Bolt Selection Guide - How to Choose the Right Bolt

Bolts (and nuts) are a ubiquitous form of fastener that we are all familiar with. But selecting the right bolt for a job takes some consideration. In this post we provide everything you need to know when choosing your bolts. The most significant developments in bolt and screw production and use have taken place over the last 150 years. Although experts have various views regarding the origins of nuts and bolts the use of helical threads can be readily traced back to 400 BC. Archimedes (287 BC–212 BC) is known to have developed the screw principle and used it to construct devices to raise water. But there is evidence that this form of water screw may have actually originated in Egypt, before the time of Archimedes. Screw threads are known to have been historically used in various devices such as the presses for grapes in wine-making. It has been argued that the threaded bolt with a matching nut, used as a fastener, dates back to the 15th century. But a form of unthreaded bolt is known to have been used by the Romans who are also credited with developing the first screws, made from bronze or silver. What are Bolts? Bolts are a form of threaded fastener with an external male thread that mates with a matching pre-formed female threaded nut. Bolts have three primary sections: the head, the shank and the threading. Some bolt descriptions also refer to the chamfer, which is the tip of the bolt that’s sometimes bevelled to make it easier to insert. The bolt head is the widest diameter section of a bolt, available in a variety of designs. The bolt head provides the surface that bears much of the load when bolts are tightened. Bolts are commonly used in conjunction with various forms of washer that distribute the load and can help prevent bolts from becoming loosened. The shaft of the bolt is called the shank. This is sometimes threaded over its entire length or it may be split into two sections: an unthreaded ‘body’ section along with a threaded section. The threading on a bolt is the helical groove that mates with a matching nut. Helical threading is what enables bolts to be tightened and loosened, as required. Bolts and nuts operate using friction and torque. When tightened, by applying torque, nuts and bolts are held together due to friction between the two components. Nuts and bolts may loosen due to vibration or the loads they are supporting. This can be overcome using locknuts or lock washers. A question that’s often asked is: what’s the difference between a bolt and a screw? The widely accepted differentiation is that a bolt is a non-tapered fastener that uses a nut (and sometimes a washer) to hold objects together. While a screw is a fastener that mates with an existing thread or creates its own thread in a material as it turns. Bolts and Machine Screws Another area of confusion exists regarding the difference between bolts and machine screws. Machine screws, like other fastenings, are used to securely join two or more objects. Although they are widely used, there is no single, agreed definition for machine screws. Characteristics of machine screws include: They are smaller in size (both length and diameter) than many other fastener types. The American Society of Mechanical Engineers (ASME) defines machine screws as featuring a diameter of up to a maximum of 0.75 inches. Unlike standard screws, machine screws are generally blunt tipped. Machine screws are generally (but not always) fully threaded. Which means the helical thread runs the full length of the shank. Machine screws are generally intended for use with pre-drilled and tapped holes. But they can also sometimes be used with nuts, like bolts. Machine screws are most widely used for securely fastening metal parts together. They are used in machinery, construction, automobiles, engines, tool assembly, electronic devices, and large-scale industrial equipment. Bolts and Set Screws The term ‘set screw’ is often used in reference to bolts. In the UK, the term "set screw" or "setscrew" simply refers a bolt which is fully threaded along its entire shank. In the U.S. this type of fastening is called a cap screw. Set screws are most commonly used without nuts, which is what distinguishes these fasteners from bolts. They attach to pre-drilled and tapped holes in just one of the two objects being fixed together. Some forms of set screw are entirely headless enabling the fastening to be secured without any protruding head. This type of set screw can be called a grub screw or blind screw. Where are Bolts Used? Nuts and bolts are used just about everywhere. Bolts, along with a wide variety of other fasteners, are widely recognised as a reliable fastener and used in a huge variety of applications and environments. They are available in various shapes, sizes and materials. The largest bolt ever made was in 1967 by the Penrith Engineering Works company in Scotland. These fasteners were manufactured for SWISSOIL in an experiment to boost the capacity of oil tankers by bolting two of them together. These massive bolts were 8.3m (27.4 feet) long, 1.27m (4.2 feet) in diameter and weighed in at 12.6 tonnes. At the other end of the scale, micro-fasteners are becoming increasingly tiny. Scientists have developed a mechanism to guide the deposition of gold atoms on pre-made