Bolts (Fastener): Definition, Types, Parts, and Uses

What is a Bolt?

A bolt is a form of threaded fastener with an external thread that requires a matching preformed internal thread such as a nut. Bolt is very closely related to screws.

Bolts are headed, externally threaded fasteners that are used to hold two objects, often made of metal or wood, in a fixed position relative to one another. Unlike screws, which cut their own threads, bolts mate with the internal threads of nuts to stay in place and maintain the desired tension in the bolted joint.

The use of nuts will help with disassembly and reassembly. In addition, bolts generally have a higher load-bearing capacity than screws but do not have a tapered shaft. The shape of the head of the bolt determines the tools required for tightening and loosening. Bolts are responsible for holding everything together, from furniture to machines to bridges.

Parts of Bolt

Parts of Bolts

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

  1. 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 of head.
  2. 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.
  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. 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.
  5. Thread length: It is the part of bolt that accommodates the nut.
  6. 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.

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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 create its own thread in material as it turns.

Bolts vs Screws

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.

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 used to connect structural and non-structural elements to concrete. The connection can be made using a variety of different components: anchor bolts, steel plates, or stiffeners. Anchor bolts transmit different types of loads: tensile forces and shear forces.

A connection between structural elements can be represented by steel columns that are attached to a reinforced concrete foundation. A common case of a non-load-bearing element attached to a structure is the connection between a facade system and a reinforced concrete wall.

2. Blind bolts

A blind bolt is a structural fastener that provides more strength and conformability than a typical rivet or weld. They were designed to forge a strong connection where traditional rivets or hex bolts were difficult to assemble or just couldn’t do the job.

On many occasions, restricted access means that using rivets or hex bolts is out of the question, and blind bolts were developed to deal with that exact problem. The effectiveness of the blind bolt solution means that, for decades, they’ve been the first choice for restricted access work in sectors like manufacturing, construction, and repair work.

3. Carriage bolt

A carriage bolt is a form of Bolt that is used to fasten metal to metal, or more commonly wood to metal. Also known as a washer head screw in Australia and New Zealand.

It differs from other bolts by its flat mushroom head and the fact that the cross-section of the shank, although circular for most of its length (as with other types of studs), is square just below the head.

This makes the bolt self-locking when inserted through a square hole in a metal band. This allows the fastener to be mounted from one side with just one tool, a wrench or wrench. The head of a carriage bolt is usually a flat dome. The shaft has no threads, and its diameter corresponds to the side of the square cross-section.

The carriage bolt was designed to be used through an iron reinforcement plate on either side of a wooden beam, with the square portion of the bolt fitting into a square hole in the ironwork. It is common to use a carriage bolt on bare wood, with the square section providing enough grip to prevent rotation.

The lock bolt is often used in security fastenings such as locks and hinges where the bolt only needs to be removable from one side. The smooth, domed head and the square nut underneath prevent the carriage bolt from being unlocked from the unsafe side.

4. Arbor Bolt

Arbor bolts are specifically designed for use with power tools, typically miter saws. These bolts hold the tool securely and keep the blade in place. The arbor is the shaft that holds the blade. On all American stationary saws that use a 10 “blade, the arbor is 5/8” in diameter where the blade is attached.

Arbor bolts differ from other bolts in that they are delivered pre-assembled. All arbor bolts have a washer that is permanently attached to the tip of the bolts. The enclosed washer gives the Arbor bolts a very distinctive appearance right from the start. These bolts also have a head with a sunken design that creates a noticeable ridge around the head of the bolt.

Typically, arbor bolts are made with a dark or black finish. They are also made with reverse threads. When you look at the shaft of an arbor bolt, it looks like it is backward as it has an inverted tread pattern like other types of bolts. This reverse thread is very important and is part of the main function of the arbor bolt.

5. Double-end bolt

Double-end studs are threaded fasteners that are threaded at both ends with an unthreaded portion between the two threaded ends.

They are mostly used when connecting flanges or pipes. The thread length varies depending on the requirement. The bolts are equally long threaded at each end to accept a nut and are Class 2A threads.

The length of the bolt is measured in total. Double-end studs are used for bolting on flanges or other applications where double-sided burning is desired.

6. Eyebolt

An eyebolt is a mechanical fastener with a threaded shaft and a head forming a ring. Eyebolts are used to attach an eye to a structure, through which rope, cable, or shackles can be secured. A common use is to create a lifting eye so that a crane can be attached to machinery, with special-purpose lifting eyes rated for their safe working load.

Low-strength eyebolts are often formed from a length of the bar with the diameter of the nominal thread size. The head is simply bent into a ring from an unthreaded section of the bar. These types of eyebolts can carry axial loads reasonably well but should not be used for off-axis loads.

The opening where the end of the bar is closed to complete the ring may be welded but is likely to remain a possible point of failure.

7. Hex bolt

Hex cap screws and hex bolts feature a hexagonal head that is used when tightening the fastener. A hex bolt uses a washer under the head and a beveled end, while the hex bolts lack these features and require a nut.

These fasteners are widely used in construction, machine parts, and maintenance repairs. Hex cap screws and hex bolts are manufactured in a variety of sizes, finishes, qualities, and materials depending on the application.

8. Machine bolt

Machine bolts are used to join two pieces of material together and are similar to hex bolts, except that they do not have a chamfered point or washer-support surface on the underside of the head. They are usually available with both a hexagonal and a square head.

Another fastener, machine screws, is often used interchangeably with machine bolts, which creates confusion. Machine screws are typically smaller than machine bolts, but they are usually evenly threaded along the length of the fastener.

Unlike other types of screws such as wood screws or sheet metal screws, the fasteners have a flat tip rather than a pointed or conical tip, as is common with these other types of screws. Screws are generally designed to either be used with pre-drilled holes or to be driven into materials where the screw creates its own thread in the material when tightened.

In contrast, bolts are designed to be tightened by applying and torquing a mating nut to the end of the bolt. While nuts and bolts are paired fasteners, nuts are typically not used with bolts that are instead screwed into pre-drilled holes.

Both machine bolts and machine screws come in a variety of head styles, including hex head, which is often combined with hex nuts, as well as flathead, oval head, and cheese head, to name a few.

9. Penta Head bolts

Penta bolts are tampered-resistant and have pentagonal heads that cannot be loosened with typical tools from the hardware store. These fasteners are commonly used to secure manhole covers.

Pentagon bolts are an example of tamper-resistant bolts which may be used in applications that want to reduce the likelihood that a person armed with standard tools can loosen or remove the screw. Because the head of the screw is pentagon-shaped, a standard wrench or socket wrench set cannot be used with these bolts.

10. Roundhead bolt

Roundhead Bolts (button heads) are often used in wood connections. Roundhead bolts are similar to carriage bolts, but without the square neck under the head.

Similar to carriage bolts, button head screws do not have a square taper under the domed head of the bolt and are typically used to join wood. The softer nature of the wood allows the bolt to compress against the surface of the wood and grip by friction to prevent the screw from turning when the nut is tightened.

11. Shoulder bolt

A shoulder screw, also known as a shoulder bolt, has a shank with a larger diameter immediately below the head, which “steps down” down to a smaller diameter for the length of the thread.

The change in diameter creates a “shoulder” that can be tightened firmly against the screwed-on part. This allows a shoulder screw to be firmly attached to one part without clamping the other part, which creates axial clearance.

The shaft provides a smooth bearing surface for a bolt connection between the parts. A shoulder screw can be used in conjunction with a sliding bearing such as nylon bushing to allow improved rotation between the parts.

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.

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.

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