Crankshaft is one of the machine elements that attracted the interest of engineers and scientists, that employed resources and time in developing design approaches able to lead to performing and economic design solutions. Indeed this topic is still of interest; this is due to the complexity of this machine element, both with regard to its geometry and the applied loads and deriving internal forces. In fact, due to its mechanical function (to transform an alternative linear motion into an almost uniform rotation motion) the geometry of crankshafts is really severe, with a strong notch effect. On the other end, the eccentricity of the mass attached to the crankshafts make it prone to a dangerous dynamic and vibration behaviour, especially in the case of multi-crank shafts. It is also peculiar that crankshafts have to transmit torque but they are particularly stressed by bending moments, at least if natural torsion modes are not excited.
In summary, the crankshaft, both for its geometry and the related severe notch effect and applied loads, is exposed to fatigue failure.
But the analysis of the fatigue behaviour of a crankshaft is really complex, since it is a typical case of out-of-phase fatigue loading, while the notch induced by the fillet radii between the web and the journal cause a steep stress gradient: I think that not many other fatigue cases are as difficult as the crankshaft. But they work thanks to the help offered by shot peening.
Indeed, high-performance crankshafts are all surface treated: nitriding, deep-rolling and shot peening are among the most popular treatments used to improve the mechanical behaviour of crankshafts. Anyway, nitriding is not an environmental-friendly treatment. Deep rolling requires dedicated machines and tools, shot peening is the most general treatment which can be applied using general machines and has a better environmental impact with respect to thermo-chemical treatments. But what about the results? It is true that the residual stress field induced by shot peening is less deep than the one caused by shot peening but this does not mean that shot peening is not useful for crankshafts, as some engineers believe.
Unfortunately not so many data are of public domain (automotive manufacturers are jealous about the experiments they make!), but some conclusions can be drawn. By looking at some data found in the literature it is clear that the application of shot peening improves the fatigue limit of crankshafts, both if the material used is a forged steel and if the material is nodular cast iron. In this latter case the increment of the fatigue limit induced by shot peening is more pronounced, and it is not rare to have an increment of the fatigue limit up to 30%-40%, depending on the peening parameters used.
But, what is (maybe) more interesting is that, without changing anything but the material, by shot peeing it is possible to obtain higher fatigue limits by using nodular cast iron than by using forged steels. That is to say that if shot peening is considered and used as final treatment it is possible to substitute a forged steels with a cheaper nodular cast iron. With a global reduction of costs.
A particular field of application of shot peening is race cars, like Formula 1 ones. In this case the requirements of crankshafts are different: they have not to last for hundreds of thousands of kms but are substituted each competition (more or less). In this case the design approach is different, since it is scheduled that crankshafts are periodically substituted and the stress level is more than the fatigue limit. So it is expected that the residual stress filed induced by shot peening relaxes and it is expected that shot peening is less effective in increasing the fatigue life of crankshafts. This is only partially true, since the better fatigue behaviour induced by shot peening is also due to the surface work hardening. This means that in these cases shot peening can be fruitfully used. And, it is true that is used also for competition cars. But, it is also true that if we want get the maximum advantage by shot peening we have to carefully think about the treatment parameters in relation to the treated material. More than this a complete experimental test program is necessary to understand the way shot peening affects the fatigue strength in the finite-life regime and the quantitative results we have to expect.

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