Microfinish optimizes new  passenger car steering systems

In order to reduce the CO2 emission in new cars the automotive industry is using more and more steering systems with electronic servo motors instead of the usual servo hydraulic systems.
The advantages are the following:

• Use of energy management tailored to suit market need

• Easy adaptation to different driving situations like parking, driving in cities, in the country or on freeways

• Use of assistance systems e.g. for parking, drive-by-wire

• Improved towing and break down behavior

Apart from the simpler electro-hydraulic systems and systems applying pinion gears manufacturers use more and more systems with recirculating ball screws. These systems feature an extremely compact and light design at high loads and are fully electronic. With this principle the balls are only in point contact with the spindle and the nut, just like with a ball bearing. Waviness of the raceways of the spindle and the nut lead to changes in the pre-load and thus to noise and tumescent resistance during actuation. Bad sensitivity and distinct noise increase the insecurity of the driver of such cars.
The waviness is generated during the manufacturing process through the narrow contact of the tools with either grinding or whirling. The waviness can only be limited by an optimum bracing of the spindle and by good cutting and well dressed tools.
Microfinishing eliminates the waviness and optimizes the running-in characteristics by reducing the roughness. The influence of waviness can be proven by noise measurement of the complete steering system or beforehand by respective measurements with a consecutive FFT analysis.

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