Types of Bolts – A Thomas Buying Guide

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Bolts are types of mechanical fasteners that are designed with external threads and used with a nut to join parts together using tightening torque. These fasteners are intended to be inserted through holes that are pre-drilled in the material being joined. Unlike screws, which are also externally threaded fasteners, nuts and bolts work together, meaning that bolts require a nut to be threaded on to it so as to function properly and require a drilled hole into which the bolt may be inserted.

The terms “bolting” or “types of bolting” are rarely used in connection with bolts in the context of fasteners, these terms being associated more with fabric bolting and bolting machines used in the fabric industry.

The Advantage of Nuts and Bolts

When compared with other types of fasteners, the use of nuts and bolts has the advantage that they can be disassembled as needed, while other fasteners such as rivets are designed for permanent installation. Bolts are often used with nuts and washers, where the washer provides an increase in surface area over which the attachment force is applied. There are a variety of different types of nuts that can be used with a bolt, the most common one being hex nuts. Other nut types include:

Locknuts
Slotted hex nuts
Flange nuts
Cap nuts
Coupling nuts
Thumb nuts (also known as wing nuts)
Square nuts
Panel nuts
Surface mount nuts

Nuts and bolts can be used to join different materials together including wood, sheet metal, steel, iron, and polymers. In other cases, such as with slotted hex nuts, they can be used with a mating bolt and a cotter pin to securely lock the nut in place.

Common hand tools or power tools are available to use with bolts and nuts, which include socket sets, open-end wrenches, box wrenches, and powered drivers. More information on socket sets can be found in our related guide.

Grades and Material Options for Nuts and Bolts

Bolts and nuts are available in a variety of different material options, depending on the intended application. Bolts, screws, and other types of fastener products are usually assigned a grade. The grade establishes the minimum acceptable standard of mechanical properties or performance that the fastener must meet. Higher grades imply bolts with greater mechanical strength characteristics.

Bolt Grade Markings

Grade identification marks are created and appear on the head of the bolt and are used to designate their grade. For commonly used ASTM and SAE steel grades, there are grade identification tables that designate these markings and specify the minimum acceptable values of proof, tensile, and yield strength that are required to meet the particular grade of bolt. Also noted are the typical material options and heat treatment used for the finished product.

For example, SAE grades for ASTM bolts range from Grade 1 to Grade 8.2. SAE Grade 1 bolts have a minimum proof strength of 33, a minimum tensile strength of 60, and minimum yield strength of 36 (all in 10³ psi). At the high end of the grade scale, SAE Grade 8.2 bolts would have a minimum proof strength of 120, a minimum tensile strength of 150, and minimum yield strength of 130 (measured in 10³ psi).

Similar grading exists for ASTM grades as well as for metric fasteners. In the case of metric bolts, ISO 898 defines the mechanical properties and grade definitions.

Common Material Options

ASTM/SAE grade bolts can be fabricated from a number of material choices. Most common are these material steels:

Low carbon steel
Medium carbon steel
Low carbon martensite steel
Weathering steel
Alloy steel
Medium carbon alloy steel

Bolts are also available in alternative material choices than the usual steels. These options include stainless steel, metal alloys such as brass and bronze, and polymers such as nylon or PEEK (PolyEther Ether Ketone). Stainless steel offers excellent corrosion resistance and strength, while the polymer options are used when intrinsic strength is of less concern but the need for a stable material that can resist chemicals is needed.

Material Treatment Options & Finishes

Both the use of heat treatments and the application of different finishes can be used to provide material stability and protection from corrosion and rust as a result of environmental conditions or exposure to corrosive substances. Treatment options include cold drawing and quenching & tempering.

Common finishes include:

Anodizing
Armor coating
Black oxide
Blue phosphate
Chrome plating
Hot-dipped galvanized
Yellow passivated
Zinc coating

Types of Bolts Video

This brief video provides a look at some of the different bolt types.

Standards for Bolts

Bolts are produced to a series of standards that assure uniformity in the dimensional characteristics and materials. When specifying standard bolts, the dimensional parameters should be designated per the tables of standards established by ANSI/ASME. These parameters include:

Nominal size in fraction or decimal equivalent
Number of threads per inch
Overall length
Material
Finish

For related information on screw fasteners, consult our guide covering types of screws.

