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Last edited 26 Apr 2018
Types of bolts
A bolt is a type of fastener, usually made from metal, that commonly comprises a head at one end, a chamfer at the other, and a shaft characterised by an external helical ridge known as a ‘thread’. Bolts are typically used to hold materials or objects together, or to position objects.
The chamfer at the opposite end of the head provides a slightly bevelled edge which helps with inserting the bolt into holes and nuts. Bolts typically (but not always) require a nut which is applied via torque while the bolt is held in place (or vice versa). Vibration or dynamic loads may loosen nuts, necessitating the use of locknuts, lock washers or thread lockers which can provide resistance to loosening.
The terms ‘bolts’ and ‘screws’ are sometimes used interchangeably, and the distinction between them can be complex, however, in simple terms, a bolt passes through the materials it is fixing together (typically into a nut at the other side), whereas a screw threads directly into them.
 Types of bolt
There are many different varieties of bolt which can be selected based on the particular requirement or the materials involved. Some of the most common types include:
 Anchor bolt
 Carriage bolt
Used to fasten metal to timber, with a squared undercut to the head which holds the bolt in place once it has been tightened.
Commonly used in conveyor systems, an elevator bolt has a flat, plain or countersunk head which holds the bolt in place when tightened.
 Hanger bolt
This type of bolt comprises two threaded ends instead of having a head, one of which contains a wood screw.
 Hexagon bolt/Tap bolt
A hexagon bolt comprises a head that has six sides, with threading that begins part-way down the shank, whereas a tap bolt’s shank is threaded the whole length.
 Lag bolt
 Machine bolt
This type of bolt has a short shank and is intended for assembling metal components through predrilled holes.
 Plow bolt
 Sex bolt
 Square head bolt
This is similar to a machine bolt in that it has a short shank, in addition to a four-sided bolt head.
 T-head bolt
Has a T-shaped head which can be gripped by a wench and can fit into a slot with ease.
 Toggle bolt
This type of bolt has an expanding wing-like nut which helps it to mount objects to walls.
 Bolt heads
Different types of head include:
- Square shoulder: A truss head, shaped so as to allow fastening with the least amount of surface obstruction, on a square shank which resists rotation.
- Indented hexagon: For use with a wrench, this head has a circular depression in its top surface.
- Indented hexagon washer: Same as an indented hexagon, but with the addition of a washer section at the base to protect the assembly finish from the wrench.
- Hexagon (trimmed): Standard type of head, with clean corners that are trimmed to close tolerances.
- Hexagon flange: Similar to the indented hexagon washer, but the washer is conical or slightly rounded.
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