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 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
This fastener is often used with nuts and washers. Bolts are made in assorted sizes and the diameter of the bolt increases with length.
Bolts have heads and threaded parts called shanks. The size of a bolt is associated with the diameter of the shank and the length.
Bolt types include:
Lag Bolts. Also known as lag screws, these bolts are typically used to connect heavy lumber and lag machinery to wood floors.
Anchor Bolts. Used in the construction industry to secure a structural component to a concrete slab or poured foundation.
Blind Bolts. For structural applications where access is restricted to one side, enabling the lock to still be completed.
Double-end Bolts. A bolt with threads on both ends, also called stud bolts. Used to fasten two flanges or pipes more effectively.
Carriage Bolts. A self-locking bolt with a domed head and square section underneath. Used mostly in hinges and locks to provide a level of security, permitting removal from the bolt’s nut side.
Eye Bolts. Used for lifting applications. Some are designed for heavy loads while others are for non-load bearing uses.
Hex Bolts. Characterized by its six-sided head, there are many different types of hex bolts. Typically used in machinery and construction applications and available fully or partially threaded.
Penta-head Bolts. Highly specialized bolt with five-sided head. Typically used for security on manhole covers and ground lids.
Shoulder Bolts. Performs as a shaft or axle that can hold a rotating part, such as a bearing. Also called a shoulder screw and stripping bolt.
T Head Bolts. Also known as a T-slot. Inserts into a recess and with the applied nut, the bolt is secured against turning. Common in construction and automotive.
U-bolt Bolts. Functions in applications ranging from supporting pipes to use on automotive drive shafts and exhaust systems.
J-Bolt. Also referred to as a hook bolt and is typically used in structural applications, such as roofing and securing walls to concrete foundations.
Elevator Bolts. Named after its original use in grain elevator systems. Today it’s used on assembly lines, furniture and various consumer products. Large circular head and low profile provide generous clearance
Flange Bolts. The flange acts as a washer to distribute the load. Commonly used to connect plumbing pipes, a vehicle’s transmission and engine and other mechanical applications.
Hanger Bolts. Connects two surfaces while hiding the bolt. Often used to suspend electrical conduits, fixtures, and sheet metal.
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.