The Basics of Bolted Joints

The Basics of Bolted Joints

This page presents the basics of bolted joints, including a description of bolted joints and why we use them and a detailed discussion of a typical bolt and the types of bolt used in bolted joints. The standardisation of fasteners is explained, with a clear definition of the key dimensions of an ISO metric thread profile. This page is divided into the following sections:

  1. Introduction
  2. Why Use Bolted Joints?
  3. Glossary of Terms Relating to Fasteners
  4. Standardisation of Fasteners
  5. The ISO Metric Thread Profile

References

  1. FEDS – Article – Screw Thread Design
  2. R.S Khurmi, J. K. Gupta – A Textbook of Machine Design
  3. ISO 724:1993 – ISO general-purpose metric screw threads – Basic dimensions
  4. BS 3643 Part 1: 2007 – ISO metric screw threads. Principles and basic data
  5. ISO 68-1:1998 – ISO general purpose screw threads — Basic profile — Part 1: Metric screw threads
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Bolted Joints / Threaded Fasteners Bolt Strength / Strength of Fasteners

 

1. Introduction

One could argue that Archimedes’ invention of the screw thread in circa 250 BC sits alongside the wheel as one of the most important human discoveries in terms of general utility. A seemingly simple concept, the screw thread has become ubiquitous in daily life and finds it use in many places; from the casing of a simple wall socket to the pressure vessel of a nuclear reactor. The bolted joint is one of the most useful, versatile and commonly used methods of connecting together parts of a structure or machine. It is therefore essential for any mechanical engineer to have a solid understanding of the strengths, limitations and applications of bolted joints, in order to be able to safely and efficiently design such connections.

1.1 Bolts

In general terms, bolts are metal cylinders with a screw thread rolled or cut into the outside surface. The bolt thread is referred to as the external thread. Bolts always engage with a reciprocal internal thread, cut into the inside surface of a bore. The external and internal threads are also referred to as the male and female threads, respectively. The matching of an external thread to a pre-formed internal thread is what distinguishes bolts from certain types of screws, which are self-tapping. Self-tapping refers to a screw’s ability to cut its own thread into another part without needing to engage with a pre-formed reciprocal internal thread. To visualise self-tapping, think of a typical wood screw that can drive itself into a piece of wood just by application of torque and axial load along the screw with a screwdriver. Finally, in a confusing twist, (pun possibly intended) there are number of bolts called “screws”, such as grub screws, which are basically mini-bolts and need to engage with a pre-formed female thread in order to work; however when you get to this point it’s probably best not to worry too much about naming conventions.

1.2 The Basic Concept

The screw thread of a bolt allows a conversion of rotational motion (or torque) to linear motion (or force). In order to apply a torque to a bolt there is generally some form of head, which is designed to allow the use of a tool to apply the torque. The application of torque causes the bolt to engage axially with the female thread until the part(s) being connected become pinched between the female threaded part (e.g. nut) and the underside of the bolt head. This applies a compressive force to the parts being connected and induces a stretch in the bolt. This stretching results in bolt tension or preload, which is reacted by the reciprocal compression force between the fastened parts. These are the forces that hold the parts of a joint together.

1.3 Types of Bolts

Although the general concept of an externally threaded cylinder remains constant, there are a variety of different types of bolt used in bolted joints, distinguished by the features of the internally threaded mating part.

1.3.1 Through-Bolt

The through-bolt is what most people would consider to be the “classic” bolt. It consists of a cylindrical bar with an external thread and (normally) an oversized head at one end. The bolt is passed through holes in the parts to be fastened together. A nut is threaded onto the bolt on the opposite side of the parts to be fastened. A through-bolt is illustrated in Figure 1.

Bolted Joints - Normal through-bolt with nut Figure 1 – Through-bolt

1.3.2 Tap Bolt

A tap bolt differs from a through bolt in that it engages directly with a threaded hole in one of the parts to be fastened. There is no nut. A tap bolt is illustrated in Figure 2.

Bolted Joints - Tap bolt

Figure 2 – Tap bolt

1.3.3 Stud

A stud is a simple threaded bar with no oversized head. It engages directly with a threaded hole in one of the parts to be fastened. A nut is then threaded over the top to secure the parts together. A stud arrangement is illustrated in Figure 3.

Bolted Joints - Stud bolt with nut

Figure 3 – Stud bolt

 

2. Why Use Bolted Joints?

So, why would one use a threaded fastener, such as a bolt, over a welded joint or other form of more permanent joint? In answer to this very good and pertinent question, they are two key advantages to bolted joints: convenience and reliability.

2.1 Convenience

The assembly of bolted joints can often be achieved without damage to the parts which are joined together and without damage to the fasteners. This allows the joint to be conveniently assembled and disassembled with standardised tools and relatively limited skill. Therefore, once assembled, bolted joints can often be dismantled for inspection and subsequent reassembly. In the case of welding or other types of adhesive joint, the fixture is permanent and cannot be disassembled without some destructive intervention.

