Lightweight construction Fighting the flab!

Just how light a vehicle should be is decided at an early stage in development. To make components lighter, ZF relies on designs, materials, production methods – or rather, on all of them together.

Less weight means lower fuel consumption and fewer emissions. So lightweight design is a feature of almost every development project in the company – especially those relating to chassis and suspension systems. In cars, lightweight construction conserves resources and significantly extends the range of, for example, electric vehicles. In trucks, reduced kerb weight adds up to heavier payloads – and also offsets the extra weight of the modern exhaust-gas treatment systems required under the stringent EURO 6 emissions standard.

Two engineers who play important roles in lightweight products “made by ZF” are Stefan Krahn and Dr. Ignacio Lobo Casanova. Stefan Krahn is in charge of lightweight plastics in the Advanced Engineering & Driving Dynamics unit in Dielingen. His thinking is product-oriented: how can we develop a rear axle for cars that feels to the driver like a sophisticated multi-link rear axle made out of steel, but weighs significantly less? This means considering all kinds of lightweight design strategies: lighter materials, alternative designs for individual components – or an integrated lightweight design that combines the functions of several components in a single unit.

Stefan Krahn, Development Engineer in Dielingen, with Dr. Ignacio Lobo Casanova, Head of the ZF Composites Tech Center in Schweinfurt, are two of the powerhouses behind the development of ZF’s lightweight engineering.

The major challenge in lightweight design is taking account of all the different dependencies involved. If you want to use lightweight materials, you must take all these interlocking dependencies into account at the earliest design stages. In the case of fiber-reinforced plastic composites (FRP), for example, which weigh much less than steel, many of the design principles that apply to steel can no longer be used. But FRPs do offer new options and opportunities. They are, for example, resistant to bending along the grain. This property can be used to improve the elastokinematic characteristics required by the engineers, thereby enhancing the interaction between the various

This is where Dr. Ignacio Lobo Casanova gets involved. For the Head of the ZF Composites Tech Center in Schweinfurt, light-weight construction is all about the materials – which in his case are fiber-reinforced plastic composites. The ZF Composites Tech Center exudes the spotlessly clean atmosphere of a laboratory: no unsightly metal shavings or oily waste here! Instead, there are rolls of carbon-fiber and glass-fiber matting, not unlike the rolls of fabric in a home furnishings store. In the middle of the room stands a massive press that uses many tons of pressure to squeeze fiber mats plus added liquid plastic into the shapes required for components.

The main tool in the ZF Composites Tech Center is a 1,000-ton hydraulic press.

“Not only do FRPs have different properties from steel, their properties also depend on the manufacturing process used,” explains Lobo Casanova. He and his team use their production expertise to support engineers working on FRP solutions in ZF development centers around the world. The Tech Center specialists don’t just focus on manufacturing processes – they also take associated costs into account. Currently, many of the company’s lightweight products based on fiber-reinforced plastic composites are still just development projects or concept studies. If ZF is to convince customers that these solutions are viable, unit prices must be factored in right from the start. So Lobo Casanova and his team try to ensure that no manufacturing processes are “designed” into products that will make volume production more expensive at a later stage.

In the composite leaf spring axle, this key component – a wheel-guiding transverse leaf spring made out of glass-fiber-reinforced plastic – replaces a steel design.

One product that has emerged from the collaboration between the Dielingen and Schweinfurt teams is the composite leaf spring axle. In this rear axle for cars, a wheel-guiding transverse leaf spring made out of glass-fiber-reinforced plastic replaces a steel design. While the innovative axle is not entirely free of steel, the main component, made out of composite materials, replaces so many different steel parts that the bottom-line result is a significant weight saving. The key principle here is functional integration: a very “composite” concept.

During the development process that culminated in this product, many things happened simultaneously. First, material samples were used that had little in common with the final component. But their rigidity and breakage characteristics provided useful data for the computerized simulation tools in Dielingen and Friedrichshafen. Finally, a powerful computer modeled the transverse leaf spring prototype down to the last detail; much of the testing was carried out “virtually”.

Only then did the team in Schweinfurt concentrate on putting together a real-world product prototype plus a related manufacturing process. “With the composite leaf spring axle, we used multiple, differently structured layers to create varying degrees of rigidity in the transverse leaf spring,” explains Lobo Casanova. Not only did this approach result in a light-weight axle – it gave them a complete modular construction kit. The handling characteristics and weight savings are impressive. In terms of driving dynamics, the composite leaf spring axle ranks in the same league as complex multi-link rear axles made out of steel. But steel axles have reached the limits of weight optimization. By comparison, the new axle weighs 13 pounds less, thanks to FRP and an entirely new design. That’s a 13 percent weight saving – an outstanding figure in a product segment where shaving off just a few ounces is enough to make automakers jump for joy.

Pictures: Dominik Gigler

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