# Bending Fatigue Behavior of 17-4 PH Gears Produced by Additive Manufacturing
## 2 Experimental Set Up  
To investigate the bending fatigue properties of gears produced via AM technologies, the STBF were performed on a Schenk mechanical pulsator capable of applying a maximum load of 60 kN. This machine, originally developed and usually used for uniaxial fatigue tests, has been equipped with a specifically designed fixture to perform the STBF tests. As shown in Figure 1, the machine's working principle is based on two coaxial springs: the inner one applies the constant part of the load, while the outer spring, thanks to rotative unbalanced masses, is responsible for the sinusoidal part of the load. A load cell and a dedicated control unit are responsible for keeping the sinusoidal load at its correct value. The equipment used to test the gears consists of two anvils, a fork, and a pin. According to the characteristic dimensions of the equipment (e.g., the distance between the anvils and the contact surface), the STBF test may assume two different configurations: asymmetric and symmetric. In the first configuration two teeth are loaded at different heights; therefore, one tooth root is more stressed. Only one tooth is effectively under test, while the other one just acts as a reacting tooth. On the opposite side, in the symmetric case, two teeth are simultaneously loaded at the same height and, therefore, both teeth are simultaneously under testing. As shown in Figure 2, the present research adopts the symmetric configuration together with the induced stress state calculated via FEM. For the purpose of obtaining the symmetric configuration, the distance between the contact surfaces of the anvils is equal to the span measure over three teeth (also called the Wildhaber measure over three teeth [32], W3), with the axis of the pin/fitting hole positioned in the middle between the two surfaces. The presence of the pin and fork is justified by the necessity of ensuring the correct mounting of the gear during the test set-up phase. As the kinematic chain composed by pin, fork and anvils gives statically indeterminate results, during the test, the pin is removed just after positioning the gear, ensuring the correct teeth/anvils contact. Then, a minimum preload (i.e., 10% of the maximum) is applied. In this way, by removing the pin, the kinematic chain gives isostatic results. The test fixture is also equipped with a spring, which oversees the gear's quick removal when the test ends. Thus, as explained above, to ensure the correct position of the gear during the whole test, a preload always has to be applied to the gear. Therefore, all the tests have been performed with a load ratio R (i.e., minimum to maximum ratio) equal to 0.1. On the basis of the authors' experience (e.g., see ([33; 34; 35; 36; 37]) this value of R is sufficient to ensure that no undesired movement will take place. All the tests were performed at a frequency of \(\approx\)35 Hz; as a mechanical pulsator was used, the test frequency was not chosen a priori, but it depends only on system mass/stiffness. Figure 3 shows the experimental set-up of the present research. Figure 1: Scheme of the mechanical pulsator. Figure 2: Scheme of the gear/anvils contact; dimensions are in mm; FEM analysis of the loaded tooth during Single Tooth Bending Fatigue (STBF) tests. Figure 4 and Table 1 report the main characteristics of the gear. The gear dimensions, especially the profile shift coefficient x, were chosen to have a tooth profile highly sensitive to the bending phenomena. The gears, made by 17-4 PH, were produced by Laser Powder Bed Fusion (PBF-LB/M). The gears were manufactured (see Table 2 for the main process parameters) and then machined with traditional machine tools (i.e., lathe and hobbing machine) to obtain the proper roughness and the right shape.