A Bolt is a mechanical device which has head on one end of a body and a thread on the other end designed for fastening of two parts.

Bolt is inserted into holes in assembly parts, it is mated with a tapped nut. Tension is normally induced in the bolt to compress the assembly by rotating the nut. This may also be done by rotation of the bolt head.

A Screw is a headed and threaded bolt used without a nut. It is inserted into an internally tapped hole and tension is induced by rotation of the screw head.

A Stud is a fastener with no head but it has threads at both ends of the shank. It, like a screw, has one end those screws into a tapped hole. A nut is used on the other end to create tension.

Major diameter – largest diameter of thread (D)

Minor diameter – smallest diameter of thread

Pitch – distance between adjacent threads.

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Types of nut bolts & Washers

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Sl No.	Fastener Type
A	BOLTS
1	Hex head bolt
a	Full threaded
b	Half threaded bolts
2	Machine screws
3	Carriage bolts
4	Shoulder bolts
5	Square head bolts
6	Allen bolts (Half threaded & full threaded)
7	Eye bolts
8	J bolts
9	U bolts
B	Screws
1	Deck screw
2	Hex lag screw
3	Self drilling screw
4	Wood screw
5	Sheet metal screw
C	Nuts
1	Hexagonal nut
2	Cap nut
3	Castle nut
4	Flange end nut
5	Hexagonal lock nut
6	Hexagonal machine nuts
7	Keps-K lock nuts
8	Knurled thumb nuts
9	Nylon hex. Nuts
10	Slotted Hexagonal nuts
11	Square headed nuts
12	Tee nuts
13	Wing nuts
D	Washers
1	Flat washer (Normal)
2	Flat washer (Extra thick)
3	Lock washer
4	Lock washer external tooth
5	Lock washer internal tooth
6	Square washer
7	Structural washer

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Types of Threads:

1. British Standard Whit Worth (B.S.W.) Threads: It is a symmetrical V thread in which the angle between the flanks is 55°.

2. British Association (B.A.) Thread: It is the B.S.W. thread with fine pitches having thread angle 47.5°.

3. American Standard Threads: Included angle is 60°.

4. Unified Standard Thread: This thread has rounded roots and crests with included angle 60°.

5. Square Thread: Used in power transmission, spindles of valves etc.

6. Acme Thread: It is a modified square thread. It is much stronger than square thread.

7. Buttress Thread: Used for transmission of power in one direction only.

8. Metric Thread: It is an Indian standard thread similar to B.S.W, but has included angle 60°.

Generally in all industries Metric & BSW threads are used

Designation of Metric size bolts:

Metric size bolts are designated based on Major diameter & Minor diameter, pitch & thread length (Bolt length)

For Example:

M12-1.25 x 50

M = Metric thread designation

12 = Major diameter, in millimeters

1.25 = Pitch (distance from thread to thread), in millimeters

50 = Length, in millimeters

OR sometimes directly written as M12 X 50 mm

Designation of Inches size bolts:

Example: ¼” X 1”

Here ¼” is the major diameter of the bolt & 1” is the length of the bolt

Note: While measuring the bolt length, head thickness is not considered

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How do you identify the Metric & Inches size bolts visually??

Sl No.	Metric thread	Inches thread
1	Threads included angle is 60⁰	Threads included angle is 55⁰
2	Thread crests are sharped edge	Threads crests are curved (small radius is given)
3	Metric threads are of fine having lesser pitch & measured in mm	Inches threads are of coarse & measured in TPI
4	Head size is smaller as compared to inches bolts	Head size is larger
5	Diameter of Metric size bolts is smaller for the same size of bolts	Diameter of Inches size bolts is larger the same size of bolts

Calculation of spanner size for Metric size nut bolts

Sl.No.	Bolt size	Calculation	Spanner size (No.)	Standard spanner size (Flat both end open & Ring)
1	M3	3 + 1 +1 = 5 (Half of 3 =1.5 1^st digit of this is to be added with 3	5	4 X 5
2	M4	4 X 1.5 + 1 =7 mm	7	6 X 7
3	M5	5 + 2 +1 =8 mm	8	8 X 9
4	M6	6 X 1.5 + 1 = 10 mm	10	10 X 11
5	M7	7 + 3 + 1 = 10 mm	10	10 X 11
6	M8	8 X 1.5 + 1 =13 mm	13	12 X 13
7	M10	10 X 1.5 + 2 = 17 mm	17	16 X 17
8	M12	12 X 1.5 + 1 =19 mm	19	18 X 19
9	M14	14 X 1.5 + 1 = 22 mm	22	20 X 21
10	M16	16 X 1.5 =24 mm	24	24 X 27
11	M18	18 X 1.5 = 27 mm	27	24 X 27
12	M20	20 X 1.5 = 30 mm	30	30 X 32