What are the 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. Deliberately omitted from this presentation are items such as lag bolts. A lag bolt while being called a bolt by name is, in fact, more accurately described as a lag screw, given the pointed end and the typical use in softer materials such as wood. You can find more information on these types of fasteners in our related guide Types of Screws – A Thomas Buying Guide.

Anchor Bolts

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Anchor bolts are bolts that are designed to be used to attach a structural member or component to a concrete slab or poured foundation. Anchor bolts may be preset into place while the concrete structure is being cast, as with the plate used on top of the poured foundation of a house. Or, anchors may be attached after the concrete has been cured through the use of a drilling operation to create a channel into which the anchor bolt may be inserted.

Blind Bolts

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Blind bolts refer to a style of bolt that allows for the use of a fastener in cases where the application does not allow access to both sides of the bolt to tighten or torque the bolt. A simple example of this is with the common toggle bolt that is used to secure items to a wall at a location between the supporting beams. Here, the portion of the bolt that is behind the surface of the drywall is not accessible and so the entire installation process of the toggle bolt must be accomplished from the external (visible) side of the wall.

Carriage Bolts

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Carriage Bolts are a form of self-locking bolt that affords a level of security by using a flush-mount domed head, which permits access to remove or loosen these bolts from the nut side of the bolt only. Beneath the domed head of the carriage bolt is a square section that can be inserted into a corresponding square cut that is slotted in the material being joined so as to provide the self-locking feature that allows the bolt to be tightened from the nut side without a need to restrain the head with a wrench. The carriage bolt heads

Double End Bolts

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Double end bolts are sometimes called stud bolts and feature a threaded portion on each end of the bolt without a traditional head. One end is designed to be threaded into a suitable hole that has been tapped with a mated thread, while the other end protrudes and is threaded to support a nut. The appearance is similar to that of threaded rod but is traditionally not threaded over the entire length of the stud and may be threaded differently on each end. Some stud bolts, such as the one shown in the image above, use a screw thread on one end in lieu of bolt threads on both ends.

Eye Bolts

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Eye bolts feature a fully closed or partially closed-looped end in place of the traditional bolt head. The loop can be used in some cases for lifting the object to which the eye bolt is attached. Some eye bolts are not designated for lifting but rather can be used to route wire, cables, or other similar elements cleanly to avoid accidental interference. Eye bolts can offer additional capabilities depending on the material composition and finish – some are suitable for low-temperature operations while others are designed to not reflect light and therefore will blend into the background.

Eye bolts are available with or without a shoulder at the base of the loop. Other options include whether the loop is open or closed (including welded), whether the thread is internal or external, and whether it is designed with a pointed end such as a wood screw or intended for use with a nut to secure it. Both metric and inch sizes and a variety of lengths are available. Different sizes of eye diameter enable these fasteners to be used for a variety of purposes.

Flange Bolts

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Flange bolts are ones that contain a washer-like surface or flange beneath the head of these fasteners that provides a means to distribute the clamping load over a larger surface area, thereby reducing potential damage to the surface with which these fasteners will mate. Flange bolts are often used in applications such as plumbing and automotive. Flange bolts are traditionally fabricated from steel or stainless steel and are available in fine and coarse threads and in a variety of grades. The flange may be smooth or serrated, the latter of which helps the fastener grip the mating surface.

Hex Bolts

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Hex bolts, also called hexagon head bolts or hexagonal head bolts, are a very common form of bolt that is available in standard dimensional inch and metric sizes. As the name implies, these bolts feature a hexagonal head or a hex head, that is suitable for tightening with a wrench or socket. A hex bolt may be fully threaded or may feature an unthreaded shoulder. Hex bolts are often used to fasten wood to wood, metal to wood, or metal to metal. Hex nuts and washers are typically used with hex bolts, the washers being valuable for situations where the material being joined is softer and may deform under the tightening force applied to the hex bolt.

Hex bolts are sometimes referred to as hex cap screws when they use a washer face underneath the head and feature a chamfered end. Hex bolts lack those features.

Machine Bolts and Machine Screws

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Machine bolts are used to fasten together two pieces of material, and are similar to hex bolts, except that they will not have a chamfered point and are not designed with a washer-bearing surface on the underside of the head. They are typically available in both hex head and square head options. A different fastener product, machine screws, is often used synonymously with machine bolts which is a source of confusion. Machine screws tend to be smaller in size than machine bolts, but usually have uniform threading along the full length of the fastener. Unlike other types of screws such as wood screws or sheet metal screws, the fasteners feature a flat tip, not a pointed or tapered tip as is common with these other types of screws.

