Screws and bolts both belong to the family of threaded fasteners. They were some of the early fasteners in use and before the advent of a wide variety of fasteners suited for different applications, the terms screws and bolts were often used interchangeably.
A crude way of differentiating a screw and a bolt is to state that a threaded fastener with a nut and a flat bottom is a bolt and one without is a screw. However, the most suitable way to distinguish a screw and a bolt is comprehending that, in essence, the main difference between a screw and a bolt arises in the way they are used.
Screws and bolts are both externally threaded fasteners. Bolts are often restricted from turning when part of an assembly, and have to be loosened by applying torque on the nut. On the other hand, screws can be placed within pre tapped holes and can penetrate materials. They are often fastened onto the materials by applying a torque onto the screw head.
Types of screws and bolts
Screws are predominantly used to fit materials together and to hold objects in place. They are distinguishable due to the presence of a circular/helical ridge, denoted as the male (external thread).
Some screws have threads that are solely designed to pair with a compatible thread, i.e. the female thread. Others are made with the purpose of penetrating into soft materials when the screw is being injected.
Primarily, screws are able to function due to the presence of the threaded part, which allows them to perforate materials. This functionality in turn gives rise to the use of screws in joining surfaces together and keeping them in place.
The various types of screws can be divided into different categories based on the driving method of the screw, the head shape, as well as what job requirement it fulfils.
Bolts are made of materials like steel, iron and other type of metals as well as wood and plastic. During installation, bolts need a nut and a pre-existing hole to carry out their required functionality.
Bolts are made up of the head at the top, followed by a shank and a chamfer at the end. The head serves a platform for tools to be able to provide a torque or withstand it. The shank is the longest part of the bolt and contains the threads. Finally, the chamfer allows the bolt to be inserted into a pre-tapped hole or nut.
Bolts are divided into different categories based on their head shapes as well as job requirement. Some bolts are also known for the shape they take, such as the U bolts and J bolts.
Classifying screws by driving methods
|Screws with slotted heads. Considered the oldest and one of the most commonly used. A linear diagonal slot on the head aids with screw driver applications.||They have crossed shaped slots on the head. Require a Philips-head screwdriver. Such heads lower chances of wear and tear, as well as slipping.||Some screws have a combination of a slotted and a Philips drive, the slot does not extend up to the edge and the shape is in between that of the two drive types.|
|Square||Socket, Hex or Allen||Star|
|This type of screw has a square indentation on the head that lessens the possibilities of a slip. Also referred to as Robertson drive.||These type of screws have hexagonal shaped holes that is typically used with an Allen wrench. Tightening or loosening is done with a socket wrench.||Six pointed star pattern indented on the head. These head types help in prevention of stripped heads.|
Classifying screws by head shapes
|Made of countersunk head with a flat top.||Made of countersunk head with a round top.||Made of round head and smooth vertical edges.|
|Round||Hex||Flange or Hex Washer|
|Made of a dome shaped head.||Made of a hexagonal head.||A hexagonal head attached to a washer.|
|Cylindrical head sockets used with socket drive.||Round head sockets used with socket drive.||Rounded screws with a broad head.|
Screws with different head types and driving methods
Figure 2: Illustration of different screws with different head types and driving methods (Blog.mutualscrew.com, 2018)
Classifying screws by job requirement
|Wood Screw||These screws are used in wood and come in various head types and sizes. Made of coarse threads to get a good grip when penetrating wood.|
|Sheet Metal Screw||These screws also come in various head types and sizes and full threads along the length of the screw aid in applications concerning sheet metals. Also used in applications that necessitate the joining of different materials together.|
|Self-Tapping Screw||Can be in the form of sheet metal, wood or drywall screws. These screws drill their way into the material, creating their own holes. Removes the need for pre-drilling.|
|Lag Screw||One of the toughest fasteners in use today. Used to attach heavy materials such as lumber together. They are different from the generic wood, sheet metal and self-tapping screws.|
|MDF Screw||Medium density fibreboard screws aid in applications such as assembling bookshelves. They are usually harder than solid wood and therefore more troublesome to perforate. Requires pre-drilling.|
|Machine Screw||Wide use in a number of applications. Used for fastening and keeping materials with pre-drilled holes in place. Also used in electrical connections, terminal strips, and motor casings.|
|Chipboard Screw||Commonly used with chipboard. Used to hold together chipboard of different densities - such as low, medium and high. Fully threaded shank aids in its application.|
|Decking Screw||Made up of coarse threads and a smooth shank much like wood screws. Made to resist rust and corrosion. Aids in fastening decking and used with pressure-treated wood decking.|
|Set Screw||Screws with no head that are used to go all the way into threaded holes. Can hold an object to another object without the use of a nut.|
Classifying different bolt types
|Carriage Bolt||Used to attach metal to wood. Square undercut of bolt head to ensure bolt is in place when fixed in place.|
|Hex Head Bolt||Consists of six sided head. Commonly used for applications in wood or metal.|
|Shoulder Bolt||Used in pulleys, gears and other moving parts as it allows for movement of linked components.|
|Structural Bolt||Typically have short thread length than hex bolts. Used in structural applications.|
|Square head bolt||Use in industrial, agricultural and construction industry. Square shape facilitates use in tight areas.|
|J bolt||Use in structural connections to fasten walls to foundations made of concrete.|
|U bolt||Use in pipework through which fluids such as liquids and gases pass through.|
|Elevator bolt||Used in elevators and conveyor belt systems. Large head diameter prevents bolt from penetrating conveyor material.|
|Lag bolt||Also known as lag screws. Does not require pre-tapped holes as makes own thread in wood during penetration.|
|Flange bolt||Consists of a washer on the underside of the head of the bolt. Helps in distribution of load on bolt.|
|Anchor bolt||Used in structural applications for fastening materials to concrete or masonry.|
|Plow bolt||Very durable bolts due to flat head followed by a square shank. Use in construction.|
|Timber bolt||Use in big wood planks and structures. Commonly used in industries of marine and lumber.|
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