rod shaped nanoparticles in a spiral structure. These "nano screws" are described as screws smaller than dust. Some of the areas in which nuts and bolts are typically used include: Prisons Hospitals Airports Stations Councils Fabricators Schools Telecoms Police Departments Playground Manufacturers Civil Engineering Designers Public Utilities MOD (Ministry of Defence) Sporting Venues Architects Retail Shop Fitters Electronic systems Museums And many more… Bolt Dimensions When considering the bolts needed for a particular application its vital to correctly determine the bolt dimensions. Primary bolt dimensions are the diameter, length and thread pitch. The diameter refers to the width of the bolt shank. This important dimension doesn’t refer to the bolt head. The bolt length generally refers to the length of the shank, from the underside of the head to the tip of the bolt. But its important to note that bolts with a countersunk head are measured over their entire length. And the thread pitch defines the distance between the apex of adjacent threads on the bolt shank. Pitch is sometimes defined by the number of threads per unit length. These dimensions are measured in both metric and imperial units and its important to remember they cannot be mixed. Metric bolts don’t mate with imperial nuts and vice versa. Bolt dimensions are an important aspect of their strength and load bearing capacity. In the UK, Europe, China and other manufacturing centres such as Taiwan, metric bolt sizes are most commonly used. But in the USA and Canada they use Unified Thread Standards (UTS), based on imperial measurements. Bolt Materials Another important bolt attribute that affects their strength and application is the material they are made from. Bolts can be made from a variety of materials each with varying properties and applications. Plastic bolts have a variety of low load applications and may be used in environments involving water, such as swimming pools. They are relatively inexpensive and don’t corrode. Aluminium bolts are also low cost, lightweight and corrosion resistant. They are also non-magnetic and are often used in consumer electronics. Brass bolts are strong, corrosion resistant and conduct electricity. Brass is also often a component in corrosion resistant alloys. Copper bolts are used in electrical applications, medical equipment, plumbing and marine applications. Copper alloys have a good load capacity and are suitable for use near magnets. Mild steel bolts are strong, but uncoated steel is vulnerable to corrosion. Steel bolts are commonly available in galvanised or bright zinc plated (BZP) finishes that resist corrosion. Hardened steel bolts are stronger than mild steel, but more brittle. Steel is hardened by tempering and quenching. Stainless steel bolts are chemical and corrosion resistant and provide an aesthetically appealing appearance. A2 stainless steel is easy to machine and A4 stainless steel is a harder, marine grade option. Titanium bolts are lightweight, hard and strong. They are corrosion resistant and when alloyed with other metals, titanium increases strength and durability. Bolts made from various superalloys provide excellent mechanical strength, corrosion resistance, temperature tolerance and surface stability. A variety of bolt finishing options also available including various forms of plating (as noted) along with brushed and polished. Bolt Standards British Standard Whitworth (BSW) is an imperial-unit-based screw thread standard, devised and specified by Joseph Whitworth in 1841 and later adopted as a British Standard. It was the world`s first national screw thread standard and is the basis for many other standards, such as BSF (British standard fine) and BSP (British standard pipe). Whitworth bolts and nuts are still commonly used by classic car enthusiasts for their vehicle restoration projects. In the UK, Europe and most of the world the metric system is used to specify fasteners. Metric fasteners are usually classified as either coarse or fine, referring to their thread pitch, although some fastener sizes are available in an extra fine thread. The ISO metric screw thread is the most commonly used type of general-purpose screw thread worldwide. This was one of the first international standards agreed when the ISO (International Organization for Standardization) was established in 1947. Metric fasteners are commonly manufactured to meet standards set by DIN (German Institute for Standardization), or ISO (International Organization for Standardization). Metric bolts are often defined like this: M12x25. M: Indicates this is a metric thread. 12: Refers to the diameter of the bolt shank (12mm) 25: Refers to the shank length (25mm). This form of specification refers to a coarse threaded bolt which is the standard and most commonly used thread pitch. A bolt specification that includes the thread pitch looks like this: M8-1.0x 20. M: Indicates this is metric. 