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UNDER CONSTRUCTION

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Threaded fasteners

A bolt is a headed and externally threaded mechanical device designed to be used with a nut or inserted in a threaded hole to generate a clamp load. According to the definition of ISO (International Standards Organisation), a bolt has a plain (unthreaded) portion of its body, while a screw is threaded all the way to its head, see below.

Alternative definitions do exist: in the USA and elsewhere, a bolt is said to be a headed fastener designed for use with a nut, while a screw is designed for use in a threaded hole. For this manual, the ISO ‘thread-to head’ definition will be used.

A bolt usually has a wrenching form or a means of preventing rotation while a nut is assembled. This is most frequently (but not always) on the head of the bolt.

Some common types of bolt are detailed in the later section entitled ‘Fastener products – Bolts & studs’.

There is a different range of screws, which usually have a pointed or tapered end. The familiar wood screw is one of these, and they mostly have a different type of wrenching form from bolts. The wrenching form is always in the head so that the screw can be driven into place.

Screws of this type are reviewed in the later section entitled ‘Fastener products – Screws and set screws’.

Two important types of externally threaded fastener that are not headed are firstly the stud which is threaded at both ends and is usually used with at least one nut.

Secondly, there is the set screw, sometimes known as a grub screw, shown below. One end of this type of screw engages with one part of an assembly, and it puts a compressive load onto the part to restrain its movement.

A nut is provided with a threaded hole, and usually has an external wrenching form, as shown above. There is also a range of internally threaded fasteners with no wrenching form called thread inserts, and these are designed to provide a threaded hole in situations where tapping a thread would be difficult, or where the existing thread is damaged. These are reviewed in more detail in the later section ‘Fastener products – Nuts & thread inserts’.

Non-threaded fasteners

There is a wide range of fasteners and fastener components that do not have threads. Many of these are described in the section ‘Fastener products – Non-threaded fasteners’. Below are some of the terms that apply in this group.

A washer is intended to be placed between two surfaces in a bolted joint. There are several ways that a washer can be used. It can prevent indentation by reducing surface pressure, or it can provide a locking feature to prevent relative movement, or it can have an attachment feature that protrudes from the washer, or it may be made of insulating or a lubricating material.

A plain rivet, shown above, has a head at one end with a plain body, and it becomes a fastener by forging a head at the other end after it has been put in place in the joint. There are also numerous varieties of multi-component rivets, many of them proprietary.

These have special functions, often in holes where access is only available from one side.

A lockbolt, above, is a headed fastener that has grooves instead of threads. It uses a locking collar swaged into the grooves to prevent movement.

More detail for all of the above fastener types and many other non-threaded types is given in the later section.

4. Special Market Conditions

Nuts and bolts are universal. They are indispensable as joining elements in virtually all aspects of life. But it would be a mistake to believe that fasteners of identical geometry and strength are interchangeable between applications. There exist rules and conditions in several markets that mean that the sourcing of fasteners has to fulfil specific conditions.

The most obvious of these is the aerospace industry. Safety concerns in commercial aerospace have led to the concept of airworthiness, and in each major aero manufacturing nation, there is an authority that regulates the production, repair and maintenance of aircraft and its component parts. In the UK this is the Civil Aviation Authority (CAA). This means that there are special rules for the production of aircraft and aero engine structural parts including fasteners. Typically, manufacturers must have a quality management system approved by the aerospace industry as a minimum. This is to the same standard across the world. For parts that require special processes (heat treatment, coating, non-destructive testing etc), there must also be a separate special process approval. And finally, for critical parts, a separate product qualification (approval) is necessary.

This may seem like a lot of approvals to produce fasteners, but a similar situation exists in automotive. There is a special quality management system qualification that must be held by the manufacturer, ISO/TS 16949. Increasingly there is a requirement for special process approval for heat treatment, CQI-9, and then there are PPAPs (production part approval process) for specific fastener product approval. So there are entry requirements for manufacturers of automotive parts that need to be satisfied.

