Shot Peening in the Automotive Industry

in Vol. 20 - March Issue - Year 2019
Contact And Bending Fatigue, Two Main Problems Of Gears

Mario Guagliano

I think that there is no need to underline the importance of gears in an automobile. Gears are the core of the power transmission of a car and will remain important even when cars will move to electric drive. Due to the continuous challenge of leading to more and more reduction of weight, the size of the gearbox has been greatly reduced. This is possible thanks to the present possibility to take advantage of the use of modern software for numerical simulation, being able to refine the design of the system and reducing the traditional safety factors without losing reliability and strength. But it is also due to the continuous research and development of treatments able to induce surface hardening, thus reducing the typical damage mechanisms of gears, contact fatigue at the tooth flank, and bending fatigue at the tooth root. These two kinds of damage, even if both induced by the cyclic variation of the load, are quite different and must be differently considered.
Contact fatigue originates at the in-contact surfaces and is initiated by the excessive value of the contact pressure, causing crack initiation and propagation. Indeed, there are different damaging mechanisms due to contact fatigue; the most common is called pitting, which initiates at the surface and causes progressive removal of small areas of material and the formation of pits. This phenomenon is very complex and influenced by different factors: however, also in this case, compressive residual stresses in the surface layer of the material improves the behaviour of gears with respect to pitting.
Bending fatigue happens at the tooth root and is caused by the severe stress concentration at that location. In this case, the crack starts at the base of the tooth, where the bending moment is at its maximum. No need to say that also in this case, compressive residual stresses are beneficial and improve the bending fatigue strength of the gears.
The favourable action of compressive residual stresses is the main reason why shot peening is done in high-performance gears, where the surface hardening induced by thermal and thermochemical treatments is not enough to guarantee an adequate endurance and/or strength.
But shot peening can be done in different ways and by using different parameters. Each combination of the relevant parameters gives a different in-depth residual stress distribution, which results in a different effectiveness of the treatment itself. In other words, shot peening must be finely tuned and optimized considering the material to be treated, the geometry of the peened part and the entity of the fatigue stress: if this step is skipped, premature and unexpected failures could happen.
In the case of a gear tooth, the final choice of the shot peening parameters is even more complicated, since there is more than one critical zone and there are two damage mechanisms to consider. And this means that the optimal shot peening for the tooth flank is different with respect to the optimal treatment regarding the tooth root. The natural consequence is that shot peening should be performed in a different way in the two zones of interest.  This means increasing the cost of the treatment and using facilities that make possible a different kind of shot peening without the need for stopping the process.
Indeed, I do not know how much this way to proceed is used in the automotive industry and whether it is compatible with mass production; however, thanks to new technological developments and the stronger and stronger integration of informatics tools and shot peening machines, a "dual" shot peening treatment could become more popular and convenient soon.

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