8: Defines the shank diameter as 8mm. 1.0: Is the pitch (distance between threads) of 1mm. 20: Is the shank length in mm. Types of Bolt Bolts are available with a wide variety of head shapes and in various designs, often specific to certain applications. The most common bolt head shapes and types include hex, square, slotted and socket bolts. Here`s a handy bolt type reference that will help when selecting the right bolt for a job. Hex Bolt or Set Screw Hex Bolts are characterised by their hexagonal head with threads that mate with a nut or tapped hole. Allen Bolt Allen bolts are distinguished by their hexagonal socket head. They require a matching Allen key to tighten and loosen them. Masonry Bolt Self tapping masonry bolts are used to securely attach things to masonry and concrete. Coach Bolt Also known as carriage bolts, coach bolts have a smooth rounded head with a small square section underneath. Tri-Head Bolt Tri-head bolts are also known as triangular head security bolts, triangular head security screws or security recess screws. The head features a distinctive flange and requires a matching tool for use. U-Bolt U-bolts have this distinctive `U` shape. They are partially threaded at either end and typically used to support pipework, poles or similar structures. J Bolt J Bolts are characterised by their distinctive `J` shape. Threaded on the longer length and typically used in structural applications. Roofing Bolt Roofing bolts have this distinctive round head. Commonly used to secure roofing material. Saddle Bolt Saddle bolts have this distinctive curved head shape. Commonly used to secure various forms of fencing. Expansion Bolt Expansion bolts are typically used in masonry substrates. As the bolt is tightened the anchor section expands in the predrilled hole, securing the bolt in place. Rambolt Asphalt Fixing Asphalt fixing bolts are a special type of expansion bolt specifically designed to securely attach objects to asphalt. Chemical Resin Stud Chemical resin studs or anchor studs are used to achieve high strength fastening to concrete using Polyester or Vinylester Resin. Pin Hex Security Bolt Similar to a standard allen socket head bolt but with the addition of a security pin that prevents the fastening from being tampered with using standard Allen keys. Also known as Tamper Pin Hex bolts. Pin Torx bolts are also available with the Torx head type along with a central security pin. Scroll Bolt Scroll bolts are high security fastenings that don`t offer any sharp edges that might be used to grip and loosen the bolt. These bolts can only be used with matching Scroll driver socket tools. Pignose security bolt Pignose security bolts, also known as snake-eye security bolts or two-hole security bolts, are another security fastening that require a special tool. Shear Bolt Shear bolts are a high security fastening that`s ideal when robust, permanent, high security fixing is required. Once tightened the hex section of the head simply shears off under torque. Shouldered Bolt A shouldered bolt is characterised by the shank that has a larger diameter that reduces to a smaller diameter for the threaded length. Selecting the Right Bolt When selecting the most appropriate bolt for a specific task its important to consider safety critical factors. Sometimes it will be necessary to consult with relevant experts to determine the required bolt specifications. The criteria that need to be considered include: The environment in which the bolt will be used. The size of the bolt. The material the bolt is made from. The load the bolt will need to withstand. Any special requirements such as anti-tamper, security or aesthetic appearance. In most cases answering the following questions will determine the best bolt for the job. In what sort of environment will the fastener be used? For example, indoor or outdoor or possibly marine, where it will need to withstand the corrosive impact of salt water. What size of bolt is needed? Its important to get the dimensions right. Consider the bolt shank length and diameter as well as the threading. When replacing bolts its important to match the required bolt dimensions. What should the bolt be made from? As noted, bolts are available in a variety of materials suitable for a range of applications and environments. Its important to consider exactly which bolt material is right for the job and be aware of how the bolt material affects the load the bolt can take. What type of bolt is required? As demonstrated in the previous section, there are many bolt types so its important to select the right type for the job. What is the load on the bolt? The bolt material, size and design are all important factors affecting a bolt’s ability to withstand stress and load. Its always important to ensure selected bolts provide the strength required. Does the bolt need to have security features? Some bolts, such as pig-nose and scroll bolts, require specialist tools to install and undo them, making them less susceptible to tampering. Will the bolt be permanently installed? Shear bolts offer the ultimate in security as once they are tightened the hex head is sheared off, making them very difficult to remove. If the bolt is a permanent fastening then it may be sensible to consider shear bolts. What should the bolt look like? Aesthetic aspects of bolt selection often need to be considered. As noted, bolts are available in wide variety of designs and finishes to suit all applications. If you have any questions about bolts or fastenings and which options are appropriate for your needs, remember we are here to help. Give us a call on 01273 475500 and we’ll provide you with free, expert advice.

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