The construction industry is now regulated across Europe, with the requirement for certain parts, including some fasteners, to be qualified against a set of standards (either harmonised Euro Norms or European Technical Assessments). This means that certain types of fasteners can only be obtained from approved sources, so once again there is a restriction on where parts may be sourced.

The Construction Products Regulation came into force in July 2013. A brief outline of the regulation as it affects fasteners is given in the construction products section later in the manual.

There is also a Pressure Equipment Directive, which defines several classifications for pressure vessels based on the level of hazard presented, and then identifies the requirements for approval, which will be accepted in all European states.

These special conditions need to be recognised by the fastener supplier because they are barriers to supply that must be satisfied.

Standard parts

Fastener standardisation is the process of developing and specifying the dimensions, tolerances, properties and manufacturing requirements of families of fastener parts. Its aim is to ensure that all manufacturers of standardised parts produce components that are interchangeable so that users who order parts to a standard can be confident that they will be functional no matter from where they are ordered. According to the British Standards Institution (BSI), the purposes of British Standards include the facilitation of trade, particularly in reducing technical barriers and artificial obstacles to trade and providing a framework for achieving economies and efficiencies. There are numerous standardisation bodies, but for the great majority of UK based purchasers of fasteners, the most important bodies are ISO (the International Standards Organisation) and BSI. ISO has standardised several families of metric fasteners, and where requirements exist outside the range covered by ISO, there are BS standards to provide for some specific UK applications. Standard parts are known by the number of the standard or drawing, and the abbreviation of the standards body, e.g. ISO 4014 for hexagon head bolts. The German standards number, DIN ***, is also commonly referred to in the UK market, in some cases for products not covered by either ISO or BS standards, but also by custom and practice even though the DIN standard has been superseded by an ISO standard.

When designers specify a fastener, they will usually select a standard part if one is available that will do the job they require, i.e. a part for which international or national standards exist. This is because it will be considerably lower cost than designing and producing something from scratch. ISO standards provide designs for hexagon head bolts and nuts, socket head products and fasteners with other drives, self-tapping and drilling screws, locknuts, washers, pins, rivets and others. They provide drawings for the specific products, and in addition, there are reference standards that give details of threads and other features, tolerances, materials allowed and properties required. BS standards have a similar approach and cover the design of inch-size products and metric products where there are UK applications outside the ranges covered by ISO.

5.2 Proprietary parts

Proprietary fasteners are made to a patented or licensed design that is owned by a company. They are non-standard parts, and they usually have a specific function that cannot be achieved by standard parts. Proprietary parts include various types of rivets, special multi-component fasteners, load indicating bolts and washers, tension control bolts, parts with special wrenching forms, self-locking fasteners, and tamper-proof systems. There are probably more proprietary designs of fasteners than there are of standard parts, but a greater volume of standard parts is produced and sold. Proprietary parts are usually known by their registered trade name.

5.3 Specials

When a designer cannot find a standard part to do the job required, and when a suitable proprietary part is not available, then a special part will have to be made. Sometimes this occurs because the international or national standards do not cover the exact range of properties or materials required, or the specific dimensions are not suitable for the application. So a special part will have its own unique drawing. This does not always mean that it is expensive to create the part or drawing. Many specials are only slight modifications of a standard part, or they may use many of the existing reference standards, such as materials, properties, thread dimensions, tolerances, etc.

However, there are industries that require special control over the parts they design. Examples are the automotive and aerospace industries, where OEMs often control almost all aspects of their drawings. In one sense these are proprietary parts because the OEM owns the drawings and specifications, but they are more usually known as special parts.

Questions

1. Name a disadvantage of welding that does not affect mechanical fasteners.

2. What is the difference between a bolt and a screw? Thread length

3. Name two types of externally threaded fastener that do not have a head.

4. Describe a lockbolt.

5. Name two industries where there is a restriction on where some fastener products may be sourced.

6. Name two standards organisations.

7. How are standard parts identified?

8. What characterises a special fastener?

There are features of a headed fastener that are generally referred to in the industry and are worth remembering. Referring to the ‘Bolt terminology’ figure below:

The ‘head’ of a bolt, screw or rivet is the increased diameter section that will be seated on the joint and apply pressure to it.

