PSI - Issue 18

Goran Vukelic et al. / Procedia Structural Integrity 18 (2019) 406–412 Vukelic, Pastorcic, Vizentin/ StructuralIntegrity Procedia 00 (2019) 000–000

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1. Introduction Gears are almost unavoidable components of every machine. Their application ranges from transmission systems in automotive, plane and marine industry, to special applications in power generating and processing plants. Gears design ranges from simple to sophisticated gear pairs and, along with their complexity, rises the need for thorough understanding of acting stresses and, consequently, possible failures. It is especially important as the data show that gear failures make up about 10% of all rotating machinery failures[1]. As for the gear failures, two common types of localized tooth defects are recognized [2]. One is the tooth fillet crack and the other tooth surface spalling. As previous research shows, fillet cracks usually develop in the tooth fillet region. They are caused by deficiencies in the gear tooth which result in stress concentration points that act as a root of subsequent damage. On the other hand, gear tooth spalling tends to develop in the region near the pitch circle of the tooth surface where the tooth pair is subjected to a higher mesh force. Gear tooth spallings are a product of extremely high localized contact stresses that act as damage initiators. There has been a considerable effort in researching the causes of gear failures and offering the propositions for design improvements. Some of the recent work on gear failures includes investigation of a failed axle of a reduction gearbox where, using experimental procedures, was concluded that at the initiation site during the case carburization quenching cracks were formed [3]. Failure analysis of a helical gear used in a bus gearbox indicated that teeth of the helical gear failed by fatigue with a fatigue crack initiation from destructive pitting and spalling region at one end of tooth in the vicinity of the pitch line because of misalignment [4]. Experimental and numerical study of microstructural degradation of a failed pinion gear at a cement plant has proven that the concentration of tensile residual stresses due to untempered core at the tip aided the micropitting and micro-cracking due to the rolling contact surface fatigue was responsible for the initiation of surface cracks and final failure of the gear [5]. Understanding the causes of gear damage and failures is important since it can contribute to the prevention of catastrophic machinery failures. This paper deals with a teeth failure of a gear shaft that serves as a part of shipyard crane drive train. Almost all of the teeth of a spiral bevel gear, part of a larger shaft, fractured during normal operation of the crane. Failure analysis presented here combines experimental and numerical research giving insight how did the damage propagated and influenced the performance of the gear pair. 2. Experimental procedures 2.1. Visual observations A gear shaft that serves as a part of shipyard crane drive train failed with 15 of its 17 helical teeth damaged. It is a spiral bevel gear, part of a larger shaft. Spiral bevel gears find their main application vehicle differentials, where the direction of drive from the drive shaft must be turned 90 degrees to drive the wheels. Design that is characterized by the helical teeth is capable of providing less vibration and noise than conventional gears with straight teeth. Spiral bevel gears come in pairs and once damaged, they should both be changed. Geometry and dimensions of considered spiral bevel pinion are shown in Fig. 1.

Fig. 1. Geometry and dimensions (in mm) of the considered pinion shaft.