A safety coupling in XXL format

R+W Antriebselemente GmbH developed a superlative safety coupling for installation in a wind turbine test facility.

Couplings are used in almost every machine. Of course, therefore, the components come in various designs and sizes. Safety couplings, for example, ensure the smooth operation of an engine. They protect equipment from overload damage that can ultimately result in expensive repairs and cause a complete shutdown. Safety first – that’s why precision and expert knowledge are required when using torque limiting couplings

Special projects involving the individual production of industrial and precision couplings are part of R+W Antriebselemente GmbH’s daily business. The in-house development department is experienced and has the know-how to handle requests from industry and science with particularly delicate, often extreme parameters. These include such renowned projects as the safety coupling for the ISS space station or the pluggable metal bellows coupling for the world’s largest machine and most powerful particle accelerator, the Large Hadron Collider (LHC).

Extreme requirement profiles make for inventive minds

A special project was traded this summer – currently the largest safety coupling in the world, the STF 20,000 with a total weight of 21.5 tons. The requirements were therefore extreme in every respect. The order developed from the tender of a Spanish coupling manufacturer. The latter, in turn, built a transmission element for the two six-megawatt motors of a new Danish wind turbine test facility. However, the additional overload protection required for this in the form of a safety coupling with manual engagement had to be provided by a specialized cooperation partner. R+W was the only manufacturer that felt up to this particular tender and was able to start the development phase in July 2016.

“Our specifications determined the combination of the enormous load cycles of wind power operation, the correspondingly high kilonewton-meter ranges that had to be handled, and the Spaniards’ structural specifications,” explains Rainer Benz, Technical Manager at R+W. In concrete terms, this means a release torque of between 15,000 and 20,000 kilonewton meters, for which the safety coupling should be designed. As a rule, up to 2800 Newton meters are achieved in the safety range – this corresponds to 7000 times the usual value. The outer diameter to be achieved was four meters. The inner diameter of the gigantic safety coupling should still be 70 centimeters, with a length of 470 millimeters. “Nevertheless, the STF 20.000 is also designed according to the same principles as all our proven industrial safety couplings, just in different dimensions than usual,” explains Rainer Benz.

Seven months for conception

The R+W development team approached the special task in seven months of intensive conception time, initially by creating a 3D model. After this virtual prototype, the gigantic coupling could be realized. The engineers mastered the unusual physical parameters using the finite element method. Particularly meticulous was the search for possible weak points on the coupling, which would not occur in this way with conventional use and usual size. “We have carefully checked the risk of deformation or cracks due to the extreme stress in the wind power test facility. The long-term functional reliability of the coupling also depends on this,” adds Rainer Benz. Accordingly, the designers reinforced the localized neuralgic points on the coupling to prevent faults caused by wear.

Three weeks large parts puzzle

As a special design, all parts of the coupling were then forged individually. Here, too, absolute precision work was required, because the subsequent, meticulously planned process of assembly was not to forgive any inaccuracy, no matter how small, in the alignment of a drill point. For the assembly of the huge safety coupling, the project team specially rented a hall in Mömbris near Aschaffenburg, 50 kilometers away, which was designed for the enormous diameter of the safety coupling and the exceptionally high crane capacity required for this. The large parts puzzle took three R+W mechanics a total of three weeks to complete. Instead of assembling the parts from all sides, the individual components were only assembled from one side: “We simply made a virtue out of technical necessity and used the topping of a pizza as a model. Individual part after individual part was thus pinned to the base frame – the ideal procedure, as it turned out,” summarizes Rainer Benz. The team also documented the exciting process with a time-lapse film.

The STF 20,000 had to be shipped to Andalusia for final assembly – another logistical challenge. In addition to the 21.5-ton weight of the safety coupling, the 3.5-ton transport frame was required for the approximately 1,500-kilometer trip, which meant a zero-loader at ground level without excess width. Nevertheless, since the use of highways for heavy transports of this type is prohibited in France, the team of drivers needed a total of three days to reach their destination. After thorough functional tests, the unit was shipped for installation in the wind turbine test facility in Denmark, whose motors have since been protected by the largest fuse coupling currently available.

Pilot project for further high-performance safety couplings

“Of course we are proud after the successful realization of this project. Such extreme requirements can only be mastered by those who have a deep understanding of processes anchored in the company and can think laterally at the same time,” summarizes Rainer Benz. Already further projects in the kilonewtonmeter range are the best confirmation for the development team. The design can be based on the first design. The freely customizable interface always ensures individualization to the respective project and its specific requirements.