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High strength steel frame keeps passengers in place

SSAB Tunnpl : 29 November, 2002  (New Product)
A maximum weight of 20 kg and the strength to withstand 60 kN, or around six tonnes, of tensile force in a collision. Those were the exacting requirements which Finnveden committed themselves to when they were assigned by Volvo's development division to produce a new strengthening frame for the centre seat in the new Volvo XC90.
By combining extra and ultra high strength steel with high strength hardened boron steel, engineers arrived at a durable solution which exceeded all expectations. The end result was a 16.4 kg heavy steel frame which ensures that the centre seat meets stringent collision requirements.

The new Volvo XC90 has been acclaimed by the press. The car, an sport utility vehicle, is packed with cutting-edge technology and safety solutions. One of these is the strengthening element for the centre seat in the seven-seat, 2.2-tonne car. The steel frame looks like a very simple structure, yet more than a year's development work lies behind the technically complex design.

The main problem has been to design the frame in such a way that it can withstand the extreme forces exerted in a collision. These must be absorbed by the beam under the seat. The toughest crash tests generate forces up to six tonnes.

'It has been problematic since we were not able to attach the seatback to the sides of the car,' says Martin Wallström, the design engineer at Volvo Car who has headed the development of the new frame. 'In the new XC90 it must be possible to fold the seatback forwards for the passengers at the back. The centre seats are therefore not attached to the car sides. Furthermore, the middle seat belt housing is fitted to the seatback. There are no side attachments, so the frame has to absorb all the forces generated.'

Three quarters the weight
The project was assigned to Finnveden in spring 2000, when they continued work on an existing initial design draft. The element weighed 22 kilograms, and did not meet Volvo's requirements which included maximum post-collision bending of 20 millimetres in the rear beam. Selecting thicker steel beams was therefore not an option. Engineers at Finnveden and Volvo instead commenced work on a completely new design.

The engineers realised at an early stage that the weight could be reduced, particularly in the two long transverse beams the material thickness of which was 2.5 and 4 millimetres.

'The thickness of the steel made the beams very heavy,' says Mats Bergman, technical director for the Finnveden business area Sheet Metal Components. 'It was evident that the weight could be reduced considerably, something which would enhance the design.'

Various high strength steels were used and the development team eventually came up with a design that was only three quarters the weight of the original frame. The 16.4 kilogram design met the exacting safety requirements and the weight reduction was far more than Volvo had anticipated. Production is now in full swing and the assembly line at Volvo's Torslanda plant in Sweden is producing the Volvo XC 90 for delivery to the first customers in the United States and Europe.

'We spent more than a year producing the final version of the frame,' says Mr Wallström. 'We must ensure that we develop cars which are unquestionably world class with regard to safety and quality. This involves time-consuming development work trying out new solutions and materials. However, we have never considered using anything apart from steel. For us, it has proved unbeatable as a design material for parts where safety, strength and price are decisive factors.'

Well thought-out
The new frame consists of 16 different parts which are welded together at Finnveden's production plant in Olofström in the south of Sweden. The two long transverse beams are already rolled and cut when Finnveden receives them.

The open-type front transverse beam is in ultra high strength steel and has a tensile strength of 1200 N/mm2. The closed-type rear transverse beam is manufactured in hardened boron steel, which has even higher tensile strength. The rear beam is rolled and hardened in the same process, a patented production method carried out by Accra Teknik in Piteå in the north of Sweden. The two side beams, which form the design's collision boxes, are pressed from high strength steel and have a tensile strength of 600 N/mm2.

The entire frame is welded automatically in a specially constructed fixture. The frame is then sprayed before being stacked in transport racks for delivery to the Volvo's Torslanda plant.

'Careful thought has been given to all the parts in both the product and the production process,' says Mr Bergman. 'We can see that our well-targeted research work has borne fruit. The new frame is a solution which is cost-efficient, secure and eco-friendly.'

The frames are sprayed so that they are clean to handle on the assembly line in Torslanda.

'We have stringent requirements here when it comes to cleanliness,' asserts Mr Wallström when he shows us how the frame is assembled in an XC 90 on the Torslanda line. The completed cars must be completely clean when they leave the line. A single dirty part can spoil the entire car. The hygiene requirements also make for a more pleasant work environment.
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