News about Bolt Science

Many people think that bolting is one of those topics that never change, all that can be known about the topic is known. This is not the case. Advances in our understanding about the subject are continuing. For example into the causes of bolt preload reduction by relaxation and self-loosening. Posted on this page will be news about what is happening both in the subject in general and with Bolt Science in particular. This will include any updates to our training courses and software.

 

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2025 April Newsletter - Using a Design Margin or a Probability of Failure Approach. Part 2
A probability assessment of failure is closer to the reality of the situation and offers a refined and data-driven approach compared to a traditional factor of safety.
The disadvantage of the probability approach is that it can be disconcerting but is likely to be closer to reality than the design margin approach. It is disconcerting since most people are uncomfortable with a risk of any magnitude. The Design Engineer may well be asked the question, will it fail or not? Replying that there is a failure probability of 0.000006 is likely to raise eyebrows and maybe concern. < Read more >

2025 March Newsletter - Using a Design Margin or a Probability of Failure Approach. Part 1
The root cause of the majority of bolt/joint failures is due to insufficient preload. It is unusual for the bolt to be overloaded. If the preload provided by the bolt is insufficient, joint separation and movement can occur resulting in possible bolt fatigue and self-loosening issues. In order that such problems do not occur it is vital that there is sufficient residual clamp force acting on the joint interface after accounting for the effects of the applied forces and embedding losses. A Preload Requirement Chart graphically illustrates this point as it looks at the forces acting on the joint interface.
I was recently asked the question that if the minimum preload value was equal to the total preload requirement, would the joint be okay. That is, the design margin would be 1, so would the joint be okay? Most Engineers are familiar with the use of a design margin or factor of safety. Crudely put, if the margin is below 1 the joint will fail, if above 1 it will be okay. It gives that certainty which we crave for, will the joint fail or not, yes or no. Reality is more nuanced. < Read more >

2025 February Newsletter - Friction Grip Joint Calculation Example Study
In the December and January newsletters, the topic of interface friction in friction grip joints was discussed. Prior to the bolt being tightened, any shear loading would cause the joint plates to slip resulting in wear issues, amongst others. By tightening, a preload is induced into the bolt that is reacted by a clamp force acting on the joint. The clamp force in conjunction with the friction that is present between the plates generates a friction grip that resists the shear force. The importance of the value of friction coefficient between the joint plates is often underestimated.
Bolt Science has been consulted on a number of occasions in which bolts were coming loose on joints. In some instances, such issues can be resolved by the use of an effective locking method, such as for example, wedge-lock washers. However, this is not always the case. The locking approach works if the joint only experiences an occasional overload that results in the joint slipping. If the joint experiences frequent loading that causes joint slip, a bolt fatigue issue substitutes for the loosening problem. Usually, some time passes before the bolt fails by fatigue, but failure is likely if the joint experiences repeated slip. < Read more >

2025 January Newsletter - The use of friction shims in bolted joints
The newsletter discusses ways in which friction on the joint interface can be reliably increased and the advantages of doing so. Friction shims are discussed, both diamond coated and the recently released MicroLock disks. The diamond coated shims have microscopic diamonds randomly distributed on the shim surface. MicroLock disks that have carefully formed microscopic spikes on their surface. In essence, the principle behind these two shims is the same. That is, the asperities on the shim dig into the joint surface significantly increasing friction so that a far larger greater shear force is needed to slip the joint for a given clamp force provided by the bolts. Preventing slip prevents self-loosening and bending fatigue issues as discussed in last month's newsletter. < Read more >

2024 December Newsletter - The Importance of Friction in Friction Grip Joints
In mechanical engineering, most bolted joints have clearance holes. That is, the hole is a larger diameter than the bolt's thread diameter. A larger hole is employed for manufacturing reasons so that the parts can be economically made and assembled. Many bolted joints have to sustain a shear loading as illustrated in the image below. Prior to the bolt being tightened, any shear loading would cause the joint plates to slip resulting in wear issues, amongst others. By tightening, a preload is induced into the bolt that is reacted by a clamp force acting on the joint. The clamp force in conjunction with the friction that is present between the plates generates a friction grip that resists the shear force. This is refered to as a friction grip joint. If friction grip is maintained, the plates will not slip, and the shear force passes through the plates themselves rather than the bolt. To prevent slip, the critical parameters are the magnitude of the bolt preload and the value of the coefficient of static friction on the joint interface. In structural engineering, the term slip coefficient is frequently used instead of the coefficient of friction in this regard. < Read more >

