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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Ghaffari et al. (2015). Apart from a precise mathematical model describing the wear process of individual teeth, including the development of fatigue crack, it is possible to derive many practical guidelines such as:  for the onset of wear process it is necessary to wipe the hardened layer at the tooth’s surface,  wear of bearing supports of the shaft, on which the gear wheels are mounted, is very important – the approaching rotation axis of the cooperating gear wheels leads to undercutting the tooth’s root, whilst by moving away, there is an increased bending moment contributing to breakage,  fatigue cracks develop directly proportional to rotational speed and the number of meshings,  to initialize cracks, instantaneous overloads are needed (increase in driving torque or braking torque). In this article, which presents the monitoring of real mechanical elements (in laboratory conditions and during normal operation of aircraft transmissions), all of these suggestions have been confirmed. However, there is also another phenomenon here – resonance impact of the damaged power plant on the pair of gear wheels. The authors believe that the conditions of weakening the structure of a tooth and creating conditions for fatigue cracks at the tooth’s root forms over a long time and thus, it should be subject to the in -depth monitoring. The subsequent process of initiating the fatigue crack is usually short, especially in the case of aircraft transmissions, angular speeds and variable loads distinguishably accelerate this process in the way which substantially hinders from taking the countermeasures. Such opinions are also shared in other publications by: Bukowski and Kłysz (1993), Padfield (1998), Żurek (2006) . Helicopter power plants are characterized by diverse rotational speeds of shafts and, thus, also the journals of rolling bearings – Fig. 1: engine n = 250 rev/min; main shaft of helicopter rotor n = 4 rev/min. The authors frequently observed the resonances of individual bearings. The most loaded element, or even overloaded, is the upper bearing in the main transmission – Fig.1, element 3a. It is the principal support of the main shaft (Fig. 1, element 3c) – the helicopter rotor with substantial structural unbalances changing significantly during flight direction control due to changing angles of attack (system of the so-called periodic control) developed by: Barszewski (1956), Padfield (1998), Żurek (2006) . 4. Resonance of bearing support system and its impact on the wear of transmission gear wheels

Fig. 1. Arrangement of transmission elements between the engine and generator in Mi-24 helicopter: 1. – TW3-117MT propulsion engine, 2. – mechanical fan, 3. – WR-24, 3a. main transmission – upper bearing, 3b. – main shaft of helicopter rotor, 3c. – rotor blade, 4,5,7. – power transmission shaft, 6. – accessory gearbox, 8. – left GT-4PCz6 generator (behind it, a right GT-4PCz6 generator is installed to the same accessory gearbox), 9. – intermediate transmission, 10. – tail transmission, 11. – auxiliary rotor. During operation, such bearing deteriorates – spallings in the inner ring raceway developed by Augustyn and Gębura (2012). Then, the waveform in the angular frequency component observed by the FAM-C method – Fig. 2, Fig. 3 is modified. From these diagrams, it can be calculated that the pulsation frequency for the upper bearing working correctly (‘positive standard’) amounts to f p = 13 ÷ 23 Hz (Fig. 2, f p = 13 since there are 13 oscillations/s), and for the bearing with a spalling, it equals f p = 51÷ 64 Hz ( Fig. 3, f p = 51 since there are 51 oscillations/s).