Published Articles on Bolting

Bill Eccles of Bolt Science has written several articles that have been published in fastener magazines. On this page links to some of the articles are presented together with a brief summary of the article content.

Click on the image next to the summary to load a PDF version of the article.

 

A new approach to the tightness checking of bolts

Published in the Fastener and Fixing magazine in November 2014, this article discusses a new approach to torque auditing of threaded fasteners. The article discusses the current tightness checking approaches, the on-torque method, the off-torque method and the marked fastener method. Each of these three methods have their deficiencies. The key assumption in each method is that the torque value measured is a true assessment of the tightness of the connection. The critical flaw in each of these methods is the assumption that the coefficient of friction has not changed between the tightening of the bolt/nut and the completion of the checking process. The method described in this article is an attempt to improve upon the present tightness checking methods to allow the clamp force being provided by a previously bolted connection to be assessed and, potentially, corrected.

There are limitations to this new approach to fastener tightness checking, principally its accuracy in high friction conditions, but it is a useful indicator as to the retained fastener clamp force in many circumstances.

A case study in torque-angle tightening

Published in the Fastener and Fixing magazine in 2006, this article discusses the torque-angle method of tightening. Because of space restraints, a manufacturer of construction equipment found that it was not possible to achieve the performance requirements for a high performance joint by using torque tightening. The joint shown in figure 1, consists of two steel outer members sandwiching an aluminium beam that are clamped together with a M12 flanged headed nut and bolt.

The article mentions the Bauschinger effect which is one of the reasons why a bolt tightened past its yield point will act elastically when loading is subsequently applied to the joint.

For critical applications, the torque-angle method of tightening bolts provides a way of achieving the maximum clamp force in a joint and is often the way forward when torque control proves to be inadequate

The Basics of Bolted Joints

Published in the Fastener and Fixing magazine in November 2011, this article presents some fundamental ideas about bolted joints. Over the last sixty 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. It is not widely understood how a bolted joint carries a direct load. A fully tightened bolt can survive in an application that an untightened, or loose bolt, would fail in a matter of seconds. When a load is applied to a joint containing a tightened bolt it does not sustain the full effect of the load but usually only a small part of it. This seems, at first sight, to be somewhat contrary to common sense but hopefully this article will explain why this is the case.

Self-loosening of threaded fasteners

Published in the Fastener and Fixing magazine in July 2011, this article looks into the causes of self-loosening of threaded fasteners and what steps can be taken to prevent loosening. A key advantage of threaded fasteners over the majority of other joining methods is that they can be dis-assembled and re-used. This feature is often the reason why threaded fasteners are used in preference to other joining methods and they often play a vital role in maintaining a product's structural integrity. However, they are also a significant source of problems in machinery and other assemblies. The reasons for such problems are due, in part, to them unintentionally self-loosening. Such self-loosening has been a problem since the start of the industrial revolution and for the last 150 years inventors have been devising ways in which it can be prevented.

Many of the common types of locking methods for threaded fasteners were invented over 100 years ago, however it is only relatively recently that the main mechanism that is considered to cause self loosening has become understood.

Failure of an M24 Engine Mounting Bolt

Published in the Fastener Technology International magazine in August 2006, this article looks into the causes of the failure of M24 engine mounting bolts. An M24 property class 8.8 bolt was used to secure one of four engine mounts to the chassis of a bus. Following the introduction of the bus into service and some operational experience, reports started to be received that bolts were occasionally found loose and, on a number of occasions, the bolts were failing. To prevent what was perceived to be a loosening problem, a split pin was introduced that passed through the bolt thread immediately below the nylon insert nut to prevent the possibility of the nut backing off. This fix proved to be only partially successful and instances were still being reported that the nuts continued to back off, leading to the split pin being completely sheared off in some instances. Fatigue failures continued to be experienced. This failure illustrates two problems that inadequate preload can manifest itself as. Fatigue failure is a common by product of inadequate preload; joint movement, because the friction grip was inadequate, results in stresses being induced into the bolt that it was never designed to sustain. This same movement, when the fatigue strength of the bolt is able to sustain the induced stresses, will result in the tendency for the fastener to self loosen.

The importance of achieving and maintaining an adequate preload is often the crucial factor in ensuring that the structural integrity of the joint is assured.