2024 November Newsletter - Helical Spring Washers
I was recently asked under what circumstances would I recommend the use of helical spring washers. These are the type of spring washers which are split. My reply was that I would not. In this months Bolt Science newsletter I thought that I would just review the reasons why I believe that all the evidence points towards them being ineffective.
It is not that this type of washer causes self-loosening, but don't necessarily expect them to help if the conditions arise that will cause self loosening. < Read more >

2024 October Newsletter - Joint bearing pressure and the use of unsuitable washers
Some updates have recently been made to our online training material on joint bearing pressure. This is the pressure acting on the joint as a result of bolts being tightened. If the contact pressure is too high, plastic collapse of the surface can occur. This in itself may not be an issue, but when such plasticity occurs after the tightening is completed, the bolt preload will reduce. As a consequence, joint integrity issues can arise such as self-loosening and fatigue.
The issue over excessive bearing pressure is likely to be encountered when softer joint materials are being used, such as aluminium, and also, when mild steel instead of hardened washers are used. The strength of aluminium is typically less than half of that of the bolting materials typically used. As a consequence without having something present to increase the contact area, such as flanged fasteners or hardened washers, the surface is likely to collapse when the bolt is fully tightened. < Read more >

2024 July Newsletter - Bolt Fatigue and a Bolt's Thread Root
Most engineers will likely come across a fatigue failure at some time in their career. Fatigue is the progressive cracking of a material due to repeated loading and unloading cycles. For fatigue to occur, there must be a repeated change in the loading condition and subsequently, the stress condition. No stress change, no fatigue. When bolts fail due to fatigue it is usually in the first thread root next to the nut-to-joint interface. This newsletter explains why it is this location on the thread that usually fails as a result of fatigue rather than at some other location on the bolt. < Read more >

2024 April Newsletter - Tightening Multiple Bolts and Elastic Interaction
Most joints consist of several bolts and since the vast majority of bolts are not tightened simultaneously, the effect of tightening one bolt in the group has an effect on the preload in other previously tightened bolts in the group. Such an effect is called elastic interaction, or alternatively, bolt crosstalk. Ensuring that the bolt loading is even is often critical in gasketed joints to minimise possible leakages. Obtaining an even bolt load distribution is not clear-cut since the final bolt load usually differs from the initial bolt load. < Read more >

2024 March Newsletter - Which Types of Bolt Failure Are Most Common?
Long ago when I worked as part of a large team of design engineers, there were people who stated that fatigue was the commonest cause of bolt failures, others said that it was insufficient preload, and fatigue was an indirect cause. I was reminded of this when I read a recent technical paper (Review on recent advances in structural health monitoring paradigm for looseness detection in bolted assemblies, in Structural Health Monitoring, March 2023). This included an interesting pie chart that I've reproduced below. This shows the distribution of articles based on bolt failures published from 2016 to 2021 obtained by keywords bolt failure, bolted joints, and machine learning. < Read more >

2024 January Newsletter - Non-Linear Torque-Tension Relationships during Tightening
This newsletter discusses linear and non-linear behaviour that can be experienced when tightening bolts.
The relationship between the torque being applied and the bolt preload is linear. That is, when you plot out the relationship between the two by measuring both, you get a straight line as shown in the image. So, for a particular bolt, if you double the applied torque, you will double the preload. Assuming that is, that the bolt material does not yield, or the thread starts to strip.
This linear behaviour forms the basis of the torque calculation processes. Occasionally, a situation arises where the graph of torque against preload is not linear, that is, the graph is a curve instead. The graph of torque against preload may curve upwards, or downwards, depending upon circumstances. < Read more >

2023 November Newsletter - ISO 68-1 The Basics of the Metric Thread Form
This newsletter discusses an important update to the ISO standard that defines the metric thread.
In October 2023 the biggest revision of ISO 68-1 in 50 years was released. This standard defines the profile, or shape, of the metric thread. It is referenced by all metric thread fastener standards. Prior to the release of this latest standard, it defined just the basic thread profile, that is, the theoretical profile of the thread defined by dimensions and angles common to both internal and external threads.
The big change the latest version introduces into this standard is the design profile. Unlike the basic profile, the design profiles for the internal and external threads are different. For the internal thread the profile is as the basic profile, that is, no radii at the thread root are specified, they are flat. For external threads, the root contour of the thread is rounded. < Read more >

