Rotor rub-impact has a great influence on the stability and safety of a rotating machine. This study develops a dynamic model of a two-span rotor-bearing system with rubbing faults, and numerical simulation is carried out. Moreover, frictional screws are used to simulate a rubbing state by establishing a set of experimental devices that can simulate rotor-stator friction in the rotor system. Through the experimental platform and its analysis system, the rubbing experiment was conducted, and the vibration of the rotor-bearing system before and after the critical speed is observed. Rotors running under normal condition, local slight rubbing, and severe rubbing throughout the entire cycle are simulated. Dynamic trajectories, frequency spectrum diagrams, chart of axis track, and Poincare maps are used to analyze the features of the rotor-bearing system with rub-impact faults under various parameters. The vibration characteristics of rub impact are obtained. Results show that the dynamic characteristics of the rotor-bearing system are affected by the change in velocity and degree of impact friction. The findings are helpful in further understanding the dynamic characteristics of the rub-impact fault of the two-span rotor-bearing system and provide reference for fault diagnosis.

With the requirement of high speed and high efficiency in modern rotating machines, clearance reduction between the rotor and stator has been widely used. Nevertheless, as gap decreases, rub-impact fault frequently occurs, which causes abnormal vibration that may affect the normal operation of machines. Therefore, the friction fault of the rotor-bearing system should be studied, and the vibration characteristics of the rotor-bearing system under different friction conditions before and after the critical speed should be observed to detect the machine fault and provide reference for fault diagnosis.

Scholars have conducted considerable work and obtained many valuable results on rub-impact fault. Muszynska [

Nowadays, in order to improve energy efficiency and higher thrust-to-weight ratio, double-disc rotor systems are widely used in rotating machinery such as aeroengines. Therefore, the study of the rotor-bearing system rubbing test is necessary. At the same time, the double-disc rotor system exhibits more complex dynamic characteristics than the single disc rotor system. In the above literature, the research focus is on the numerical simulation of a single rotor system, which is obviously not satisfactory for the actual rotating machinery. Through a numerical model and by building an experimental platform, Torkhani et al. [

In the existing research, the analysis of the rubbing failure of the two-span rotor rotating machine before and after the critical speed is insufficient. So in the present study, the dynamic model of the two-span rotor-bearing system with rubbing faults is developed, and the numerical simulation of changing the speed and rubbing condition is carried out. Meanwhile, an experimental device for simulating rotor-stator friction in the rotor system is established. The vibration characteristics are numerically studied through an experimental research on the three stages of the double-span rotor system (normal operation, slight rubbing, and severe rubbing) at different speeds to provide reference for fault diagnosis.

In order to study dynamic behavior efficiently, a simplified mathematical model of the double-disc rotor-bearing system with rub-impact fault was developed, as shown in Figure

Two-span rotor-bearing system.

The double-disc rotor model studied in this section is shown in Figure

The normal and tangential impact forces of the rotor during rubbing can be expressed as

The rubbing force in the

Combined with these two cases, the previous formula can be expressed as

Therefore, the impact forces on the left and right disks are as follows:

On the basis of the motion theorem, the differential equations of motion of the rub-impact rotor are obtained. Only the rubbing fault of a single disc is considered here:

In order to facilitate calculation and avoid excessive truncation errors, the dimensionless transformations are given as follows:

Changes in rotational speed and rubbing conditions have a significant effect on the motion of the rotating machine. In the simulation, by changing the speed and the stiffness of the rubbing, the rotor is numerically simulated before the critical speed, near the critical speed and after the critical speed. The motion characteristics of the rotor under different rubbing conditions are also studied. The main parameters of the system are shown in Table

Main parameters of the system.

Parameter | Value |
---|---|

Quality, |
32, 30 |

Equivalent stiffness, |
2.5 |

Damping, |
2100 |

Rubbing clearance, |
0.11 |

Friction coefficient, |
0.1 |

Rotor eccentricity, |
1.1 |

According to equation (

Figure

Dynamic response of the speed change to the system.

In summary, the response of the rotor-bearing system is affected by the speed. When the speed exceeds the critical speed, an increase in the speed range will further stabilize the system.

The dynamic response of the system is shown in Figure

Dynamic response of rubbing stiffness change to the system.

In summary, the rubbing stiffness has a great influence on the motion state of the rotor. As the rubbing stiffness increases, the motion state of the system will change.