Fatigue Failure of Bolts

Published in the Fastener and Fixing magazine in July 2004, this article provides some background to the fatigue failure of bolts. it used to be thought that metal fatigue was due, in some way to the metallurgical nature of the material changing. Somehow the metal got weaker, became tired (i.e. fatigued) and failed prematurely when subjected to alternating loading. For the last two hundred years it has been a known problem and has been the cause of numerous product failures including some involving the loss of life. Nowadays, following a great deal of research, it is known that fatigue is due to one or more cracks progressing through a material.

The crack can start at some existing defect, such as an inclusion in the metal, or at point of high stress, such as a notch, and slowly grow in length at each loading. It may take millions of loadings and unloadings (known as load or stress cycles) before the crack is actually detectable. As the length of the crack increases, the material remaining is placed under increasing stress because there is less area to sustain the loading. When the crack actually reaches a critical length it progresses all the way through the material resulting in complete failure.In the vast majority of applications, the most effective way to ensure that the bolt is fatigue resistant is to ensure that it is tightened sufficiently.

The Loosening of Prevailing Torque Nuts

Published in the November 2009 issue of Fastener and Fixing magazine, this article looks into the causes of why prevailing torque type nuts can sometimes experience complete self-loosening. The work is based upon some original work on the subject. Prevailing torque nuts, often referred to as stiff nuts, are one of the commonest methods of providing resistance to self-loosening. Patents for this type of nut began to appear in the 1860's and many of the principal types can be traced back over 100 years. One advantage of this type of nut is that the locking feature can be checked at the time of assembly by measuring the prevailing torque. The present standard test code (ISO 2230) specifies performance requirements to ensure that nuts meet a certain minimum standard. Key requirements are that the first prevailing torque in the tightening direction must not exceed a maximum value and that the first and fifth prevailing torque in the untightening direction must achieve certain minimum values.

There have been a number of cases of prevailing torque nuts becoming detached from bolts leading to a catastrophic failure of the joint. The cause of such detachments has not been understood and one reason for this is that such complete loosening has not been able to be reproduced in a standard Junker test. The article reports on how a Junker vibration machine was adapeted so that axial loading as well as transverse joint movement could be induced into a joint. The experiments conducted using the modified Junkers machine have demonstrated that the combination of axial and transverse load have a profound effect on the loosening of prevailing torque nuts.

The re-use of threaded fasteners

Published in the Fastener Fair magazine in March 2005, this article provides some background as to the re-use of threaded fasteners. Many OEM’s specify that if a part is removed, the fasteners should be replaced. On the face of it, this seems like a policy designed to increase spare part sales. However just what are the factors influencing whether or not a fastener should be re-used? In the majority of applications, there are three major considerations:

1. Has the strength of the fastener been impaired by the first tightening in some way such that it is weaker if it is re-tightened again?

2. Has the fastener been affected by corrosion to a degree that its performance will be impaired or from an aesthetic standpoint requires replacement?

3. Has the torque-tension characteristics of the fastener changed so that its performance would be impaired if it was to be re-tightened?

This article reports upon a study investigating the third point – is the torque-tension relationship affected by repeated tightening. This, in many applications will be the determining factor on whether a fastener should be re-used or not.

The stronger the better is not necessarily the case for fasteners

Published in the October-December 2013 issue of Fastener and Fixing magazine, this article looks at why having a fastener that is 'too strong' may actually increase overall risk. The maxim 'the stronger the better' is frequently used in Engineering since structural failure can often be catastrophic in terms of not only material/replacement costs but also the cost of the loss of reputation of a company. In regard to fasteners, when a fastener fails the cost of the actual fastener is usually insignificant relative to the overall cost of the failure. But in regard to fasteners, given the adverse effect that hydrogen can have on high strength fasteners, stronger is often certainly not better.

Hydrogen embrittlement, is to some degree, unpredictable and it is sensible to, whenever possible, specify fasteners that are, to a large degree, inherently less prone to this type of failure. Experience indicates that if property class 12.9 fasteners are electroplated, measures can be taken to reduce, but not fully eliminate, the risk from hydrogen embrittlement. In some applications, the consequences of joint failure as a result of brittle fracture are such that a campaign change to replace all potentially defective fasteners from a product is deemed essential even when the failure rate is very low. Usually the extent of the problem is unknown when a decision needs to be taken whether to replace the fasteners. It may be possible to identify a batch of fasteners affected and reduce the extent of the campaign but this can often be problematic. Typically, the cost of the fasteners is usually minimal compared to the other costs involved in such a campaign. Considering that there are other valid options available, high strength fasteners (10.9 and 12.9) should not be electroplated.

