Shot peening is widely used in the automotive industry: springs, gears and shafts are some of the car components usually peened. This is not a novelty, everyone knows, I suppose.

Shot peening of gears is a traditional application of this treatment, since it allows increase in strength of the gear tooth both regarding the so-called "contact fatigue", which is due to the cyclic repetition of the Hertzian contact pressure distribution, as well as concern for the bending fatigue at the gear root, due to the bending stress at the root of the fillet of the gear tooth. In both cases, the result of shot peening is surprising. It is not rare to read that the bending fatigue limit after shot peening is more than twice that before shot peening, or that contact fatigue endurance after shot peening has increased even ten-fold the number of cycles of un-peened gears.

Indeed, the result is strictly related to the correct design of the shot peening treatment. This means that a series of experimental tests and analysis should be done, at least when a new case is presented. But we know that this means adding time and cost to the project, and the desire is to cut costs!

Another way to proceed is to consider the international standards, which for gears means to look at the AGMA and the ISO codes.  Indeed, most of the standards more consider the aspects related to the way shot peening is performed (in other words the correct set-up of the treatment) rather than the relationship between treatment parameters and effect on the performance of the treated parts.

If we consider the ANSI/AGMA code “Gear Materials and Heat Treatment Manual”, it includes a paragraph dedicated to shot peening. The standard informs us that bending fatigue resistance can increase about 25% due to shot peening and consequently, this surface treatment can be utilized for upgrading existing gears. If contact fatigue of the flank is considered, the standard states that resistance can be improved; however, it states also that quantitative data to substantiate this condition is poor. 

The ANSI/AGMA 2001- D4 "Fundamental Rating Factors and Calculation Methods for Involute Spur and Helical Gear Teeth" states that compressive residual stresses should increase the bending load limit of gears. In the Appendix of the code, the effects of shot peening on automotive gears are quoted: an S-N curve taken from a scientific paper is reported, showing that the improvement of bending fatigue limit is around 30%.

If the ISO Standards are considered, it can be noted that they do not give little evidence and data concerning shot peening as a method to improve the fatigue resistance of gears; quantitative data is poor. The ISO 6336-3 "Calculation of load capacity of spur and helical gears" states that, in general, the allowable limits depend on residual stresses. 

Whatever is the way to proceed, there are some critical issues, and by performing experimental tests, it is possible to optimize the treatment and by following the standards, we underestimate the benefit of shot peening.

So, what can we do to accurately consider the effect of shot peening without running many experimental tests? The development of a digital twin of the process could be an interesting solution.

Contributing Editor MFN and 

Full Professor of Technical University of Milan

20156 Milan, Italy

mail: mario@mfn.li