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.

 

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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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. ........
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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........
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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......
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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.
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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?
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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.
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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.
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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...
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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.
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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.
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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 >

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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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