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GLOSSARY of Blind Threaded Insert Terminology


Anneal: The process of reducing the hardness and increasing the ductility of metal parts by heat treatment. Steel, stainless steel and brass blind threaded inserts are typically annealed to reduce the upset or installation load required and to reduce the risk of fracture at the outer perimeter of the bulb.

Anvil: An alternate name for nosepiece.

Blind Threaded Insert: The generic term for a threaded fastener capable of being installed into a panel with access to one side only, hence the use of the word blind.

Bulb: The feature created when the free counterbore yields in compression and buckles radially outward. The bulb captivates the insert in the panel. The shear strength of the bulb determines the pull-out strength of the insert. Also known as bulge.

Bulb Diameter: The diameter of the bulb. When an insert is used near max grip the bulb diameter will be smaller than when the insert is used near min grip. Thick wall parts have historically been designed to provide a bulb diameter of 125% of shank diameter at max grip and 135% of shank diameter at min grip.

Bulge: An alternate name for bulb.

Closed End: The term used in the blind threaded insert industry to describe an internal threaded insert with metal enclosing the end of the thread opposite the head. In the self-clinching fastener industry this style of part is said to be blind or have blind thread. Closed end parts are used to prevent long screws from interfering with other components or for protecting the mating threads from corrosive environments.

Counterbore: The hole in a blind threaded insert which clears the major diameter of the thread in the radial direction and extends axially from the head end of the part to the thread. For inserts with round shanks the counterbore is round. For inserts with hex shanks the counterbore may have a hex shape.

Countersunk Head: A style of blind threaded insert with an angle, typically 100 degrees included, on the shank side of the head. If a proper size countersink is applied to the mounting hole, this style of part will be flush in the panel on the head side after installation.

First Grip: For a given type and thread size, the part with the lowest value of grip range is commonly referred to as a first grip part.

Flat Head: A style of blind threaded insert having a head with a rectangular cross section. Flat head inserts typically have relatively thick heads and therefore provide high levels of push-out strength. For this style of part the head projects above the panel after installation.

Free Counterbore: That portion of the counterbore which is not contained within the panel when the part is properly positioned in the panel. The free counterbore is available to form the bulb.

Full Hex: A style of blind threaded insert in which the shank has a hex shape for the entire part length. The hex shank is designed to be installed in a hex hole and offers improved spin-out resistance. The need to orient the part in the mounting hole during installation is generally considered a disadvantage.

Grip: The actual thickness of the panel or stack up of panel into which the insert is installed. All performance testing of blind threaded inserts should report the grip used, as it can have a significant effect on performance

Grip Range: Range of panel thickness values into which a given part can be properly installed. It is common practice in the blind threaded insert industry to embed the maximum grip into the end of the part.

Half Hex: A style of blind threaded insert in which the shank has a hex shape for roughly half of the entire part length. The hex portion is adjacent to the head and typically is the same length as the counterbore, followed by a transition to a round shank in the thread area. The hex shank is designed to be installed in a hex hole and offers improved spin-out resistance. The need to orient the part in the mounting hole during installation is generally considered a disadvantage.

Hard Metric: A term applied to a mounting hole scheme for inserts with metric threads in which the specified mounting hole sizes are in whole millimeter increments.

Head: A mechanical feature present on all blind threaded inserts at the opposite end of the counterbore from the threads. The function of the head is to prevent the insert from completely entering the mounting hole in the panel. After the bulb is formed, the insert is restrained from axial motion relative to the panel by the head in one direction and by the bulb in the opposite direction. Heads come in various styles including flat, low profile and countersunk.

Installation Load: The force required to completely form the bulb during installation. Thick wall parts have higher installation loads than thin wall parts. The installation mandrel must transmit the installation load and therefore blind threaded fasteners are typically not offered in sizes smaller than #4-40 or M3. Also known as upset load.

Key: A single lug of metal applied at the intersection of the head and shank. It engages a mating notch in the panel to provide increased spin-out resistance. The required notch in the panel is often difficult to produce and the insert must be carefully aligned during installation.

Keyed: A style of blind threaded insert that uses a key for increased spin-out resistance.

Mandrel: The threaded member of the installation tool that engages the threads of the insert and applies the installation load. Spin-spin tools typically use a standard socket head cap screw as a mandrel. Most spin-pull tools use a more complex mandrel, but some newer tools use a standard socket head cap screw. To install an insert with external threads, the mandrel has internal threads.

