenance checks?

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 designed.

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 early on.

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.

RVmangetics has a demonstration of the Contactless Bolt Tension Monitoring, which is publicly available for review:

For more information about the technology, feel free to watch the following 3-minute video about MicroWire sensors or simply contact us.