Experimental study of nonlinear vibration characteristics of a two-rotor turbofan engine system with an intershaft support

The layout of the turbofan engine with coaxial rotors and intershaft rear support of the high-pressure rotor is the most compact one since the rear supports of both rotors are located in one oil cavity, provided with a joint system of communications for oil supply and suction, and the force diagram of the aircraft engine stator is simplified. The disadvantage of this layout is the mutual influence of high-pressure (HP) and low-pressure (LP) rotors through the intershaft support, which leads to significant vibration activity due to the operating conditions of the intershaft bearing. The purpose of this work was to identify the characteristic features of vibration instability during the testing of turbofan engines with intershaft support, the factors influencing it, and the methods of elimination within the framework of the general reduction of vibration levels and the improvement of the efficiency of engine vibration diagnostics. The research was performed on serial engines manufactured by MOTOR SICH JSC. Based on the vibration recording data, approximate curves of the vibration response in the near-resonance area are plotted. The directions of the rotation frequency change and hysteresis zones with stable and unstable oscillations are marked. Bench tests with registration of broadband vibration sensor signals were performed. The moment spectra of the vibration signals for two stable oscillation modes with the same rotor speed in the resonance region were calculated. The phenomenon of vibration instability during the operation of a turbofan engine with intershaft support in the steady-state mode of operation is analyzed. The main features of the vibration state of such an engine are identified as follows: 1) the presence of the vibration jump phenomenon when passing the resonance frequency of HP rotor; 2) the presence of combination frequencies of the rotors and characteristic frequencies of the intershaft bearing in the vibration signal spectrum; 3) the presence of a constant phase shift of the signal from the front and rear suspension plane sensors with the frequency of HP rotor in modes with vibration instability; 4) the occurrence of a chaotic phase shift in the vibration signal of the rear suspension plane when vibrating with the frequency of LP rotor. An assumption was made about the presence of rolling element skidding in the intershaft bearing as the main cause of the described vibration instability. The influence of the inlet oil pressure on the instability occurrence was detected.