2.1.1 A Warning Note

It is not always possible to safely reassemble bolted joints for re-use and there are many conflicting opinions on how and when it is safe to reassemble a joint using the same fasteners, for example in the case of pre-loaded / pre-tensioned joints. Please see the page discussing the re-use of bolts / fasteners for further information.

2.2 Reliability

Properly designed and executed bolted joints are highly reliable. Fasteners are cheap to produce and widely available in a range of types due to standardisation and efficient manufacturing processes.

2.3 Summary

In summary, these advantages make bolted joints an obvious choice for many mechanical and structural assemblies, especially those that require parts to be inspected, maintained, replaced or cleaned. However, it must be noted that a key disadvantage of using threaded fasteners is their vulnerability to fatigue failure when used inappropriately due to inherent stress concentrations in the thread profile.

Whilst at first glance the ubiquitous bolted joint may seem to be a straightforward concept, the developing engineer may quickly realise that the analysis of a particular joint is not quite as simple as it first seemed. Personally, I have found myself scratching my head when trying to design or analyse a particular set of bolted joints, and sometimes still do. When trying to find a solution to a problem, the sheer volume of information available on the topic can be quite overwhelming.

3. Glossary of Terms Relating to Fasteners

Before attempting to design new bolted joints or analyse an existing joints it is worth giving ourselves a more detailed reminder of the key features of a threaded fastener. Please note that bolts, nuts or screws (or any combination thereof) are often also referred to as fasteners. You’ll find that this website use the terms interchangeably.

So what are we talking about when we look at a threaded fastener? Refer to Figure 4 and the list below (in alphabetical order) for a description of the key features.

Bolted Joints - fastener features

Figure 4 – Illustration of fastener features

3.1 Bolt

A cylindrical piece consisting of an external threaded section and a hexagonal head. The screw thread is formed by cutting or rolling a helical groove into the surface of the cylinder. The Discovery Channel video here provides a good overview of how typical fasteners are manufactured. The external thread of standardised bolts are generally rolled. As described in reference [1], rolling has several advantages, including, but not limited to the following:

  • More accurate and uniform thread dimension.
  • Smoother thread surface.
  • Generally greater thread strength, particularly in fatigue and shear strength.

Thread cutting is generally only used for large diameter or non-standard externally threaded fasteners; however it is still the most commonly used method for internal threads.

3.2 Crest

The crest is the “top of the “hill” of the thread.

3.3 Fundamental Triangle

This is the equilateral triangle on which the thread profile is based.

3.4 Handedness

The helix of the thread on all standardised fasteners is right-handed, meaning that rotating the bolt (or nut) clockwise will result in tightening of the fastener and vice-versa for a left-handed thread. This gives rise the common mnemonic “righty-tighty, lefty-loosey“.

3.5 Head

The head is (usually) hexagonal and allows the use of a spanner / torque wrench / fingers / teeth to tighten or release the bolt. The head of standardised fasteners will generally show two pieces of information: the manufacturer’s mark, and the grade of the bolt. The grade signifies the material properties of the fastener in accordance with a particular standard.

3.6 Lead

The lead of a screwed thread is the distance between two corresponding points on the same helix. It is the distance along the screw’s axis that is covered by one complete rotation of the screw. The vast majority of screwed thread forms are single start threads and in this case the lead of the thread is exactly the same as the pitch.

3.7 Pitch

The pitch of a screwed thread is the distance from the crest of one thread to the adjacent crest.

3.7.1 Coarse and Fine Pitch

It is worth noting that ISO metric standard threads are manufactured in either “coarse” or “fine” pitch threads, and even “extra fine” pitch threads. The majority of threads are coarse; in fact the default thread specification is for a coarse pitch. Again, I have only ever had reason to use coarse pitch threaded fasteners in my work to-date. There are however, some good reasons you might want to use a finer thread pitch. For example:

  • The smaller pitch of a fine or extra fine pitch thread allows finer adjustment in the axial direction of thread travel per turn of the nut.
  • Finer threads are easier to tap into harder materials and thin plate or tubes.
  • Fine threaded bolts are stronger across their cross-section than the corresponding coarse threaded bolts made from the same strength material. This additional strength is found in both tension and shear due to fine threaded bolts having a slightly larger minor diameter and therefore a larger tensile stress area.
  • Fine threads have less tendency to loosen under vibration due to the smaller helix angle when compared with coarse threads, giving rise to a smaller off-torque.

However, it should be noted that in general, coarse threads are much more common and are specified for most industrial applications due to the following advantages of coarse threads:

  • They are less likely to jam during installation and faster to install than finer pitch threads.
  • The coarser thread is less susceptible to damage during handling and installation, due to it being a bigger and “more chunky” thread.
  • They are more commonly available, which results in a self-fulfilling feedback loop of increasing availability.

On metric fasteners, generally the coarse sizes are the most commonly used with the finer pitches being less readily available. KATO, a company specialising in helical screwed thread inserts recommends that coarse threads be used over and above fine threads where possible and I have no reason to disagree with them. In any case, most bolts are provided in coarse thread form and most design standards will expect the engineer to be using coarse thread form.

