Maybe it is trivial to recall that gears are among the machine elements that have been shot peened from a long time ago, and that most of the shot peened gears are used by automotive manufacturers to improve the performance of power transmission systems of cars. Indeed, if shot peening is applied to a gear power transmission designed without considering shot peening and then improved by applying this treatment to gears, the real advantage could be limited by the fact that all the other components of the systems are not changed and so cannot follow the new, improved performances of gears without some problem. That is to say, that it would be better to re-design all the gearbox to get the maximum advantage from the application of shot peening. Anyway, a greater advantage could be obtained also with a proper design of the shot peening parameters used in this application.
In fact, we know that gears are subjected to three main problems; contact fatigue in its multiple forms (pitting, micropitting, spalling…), wear, and bending fatigue.
The first two damage mechanisms develop on the tooth flank and are directly related to the contact conditions of the mating gears, while the third one happens at the tooth root and development follows the classical stages of metal fatigue.
Shot peening is effective both in preventing (or delaying) contact fatigue and bending fatigue while there is little data about shot peening against wear (note: someone confuses contact fatigue with wear and writes that shot peening is very useful against wear: but he is only confusing two different damage mechanisms!).
Most times, contact fatigue and wear are the most dangerous, damaging mechanisms in gears, but the greatest amount of knowledge about the way shot peening improves the mechanical properties of the treated parts is about bending fatigue.
The truth is that contact fatigue and wear are very complex phenomena, not fully understood (also bending fatigue is not easy, but many more research studies were developed for a better understanding of this damage mechanism) and residual stresses and surface work hardening due to shot peening makes the development of a criterion for assessing strength and in-service life much more difficult.
But there is another effect of shot peening that has a great importance for contact-related damage mechanisms; it is roughness.
Roughness is generally considered as an unwanted consequence of shot peening, since it increases with shot peening with a negative effect on fatigue strength. But, as regards contact fatigue, it can have a useful effect on the contact conditions. The indentations left by the impact of the shots against the target surface, in fact, can act as a sort of "bag" where lubricant remains and that is thrown out during the contact cycle, thus improving the lubrication conditions and reducing friction between the in-contact teeth.
This is not new, but I think that nobody would be able to describe what really happens and describe with an analytical formula the relationship between the roughness, the shot peening parameters, and the improved lubrication conditions. This means that a clear understanding of what happens has to be obtained, and no design criteria about this are available.
A better understanding of this phenomenon could result in a fruitful application of shot peening against wear. Indeed, we know that wear is not so strongly influenced by residual stresses and by the surface work hardening due to shot peening. But wear is influenced by the contact conditions and by friction between the in-contact surfaces. And, among the different wear mechanisms, adhesive wear should be the one most influenced by the improved contact conditions.
The topic is not easy, as many variables have a role in this engineering problem (i.e., the lubricant characteristics could modify the effect of shot peening with respect of wear, making the solution of the problem even more difficult), but the importance of wear in all the industrial fields would justify an effort devoted to look in-depth at this peculiar aspect of shot peening and to better understand the relations between shot peening indentations and the real lubrication conditions.
This is particularly true in the automotive field, where competition among the surviving actors is becoming increasingly aggressive.

Shot Peening in the Automotive Industry
by Mario Guagliano
Contributing Editor MFN and
Associate Professor of Technical University of Milan
20156 Milan, Italy
E-mail: mario@mfn.li

Author: Mario Guagliano