2. • A MECHANICAL ELEMENT WHICH HOLDS TWO OR MORE PARTS
OF MACHINE OF A STRUCTURE IS KNOWN AS FASTENER. THE
PROCESS IS CALLED FASTENING.
INTRODUCTION
FASTENER DEFINATION
2
3. Basic purposes to be served by Fasteners
3
They simplify manufacture
They simplify repairs.
They provide safety.
4. When selecting a fastener for particular use, consider these factors :
4
Strength: Will it hold the loads and pressures?
Security: Will it remain attached?
Cost: Realistic?
Installation: Appropriate for situation?
Skill: Is specialized training needed?
Equipment: Is specialized equipment needed and available?
Appearance: If the fastener shows, which kind looks best?
11. A nut is a type of hardware fasteners with a threaded hole.
Nuts are almost always used opposite a mating bolt to fasten a
stack of parts together.
The two partners are kept together by a combination of their
threads' friction, a slight stretch of the bolt, and compression
of the parts.
A washer is a thin plate typically disk-shaped with a hole that
is normally used to distribute the load of a threaded fastener.
THREADED FASTENER: NUTS AND BOLTS
11
16. THREAD NOMENCLATURE
Crest: It is the outermost portion which joins the two sides of a thread.
Root: It is the innermost portion which joins the adjacent sides of a thread.
Pitch: It is the distance from a point on a screw thread to a corresponding point on
the next thread measured parallel to the axis e.g. distance between crest to crest or
distance between root to root.
Lead: It is the distance moved by a nut or bolt in the axial direction in one
complete revolution.
For single start thread; Lead = Pitch
For multi start thread,
Lead = n x pitch where n = No. of starts
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17. THREAD NOMENCLATURE
Flank: It is the straight portion of the surface, on either side of the screw
thread.
Angle of Thread: It is the included angle between the flanks of the thread
measured in an axial plane.
Helix Angle: Angle that thread makes with plane perpendicular to thread axis.
Pitch Diameter: It is the diameter of an imaginary cylinder, the surface of which
would pass through the thread at the points where the thread width is equal to
the space between the threads.
Depth of the thread: It is thedistance between the crest and root of a
thread measured perpendicular to the axis.
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18. THREAD NOMENCLATURE
SINGLE START THREAD…..
Single Start Thread: When only one
helix forming the thread run on a
surface it is called single start
thread.
A single start thread consists of a
single, continuous helical groove for
which the lead is equal to the pitch.
Only one starting point is seen in the
beginning of the threaded portion.
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19. THREAD NOMENCLATURE
MULTI START THREAD…..
Multiple-Start Thread: When two
or more helices forming the
threads run on a surface it is
called Multiple start thread.
The lead is equal to (pitch x no. of
starts)
Two or more than two starting
points are seen in the beginning
of the threaded portion.
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20. THREAD FORMS
There are main two types of
thread are which are,
V-thread
Square thread
V-THREAD
British Standard Whitworth Threads (BSW)
British association threads (BA)
Sellers threads
Unified standard threads
ISO Metric Threads
ISO metric Trapezoidal Threads
SQUARE THREADS
Square thread
Acme thread
Knuckle thread
Buttress thread
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21. TYPES OF V-THREAD FORM
BRITISH STANDARD WITHWORTH(BSW)
Used as a standard thread in Britain Give an
effective leak-proof joint due to their fine
pitches and form Used on bolts, nuts and
screws for general purpose fastening.
BRITISH ASSOCIATION(BA)
Modified form of BSW thread with included
angle 47.50 Used for precision fastening like in
mechanical instruments, aircraft construction
etc.…
21
22. TYPES OF V-THREAD FORM
SELLERS THREAD
These thread have flat crest and root and can
withstand more rough usage Adopted as a
standard form in America Used on fasteners
for making adjustment.
UNIFIED STANDARD THREAD
These threads have been standardized by the
ISO and are available in inch and metric
series.
22
23. TYPES OF V-THREAD FORM
ISO-METRIC THREAD
Recommended by Bureau of Indian Standard
and adopted as a standard form.
