Thread insert
With the threaded insert , internal threads for screw connections are created by inserting a hollow cylinder ("insert") with an internal thread into the material. The process is related to dowelling . There are different types of thread inserts. If the thread in the receiving hole has already been cut with the tap, flexible wire spools, the thread inserts, are used.
Wire thread inserts
introduction
Wire thread inserts create heavy-duty connections in low-strength metallic materials and have been tried and tested in practice for around 50 years. The wire thread inserts are formed from profiled wire into a resilient spiral. The driving pin, which is required for assembly, is cut off at the notch (predetermined breaking point) after installation, so that a through thread is created which, depending on the design of the receiving thread, corresponds to an ISO nut thread of tolerance class 6H or 5H. The most common German standard is DIN 8140-1 to -3. It specified nut threads, which are produced using wire thread inserts, as EG = insert threads.
Inserting a wire thread insert creates a threaded connection that is more wear-resistant and corrosion-resistant than a single screw connection.
The prototype of the wire thread insert bears the trademark Helicoil and was developed in the USA in the late 1940s. Today the Helicoil is manufactured, developed and sold by the companies Böllhoff and Emhart Teknologies .
Other manufacturers of wire thread inserts include the Baercoil company with the product of the same name, the VÖLKEL company with V-COIL and the Arconic company with the Recoil.
If, however, in wood or similar materials such as z. B. soft plastic a metric thread are introduced, mostly ramp sleeves are used.
Basics
Wire thread inserts are characterized by high wear resistance, low thread friction within narrow tolerances, high surface quality, and corrosion and heat resistance. The performance limits specified by the nut thread and the strength of the material are increased by the internationally proven technology. When using a wire thread insert, the following work steps are necessary:
- Core hole drilling (commercially available twist drills are used) - Specifications for the core hole diameter of the individual dimensions can be found in DIN 8140-2.
- Generate mounting thread - System-specific taps are to be used to cut the mounting thread. The accuracy of the holding thread to the gauge must be checked with thread limit plug gauges. Thread milling and thread forming are also common manufacturing processes for generating the receiving thread.
- Installation process
- Insert thread insert - Installation is possible with manual and mechanical installation tools or automatic installation. The wire thread insert is screwed with its driver pin down onto the installation spindle, inserted into the preload cartridge or attached to the union tool and the device is placed on the threaded hole.
- Installation - The threaded insert is screwed in by turning the threaded mandrel, the spindle or the union tool by hand or by releasing the drive. He is with min. 0.25 P below the surface.
- Tang breaking - To create a through thread, the driving pin is broken off at the notch. This is done with a tang breaker. For threads from M 14 fine and normal pitch, the driving pin can be removed with needle-nose pliers. In the case of blind hole threads, the removal can be omitted if the maximum screw-in depth of the screw is observed.
According to DIN 8140-1, a distinction is made between two different versions.
- Form A : thread from M2 to M39 and from M8 × 1 to M39 × 3
- Form B : with polygonal elastic windings, arranged in the middle, for clamping / thread from M2.5 to M24 and from M8 × 1 to M33 × 2
Note : Clamping is the creation of an increased frictional connection in the thread, which counteracts the automatic loosening of the screw connection. The torque required to turn the screw in the thread, which is measured without any axial load during the turning process, is considered an additional friction torque.
Threaded inserts have a notch that enables the driving pin to be broken off.
In addition to DIN 8140 parts 1 to 3, the aviation standards LN9039 for wire thread inserts made of stainless steel, Form A, LN 9499 for wire thread inserts made of stainless steel, form B, and DIN 65536 for wire thread inserts made of bronze are the most widely used standards in Europe.
Mounting thread design
A detailed explanation of the calculation and design of a screw connection can be found in VDI guideline 2230.
Construction guidelines
The dimensioning of the minimum wall thickness is largely determined by the individual operating data. These in turn determine the strength of the material and the screw length. The specified guideline value formulas apply to aluminum, cast and wrought alloys and a thread length of the wire thread insert of 1.5 d.
- d = nominal diameter
- D HC = outside diameter of the receiving thread
- a = remaining wall thickness
surfaces
To avoid cold welding between, for example, CrNi screws and wire thread inserts made of stainless steel, as well as the galvanic separation of material pairings at risk of contact corrosion, the wire thread insert can be coated with different surfaces (e.g. silver, various dry lubricating films, tin).
friction
The thread friction can be reduced by using a wire thread insert and the range of variation can be limited (e.g. µG of a steel screw of strength class 10.9 is in the delivery state screwed into a cut nut thread between 0.12 ... 0.18, whereas when using a wire thread insert the values for µG lie between 0.11 ... 0.13). With a torque-controlled screw tightening process, this results in a more precisely adjustable screw pretensioning force or better utilization of the screw yield point.
Areas of application
Originally intended as a repair solution, the wire thread insert is now recognized as a design element. The areas of application are diverse due to the different designs in terms of material, type and surface coating. It is primarily used in the course of lightweight constructions in low-shear materials such as aluminum and magnesium alloys. Here it is used for thread reinforcement and has a true-to-gauge thread even after a large number of repeated screw connections.
The use of wire thread inserts, based on the standard ISO nut thread, gives rise to different options for structural design:
- A: The thread length can be reduced with the same screw diameter and strength class.
- B: The screw diameter can be reduced with the same thread length. (It should be noted that the strength of the screw may have to be increased.)
- C: A combination of A and B - For screw connections subject to high thermal loads, wire thread inserts made of nickel-based materials with and without a coating are available. Elasticity and spring force are retained even under high temperatures. Since the base body is not exposed to the high temperature directly through the use of the wire thread insert, it may be possible to use a material that has a lower temperature resistance. The above properties of the wire thread insert can be used to reduce costs and weight when used accordingly.
further reading
- [Bol10a] Technology around screws, Böllhoff, 6th edition, 2010
- [Bol10b] Böllhoff aktuell, Securing screw connections, issue 27
- [MB302] Strelow, O .: Leaflet 302: Securing devices for screw connections, Advice center for the use of steel, Düsseldorf
- [Rol09] Wittel, H .; Jannasch, D .; Vossiek, J .; Spura, C .: Roloff / Matek machine elements, book of tables, 24th edition, 2019
- [VDI2230] Systematic calculation of highly stressed screw connections Cylindrical single screw connections, sheet 1, October 2001