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News

Safer bus seat weighs 20% less

SSAB Tunnpl : 22 June, 2006  (New Product)
A new bus seat built from high-strength steel has been a success for Grammer S.r.l. from Italy. Improved comfort, better safety and significantly lower weight are among the advantages of the new seat, to say nothing of affordable production costs. Grammer has now been nominated for the Swedish Steel Prize 2004 for the seat's high-strength steel frame.
Today it is ready to go, with safety characteristics that exceed EU requirements. The first prototype for the Gran Turismo, as the seat has been named, was presented at a bus exhibition in Belgium, when a number of requests were put forward by bus manufacturers. As a result, Grammer drew up a list of goals to achieve:

pleasing design
a price comparable to existing designs
high standard of comfort and safety
lighter weight than seats currently on the market
ability to use the same frame in different seat models
frame to be easy to mount in different vehicle types

The key to reconciling these apparently conflicting requirements turned out to be the choice of material for the frame. After trying out several alternatives including magnesium and aluminium, the design team hit on the idea of using advanced high-strength steel.

'The price was the killer for the other materials, they simply cost too much,' says Sandro Guidoni, who with Agostino Mengoni and Eleonora Mancinelli led the development project at Grammer S.r.l. (Monsano, Italy). 'We could build a lighter seat with magnesium than with steel, but it's much more expensive. Aluminium doesn't save that much weight at all, because the aluminium shapes need to be so heavy to meet EU requirements.'

Each backrest must be capable of withstanding an impact of over one tonne during crash testing. For a double seat, the most usual kind, this means the design must withstand impact forces exceeding two tonnes.
Given the narrow time frame and the variety of demands to be met, Grammer decided to seek help from a number of partners. The steel supplier, the tube manufacturer and others contributed their expertise to the project, enabling development to proceed rapidly despite a series of difficult problems including forming analysis, tool analysis, manufacturing step design, jointing, and others.

'Integrating our partners into the development process was a natural extension of our own team-based approach,' explains Innocenzo S. Carbone, Grammer's CEO. 'With all parties involved in the conversation, the project was soon taking giant strides.'

The new frame is based entirely on advanced high-strength steels.
The seat bottom is constructed from two hollow sections of ultrahigh-strength cold-rolled two-phase steel with a minimum tensile strength of 800 N/mm. The same grade of steel is used for the brackets by which the seat is fixed to the bus floor and which also serve as hinges to adjust the tilt of the backrest. The brackets are designed to withstand the severe forces that occur in an accident as well as the hard wear and tear of commercial passenger traffic. They also hold the lower attachment points of the seat belt.

The remainder of the frame, including the backrest with integrated three-point belt, is mostly constructed from round tubular cold-rolled extra high-strength two-phase steel with a minimum tensile strength of 600 N/m. In the event of an accident, each seat must be able to withstand the force from a belted passenger plus that of an unbelted passenger sitting in the seat behind.

The choice of high-strength steel for the new seat has been a success and has opened up new paths for further product development at Grammer.

'At the moment Gran Turismo is the only product in which we are using high-strength steel, but we are interested in studying its potential for use in other products,' says Fabrizio Dardani, Grammer's QA manager.
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