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What fastener terms should you know?

clock 10 minutes | 03 Sep 2021

What is the difference between bolts and screws? What is the bolt definition? The tap-bolt definition? What tool is used to tighten bolts?

We’ve put together a glossary of fastener terminology to help you answer these and other questions.

A

Allen key: Also called an Allen wrench or hex key. Allen keys are ‘L’ shaped wrenches that drive bolts and screws with hex sockets.

ANSI: American National Standards Institute. They set standards for products, processes, systems, services and resources in the U.S.

ASME: American Standard of Mechanical Engineers who produce around 600 codes and standards, including those for fasteners.


B

Bearing surface: The part of a fastener that carries the load. On a screw, for example, this can refer to the underside of the head. Washers can be used to increase the bearing surface.

Binder machine screw: Type of screw with a thick head and deep slot, typically used for binding books, large manuals and material swatches.

Bolt: A non-tapered fastener with part of the shaft threaded. Mates with a nut or uses a washer to secure fastening.

Bolt shank: Bolts with smooth shafts, or the unthreaded part of the bolt’s shaft. A longer bolt shank can increase the shearing capacity while a smaller bolt shank increase the joint’s elastic resilience. Some bolts are fully threaded.

BS: British standards. British Standard Whitworth (BSW) was the world’s first national screw thread standard established in 1841 but with a few exceptions, is no longer used.

Button head: A type of socket head with a domed head and internal hex drive. Works well when a larger bearing surface is needed.


C

Cam out: The stripped screw definition. Cam out happens when the screwdriver slips out of the head’s slot once the necessary torque has been exceeded, which can damage the surrounding surface.

Chamfering: The process of grinding off the first thread on a rod and creating an angle. The purpose is to make it easier to fasten a nut to the threaded rod.

Clamp force: The amount of pressure a bolt delivers when tightened.

Countersink: A tool used to create a countersink threaded hole. The countersink increases the size of the outer end of a drilled hole.

Countersunk: Fastener heads, such as countersunk screws, that sit flush with the surface where they’re installed. Countersunk hex nuts are used for applications where the head sits flush with the surface. Flat head screws, as seen here, are also countersunk.

Curved flange nut: The flange curves toward the nut’s barrel, which allows it to be welded on round surfaces with ease.

Cut threaded bolts: Bolts where threads are formed by cutting away steel, as opposed to rolled threaded bolts.


D

DIN: The German standards body, Deutsches Institut für Normung. The standards are reviewed every five years. Changes are common, depending on technological advances.

Dome nut: A nut with a domed top used to cover threaded studs, bolts or rods for safety and aesthetics. Sometimes listed as round caps bolts.

Double-ended threaded screw: Used to attach two pieces of wood together. To install, place one end of the screw in the pilot hole of the larger piece of wood. Then take the smaller piece of wood and place its pilot hole over the other end of the screw. Spin the smaller piece of wood until the two pieces of wood meet.


F

Fillister head: Fillister screws are also known as raised cheese head screws. The head is similar to a pan head but has a deeper slot and higher profile. Once very popular, today they tend to be used as a replacement screw.

Flange: In fasteners, this is a protruding rim designed to spread the load. An example is a flange bolt or as shown here, a nylon flange nut.

Flat head: A countersunk head that sits flush with the surface in which the screw is installed.


H

Hex head: A non-countersunk head with six sides, which enables greater torque than other screw heads.


I

Inside and outside threaded bolt: A bolt with both internal and external threads. Also known as internal external threaded bolts.

ISO metric screw thread: International Standards Organization. The most popular general-purpose screw thread worldwide.


J

Jam nuts: A bolt with two nuts called jam nuts can be used to resist loosening, but frequent inspection and maintenance is often required.


L

Left-handed thread: Also called reverse threads. These atypical threads tighten counter-clockwise. If, for example, the threat of applied pressure will loosen a typical thread, then a left-handed thread set screw might be used instead.


M

Major diameter: Major diameter measures the largest diameter of the screw from the top of the threads. The minor diameter is the lower extreme diameter of the thread.

Mushroom head: Also known as a truss head. These decorative heads have a wide bearing surface and low profile.