The rotor system is a core component of aeroengines and gas turbines. The two-span rotor-bearing system is a form of construction often used in modern aeroengine and gas turbine rotor systems. In this paper, the two-span rotor test bench model is taken as the research object, and the dynamic characteristics of the rubbing response of the model are studied.

As shown in Figure

Rotor test rig.

Main parameters of the two-span rotor-bearing system.

Component name | Value | Unit |
---|---|---|

DC motor | 0–10000 | rad/min |

Sliding bearing | 24 × 30 (diameter × length) | mm |

Graphite bearing | 21 × 14 (diameter × length) | mm |

Long shaft | 10 × 500 (diameter × length) | mm |

Short shaft | 10 × 400 (diameter × length) | mm |

Large disk | 80 × 16 (diameter × length) | mm |

Small disk | 80 × 20 (diameter × length) | mm |

Flexible coupling | 24 × 30 (diameter × length) | mm |

Sampling frequency | 4000 | Hz |

Lubrication oil | 30 |

The entire rotor test bed is based on digital speed governor, sensors, multifunction filter amplifier and acquisition instrument, and computer software as auxiliary equipment. The working mechanism is that the digital speed governor adjusts the rotating speed of the test bed, the sensors measure the various signals produced during the vibration of the test bed, and the acquisition instrument collects all types of signals. The computer software displays the collected data to the terminal screen. Figure

Schematic diagram of the experimental data acquisition system.

To simulate the rub-impact fault that often occurs in practical rotor systems, several movable sensor brackets are designed, as shown in Figure

Rub-impact device.

The rotor vibrates sharply at a resonant speed. Therefore, this experiment simulated the rubbing fault of the actual rotor system before and after the critical speed. This study verified that the first critical speed of the test bench is approximately 3100 rev/min. For the rub-impact experiment, the following tests were performed: under normal conditions, local slight rubbing, and severe rubbing throughout the week at speeds of 2000, 3000, and 4000, respectively. In the first experiment, the actual rotor speed-up process was simulated and the system stability from the speed before and after the critical speed was observed without any rubbing conditions. In the second experiment, a partial rubbing test was conducted and the thimble was gently pushed to the rotor surface, resulting in a partial slight rubbing effect. In the third experiment, the thimble will be immediately pushed into a tight coupling with the rotor, which will cause rubbing during the entire week.

Rotational speed has a significant influence on rotary movement. Rotors generally start with slow rotations under low rotational speed. In the experiment, the rotor was simulated under normal working conditions, local slight rubbing, and full-cycle severe rubbing by changing the rotational speed, and the motion characteristics of the rubbing rotor were analyzed. Through multiple simulation experiments, representative data of the rubbing experiment were collected, and dynamic trajectories, frequency spectrum diagrams, chart of axis track, and Poincare maps were used to reflect the rotor motion characteristics.

To facilitate comparison of the features, the experiment provides the vibration characteristics of the two-span rotor under normal operation of the power frequency, as shown in Figures

Normal state at 2000 r/min. (a) Time-domain waveform. (b) Spectrum graph. (c) Axis orbit. (d) Poincare orbit.

Normal state at 3000 r/min. (a) Time-domain waveform. (b) Spectrum graph. (c) Axis orbit. (d) Poincare orbit.

Normal state at 4000 r/min. (a) time-domain waveform. (b) spectrum graph. (c) axis orbit. (d) Poincare orbit.

Figures

Local slight rubbing at 2000 r/min. (a) Time-domain waveform. (b) Spectrum graph. (c) Axis orbit. (d) Poincare orbit.

Local slight rubbing at 3000 r/min. (a) Time-domain waveform. (b) Spectrum graph. (c) Axis orbit. (d) Poincare orbit.

Local slight rubbing at 4000 r/min. (a) Time-domain waveform. (b) Spectrum graph. (c) axis orbit. (d) Poincare orbit.

When the thimble is pushed immediately into the rotor, severe impact throughout the entire cycle will occur. In the entire cycle under severe rub impact, frictional vibration will cause large system stiffness changes. The stable full-cycle rub impact will not last for a long time, but it will immediately damage the structural surface, change the rub-impact state, and have transient response characteristics. Figures

Severe rubbing at 2000 r/min. (a) Time-domain waveform. (b) Spectrum graph. (c) Axis orbit. (d) Poincare orbit.

