Power steering with plastic wheels

Automotive systems maker ZF Lenksysteme of Germany is the first company to use a plastic gear wheel in electric power steering units now being fitted to medium-sized passenger cars.

Automotive systems maker ZF Lenksysteme of Germany is the first company to use a plastic gear wheel in electric power steering units now being fitted to medium-sized passenger cars such as the new Golf, Touran by Volkswagen and the Audi A3 (the system first appeared on the BMW Z4 roadster).

Conventional hydraulic power steering assemblies are relatively bulky, requiring a steering pump and valve, oil reservoir and high-pressure hoses. What’s more, the engine-driven steering pump runs constantly – and that requires energy and therefore precious fuel.

By contrast, electric power steering units – ZF calls theirs ZF-Servolectric – which employ an electric servo-motor, are more compact and only operate when assistance in turning the road wheels is required. During straight-ahead driving, the system is inactive and so consumes no power, resulting in fuel savings of around 0.2 litres per 100 km (to achieve a similar saving, a vehicle would have to be some 100 kilos lighter).

In addition, electric steering is largely maintenance free, and unlike hydraulic power steering, can be tested as a complete module before being fitted to the vehicle. And being modular, electric power steering is easy to adapt to different types of vehicle.

Another benefit is the ability to program (electronically) the steering response to produce different vehicle “feel” and handling characteristics by taking into account not just vehicle speed, but also the steering angle and the steering wheel rotational speed. Such microprocessor control also allows special steering characteristics to be incorporated into vehicles intended for disabled drivers.

The advantage of electric steering is that the electric servomotor that provides the power assistance can be linked to the steering system at various points: on BMWs, it is on the steering column; on Volkswagen models, the servo-motor is connected to the steering rack via a second pinion wheel.

The ZF-Servolectric power steering unit is the first for medium-sized passenger vehicles in which the toothed wheel of the servo-motor’s worm gearset is not made of metal but an engineering plastic.

This plastic is Ultramid A4H, an unreinforced nylon 6.6 with excellent tribological properties. The plastic worm wheel must be capable of transmitting large forces at temperatures between -40 degrees and +120 degrees Celsius for the lifetime of the vehicle.

Thanks to its high melt viscosity, the resin is especially suitable for moulding thick-walled components. Ultramid A4H is used in applications in which frictional properties and temperature are important factors: for chain tensioner blades and guides inside engines, actuators for electric seat adjustment and windows, as well as roller bearing cages.

ZF Lenksysteme GmbH is a 50:50 joint-venture between Robert Bosch GmbH and ZF Friedrichshafen AG.

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