2023 September Newsletter - Recent Update to ISO Nuts
An updated version of the standards ISO 4032 on hexagon regular nuts, and ISO 4033 on hexagon high nuts has been released over the last month. These latest versions replace the previous 2012 standards. Several changes have been made to the standard, the introduction of additional property classes and thread sizes and the addition of duplex stainless steel nuts D4 and D6 and the addition of property class 80 to ISO 4032 standard. < Read more >

2023 August Newsletter - The Reduced Loadability of DIN compared to ISO Nuts
Some years ago, on a Bolt Science training course given to largely Design Engineers at a major manufacturer, they spoke about a thread stripping issue they had on one application. The nuts were stripping, and they didn't know why. Looking at the nuts they had a |8| marking on them, they had assumed that they were property class 8 to the relevant ISO standard (which is ISO 898 Part 2). They thought that the vertical bars at either side of the 8 was just decorative and of no significance. In fact, they are essentially a separate property class, property class |8| to a withdrawn DIN standard. Such nuts have what is referred to as reduced loadability, the nut's proof load is lower than that of an ISO nut. The reduced strength of the DIN nut was a likely cause of the thread stripping issue that they were experiencing. < Read more >

2023 July Newsletter - The Goldilocks Zone and the Coefficient of Friction of Fasteners
In July 2023, Bill Eccles of Bolt Science presented a paper titled: 'The Goldilocks Zone and the Coefficient of Friction of Threaded Fasteners' at the LUBMAT 2023 conference in the UK. This summarises the paper and adds some background. In brief, there exists a range of fastener friction values, the Goldilocks zone, that is not too low so that self-loosening becomes a concern, and not too high so that concerns about high torque, high thread torsion and low preload arise, but just right to use Goldilocks words. < Read more >

2023 June Newsletter - The Joint Decompression Point - Why Bolt Preload is Important!
The most common reason why bolted joints fail is due to the bolt failing to provide sufficient preload to prevent the external applied forces overcoming the clamp force acting between the joint faces. The joint decompression point is when the clamp force acting between the joint faces, that has been provided by the bolt's preload, has been reduced to zero by the applied forces. < Read more >

2023 May Newsletter - What is a Residual Torque Measurement?
It is not unusual for there to be uncertainty as to the installed integrity of a bolted joint. A common check that is completed to assess whether the nut has been tightened to the correct torque, or whether some loosening occurred, is a residual torque measurement.
The residual torque method involves the measurement of the torque needed to rotate the bolt or nut by a small angle, typically 2 to 5 degrees, in the tightening direction. The residual torque method is the approach normally used by most industries to assess the torque value of a previously tightened fastener. < Read more >

2023 April Newsletter - Case study of fastener issues causing a rail accident?
A new case study has been added to Bolt Science’s distance learning course on Bolting Technology for Engineers and Designers. The case study is on a major rail accident which occurred in the UK due to a problem with a joint secured with two M10 studs.
On the 26 August 2020, a freight train travelling from Milford Haven in Wales derailed near Llangennech in Carmarthenshire. The freight train comprised 25 tank wagons, each with 75 tonnes of diesel and fuel oil. < Read more >

2023 March Newsletter - Is it okay to use a mild steel nut with a high tensile bolt?
Nut thickness standards have been drawn up on the basis that the bolt will always sustain tensile fracture before the nut will strip. If the bolt breaks on tightening, it is obvious that a replacement is required. Thread stripping tends to be gradual in nature. If the thread stripping mode can occur, assemblies may enter into service which are partially failed, this may have disastrous consequences. Hence, the potential of thread stripping of both the internal and external threads must be avoided if a reliable design is to be achieved. When specifying nuts and bolts it must always be ensured that the appropriate grade of nut is matched to the bolt grade. < Read more >

2023 February Newsletter - Bolt Preload Variation due to the Tightening Method.
The effect that the method of tightening has on determining what size of bolt is required to fulfil a specific function is largely underestimated. If several bolts of the same size are tightened by the same method then there will be variation in the bolt's preload - they won't have all the same value. This variation is influenced by such factors as variation in friction characteristics in the thread and under the nut face, thread form and pitch variations, variations in the surface flatness etc. Hence for any particular tightening method there will be a maximum anticipated preload and a minimum given a set of conditions. < Read more >

