An IntroductionTo Fasteners,Bolt Types ,Materials ,Threads And Selection

An IntroductionTo Fasteners,Bolt Types ,Materials ,Threads And Selection

Introduction

A mechanical fastener is a device that is used to mechanically join (or fasten) two or more objects together
Bolts, screws and studs are the most common types of threaded fasteners. They are used in both permanent or removable joints.

There are many different types of mechanical fasteners, but, in general, fasteners can be divided

into two main categories; non-permanent and permanent fasteners. Non-permanent fasteners (they enable parts to be assembled and disassembled repeatedly) can further be divided into two groups:

     1-  General fasteners such as keys, pins, retaining rings, etc. (they are often associated with                       shafts)
     2 - Threaded fasteners such as bolts, screws, studs, setscrews, etc. They are the most widely used             type  of non-permanent fasteners since they can easily be removed then reused.

Threaded Fasteners

Threaded fasteners are the principal devices used for assembling components and they are usually grouped into three main categories as shown above:

  1-  Bolts: A bolt has a head on one end and threads on the other end and it is paired with a nut.

  2-  Screws: Screws are used to join two mating parts together and similar to bolts, they have head            on one end and threads on the other end. However, screws usually have longer threads than                  bolts, also they can be made with slotted heads.

               Screws are sometimes divided into two sub-categories; Cap Screws and

               Machine Screws. Machine screws are generally smaller in size than cap

               screws and they are used for screwing into thin materials.

  3- Studs: A stud is a rod that is threaded on both ends and joins two mating parts. A nut may be                used on one end.

The terms bolt and screw are sometimes used interchangeably and they can refer to the same element. In practice, the basic difference between a bolt and a screw is that a bolt is usually intended to be used in conjunction with a nut where it will be tightened or loosened using the nut, while a screw is usually intended to be mated with an internally threaded hole and the screw head is used for tightening or loosening.

Definitions and standards
The terminology of screw threads is illustrated in the figure 1:

FIG 1

Major diameter (𝐷, 𝑑):     the largest diameter of the screw thread.
Minor diameter (𝐷1, 𝑑1): also called “root diameter”, is the smallest diameter of the screw thread.
Mean diameter (𝐷2, 𝑑2): also called “pitch diameter”, the average diameter of the screw thread                                                    (considering the theoretical full height of the threads).
Pitch (𝑝): the distance between adjacent threads measured parallel to thread axis.
Thread angle (2𝛼): the angle between the mating faces of two adjacent threads.
Bolt and screw threads are standardized and there are two major standards: (ISO) Metric and (ANSI) Unified. In both standards the thread angle is 60°.
Metric (ISO):
             There are two standard profiles M (figure 2) and MJ (figure 3) where both have a similar                     geometry but the MJ has a rounded fillet at the root and a larger minor diameter (it is
             better in resisting fatigue loading).
Unified (ANSI):
           There are two standard profiles UN and UNR where the UNR (figure 4) has a filleted root.

FIG 2

FIG 3

FIG 4

Metric bolts are specified by the major diameter and the pitch (both in mm).
Unified threads are specified by the major diameter (in inch) and the number of threads per inch (𝑁).

FIG 5

Thread forms
Basically when a helical groove is cut or generated over a cylindrical or conical section, threads are formed. When a point moves parallel to the axis of a rotating cylinder or cone held between centers, a helix is generated. Screw threads formed in this way have two functions to perform in general:
     (a) To transmit power - Square. ACME, Buttress, Knuckle types of thread forms are useful for                   this  purpose.
     (b) To secure one member to another- V-threads are most useful for this purpose.
Some standard forms are:
1- V-thread form

2- American national standard form

3- Basic Whitworth (55°) thread form

4- Acme thread form

5- Square thread form

6- Buttress thread form

7- Iternational standard metric form

FIG 6

V-threads are generally used for securing because they do not shake loose due to the wedging action provided by the thread. Square threads give higher efficiency due to a low friction. This is demonstrated in figure 7

