Threaded insert

Last updated
TIME-SERT insert TIME-SERT thread insert.gif
TIME-SERT insert

A threaded insert, also known as a threaded bushing, is a fastener element that is inserted into an object to add a threaded hole. [1] They may be used to repair a stripped threaded hole, provide a durable threaded hole in a soft material, place a thread on a material too thin to accept it, mold or cast threads into a work piece thereby eliminating a machining operation, or simplify changeover from unified to metric threads or vice versa. [2]



Thread inserts come in many varieties, depending on the application. Threaded inserts for plastics are used in plastic materials and applied with thermal insertion or ultrasonic welding machines.

Manufacturers of ready-to-assemble furniture often ship the parts with threaded inserts and other kinds of knock-down fasteners pre-installed.

People who use sheet metal or sandwich panel or honeycomb sandwich-structured composite often install threaded inserts to spread shear, tension, and torque loads over a larger area of the material. [3]

Captive nut

Captive nuts come in two basic styles. One type, the cage nut or clip-on nut is a conventional nut held captive by a sheet metal carrier that clips onto the part to be connected. These are generally used to attach screws to sheet metal parts too thin to be threaded, and they can generally be attached, removed and reused with simple hand tools.

The second type of captive nut is a threaded insert. These are either pressed into holes in the material to be joined or moulded in. In either case, part of the insert is generally knurled to get a good grip on the material supporting the insert. One variant, the swage nut, has a knurled portion that swages the sides of a soft metal hole to more tightly grip the nut. Press fit and swaged captive nuts are used in panels that are too thin to be threaded or in soft materials that are too weak to be threaded. They are installed by pressing them in with an arbor press. [4]

Threaded inserts are commonly used in plastic casings, housing, and parts to create a metal thread (typically: brass or stainless steel) to allow for screws to be used in the assembly of many consumer electronics and consumer products. These may be cast in place in injection molded parts or they may be added by thermal insertion. In the latter, the insert is heated and then pressed into a hollow in the plastic part. The heat causes local melting in the plastic. [5] Ultrasonic Insertion is the process used to apply vibration and pressure to install the threaded insert into a molded hollow boss (hole) of a plastic part. The ultrasonic vibrations melt the thermoplastic material where the metal insert is in contact, and pressure is applied to press it into position. The material typically reforms around the knurled body of the threaded insert to ensure a good retention.

Externally-threaded inserts

A self-tapping insert Taplok Insert.jpg
A self-tapping insert

Externally threaded inserts have threads on the outside and inside of the insert. The insert is threaded into a pre-tapped hole, or some inserts tap their own threads in a drilled or molded hole. It is then anchored by various means, such as a nylon locking element. Inserts that are anchored via Loctite are more commonly known by the trademarked name E-Z Lok. [6] A thin walled solid bushing insert by the trademarked name TIME-SERT is locked in by rolling the bottom few internal thread into the base material with a special install driver which will permanently lock the insert in place.[ citation needed ] Key locking inserts, more commonly known by the trademarked name Keenserts, use keys that are hammered into grooves through the threads, permanently locking the insert. [7] [8] [9] Inserts that are self-tapping and lock via friction are more commonly known by the trademarked names Tap-lok or Speedserts.

Helical insert

A helical insert Helicoil plus.jpg
A helical insert

A helical insert is an insert made of coiled wire. The helically formed coils of diamond shaped stainless steel or phosphor bronze wire screw into a threaded hole to form a mating internal thread for a screw or stud. These inserts provide a convenient means of repairing stripped-out threads and are also used to provide stronger threads in soft materials such as aluminium, zinc die castings, wood, magnesium etc. than can be obtained by direct tapping of the base metal involved. Another common generic name is screw thread insert (STI), although many users call them all by a prominent brand name, the registered trademark Heli-Coil. Applications include engine cylinder head repair after unintentional over-torquing or cross-threading of spark plugs strips the thread of the socket. Kits with matched tap and coil exist for this. The straight radial piece in the photo is the driver tang which is used as a key to grip with pliers for driving the coil into place and is discarded after installation.

Mold-in inserts

Mold-in inserts are internally threaded and have a specially shaped outer diameter to anchor the insert in plastic. The insert is placed in the mold of an injection molded part beforehand. The mold is then closed and filled with the plastic filling in around the insert. These inserts can also be heated and pressed into pre-made thermoplastics. [10]

For softer more pliable plastics, hexagonal or square inserts with deep and wide grooves allow the plastic to flow and adhere. The process allows large product manufacture i.e. fuel tanks, boats etc., so the torque inserts may be of large thread sizes.

Press fit inserts

Press fit insert Press-in Insert.png
Press fit insert

Press fit inserts are internally threaded and have a knurled outer diameter. They are pressed into a plain hole with an arbor press. [10]

Potted inserts

An insert that is potted-in refers to the application of epoxy to fix it to a panel, such as a honeycomb sandwich panel, which are often used in commercial aircraft. [12]

Strength factor for threaded inserts

Resistance is the key strength factor in case of inserts, pull-out & torque-out are the two parameters to judge inserts.


Knurling is the metalworking which is done on the outer side of the component. In case of Brass Insert, knurling plays an important role in increasing pull-out & torque-out resistance. Types of knurling and its benefit are as follows:

Installation methods

For industrial purposes, following installation methods are the standards:

See also

Related Research Articles

Taps and dies are tools used to create purpose screw threads, which is called threading. Many are cutting tools; others are forming tools. A tap is used to cut or form the female portion of the mating pair. A die is used to cut or form the male portion of the mating pair. The process of cutting or forming threads using a tap is called tapping, whereas the process using a die is called threading.

Washer (hardware) Thin plate with a hole, normally used to distribute the load of a threaded fastener

A washer is a thin plate with a hole that is normally used to distribute the load of a threaded fastener, such as a bolt or nut. Other uses are as a spacer, spring, wear pad, preload indicating device, locking device, and to reduce vibration. Washers often have an outer diameter (OD) about twice their inner diameter (ID), but this can vary quite widely.

Bolted joint

Bolted joints are one of the most common elements in construction and machine design. They consist of fasteners that capture and join other parts, and are secured with the mating of screw threads.

Self-tapping screw

A self-tapping screw is a screw that can tap its own hole as it is driven into the material. More narrowly, self-tapping is used only to describe a specific type of thread-cutting screw intended to produce a thread in relatively soft material or sheet materials, excluding wood screws. Other specific types of self-tapping screw include self-drilling screws and thread rolling screws.

Knurling Patterning of metal

Knurling is a manufacturing process, typically conducted on a lathe, whereby a pattern of straight, angled or crossed lines is rolled into the material.

Spacers and standoffs

In general, a spacer is a solid material used to separate two parts in an assembly. Spacers can vary in size from microns to centimeters. They can be made of metal, plastic, glass, and other materials. Shapes include flat sheet, cylindrical and spherical.

Set screw Type of screw

A set screw, also known as a blind screw or a grub screw, is a type of screw generally used as a detent to secure an object within or against another object, usually without using a nut. The most common examples are securing a pulley or gear to a shaft. Set screws are usually headless, meaning that the screw is fully threaded and has no head projecting past the thread's major diameter. If a set screw does have a head, the thread will extend all the way to the head. A set screw is almost always driven with an internal-wrenching drive, such as a hex socket (Allen), star (Torx), square socket (Robertson), or a slot. The set screw passes through a threaded hole in the outer object and is tightened against the inner object to prevent it from moving relative to the outer object. It exerts compressional or clamping force through the bottom tip that projects through the hole.

Sex bolt

A sex bolt,, is a type of fastener (nut) that has a barrel-shaped flange and protruding boss that is internally threaded. The bolts sits within the components being fastened, the flange provides the bearing surface. The sex bolt and accompanying machine screw sit flush on either side of the surfaces being fastened. It is normally chosen because of its low profile compared to other nuts. The sex bolt often has a built-in feature, such as a slot, to aid in tightening the fastener. Some sex bolts, more commonly known as "architectural bolts", have knurled barrels to allow one-sided assembly. "Binding posts" are similar to architectural bolts in that they are designed to be assembled from one side, but they have teeth on the flanged surface to keep them fixed.

Nyloc nut Type of lock nut

A nyloc nut, also referred to as a nylon-insert lock nut, polymer-insert lock nut, or elastic stop nut, is a kind of locknut with a nylon collar that increases friction on the screw thread.

Rivet nut

A rivet nut, also known as a blind rivet nut, or rivnut, is a one-piece internally threaded and counterbored tubular rivet that can be anchored entirely from one side. It is a kind of threaded insert. There are two types: one is designed to form a bulge on the back side of the panel as a screw is tightened in its threads. The other is similarly drawn in using a screw, but is drawn into the sleeve instead of creating a bulge.

Swage nut

A swage nut or self-clinching nut is a type of nut or threaded insert that is used on sheet metal.

A clip-on nut, also known as a sheet metal nut or a speed nut, is a type of nut designed to be clipped to sheet metal. It is a type of captive nut commonly made as a cage nut.

Screw Type of fastener characterized by a thread wrapped around a cylinder core

A screw and a bolt are similar types of fastener typically made of metal, and characterized by a helical ridge, known as a male thread. Screws and bolts are used to fasten materials by the engagement of the screw thread with a similar female thread in the matching part.

Nut (hardware) Type of fastener with a threaded hole

A nut is a type of fastener with a threaded hole. Nuts are almost always used in conjunction with a mating bolt to fasten multiple parts together. The two partners are kept together by a combination of their threads' friction, a slight stretching of the bolt, and compression of the parts to be held together.

Threading is the process of creating a screw thread. More screw threads are produced each year than any other machine element. There are many methods of generating threads, including subtractive methods ; deformative or transformative methods ; additive methods ; or combinations thereof.

Inserts are pins, bolts, screws, joints and other structures that are used to transfer localized loads to a composite panel or to join two composite panels together. Metallic inserts are commonly used in the aerospace and marine industries to attach objects to sandwich composite panels.

Screw extractor

A screw extractor is a tool for removing broken or seized screws. There are two types: one has a spiral flute structure, commonly called an easy out after the trademarked name EZ-Out; the other has a straight flute structure. Screw extractors are intentionally made of hard, brittle steel, and, if too much torque is applied, can break off inside the screw that is being removed. Since the extractor is an extremely hard material, and a typical home shop drill bit will not be able to drill into it, a larger element of difficulty is added to the original screw extraction project. One way to avoid this added difficulty is to drill a hole completely through the screw. Thus, if the fastener breaks, a punch can be used to drive out the easy out from the screw, via the back, or end, of the fastener.

Pentalobular screw thread

A pentalobular screw thread is a form of self-forming thread used for screws. Self-forming screws are used in ductile materials, such as aluminium and plastics.

Bolt (fastener)

A bolt is a form of threaded fastener with an external male thread requiring a matching pre-formed female thread such as a nut. Bolts are very closely related to screws.



  1. "Repairing damaged threads: Types of thread repair inserts |". Retrieved 2017-09-30.
  2. "Newman Tools Inc". Retrieved 2012-02-29.
  3. "Delron Inserts for Honeycomb and Sandwich Panels" (pdf). Retrieved 2013-06-03.
  4. McMaster-Carr, p. 3150.
  5. Sullivan, G. & Crawford, L. "The Heat Stake Advantage" (PDF). Plastic Decorating Magazine (January / February 2003). Archived from the original (PDF) on 2015-06-10. Retrieved 2013-04-26.
  6. "Inserts For Metal". E-Z Lok. E-Z Lok.
  7. McMaster-Carr, p. 3204.
  8. McMaster-Carr, p. 3206.
  9. McMaster-Carr, p. 3207.
  10. 1 2 McMaster-Carr, p. 3209.
  11. "Press-In Insert". Himalaya Brass Products. Retrieved 10 February 2021.
  12. "Composite Potted-In Inserts". Marketing Masters. Retrieved 25 June 2020.
  13. 1 2 3 "Threaded Inserts". Himalaya Brass Products. 2020. Retrieved 10 February 2021.