2023 January Newsletter - Some bolts are deliberately tightened past their yield point. Is this a problem?
When a bolt is tightened into its plastic region, yielding is the result of the combined effects of both tensile/axial stress and the torsional stress exceeding the yield point of bolt material. The tensile/axial stress is the result of the extension/stretching of the bolt and the torsional stress as a result of the thread friction and stretch torque acting on the thread. When the joint sustains an external loading, there are two effects that allow the bolt to be axially loaded without sustaining further plastic deformation: < Read more >

2022 December Newsletter - What is the Best Way to Check the Torque Value on a Bolt
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 coincide.
For methods 1. and 2. the breakloose torque is normally slightly higher than the installation torque since static friction is usually greater than dynamic friction. In my opinion, the most accurate method is method 3 - however what this will not address is the permanent deformation caused by gasket creep. An alternative is to measure...... < Read more >

2022 September Newsletter - World Standards Day
On the 14th October, it is World Standards Day, a day celebrated since 1970. Supported by the main international standards bodies (ISO, IEC and ITU) it is a day in which the importance of standards to global prosperity is celebrated. It is also a day in which tribute is paid to all the thousands of expert volunteers who give up their time to create these voluntary technical agreements.
Is it to spec.?; a saying often heard when a part does not fit, or it fails. A standard provides the information on which to judge whether a part is fit for purpose or not. Even though fasteners are standardized products, the central importance of standards are often undervalued. If we go back to before 1840, threaded fasteners were generally bespoke, that is a bolt of a particular size was made to fit a particular nut. Obtain a bolt from some other source, of the same diameter, and the nut was unlikely to fit, and vice-versa. To quote from the paper by Joseph Whitworth: On a Uniform System of Screw Threads, published in June 1841: Engineers have adopted their threads without reference to a common standard. Whitworth described his proposed thread in a technical paper since it was before official standards as we know them today. His description of his standard thread allowed bolts, and nuts, to be made as standardized products thus allowing taps, dies and gauges to be mass produced for the benefit of all. < Read more >

2022 August Newsletter - Quality Control in the Tightening of Bolts
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 here 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. < Read more >

2022 July Newsletter - 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 below the standard thickness nut and on other installations, it’s on top. 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. In this newsletter the effectiveness of this locking method is investigated and the tightening procedure that should be used if effective locking is to be achieved. < Read more >

2022 June Newsletter - Analysis for a Bolt Fatigue Failure in a Clevis Joint
A clevis type joint consists usually of two parts, a rod or member that is trapped between two supports. Such joints are used to transmit primarily shear loads. There are a wide range of uses of such joints, the one shown in the image consists of a spring on a vehicle suspension being secured between two supports. There must initially be a gap between the bush in the spring and the supports to allow it to be installed. The gap is closed by tightening the through bolt so that there is metal to metal contact through the joint. Accordingly, the gap is usually small so that the force needed to pull the plates together is achievable and that the supports are not over-stressed by the clamp load from the tightening process. The advantage of clamping the parts together is that the loading from the spring is transmitted by friction grip rather than by placing the bolt into direct shear. This allows dynamic loads to be transmitted without joint slip or movement occurring. If the direct shear approach is used, the shear load is carried into the supports by a shear stress carried in the bolt. In such circumstances, the hole clearance needs to be minimised and the thread not to be in the shear plane, ideally. Clevis joints in direct shear are extensively used for applications in which the loading is not highly dynamic and having little or no requirement to transmit any side loading. < Read more >

2022 May Newsletter - Bolt Self-Loosening and the Critical Slip Distance
It’s been known for over 50 years that if movement of joint plates occurs, the securing bolts can self-loosen. The research completed first by Gerald Junker followed by many subsequent researchers found that if transverse joint slip occurs, simultaneous slippage of the bolt head, or nut face, and the thread interface, momentarily frees the fastener from friction. Free from the restraints of friction, the load present in the bolt, acting on the inclined plane of the thread, allows the bolt to rotate by a small increment. Recurring joint movement repeats this self-loosening process until the bolt is completely relieved of its load.

