Threaded fasteners such as bolts and screws join components together through the transformation of rotational motion into linear motion. There are various thread standards that specify attributes like diameter, pitch, class of fit, and thread type. Early threaded fasteners lacked standardization but efforts in the 18th-19th centuries established conventions for sizes. Modern standards include metric and unified external and internal thread systems.

1. Threads and Fasteners
Engineering Graphics

2. Threads
• Threads are
essentially an inclined
plane wrapped
around a cylinder.

3. Screw Threads
• There are two distinct uses for screw threads that require
different thread characteristics:
• a power screw such as a lathe leadscrew or the screw
in a lifting jack which transforms rotary motion into linear
motion
• a threaded fastener as below which joins components
together, again by transforming rotary motion into linear
motion, though in this case the translation (linear
movement) is small. Although tightly fastened the
attachment is not permanent.

4. Power Screw
• Rotation of the screw is converted to linear
motion (lift)

5. Power Screw
• Rotation is transformed into linear motion

7. Threaded Fasteners
• Rotation of the screw advances the
fastener into the material

8. Threaded Fasteners
• The fastener and accompanying nut
fastens two or more parts together

9. Threaded parts were originally made by manufacturers in random
sizes to suit a particular application.
These fasteners were not interchangeable
Two Englishmen are usually credited with starting a movement toward
standards for nuts, bolts and screws. H. Mandslay, working in the machine
construction industry, tried to introduce a standard between 1800 and 1810.
The number of different sized nuts and bolts was relatively small at this
time. Mandslay influenced his apprentice J. Withworth to continue to
develop standards, and in 1841 the Withworth thread was adopted in
England as a standard in some industries.
Various industries in the US, UK, and Europe began to standardize
threaded parts to ease maintenance, interchangeability, and repair
issues. Industries manufacturing aircraft, automobiles, carriages,
railroad, munitions and household appliances all developed industry
specific standards.

10. .
• In the US and UK the lack of standardization
between industries became a serious problem at
the beginning of World War II, especially in the
areas of munitions, aircraft and vehicle
maintenance.
• As a result, in 1949 the Standards Committee of
the American National Standards Institute
standardized threadforms into two broad
categories, metric and unified series threads.

11. Unified Threaded Fasteners
• A threaded fastener is a fastening device
used to mechanically join two parts of an
assembly together( as contrasted with
welding or brazing)
• Threaded fasteners are now standardized to
allow interchangeability
• To specify a thread using the Unified
system, you must provide five pieces of
information:
• Thread form
• Thread series
• Major Diameter
• Class of fit
• Threads per inch

12. .
• So, for example, in the United States one
might ask for a “2-inch quarter-twenty
bolt,” which would be:
 2 inches long,
 have a nominal diameter of a quarter of an inch,
and
 have twenty threads to the inch.
• A typical smaller fastener could be “a 1-inch 8-
32,” “8” being a gauge number and “32” the
number of threads per inch.

13. The Fastener Description: Details
• A machine screw or its larger cousin, a bolt, is described by
length, the type of head and the thread type
• In the United States, the convention for describing threads is to
give the number of threads per inch, preceded by a gauge
number if the bolt is smaller than a quarter-inch in diameter,
otherwise by the diameter in fractions of an inch.
Machine Screw Bolt and Nut

16. Threadforms
• Coarse Thread Series, UNC: The coarse
thread series UNC is the most commonly
used thread system used in the majority of
screws, bolts, and nuts for quick assembly or
disassembly
• Fine Thread Series, UNF: This is used for
applications that require a higher tensile
strength than the coarse thread series and
where a thin wall is required.
• Extra Fine Series, UNEF: where length of
engagement is short and extra strength is
required.