ISO-METRIC TRAPEZOIDAL THREAD
Standardized by ISO and same to ACME
threads.
23
24. ISO METRIC THREAD
THE STANDARD DIMENSION FOR DIFFERENT SIZES OF THREAD ARE
GIVEN BELOW,
24
25. TYPES OF SQUARE THREAD FORMS
SQUARE THREAD
These threads have their flanks at right angles
to the axis Offers less resistance to the motion
Generally used for power transmission. Ex.
lead screw of lathe, screw jack The pitch is
twice that of BSW thread for same diameter.
ACME THREADS
Modified form of square threads. The thread
angle is 290 ,These threads are particularly
used in cases where engagements and dis-
engagements are frequent e.g. brass valves,
lead screw of lathe and bench vices.
25
26. TYPES OF SQUARE THREAD FORMS
KNUCKLE THREAD
Modified form of square threads Rounded at
top and bottom. These threads are used in
electric bulbs, bottles, railway couplers
etc.…
BUTTRESS THREAD
Combined form of V-threads and square
threads. It combines the low frictional
resistance of square threads and the strength
of V-threads Used for power transmission in
one direction only like carpenter wise, air
plane propellers etc.…
26
27. THREAD MANUFACTURING
INTERNAL THREAD
• Internal thread manufactured by taping tool on milling machine.
The tap drill hole is a little
bigger than the minor
diameter.
27
29. THREAD DESIGNATION
(1) ISO-METRIC DESIGNATION (2) CONVENTIONAL DESIGNATION
ISO-METRIC DESIGNATION: The diameter-pitch combination of an ISO
metric screw thread is designated by the letter ‘M’ followed by the value of
nominal diameter, pitch and thread length in mm.
The two values are separated by the sign ‘x’ e.g.
M10 X 1.25 X 24
METRIC THREAD
NOMINAL
DIAMETER(MM)
PITCH(MM) THREAD
LENGTH(MM)
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30. CONVENTIONAL REPRESENTATION
True projections of threaded portion of a part consists of series of helices and it
takes considerable time to draw them,
Hence some conventional methods are used to represent the threads.
Notes may be given at the bottom of the threads stating the particulars e.g.
thread form, pitch,diameter, left or right hand threads etc.
It is generally consist schematic representation and simple representation of
external thread, internal thread, v-thread.
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31. CONVENTIONAL REPRESENTATION
EXETERNAL THREAD (SIMPLE REPRESENTATION)
The crests of threads are indicated by thick continuous line and the roots are indicated by
thin continuous line.
In side view, the threaded roots are represented by portion of a circle drawn with a
continuous thin line of length approximately three-quarters of the circumference
The limit of useful length of screw threads is represented
by a continuous thick line.
The length up to which the incomplete threads are formed beyond useful limit is
represented by two inclined lines.
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32. CONVENTIONAL REPRESENTATION
INTERNAL THREAD (SIMPLE REPRESENTATION)
For internal screw threads, the crests and roots are indicated by hidden lines.
In side view, the threaded roots are represented by portion of a circle drawn with
a continuous thin line of length approximately three-quarters of the
circumference.
The limit of useful length of screw threads is represented by hidden line
The length up to which the incomplete threads are formed beyond useful limit is
represented by two inclined hidden lines.
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33. CONVENTIONAL REPRESENTATION
V-THREAD (SCHEMATIC REPRESENTATION)
The crests are represented by continuous thin lines extending up to the major
diameter.
The roots are represented by thick lines extending up to the minor diameter.
These lines are drawn inclined with a slope equal to half the pitch as shown in
figure,
33
34. THREADS IN ASSEMBLY
Figure (a) shows schematic representation of thread assembly.
Figure (b) & (c) shows simple representation of thread assembly.
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35. BOLT
A cylindrical piece/job with a head
on one side and threaded length on the
other side is called a bolt.
Bolts are used to join two or more
parts temporarily.
The function and the purpose of a
bolt decides the shape of the bolt.