FIG 7

Properties of a threaded fastener
The shank diameter of a 'waisted' bolt is less than the thread diameter; allows a thread run out which reduces stress concentration.
A Washer under the nut ensures uniformity of a contact.
A bolt's 'grip'is the combined thickness of the fastened parts

FIG 8

Bolt-has a nut which turns to tighten
Screw-turns itself in the threaded hole
Stud-has no head and is threaded on both sides
Clearance hole-        15-20% larger than a bolt/stud size
Taped hole-               drilled smaller than the minordia. extends deeper than the stud
Stud depth-               1.5 times the major diameter
Thread length-          only a couple of threads longer than a bolt (𝐿𝑇 ) is usually determined
according to the length of the bolt using the relation:

FIG 9

The length of a bolt (𝐿) is usually chosen from the preferred ISO sizes.

FIG 10

Note that there is Coarse-pitch and Fine-pitch (more threads) where the finepitch has better tensile strength.
The tables gives the preferred standard sizes

External and Internal Threads
Threads can be either external or internal:
1- External Thread: External threads are on the outside of a member (such as the treads of bolts and                                     screws). A chamfer on the end of the screw thread makes it easier to engage it                                       into a hole or a  nut.
 An external thread is usually cut using a die (such as seen in the figure 11) or a lathe.

FIG 11

2-  Internal Thread: Internal threads are on the inside of a member (such as the threads of nuts and                                       holes). Usually, threaded holes have a chamfer on the side from which the                                             screw will enter to  make its engagement easier. 
An internal thread is usually cut using a tap.

FIG 12

Types of fasteners

There are many different types of bolts and screws where each is suitable for different types of applications.

Types of bolt

There are many different varieties of bolt which can be selected based on the particular requirement or the materials involved. Some of the most common types include:

Anchor bolt

Usually embedded in concrete or masonry for structural applications.

FIG 13

Carriage bolt

Used to fasten metal to timber, with a squared undercut to the head which holds the bolt in

place once it has been tightened.

FIG 14

Elevator bolt

Commonly used in conveyor systems, an elevator bolt has a flat, plain or countersunk head

which holds the bolt in place when tightened.

FIG 15

Flange bolt

Also known as frame bolts, this type of bolt distributes the bearing load using a washer on the

undercut of the head.

FIG 16

Hanger bolt

This type of bolt comprises two threaded ends instead of having a head, one of which contains

a wood screw.

FIG 17

Hexagon bolt/Tap bolt

A hexagon bolt comprises a head that has six sides, with threading that begins part-way down

the shank, whereas a tap bolt’s shank is threaded the whole length

FIG 18

Lag bolt

Also known as lag screws, this is a heavy-duty fastener that creates its own mating thread in

timber and other soft materials when tightened.

FIG 19

Machine bolt

This type of bolt has a short shank and is intended for assembling metal components through

predrilled holes.

FIG 20

Plow bolt

This type of bolt is commonly used in construction tools and other devices due to its durability,

and is characterised by its flat countersunk head and square shank neck.

FIG 21

Sex bolt

Rather than requiring a nut, the shank of sex bolts are covered with a ‘mating’ female

component. These are useful for fastening components that cannot be exposed to abrasive

threads.

FIG 22

Square head bolt

This is similar to a machine bolt in that it has a short shank, in addition to a four-sided bolt

head.

FIG 23

Stud bolt

This type of bolt has hexagon nuts on both ends. Components are fastened between the two bolts

FIG 24

Timber bolt

Bolts that are meant for use with large timber components.

FIG 25

T-head bolt

Has a T-shaped head which can be gripped by a wench and can fit into a slot with ease.

FIG 26

Toggle bolt

This type of bolt has an expanding wing-like nut which helps it to mount objects to walls.

FIG 27

U-bolt

Similar to staples, U-bolts are bent in the shape of a ‘U’ and are partially threaded on both

ends.

FIG 28

eye bolt

eye bolt is a bolt with a loop at one end. They are used to firmly attach a securing eye to a structure, so that ropes or cables may then be tied to it.

FIG 29

J-Bolts 

are designed for use mounting instrumentation, equipment stands, and other objects to varying types of grating.

