## 3 The Pulsator to Running Gears Problem  
In the previous section, the typical experimental pulsator test rig configurations were presented. The fact that gear teeth are loaded by jaws rather than by rotating gear meshing implies that differences occur between the pulsator test case and the actual application case of MG. Recently, [PERSON] et al. [37], whose testing rig is an asymmetric one, summarized those differences as follows:  
1. Pulsator tests and MG have different loading conditions, as, in the first, a sinusoidal load is applied at a fixed position, while in the latter, the load changes both the magnitude (due to the load sharing between teeth pairs) and the point of application (as the contact point between teeth changes during gear meshing). 2. The stress ratio R is different as pulsator tests are required to work at \(R>0\), while in the MG gear case, the tooth root stress moves to a positive maximum (typically when the contact is at the outer point of a single tooth in contact) to a negative minimum (due to the extension of the compression field from the adjacent tooth, e.g., [4; 5; 38]), thus at a load ratio \(R<0\) (e.g., [39]). 3. The crack initiation location seems to be different. Indeed, certain experimental findings of [PERSON] et al. [40] and [PERSON] et al. [36; 41; 42; 43; 44] possibly imply that in the pulsator test case, crack initiation is much more likely to occur at the surface, while in the MG case, a slightly increased risk for subsurface crack initiation can be observed. However, their findings are based on investigations on shot-peened gears made from high-cleanliness steels. Furthermore, a different runout level was considered for pulsator and MG tests. Moreover, considering all the data available [36; 40; 41; 42; 43; 44], and further results from experimental investigations at FZG that have not been published yet, it can be assumed that there is not a different crack location for common case-hardened gear steels for both compared testing methods (i.e., pulsator and MG). Therefore, the effect of the crack initiation location will not be discussed here. 4. The statistical behavior of pulsator and MG tests is different. In pulsator tests, the failed teeth are predetermined and are those that are loaded within the pulsator test rig (for each test run, two teeth are in the symmetric configuration); furthermore, not all the gear teeth can be tested. On the other hand, in the MG case, all the gear teeth are loaded during a test run, and the gear failure is the result of the failure of the weakest gear tooth (e.g., [22; 29; 37; 45; 46; 47]). Therefore, strength numbers determined by pulsator tests should be elaborated to ensure a reliable estimation for gear design. In 1964, with a slightly different test rig, [PERSON] and [PERSON] [29] already suggested that pulsator experimental strength numbers should be reduced by a factor of 0.77 to account for those differences. In 1987, [PERSON] [22] dealt with the problem of the different loading conditions and stress ratio between the test methods. He suggested that, in order to consider the differences in the loading conditions, pulsator test results should be reduced by a factor of 0.9. The validity of [PERSON]'s coefficient has also been reaffirmed recently by [PERSON] et al. [48; 49; 50], who adopted high-cycle multiaxial fatigue criteria to estimate the effect of the different stress histories occurring at the tooth root. In 1999, [PERSON]'s work [22] laid the groundwork for the estimation procedure reported by [PERSON] et al. (FVA research project 304) [23]; this approach is discussed in detail in Section 4. This method is completed by the FVA guideline 563 I [25], in which additional information (e.g., the sampling strategy) is provided to the experimenter. According to [PERSON] et al. [23], pulsator tests should be reduced by a factor of 0.83 for peened and 0.77 for unpeened gears. Those values are completely comparable to what [PERSON] and [PERSON] [29] found. In 2003 and 2008, [PERSON] and [PERSON] [45; 46] proposed a different approach.