Crash tubes for steering columns and bumpers
As the highest safety relevant components, crash tubes must meet highest requirements. During the development and production of the complex parts for steering columns and bumpers, Felss works closely with the customers.
Complex geometries with maximum precision.
Many technical details guarantee the safety in the car. Quite a few of these solutions are hidden and the driver doesn’t even know they are there – if no accident happens. There is a particularly interesting safety component on the steering column. To make sure that the driver isn’t unnecessarily seriously hurt by the steering wheel in the event of a possible collision, many steering columns have a so-called crash tube.
This sophisticated component is produced by tube experts at FELSS Rotaform Switzerland on behalf of ZF Lenksysteme. In the event of a crash, the steering wheel is a hazard to the driver. If the driver’s body hits it at full force, this can cause serious injury even when a seatbelt is worn. At ZF Lenksysteme in Bremen, an inconspicuous tube is therefore being produced that is located in the top section of the steering column below the steering wheel. If the driver hits the steering wheel with great impact, the short tube splits into several strips through the impact and the tube peels outwards like a banana skin. The whole steering wheel therefore yields slightly in just a fraction of a second and the impact energy is minimized – a simple yet ingenious concept and an integral part of sophisticated safety components in vehicles. The construction of the crash tube poses a whole number of challenges to the process developers at FELSS Rotaform: It is extremely thin-walled and at the same time it has five symmetrically arranged grooves inside it. They form the aforementioned required breaking points. After rotary swaging, the tube is widened at one end and indented – four tongues are slightly bent outwards. “Because the design is so sophisticated, we had to develop a new production process with new systems “, says Rolf Käser, Managing Director of FELSS Rotaform Switzerland.
Crash tube sawing is the first challenge
The challenge already starts in the first production step: the cutting to length of the blank. In order for the extremely thin-walled tube to be cut precisely and with a clean cut during the sawing process, a saw blade is used at Rotaform where the saw teeth face inwards. The crash tube is therefore fed through a kind of “saw hole”. The machine spindle at the same time makes a walk movement. “Many teeth are engaged, thus producing a soft cut”, explains Eckhard Morlock, Managing Director at Felss Rotaform.
Controlled splitting of the tube
An alignment station then captures the tube optically and places it in the correct position for the next production steps: After five holes have been punched, one side is widened. The tube hereby splits precisely down to the holes. “In order to be able to guarantee this sensitive process, a kind of outer ring encapsulates the tube before the flaring tool penetrates into the tube”, Jecklin explains about another system solution at Felss Rotaform.
Full responsibility for the crash tube at Felss Rotaform
“Together with our machine construction partners, we have developed a complex production process for this component that had not been seen before”, explains Käser. Since the start of production in 2004, Felss Rotaform is responsible for the whole production of the crash tube. This does not only involve the use of optimal processes such as sawing, rotary swaging or flaring. Felss Rotaform defined, for example, the surface condition of the blanks together with the tube suppliers. The confidence that the customer ZF Lenksysteme had in the process and production know-how of the Swiss was evident in the continuous development, as the Felss Rotaform Project Manager Daniel von Deschwanden explains: “The forces that affect the driver through the steering wheel in a collision were still a bit too high to start with during a crash. Two years later and with help from the ZF designers, we therefore looked for a better solution and have since implemented it in a production-technical way.” Just one small detail has been changed: the shape of the holes that were punched. They are now shaped like drops, thereby simplifying the splitting of the crash tube in the event of a crash.
Reaching the “limits of what can be done": up to 500,000 crash elements for bumpers per year
Safety technology in and on the car: It is often a matter of millimeters. The components in question must be manufactured to the highest level of precision in order to be able to guarantee the functionality that is required. An example of particular interest is currently being manufactured by the forming experts at Felss Rotaform on behalf of ZF Friedrichshafen AG. The crash tube ensures that in the event of a collision, parts of the bodywork and drive train are protected against damage. Its unusual shape challenges the production planners at Felss Rotaform in many different ways. At a first glance, the crash tube looks like a funnel that has been pulled lengthways. Wide on one side, slightly narrow at the other end. Straight in the middle. The tube finished using rotary swaging, axial forming and expanding doesn’t look like anything spectacular and yet its process-reliable and effective production is a real challenge, with a lot riding on it in the end”, explains Eckhard Morlock, Managing Director at Felss Rotaform in Germany, at the Bretten-Gölshausen site.
The wall thickness of safety components is key
The front bumper of a car is suspended from two of these crash tubes. In the event of a collision at up to 16 kilometers an hour, they deform and absorb the force of the crash. The outcome: If repair work is needed, only the bumper needs to be replaced, the vehicle itself is protected against any greater damage. Repair costs remain low and the vehicle is classed in a lower insurance class. Even the other safety components in the vehicle benefit thanks to the deformation force of the crash absorber that is constantly set, the activation of the airbags can be set with precision. The demands on the crash tube itself and on its material properties are high. In the event of a high-speed crash, it must not break, just as the bumper must not break. This is the only way to ensure that the front end of the vehicle is deformed as little as possible. “The wall thickness and dimensional stability of the tube are therefore crucial to the overall innovative design. This is why its dimensions are very precise”, says Morlock.
Forming is recursive-modulated
An added challenge that is quite the norm to component manufacturers since up to 500,000 crash tubes are manufactured every year. What a large volume production demands, is a high level of repetitive accuracy. From a production-technical point of view, the so-called frequency-modulated axial forming takes center stage at Felss Rotaform (see image 4). The whole tube shape is manufactured in just one operation. So that the blank is not compressed, the axial forces are reduced during forming using a process patented by Felss. The die is fed in frequency modulated. This changes the flow behavior of the material and the necessary force is reduced by up to 40 percent.
Changing the surface of the internal mandrel
“We must make sure that we adapt the process very accurately in this case to the component”, explains Morlock. “In the case at hand, it was for instance a big challenge that the crash tube is widened at one end and that the material has to also flow lengthways during rotary swaging. To guarantee optimum flow behavior, we have changed the surface condition of the mandrel that is in the component during forming. Simply put, it is slightly rougher. This improves the flow of the material.”
As far as technical forming is concerned, tube specialists are reaching the limits of what can be done – the tube cannot be widened any further at the funnel-shaped end. The wall thickness of the blank has its own natural limits. The level of difficulty of the process was demonstrated by a different supply company who, on behalf of ZF Friedrichshafen AG, wanted to manufacture the crash tube using a conventional impact extrusion process. “Micro-cracks appeared in the component that were sometimes very difficult to detect. This is not just unacceptable for safety systems. Within the framework of frequency-modulated forming, we have found a way that guarantees a process-safe and highly effective production”, concludes Morlock.