Gears were among the first mechanical elements that were shot-peened and they keep on being widely shot-peened at the present time. Gears of automotive gearboxes are frequently shot-peened if better performances are needed and gearbox-downsizing is the main objective of the project. This is even more important today since the design of light cars is a key point to reduce fuel consumption and emissions in the environment. 

Despite the application being quite old, gears remain one of the main subjects for research on shot peening. Look around you. In every conference about shot peening or surface treatments, there is at least one session dedicated to this topic. The aim of these presentations is quite similar and is related to the assessment of the improvement that can be reached by applying shot peening in terms of pitting or fatigue strength, and research of the optimal peening parameters with different materials of interest. In recent years these studies have also considered the simulation of shot peening to determine the residual stress state induced and finally, applying this data for fatigue or contact-fatigue strength assessment.

However, if we read the books on gears, it is possible to find that shot peening is also able to improve lubrication conditions thanks to the modified surface texture and its ability to trap lubricant in the indentations. This means that wear will be reduced, and mechanical efficiency will be improved. Even if this fact is generally accepted, to my best knowledge, great effort has not been dedicated so far to investigate and understand the phenomenon and to address shot-peening solutions able to analytically consider this aspect.

This is probably due to different factors: this topic was not among the priorities of gearbox design; the ability to control and analyze the surface texture induced by shot peening was poor; and the numerical tools for computational fluid dynamics (CFD) analysis were not easy to use and not well developed. 

Today, the panorama has changed: the experimental surface analysis tools have greatly improved and the possibility to digitalize the surface allows a deeper understanding of the surface behaviour. The CFD codes are now much more powerful and user-friendly at the same time, and can be considered a practical tool for this kind of analysis. Finally, the new shot-peening systems allow better control in terms of final surface state. 

Times are mature for these kinds of investigations that would clarify whether shot peening can really improve efficiency of the gearbox while not only increasing its fatigue and contact-fatigue strength.  By means of such research, shot peening would add another reason for its applications and would probably lead to new treatment parameters.

For sure, it is not a short and easy way to go. Experimental and numerical investigations are time-consuming and expensive, and the final results not known a priori. But, in an ever-more competitive environment, the know-how coming from these studies can add another quality to shot peening that can be used to reduce the environmental impact of cars (and not only cars). Today, this is not a trivial aspect to consider.  

Contributing Editor MFN and 

Full Professor of Technical University of Milan

20156 Milan, Italy

E-mail: mario@mfn.li