Screws generally are designed to be used either with pre-tapped holes or to be driven into materials where the screw will create its own thread in the material as it is tightened. By contrast, bolts are designed to be tightened by applying and torquing a mating nut on the end of the bolt. So while nuts and bolts are paired fastener items, nuts are not typically used with screws which instead are threaded into pre-tapped holes. Both machine bolts and machine screws are available in different head styles, including hex head which is frequently paired with hex nuts, as well as flat head, oval head, and cheese head, to name a few.

Penta-Head Bolts

Penta-head bolts are an example of a tamper-resistant bolt, which may be used in applications where it is desired to reduce the likelihood that a person armed with standard tools can loosen or remove the bolt. With the head of the bolt in the shape of a pentagon, standard wrench or socket sets will be incapable of being used on these bolts.

Round Head Bolts

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Similar in appearance to carriage bolts, round head bolts do not have the square taper beneath the domed head of the bolt and are typically used to join wood. The softer nature of wood permits the bolt to compress against the surface of the wood and grip by friction to preclude the bolt from turning when the nut is tightened.

Shoulder Bolts

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Shoulder bolts, also called stripper bolts, feature a threaded bolt section of smaller diameter than the shoulder of the bolt (the section of the bolt between the head and the beginning of the threaded portion). Shoulder bolts are useful for functioning as a shaft or axle that can contain a rotating part, such as a bearing or a bushing.

Socket Head Bolts

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Socket head bolts feature a recessed head that is tightened using an Allen wrench or hex socket tool. The head style of these bolts can vary from a cylindrical profile to a flat-head countersunk style to a domed button head design, depending on the desired use. The deeper recessed Allen socket head affords the ability to increase the torque applied to these fasteners while minimizing the risk of stripping or damaging the head.

Socket head bolts are often referred to as socket head cap screws, depending on the supplier.

Square Head Bolts

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Square head bolts, also called square bolts, were once very commonly used predating the creation of hex head bolts. While square bolts are still available, they are primarily used in railway applications or to achieve the aesthetic of an older or more traditional appearance.

The hex head bolt is much more commonly utilized given the prevalence of tool options such as socket sets, but many tools can still be used to tighten square head bolts. Square head bolts are available in low, medium, and high strength grades and have both a square head and T-slot head option available. Steel and stainless steel are the most common material options. Both fully and partially threaded options are available. Some suppliers refer to these using the term "screws” as well as bolts.

The square heads are sometimes fashioned to a decorative appearance so as to mimic the styles common in older construction. Square heads can be pyramid-shaped, hammered, or have an irregular appearance that reflects the way in which these fasteners would have been created by using hand-forging techniques before the age of mass production.

T-Head Bolts

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T-head bolts or T-slot bolts feature a head design that enables it to be inserted into a slot or recess, thereby securing the bolt in place to prevent it from turning when the securing nut is tightened. T-bolts are used in applications such as securing fuel tanks in place where having access to both sides of the fastener may not be possible in all cases. The T-slot variety of T-head bolt can be fed into a channel in a machine called a T-slot track and may be used to secure an object at any position along the length of the channel.

U-Bolts

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U-bolts are fasteners that are in the shape of the letter U, which contain two male threads, one on each end of the bolt on to which a mounting plate bracket and attachment nuts are placed. While most U-bolts feature a semi-circular profile, some have a squared-off shape. These fasteners are typically not fully threaded, as they function to clamp an object in place. They are used in a range of applications from supporting pipe to automotive use on drive shafts and exhaust systems. Some designs include rubber coating to preclude wear from the movement of metal against metal. Other designs of U-bolt add thick rubber gaskets to reduce the effect of vibration and to control noise. Non-metallic designs are also available for use in applications where there is a risk of the U-bolt making contact with an electrically powered conductor.

Summary

This article presented a brief summary of some of the common types of bolts. For more information on related products, consult our other guides or visit the Thomas Supplier Discovery Platform where you will find over 2200 suppliers of bolts.

Sources:

https://www.engineeringtoolbox.com/
https://www.oldwestiron.com/collections/square-head-bolts
https://www.mcmaster.com/
https://www.fastenal.com/
https://www.grainger.com/
https://cnb-inc.com/complete-nut-and-bolt-guide/
https://thecoatinghouse.com/paacd-pre-applied-adhesive-coating/