| The rotor is the main part of turbocharger, whose rotating speed can reach tens of thousands rpm to more than200000rpm. It’s investigated that rotor system works under the circumstance of high speed, high temperature and high press, highly corrosion, so a tiny crack occurs easily on the shaft. Under function of variable stresses, the crack begins to extend, and finally leads to the fracture of the shaft. It’s on doubt that bad results will take place. The worse is the losses of the wealth, and the worst is the injury of personal safety. Crack, which is the common and extremely dangerous malfunction of rotating machinery, is usually tested by the methods of ultrasonic, infrared, magnetic powder flaw detection. But these methods all are useful only when the machine is in a state, but they are unfavorable for on-line identification and detection of crack. However, the important discovery is found that the generation and propagation of crack have a great influence on the vibration characteristic of the rotor system. Thus research on the dynamic characteristic of the turbocharger rotor with a crack provides the theory reference for online fault diagnosis and detection, and it has a certain theoretical significance and engineering application value.Based on the case that turbocharger supported on the oil film, regarding a crack at the turbine cantilever end, the mechanical model and the differential equations considering single oil film supporting and considering the floating rings supporting (double oil film) are both built. Then the effect of crack on the nonlinear dynamic characteristic and its stability of the rotor system are analyzed by use of the nonlinear theory and rotor dynamic theory. The main content of the paper are as follows:(1) Considering the single oil film supporting and simplifying the model as journal, the mechanical model and its mathematical model of the turbocharger rotor system are built. Based on the models, the matrix transfer method is applied to calculate the first critical speed and the second critical speed and the figure which reflects the change of the critical speed with supporting stiffness on the rotor system without a crack; Taken the open-slant crack at the turbo-disk into consideration, the diagrams of the cracked shaft stiffness and the critical speed with change of the depth of crack and rotation of the axis are provided.(2) Based on the models built above, the fourth order Runge-Kutta numerical method is used to calculate the dynamic equation of the rotor system without a crack and with a open-slant crack. Some interesting phenomenon can be found by analyzing the effect of crack on the turbocharger rotor system, which are showed as follows:①the cracked system has rich nonlinear behavior such as periodic motion, quasi periodic motion at the speed range of1/3Ωc and2/3Ωc;②The crack restrain the rotating speed range that oil film begins to force;③Crack have little influence on the rotor system at high speed area.(3) Considering dynamics of the floating ring, the differential equations of the turbine rotor system with a open-close crack on shaft are established. The system was analyzed by numerical calculation based on the fourth order Runge-Kutta method. And research on the influence of the crack on the dynamic characteristics of the turbocharger rotor system was done. The results showed:①The crack caused subharmonic vibration of the system, and increased the magnitude of the imbalance force when the speed was low;②The crack also leads to reduce of the speed area, but the increase of the amplitude of the inside oil film instability;③When speed is in the area, the inside and outside oil film damp had no influence on the amplitude of the vibration. And the crack showed little effects on the vibration of the system when the speed is high.(4) The main work done in the paper is summarized, and the further research on the subject is discussed. |
PDF Full Text Request