N

Non-countersunk: Refers to any style of screw head where the head protrudes from the surface. The example shown here is a round head screw.

Non-standard threads: Atypical fasteners. In the UK, fasteners with non-standard threads can be found off the shelf. In the U.S., non-standard fasteners are custom made. Or, a non-standard screw head, for example, can be created by modifying an existing standard head.

Nuts: A fastener with internal threads that mate with the external threads of bolts and some screws. Usually consists of a hex shape, as seen in these nylon hex nuts.



O

Oval head: A non-countersink, slightly curved head screw. Typically used for switch covers for their aesthetic appeal.


P

Pan head: A non-countersunk, slightly domed screw or bolt head. The rim of the head enables superior torque for fastening and removing.

Pass-through bolt: Also known as a through bolt, which goes all the way through thick materials and is fastened with a nut. Used for heavy-duty jobs.

Phillips: A type of screw drive with a cross-shaped slot for turning.

Pilot hole: A small hole drilled into a material to help prevent the screw from slipping during installation.

Pitch: The space between a point on a fastener thread to the point on the next thread.

Preload: The tension created in a fastener when tightened. Also called clamp load.


R

Rectangular nuts with threaded hole: Also called slide nuts. Used in struts, where its shape prevents turning.

Right-handed screw: Right-handed thread runs clockwise, which is the norm for screws.

Rolled thread bolts: Bolts formed through the process of roll threading, where steel is extruded to form the bolts’ threads.

Root: The groove of the thread, from which the minor diameter is measured.


S

Set screws: Used to hold non-parallel surfaces against another surface. These machine screws are also called grub screws and lack heads.

Screws: A fastener with a threaded shaft that screws into a surface. The smooth, unthreaded part of the screw’s body, above the threaded part is known as the screw shank. Many screws, however, have threaded shanks, meaning the entire body is threaded. The top of the screw is the head and contains the screw head hole, known as the drive.

What is the difference between bolts and screws?

Bolt or screw? Bolts are designed to be used with a nut. For that to happen, the bolt is inserted into a hole so that it protrudes from the other side, enabling a nut to be fastened. Screws are either self tapping or inserted into pre-drilled holes and tighten into the material in which it’s been installed.

Screw tightener: tools used to tighten screws, such as a screwdriver. Some screws, such as plastic knurled head thumb screws, can be tightened manually.

Self-tapping screw: A screw that can dig, or “tap,” its own hole as opposed to being inserted into a pre-drilled hole.

Slotted: A type of screw drive that uses a single slot. Prone to cam out if attempted to fasten with a power tool.

Socket head: a non-countersunk screw head with an internal hex drive.

Stud bolt caps: Caps used to cover exposed bolts and studs in scaffolding to protect workers and pedestrians from injury. Also known as scaffold stud caps.


T

Tap bolt: A fully threaded hex bolt used without a nut and screwed by hand into a hole.

Tensile strength: The maximum tension-applied load that a fastener can withstand before failing.

Thread angle: Also called the flank, this is the distance between the angled sides of the thread.

Threads per inch (TPI): The number of complete threads within one inch of a screw’s threaded length.

Threaded stud: Also known as a rod, threaded rod or all thread rod. A threaded stud with a hole uses a cotter pin to join materials together for light-duty holding.

Threaded screw caps: An aesthetic cover for countersunk screw heads.

Torque: Twisting force that causes rotation.

Truss head: Also known as a mushroom head screw.


U

Unified Coarse Threads (UNC): A series within the Unified Thread Standard, which allows for tolerances and allowances. The most common fastener thread and has higher strength than UNF threads (see below) and is easier to remove.

Unified Fine Threads (UNF): A series within the Unified Thread Standard, which allows for tolerances and allowances. Fine threads provide better sealing than UNC, but are not as strong nor can they be as easily removed.

Unified Thread Standard (UTS): Standard thread forms and series as set by the ASME, consisting of symmetric V-shaped threads with an angle of 60° to each other, mirroring ISO Metric screw threads.


W

Washers: Fasteners that distribute the load of a screw or nut to prevent damage to surfaces.

Wing nut: So called because of the “wings” on either side of the nut. These wings make the nut easy to grip for installation or removal, with no need for tools.

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