Thread grooving saves energy

Space is at a premium in many automotive applications. As a result, less and less installation space is available for bolted connections. At the same time, however, the requirements for preload forces and the reliability of the connections are increasing. And the lowest possible overall connection costs are also important. A new thread-forming screw for direct metal fastening has been developed to meet precisely these challenges.

In the case of light metal screw connections, the focus is on optimizing installation space, performance compression and cost optimization. A very important point against this background is the load-bearing capacity optimization of the connection. With regard to thread-grooving metal screw connections, especially light metal screw connections, this means that better utilization of the nut material must be achieved by increasing the thread overlap of the thread flanks of the screw and grooved nut thread.

Arnold Umformtechnik GmbH & Co. KG has developed Powertite for precisely these applications. “The thread-forming Powertite is characterized by a higher load-bearing capacity of the screw itself compared to conventional products on the market. Thanks to a slightly increased stress cross-section, which corresponds to the level of the standardized stress cross-section of a metric screw, Powertite can generate higher tensile breaking forces,” says Thomas Jakob, Application Sales Engineer at Arnold Umformtechnik.
The special geometry of the fastener, in combination with adapted surface systems, allows relatively high assembly preload forces to be achieved during screw assembly. These are generally on a par with metric screws, both for elastic and overelastic assembly.

Geometry plays a decisive role in the function of the fastener

The Powertite screw is a round thread-forming screw with a trilobular groove. This means that where the forming work is done, the screw has a slightly triangular cross-section (with rounded corners) to reduce the forming moments. With a length of 3 x p (p = thread pitch), the groove zone length is optimized for installation space and enables the forming work during the thread forming process to be distributed over several thread turns as well as optimum screw seating and centering. The round load-bearing cross-section has a defined oversize; the outer diameter of the screw is always larger than the nominal nominal dimension. This ensures greater overlap between the thread flanks of the screw and the grooved nut thread.

Basically, this special geometry ensures that the screw achieves an increased stress cross-section and therefore slightly higher tensile breaking forces than conventional metric screws. The combination of trilobular cross-section in the grooving zone and round cross-section in the load-bearing area significantly optimizes thread-grooving screw connections because, on the one hand, the grooving torques are low and, on the other, the overtorques and load-bearing capacity of the grooved nut thread are significantly increased.

Areas of application for self-tapping screws

With the Powertite, Arnold Umformtechnik wants to provide an answer to the new market requirements: transferring higher assembly preload forces in less installation space. There are numerous examples of the use of thread forming Powertite. These are, for example, applications in which the highest preload forces must be realized through overelastic assembly. It is also suitable for assemblies where only a small screw-in depth can be realized and high hole tolerances are required. In addition, weight reduction can be achieved in many cases by reducing screw lengths and weight-optimized screw heads.

Compared to other thread-forming screws, Powertite is a load-bearing capacity-optimized thread-forming screw. While the entire load-bearing and thread-forming area of Taptite is trilobular, Powertite is round with oversize in the load-bearing area and only trilobular in the thread-forming area. As a result, the preload force level of metric screws was achieved even with overloaded assembly and installation space optimization was also possible. In both cases, the Fast Designer Metals tool can be used to make a mathematical prediction for the specific application. This is always preceded by a recommendation from Arnold Umformtechnik for a suitable core hole design.

Diverse optimization and savings potential

The focus of Powertite applications is on electromobility in particular. From the battery area to the engine and axle steering area as well as the transmission, it offers great potential. This applies in particular to light metal bolted joints, but also in combination with cast components.

Thomas Jakob has been familiar with precisely these topics through his work in various areas for many years. Market proximity is very important to him: knowing the interface to the customer, advising with a view to the specific application and driving product development forward together with the customer. He cites three examples in which assemblies were optimized through the use of Powertite thanks to targeted advice and extensive calculations and investigations: “In a gearbox housing screw connection, an M8 thread-forming screw, which was originally mounted elastically, was replaced by a Powertite M7, which is mounted overelastically. This enabled significant downsizing to be achieved. In a high-voltage battery application, the use of Powertite M8 x 33 for bracket bolting in cast and drilled hole areas resulted in significant cost savings compared to the metric bolts previously used. An actuator screw connection in the steering column also became more cost-effective thanks to the use of a Powertite M6 x 20. Previously, a metric screw was screwed into a press-in nut. By eliminating the press-in nut, there are now no costs for the element and the setting process.”

Calculation of weight and CO2 savings

Powertite thread-forming is a fundamental further development of thread-forming technology. By replacing conventional M8x40 thread-rolling screws (nominal diameter x length) with M7x40 screws from the Powertite series, significant CO2 savings can be achieved. This is demonstrated by two specific examples.

For example, the Powertite M7x40 has been used since June 2022 in a load-distributing transmission that is currently still used in conjunction with an internal combustion engine. This alone can achieve a weight saving of around 42 g per gearbox and therefore a CO2 saving of around 72 g per gearbox. With 13 screws per gearbox and a requirement of around 6 million screws per year, this results in a weight saving of around 19,400 kg or a CO2 reduction of 33.1 tons per year for this gearbox alone.
The saving is even greater for an electric motor gearbox unit, in which 33 screws are used per gearbox unit. Of the 50,000 vehicles planned per year, 30,000 are expected to be equipped with two e-motor transmission units per car. In this case, the weight saving through the use of Powertite is around 213 g per vehicle and the CO2 saving is around 364 g. Extrapolated to all 50,000 vehicles and the necessary 80,000 gearboxes, this results in a total requirement of approx. 2.64 million screws per year. The use of Powertite alone can then achieve a weight saving of around 8,519 kg and a CO2 reduction of 33.1 tons per year in the production of these vehicles. This means a CO2 saving of 17 percent per vehicle for the fasteners replaced by Powertite.

Technical and commercial advantages

With the thread-rolling Powertite, the load-bearing capacity of the grooved nut thread is increased by more than 20 percent compared to standard trilobular screws on the market. This means a number of technical advantages. These are: standard-compliant thread forming torques, overelastic assembly even in conically cast core holes in GD-Al, a preload force level similar to metric screw connections and the possibility of larger core hole tolerances to optimize casting processes for light metals. In addition, there is a high level of assembly safety and applicability in steel as well as aluminum and other light metals. In addition, repeat hollow bolting is possible and replacement with metric bolts is possible during servicing.

In addition to the technical advantages, Powertite screws offer a number of commercial benefits: a reduction in overall connection costs of up to 85 percent, the possibility of downsizing, space and weight optimization and leaner upstream processes in the production of light metal or steel components.

Intensive development before market launch

Arnold Umformtechnik spent more than three years in an intensive development process designing, optimizing and adapting the Powertite to meet requirements  always with a view to specific applications. It was presented to the public for the first time at the end of June at the Industrial Screwdriving Assembly Workshop in Dresden.
“We made the most of the development time, took on board all of our customers’ suggestions and implemented them in the best possible way,” reports Thomas Jakob.

Potential customers can draw on the complex product knowledge of the Forchtenberg-based fastener manufacturer in the usual way, as well as a comprehensive understanding of the performance of the product in the various applications and application areas. This is complemented by the corresponding calculation tools and extensive manufacturing expertise. Arnold Umformtechnik has developed a completely new manufacturing process specifically for Powertite in collaboration with development partners from the tool manufacturing sector.

Author: Annedore Bose-Munde

Web:
www.arnold-fastening.com