The ultimate safety coupling

The ultimate safety coupling

R+W Antriebselemente GmbH has developed an XXL safety coupling for use in a wind energy test facility. An application is currently pending for this to be included in the Guinness Book of Records.


Safety couplings are a decisive factor in guaranteeing seamless operation in rotating equipment. They protect delicate machine components from overload damage which can lead not only to expensive repairs but also to extensive downtime. Every millisecond counts when shutting a machine down due to failure. Safety first – expert knowledge and precision are therefore required when selecting and applying safety couplings.


Customized manufacturing of precision and industrial drive couplings is part of the day-to-day business at R+W Antriebselemente GmbH. The German-based company regularly receives inquiries from the business and scientific communities with immensely detailed, often extreme parameters, due to the reputation for experience and expertise the in-house development department has earned it. This includes well-known projects such as safety couplings for the International Space Station (ISS) and blind-mate metal bellows couplings for the world’s largest machine and most powerful particle accelerator, the Large Hadron Collider (LHC).


Extreme demands generate creativity

Another flagship project was completed and installed in the summer of 2017 – the STF 20,000, which is currently the largest safety coupling in the world, weighing in at 21.5 metric tons. The demands were therefore extreme – in every respect. The order came from a Spanish coupling manufacturer which was building a transmission shaft for two six megawatt motors at a new Danish wind energy test facility. The additional overload protection required here in the form of a safety coupling with manual engagement definitely needed the help of a specialist partner. R+W was the only manufacturer that could fulfil this special requirement, and was able to start development in July of 2016.


“Our design specification corresponded to the combination of the enormous load cycles from the action of the wind turbine, the correspondingly high kilonewton meter ranges which had to be overcome, and the structural specifications from the Spaniards,” explains Rainer Benz, Technical Director at R+W. What this actually means is that the safety coupling is designed to support a disengagement torque of between 15,000 and 20,000 kilonewton meters. Typical safety couplings offer disengagement torque levels of up to 2,800 newton meters – so this corresponds to a 7,000-fold increase in the usual value. The required outside diameter was four meters. The inside diameter of this gigantic safety coupling was still to be only 70 centimetres, with an overall safety coupling length of 470 millimetres. “The STF 20,000 is constructed using the same principles as all our other proven industrial safety couplings, but in non-standard dimensions,” continues Rainer Benz.


Seven months of design work

At the start of the seven months of intensive design work, the R+W development team approached this special task initially by producing a 3D model. The actual gigantic coupling was based on this virtual prototype. The engineers also used finite element methods to master the unfamiliar physical parameters. Great care was taken over finding possible weaknesses in the coupling which would not usually occur in conventional use and standard sizes. “We have been extremely meticulous in checking for the danger of deformation or cracks due to extreme loading in the wind energy test facility. The long-term functional safety of the coupling depends on this,” adds Rainer Benz. The designers therefore strengthened specific critical points in the coupling in order to prevent failure due to wear.


Three weeks working on a giant jigsaw puzzle

After that, all of the individual parts of the coupling were forged as a special production run. Absolute precision was required in this work as the subsequent meticulously planned process of putting the parts together would not tolerate even the slightest inaccuracy in the location of a drilled hole. The project team rented a production space 50 kilometers away from Mömbris at Aschaffenburg, specifically for the assembly of the giant safety coupling, as an especially high ceiling was needed to accommodate the large diameter of the safety coupling and the cranes used to handle it. It took three weeks for three R+W engineers to complete this giant jigsaw puzzle. Instead of assembling the parts from all sides, the individual components were mounted from one face only: “We made a virtue of the technical necessity and used the idea of stacking of a pizza as our template. Individual parts were therefore placed on the base frame one at a time – the ideal procedure as it transpired,” recalls Rainer Benz. The team also documented this exciting process using time-lapse photography.


The STF 20,000 had to be sent to Andalusia for final assembly – a further logistical challenge. The weight of the 21.5 ton safety coupling was further increased by the 3.5 ton heavy-duty transport frame. Once ready for shipment, the ground level low-loader had no width capacity to spare for the 1,500 kilometer journey. As this type of heavy-duty transport is not allowed on the motorways in France, the team of drivers took a total of three days to reach their destination. After intensive functional tests, the unit was shipped to Denmark for installation in the wind energy test facility whose motors have since then been protected by what is currently the largest safety coupling.


Pilot project for other high-capacity safety couplings

“We are obviously proud of the successful completion of this challenging project. These type of extreme demands can only be met by businesses who have an in-depth understanding of process whilst being able to think laterally at the same time,” concludes Rainer Benz. His team therefore sees the pending application for an entry in the Guinness Book of Records for the world’s largest safety coupling as the icing on the cake. Other projects in the multi-million Newton meter range have already appeared as a result of this accomplishment, which is the real seal of approval for the development team. Concepts for these new projects can refer back to the initial design. This freely adaptable interface ensures the individual and specific demands of each project are met.