The ‘face’ is the flat part of the head pointing away from the body of the bolt.

The opposite side of the head, which will engage with the joint and apply pressure to it, is known as the ‘bearing face’.

The ‘body’ or ‘shank’ is the unthreaded cylindrical portion of the bolt.

The transition between the head and the body is called the ‘fillet’, and the ‘fillet radius’ is an

important parameter because it determines the stress concentration in this transition area.

The transition between the thread and the body is the ‘thread run-out’.

The end of the thread is known as the ‘point’ because it is usually angled during manufacture

before the thread is formed.

The thread shown here is an ‘external thread’. Nuts have ‘internal threads’.

‘Flange bolts’ have a skirt or flange at the base of the wrenching form. (The diagram below is

taken from ISO 4162 for hexagon bolts with a flange.)

A bolt, or other externally threaded fasteners, is usually referenced by its ‘nominal diameter’, for

example, a 6mm bolt. There may not be any dimension on the bolt that actually has this nominal

diameter, because it refers to the thread designation, and as you will see below, none of the

actual thread dimensions may comply with this diameter.

Nomenclature

The ‘nominal diameter’ is the way a thread is designated, and for external thread refers to the

diameter of the plain bar from which threads would be machined. At one time all external

threads were machined so much of the terminology comes from that. Most threads are now

rolled or formed so the definition no longer applies. So a 6mm thread would have been

machined from a 6mm diameter bar.

The maximum external diameter or thread crest diameter of an external thread is the ‘major

diameter’. Because of tolerancing, the major diameter of any thread is slightly less than the

nominal diameter.

The thread root diameter is called the ‘minor diameter’. There is also a ‘pitch diameter’ or

‘effective diameter’, which is that diameter at which the width of the thread is equal to the gap

between the threads. This is the diameter at which the external thread should contact with its

equivalent internal thread, so this is an important dimension. Major diameter, minor diameter

and pitch diameter, together with thread pitch (see below) are the most important dimensional

parameters of a thread form.

There are two important angle measurements of threads. The ‘thread angle’ is the angle

between the ‘thread flanks’, the two straight surfaces from root to crest. In other words it is the

angle of the ‘V’ of the thread form. The ‘thread helix angle’ is the angle of the thread viewed side on, relative to its axis – see the drawing for nominal diameter above.

For internal threads, the thread crest is the minor diameter because this is the smallest dimension. The thread root diameter is the major diameter. Thread pitch diameter is the same definition as for external threads.

Thread pitch’ is a measure of the coarseness or fineness of threads. There are two systems of measurement. The pitch of metric threads is determined by the distance between thread crests, so it is typically measured in mm (millimetres).

Inch threads are measured as threads per inch, so it is a ratio, usually kept as a whole number.

Standardisation has had major benefits in controlling the types and tolerances of thread forms around the world. The ISO metric thread series is the now the most widely used system and manufacturers in all parts of the world make fasteners with metric threads complying with the ISO standards. As a result, most designers specify ISO metric threads, and fasteners may be procured from any country manufacturing such parts, provided the buyer has confidence in the quality of the source.

There are, of course, exceptions. The aerospace industry worldwide has only partially converted to a metric system, and inch sizes are still specified in fasteners for commercial aircraft and engines. Some metric fasteners are used in military aircraft but it is by no means universal. The most commonly used inch thread for aircraft applications is the American Unified fine pitch (UNF) system with a large root radius for increased fatigue properties, designated ‘J’, hence UNJF threads. Where metric threads are specified, they are usually ‘MJ’, again with large root radius.

In the USA and Canada, most fasteners are still specified in inches, to the unified system, either UNC or UNF, for coarse or fine threads. These two systems of specification, ISO metric and Unified inch, are the most widely used, and each utilises a thread angle of 60˚, though this does not make them interchangeable because all other dimensions are different.