Bolt is used along with nut to tighten
two parts.
Following types of bolts are generally
used in engineering applications:
a) Hexagonal head bolt
b) Square thread bolt
c) Cup thread bolt
d) Cylindrical head
e) T-headed bolt
f) Eye bolt
g) Lifting eye bolt
h) Hook bolt
i) Shackle bolt
j) Head less tapered bolt
k) Counter sank headed bolt
35
36. TYPES OF BOLT…
HEXAGONAL HEADED BOLT
• This is one of the most commonly used bolts.
• The shape of the head is hexagonal.
• The hexagonal head is chamfered at its upper end at an angle of 300 to itsbase.
SQUARE HEADED BOLT
• This bolt is used where a head is to be accommodated in a recess.
• The recess is also in the form of a square shape.
• This prevents the bolt from turning when the nut is screwed on or off it.
CUP HEADED BOLT
• It is square neck bolt which fits in a square recess.
• This fit prevents the bolt from rotating.
• Used largely in tank construction and in certain parts of locomotive constructions
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37. TYPES OF BOLT…
CYLINDRICAL HEADED BOLT
Used when limited space is available and it also
avoids the use of spanner.
The rotation of the bolt is prevented by means of
a pin inserted in to the shank just below the head
These bolts are commonly used in big ends of
connecting rod, eccentrics etc…
T-HEADED BOLT
This bolt is mainly used in machine tool tables that are
provided with T-slots
This makes it possible to mount jigs and fixtures
anywhere on the slide by using T-headed bolts
The neck of the bolt is made square fitting in to the slot
provided on the slide.
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38. TYPES OF BOLT…
HOOK BOLT
This bolt passes through a hole in one part
only while the other part is gripped by the
hook shaped bolt head
It is used where there is no space for making
a bolt hole in one of the parts. The square
neck prevents the rotation of the bolt.
SHACKLE BOLT
This bolt is having head just like a fork
having holes to receive a pin.
Square neck is usually provided with
the head to prevent the rotation.
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39. FOUNDATION BOLT
Foundation bolts are used for fixing machines to their foundation
Foundation bolts are made by forging from mild steel or wrought iron
rods
The bolt size depends up on the size of the machine and the magnitude of
the forces that act on them when the machine is in operation
For setting the bolts in position, their positions are marked and then
suspended in the holes made in the ground
Afterwards, cement concrete is filled in the space around bolts
Once cement concrete mixture sets, the bolts are firmly
secured to the ground
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40. TYPES OF FOUNDATION BOLT
Various types of foundation bolts used for fixing the heavy machines are as
follows:
(1) Eye or Hoop foundation bolt
(2) Bent or Curved foundation bolt
(3) Rag foundation bolt
(4) Lewis foundation bolt
(5) Cotter foundation bolt
(6) Square headed foundation bolt
(7) T-headed foundation bolt 40
41. BOLT AND SCREW CLEARENCES
Bolts and screws attach one
material with a clearance hole to
another material with a threaded
hole.
The size of the clearance hole
depends on;
the major diameter of the fastener
→ and the type of fit
• normal
• close
• loose
41
42. BOLT AND SCREW CLEARENCES
Other fits found in table given below..
42
43. BOLT AND SCREW CLEARENCES
• Sometimes bolt or screw heads
need to be flush with the surface.
This can be achieved by using
either a counter bore or
countersink depending on the
fasteners head shape.
43
44. BOLT AND SCREW CLEARENCES
Counter bores:
Counter bores are holes designed
to recess bolt or screw heads
below the surface of a part.
Typically,
CH = H + 1/16 (1.5 mm)
and
C1 = D1 + 1/8 (3 mm)
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45. BOLT AND SCREW CLEARENCES
Countersink:
Countersinks are angled holes
that are designed to recess
screws with angled heads.
Typically,
C1 = D1 + 1/8 (3 mm)
Appendix B gives other
counter bore, countersink
and shaft clearance holes.
