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Superfinishing of transmission gears

The gears in racing transmissions are subject to loads that far exceed those in production cars. As such, careful attention needs to be paid to gear construction and design to ensure they are reliable and efficient. In the past we have looked at gear materials and manufacturing methods, as well as the benefits of surface coatings such as DLC. However, there is another useful technique that can be used to improve the performance and reliability of gears: superfinishing.

It is well documented that finishing the working surfaces of gears and their root fillet regions to a very low roughness can result in a considerable increase in surface durability, as well as reducing friction and operating temperatures. Achieving such finishes using traditional methods such as surface grinding and honing is very time-consuming though, and carries considerable risk that the profile of the gear teeth can be irreparably changed. The use of chemically accelerated vibratory finishing methods, generically referred to as superfinishing, can avoid such problems while also resulting in a much smoother surface finish. The average roughness achievable with grinding techniques is about 6.0-12 µin, whereas superfinishing can produce a roughness in the region of 1-3 µin.

Superfinishing is undertaken in vibratory finishing tubs, of the same type that have been used for many years in other abrasive media finishing processes. The media used in these tubs is a high density, non-abrasive ceramic material, with the shape and size of the media selected to match the geometry of the gears being finished. The media does not itself remove any material from the gears, it is only once a reactive agent is introduced into the finishing tub that changes in the surface finish of the gears occur.

The reactive agent produces a stable, soft conversion coating across the asperities of the gear surfaces. As the media in the finishing tub rubs across this coating, the peaks and valleys of the material are gradually smoothed out, until the surface is, to all intents and purposes, free of asperities. The reactive agent is mildly acidic and, depending on the concentration used, stock removal occurs at 0.00005-00040 in/h. This removal is beneficial, allowing gears with an initial surface roughness of around 60 µin to be finished to a final roughness of 3 µin.

Interestingly, a consequence of using hard ceramic media in the process is improved wear resistance in use. Although the material is non-abrasive, it still leaves a micro-textured surface on the material being finished. Testing undertaken by the University of Cardiff, Wales, to look at scuffing performance of test discs treated with ceramic and plastic media respectively showed that the ceramic media-treated parts had a much higher resistance to scuffing than those treated in plastic media. This was surprising, given that the plastic media leaves a smoother surface. It therefore follows that there is an optimum surface roughness that needs to be achieved in order to reap the greatest benefits from the process.

Overall then, superfinishing can improve durability and performance in transmissions compared with traditional ground finishes. As a result, many high-performance transmission manufacturers now use the process to ensure their gears are better able to survive the rigours of competition use.

Written by Lawrence Butcher

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