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.
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
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.
|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.|
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||–|
Finishes on a bolt is described as the coating used on a bolt material. Depending on the kind of application required, finishes can be of very high to minimal quality.
An anti-corrosive finish is a common example of a very high quality finish whereas decorative finishes which serve the purpose of giving the bolt some color, are classified as low quality finishes. At the very least, some bolts may be available with no coating.
There are various finishes for bolts in use today and they are classified as follows:
|Plain finish||No coating. Also called natural, self-colour, black. When a finish isn’t specified it is automatically understood that the bolt has no coating. Plain finish products provide no resistance against corrosion.|
|Zinc plated||Zinc coated bolts. Thicker coatings provide higher resistance to corrosion and vice versa. Widely used as it is inexpensive. Coating is smooth and the finish looks good on bolts. These finishes are also clean to handle and provides a surface for painting, if required.|
|Zinc and clear chromate||Also known as zinc, clear zinc, bright zinc or clear. Very silvery in appearance. Offers resistance to corrosion.|
|Zinc and yellow chromate||Also known as zinc and yellow, z/chr, or zinc and chromate. This mixture is heavier than that of clear chromate and therefore offers better resistance to corrosion.|
|Hot dipped galvanized (HDG)||A process in which bolts are submerged in molten zinc. Zinc sticks to fasteners when dipped. Coating thickness is much more than that of zinc plating due to submerging of bolts. Therefore, provides superior resistance to corrosion.|
|Class 3||Bolts that have a zinc/tin coating. Corrosion resistance properties similar to that of hot dipped galvanized finish. Provides better socket fit than HDG.|
|Cadmium plated||Not used in present day due to environmental issues surrounding the poisonous cadmium. Replaced by zinc plating.|
|Decorative finish||Fundamentally used to produce a decorative or colorful outlook. Does not provide resistance to corrosion. Ideal for indoor applications. Important examples of such finishes include bronze metal antique (BMA), florentine bronze, brassed and nickel plated.|
Selection of a suitable fastener material for a particular application is vital. This is because bolts are used for various heavy duty as well as light weight applications and therefore the materials of these bolts have to be chosen accordingly.
A bolt made of steel as opposed to one made of aluminium can hugely affect the quality and durability of the joint it forms. Other factors such as environmental situations, presence of corrosive components as well as structural stability can alter a material’s effectiveness.
Some common bolt materials include:
Most commonly used material due to high formability, tensile strength and durability. Is inexpensively fabricated and widely available.
Available in various alloys to provide varying degrees of strength to suit different applications.
An alloy that mixes the attributes of low carbon steel and specific amounts of chromium and nickel.
Martensitic stainless steel is strong and durable and can be strengthened via heat applications but provide low resistance to corrosion.
Austenitic stainless steel is the most common kind of stainless steel. It is made up of high levels of nickel and chromium and therefore provides superior resistance to corrosion. This kind of stainless steel can also bear considerable loads without fracture.
Bronze and Brass
Bronze is an alloy made up of tin and copper. Provides high resistance to corrosion. Therefore, ideal for aquatic applications such as submarines, ships, boats and other underwater constructions.
Brass is an alloy made up of copper and zinc. It also provides high resistance to corrosion and forms a soft material.
A lightweight, synthetic material made of plastic.
Along with being corrosion resistant, it also has superior thermal and electrical insulating properties and can be easily dyed. However, nylon can melt if exposed to extremely high temperatures and can weaken in extremely low temperatures.
Everyday millions of bolts are keeping tall structures as well as small components in place. Their vast variety has emerged from the type of applications that demands their use.
There are various ways to classify the types of bolts, and all the considerations mentioned above make up the final choice of a bolt to be used in a certain application.
- Globalspec.com. (2018). Bolts Information | Engineering360. [online] Available at: http://www.globalspec.com/learnmore/mechanical_components/mechanical_fasteners/bolts [Accessed 29 Jan. 2018].
- Boltdepot.com. (2018). Fastener Type Chart. [online] Available at: https://www.boltdepot.com/fastener-information/printable-tools/type-chart.pdf [Accessed 29 Jan. 2018].
- Aallamericanfasteners.com. (2018). Bolts | AALL AMERICAN Fasteners. [online] Available at: http://www.aallamericanfasteners.com/fasteners/bolts [Accessed 30 Jan. 2018].
- Blog.mutualscrew.com. (2018). Different Types of Bolts. [online] Available at: http://blog.mutualscrew.com/2015/07/07/different-types-of-bolts/ [Accessed 31 Jan. 2018].
- Boltmasters.com.au. (2018). Fastener Finishes. [online] Available at: http://www.boltmasters.com.au/webfiles/BoltmastersAU/files/Boltmasters_Pty_Ltd_Fastener_Finishes.pdf [Accessed 31 Jan. 2018].
- Thomasnet.com. (2018). Fastener Material Selection. [online] Available at: https://www.thomasnet.com/articles/hardware/fastener-materials [Accessed 1 Feb. 2018].
- Featured photo by Josh Welton via Flickr, licensed under Creative Commons (the bike was made for www.yearofthechopper.blogspot.com) [Accessed 22 Feb. 2018].