3.8 Root

This is the bottom surface of the thread i.e. the lowest point in the “valley” of the thread.

3.9 Run-Out

The thread at the end of the shank beyond which a nut cannot travel. Underneath the head, at the top of the shank, the corner of the bolt is formed into a radius. This radius is large enough to reduces the stress concentration under the head, improving the fatigue resistance of the bolt.

3.10 Shank

The non-threaded portion of the bolt between the head and the threaded is called the shank.

3.11 Thread

The helical, triangular-shaped ridge wrapped around the outer surface of a bolt, or the inner surface of a threaded hole. All standardised, readily available fasteners are single threaded, meaning that there is only one “ridge” wrapped around the surface of the screw. This is also known as a single-start thread. It is possible to have multiple start threads although these are much less common and will not be dealt with here. I certainly haven’t come across a multi-start thread (yet) other than on a machine (jacking) screw.  For more information on multiple start threads, see the very useful Textbook of Machine Design, by R.S Khurmi and J. K. Gupta [2].

The helical property of the thread allows a conversion of rotational motion (or force) to linear motion (or force). The mechanical advantage of a screw thread depends on its lead, which is the linear distance the screw travels in one revolution. In most applications, the lead of a screw thread is chosen so that friction is sufficient to prevent linear motion being converted to rotary motion. I.e. no matter how much axial force you apply to the screw it will not slip and rotate, so long as no external rotational force is present. This characteristic is essential to the vast majority of its uses. The tightening of a fastener’s screw thread is comparable to driving a wedge into a gap until it sticks fast through friction and some slight local plastic deformation.

4. Standardisation of Fasteners

Standardised fasteners available in Europe and many other parts of the world are produced in accordance with the International Standards Organisation (ISO) metric system. The sizing of a metric fastener is characterised by the Major Diameter and the Pitch of the thread. For example a bolt with a designation “M20” has a Major Diameter of 20mm. A particular bolt size can be provided in a range of thread pitches. For example, in accordance with BS 3643 Part 1: 2007, a standardised M20 fastener can be provided with thread pitch of 2.5mm (coarse) and 2.0mm, 1.5mm or 1.0mm (all referred to as fine). Fasteners, i.e. bolts are produced in a range of materials called the fastener classification. Further information on fastener classification can be found here. The nominal axial load capacity of a bolt is a function of its diameter and the fastener material strength.

To aid standardisation, a preferred size range has been established. Preferred fastener sizes are approximately based on the R10 series of preferred numbers, as defined in ISO 3, while the second choice sizes are based on rounded off values from the ISO 3 R20 series. The third choice sizes are based on ISO 3 R30 series and are probably best avoided due to risk of reduced availability. For design purposes, always try to stick to the first preferred fastener sizes, as the likelihood of these being in stock at suppliers is naturally much greater. Avoid special items wherever possible.

5. ISO Metric Thread Profile

There are a number of different thread types available, including British Standard Whitworth (BSW), British Association (BA), American National Standard Thread (US), Unified Thread Standard (UTS) and the ISO Metric Thread. They each have subtle differences in the thread form through variations in thread angle, crest radius and form of crest / root. These days however; most threads worldwide are of the ISO metric or UTS type, which share the same thread form. Certainly the only threaded fasteners I have personally specified for new designs in Europe are of the ISO metric / UTS type.

The basic profile of a single-start ISO metric general purpose screw threads is presented in Figure 5. The basic dimensions of the thread are specified in ISO 724:1993 [3], and are repeated in BS 3643 Part 1: 2007 [4].

Bolted Joints - ISO metric screw thread profile

Figure 5 – ISO metric screw thread dimensions in accordance with ISO 724:1993 [3] and BS 3643 Part 1: 2007 [4]

5.1 Definitions

The dimensions in Figure 5 are defined below.

d is the basic major diameter of external thread (also referred to as the outside or nominal diameter). This is the diameter commonly referenced when describing a bolt, i.e. it’s M-number (e.g. for an M20 bolt, d = 20mm).
d_1 is the basic minor diameter of the external thread (nominal diameter)
= d - \frac{5}{4}H
d_2 is the basic pitch diameter of the external thread. Sometimes it is referred to as the pitch circle diameter in which case it is often given the nomenclature d_p
= d - \frac{3}{4}H
 d_3 is the minor diameter of the external thread (also referred to as the core or root diameter). It can be defined as the diameter of a cylinder which passes through the thread profile at the diameter where the widths of the ridge and groove are equal on both sides of the thread, as presented in Figure 5. It is the diameter at which the ridges on the bolt are in complete contact with the ridges of the nut. It is also referred to as the effective diameter.
= d_1 - \frac{H}{6}
 H is the height of the fundamental triangle according to ISO 68-1 [5]. The fundamental triangle is the equilateral triangle on which the thread is based.
= \frac{\sqrt{3}}{2}P
= 0.86604P
 P is the thread pitch
= {(No.\;of\;threads\;per\;unit\;length\;of\;screw})^-1