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Safe backrest uses tomorrow's material

SSAB Tunnpl : 26 November, 2003  (New Product)
Same weight and safety at a lower cost, this was part of the specification when the Autoliv Mekan automotive safety company was entrusted with the development of a new backrest for the rear seat of a car. The task was complicated by a tight time schedule and the fact that the new backrest was to be incorporated into an existing production line. Everything needed to work well immediately, without any hitches and the project was completed after only 18 months.
The new rear seats manufactured from high strength steel are now being manufac-tured on a production line at the Autoliv plant in the town of Hässleholm in southern Sweden.

Autoliv Mekan specialises in car seat structures. The company's plant in Hässleholm, where most of the products are made, is permeated by the safety approach. The company's area of operation is ob-vious as soon as you enter the reception area. Visitors are greeted by a receptionist and invited to take a seat in a comfortable armchair in the form of a car seat. Sounds of production are just audi-ble, and there is no doubt that there is intensive activity.

Tough challenge
A couple of years ago, the company was faced with the challenge of developing a new backrest for the rear seat of an estate car. The car was already established on the market, and production was in full swing. But for cost reasons, the carmaker wanted to develop a new rear seat as the existing alu-minium structure was simply too expensive to produce.

'Aluminium is much more expensive than steel,' explains Sven-Göran Johnsson, Purchasing Man-ager at Autoliv Mekan. 'The difference is substantial and cannot be justified if the need can be met by some other material.'

But the problem lay not only in designing a new seat that was less expensive to produce. The design also had to meet all of the other requirements and specifications that applied to the earlier seat. So the new backrest had to weigh roughly the same as its predecessor with no compromises on safety and strength allowed either. In addition, the new design would have to suit all other compo-nents parts of the rear seat.

'Adaptation to other parts of the car is an important element, since there are quite a few parts linked to the backrest structure,' continues Ingvar Persson, 'Including plastic parts, seatbelts, load nets, armrests and upholstery. So nothing could be moved or modified and everything was dictated by the previous seat.'

Additional difficulty of parallel development
A further complication was that the design and production of the new backrest had to be developed in parallel with the assembly of the existing aluminium backrest.

'When the time came for the actual switch, the new backrest was taken into production without interruption,' explains Ingvar Persson, as we watch a complete backrest structure being welded to-gether by one of the 70 or so welding robots at the company.

Welding is continually in progress, and a double fixture supplies the welding robots with a steady stream of backrest structures. Everything is done extremely accurately and quickly, and it is easy to see the difficulties posed by the transition to the new design, particularly since the car model had already been launched on the market and was selling well.

'There was no time for running in. Everything had to perform perfectly right from the start,' recalls Ingvar Persson.

High strength steel is the obvious choice
As soon as Autoliv Mekan engineers began poring over the design of the new backrest, the choice quickly fell to high strength steel.

'Our own production, and probably that of many others, is largely set up for steel,' says Sven-Göran Johnsson. 'We have the machines, equipment and competence for steel, which we could ap-ply to the ultra-high strength steels that have emerged in recent years. When we use aluminium in our products, we have to rely on others to a greater extent.'

To meet the customer's requirements, an entirely new backrest was designed. The existing alumin-ium backrest was built as a box structure, with many concealed beams.

'Aluminium is lighter than steel, but much more material is needed in the reinforcing beams to achieve the required strength,' explains Ingvar Persson. 'The box construction concealed the somewhat complex aluminium structure.'

High strength steel opened new opportunities, and the designers chose instead an open and slimmer design. The use of ultra-high strength steel with a tensile strength of 1000 MPa enabled the number of reinforcing beams to be substantially reduced. The new backrest, which is split 40/60, has a total of five vertical beams. The beams provide the backrest structure with the necessary strength and stiffness. At the point where the backrest is split, it is not secured to the body. In the event of a crash, the backrest must be able to withstand both the load applied by the passengers whose seat-belts are secured to the backrest structure, and also any contents of the luggage compartment that apply a load from the rear. Thus, the stresses are highest at the point where the backrest is split. In addition to withstanding the load without deformation, the backrest must not deflect significantly. This is where the ultra-high strength beams play a decisive role.

'Steel of such high strength levels has good properties for this type of structure. We can use very thin material, even though the forces are high,' explains Ingvar Persson. 'We press the beams in-house in Hässleholm.'

The beams are secured to a strong steel tube at the bottom of a backrest structure, which counteracts torsion and deflection. In addition, a transverse beam is fitted to the larger part of the backrest to provide the structure with the right stiffness.

Short development time
The entire development work from order to the beginning of production took 18 months, which is a short time in this context.

'We met all of the weight, price and safety requirements, and we have found out a great deal about the opportunities offered by ultra-high strength steels,' confirms Sven-Göran Johnsson. 'We realise that advanced high strength steels can replace aluminium in many products. We have proved this by our design and so it is also natural for us to continue working with steel as the first choice when selecting materials for future products.'
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