Bolt(Fastener): Definition, Types, 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.

These types of fasteners are typically inserted through two parts, with aligned holes.

According to some definitions, whether something is a bolt or a screw depends on how it is used. A bolt is inserted through parts that all have unthreaded holes and a nut is then screwed onto the bolt to provide a clamping force and prevent axial movement.

A screw may first pass through a first part with a clearance hole but its threads mate with threads in one of the parts being fastened. A screw may cut its own threads or mate with a threaded part.

In practice, this definition is rarely used. The term bolt is usually used for a fastener that has only part of its shaft threaded. Fasteners with their entire shaft threaded are normally called screws. The unthreaded part of the shaft of a bolt is called the shank.

The shaft of the bolt prevents radial movement of the parts, while the head of the bolt and the nut if fitted, prevents axial movement.

The unthreaded shank provides an interface with the parts that are more precise and less abrasive. The shank also does not contain stress concentrations that could lead to failure, it is therefore important that the shank extends well beyond the interface between parts if a significant shear force will be placed on the bolt.

A bolt fastening a part to another part with a threaded hole, technically in this application, it is acting as a screw.

Bolts often rely on axial force causing sufficient friction at the threads to remain in place. A torque is applied to the head to generate this axial force. The force acts between the bolt head and whatever the bolt is screwed into, whether that is a nut or one of the parts being fastened.

This causes elongation of the bolt and compression of the parts containing clearance holes. Alternatively, some form of locking nut or thread-locking adhesive may be used to prevent the bolt from loosening.

Parts of Bolt

Parts of Bolt

As shown above, Different parts of the bolt describe as following.

  1. Head: It is the part of a bolt from where a spanner can hold it (to make it tight or loose).
  2. Shank: A bolt can broadly divide into two parts head and shank. Shank is partially threaded (as shown in the figure) to accommodate a nut.
  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. Thread length: It is the part of bolt that accommodates the nut.
  5. 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 creates its own thread in material as it turns.

Bolt and Screw

Machinery’s Handbook explains that 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.

Some objects have pre-threaded, while others create the threads when the screw is installed. So, the basic difference between screws and bolts is that the former is used to assemble threaded objects, while the latter is used to assemble untreated objects.

However, screws can make their own threads during the installation process. It’s also worth noting that screws need to be turned to assemble a joint, while the bolt can be tightened with a tool or carriage bolt.

Bolts are typically used to create a bolted joint by using a nut to apply force while the shank is used as a dowel. As a result, the joint is essentially supported against sideways forces. For this reason, many bolts have an unthreaded shank (known as the grip length); This will make them more effective for dowels.

There are dozens of different types of bolts, some of which include anchor bolts, arbor bolts, elevator bolts, hanger bolts, hex bolts, J bolts, lag bolts, rock bolts, shoulder bolts, and U bolts.

In addition, bolts are available in a variety of materials including steel, stainless steel, bronze, brass, and nylon. However, statistics show that up to 90% of all bolts are made of steel, making it the preferred choice among manufacturing companies.

There are also dozens of different types of screws, include chipboard screws, particleboard screws, deck screws, drive screws, hammer drive screws, drywall screws, eye screws, dowel screws, wood screws, twin fast screws, security head screws, and sheet metal screws.

Some of the different head shapes that screws come in are pan, button, round, mushroom, oval, bulb, cheese, larynx, and flange head shapes. And like their bolt counterparts, screws are available in a range of materials.   

Types of bolts

There are many different types of bolts, each having different characteristics that make them suitable for use across a wide range of applications. The principal types of bolts commonly used include:

  • Anchor Bolts
  • Blind Bolts
  • Carriage Bolts
  • Double End Bolts
  • Eye Bolts
  • Flange Bolts
  • Hex Bolts
  • Machine Bolts and Machine Screws
  • Penta-Head Bolts
  • Round Head Bolts
  • Shoulder Bolts
  • Socket Head Bolts
  • Square Head Bolts
  • T-Head Bolts
  • U-Bolts

Below are summaries of each of these common types of fasteners and a description of their important characteristics and uses.

1. Anchor bolt

Anchor bolts are bolts that are designed to secure a building element or component to a concrete slab or poured foundation.

Anchor bolts can be preset when pouring the concrete structure, is being cast, as with the plate used on top of the poured foundation of a house. Or anchors can be attached after the concrete has hardened by a drilling operation to create a channel into which the anchor bolt can be inserted.

2. Blind bolts

Blind bolts refer to a style of a bolt that enables a fastener to be used in cases where the application does not allow access to either side of the bolt to tighten or tighten the bolt. A simple example of this is the conventional toggle bolt, which is used to secure objects to a wall at a point between the support beams.

Here the part of the bolt that is behind the surface of the drywall is not accessible and therefore the entire assembly process of the toggle bolt must be carried out from the external (visible) side of the wall.