17. Future threads
It might be thought that after 150 years so seemingly
simple a thing as the bolt could have been brought to
perfection, but that is not the case.
All the thread forms discussed so far share one
characteristic: they are symmetrical. But when a bolt is
tightened, the forces on the two sides of the thread are
different. That alone suggests that an asymmetrical
thread form might be better, and so it seems.
The space shuttle, for example, uses bolts with an
asymmetric thread form. In time all threaded fasteners
may have forms subtly different from those we use
today.

18. An Asymmetrical Threadform

19. Thread Specifications
• A bolt's diameter isn't all you need to know to buy a
nut to fit it. A bolt a quarter-inch in diameter might
have 20, 28 or even 32 threads per inch, and a nut
that fit one would not fit another.
• The relationship between diameter and number of
threads per inch is standardized in a number of
series, the most common in the United States being
UNC and UNF. A more complete sizing of a quarter-
inch bolt's thread might be ¼-20 UNC or ¼-28 UNF.
A few examples:
• UNC UNF
• ¼-20 ¼-28
• 5⁄16-18 5⁄16-24

20. Thread Terminology
• .
• The major diameter of a thread is the
diameter of the imaginary co-axial
cylinder that just touches the crest of
an external thread or the root of an
internal thread.
• The minor diameter is the diameter of
the cylinder that just touches the root
of an internal thread
• The crest of a thread is the prominent
part of the thread
• The root is the bottom of the groove
between the two flanking surfaces
• The flanks of a thread are the straight
sides that connect the crest and the
root.
• The angle of a thread is the angle
between the flanks,
• The pitch of a thread is the distance,
measured parallel to its axis, between
corresponding points on adjacent
surfaces

21. Thread Pitch
• Distance from crest of
one thread to the
crest of the adjacent
thread
• Used to determine
threads per inch.

22. Metric Thread Pitch
• Metric thread pitch
is indicated by the
distance from one
thread to the next
thread.
• 8 X 1.25 = 8mm
diameter with
1.25mm distance
between threads.

23. Measuring Thread Pitch
• Use Thread Pitch Gauge

28. Typical “UN” Series Designation
1/2” - 13 UNC - 2A
external thread
(A =external- B =internal)
Class of fit
(1 is loosest tolerance, 3 is tightest)
Pitch (threads/inch)
Nominal Diameter
(also shown as decimal - or screw gage # if less than ¼”)
Thread form
Thread series (e.g. coarse or fine)

32. .
• The basic profile of ISO Metric threads is built up from
contiguous equiangular triangles of height h disposed
symmetrically about a pitch line which becomes
the pitch cylinder of diameter d2 when the profile is
rotated about the axis to form the thread. The distance
between adjacent triangles - the pitch - is p = 2 h /√3.
The tips of the triangles are truncated by h/8 to form the
major diameter ( size ) d of the thread, and the bases
are truncated by h/4 to form the minor diameter d1 . It
follows that d1 = d - 5 h/4 = d - 1.08 p. This leads
to the rule of thumb for suitable tapping size drills in
normal materials : dtapping = d - p.

33. .
• The 60o thread form is not suitable for power screws
which transform motion and which therefore must have
high efficiency. The 'square' thread offers the best
efficiency but is generally impractical. The 'Acme' thread
form offers the best compromise between efficiency,
ease of manufacture, assembly and wear take-up using
split nuts. The stress area of Acme threads is based
upon the average of the minor and mean diameters :
• ds = d - 3/4 p .

34. The American thread series, with their
symbols:
• Historically Used (obsolete)
• NC American National Coarse
• NF American National Fine Thread
• NEF American National Extra Fine Thread
• 8N American National 8 Pitch
• 12N American National 12 Pitch
• 16N American National 16 Pitch
• NS Special Threads of American National Form
• SB Manufacturers Stove Bolt Standard Thread
• Commonly Used
• UNC Unified Coarse Thread
• UNF Unified Fine Thread
• UNS Unified threads of special diameters, pitches, and length of
engagement
• UNM Unified Miniature

35. Specialized application threads
• A knuckle thread is
used to manufacture
threaded container
caps and frequently
has multiple lead
threads (discussed
below).

