Development of a methodology to increase the torsional fatigue strength of drive shafts with cross bore (shaft bore)

 

In the powertrain of passenger cars, shafts, such as input and output shafts in transmissions, but also crankshafts, are required to have cross bores with diameters between 3 mm and 5 mm to supply oil to bearings. Under cyclic torsional loading, these bores are typically the failure-critical areas of the shaft. The fatigue strength of the bores is therefore the design criterion for shafts in most applications. By means of surface treatments, a significant increase in fatigue strength of the bores is possible. Currently, the fatigue strength increasing effect of mechanical surface treatments of the cross bores is determined experimentally for each application. These expensive and time-consuming experiments are necessary because no reliable numerical or analytical design concepts exist for the design of shafts with surface strengthened cross bores. Particularly for small and medium-sized companies, component design therefore turns out to be extremely costly. Fatigue strength assessment is either based on expensive experiments or surface treatments are not used, and thus lightweight design and savings potential is not exploited to the full.

To increase the fatigue strength of shafts with cross bores, this project primarily investigated two mechanical surface treatments, (1) deep rolling and (2) shot peening. The principle of effect of these treatments is based on the induction of compressive residual stresses in the peripheral zone. In addition, induction hardening and the combination of induction hardening and subsequent deep rolling were investigated. Within the research project, the potential of the different surface treatments of the cross bores with regard to fatigue strength enhancement was demonstrated.

The investigations were carried out on a cast iron (EN-GJS-700-2) and a steel material (34CrNiMo6 +QT). In the case of the cast iron material, it was shown that the mechanical surface treatments lead to a lower slope of the finite life fatigue curve and an increase in the load-bearing capacity at the buckling point. Induction hardening shifted the location of the crack deeper into the cross bore, thereby increasing the fatigue strength. In combination with deep rolling, a further increase in fatigue strength was achieved by combining the effects that occur during deep rolling and inductive hardening. No significant increase in fatigue strength was observed for the steel material specimens that were only mechanically surface-treated. Based on experimental as well as numerical investigations, an explanation for these results was the complete redistribution of the compressive residual stresses induced by the applied loads during fatigue testing. A slight increase in fatigue strength was observed for the induction hardened specimens. Only the combination of induction hardening and deep rolling resulted in a decisive increase in fatigue strength.

Furthermore, a calculation method based on finite element simulations, which was further developed in this project, made it possible to predict component properties and thus evaluate the fatigue strength.
The results of this research project can be used

• for better material utilization,
• for more reliable dimensioning,
• for cost-, time- and resource-efficient product development through the computer-aided evaluation method, and
• to ensure the reliable use of drive shafts.

The IGF project 20407 N of the Research Association Forschungskuratorium Maschinenbau e.V. - FKM (Combustion Engines), Lyoner Straße 18, 60528 Frankfurt am Main, Germany, was funded by the German Federal Ministry of Economic Affairs and Climate Action through the German Federation of Industrial Research Associations (AiF) as part of the program for Industrial Collective Research (IGF) on the basis of a decision of the German Bundestag. The final report of the project can be obtained from the Forschungskuratorium Maschinenbau (FKM) (FKM) e. V. (postal address: Lyoner Str. 18, 60528 Frankfurt am Main, e-mail: info@fkm-net.de, phone: +49 69 6603 1681).