High strength steels help reinvent the wheel

A novel design of off-highway wheel disc has been developed by GKN Wheels, Corus and SSAB Swedish Steel, using high strength steels that are up to four times stronger than ordinary mild steel.

A ground-breaking new design of off-highway wheel disc has been developed by GKN Wheels, Corus and SSAB Swedish Steel, using advanced high strength steels (HSS) that are up to four times stronger than ordinary mild steel.

The new design, which is believed to be the first off-highway wheel disc to use HSS, is 30% thinner and 30% lighter than a conventional wheel disc, as well being more cost-effective and environmentally friendly.

One of the major drivers behind the development of the new wheel disc has been the increasing specification levels of agricultural, construction and other off-highway vehicles over recent years.

Manufacturers have made big advances in engine power, speed and in-vehicle equipment, such as satellite navigation, to meet the increasing demands that are being placed upon these vehicles.

At the same time, the supply-chain has been subject to increasing pressure to help reduce costs and weight in order to minimise the impact of rising specification levels on these critical areas.

David Eastham, Sales and Marketing Director for GKN Wheels, said: "We worked on the principle that the biggest gains were not to be made by trying to take cost and weight out of existing products but by ensuring that it wasn't there in the first place.

Essentially, this meant starting afresh and taking a holistic look at the wheel disc design and manufacturing process to identify how we could develop a lighter and more cost-effective product without compromising performance and safety".

The availability of advanced high strength steels with minimum tensile strengths ranging typically between 400 and 1400N/mm2 and yield strengths in excess of 550N/mm2, offered a potential solution.

For several years, Corus Service Centres has been at the forefront of developing the UK market for these products.

Alongside high strength steels from Corus, it offers a range of materials from other leading European manufacturers including SSAB Swedish Steel.

Paul Taylor, Technical Services Manager for Corus Service Centres said: "When the dynamic properties of these steels are taken into design calculations, significant reductions in weight, material and production costs can be achieved.

The first step at GKN Wheels was to undertake a comparison benchmarking exercise, using a 12.5mm gauge wheel disc in grade 355 hot rolled steel from the existing product range, to assess whether high strength steels could be used to 'reinvent the wheel'.

Using a rule of thumb equation (see note below), which links yield strength to thickness in terms of comparable performance, we found that in principle it would be possible to produce a lighter, thinner wheel disc, subject to changes in the design".

The next phase was to undertake detailed feasibility studies to investigate whether GKN Wheels existing cold forming presses would be able to press a HSS component, and to determine if a disc manufactured from higher strength, thinner gauge material, albeit to a revised design, would be able to resist the bending moments to which it would be subject.

Critically, it was also necessary to discover whether it would have sufficient fatigue resistance to perform without risk of premature failure.

Using advanced mathematical modelling techniques SSAB Swedish Steel was able to successfully validate the initial findings and provide sound theoretical answers to these questions.

In all cases, the calculations gave clear, positive results indicating it would be possible to down-gauge the disc significantly from 12.5mm to 8mm by substituting grade 550 hot rolled steel for the grade 355 material, and incorporating stiffening ribs into the design to provide the required levels of rigidity.

Before embarking on the costly process of manufacturing production tooling, two further tests were undertaken.

Using existing tooling, plain discs were manufactured from 8mm material in grades 500 and 650 and subjected to cyclic fatigue tests.

The results from these tests initially gave cause for concern, but the root cause of the failure was quickly traced back to the poor geometry of several punched holes in the disc.

Once this was rectified, the project was able to proceed quickly to the next phase.

Pre-production tooling incorporating the stiffening ribs was commissioned and sample discs were successfully produced from 8mm hot rolled material in grades 500, 550, 600 and 650.

Paul Taylor said: "At this stage we have demonstrated how it is possible to produce a wheel disc that is 30% thinner and therefore 30% lighter than a conventional component by using high strength steels and the latest CAD design and modelling techniques.

This new disc will help to maximise the load-bearing capacity and fuel-efficiency of the vehicle.

We have also succeeded in reducing total supply-chain costs which provides a significant competitive advantage that is difficult to ignore".

David Eastham added: "The work to date has shown the major advantages of closer co-operation throughout the supply-chain and the involvement of key supply partners at the concept stages of projects.

This has huge benefits for all parties, including most importantly, the end customer who gets a product that helps to improve the overall performance of his vehicle.

We are now looking to engage the support of the remaining members of the supply-chain in order to realise the true potential of this project and bring the new disc into full production".

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