PSI - Issue 16

Andrzej Gębura et al. / Procedia Structural Integrity 16 (2019) 184 – 191 Sylwester Kłysz, Andrzej Gębura, Tomasz Tokarski / Structural Integrity Procedia 00 (2019) 000 – 000

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Contact edge and the root of two teeth of wheel Z30 are subject to intensive abrasion. It may result in the fracture of these two teeth. Then, it may end in the loss of the continuity of transferring the mechanical power to the tail rotor. In consequence, the helicopter loses its directional maneuverability, and its fuselage starts to rotate around the vertical axis in the opposite direction to the direction of rotations of the main rotor. Such a case (helicopter crash) was reported in 2011. 7. Development of destruction process of gear wheels according to AFIT research Due to the disassembly and investigations conducted in Air Force of Institute of Technology (AFIT) of helicopter destroyed in 2011 – Fig. 9 – the following conclusions were formulated: 1. the reason for the loss of kinematic attachment of helicopter transmission between the main transmission and tail rotor hub was the destruction of gear wheels of the drive shaft and intermediate shaft of the accessory gearbox; 2. the destruction of the gear wheels consisted in fatigue fracture of two subsequent teeth of drive wheel mounted on the intermediate shaft (Fig. 5, element 6). The initiation of fracture occurred most likely under the influence of a short-term growth of loading of a gear wheel. A crucial factor, which impacted the initiation of cracks, was a defective shape of the teeth of this wheel done during the mechanical treatment – the root of all 30 teeth was not polished, and as a result, a dent was created, which contributes to fatigue failures. Driving wheel mounted on the intermediate shaft transmits torque on the above – mentioned drive wheel. Driving gear wheel (Fig. 5, element Z30) was subject to greater destruction than the drive wheel (Fig. 5, element Z70), which is illustrated in figure 10. In gear wheel Z30, all 30 teeth of this wheel were damaged. Due to the macroscopic observations developed by Dudziński et al. (2011), two modes of teeth failure were identified. First of them is milling of 14 teeth on the length of the junction with the teeth of wheel Z30 – Fig. 5, element Z30. The second mode of teeth destruction was its breakage at the tooth’s root. In thi s way, 16 teeth of wheel Z30 were damaged. Macroscopic observations of fracture surfaces found out that 14 fractures, amongst 16 fractures of broken teeth, are distinguished by features of temporary destruction. The surface of these fractures is characterized by coarseness and rel ief in the form of ‘ridges’ and ‘valleys’ of ‘rivers’ crossing perpendicularly to the length of teeth, i.e. circumferential relative to the axle of the gear wheel – Fig. 7, elements a2. Such form of fracture indicates temporary destruction (breakage) of teeth. This type of failure and the afore-mentioned milling is known as secondary teeth damage.

fatigue fracture

temporary fracture

a1

a2

Fig. 7 . Macrostructure of gear wheel’s fractures – upper view: a1 – fatigue fracture a2 – temporary fractures of dynamic destruction ( Dudziński et al. (2011)).

The fractures of two teeth show macroscopic features of fatigue cracking – Fig. 7, elements a1. In this elaboration, it was concluded that: