Threaded insert

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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

An externally threaded insert has threads on the outside and inside. The insert can be threaded into a pre-tapped hole, or a self-tapping insert creates its 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. The straight radial piece is the driver tang which is used to grip the coil while driving it into place. It is broken off and is discarded after installation. Helicoil plus.jpg
A helical insert. The straight radial piece is the driver tang which is used to grip the coil while driving it into place. It is broken off and is discarded after installation.

A helical insert (also called a screw thread insert (STI), although most users call them all by one of the prominent brand names: KATO®, Heli-Coil® or Recoil®) is an insert made of stainless steel or phosphor bronze wire, with a diamond cross section, coiled to form inner and outer threads. The coil of wire screws into a threaded hole, where it forms a smaller-diameter internal thread for a screw or stud. [10] These inserts provide a convenient means of repairing stripped internal threads. These inserts are commonly sold in kits with matched taps and insert tools.

In soft materials, they are used to provide stronger threads than can be obtained by direct tapping of the base materials, e.g. aluminium, zinc die castings, wood, magnesium, plastic.

An example application is engine repair after unintentionally destroying the threads in a socket for a spark plug by over-torquing or cross-threading.

Mold-in inserts

A mold-in insert has a specially shaped outer surface to anchor the insert in plastic. For injection-molded plastic, the insert is placed in a mold before it is filled with plastic, making an integral part. An insert can also be heated and pressed into pre-made thermoplastic material. [11]

For softer, more pliable plastics, hexagonal or square inserts with deep and wide grooves allow the softer plastics to hold the inserts sufficiently. 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

A press-fit insert is internally threaded and has a knurled outer surface. It is pressed into a plain hole with an arbor press. [11]

Potted inserts

A potted insert is set in epoxy to fix it, such as in a honeycomb sandwich panel, often used in commercial aircraft, and is said to be potted in.

Strength factors of threaded inserts

Pull-out resistance & torque-out resistance are the two main strength factors of threaded inserts.


Knurling is the grooved texture on the outside of the insert. Types of knurling and their benefits are as follows:

Installation methods

For industrial purposes, the following installation methods are the standards:

See also

Related Research Articles

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<span class="mw-page-title-main">Bottle cap</span> Top for holding liquid inside a bottle

A bottle cap or bottle top is a closure for the top opening of a bottle. A cap is sometimes colorfully decorated with the logo of the brand of contents. Plastic caps are used for plastic bottles, while metal with plastic backing is used for glass; plastic caps are commonly made from polyethylene or polypropylene, while metal caps are usually either steel or aluminum. Plastic caps may have a pour spout. Flip-Top caps like Flapper closures provide controlled dispensing of dry products. Caps for plastic bottles are often made of a different type of plastic from the bottle.

<span class="mw-page-title-main">Fastener</span> Hardware device that mechanically joins or affixes two or more objects together

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<span class="mw-page-title-main">Washer (hardware)</span> 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.

<span class="mw-page-title-main">Bolted joint</span> Mechanical joint secured by a threaded fastener

A bolted joint is one of the most common elements in construction and machine design. It consists of a male threaded fastener that captures and joins other parts, secured with a matching female screw thread. There are two main types of bolted joint designs: tension joints and shear joints.

<span class="mw-page-title-main">Self-tapping screw</span> Type of 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.

<span class="mw-page-title-main">Countersink</span> Conical hole cut so a fastener can be inserted flush with the surface

In manufacturing, a countersink is a conical hole cut into a manufactured object, or the cutter used to cut such a hole. A common use is to allow the head of a countersunk bolt, screw or rivet, when placed in the hole, to sit flush with or below the surface of the surrounding material. A countersink may also be used to remove the burr left from a drilling or tapping operation, thereby improving the finish of the product and removing any hazardous sharp edges.

<span class="mw-page-title-main">Set screw</span> Type of screw

In American English, a set screw is a screw that is used to secure an object, by pressure and/or friction, within or against another object, such as fixing a pulley or gear to a shaft. A set screw is normally used without a nut, being screwed instead in a threaded hole drilled in only one of the two objects to be secured. A set screw is often headless and threaded along its entire length, so that it will sit entirely inside that hole; in which case it may be called a grub screw or blind screw.

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<span class="mw-page-title-main">Sex bolt</span> Type of mating fastener

A sex bolt is a type of mating fastener (nut) with a barrel-shaped flange and protruding boss that is internally threaded. The bolts sits within the components being fastened, and the flange provides the bearing surface. The sex bolt and accompanying machine screw sit flush on either side of the surfaces being fastened.

<span class="mw-page-title-main">Anchor bolt</span> Connection elements that transfer loads and shear forces to concrete.

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<span class="mw-page-title-main">Pilot hole</span> Small hole drilled into a workpiece to guide a larger drill or other tool

In construction, a pilot hole is a small hole drilled into a piece of construction material. Its purpose may be:

  1. to guide a larger drill to the appropriate location and ease the job of the larger drill
  2. allow for the insertion of another hole-making tool, such as a knockout punch, that will produce the final-sized hole, or
  3. locate, guide, and provide clearance for a self-threading screw in wood or plastic to prevent damaging the material or breaking the screw.
<span class="mw-page-title-main">Swage nut</span> Type of fastener

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.

<span class="mw-page-title-main">Screw</span> Type of fastener characterized by a thread wrapped around a cylinder core

A screw is an externally helical threaded fastener capable of being tightened or released by a twisting force (torque) to the head. The most common uses of screws are to hold objects together and there are many forms for a variety of materials. Screws might be inserted into holes in assembled parts or a screw may form its own thread.

<span class="mw-page-title-main">Nut (hardware)</span> 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.

In manufacturing, 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.

<span class="mw-page-title-main">Bolt (fastener)</span> Threaded fastener with an external male thread requiring a matching female thread

A bolt is an externally helical threaded fastener capable of being tightened or released by a twisting force (torque) to a matching nut. The bolt has an external male thread requiring a matching nut with a pre-formed female thread.



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  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.
  7. McMaster-Carr, p. 3204.
  8. McMaster-Carr, p. 3206.
  9. McMaster-Carr, p. 3207.
  10. "What is a helicoil insert?". 2023-09-21. Retrieved 2023-10-12.
  11. 1 2 McMaster-Carr, p. 3209.