Nonlinear vibration behaviors of marine rotor system coupled with floating raft-airbag-displacement restrictor under ship heaving motion

To study the nonlinear vibration behaviors of rotor system coupled with floating raft-airbag-displacement restrictor under ship heaving motion, the dynamic model is established considering the effect of heaving motion, its steady-state responses are numerically obtained using Runge-Kutta method and the results are surveyed by tools such as the spectrum waterfall diagram, time-domain response, frequency-domain response, axis orbit, and Poincaré map. The effects of rotating speed, ship heaving amplitude, and its frequency on the nonlinear dynamic behavior of the system are mainly studied. The results show that the responses of the rotor and raft are of obvious nonlinear behaviors such as amplitude jumping, bifurcation, and chaos due to the effects of nonlinear oil film force and ship heaving motion. With the increase of rotating speed, the motion of rotor and raft presents quasi-periodic and chaotic vibrations. Ship heaving amplitude and its frequency all have great effect on the vibration of rotor and raft; as heaving amplitude or frequency increases, the motion state of rotor and raft changes, and the amplitude of raft increases significantly. The displacement restrictor can effectively limit the vibrating displacements of the raft when ship heaving amplitude or its frequency is large.

Reliability Test Rig of the Motorized Spindle and Improvements on Its Ability for High-Speed and Long-Term Tests

To realize the accurate performance test under high-speed operation and the long-term stable reliability test of the motorized spindle, a reliability test rig (RTR) which can simulate the cutting force in the actual machining process is presented. Firstly, a reasonable prototype integrating dynamic force loading devices and torque loading devices is designed and established based on the load analysis of the spindle, and a complete and explicit control strategy of the reliability loading test is designed. Secondly, the effects of misalignment of the diaphragm coupling caused by assembling are analyzed, and experiments are conducted to test the axis orbit of the motorized spindle. The experimental results illustrate that the axis orbit can identify the occurrence of misalignment, which ensures timely adjustment of misalignment and the accurate performance test under high-speed operation. Lastly, a damper is added in the mechanical structure of the electrohydraulic servo loading system (EHSLS), and the comparison of Bode maps before and after optimization is analyzed by the Nyquist criterion. After the optimization, the gain and phase margin of the Bode diagram are 12.9 dB and 57.2°, respectively, which are both within the stable range and validate the improvements on the ability of long-term reliability tests of the motorized spindle. The presented RTR is able to simulate the actual cutting force and provides an efficient loading approach to guarantee the accuracy and stability of motorized spindle tests.

Feature frequency extraction algorithm based on matching pursuit and its application in axis orbit

Rotor displacement signal is an important state characteristic parameter of rotating machinery, which contains abundant operation information of rotating machinery. Aiming at the problem of purifying axis orbit in large -scale rotor test-bed, a new method of extracting the characteristic frequency and synthesizing axis orbit is proposed based on the matching pursuit algorithm (MP). In this paper, the sparse decomposition of signals in the algorithm is implemented by MP algorithm. The algorithm can accurately extract single or multiple feature frequencies. The purified curve is clearer and more efficient. Fault types can be found by extracting different characteristic frequency synthetic axis trajectories, so as to understand the operation status of the large-scale rotor test-bed.

A Method for Monitoring Lubrication Conditions of Journal Bearings in a Diesel Engine Based on Contact Potential

Wear of the journal bearings in a diesel engine is usually caused by asperity contact. Increased contact potential is caused by the asperity contact between the journal bearing and the shell. This paper analyzes the relationship between the contact potential and asperity contact and presents a method based on contact potential to monitor the bearing wear caused by asperity contact. A thermo-elastic hydrodynamic lubrication (THL) model of the journal bearing on the test bench was established and was verified by measuring its axis orbit. The asperity contact proportion was calculated based on this THL model, and its relationship with the measured contact potential was determined. The main contribution of this paper is to present a new method for monitoring the lubrication conditions of journal bearings in a diesel engine based on contact potential. The results showed that (a) when the minimum oil film thickness was less than 5 μm, asperity contact occurred between the bearing shell and the journal, which led to a sharp increase in contact pressure and a rapid increase in friction power consumption. Further, (b) there was a positive correlation between contact potential and asperity contact. The contact potential was greater than 0.75 mv when asperity contact occurred. These results proved that asperity contact could be accurately monitored using the contact potential, and the feasibility of using the contact potential to monitor the lubrication condition of a bearing was verified.

Application of Axis Orbit Image Optimization in Fault Diagnosis for Rotor System

The shape characteristic of the axis orbit plays an important role in the fault diagnosis of rotating machinery. However, the original signal is typically messy, and this affects the identification accuracy and identification speed. In order to improve the identification effect, an effective fault identification method for a rotor system based on the axis orbit is proposed. The method is a combination of ensemble empirical mode decomposition (EEMD), morphological image processing, Hu invariant moment feature vector, and back propagation (BP) neural network. Experiments of four fault forms are performed in single-span rotor and double-span rotor test rigs. Vibration displacement signals in the X and Y directions of the rotor are processed via EEMD filtering to eliminate the high-frequency noise. The mathematical morphology is used to optimize the axis orbit including the dilation and skeleton operation. After image processing, Hu invariant moments of the skeleton axis orbits are calculated as the feature vector. Finally, the BP neural network is trained to identify the faults of the rotor system. The experimental results indicate that the time of identification of the tested axis orbits via morphological processing corresponds to 13.05 s, and the identification accuracy rate ranges to 95%. Both exceed that without mathematical morphology. The proposed method is reliable and effective for the identification of the axis orbit and aids in online monitoring and automatic identification of rotor system faults.

A New Concept of Instantaneous Whirling Speed for Cracked Rotor’s Axis Orbit

At present, the axis orbit (whirling) and the instantaneous angular speed (spinning) are important symptoms in the condition monitoring of rotor systems. However, because of the lack of research of the transient characteristics of axis orbit within a whirl cycle, the axis orbit cannot reflect the instantaneous characteristics of the rotation during one whirling cycle like the instantaneous angular speed. Therefore, in this paper, a new concept of instantaneous whirling speed of axis orbit within a whirling cycle is proposed and defined. In addition, the transient characteristics of instantaneous whirling speed are studied. Meanwhile, the response mechanisms are qualitative analyzed through the study of the work of the additional stiffness excitation and the conversion relationship between the kinetic energy and the potential energy. Then, the minimum of the relative instantaneous whirling speed (RWS) is proposed as a potential monitoring index for crack severity. The instantaneous whirling speed is a new attribute of axis orbit and a new perspective for the vibration analysis of cracked rotors. The addition of this new attribute significantly increases the effect of axis orbit for distinguishing normal and cracked rotors. The new analysis perspective and the new diagnosis index are potential supplements for crack diagnosis.