FIG 30

Shoulder Bolt

are machine screws with an integral shoulder or journal between the head and thread. shoulder is described by its diameter and length, and the thread has a major diameter slightly smaller than the shoulder diameter.

FIG 31

Bolt And Screw Head Shapes

Bolts as well as screws are available in a vast variety of head shapes. These heads are made in

order to grip the tools that are used to tighten them.

The most common type of bolt head types includes square, hex, slotted hex washer and

socket cap.

There are numerous other head shapes in use as well, namely figure 32 

FIG 32

Bolt Material

Selection of a suitable fastener material for a particular application is vital. This is because bolts are used for various heavy duty as well as light weight applications and therefore the materials of these bolts have to be chosen accordingly.

A bolt made of steel as opposed to one made of aluminium can hugely afect the quality and durability of the joint it forms. Other factors such as environmental situations, presence of corrosive components as well as structural stability can altera material’s efectiveness.

Steel: Steel is the most common fastener material and is available in plain as well as various surface finishes

When selecting steel fasteners it is important to consider the grade or class of material. In metric systems, material strength is organized into classes. 
With imperial systems, material strength is organized into various grades.

Stainless Steel: Stainless steel is a special steel alloy containing at least 18% chromium and 8% nickel which provide significant corrosion resistance. 

Other Alloys: When steel and stainless steel fasteners don’t meet design requirements other types of alloys may be used. Aluminum for instance, is a lightweight with an impressive strength to weight ratio. However, due to strength limitations and a susceptibility to fatigue use of aluminum in critical applications should be carefully considered. Other high nickel alloys are commonly used when high temperature resistance is a requirement. 

Bolt Grades

This is a technical term which determines the properties of a bolt. The grade of a bolt determines the maximum amount of stress that the bolt can handle (figure 33). It also helps determine which tools are suitable for tightening these bolts. Moreover, the type of method used for tightening the bolt is also dependent upon the grade of the bolt. Therefore, it is very important to understand the grade of a bolt in order to use it properly.

FIG 33

SAE American Grades
The SAE marking standard starts with grade 2, indicated by a bolt head with no markings whatsoever. A grade 2 bolt made of low-carbon steel has a tensile strength of 64,000 pounds per square inch or less. Tensile strength is the amount of pull the bolt can withstand before breaking. A bolt head with three raised dashes in a radial pattern marks an SAE grade 5 bolt of tempered medium carbon steel with a tensile strength of at least 105,000 pounds per square inch. The strongest commercial-quality bolt is grade 8, marked by six raised dashes; its medium-carbon alloy steel has been quenched and tempered to achieve a tensile strength of 150,000 psi.

Metric Grades
The strength grade of a metric bolt, known as its property class, consists of two numbers separated by a dot. The property class is expressed in raised or depressed numerals on top or on the side of the bolt head, according to rules set by the International Standards Organization. The first number represents the load in megapascals -- a pascal unit of measurement for internal stress -- required to break the bolt. The second number represents a ratio between breaking load and bending load.

ASTM Grades
Another widely-used system of bolt strength grades comes from ASTM International, formerly the American Society for Testing and Materials. Its strength grades are indicated by the letter A plus three numerals stamped on the bolt head. Common ASTM grades include A307, which roughly corresponds to SAE Grade 2. An ASTM A325 bolt is roughly equivalent in strength to SAE Grade 5 and an A490 bolt is about equivalent in strength to SAE Grade 8.

US and Metric Thread Sizes

Measuring Diameter

For most types of fasteners, the diameter is measured on the outside of the threads.

FIG 34

Measuring Length

FIG 35

Thread Count & Thread Pitch

Machine threaded fasteners specify a thread density in Threads Per Inch (US) or as a Thread Pitch in mm (Metric) figure 36. For a given diameter, a fastener may be available in coarse (standard), fine and sometimes super fine thread.

