## 2 Experimental Procedure for Tooth Root Bending Fatigue Testing  
Pulsator or STBF tests are performed with the idea of loading one or two teeth at a fixed location to ensure tooth root bending fatigue is the only present failure mechanism. This approach leads to a variety of testing configurations such as the one adopted by [PERSON] and [PERSON] [29], where torque is applied to a toothed shaft that is in contact with a fixed anvil, the three-point bending configuration (e.g., [1; 30]), and the one-tooth (also known as asymmetric) and two-teeth pulsator (also known as symmetric) test configurations; the last two seem to be the most commonly adopted.  
The one-tooth test configuration was first described by [PERSON] et al. in 1982 [31] and is now included in the SAE recommendation practice J1619 [32]. Here, the force is applied only on a single tooth while another tooth and a centering pin provide the reaction to the applied load. Therefore, only a single tooth is fully loaded. Hence, this configuration is typically called asymmetric (the same is applied here). It is worth mentioning that STBF (Single-Tooth Bending Fatigue) has been developed to describe this type of test. However, now this term is also adopted as synonymous with a pulsator test (e.g., [15; 33]), therefore describing all the possible test methodologies that aim to perform tooth root bending fatigue tests by loading the gear teeth (or tooth) by an anvil(s) (rather than in MG conditions).  
In the two-teeth configuration, the load is transmitted from one tooth to the other without the interaction of any other parts of the testing equipment. Here, the force is applied by two clamping jaws, typically spanning over three to five teeth. As a result of the principle involved in the Wildhaber measurement [34] and by proper design of the text fixture (i.e., the distance between the anvils and the gear hub centerline equal to the span measure over two), these jaws load the teeth at the same time at the same nominal diameter in a direction that is tangential to the base diameter. Therefore, both the tested teeth are subject to the same nominal tooth root bending stress. Hence, this configuration is typically called symmetric (the same is applied here). This configuration can still be referred to as STBF but, differently from the previous case, represents Single-Teeth Bending Fatigue. Research is now underway with the aim to investigate the effect of dynamic loads on tooth root stress via a specific testing rig (e.g., [35]).  
Here, it is worth mentioning that by properly setting the distance between the anvils and the gear hub centerline, it is possible to obtain an asymmetric two-teeth test configuration (e.g., [33]). This configuration is adopted to maximize the too root stress of one of the two tested teeth; it is rarely used.  
Figures 2 and 3 show two examples of pulsator machines working in symmetrical configurations. All the data discussed here have been obtained with this methodology.  
To maintain the specimen in position, pulsator tests must be run with a fixed preload. For instance, the SAE recommendation practice J1619 [32] suggests performing tests at  
Figure 3: The Schenck mechanical resonance pulsator adopted at POLIMI.  
Figure 2: Pulsator test rig (schematic) adopted at FZG [36].  
\(R=0.1\), while within FVA guideline no. 563 I [25] a load ratio between 0.03 and 0.075 is suggested.