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

 

In the powertrain of passenger cars with combustion engines, shafts such as input and output shafts in gearboxes, but also crankshafts having typically cross-bore diameters between 3 mm and 5 mm for supplying oil to the bearings. Under cyclical torsion loading, these bores are the failure critical areas of the shaft.

The fatigue strength is the design criterion for shafts in most applications. Since the shafts often require cross bores, these are the critical areas with regard to fatigue strength. By means of surface treatments, an increase of the fatigue strength by a factor of five 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 there are no reliable numerical or analytical design concepts for the design of shafts with surface treated cross bores. Especially for small and medium sized companies the component design is therefore extremely costly. Either the fatigue strength evaluation is based on expensive experiments or surface treatments are omitted, thus wasting potential for lightweight construction and savings. In order to increase the fatigue strength of such shafts with cross bores, two mechanical surface treatments, internal deep rolling and shot peening, are therefore being investigated in this project. The mode of operation of these processes is based, as in the case of deep rolling of radii, on the introduction of residual compressive stresses in the peripheral zone near the surface.

The research project will demonstrate the potential of mechanical surface treatments of cross bores in shafts by shot peening and internal deep rolling with regard to the increase of fatigue strength. Furthermore, a calculation method based on finite element simulations to be developed in this project will enable the prediction of component properties and thus the evaluation of the fatigue strength.

The expected 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 reliable operation

of drive shafts.