What is Bolt Fastener – types of Bolts

What is Bolt Fastener – types of Bolts

A bolt is a metal having a head at one end and a threaded portion to a definite length on other end. The head is formed by forging or machining. The bolt is admitted through holes in the parts, which are to be fastened. The projected thread end of the bolt admits a corresponding nut from the other side. Tightening the bolt by turning gives necessary clamping grip to hold the parts together.

Bolts and nuts of various shapes are used for different purpose but the hexagonal head and square head are very common. Although, the square shape provides better spanner grip than the hexagon, but needs one fourth of a turn to bring it into the same position for inserting spanner again, whereas a hexagon need only one sixth of a turn and hence provided.

The sharp corners on the external flat end faces of bolt heads and nuts are chamfered conically at 30ᴼ to ensure safety of the user. To facilitate early insertion of the nut over the bolt, the threaded holes in the nut are countersunk.

Three dimensions are usually sufficient for simplified representation of a bolt

  • The bolt shank diameter (d)
  • The bolt length (l)
  • The length of a threaded portion of the shank (b)

Read more : What is riveted joints- Forms Of Rivets Head Shapes



  • Nominal diameter d=size of bolt or nut,mm
  • Width across flats s=1.5d+3mm
  • Width across corners e=2d
  • Thickness of bolt head k=0.8d
  • Thickness of nut n=0.9d
  • Root diameter d1=d-(2*depth of thread)
  • Length of the bolt l=as specified
  • Thread length b=2d+6mm (for l<150mm) =2d+12mm (for l>150mm)
  • Chamfer of bolt end Z=depth of thread*45(degree) (or) =0.1d
  • Chamfer angle of bolt head & nut 30 (degree)
types of bolts
types of bolts

Special Types of Bolts :

In practice various types of bolts than the hexagon and square head bolts are used in where the bolt head cannot be held by the spanner when the nut is turned on or off the bolt. The rotation of the bolt prevented by a stop pin or a snug or a square neck provided below the head.

 Cylindrical or Cheese Headed Bolt:
The head of this type of bolt is of cup shape and the rotation of the bolt head is prevented by a stop pin. The stop pin may be driven into the shank with its axis perpendicular to the axis of the bolt. The stop pin may also be driven into the head adjacent to the shank with its axis parallel to the axis of the bolt. These types of bolt heads are used in the big ends of the connecting rods, eccentrics, cross heads etc…

 Cup Or Round Headed Bolt:
Two types of cup head bolts are available. In one type, a snug is provided which prevents the rotation of bolt head. The other type, a square neck is provided which will fit into the square hole provided in the bearing surface and thus prevents the rotation of the bolt head.

 Counter Sunk Head Bolt:
The counter sunk head bolts are used when the bolt head must not project and foul with surfaces. The counter sunk bolt is provided with a stop pin of square cross section integral with the head. The other type of counter sunk bolt is provided with the square neck below the head. This type of bolt is also called as “coach bolt”.

Head Bolt:
The head of this bolt is like letter „i‟. These bolts are used in bearing housing and in glands packing. These bolts are used in setting work on machine tool tables. The i head of the bolt can slide, to the required position through the i slots cut on the m/c table.

Eye Bolt:
The head of the bolt is in the form of circular form of rectangular cross section. It is generally used in the inspection covers, lids etc…, Which have to be opened and closed frequently.

Lifting Eye Bolt:
The lifting eye bolt, having a circular ring of circular cross section as head. A flat circular portion, integral with a head is also provided. This type of bolt is used for lifting a heavy machine such as motors, pumps, turbine, electrical generators etc., This bolt is screwed in a threaded hole provided for this purpose , on the top of the machine directly above the centre of gravity so that while lifting the machine does not change from its usual working position.

Hook Bolt:
The hook bolt has its head comprising of a square neck and projection. The shank of the bolt passes through a hole in one of the fastening pieces and the other piece comes under the bolt head and is supported by it.

Sachin Thorat

Sachin is a B-TECH graduate in Mechanical Engineering from a reputed Engineering college. Currently, he is working in the sheet metal industry as a designer. Additionally, he has interested in Product Design, Animation, and Project design. He also likes to write articles related to the mechanical engineering field and tries to motivate other mechanical engineering students by his innovative project ideas, design, models and videos.

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