What is a Bolt?- Definition, Parts, And Uses

What is a Bolt?

A bolt is a form of threaded fastener with an external male thread requiring a matching pre-formed female thread such as a nut. Bolts are very closely related to screws.

Bolts are a type of fastener. Fasteners are used to secure or fasten materials together. There are many types of bolts and hardware nuts. Most, if not all, bolt types have machine threads. A threaded bolt screws into nuts to hold or fasten materials together. Bolt types include eye bolts, wheel bolts, and machine bolts while types of nuts include cap nuts, expansion nuts, and u-nuts.

A bolt is a mechanical fastener that is usually used with a nut for connecting two or more parts. A bolted joint can be readily disassembled and reassembled; for this reason, bolts or screw fasteners are used to a greater extent than any other type of mechanical fastener and have played an important part in the development of mass-produced articles and steel structures.

The bolt consists of a head and a cylindrical body with screw threads along a portion of its length. The nut is the female member of the pair, having internal threads to match those of the bolt. Washers are often used to prevent loosening and crushing. See also nut; washer and Screw.

Parts of Bolt

Parts of Bolts

These are different parts of the bolt describe as follows.

  1. Head: The head is the top part of a bolt. It serves as a gripping surface for tools. To tighten or loosen a bolt, a tool with the appropriate bit must grip the head. Most bolts have a wrench-type of head.
  2. Shank: The shank is located under the head. The shank is the smooth part of an unthreaded bolt. It should prevent radial movements of the joined workpieces. Without a shank, there is a greater chance that the bolt will loosen. Some workpieces generate vibrations, others are exposed to vibrations in their environment.
  3. Grip length: It is the part of bolt that accommodates the parts which are to assemble. Grip length should be equal to the combined thickness of the joining parts.
  4. Threading: All bolts are threaded. Threading is what allows a bolt can be driven into or out of workpieces. Most bolts, however, are not entirely made up of threads. They have a head followed by a smooth shank and finally the threading.
  5. Thread length: It is the part of bolt that accommodates the nut.
  6. Nominal length: It is the sum of thread length and grip length (as shown in figure).

Type of bolt heads

Bolts, as well as screws, are available in a vast variety of head shapes. These heads are made in order to grip the tools that are used to tighten them.

The most common type of bolt head types includes square, hex, slotted hex washer, and socket cap.

The earliest bolt heads in use were the square heads. Square heads consist of a square indentation on the head followed by a shaft that withstands rotation when a torque is applied to it. Square heads are still in use today but hex heads have become more common. Hexagonal heads are used with a wrench or a spanner to provide torque.

There are numerous other head shapes in use as well, namely:

  • Flat bolt head: A counter shank head with a flat top.
  • Oval bolt head: A counter shank head with rounded head top.
  • Pan bolt head: A slightly rounded head with a short vertical side.
  • Truss bolt head: An extra-wide head with a rounded top.
  • Round bolt head: A Dome’s head.
  • Hex bolt head: A hexagonal head.
  • Hex washer bolt head: A Hexagonal head with a round washer at the bottom.
  • Slotted hex washer bolt head: A hexagonal head with a built-in washer and slot.
  • Socket cap bolt head: A small cylindrical head using a socket driver.
  • Button bolt head: A low-profile rounded head with a socket driver.

However, there are many similarities between bolts and screws, there are some differences too.

Difference between Bolt and Screws

A bolt is a non-tapered fastener that uses a washer and nut to hold objects together. A screw is a tapered fastener that mates with an existing thread or create its own thread in material as it turns.

Bolts vs Screws

Machinery’s Handbook explains that a bolt is used to assemble unthreaded objects, usually using a nut. In comparison, screws are used to assemble objects with threads. The thing is though: not all items that use screws are already threaded.

What is a Bolt Uses for?

Bolt uses the following conditions when:

  • The parts that are fastened, require frequent dismantling and reassembly.
  • When the parts that are fastened, are made of a material which is too weak to make durable threads.
  • The parts that are fastened have medium thickness. For example, beams, flanges or plates, etc.
  • When there is a place available for bolt head and nut.
  • There is a place available for a spanner.

advantages and disadvantages of screws and bolts fastener

  • Screws are cheaper compared to bolts.
  • Bolts are good for frequent dismantling and reassembling, unlike screws.
  • Bolts carry the load on a larger shank area when compared to the screw.