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A bolt is a type of threaded hardware fastener that is used to position two workpieces in specific relation to each other. Bolts come in several configurations for their application and specification variances.

 

Bolt vs. Screw

Since these terms were in use before the advent of easily-produced helix fasteners, they are often synonymous. However, several standards bodies have attempted to differentiate the terms, with many concluding that it is not the devices which are different, but how they are used. As provided by Machinery's Handbook and ASME B18.2.1, bolts are externally threaded fasteners that are prevented from being turned during assembly, but are positioned or released by torqueing a nut. Screws are externally threaded fasteners that can be inserted into pretapped holes or can perforate a material and create their own internal threads. Screws are fastened by torque applied to the head. This definition is still somewhat ambiguous and is not all-encompassing, but does provide a basis to begin differentiation.

 

bolt screw vs. differences terminology synonym

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Bolt Operation

The components that form a bolt can be identified in three sections. The head is the part of the bolt with the largest diameter, which provides a mount for tools to either apply or resist torque. It also provides part of the bearing surface for substrates being bolted. The shank of the bolt is the longest part of the bolt and has external, helical threads on its circumference. This piece is responsible for the alignment of the workpieces. Finally, the end opposite of the head is known as the chamfer, which provides a slightly beveled edge to aid the bolt's insertion into holes and nuts.

Successful bolt implementation almost always requires a nut to retain components on the bolt's axis. Nuts are applied via torque while the bolt itself is held in place, or vice versa. Nut position and bolt employment are dependent upon the same physical principles: the friction of the bolt and nut threads against one another, which is comprised of a slight stretch of the bolt from the compression resistance of the components being fastened, and a slight elastic deformation of the parts held together.

Nut and bolt interaction explainedBolt/nut operation against workpieces substates material compression extension

 

Nuts may loosen with vibration or dynamic loads, in which case locknuts, lock washers, or thread locker can provide jostle-resistant or permanent nut placement.

 

Bolt Production

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Bolt Dimensions

The dimensions of a bolt can be expressed in metric or imperial units, both of which are better explained in the Bolt Standards section of this selection guide.

 

Image credit: UC Davis Engineering

  • Pitch is the measurement between the apex of adjacent threads on the bolt's shank.
  • Diameter is the width of the screw shank. This should not include the bolt head.

  • Length is the measurement of the bolt from the edge of the chamfer to the undercut of the head. Against this measurement should not include the head.

Bolt Material

  • Aluminum screws are light, resistant to oxidation, thermal and electrical conducive, and easy to manufacture.
  • Brass screws are strong, conductive, and corrosion resistant, with low magnetic permeability.
  • Copper alloy screws have good load capacity, wear resistance, and are suitable for use near magnets.
  • Plastic screws are inexpensive and corrosion resistant for light loads. They are common for applications near water, such as pools.
  • Steel screws are produced of strong, carbonated iron. Uncoated steel is vulnerable to corrosion.
  • Hardened steel screws are stronger than steel screws, but more brittle. They are made of steel treated by tempering and quenching methods.
  • Stainless steel screws are chemical and corrosion resistant with an appealing finish. They cannot be hardened like carbon steel.
  • Screws consisting of superalloys exhibit good mechanical strength, surface stability, corrosion resistance, and resistant to creep at high temperatures. Common super alloys include Hastelloy®, Inconel®, Incoloy®, and Monel®.

  • Titanium screws are hard and strong, light, and corrosion resistant. When alloyed with other metals, it increases strength and durability.

Bolt Strength

Bolts are regularly marked to indicate their strength, which is dependent on the material and dimensions of the bolt.

Material

Metric class and marking

Imperial grade and marking

Metric/imperial proof load

Low or medium carbon steel

(no class)

 

Grade 2

Imperial bolt SAE no marking

Medium carbon steel, quenched & tempered

Class 8.8

metric bolt

Grade 5

Grade 5 bolt

Medium carbon alloy steel, quenched & tempered

Class 10.9

Call 10.9 metric bolt

Grade 8

Grade 8 imperial bolt

Alloy steel, quenched and tempered

Class 12.9

(no grade)

Stainless steel

A-2

(markings vary)

18-8

(markings vary)

 

Screw Finishes

Finishes applied to the base metal of the bolt enhance the durability and corrosion resistance of the material.

  • Black oxide finish does not enlarge the dimensions of the screw and is a processed black rust. It is mostly used for aesthetic purposes.
  • Chrome coating is a bright, reflective finish that is decorative and very durable. It is applied via electropating.
  • Zinc plated coatings act as a sacrificial anode, protecting the underlying metal. It is applied as a fine white dust.

  • Other coatings like galvanization and phosphating are common for particular types of hardware, like screws meant for fence or window applications.