The speed in which the bolt can loosen depends upon the magnitude of the joint movement, the fastener friction, and the grip length of the bolt. If you prevent the threads from slipping, by use perhaps of an adhesive, or the head from rotating by for instance a wedge-lock washer, self-loosening will not occur. < Read more >

2022 April Newsletter - The Effect of Joint Face Angularity on the Fatigue Life of Bolts
It is well known that it is good design practice to ensure that the joint faces, which the nut or the bolt head seat on, are at right angles to the bolt axis. Angle errors on the joint face do have an influence on the static strength of bolted joints, however for small angles, this is usually not significant. What is less well known is that even small angularity errors can have a catastrophic effect on fatigue life. < Read more >

2022 March Newsletter - Information on Screw Threads
The basic profile is the theoretical profile of the thread. An essential principle is that the actual profiles of both the nut and bolt threads must never cross or transgress the theoretical profile. So bolt threads will always be equal to, or smaller than, the dimensions of the basic profile. Nut threads will always be equal to, or greater than, the basic profile. To ensure this in practice, tolerances and allowances are applied to the basic profile. < Read more >

2022 February Newsletter - When to use a back-up wrench
A back-up wrench or spanner is placed on the bolt head to prevent bolt rotation when the nut is being tightened. Once the bolt is tightened and the nut is then attempted to be tightened, the whole nut-bolt assembly can spin without further tightening occurring at a torque lower than the initial tightening torque. The concern sometimes expressed is whether or not this is a problem?

When a nut is tightened, the torque applied to the nut can be split into three components: The torque needed to stretch the bolt (the thread extension torque); the torque needed to overcome thread friction; and the torque needed to overcome nut face friction. The torque needed to stretch the bolt and the torque needed to overcome thread friction is transmitted through the bolt shank and is reacted by the bolt head. Whether a backing spanner is needed depends upon whether the torque needed to rotate the bolt head is greater than the torque being transmitted up the bolt shank. This torque is the sum of the torque needed to stretch the bolt plus the torque needed to overcome thread friction. If the reaction torque provided by friction grip of the bolt head on the joint surface is greater than the torque being transmitted up the shank, then the bolt will not rotate and the nut would tighten the assembly. < Read more >

2022 January Newsletter - Helical Spring Lock Washers
Helical spring lock washers have been in use for well over 100 years. They are still used on many applications in the belief that they will will "lock" the nut/bolt to the joint and prevent loosening. The body of evidence, based upon both experience and experimental results, is that they do not prevent loosening and can be shown to actually speed up the rate of loosening in many cases. Junker originally showed in his work published in 1969 that these washers are ineffective in preventing loosening. Joints containing these washers have come loose resulting in structural failure leading to both material and human loss. < Read more >

2021 December Newsletter - Thread Stripping Case Studies
Thread stripping or shearing of threads is a common problem. Thread stripping is gradual in nature and hence is not necessarily detectable at the time of assembly. For the threads to be fully sheared, a displacement of at least half a thread pitch is needed. If the threads fully shear on tightening, the bolt will detach from the nut. This is likely to be noticed before the product goes into service. If however the thread is only partially sheared, the nut will be retained on the bolt thread and the potential for failure may not be noticed. When subsequently a significant load is applied to the joint, the partially failed threads could shear, and the nut become detached from the bolt with potentially catastrophic results. A partially stripped bolt thread is shown in the image. Thread stripping can affect both internal and external threads, which thread will shear first depends upon their relative strengths. < Read more >

2021 November Newsletter - Case Study on a Bolt Self-Loosening Investigation
Self-loosening of bolts and nuts is extremely common. In some situations it is an inconvenience, in others, disastrous consequences can arise. When bolts do come loose, it is due to specific causes, address those causes, and loosening will not occur. A case study investigating the reason why bolt loosening was occurring on a drive sprocket on a tracked vehicle has just been published on the Bolt Science website.
It is known that self-loosening is usually the result of the bolts providing insufficient bolt preload to prevent joint movement. Relaxation losses will reduce the preload, to some extent. If the shear forces applied to the joint then exceed the friction grip capability, some transverse joint displacement will occur. What the case study investigates using the BOLTCALC program is whether there is sufficient bolt preload to prevent joint movement. In general for the vast majority of joints, preventing joint movement from occurring will prevent self-loosening. < Read more >

