The paper presents results of complex research of vibration behavior of the differential reduction gear unit of a turboprop engine on the basis of statistics of products with repaired reduction gears and those placed under repair. The conducted research made it possible to establish the source of resonant oscillations of the compressor design elements leading to their breakage. It is established that this source is determined by the kinematics of the reduction gear unit and is associated with the wear of its gear teeth. Standards for the intensity of the appropriate discrete component of the reduction gear vibration spectrum and those for the amount of permissible wear are substantiated and established on the basis of the work carried out. A complex of diagnostic parameters of wear based on intensity components of the auto-spectrum and the spectrum of maxima, the depth of amplitude modulation in some narrow-band processes, their characteristics based on the parameters of the current frequency and mutual characteristics was identified. It is shown that for the whole complex of the diagnostic parameters obtained their dependence on the wear value is satisfactorily approximated by a linear characteristic. The obtained results can serve as a basis for the development of a complex of methods of differential reduction gear vibration diagnostics.

Defects of planetary reduction gearboxes of turboprop engines often cause excitation of high-frequency oscillations that bring about failures of compressor structural elements. The vibration behavior of these reduction grarboxes is a broadband process with the presence of a large quantity of vibration components, which significantly complicates the search for informative diagnostic indicators of defects. In order to simplify the procedure of identifying the vibrations caused by defects of toothed gearing a simulation model of the vibration behavior of a turboprop engine differential gear unit is proposed. The model takes into account kinematics and design of the gearbox, determines the structure of its vibration behavior in the presence of defects on the tooth flanks. Using the example of a commonly occurring defect in the form of wear of the teeth of the “solar gear – satellites” pair the adequacy of the model to the actual vibration behavior of the mechanism is shown.

The peculiarities of the structure of the vibration spectrum of the turboprop engine differential gear in the case of wear of the central gear teeth are discussed and a technique of vibration diagnostics of the defect is proposed in the paper. It is found that the wear leads to multiple harmonics, including tooth harmonics, in the broadband vibration maxima spectrum. The frequency of the first harmonic of the series is determined by multiplying the speed of rotation of the outer central gear by the number of the worn teeth. It has been shown that increased intensity of one of the components of this series is the cause of the fatigue failure of a rotor blade of one of the compressor stages. Comparative analysis of the autospectra, the spectrum peaks of the spectrum envelope and the data of cascade demodulation show that in this case the autospectra is most informative. On the basis of the statistics of reworked gearboxes and a gearbox with the level of wear twice the norm threshold values were obtained for two of the most informative spectral components. These values make it possible to identify the gear unit with inadmissible degree of wear of its teeth. Statistical analysis of gear vibration of the repaired items revealed the presence of a component that characterizes relative beating of the outer central gear and satellites. It was found that its intensity differs from engine to engine by more than an order of magnitude. The impact of this beating is considered to be one of the main causes of increased wear. The investigation of the effect of this factor on the development of the defect is determined as the line of further research.

It is shown that analyzing the signals from standard sensors of gas-turbine engine shaft speed we can resolve some issues of diagnostics of the technical condition of the reduction gearbox without installing any additional vibration transducers. It was found, in particular, that the vibration spectrum component generated by the wear of the differential reduction devices gear teeth causing resonant oscillations of the turbocharger blades produces torsional vibration at the appropriate frequency. The analysis of the parameters of the instantaneous rotational speed of the reduction gear output link (the shaft of the rear screw) made it possible to obtain some diagnostic indicators of the value of the reduction gear teeth wear on the basis of evaluation of the frequency deviations variance. It was established that the value of the diagnostic indicator is linearly dependent on the value of wear. The norm for this indicator was established for the engine under investigation. Significant influence of the load on its level was shown.

The paper shows certain ambiguity of the narrow-band boundary of random processes obtained by different authors. The use of filters of different widths in obtaining the envelope spectrum makes it difficult to compare the obtained results by estimating the depth of amplitude modulation in vibration diagnostics of defects in rotary machines. We studied the results of bandpass filtering of the noise process only, as well as of amplitude-modulated noise. The analysis of the results of filtering of a broadband normal random process with constant spectral density and the representation of the spectrum width of the resulting oscillations through spectral moments showed that processes selected by a filter of no more than 1/3 octave bandwidth should be considered narrowband. Simulation of the amplitude of a broadband noise by the harmonic process and estimating the dependence of the modulating harmonic amplitude, the depth of amplitude modulation, skewness and kurtosis, as well as envelope characteristics on the width of the selecting filter allowed us to establish that a relative filter width of 30% should be applied when the envelope spectrum is used.