Types of Bolts and Their Uses

Bolts belong to the family of threaded fasteners and are paired with a threaded nut when in use, usually to join two or more components together. Bolts are classified into different types according to their distinct head shapes, strengths, finishes and materials. Types of bolts and their uses.

These classifications all play a role in the final selection of a bolt type for a certain application. It is imperative to have the required knowledge to choose the correct bolt type for a specific job at hand, as there are various styles of bolts that are more effective than others.

Types of Bolts and Their Uses

Head Shapes

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 the hex heads have become more common. Hexagonal heads are used with a wrench or a spanner to provide a torque.

There are numerous other head shapes in use as well, namely flat, oval, pan, round, button and truss.

Bolt Types

Anchor bolts Bolts with a threaded part in one end and a non-threaded L shaped part on the other. Usually found with a washer and a nut. Made to be rust-resistant. Securing light poles, structural beams and to fasten equipment to concrete.
Carriage bolts Fully threaded bolts with a smooth head and a square or ribbed undercut that prevents the bolt from turning when tightened. Fasten metal to wood.
Elevator bolts Fasteners with a thin flat or countersunk head and a square undercut that prevents the bolt from turning when tightened. Elevators and conveyor systems.
Eye bolts Rod shaped fasteners with threads on one end and bent into a loop on another. Lifting applications such as wire and rope in light rigging.
Flange bolts Washer on the underside of a hex head. Helps in distribution of force from the bearing load. Also referred to as frame bolts. Frame applications such as truck and bed frames.
Hanger bolts Does not come with a bolt head. Both ends of the bolt are threaded. One of the ends is in the shape of a wood screw. In overhead applications and fastening metal to wood.
Hex bolts Bolts that are fully or partially threaded with six-sided heads. Broad range of use such as construction and repair of bridges, docks, highway elements and buildings.
J bolts Shaped like the letter J. Non curved section is threaded Structural applications such as fastening walls to concrete.
Lag bolts One of the toughest fasteners. Creates its own thread when it penetrates wood and other soft material. Connect heavy materials that are bearing extreme loads.
Machine bolts Square heads paired with a semi-cone point and a fully threaded shaft. Fastens wood to wood, wood to metal and metal to metal. Also found in general hardware use.
Plow bolts Similar to carriage bolts. Contains a countersunk flat head followed by a square undercut and a threaded shaft. Used in applications where top surface of where bolt is fastened to, needs to be smooth. Used in manufacture of plow in early days.
Roof bolts Made of a shaft that is to be placed in a pre-drilled hole. Tunnelling and underground mining. Provides support to the roof.
Shoulder bolts Also known as shoulder screws.
Consist of a long unthreaded, cylindrical shank that helps rotation of attached moving parts.
Pulleys, moving engine parts and mechanical assemblies, gears and rolling wheels.
Square head bolts Consists of a square head, followed by a smooth shank and a machine screw thread. Can also be fully threaded with the absence of a smooth shank. Design of the head facilitates easier wrench grip when tightening. All kinds of industrial, agricultural as well as construction applications.
Step bolts Shank may be plain or textured. High strength, threaded bolts. Used as steps for climbing on steel communication and electrical transmission towers.
Structural bolts Are similar to the standard hex head bolts but have shorter thread lengths, in order to be used in structural applications. In all types of structural connections.
T-bolts Has a T shaped head in order to be held by a wrench or to be easily fixed in place. Provides a long lasting connection. Use in buildings, instruments, furniture, automobiles, etc.
Timber bolts Sometimes referred to as mushroom head bolts or dome head bolts. Treated lumber, marine applications and wood construction.
Toggle bolts Sometimes referred to as a butterfly anchor. Full threaded shafts with a nut in the shape of an expanding wing. Hanging heavy elements in plaster and drywall.
U-bolts Shaped like the letter U. Threaded on both ends and non-threaded in curved section. Primary use to support pipework, especially pipes via which liquids and gases flow.

Bolt Strength

Bolts have markings to specify their strength, which depends on the material that the bolt is made up of and the dimensions that have been used to make the bolt.

Below is a table showing some typical materials via which common bolts are made and their corresponding markings. Materials mentioned are getting stronger progressively down the table.

Material Metric class marking Imperial grade marking
Low or medium carbon steel Grade 2
Medium carbon steel - tempered and quenched Class 8.8 Grade 5
Medium carbon alloy steel - quenched and tempered Class 10.9 Grade 8
Alloy steel - quenched and tempered Class 12.9
Stainless steel A-2


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