2021 October Newsletter - The Joint Decompression Point - Why Bolt Preload is Important!
This is the second part in a series where we are getting back to basics and looking at the importance of bolt preload. The most common reason why bolted joints fail is due to the bolt failing to provide sufficient preload to prevent the external applied forces overcoming the clamp force acting between the joint faces. The slide in the newsletter illustrates the joint decompression point. This is when the clamp force acting between the joint faces, that has been provided by the bolt's preload, has been reduced to zero by the applied forces. < Read more >

2021 September Newsletter - - Back to Basics - Why Bolt Preload is Important!
Over the last fifty years great improvements have been made by the fastener industry in improving the design and reliability of their products. However, no matter how well designed and made the fastener itself is, it cannot alone make the joint more reliable. Fastener selection based upon an understanding of the mechanics of how a threaded fastener sustains loading and the influence that tightening procedures can play is also needed. This article provides an introduction to the basics of bolted joints and the major factors involved in the design of such joints. < Read more >

2021 August Newsletter - Historical Background of Screw Threads
It is considered by some that the screw thread was invented in about 400BC by Archytas of Tarentum (428 BC - 350 BC). Archytas is sometimes called the founder of mechanics and was a contemporary of Plato. One of the first applications of the screw principle was in presses for the extraction of oils from olives and juice from grapes. The oil presses in Pompeii were worked by the screw principle. < Read more >

2021 July Newsletter - The Effect of Joint Face Angularity on the Fatigue Life of Bolts
It is well known that it is good design practice to ensure that the joint faces, which the nut or the bolt head seat on, are at right angles to the bolt axis. Angle errors on the joint face do have an influence on the static strength of bolted joints, however for small angles, this is usually not significant. What is less well known is that even small angularity errors can have a catastrophic effect on fatigue life. < Read more >

2021 June Newsletter - Non-rotational Loosening - Stress Relaxation Loss
In this third newsletter on the non-rotational loosening of threaded fasteners, I will be looking at a mechanism called stress relaxation. Stress relaxation results in a loss of bolt preload, without any rotation of the fastener occurring. At an elevated temperature, if a metal part is subjected to a tensile stress, creep or cold flow can occur resulting in the part increasing in length. The amount of deformation the part will experience depends upon the magnitude of stress, the properties of the metal and how long the part is exposed to the elevated temperature. ........ < Read more >

2021 May Newsletter - Non-rotational Loosening - Preload Loss from Embedding
This is the second article in a series discussing the non-rotational loosening of threaded fasteners. Embedding sometimes referred to as settlement or permanent set can be shoulded in confusion. Embedding can result in a loss of bolt preload without any rotation of the fastener occurring. So, what is embedding? Surfaces may appear to be smooth to the naked eye but when sufficiently magnified, there are high spots on the surface. These are referred to as asperities. Embedding is a localised deformation process that involves the plastic flattening of the asperities at the contact areas within a joint........ < Read more >

 

April 2021 Newsletter - Non-Rotational Loosening of Threaded Fasteners - Part 1
This is the first part in a series of articles discussing non-rotational loosening of threaded fasteners. One feature of nuts and bolts is that they allow complex assemblies to be built and dismantled, as necessary. Under certain conditions, a nut or a bolt can come loose with obvious undesirable consequences. Self-loosening is when the nut, or bolt, rotates under the action of applied forces. The detailed mechanisms that result in self-loosening have been a subject of research for the last sixty years and is explained elsewhere on the website. Self-loosening of threaded fasteners can lead to complete disassembly and catastrophic failure of a product. The numerous locking devices available are all trying to eliminate, or at least reduce, this possible fastener rotation...... < Read more >

 

March 2021 Newsletter - Is a Torque Value Really Needed?
A question often asked on training courses is whether it is necessary to specify a tightening torque? The answer somewhat depends upon what industry sector you are working in. In structural engineering, on buildings for example, many joints do not rely upon the bolt’s clamp force for the transmission of loads. The bolts in the joint sustains the load directly in tension if the loading is axial, or directly in shear, if the loading acts transversely to the joint. This works satisfactorily when the loading is largely static, and it does not reverse. Essentially, the requirement is that the nut must be just fully engaged on the bolt thread with the joint plates in metal-to-metal contact.

