PSI - Issue 16

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

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movements of the rotation axis of the Z70 wheel will contribute to the oscillations of angular speed of tail rotor, thus, the oscillatory movements of the direction of helicopter flight.

Z70.

Fig. 5. Block diagram of a complete accessory gearbox (GT-4PCz6 generators) from Mi-24 helicopter: 1 – right generator-converter GT-4PCz6 No. 1, 2 – accessory gearbox, 3 – generators’ fan, 4 – left generator-converter GT-4PCz6 No. 2.

6. Spatial resonance between the upper bearing of the main transmission and intermediate shaft of accessory gearbox of Mi-24 helicopter Phenomena described in the previous chapters include:  resonance of the upper bearing of the main transmission – causing false brinelling on its inner ring raceway (in the form of spallings with the depth of 2,1 mm) – they induce the increase of free-running frequency of this bearing f ≥ 51 Hz,  specificity of the asymmetric dynamic load of the intermediate shaft (in the accessory gearbox) results in its skew. This skew, in combination with the increased radial clearances, manifests itself in the form of the decrease in angular frequency (Fig. 4) of this node with f p = 102 Hz to f p = 51 Hz. It causes the synchronous acceleration and deceleration – Fig. 6 – of the angular speed of intermediate shaft (Fig. 6, element 6). On this shaft, from the side of a fan (i.e. from the greatest radial clearance of the bearing support), a gear wheel Z30 is mounted – Fig. 5, element Z30. The toothed-wheel rim is subject to asymmetric abrasion due to: 1. synchronous acceleration and deceleration of the angular speed of the intermediate shaft (Fig. 6.), 2. wheel Z30 moving closer and further away from wheel Z70.

Instantaneous increase of the frequency and amplitude of the envelope of the instanteous frequency course

f i [Hz]

t [s]

Fig. 6. Waveform of frequency in time function obtained from the pilot exciter generator Mi-24 No. 456 test No. 31, measurement No. 2 – visible instantaneous increase in frequency and amplitude of envelope of frequency with f p ≈ 100 Hz – observation time: 0,2 s.