The use of two nuts to prevent self-loosening

Published in the November 2008 issue of Fastener and Fixing magazine, this article looks at the use of two nuts to prevent self-loosening. Many types of old machinery have two nuts on the bolts. A thin nut is frequently used in these applications. Sometimes the thin nut can be observed on top of a standard thickness nut and on other installations, it’s next to the joint, under the thick nut. Although it may seem counter-intuitive, the thin nut should go next to the joint and not be put on last. In other applications, for example on column attachments, two standard thickness nuts are frequently used.

A series of tests were conducted to investigate the effectiveness of the two-nut method in terms of resistance to self-loosening. A Junker transverse vibration test machine was used with M10 nuts and bolts. The results are illustrated in figure 6. With the small nut on top, both nuts can be observed to rotate together and can subsequently come completely loose. The results are slightly better than is normally observed with a single plain nut. With the small nut next to the joint, some relaxation occurs but not a significant amount of self-loosening . The performance of the two-nut method, when properly applied, provides a superior locking capability when compared to many so-called lock nuts. The proper application of the two-nut method is time intensive and requires a degree of skill and is hence unlikely to make a major comeback on new machinery any time soon.

Torque control and torque-angle control tightening methods

Published in 2004, this article looks at the torque control and torque-angle tightening methods. Using torque to indirectly control a bolt's preload is by far the most popular controlled tightening method. This is due to its simplicity and its widespread understanding by all engineers. One of the central problems of using threaded fasteners for structural purposes is that it is difficult to maintain a consistent clamp force or preload. It is widely known that torque tightening can result in signifcant variability, however the structural integrity of many designs is based upon ensuring that a certain level of minimum preload is achieved and maintained.

This article discusses the advantage that a torque-angle tightening approach can bring. Specifically, the torque-angle method can allow a consistently high bolt preload to be acheived.

What should be tightened the nut or the bolt head?

Published in March 2006 issue of Fastener and Fixing Europe, this article discusses the question of whether the nut or bolt head should be tightened and whether it makes any difference? The answer depends upon which tightening process is being used. For torque controlled tightening whether the nut is tightened and the bolt head held, or the bolt head tightened and the nut held, can be of importance.

The general objective from a tightening process is to achieve a consistent bolt preload. Controlling the torque during tightening and completing subsequent inspection checks to ensure that the specified torque is being achieved, are common ways that this objective is implemented.

This article shows that, in general, when using torque control, tightening the bolt by rotating the bolt head or the nut can matter. It is good practice to specify which part should be tightened so that the bolt preload variation is minimised. The issues involved are discussed.

Why use washers?

Published in the July 2007 issue of Fastener and Fixing Europe, this article discusses the use of washers in bolted assemblies. Washers have traditionally been used to protect the joint surface from damage during the tightening process and to distribute the load under the bolt head and nut. The stress under the face of a standard hexagon headed nut can result in indentation into standard strength steel surfaces. In recent years washers have started to be replaced by flanged headed fasteners for several reasons that will be discussed in this article.

The effect of using washers instead of flanged nuts and bolts is to increase the number of interfaces within the joint. Since each face will sustain an embedding loss, the net effect of using washers is to increase the loss of bolt tension due to this effect. On joints that use bolts that have a relatively short grip length, typical of chassis joints, the additional embedding effect can be particularly pronounced. It is not unusual for short grip length joints, with several interfaces, to experience a bolt tension reduction of up to 50%. A joint that may perform satisfactorily when flanged nuts and bolts are used may suffer problems when washers are inserted into the joint. Such a change can invalidate all the calculations and testing pertaining to the joint completed by the manufacturer.

Using a plain hexagon headed nut and bolt without washers will increase the surface pressure under the bolt head and nut face well beyond that which a standard structural steel surface can adequately support. The consequence of this is that the amount of embedding loss is significantly increased resulting in a higher than anticipated bolt tension loss. Essentially the risk is that for short grip length joints, the bolts can come loose without nut rotation occurring. The adverse effect that this can have on the joint’s structural integrity is obvious.

 

 

 

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