# Bending Fatigue Behavior of 17-4 PH Gears Produced by Additive Manufacturing
###### Abstract  
The introduction of Additive Manufacturing (AM) is changing the way in which components and machines can be designed and manufactured. Within this context, designers are taking advantage of the possibilities of producing parts via the addition of material, defining strategies, and exploring alternative design or optimization solutions (i.e., nonviable using subtractive technologies) of critical parts (e.g., years and shafts). However, a safe and effective design requires specific resistance data that, due to the intrinsic modernity of additive technologies, are not always present in the literature. This paper presents the results of an experimental campaign performed on gear-samples made by 17-4 PH and produced via Laser Powder Bed Fusion (PBF-LB/M). The tests were executed using the Single Tooth Bending Fatigue (STBF) approach on a mechanical pulsator. The fatigue limit was determined using two different statistical approaches according to Dixon and Little. The obtained data were compared to those reported in the ISO standard for steels of similar performance. Additional analyses, i.e., Scanning Electron Microscopy SEM, were carried out to provide a further insight of the behavior 17-4 PH AM material and in order to investigate the presence of possible defects in the tested gears, responsible for the final failure. 17-4 PH; additive manufacturing; bending fatigue; gears; gearbox design Article Bending Fatigue Behavior of 17-4 PH Gears Produced  
Aditive Manufacturing 1 Faculty of Science and Technology, Free University of Bolzano/Bozen, piazza Universita 1, 99100 Bolzano, Italy; [EMAIL_ADDRESS]
gears are produced by using AM technology is presented. The American Gear Manufacturing Association (AGMA) has recently published a document calling for the necessity of investigating all the aspects related to AM steel gear production [12]. However, despite the fact that static and fatigue properties of AM materials were deeply investigated [13; 14; 15; 16; 17; 18; 19], the literature is lacking in terms of reliable data that can be used to design such critical components (e.g., tooth root bending properties). In any case, the design of critical components, such as shafts or gears, must pass through analytical standardized codes (e.g., see [20; 21; 22] for gears). Those require reliable resistance data which, if not directly known, can be easily found within the standards too (e.g., see [20; 23] for gear materials resistance). Unfortunately, due to the modernity of AM technologies, the standard codes do not present any resistance values that can be used to design components produced via AM. Only a few piece of data about the wear behavior can be found in the literature [24; 25]. In any case, the standards present the possibility of us resistance data coming directly from experimental campaigns. Nevertheless, it is common practice in gear design to perform the resistance tests directly on the gear specimens. This is related to the specific manufacturing technology, typically hobbing, which produces, for example, characteristic surface patterns and residual stresses that cannot be accurately considered with tests on traditional samples. Therefore, provided that tooth root fatigue (i.e., bending) is one of the most relevant gear failure phenomena [26], this work presents the results of an experimental campaign aimed at determining the tooth root stress limit obtained directly by testing AM gears via the Single Tooth Bending Fatigue (STBF) approach. In the STBF tests, the gear-specimens are tested by applying a normal cyclic load to two flanks taking advantage of the involute properties and the Wildhaber distance. While two teeth are tested at the same time, the friction between the teeth flanks and the anvils (used for applying the load), ensures that the forces acting on the gear-sample are automatically balanced and no additional supporting structures are required.