Severe rubbing at 3000 r/min. (a) Time-domain waveform. (b) Spectrum graph. (c) Axis orbit. (d) Poincare orbit.

Severe rubbing at 4000 r/min. (a) Time-domain waveform. (b) Spectrum graph. (c) Axis orbit. (d) Poincare orbit.

When the speed is 2000 r/min, the rotor moves from a normal power frequency to severe rubbing and time-domain waveform irregularity, and the rotor amplitude decreases consistently. Hence, the thimble remains close to the outer edge of the rotor in the entire process, which limits the amplitude pulsation. The reverse friction of the thimble reduces the main vibration frequency of the rotor, and the frequency component is different from that of local rubbing. In comparison with local rub impact, no pulsation of the axis track of the rotor during the entire cycle of rub impact is observed. As the thimble is sticking closely to the rotor surface, although the figure becomes irregular, the trajectory is relatively stable and the amplitude decreases. When the rotational speed is 3000 r/min, which is close to the critical speed, the vibration is intense. Serious rubbing fault occurs in the rotor system, and the phenomenon of clipping wave peak can be seen from the waveform. As shown in the spectrogram, in addition to the fundamental frequency components, significant half-frequency, 3/2X and 5/2X fractional octave, and high-frequency components, such as 2X, 3X, 4X, and 5X, also appear. The axis track is relatively disordered, and the system response Poincare is concentrated around two points. Hence, when the critical speed is reached, the serious rubbing fault will be that the motion state of the system is multiperiodic. The rotational speed is increased to 4000 r/min, which is larger than the critical speed and vibration decreases. At this time, the time-domain waveform is irregular due to severe rubbing, and the waveform “clipping” phenomenon occurs. The amplitude of the second octave in the spectrum is higher than the fundamental frequency, and the high-power spectrum, such as 3X and 4X, appears. The axis orbit presents a sharp angle; the rotor is in a slight bending state. The Poincare of the system response is concentrated near 1 point, and its motion can still be regarded as a single cycle.

The dynamic equations of the two-span rotor-bearing system with rubbing faults are established and analyzed, and the numerical simulation of changing the speed and rubbing condition are carried out. The numerical analysis indicates that changes in speed and the interference of the rubbing force increase system instability, and the dynamic characteristics become further complex. Moreover, experimental investigations are conducted intensively on the basis of the numerical simulation in this study.

The experimental analysis indicates that rotational speed has a significant influence on rotary movement. Under similar rubbing conditions, the dynamic characteristics of system stability are dissimilar at different speed stages. The speed is low at 2000 r/min before the critical speed, rubbing fault occurs at this time, and the system has a relatively small degree of response. When the rotational speed is 3000 r/min near the critical point, the system responds violently once the rubbing fault occurs, and when rubbing is severe, the system will be in a multiperiodic motion. However, after the first-order critical speed is reached and the speed continues to increase to 4000 r/min, the rotational motion will gradually change from a nonstationary phase to a relatively stationary phase. This situation reflects the principle of supercritical unit operation: when the speed exceeds the critical speed, an increase in a certain speed range will further stabilize the system.

The response of the rotor-bearing system is affected by rubbing conditions. The rotor’s various dynamic characteristics are stable when no rubbing fault appears. When the system is subjected to a local slight friction, with the increase in rotational speed, waves, troughs, and sharp corners appear in the vibration waveforms, and the high-frequency harmonic envelope appears. Slight friction causes a nonlinear runout at the impact position, causing the axis trajectory to become distorted and irregular, and the Poincare response is a single point, which is a single-cycle motion. Under severe rubbing, the amplitude fluctuation is limited and the amplitude decreases. In comparison with slight rubbing, as the rotational speed increases close to the critical speed, severe rubbing will cause the occurrence of a fractional frequency multiplication component, and the axis trajectory becomes abnormally disordered. Moreover, the Poincare response is near two points, indicating that it is in a multicycle motion state at this time. The analysis results verify the accuracy of the numerical simulation.

The data used to support the findings of this study are available from the corresponding author upon request.

The authors declare that they have no conflicts of interest.

This work was supported by the National Natural Science Foundation of China (Grant no. 11502140) and Science & Technology Commission of Shanghai Municipality (no. 16020500700).