3. Carriage bolt

Carriage bolts are a form of self-locking bolts that provide a level of security by using a flush-mounted domed head that allows access to remove or loosen these bolts only from the nut side of the bolt.

Underneath the domed head of the carriage bolt, there is a square profile that can be inserted into a corresponding square cut that is slotted in the joining material to ensure self-locking, which means that the bolt can be tightened from the nut side without the need to remove the head with a wrench to hold on.

4. Arbor Bolt

The bolt permanently attaches to the washer and the threading is reversed. The Matter design is for use in saws and other means to harden the auto during use to prevent blades from falling.

5. Double-end bolt

Double-end bolts are sometimes called stud bolts and without conventional heads have a threaded part at each end of the bolt. One end design to be threaded into a suitable hole that is tapped with a mate and the other end is threaded to support the nut.

The appearance is similar to a threaded rod, but is not traditionally threaded over the entire length of the stud and can be threaded separately at each end.

6. Eyebolt

The eye bolt has a fully closed or partially closed-looped end in place of the conventional bolt head. The loop can in some cases use to lift the object to which the eyebolt is attached. Some eye bolts are not designate for lifting but may use to route wires, cables, or other similar elements to avoid accidental interference.

Eyebolts may offer additional capabilities depending on material structure and finish – some are suitable for low-temperature operation while others design to not reflect light and will therefore blend into the background.

7. Hex bolt

Hex bolts, also known as hexagon head bolts, are a very common form of bolt available in standard dimensional inches and metric sizes.

As the name itself suggests, these bolts have a hexagon-shaped head suitable for tightening with a wrench or socket. Hex bolts can fully thread or show an unread shoulder.

8. Machine bolt

Machine bolts use to fasten two pieces of material together, and are similar to hex bolts, except that they will not have a chamber point and design with a washer-bearing surface under the head has gone. They are usually available in both hex head and square head options.

9. Penta Head bolts

Penta-head bolts are an example of a tamper-resistant bolt, which can use in applications where it is desired to reduce the likelihood that a person equipped with standard equipment may loosen or remove the bolt.

With bolt heads in the shape of a pentagon, standard wrench or socket sets will be unable to be used on these bolts.

10. Roundhead bolt

Similar to carriage bolts, the round head bolt does not have a square taper under the domed head of the bolt and is typically used to attach to the wood.

The soft nature of the wood allows the bolt to compress against the surface of the wood and grip by friction to prevent the bolt from turning when the nut tightened.

11.Shoulder bolt

The shoulder bolt, also called a stripper bolt, is a threaded bolt section of a smaller diameter than the shoulder of the bolt (the part of the bolt between the head and the beginning of the threaded part).

Shoulder bolts are useful to act as a shaft or axle that may include a rotating part, such as a bearing or bushing.

Selection of Bolts Material

Following materials are generally used for manufacturing the bolts:

  • Nylon Bolts: They are lightweight and water-resistant
  • Bronze and Brass Bolts: They are water-resistant
  • Stainless Steel Bolts: They have good strength and are corrosion resistant
  • Steel Bolts: They have good strength
  • Titanium Bolts: They are strong, light, and corrosion resistant
  • Plastic Bolts: They are inexpensive and corrosion-resistant. They are generally used for light loads.
  • Copper alloy Bolts: They are wearing resistant and have good load capacity.
  • Aluminum Bolts– They are thermally and electrically conductive. They are light and easy to manufacture. Apart from the above material, sometimes finishing material is also applicable to the bolts. The finishing material provides durability and corrosion resistance to the bolt. Here are some finishing materials used for bolts.
  • Zinc: Its coating acts as a sacrificial anode, protecting the underlying metal. It applies as fine white dust.
  • Black oxide– Its coating mostly uses for aesthetic purposes. It does not enlarge the dimensions of the bolt. It processes black rust.
  • Chrome: Its coating gives a bright, reflective finish. It is decorative and very durable. It applies by electroplating.

Manufacturing of Bolt

There are three major steps in manufacturing a bolt:

  • Heading
  • Thread rolling
  • Coating

Bolts are normally made from wire. The wire is then cut to the proper length for the type of bolt being made. Heading produces the head of the bolt. The shape of the die in the machine dictates the features to be pressed into the bolt head, for example, a round head bolt uses a round die. The threads are generally produced via thread rolling. However, some machined.

Finally, a coating, such as electroplating with zinc or black oxide, is applied to prevent corrosion.

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.

Relative 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.


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.

What are the types of bolts?

The principal types of bolts commonly used include:
1. Anchor Bolts.
2. Blind Bolts.
3. Carriage Bolts.
4. Double End Bolts.
5. Eye Bolts.
6. Hex Bolts.
7. Machine Bolts.
8. Penta-Head Bolts.

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