Calculation of spanner size for Inches size nut bolts

Sl.No.	Bolt size	Calculation	Spanner size (No.)	Standard spanner size (Flat both end open & Ring)
1	2 soot or 1/4”	2 X 5 + 1 =11 mm	11	10 X 11
2	3 soot or 3/8”	3 X 5 = 15 mm	15	14 X 15
3	4 Soot or 1/2”	4 X 5 =20 mm	20	20 X 21
4	5 soot or 5/8”	5 X 5 + 1 =27 mm	27	24 X 27
5	6 soot or ¾”	6 X 5 = 30 mm	30	30 X 32
6	7 soot 7/8”	7 X 5 + 1 =36 mm	36	36 X 41
7	8 soot 1”	8 X 5 + 1 = 41 mm	41	36 X 41 or 41 X 46

Allen bolt key size (Metric)

Allen bolt size	Allen key size mm
M3	2.5
M4	3
M5	4
M6	5
M8	6
M10	8
M12	10
M14	12
M16	14
M18	14
M20	17
M22	17
M24	19

Bolts manufacturing process

Bolts manufacturing process involves following sub processes

A-Wire: A long coiled steel rods of required grades are first uncoiled & cut into required length

B-Cold forging:

C-Manufacturing of bolt head by pressing the steel into required die

D-Heat treatment

E-Threading: Generally formed by rolling & cutting

Other general methods employed for manufacturing of nut bolts are Machining & Forming

What heat treatment processes are carried out on nut bolts?

Annealing
Stress relieving
Case hardening
Quenching & Tempering

What properties of materials are considered while designing the nut bolts???

Tensile strength
Ultimate strength
Yield strength
Hardness
What are the different materials used for manufacturing of nut bolts?

Nut bolts are manufactured by different materials based on applications, some of them are

Carbon steel
Stainless steel
Alloy steel
Brass
Silicon Bronze
Gun metals

Practical QnA on power plant Maintenance

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Half threaded bolts threading length = 2D + 6 mm for bolts length < 150 mm
Half threaded bolts threading length = 2D + 12 mm for bolts length >150 mm
Nut thickness t = 0.9 X D
Bolt head thickness = 0.8 X D
Plain washer internal diameter = D+1
Plain washer outer diameter = 2D+3
Plain washer thickness t =D/8
Where D is the major diameter of bolt
Spanners are made by Chromium & Vanadium alloy steel by the process forging
The angle of chamfer for nuts & bolts head is 30⁰
Aluminium & stainless steels should not be mixed while fastening. This leads to galvanic corrosion by transfer of electrons.
Lubricant/or grease should be lapped on SS bolts before inserting into threaded holes to avoid initial friction & damages of the threads, as SS bolts are coated with thin invisible coating to prevent oxidation
Salt water has tendency to cause fast oxidation/rusting even on SS nut bolts
Do not mate galvanized fasteners with other (carbon steel, SS etc) to avoid thread damage as galvanized iron fasteners have thick anti-rust coatings.

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Nut bolts grades:

A-Imperial standard grades:2,5,8 etc

Grade 2 bolts is of low carbon steel having no any marks on its head
Grade 5 bolts have 3 marks on its head
Grade 8 bolts have 6 marks on its head

B-Metric standards grades:

Generally used carbon steel nut bolts of grades 4.6, 4.8, 5.8, 8.8, 10.9 etc

Here 1^st digit of grade indicates the tensile strength of the bolt & second digit indicates the ratio of Yield strength to ultimate strength.

For example: A bolt of grade 10.9 implies that,

Tensile strength = 10 X 100 = 1000 N/mm2

Efficiency or with standing load = 0.9 X 100 =90%

This means that, a bolt of metric grade 10.9 with stands a load 101 kg/mm2 of (1000/9.81) & fails at 90% load (90 kg)

Stainless steel bolts grades

Generally used nut bolts are of SS304 & SS316 materials

Grades are ,A1-50,A2-70 & A4-80

A2-50 : Soft graded used for lower loads
A2-70 : Austenitic steel, cold worked & tensile strength 70 X 10 = 700 Mpa
A4-80 : Austenitic steel, cold worked & tensile strength 80 X 10 = 800 Mpa
C4-70: Martens tic 12% Cr steel, hardened & tempered & having tensile strength 70 X 10 = 700 Mpa

Other high temperature nut bolts grades grades:

SA 193 grade B-7: For alloy steel bolts Quenched & Tempered
SA 193 grade B-16: For alloy steel bolts Chromium & Vanadium
SA 193 grade B-8 & B8M: For SS 304 carbide solution treated
SA194 2H: For alloy steel nuts
SA 193 & A 193 grade materials properties are same, however SA refers to the ASME grade & A refers to the ASTM grade materials
ASTM-A 325 or A-325 -Medium carbon steel
ASTM-A 354 Grade BB, BC & BD-Alloy steel quenched & tempered
ASTM-A490-Alloy steel quenched & tempered