Screwing Technology Thread Forming Screws Require Matching Surfaces
Thread-forming screws are frequently used in lightweight construction. In order to ensure process reliability and reliable functionality in assembly, both the screw design and the exact specification of the surface system are essential.
Thread-forming screws are fasteners that form their own counter-thread without cutting into a pre-drilled or cast bore hole by means of a special thread geometry. This is why they act as a force-locking connection system including loss protection by positive locking. The screws are used in through-holes and very frequently in cast core holes in die cast aluminum, and sometimes also in magnesium or zinc die cast components.
Defined Assembly Requirements
To ensure safe installation, experts rely on DIN 267 Section 30. This standard defines the mechanical properties of metric thread-forming screws of strength class 10.9. These include the minimum breaking torque, screwing or grooving torques in special test plates with precisely defined bores and tensile breaking forces. In addition, the penetration zone of the screw is defined. The ratio or compatibility to the metric screw is also defined. This means that the grooved nut thread must be able to accommodate a commercially available metric screw.
In addition to this DIN standard, there are special company standards, for example those of Magna, Ford, the BMW Group or Bosch. These often set higher requirements for certain applications. In order to react as a supplier, precise knowledge of the entire screw assembly is required.
The Field of Application is Light Metal
Thomas Jakob is Head of Product Engineering at Arnold Umformtechnik GmbH & Co. KG. He knows both the customer requirements and the possible joining solutions to meet them: "Thread forming screws are primarily used for light metals. As the latest product in the Taptite family, Taptite 2000, which is protected by trademark law, exhibits very good penetration performance. It is characterized by a radius profile which facilitates the flow of the nut material in the direction of the screw thread core. This results in less friction and a lower moment of inertia," explains Jakob.
Matching Torques are Decisive
Trilobular Taptite screws have been on the market for over 30 years. The basic objective of this trilobular, i.e. slightly triangular, cross section is to reduce the creasing moments in order to ensure a stable assembly process. The aim is to achieve a large differential between the tightening torques as well as a sufficient distance between the tightening torque and the failure torque (overtorque or bolt breaking torque).
There is a wide range of applications for thread-forming screws. They are frequently used for light metal applications, i.e. preferably in aluminum or aluminum die casting, magnesium die casting, other light alloys, but also in solid steel screw connections. Good examples of the use of thread-forming screws are gear and sensor screw connections, fastenings in the motor area, mounting plates, actuators or pumps.
Low Tapping Torques Increase the Importance of the Screw Surface
However, if, for example, in the field of upmarket customer requirements, lower torques are to be used, the surface system also plays a decisive role. The surface of the screw is usually treated with a base coating to ensure corrosion protection — usually a zinc, zinc-nickel or zinc flake coating. "OEMs have to comply with regulations for surfaces, which usually include the base surface and corrosion protection. We then define the corresponding anti-friction coating required for the screw. This is important for a prestress-oriented design of joined metal components," Jakob explains.
Basic Layer, Anti-Friction Coating and Sealing Must Harmonize
With regard to surface systems, there are guideline values according to VDA which refer to corrosion protection and friction values for metric screws. Although these values cannot be directly applied to thread-forming screws, they do serve as a reference. In addition to this — and also functionally relevant for thread-forming screws - is usually the anti-friction coating. Explicit reference is also made to this in DIN 267 Part 30
The key is that the entire surface system and the assembly must be precisely aligned - i.e. the screw geometry itself and optimum alignment with the counterpart. The base surface and the corrosion protection requirements must also be implemented accordingly. Usually, this is achieved by the above-mentioned coatings. "The entire surface system in interaction affects the prestressing force, i.e. both the base and anti-friction coating as well as the sealing," explains Jakob.
Cleaning Processes Can Have Negative Effects
The experts at Arnold have achieved good results with a Taptite 2000 and a zinc flake coating as well as sealing and lubricant coating. "We have investigated a large number of surface systems and on this basis can give concrete recommendations for various applications. We also examined the technical cleanliness, but found that the cleaning processes often have undesirable effects on the surface system because lubricants and sealants are partially removed again," says Jakob.
Smaller Screws Take More Thought
A further challenge is posed by small screws below a diameter of M6 that have to meet high corrosion requirements, as they are often coated with zinc-nickel, which is more suitable for further processing, but sometimes has an inhibiting effect on the creasing process. The definition of the prestressing force also requires a great deal of expertise: While the requirements and calculation methods for defining the prestressing force for metric screws are clearly specified, only the test plate and the tapping torque in the test plate are specified in the case creasing screws.
So Hedge Therefore, Who Forms a Thread
Arnold's experts closely collaborate with customers from the very beginning to create a cost-optimized direct metal fasteners that meets specific customer applications. To do so, a number of key data must be taken into account: In principle, it is necessary to check whether a thread-forming screw can be used for the intended connection with regard to the required pretensioning forces. Another point, of course, is the available installation space and the associated possible screw-in depth. Depending on the planned application, appropriate recommendations for its implementation can then be provided.
Pre-Cast Core Holes Require Attention
"Particularly in the case of light metals, the core drill holes are frequently pre-cast. Then we make recommendations on how the core hole must be designed and what tolerances must be taken into account. However, the customer is the final link in the chain and has to agree with the foundry whether the proposed solution also works in terms of casting technology," Jakob notes. The actual situation of the assembly also plays a role, i.e. operating loads and temperatures. Naturally, it must also be taken into account whether the manufacturer requires specific instructions for the assembly.
Special Tools Help In the Design of the Screw System
Many companies also use factory standards that need to be observed, especially with regard to overtorque, torque, or tapping force. The grade of strength, head or thread design also play a role. "By conducting tests in our Fastener Testing Center, we can often support our customers with screw joint analyses on the original customer components in order to define an optimum tightening torque so that precise assembly is guaranteed under series conditions," says Jakob.
In principle, there are guideline values for the design of screw fastenings in order to ensure a reliable assembly process. At Arnold, various additional tools, such as the core hole design and the "Fast Creator", support the screw design. A tool is also currently being developed that can be used to provide well-founded recommendations regarding the torque-preload behavior of exactly fitting, thread-forming Taptite 2000 screws, says Jakob.
Constant Loosening and Tightening Can Be Problematic
Arnold also considered repair cases. "It is unlikely that thread-forming screws will loosen themselves during the first fastening. In the case of repeated screw fastenings or repairs, however, the entire surface system should be checked in order to guarantee the safety of the connection," Jakob notes.
Regardless of the application in which the thread-forming screws are ultimately used, in order to achieve a reliable fastening solution, it is just as important to carry out a detailed analysis beforehand as it is to accurately design the surface system. At the testing centre in Dörzbach, customers can have their repair concept evaluated, taking into account the original components.
This article was first published by MM MaschinenMarkt.