Bolt Standards

Manufacturers produce bolts in both imperial and metric units, and due to their explicit correspondence with nuts and pretapped holes, the units cannot be mixed. Furthermore, screws are produced with fine or coarse threads, which are a designation of the screw's thread pitch--not the quality of the product. Coarse-thread bolts are less susceptible to galling, thread crossing, and seizing, while fine-thread bolts are less likely to loosen from jostling, and are more easily tapped and adjusted. 

 

Imperial

For imperial sizes, bolt producers will provide a designated number or fractional inch for the dimension of the screw. North American countries follow this system, called the Unified Thread Standard. Also included will be a "threads per inch" (TPI) count which will indicate a course or fine thread.

UNC UNF thread types of imperial hardware bolts

 

Metric

ISO, JIS, and DIN standards are based upon the metric system and are closely related (ISO guideline 261). Most hardware measured in metric units is subject to Society of Automotive Engineers (SAE) edict as well. These standards are common in the majority of the world, and speculation exists that this may become a global hardware standard. The threads on metric bolts can be available in both fine and course pitch configurations.

Coarse fine thread table metric bolts

 

Bolt Configurations

 

Bolt Head Styles

Table credit: Engineering Parts Sourcing Inc.

 

Bolt Types

Anchor bolt

Anchor bolt selection info bolt types configurations

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Anchor bolts are embedded in concrete or masonry for structural applications.
Carriage bolt

Carriage bolt selection info

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Carriage bolts are used to fasten metal to wood. The undercut of the bolt head is square to hold the bolt in place once tightened.
Elevator bolt

Elevator bolt design type

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A flat, plain or countersunk head with a squared undercut holds the bolt in place when a nut is tightened. These are common in conveyor systems.
Flange bolt

Flange bolt information fastener hardware info

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Flange bolts have an integral washer on the undercut of the bolt head that distributes the bearing load. These are also known as frame bolts.
Hanger bolt

 Hanger bolt selection info

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A hanger bolt does not contain a bolt head. Both ends are threaded, but with one end containing a wood screw.
Hexagon bolt/Tap bolt

Hex bolt selection info guide Tap bolt selection info thread bolt type configuration

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Hex bolts have six-sided heads. Tap bolts are threaded the entire length of the shank.
Lag bolt

Lag bolt industrial fastener info

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Lag bolts are also called lag screws. They create their own mating thread in wood and other soft materials when tightened.
Machine bolt

Machine bolt selection screw

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Machine bolts are meant for assembling metal components through predrilled holes with an accompanying screw. These are more commonly called machine screws.
Plow bolt

Plow bolt hardware 

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With a countersunk flat head, a square shank neck, and a unified thread pitch, plow bolts are durable and common in construction devices.
Sex bolt

Sex bolt male female mating threads industrial fastener

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Sex bolts do not require a nut. They come with a mating female component that covers the bolt shank. They are useful for fastening items that cannot be exposed to the abrasive threads.
Shoulder bolt

Shoulder bolt screw

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More commonly know as shoulder screws. Visit GlobalSpec's Shoulder Screw Selection Guide and Specification Filter for tutorial information.
Square head bolt

Square head bolt selection

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Square head bolts a four-sided bolt head with a machine screw thread.
Stud bolt

Stud bolts

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Stud bolts utilize hex nuts on both ends of the bolt. The workpieces are held between the two bolts.  
Timber bolt

Timber bolt

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Timber bolts are explicitly meant for use with large wood planks and structures. They are common in lumber and marine industries.
T-head bolt

T-Bolt head selection how to

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A T-head bolt has a T-shaped head which can fit into a slot or can be gripped easily by a wrench.
 Toggle bolt

Toggle bolt selection info hardware

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With an expanding wing-like nut, toggle bolts are used to mount objects to walls. The nut inserts through a hole before it expands to lay flat against the surface.  
 U-bolt

U-bolt

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U-bolts are partially threaded on both ends and are bent in the shape of the letter 'U', with either a radius or two right angles.

 

Resources

 

UC Davis Engineering - To Determine the Size of a Bolt.pdf

 

Engineered Parts Sourcing Inc - Types of Bolt and Screw Heads

 

Keeping It All Together - How a Bolt Works

 

Global Fastener Market - Types of bolts

 

Fastenal - Bolts

 

Portland Bolts - Headed Bolts

 

Bolt Depot - Standard U.S. Hex Bolt Sizes and Thread Pitches.pdf

 

Bolt Grade Markings and Strength Chart

Read user Insights about Bolts

Related Products & Services

  • Captive Screws

    Fasteners designed for ease of installation - often knurled for finger use - and retained within their functional environment when not engaged. Captive screw (or bolt) retention prevents loss or damage within the captive screw's parent assembly, or damage to components in the nearby working environment.

  • Screws

    A screw is a wide-encompassing type of hardware fastener that attributes its mechanical capabilities to the helical groove that extends around the circumference of the device's shank. These threads provide the friction and traction that serves a screw's purpose: to assemble or position two workpieces in relation to each other. While used synonymously, screws and bolts are not mechanical clones.