Max Grip: Maximum panel thickness into which a given blind threaded insert can be properly installed. If the panel is thicker than this value there will not be enough free counterbore to form an adequate bulb. It is common practice in the blind threaded insert industry to embed the maximum grip into the end of the part.

MaxTite®: A trademarked name for thick wall product. The name is intended to highlight the higher level of performance that thick wall product provides. The trademark is used by Atlas Engineering, a PennEngineering company.

Min Grip: Minimum panel thickness into which a given blind threaded insert can be properly installed. If the panel is thinner than this value there will too much free counterbore and the bulb will form in an uncontrolled manor.

Nosepiece: The structural member of the installation tool that supports the head of the fastener during installation. With the head being supported and an axial load applied to the threads, the counterbore is placed in compression causing it to yield and create the bulb. For spin-spin tools the nosepiece has radial serrations to prevent the insert from rotating relative to the tool. Spin-pull tools have smooth nosepieces. The nosepiece is sometimes referred to as an anvil.

Nutsert®: The original trade name used by Aerpat A. G. of Sweden to describe what is now commonly known as “T” series parts. This style employs a different technology than other blind threaded inserts. A fracture point is designed into the insert causing the part to break into two separate parts when the installation load is applied. The exterior of the threaded portion has a taper which is forced into the counterbore causing it to expand radially against the wall of the mounting hole.

Panel: The material into which a blind threaded insert is installed. It must have a thickness within the specified grip range for the intended insert. Multiple panels can be used as long as the thickness of the total stack up is within the grip range. Unlike self-clinching technology, high hardness low ductility panels are not a problem for blind threaded inserts. Extremely soft thick panels are a concern, as they may not provide adequate radial support to the counterbore in a near max grip application.

Plusnut®: A trademarked name for slotted shank product. Originally issued to BF Goodrich, the trademark is currently owned by Rivnut-Bollhoff.

PlusTite®: A trademarked name for slotted shank product. The trademark is used by Atlas Engineering, a PennEngineering company. The name identifies with the Plusnut trademark and with the “Tite” suffix of the SpinTite and MaxTite trademarks in the Atlas Engineering product line.

Pre-bulbed:The practice of starting the bulb during the manufacture of the insert. This is done to reduce the installation or upset load required. The major disadvantage of this practice is that it requires a larger mounting hole to clear the bulb. Unless a positioning step is provided under the head, the installed insert may not be concentric to the mounting hole.

Pull-Out: The force required to significantly yield or fracture the insert or the panel when an axial load is applied in a pulling direction from the head side. For thin wall parts the industry standard is to support the panel with a bushing having a hole diameter of three times the fastener shank diameter. If the panel strength is adequate, the mode of failure is shear of the bulb.

Push-Out: The force required to significantly yield or fracture the insert or the panel when an axial load is applied in a pushing direction from the head side. For thin wall parts the industry standard is to support the panel with a bushing having a hole diameter of three times the fastener shank diameter. If the panel strength is adequate, the mode of failure is sure of the head. Push-out and pull-out are heavily dependent on panel material and thickness and test results should always include panel thickness, material type and hardness.

Ribbed Shank: A style of insert with a knurl on the shank OD. The knurls are typically aligned with the long axis of the part and begin just enter the head and extend toward the thread. Knurl length is typically equal to counterbore length.

Rivnut®: A trademarked name applied to the first blind threaded inserts developed by BF Goodrich to attach rubber flaps to airplane wings. Originally registered in 1975, the trademark is currently owned by Rivnut-Bollhoff. The fastener business was sold by BF Goodrich and became known as Rivnut Engineered Products, sometimes referred to as REP. In 2000 Rivnut Engineered Products was purchased by Bollhoff, a 125-year-old European fastener company, and now uses the Rivnut-Bollhoff name.

Sealed Head: A term used to describe any style of insert that has had a ring of elastomer applied at the intersection of the head and shank. A typical application of a sealed head part is to fastener a roof rack to an automobile without creating a leak point.

Second Grip: For a given type and thread size, the part with the second lowest value of grip range is commonly referred to as a second grip part. The next grip range is a third grip part and so on. For some thread sizes of thick wall parts, the catalog lists six grip ranges, in which case the highest grip range part would be referred to as a sixth grip part.

Shank: The entire outer portion of a blind threaded insert excluding the head. The counterbore and the threads are both contained within the shank. Shank diameters are typically just under the minimum mounting hole diameter with a modest minus tolerance applied.