FIG 36

Related Iso Standards
ISO 68:1973 ISO      general purpose screw threads - Basic profile
ISO 261:1973 ISO    general purpose metric screw threads - General plan
ISO 262:1973 ISO    general purpose metric screw threads - Selected sizes for screws, bolts and nuts
ISO 724:1993 ISO    general-purpose metric screw threads - Basic dimensions
ISO 965-1:1980 ISO general purpose metric screw threads - Tolerances - Part 1: Principles and basic                          data
ISO 965-2:1980 ISO general purpose metric screw threads - Tolerances - Part 2: Limits of sizes for                                      general purpose bolt and nut threads - Medium quality
ISO 965-3:1980 ISO general purpose metric screw threads - Tolerances - Part 3: Deviations for                                            constructional threads
ISO 1502:1996 ISO  general-purpose metric screw threads - Gauges and gauging

Bolt Design Recommendation

Bolt, one of the most widely used fasteners in the industry, is usually tightened by applying torque to the head and/or nut. As the bolt is tightened, it is stretched (preloaded).

FIG 37

Preload tension is necessary to keep the bolt tight, increase join strength, create friction between parts, and improve fatigue resistance. The recommended preload force Fi (N.lb) is
For reusable connections:                           Fi = 0.75 × At ×Sp
For permanent connections:                      Fi = 0.9 × At × Sp

where
At is the tensile area of the bolt  (mm².inch²)
Sp is the proof strength of the bolt. (Mpa.ksi)

         Sp = 0.85 × Sy

where
Sy : yield strength (Mpa)

UN and UNR Bolts Tensile Stress Area

FIG 38

The tensile stress area can be expressed as

At = 0.7854 (dn - 0.9743 / n)²      (2)

where

dn  = nominal diameter of bolt (inches, m)

n    = number of threads per inch (pitch)

ISO 898 Bolts Tensile Stress Area

Tensile stress area in bolts according ISO 898-1 Mechanical properties of fasteners made of carbon steel and alloy steel:

At,nom = (π / 4) ((d2 + d3)/ 2)² 
or
 At,nom = 0.785 ( dn – 0.9382P)² 

where :

At,nom = nominal stress area (m, mm².)

d2              =  the basic pitch diameter of the external thread according ISO 724 ISO general-                                    purpose metric  screw threads -- Basic dimensions (m, mm.)         

d3        = d1 - H / 6 = the minor diameter of external tread (m, mm)

d1          = the basic minor diameter of external thread according ISO 724

H         = the height of the fundamental triangle of the thread according ISO 68-1 ISO general                            purpose screw threads (m, mm )
p          = thread  pitch  (mm)     

The applied torque T. This torque, usually achieved by a torque wrench, the turn-of-nut, or an indicating washer, is where d is the nominal outside diameter of the bolt and K is the correction

T = K×Fi × d

where
d   is the nominal outside diameter of the bolt(mm.inch)
K  is the correction factor that depends on the material, size, surface friction, and threading of the           bolt. For most small to mid size bolts, K is between 0.15 and 0.3.

As a rough approximation:
dry (un-lubricated) mid-size steel bolts:       K = 0.2
non-plated black finish:                                K = 0.3
zinc-plated:                                                   K = 0.2
cadmium-plated:                                           K = 0.16
lubricated:                                                    K = 0.15 ~ 0.18

Bolt  subjected  to shear

The tension-type shear test was observed to provide a lower bound shear strength. The shear strength (in kilopounds per square inch) of a fastener was found to be independent of the bolt grade and equal to 62% of the tensile strength of the bolt material;

Sus = 0.62 × Su

where 

Sus =  ulimite shear stress (Mpa.Ksi)

Su   =  ultimate tensile stress,(Mpa.Ksi)

And

Shear yield stress Ssy (Mpa.Ksi) :

Ssy = 0.58 × Sy

where

Sy  = yield stress (Mpa .Ksi)

The shear resistance of a bolt is directly proportional to the available shear and the number of shear planes. If a total of m shear planes pass through the bolt shank, the maximum shear resistance Fsu of the bolt is equal to

maximum shear resistance Fsu (N.lb)

