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.
A bolt is a type of mechanical fastener that consists of a shaft with threads and is used to hold two or more parts together. The shaft of the bolt is inserted through aligned holes in the parts that need to be fastened, and a nut is then screwed onto the threads of the bolt to provide clamping force and prevent axial movement.
According to some definitions, whether a fastener is a bolt or a screw depends on how it is used. A bolt is inserted through unthreaded holes in the parts being fastened, while a screw may pass through a hole with threads that mate with threads in one of the parts.
In practice, however, the distinction between bolts and screws is not always clear, and the term “bolt” is often used to refer to any fastener with a threaded shaft.
Bolts are designed to prevent both radial and axial movement of the parts they are fastening. The unthreaded shank of the bolt provides an interface with the parts that are more precise and less abrasive than the threaded portion.
The shank also does not contain stress concentrations that could lead to failure, so it is important that it extends well beyond the interface between parts if a significant shear force will be placed on the bolt.
When a bolt is tightened, torque is applied to the head to generate axial force. This 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, a locking nut or thread-locking adhesive can be used to prevent the bolt from loosening.
Parts of Bolt
These are different parts of the bolt described as follows.
- 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 head.
- 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.
- Grip length: It is the part of the bolt that accommodates the parts which are to assemble. The grip length should be equal to the combined thickness of the joining parts.
- 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.
- Thread length: It is the part of the bolt that accommodates the nut.
- Nominal length: It is the sum of thread length and grip length (as shown in the figure).
Difference between Bolts 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.
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 a 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.
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.
Types Of Bolts
Below are different types of bolts.
1. Anchor bolts
Anchor bolts are used to secure structures to concrete foundations. They are typically made of steel and come in a variety of shapes and sizes. The most common type of anchor bolt is the L-shaped anchor bolt, which has a hook or bent end that keeps the structure in place.
They are also used to secure equipment to concrete or masonry surfaces. They can be installed either before or after the concrete is poured.
2. Carriage bolts
Carriage bolts are used to attach metal parts to wood. They have a smooth, rounded head and a square shoulder that digs into the wood when driven in. The square shoulder prevents the bolt from turning once it is in place.
They are commonly used in furniture and cabinetry construction. They have a more finished look and are less likely to cause injury than other types of bolts.
3. Eye bolts
Eye bolts are used to attach cables or ropes to a surface. They have a circular loop at one end, through which a rope or cable can be passed. The other end of the eye bolt is threaded, and the bolt is tightened into a drilled hole in the surface.
They are commonly used in rigging, lifting, and material handling applications. They come in a variety of materials, including stainless steel and galvanized steel, to suit different environments.
4. Hanger bolts
Hanger bolts are used to attach items to a surface. They have a machine screw thread on one end and a wood screw thread on the other. They are commonly used in furniture and cabinetry construction, to attach legs or other components to a wooden surface.
They are also used to secure electrical fixtures to wood. They come in a variety of sizes and are typically made of steel or stainless steel.
5. Hex bolts
Hex bolts, also known as hex cap screws, have a six-sided head and are used for a wide variety of construction and industrial applications. They come in a variety of sizes and lengths and are typically made of steel or stainless steel.
They are tightened with a wrench and are commonly used in machinery, equipment, and structures. They have a variety of grades, with the most common being grades 2, 5, and 8.
6. Square head bolts
Square head bolts have a square head and are used in applications where the bolt head needs to be tightened with a wrench. They are commonly used in heavy machinery and equipment, as well as in mining and construction.
They have a square-shaped head and a smooth shank and are made of steel. They are available in a variety of sizes and lengths and are typically used in conjunction with square nuts or square plates.
U-bolts are shaped like the letter “U” and are used to secure pipes and poles in place. They are typically made of steel or stainless steel and come in a variety of sizes and lengths. They are commonly used in plumbing, electrical, and transportation applications.
They are used to clamp pipes or other round objects to a surface. They are available in a variety of materials and finishes.
8. Lag bolts
Lag bolts, also known as lag screws, are large wood screws that have hexagonal heads. They are used to secure wood to wood or wood to concrete. They have a coarse thread and are typically made of steel, and come in a variety of sizes and lengths.
They are commonly used in deck and fence construction, as well as in heavy timber construction. They are also used to secure heavy machinery and equipment to wooden surfaces.
What is a Bolt Used 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 that is too weak to make durable threads.
- The parts that are fastened have medium thicknesses. 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.