Shortening: The amount of decrease in overall length that occurs when a blind threaded insert is installed. It is typically slightly less than the free counterbore length minus two times the wall thickness. Inserts shorten more when installed in min grip than when installed near max grip, as a result, the installed height on the blind side of the panel is essentially constant, regardless of grip.

Slotted Shank: The generic term for a Plusnut or PlusTite insert. Slotting the shank allows larger grip range and also produces a larger bulb diameter than could be achieved with a slotted shank. Slotted shank parts typically have very thick walls and therefore high upset loads. They are sometimes pre-bulbed to reduce the upset load.

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Spin-Out: The torsional holding power of an insert relative to the panel in the absence of any clamp load. It is typically measured by holding the panel, inserting a screw from the thread end of the part until it bottoms against the part and applying torque until failure. For inserts with round shanks the typical failure mode is the insert spinning in the panel. For keyed inserts or inserts with hex shanks the values will typically be higher and the mode of failure may switch to torsional failure of the insert or thread stripping due to the induced load.

Spin-Pull: A type of tool that applies the upset load by first turning a mandrel into the threads of the insert with a low torque and then apply a pulling force to the mandrel. After the part has been upset by the pulling action, the mandrel is turned in the opposite direction to remove it. For this reason, a more proper term for this type of tool that is sometimes used is spin-pull-spin. Compared to spin-spin tools, these tools are more complex, heavier, more expensive and more difficult to setup. However, a smooth nosepiece that will not mar the insert head can be used.

Spin-Spin: A type of tool that applies the upset load by turning a threaded mandrel into the threads of the insert. The mandrel is driven by an air motor through reduction gearing. A thrust bearing is used to reduce frictional torque resulting from the applied load. Spin-spin tools are light weight and inexpensive and come with varying amounts of gearing. As thread size increases additional gearing is used to increase torque output at the expense of speed. The stall torque is adjusted by air pressure. Operation is simple, spin in to stall and then reverse the trigger and spin out. Hence the spin-spin terminology. To prevent the insert from rotating, a serrated nosepiece must be used and therefore significant marring of the insert head occurs.

SpinTite®: A trademarked name for thin wall product. The name is derived from the fact that thin wall product can be installed with a spin-spin tool. The trademark is used by Atlas Engineering, a PennEngineering company.

Spinwall Technologyä: A trademark used by AVK to describe how the technology of thin wall parts differs from that of thick wall parts. The major difference they claim is the radial expansion of the counterbore to completely fill the mounting hole before the bulb is formed.

Stroke: The amount of decrease in overall length that occurs when a blind threaded insert is installed. Therefore, it is numerically equal to the shortening. However, the term stroke is typically applied to spin-pull tools whereas the shortening term is applied to the insert. Most spin pull tools have an internal stop which limits the stroke. To setup these tools, the stroke must be properly adjusted so that the insert will be installed properly.

Thick wall: A style of part having a wall thickness of roughly 14 % of the counterbore diameter. Thick wall parts have higher installation loads and typically can not be installed with spin-spin tools. Although they have narrower grip ranges than thick wall parts, they provide a higher level of performance.

Thin Wall: A style of part having a wall thickness of roughly 8 % of the counterbore diameter. Thin wall parts have lower installation loads allowing them to be installed with spin-spin tools. They also have wider grip ranges than thick wall parts, but provide a lower level of performance.

Ultimate thread strength: The force required to cause significant yielding or fracture of an insert when the head is supported and a load is applied to the threads in a pulling direction from the head side. It is equivalent to the insert being over-installed to failure. The difference between ultimate thread strength and installation load is significant in that it represents the size of the force window needed for tools that install to a constant force. Spin-spin tools install to a constant torque, but for consistent thread friction conditions, they in fact install to a constant force.

Upset: A term sometimes used as a verb to describe the act of creating the bulb when installing a blind threaded fastener. It is commonly used in cold heading and in solid riveting and has been applied to inserts because forming the bulb is similar to forming a fastener head by upsetting.

Upset Load: An alternate term for installation load

Wall Thickness: The thickness of metal, per side, between the shank diameter and the counterbore diameter.

Wedge Head: A style of insert, usually thin wall, that has a number of wedge shaped protrusions added at the intersection of the head and shank to increase spin-out from softer materials. Can also be used to improve the electrical connection between the insert and the panel for current carrying applications.

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