Fsu = m ×Ab×Sus

where 

Ab  = Shear stress area  (mm².inch²) if same shank area or nominal diameter 

m   = Number shear planes

When shear planes pass through the threaded portion of the bolt, the shear area is equal to the root area of the bolt, which is about 70 to 75% of the nominal bolt area. A lower bound to the maximum shear capacity of the bolt can be expressed as

Fsu = 0.7 × m × Ab ×Sus

Bolt  subjected  to tension

The tensile capacity of a fastener is equal to the product of the stress area At and its tensile strength

 Su. However, it is convenient for design purposes to specify permissible forces and stresses on the basis of the nominal area of the bolt Ab rather than on the stress area At.Such a transformation is readily performed because the ratio of the stress area to the nominal bolt area only varies from 0.75 for ¾-in. diameter bolts to 0.79 for l 1/8-in. diameter bolts. The maximum tensile load Fu of a fastener is given as

Fu= At × Su

Expressed in terms of the nominal bolt area and using the lower bound,For most bolt diameters,  yields a slightly conservative estimate of the tensile capacity of a bolt

Fu = 0.75 × At× Su

Shear Failure of Threads

FIG 39

Shear strength Fsu (N.lb) :

Fsu =  Sus × Ash

Ash = Surface area through which shear occurs (tubular in shape for bolt/nut)

Depending on the relative strength of bolt and nut, the tread failure will occur either in nut or bolt threads, or in both simultaneously. The shear stress area of failure is different for each of the failure types.

Full strength of bolt                               Fu = At × Su

This must equal the resistance from shear area of length LE

 ASh = p DP (1/2 × LE )

where :

LE  =   Effective length of shear area. This is the length of the treaded are required to develop full                    strength.(mm)

Dp  =  Pitch diameter (mm . inch)

p     =  pitch, distance between two threads (mm)

1/p =  number of threads per inch

Note the shear width of thread at pitch diameter is half the pitch distance.

Thus, for equal shear and tensile strength

 Fsu =  Sus × Ash =    Fu = At × Su

Bolt alowable loads 

on shear strength

FIG 40

Fsy = m × Ab × Ssy (N.LB)

where :

Fsy = yielding strength each bolt (N .Lb)

Ab = Shear stress area  (mm².inch²) if same shank area or nominal diameter 

m  = Number shear planes

Ssy = Shear yield stress Ssy (Mpa.Ksi)

Number of bolts  = Required strength/Strength per bolt

Nb = F /Fsy

where :

F :   Required strength (N. Lb)

Nb = Number of bolts

on tensile strength

FIG 41

Force on the cover caused by the pressure P (kg/m²): 

Fe  = P × A  (N .Lb)

where :

A : pressed fluid surface area (m²)
reqiured bolt area :

As = Fe /Sa

where :
As  : total required  effective cross-section area (m² .mm².inch²)
Sa   :  Bolt material maximum allowable stress (Mpa .Ksi)

Sa = Sy /  safety factor 

where
Sy : bolt material yield stress
safety factor 

So number of required bolts Nb :

Nb =  As / At 

From tables: you can find  Tensile stress area  At  and Proof strength Sp

Fastener's Holes
The holes into which threaded fasteners are inserted are of two basic types; clearance holes (unthreaded) and threaded holes.
-   Clearance holes are larger than the nominal diameter of the bolt or screw and the amount of               clearance depends on the desired type of fit.
The table gives the diameter of clearance holes for the different types of fits.

-  Threaded holes are drilled at a diameter smaller than the nominal diameter of the screw that will        go  into it (almost equal to the root diameter of the bolt), then a tap is used to cut the thread.

The table gives the drill size to be used for the different sizes of threaded holes.

Thread Fits

In some cases, the required looseness or tightness of fit between the internal and external threads may vary.

There are two classes of metric thread fits that are generally used:

   1- General purpose fit (6H/6g). A tolerance class of 6H/6g is assumed if it is not specified.

   2-  Closer fit (6H/5g6g).

Letters: specify the amount of allowance.

Upper case letters: specify internal threads.

Lower case letters: specify external threads.

Numbers: specify tolerance grade (smaller numbers indicate a tighter fit)