Eccentric rack bearing

Smoother ride

Who

Electric power steering specialist Nexteer Automotive has developed a polyamide eccentric rack bearing that it claims has superior NVH performance compared to conventional yoke-style bearings. The component will help vehicle manufacturers improve the feel and noise of electric power steering (EPS) systems in vehicles of all sizes, it says.

Technology

The need to cut C02 is making electric power steering a key technology. According to Nexteer, replacing hydraulic steering with an on-demand electrically powered system can reduce a vehicle's C02 emissions by 5-6g/km. But as manufacturers improve their vehicles' efficiency they must also maintain steering feel. Steering friction is a key factor in this, determining the purity of the feedback. A system with lower friction makes the drive feel more assured, communicating what's happening at the contact patch to the driver's fingertips.

"Controlling lash, the play between mating components, in EPS systems is a significant part of our advanced development work," says Paul Poirel, Nexteer's chief product engineer. "A balance between tight tolerances and low friction is what gives steering systems a feeling of directness. The new bearing's reduced friction ensures that feel is not lost in the system. It also noticeably improves steering noise."

Nexteer's solution eliminates traditional yoke-style bearings that use a spring-loaded bushing to protect against lash but which can resonate, creating rattles and other NVH problems. The polyamide bearing makes the steering system quieter, more durable and easier to package. Reducing the friction also makes it quicker and easier to finetune the system to suit customers' requirements.

"The bearing underwent countless painstaking iterations to produce a mechanical design that is both simple and robust," says Poirel. "It has to absorb shocks and be insensitive to moisture."

Application

Nexteer says it is receiving considerable interest in the development. The eccentric rack bearing recently entered production in the EPS system for the premium B-segment Citroen DS3 and the firm expects other applications to follow.