Study On Nonlinear Characteristics Of Friction Vibrations Of Reciprocating Sliding Pairs In Running-in Operation

Wear exits between the friction pairs in the process of machinery and equipment operation, during which the surface materials touching each other continuously damage. Wear is the main cause of parts failure and material loss, and 60-80% failure is caused by the wear of mechanical parts. About 30% energy is consumed in different forms of wear in the industrialized countries in the world. Therefore, it is very important to monitor the wear state of mechanical equipments and identify the early failure of equipments for the purpose of energy conservation, consumption reduction, reasonable operation and safety production.Friction vibration is a phenomenon originated from the friction and wear procedure of the friction pair during the mechanical equipment operating. The signals of friction vibration contain a lot of information, which can reflect the wear states of the friction pair. At the same time, the signals of friction vibration can be real-time online collected without affecting the normal operation of equipments. Therefore, it is an available way to describe and differentiate the wear states of the friction pair by making use of the friction vibration and extracting its characteristic, which can provide a theoretical basis and a new method for the equipment wear status monitoring and fault diagnosis.Friction vibration signals can reflect the wear state of the friction pairs, but the friction vibration signals with noise cannot truly reflect the wear state of the friction pairs. In this paper, the noise reduction method of the friction vibration signals was discussed by the harmonic wavelet packet transform (HWPT), moreover the effect of this method was analyzed and testified by using the friction coefficient and worn surface morphology of the friction pairs. The result shows that the method of harmonic wavelet packet transform can be used to reduce the noise of the friction vibration signals.Based on the study of signals noise-reduction method, the time domain characteristic and the frequency domain characteristic of friction vibration signals were investigated by means of wear tests. The analysis of the time domain shows that the friction vibration signals are characterized by weak and low energy and the amplitude of it gradually lowers and tends toward a steady state in the running-in process. The amplitude of the friction vibration signals was affected by the load, Jubricant, and material, and it increases as the loads raises and presents the difference in various lubricants and materials tests. The amplitude of the friction vibration signals was obviously affected by the initial surface morphology, but the influences will weaken as the wear goes on. The analysis of the frequency domain shows that the friction vibration signals are characterized by nonlinear feature, and there are a large number of side frequencies and no apparent natural frequencies in its spectra. The frequency distribution range of friction vibration signal is affected by the materials of the friction pairs, and is independent of the loads, initial surface morphology and lubricants. The experimental results show that the friction vibration frequency distribution range is 2000-30000Hz for the material of High entropy,2000-3000Hz for the material of medium carbon steel, and 3000-4000Hz for the material of cast alloy.The friction and wear is an extremely complex nonlinear dynamic process. It is an effective and available way to solve the nonlinear problems by making use of chaos theory. In this paper, the nonlinear feature of the friction vibration signals was investigated using chaos theory. The Largest Lyapunov Exponent (LLE) method, Pseudo-phase space method, Power spectrum method, Poincare section method and Stroboscopic sampling method were employed respectively to analyze the chaos nature of the friction vibration signals. The correlation dimension and K entropy of friction vibration signals were calculated. The chaotic attractors of friction vibration signals were extracted and the evolution of them in the phase space was investigated. The results are as followed. The friction vibration attractor in the phase space is an always-open trajectory with a specific hierarchy and structure. The power spectrum of friction vibration signals has broad peaks. The trajectory of the friction vibration signals has a fine structure in the Poincare section and stroboscopic section. The diagram of principle component spectrum is a straight line with negative slope that passes a fixed point. Therefore, the friction vibration signal has the chaos nature. In the running-in process, the chaotic attractor of the friction vibration gradually converges and tends toward a balanced state. The evolvement of the friction vibration chaotic attractors indicated that the wear state changes from the running-in wear stage to the stable wear stage.