In mechanical engineering, the loading on the joint is usually dynamic and can often reverse in direction. In such circumstances, the bolt would move in the hole if any transverse loading were reversed..... < Read more >

 

February 2021 Newsletter - Understanding by how much the thread should protrude past the nut
A question frequently asked is by how much should a thread protrude past the nut. There are some building codes that stipulate that there must be at least one thread protruding through the nut. However it is common practice to specify that at least one thread pitch must protrude across a range of industries. Typically the first few pitches of the thread can be only partially formed because of a chamfer etc. < Read more >

 

January 2021 Newsletter - Background to Bolt Science and a new feature, Ask a Question?
This months newsletter is a bit different. Dr Bill Eccles tells the story of how Bolt Science started. Also included is a new feature, Ask a Question. This feature will provide an answer to a question raised either via email or from discussions during training. This month's question: I am tightening stainless steel fasteners and I am finding that they are seizing, sometimes during tightening but often when they are attempted to be untightened. What is happening? < Read more >

 

November 2020 Newsletter - Should there be a tolerance on a torque value?
A question that is sometimes asked is what tolerance should be placed on a torque value. I remember in my early days in engineering that the quality department requested that a tolerance be placed on a torque value that I had specified. I was told that without a tolerance, how were they expected to know what was acceptable to the Design Department. Many organisations, probably most, specify a single torque value with no tolerance. Other companies have some documentation for quality inspection may well specify a tolerance on that single value for checking purposes. Placing a tolerance on the drawing runs the risk of a person setting the torque tool to the high or low value provided. The torque value applied could then be higher or lower than the specified range due to the additional error from the tool accuracy. Without an acceptable torque range or a tolerance, problems tend to occur if a dispute arises on what is acceptable. < Read more >

 

October 2020 Newsletter - What is short bolting?
Short bolting is a term frequently used to describe the situation when a bolt is installed and the thread does not fully protrude through the nut. In order that the full strength of the nut is to be realised, it is vital that the bolt thread protrudes through the nut. Failure to do so runs the risk that thread stripping will occur. It is common practice to specify that two thread pitches must protrude. Typically the first few pitches of the thread can only be partially formed because of a chamfer etc. Nut thickness standards have been drawn up on the basis that the bolt will always sustain tensile fracture before the nut or bolt thread will strip. If the bolt breaks on tightening, it is obvious that a replacement is required. Thread stripping tends to be gradual in nature. If the thread stripping mode can occur, assemblies may enter into service which are partially failed, this may have disastrous consequences. Hence, the potential of thread stripping of both the internal and external threads must be avoided if a reliable design is to be achieved. When specifying nuts and bolts it must always be ensured that the appropriate grade of nut is matched to the bolt grade. < Read more >

 

September 2020 Newsletter - Tightening bolts past their yield point
Some bolts are deliberately tightened past their yield point. Why don't they further yield when an external load is subsequently applied to the joint and come loose? When a bolt is tightened into its plastic region, yielding is the result of the combined effects of both tensile/axial stress and the torsional stress exceeding the yield point of bolt material. The tensile/axial stress is the result of the extension/stretching of the bolt and the torsional stress as a result of the thread friction and stretch torque acting on the thread. When the joint sustains an external loading, there are two effects that allow the bolt to be axially loaded without sustaining further plastic deformation... < Read more >

 

August 2020 Newsletter - Vibrational Detachment of Threaded Fasteners
A question was recently asked if it was possible for a loose nut to act against gravity and rotate off a bolt. Such instances can happen, but only under dynamic load conditions, i.e. the bolt being subjected to some degree of vibration. The process of previously tightened nuts coming loose is now reasonably well understood. Relaxation effects can occur (such as creep, stress relaxation and embedding) in which the preload created by the tightening process is lost, or partially lost, without the nut rotating. Self-loosening is when the nut rotation results in the preload being lost. < Read more >

 

July 2020 Newsletter - How do we know what size of bolt to use?
How do we know for sure which bolt is the right size for each unique joint interface? Could we get away with a smaller bolt without risking the integrity of the bolt/joint? If so how? Joint diagrams can display a significant amount of information about the joint but in our experience many people find them difficult to interpret and understand. Preload Requirement Charts are a way to graphically display the results of a joint analysis in a clear and understandable manner. < Read more >

 