36. Multiple Lead Threads
• Fasteners can be made
with two or three parallel
threads instead of the
usual one, although no
standard series includes
such a thread.
• The advantage of such
threading is that the bolt
will be stronger than a
single thread bolt whose
nut would advance the
same distance in one turn

37. Lead and Start
• When a nut on a screw
is rotated by one turn,
it travels along the
screw a distance
known as the lead L.
• Power screws may
employ multiple
threads, or starts,
so L = p ∗ number of
starts as illustrated.
• Fasteners on the other
hand are almost
invariably single start (
L = p ).

38. Tolerance Classes (Classes of Fit)
• The standards for a thread series include
specifications of tolerances.
• Most specify several different classes, because
for some uses a close fit is essential, while
achieving it for other uses would be a waste of
money.
• UN series has four classes of tolerance: Loose-
fit (class 1), Free-fit (class 2), Medium-Fit (class
3), and Close-fit (class 4).

39. Handedness
• Almost all threaded
fasteners tighten when
the head or nut is rotated
clockwise. That is, as a
viewer turns a nut
clockwise it moves away
from them. Such a
fastener is said to have a
right-hand thread; all
screw fasteners are
assumed to be right-hand
unless otherwise
specified.
• Left-hand threads are
usually found only on
rotating machinery,
turnbuckles, and as a
safety feature on welding
equipment.
“Right hand Rule” for threads

40. • Threaded fasteners are typicallly “right
handed” to avoid confusion in
tightening
• LH screws appear in turnbuckles and
in certain bicycle parts where the
prevailing torque would tend to loosen
RH fasteners.

41. .
• Precession can cause fastenings under large torque loads to
unscrew themselves.
• Automobile lug nuts
• Automobiles have also used left-threaded lug nuts on left-side
wheels, but now commonly use tapered lug nuts.
Bicycle pedals
• Bicycle pedals are left-threaded on the left-hand crank so that
precession tightens the pedal rather than loosening it.
• Safety Issues
• In industrial application where similar-looking connections must be
made, in parts and hose connections. LH and RH threading
prevents incorrect attachments.
• Turnbuckles
• Two threaded hoops connected to a shackle can be drawn together
if LH and RH threads are used at each end

42. Types of Fasteners
• There are a variety of fasteners
manufactured to meet the need of
designers in different applications
• Following are the most common:

43. Thread Types
• Machine screws
• Wood screws
• Tapping screws

45. Bolt Grades
• Grade indicates the tensile
strength of the bolt
• Determined by bolt material
and heat treating

48. Bolthead Symbols
• Bolt heads are maked with information
about the bolt:
– Strength
– Manufacturing process
– Manufacturer
– Material composition

50. SAE Bolt Designations
SAE
Grade
No.
Size
range
Tensile
strength,
ksi Material Head marking
1
2
1/4 thru 1-1/2
1/4 thru 3/4
7/8 thru 1-1/2
60
74
60
Low or medium
carbon steel
5 1/4 thru 1
1-1/8 thru 1-1/2
120
105
Medium carbon steel,
quenched & tempered
5.2 1/4 thru 1 120 Low carbon
martensite steel,
quenched & tempered
7 1/4 thru 1-1/2 133 Medium carbon
alloy steel,
quenched & tempered
8 1/4 thru 1-1/2 150 Medium carbon
alloy steel,
quenched & tempered
8.2 1/4 thru 1 150 Low carbon
martensite steel,
quenched & tempered

51. • A typical hexagon headed
bolt and nut are shown.
• The diameter of the bolt
shank is usually the same
as the outside diameter -
the major diameter or
briefly size - of the
thread. The radiused fillet
at the junction of shank
and head reduces stress
concentration

