Page 133 - CITS - ED - Mechanical

ENGINEERING DRAWING - CITS

Screwed fasteners

In joining number of parts together and dismandling without damaging any parts, devices called Bolts, nuts, screws
etc are made use of. These are called “Screwed fasteners”. Bolt is a metallic cylindrical rod having a specific
shape on one end called “Head” and the other end called the shank with screw threads cut on it. All the fasteners
are generally made of steel of good tensile strength.
Bolts are known by the shape of head viz., Hexagonal, Square, Cylindrical or cheese headed, cup or round, ‘T’
hook, eye bolt etc. and shank dia. The shape of head is selected depending upon the purpose for which it is used.
While engaging or dismantling a nut on to bolt, to prevent the rotation of bolt, bolt head is held by another spanner.
All the fasteners size/specifications follow letter M, stands for Metric (size) e.g. Hex.bolt M 20 x 100 i.e hexagonal
bolt shank dia 20 mm, 100 mm long.
Hexagonal head bolts: For drawing purpose, irrespective of shape of head, bolt head thickness is taken as
0.8d where d is the diameter of the shank. The length of bolt varies according to dia and the requirements. Figs
1 & 2 show the bolt head and bolt. The top corners of the hexagon are chamfered to avoid sharp corners which
get damaged while using spanner and also injurious while handling.

Fig 1 Fig 2

There are three grades of hex.head bolts viz (i) Precision, (ii) Semi precision and (iii) Black denoted by letters A,
B & C respectively according to their dimensional accuracies.
Hexagonal bolts Grades A and B IS:1364 part-1 M3 to M36. (12 Sizes)
Grade C IS:1363-part-1 M5 to M36 (10 sizes)
Grade C Black IS:3138 M42 to M156 (23 Size)are available.

Hexagonal bolts Grades A and B IS:1364 part-1 M3 to M36. (12 Sizes)
Grade C IS:1363-part-1 M5 to M36 (10 sizes)
Grade C Black IS:3138 M42 to M156 (23 Size)are available.
Square head bolt: Square head bolt is the second commonly used bolt, when the bolt head is not accessible.
The square head generally sits with in a square cavity, which prevents its rotation while the nut is handled. Fig 3
shows a square head bolt of plain shank and with square neck. They are of grade C (black) IS: 2585 Diameters
available from 6 to 39 mm (15 sizes).
Cylindrical or Cheese head bolts: It is also called as round head bolts. As the name implies, its head is cylin-
drical. To prevent rotation while handling nut, the head is fitted with a small pin of 0.12 d projecting out by 0.2 d.
The pin is fitted close to the head perpendicular to the axis or on to the head close to the shank parallel to the axis.
The pin fits into corresponding recess in the adjacent part. There is no need to use spanner to hold the head. It is
most commonly used on big ends of connecting rods, eccentrics etc. (Fig 4)
Cup head or round head bolt: It has hemispherical head with a snug or square neck. The snug is formed while
forging. Its function is same as round head bolt. (Fig 5)
‘T’ head bolt: Its head is in the shape of `T' which is accommodated in ‘T’ slots of machine tool tables. The neck
is usually of square cross section. Apart from their use in machine tools, they are used in stuffing box and gland
in boiler mountings etc. (Fig 6a)
Standard ‘T’ bolts: There are T bolts as per IS;2014 to suit T slots according to IS:2013. (Fig 6b)

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CITS :Engineering Drawing (Mechanical) - Exercise 8