PSI - Issue 39

Aljaž Ignatijev et al. / Procedia Structural Integrity 39 (2022) 89 – 97 Author name / Structural Integrity Procedia 00 (2019) 000–000

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1. Introduction Powder Metallurgy (PM) is useful for producing parts that have irregular curves (i.e. sintered gears). As presented by Flodin et al (2011), sintered gears have initially been used only for light duty applications. However, powder metal gears are a cost efficient alternative for machined gears in larger series in the Automotive industry. The next step should be power transmission gears. As published by Dizdar et al (2012), the idea of sintered gears dates back to 1985. However, due to low Young`s modulus and low surface hardness, which led to significant wear, the usage of sintered gears was limited to low load applications. Investigations of RCF (Rolling Contact Fatigue) on sintered steel led to the selective surface densification of gear flanks, which improved the surface hardness and, therefore, reduced wear (Lawcock 2006). However, surface densification of gear flanks is done after sintering, and thus causes an additional procedure, which raises the cost of the final product, consequently, the reducing economic advantage over conventional gear production with milling. Recent development in powder metallurgy production technology increased the strength of final PM-products to an again interesting level for sintered gears applications without additional procedures (Dlapka 2012, Straffelini 2014). This study is focused on the fatigue failures of sintered gears. Although two kinds of fatigue failures (surface pitting and tooth breakage) should be taken into account when dimensioning gear drives, only the tooth breakage is addressed here. The presente d study is the continuation of the authors previous work (Glodež 2014), where the stress -life approach was used to determine the fatigue life of sintered gears in regard to the bending stress in a gear tooth root. In that respect, the additional elastic-plastic computational analyses are reported here to determine the fatigue life up to the formation of initial crack in a gear tooth root. The subsequent fatigue crack growth from initial to the critical crack length is also considered to obtain the total fatigue life of sintered gear and subsequent comparison with the available experimental results. 2. Computational model Computational model was built in program package Ansys. As shown in Fig. 1, two geometrical models were made. First geometrical model (Fig. 1a) consists of gear pair with a frictional contact at the outermost single contact point, considering the coefficient of friction equals to 0.04. In the second geometrical model (Fig. 1b), only pinion is modelled which represents some simplification if compared to the model 1. It was designed due to limitations of the crack propagation simulation tools. The boundary conditions of model 1 and 2 were defined at reference points (RP 1 and RP-2) which connect the inner cylinder of the gear and the side-cut surfaces as shown with orange in Fig. 1. In RP-2, there is a fixed support in all directions of the coordinate system. In point RP-1, only rotation around the axis of the gear is free. Torque is defined on RP-1 and rotate gear around its axis.