The UK has separate historic thread standards, which although they are rarely specified in original designs, still have some residual applications so there are still requirements for such fasteners mostly for service and repair. These are the BSF and BSW inch systems (British Standard Fine and British Standard Whitworth) that use a 55˚ thread angle. There is also the rarely used BA (British Association) system for small metric screw sizes, using a 47.5˚ angle.

The table below summarises the differences in thread systems. Also included are pipe thread

systems showing differences between the US and British designs. Although the old British

threads are still made and the standards maintained, BSF, BSW and BA parts are likely to be

more expensive than the more common systems because they are made in smaller volumes.

The normal choice of ISO metric thread is usually coarse, and unless a fine pitch is shown in the

thread designation, a coarse pitch can usually be assumed. This is also true of inch thread systems, for the reasons given below.

Fine pitch threads are sometimes more difficult to assemble because they have tighter thread fit, so small amounts of raised metal on the thread crest (‘dinks’) will interfere with assembly into an internal thread.

On the other hand, fine threads will allow higher clamp loads than the equivalent coarse threads, so they are often used in situations where fatigue life is important, such as connecting rod bolts and aerospace applications. Fine threads also have better resistance to vibration loosening, because of the smaller thread helix angle.

Coarse threads have more open thread fits, so faster rundown speeds can be used in assembly. They also have greater resistance to thread stripping, so they are the general purpose choice. Thread form System Thread Angle Designation Example

ISO Metric Course

ISO Metric Fine 60o M* x pitch M10 x 1

Unified National Coarse

Unified National Fine

National Pipe Taper Fine

60o

UNC

UNF

NPTF

½” UNC

¼” UNF

¾” NPTF

British Standard Whitworth

British Standard Fine

British Standard Pipe - Taper

Thread

55o

BSW

BSF

BSPT

¼” BSW

½” BSF

¾” BSPT

British Association 47.5o BA 0BA

(=6mm)

By far the most common type of drive or wrenching form on a bolt or screw is the hexagon. Shown above are the hexagon bolt, hexagon flange bolt and hexagon socket screw. These are widely available along with the wrenches, spanners, driving bits and hexagon keys used with them. There can be problems in the use of such drives, because they are not very easy to engage unless the fit between the hexagon and the driver is relatively open, and in that situation the hexagon is susceptible to ‘cam-out’, where the corners of the hexagon (or the flats with a socket) are formed over so the drive can no longer apply a torque. For this reason other types of drive are used industrially to facilitate engagement.

The ’12-point’ or ‘double hexagon’ drive is used for easier wrench engagement. It is widely used in the aerospace and power generation industries, most frequently in a flange head.

The ‘hexalobular’ drive is a radius spline type of drive that has good engagement properties while at the same time giving high wrenching torques. It is used as an internal drive in sockets, and also as an external drive on flange bolts. It is widely used for automatic assembly in the automotive industry but can increasingly be found in fasteners for many other applications. ISO 10664:2005 is the international standard specifying the dimension of the hexalobular internal drive.

Probably the best know proprietary version is TORX®,  Other types of drive are used in special situations, such as the square drive, the spline drive, the Pentagon drive, and others (see also section 19 ‘Screws & Set Screws).

ISO 1891 ‘Fasteners – Terminology’ lists 24 types of drive, 13 of which are external and 11

internal. The most common drives, however, are those listed above.

Questions

1. Where is the fillet radius on a bolt and why is it important?

2. What is the nominal diameter of a bolt?

3. What are the three important diameters of the thread form?

4. What is the thread angle on ISO metric and Unified inch

fasteners?

5. What is thread pitch and how is it measured?

6. What are the main attributes of coarse threads that make them the

most common general purpose choice?

7. Under what circumstances might a fine thread be chosen?

8. What is the most commonly used thread system?

9. What is a flange head bolt?

10. What is the Torx® drive and where is it widely used?