May 2020 Newsletter - When is a back-up wrench needed?
A back-up wrench or back-up spanner is placed on the bolt head to prevent bolt rotation when the nut is being tightened. Once the bolt is tightened and the nut is then attempted to be tightened, the whole nut-bolt assembly can spin without further tightening occurring at a torque lower than the initial tightening torque. The concern sometimes expressed is whether or not this is a problem? < Read more >

 

April 2020 Newsletter - The use of two nuts to lock a fastener
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 below the standard thickness nut and on other installations, it is on top. 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. < Read more >

March 2020 Newsletter - Fatigue Failure of an M16 Flanged Bolt
Investigating fastener failure can provide invaluable insight into the correct application and assembly of fasteners. In the image below we can see the failure of an M16 flanged fastener of strength grade 12.9. This fastener failed as a result of fatigue failure.
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January 2020 Newsletter - A new approach to the tightness checking of bolts
A frequent question that is asked relating to bolting is: 'We've just checked a nut that we tightened and it's well below what we tightened it to just minutes earlier ‑ what's wrong?'. It's not widely known that the torque for a newly tightened nut or threaded fastener, in general, is different depending upon whether you attempt to untighten it or tighten it a bit more. Typically the torque needed to untighten a newly tightened fastener is around 10% to 30% less than the torque  to tighten it further. When you are tightening a threaded fastener a significant amount of torque is needed to overcome friction in the threads and under the nut face (or the bolt head, if the bolt is rotated). The proportion of the torque that is used to overcome friction depends upon the friction value but is typically in the 85% to 90% region. This is illustrated in Figure one, which shows that when tightening a nut/bolt with a coefficient of friction of 0.12, only about 14% of the torque is used to stretch the fastener producing the clamp load with 86% of the torque being lost overcoming friction. The torque needed to stretch the fastener always acts in the untightening direction and it's for this reason that the untightening torque is less than the tightening torque.
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December 2019 Newsletter - Problem with loose screws
On 22 August 2019, a train service from London in the UK travelled 16 miles at speeds of up to around 80 mph while a coach door was open. At the time, the coach was occupied by passengers. The doors were remotely operated by the driver. Screws that had been securing a bracket to the door had fallen out. Following this incident, checks on the fleet of trains were completed. Of the 480 doorways on the refurbished units, it was identified that at least one screw was deemed to be loose on at least 60 doorways.
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November 2019 Newsletter - HE and 10.9 Bolts
Earlier this year, the International Organization for Standardization, known as ISO published a technical report on hydrogen embrittlement. The technical report ISO/TR 20491 (Fasteners - Fundamentals of hydrogen embrittlement in steel fasteners) was published earlier this year (2019). Fasteners sustaining hydrogen embrittlement (HE) experience a permanent loss of ductility due to atomic hydrogen coupled with tensile stress affecting the cohesion of the metal. One controversial aspect of the technical report is that it asserts that property class 10.9 fasteners have no significant susceptibility to hydrogen embrittlement failure assuming that the fasteners are produced using appropriately selected steel, well-controlled steel making and fastener manufacturing processes.
<Read more>

Bolt Science Online Quiz
A new online quiz on bolting has just been launched by Bolt Science, designed to test an engineer and manufacturer's knowledge on bolting and torque tightening. The quiz, which may highlight gaps in a person's knowledge on bolting can be found on the Bolt Science website. < Read more >

Previous News

Mobile Training App Enhanced
The mobile iPad and Android App that Bolt Science uses for distance learning has recently been enhanced. The App allows you to download and view course content offline. Wherever the students are,they will be able to take courses even without Internet connection. Read more >

Technical Article Published in the August 2015 Edition of Fastener Technology International
Article in Fastener Technology International
Bill Eccles of Bolt Science had an article published in the August 2015 edition of the Fastener Technology International magazine. The article titled 'Bolt Fatigue Failure due to Insufficient Tightening' is a case study discussing the consequences of not adequately tightening bolts. Read more > 

Bolt Science releases a new distance learning course for Engineers and Designers
Online Distance Learning Course
Bolt Science has released a new distance learning course titled: 'Bolting Technology for Engineers and Designers'. The training is completely online and can be taken whilst at work on a PC, or if you wish, from home or other location using a laptop,or a tablet device such as an iPad. Read more > 

 

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