5.5.2. Pitting Tests

In order to shorten the test time, the test conditions were changed to the standardized test according to VW PV 1483 [35]. In the so-called pitting test, the axial load was 60 kN and was thus 50% higher than the previously mentioned conditions. This rolling bearing fatigue test usually aims to differentiate gear oils with regard to their pitting resistance. The test parameters are summarized in Table 4. The main criterion is a target running time of 200 h, divided into two phases:



**Table 4.** Test conditions for fatigue testing of 81212 type tailored forming bearings.

The fatigue life evaluation was carried out using Weibull analysis. Accordingly, the failure probability was calculated for *n* = 6 samples using maximum likelihood estimation and plotted in a double logarithmic diagram over the lifetime in Figure 14. A two-parameter Weibull distribution was assumed. The probability of error was 10%. The shape parameter, which indicates the slope of the regression line, was β = 3.47. The characteristic lifetime parameter indicated the lifetime at a failure probability of 63.2% and had a value of *B*63 = 6.32 × 10<sup>6</sup> revolutions. The experimentally determined nominal life, which corresponds to a 10% probability of failure, is *B*10 = 3.88 × 10<sup>6</sup> revolutions. This result serves as a baseline for future research with tailored forming bearings.

**Figure 14.** Bearing fatigue testing results: (**a**) Weibull plot with 95% confidence band for AISI 52100 tailored forming washers; (**b**) fatigue damage analysis by laser-scanning microscopy (rolling direction: right to left).

The theoretical rolling bearing life for conventional bearings can be calculated according to DIN ISO 281 [36], based on the work of Ioannides and Harris [37]. Since the operating speed of 500–750 min−<sup>1</sup> exceeds the thermally permissible speed, an adjusted reference speed of 300 min−<sup>1</sup> was assumed for the lifetime calculation of conventional bearings as a reference. The additional factors are summarized in Table 5. The extended modified lifetime at a 90% survival probability was calculated to *L*10m = 4.75 × 10<sup>6</sup> revolutions. The deviation of the experimentally and mathematically determined lifetime is *B*10/*L*10m = 81.68%. Thus, the bearing failures of AISI 52100 tailored forming bearings can be considered as premature failures due to small pores below the surface.


**Table 5.** Results of the fatigue life of tailored forming bearings and comparison with the calculated life of conventional bearings.