45
46. WHAT IS THE NORMAL FIT CLEARANCE HOLE
DIAMETER FOR THE FOLLOWING NOMINAL BOLT
SIZES.
NOMINAL SIZE CLEARRENCE HOLE
¼ 9/32
¾ 13/16
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47. GRIP OF FASTENER
The thread length of inch-series bolts, where d is the nominal diameter, is
𝐿𝑇 =2𝑑 + 0.25", 𝑓𝑜𝑟 𝐿 ≤ 6“
=2𝑑 + 0.50", 𝑓𝑜𝑟 𝐿 > 6"
For metric bolts it is
𝐿𝑇 = 2𝑑 + 6𝑚𝑚, 𝑓𝑜𝑟 𝐿 ≤ 125𝑚𝑚
=2𝑑 + 12𝑚𝑚, 𝑓𝑜𝑟 125𝑚𝑚 < 𝐿 > 200𝑚𝑚
=2𝑑 + 25𝑚𝑚, 𝑓𝑜𝑟 𝐿 > 200𝑚𝑚
The ideal bolt length is one in which only one or two threads project from the nut after it is
tightened.
During tightening, the 1st thread of the nut tends to take the entire load
But yielding occurs & load is eventually divided over about 3 nut threads.
The grip l of a connection is the total thickness of the clamped material.
Due to a few threads taking the load, 𝑙 ≠ 𝐿𝑇
47
50. Twisting the nut stretches the bolt to produce the clamping force. This
clamping force is called the pretension or bolt preload.
Since the members are being clamped together, the clamping force that
produces tension in the bolt induces compression in the members.
The spring rate is the ratio between the force applied to the member and the
deflection produced by that force. (𝐹 =𝑘bΔ𝑙⇒ 𝑘𝑏 =(F/ Δl)=(EaΔl/ (lΔl)=(Ea/l)
Stiffness of bolt or
screw clamped zero
consist
a) Unthreaded portion
Kd=(Ad*E)/Ld
b) Threaded portion
Kt=(At*E)/Lt
c) Two spring in series
(1/Kb)=(1/Kd)+(1/Kt)
STIFFNESS OF THE BOLT
50
51. Since the members are being clamped together, the clamping force that produces tension
in the bolt induces compression in the members.
Clamped members also act like springs in series
The spring rate of the members being joint by the fastener is,
Where E is the modulus of elasticity of the material of the members. In this formula we assume all the
members being jointed are of the same material
If one of the
members is a soft
gasket, its
stiffness relative
to the other
members is
usually so small
that for all
practical
purposes the
others can be
neglected and
only the gasket
stiffness used.
STIFFNESS OF THE JOINT
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52. The proof load is the maximum load (force) that a bolt can withstand without
acquiring a permanent set.
The proof strength Sp is the ratio of proof load and tensile-stress area.
In the specification standards for bolts, the strength is specified by stating SAE or
ASTM minimum quantities, the minimum proof strength, or minimum proof load,
and the minimum tensile strength.
Min proof load is load at which 1% fasteners fail. 99% fasteners exceed it
The bolt grades are numbered according to the tensile strengths.
The grade of the nut should be the grade of the bolt.
Refer to Tables 8-9, 8-10 and 8-11 for minimum strength of steel bolts
ASTM threads are shorter because ASTM deals mostly with structures;
structural connections are generally loaded in shear, and the decreased thread
length provides more unthreaded shank area (remember 𝐴𝑑 > 𝐴𝑡).
STRENGTH OF BOLT
52
53. The bolt grades are numbered according to the tensile
strengths.
Grades are printed (grades or some geometric code) along
with manufacturer logo on the bolt head.
Unmarked bolts should be avoided as they may be
unstandardized.
BOLT HEAD MARKING
53
57. 57
BOLT FAILURES
Shear loaded joints are handled the
same for rivets, bolts, and pins
Several failure modes are possible
(a) Joint loaded in shear
(b) Bending of bolt or members
(c) Shear of bolt
(d) Tensile failure of members
(e) Bearing stress on bolt or members
(f) Shear tear-out
(g) Tensile tear-out