52. Drawing Threads
• Threads are typically drawn in one of three forms:
• Detailed
• Simplified
• Schematic

53. Drilled and Tapped Holes
• You specify tapped (threaded) holes by a note giving the
diameter of the tap drill (21/64"); depth of hole (1 3/8”);
thread information (½” diameter, Unified National Course
threads, Class 2); and length of thread (1"), as:
21/64 DRILL x 1 3/8 DEEP
1/2-13 UNC - 2 x 1 DEEP

54. Thread Notes

59. Hole and Thread Formation
• .

61. Tightening Torque
It is typical on engines for bolts to have a
specified tightening torque.
• It results in a quantified preload on the bolts
• Insures that parts never separate
• Maintains friction (no sliding to shear forces)
• Insures even distribution of loading
– prevent warpage of mating parts
– uniform pressure distribution over seal or
gasket
• Prevents bolt from loosening
• Reduces fatique effects

62. Bolt Manufacturing Processes
• Forging (upsetting)
• Rolling
Thread-rolling processes: a) reciprocating flat dies; and b) two-roller dies.
Threaded fasteners, such as bolts, are made economically by these
processes at high rates of production
a) b)

63. Manufacturing Processes -
continued
• Turning on screw machines
(a) Differences in the diameters of machined and rolled threads. (b) Grain flow in machined
and rolled threads. Unlike machining, which cuts through the grains of the metal, rolled threads
have improved strength because of cold working and favorable grain flow.

64. Schematic
2.
1.
End
View
Detailed
Representing External Threads
• Options for representing external
threads in a drawing

65. Representing Internal Threads
• Options for representing internal threads
Schematic
(in section)
Detailed
(in section)
Simplified End View

66. SAE Bolt Grades
• Grade refers to strength
• Count lines +2 to get grade
• Higher the number – the stronger the
bolt

67. Metric bolt property class
• Property Class numbers
• 1st # - tensile strength
• 2nd # - yield strength

68. SAE Bolt Grades
• Grade refers to strength
• The stronger the bolt the
higher the grade

69. Metric bolt property class
• Property Class numbers
• 1st # - tensile strength
• 2nd # - yield strength

70. Machine screws
• Bolts that are less than ¼
“
• Use numbers to identify
them instead of fractions
• Sized from 0 – 12
• Also used threads per
inch
• Example of 6-32
• 6=size 32=threads per
inch

71. Sheet metal screws
• Self tapping
• Use a punch
to make a
hole for them
• Don’t confuse
them with
wood screws

72. Nuts
• Standard = hex, wing,
speed nuts
• Locknuts = collar, slotted,
crimped (do NOT re-use)
• Castle and Pal-nuts –
used with cotter pins.

73. Nut Diameter

74. Non-Threaded Fasteners
• Rivets
• Pins
• Keys
• Retaining rings
• Locking washers
• Lock screws

75. Lock Washers
• Plain
• Internal
• External
• Wave

76. Lock Nuts

77. A stud has no head and is threaded at both ends.
The ends are not necessarily the same.
clearance hole through a component, typically
15-20% larger than the bolt/stud size to facilitate
assembly and to clear any shank/ head fillet;
tapped hole which is drilled smaller than the root
or minor diameter of the fastener

78. Retaining Rings
• Used in U-joints, etc.

79. Retaining Ring Pliers
• Both internal and external
models available
• Used to remove rings

81. Cotter pins
• Used on castle &
slotted nuts
• Used on linkages
• Do NOT replace with
nails or anything else

82. Internal Hair Pin Cotters
• Used like cotter pins

83. Keys
• Used to keep two
items stationary to
each other by the use
of slots
• Crankshaft to
dampener

84. Roll Pins
• Pins are a unique
type of fastener that
can hold two
materials together
non-permanently.
Unlike bolts and nuts,
which press the
materials together for
holding power, pins
can hold the
assembly together
with no torque
pressure.

85. Set Screws
• Used to hold a part in
position only.
• Usually has an Allen
head

86. Standard Set Screws

87. Rivets
• Used in door panels,
window motors, and
upper ball joints

89. Nut-serts
• Used in body
applications