Papers by Author: Feng Zhang

Abstract: Vibration signal analysis has been widely used in the fault detection and condition monitoring of rotation machinery. But the practical signals are easily polluted by noises in their transmission process. The raw signals should be processed to reduce noise and improve the quality before further analyzing. In this paper an improved wavelet threshold denosing method for vibration signal processing is proposed. Firstly, a new threshold is developed based on the VisuShrink threshold. The effect of noise standard deviation and wavelet coefficient is retained, and the correlation of wavelet decomposition scale is considered. Then, a new threshold function is defined. The new algorithm is able to overcome the discontinuity in hard threshold denoising method and reduce the distortion caused by permanent bias of wavelet coefficient in soft threshold denoising method. At last five kinds of threshold principles and three kinds of threshold functions are compared in processing the same signal, which is simulated as the mechanical vibration signal added white noises. The results show that the improved threshold is superior to the traditional threshold principles and the new threshold function is more effective than soft and hard threshold function in improving SNR and decreasing RMSE.

Abstract: In this article, a new type of visualizer for point contact flow field has been developed. The principle, constitution and application are introduced in details. The flow field of point contact area are captured and analyzed.

Abstract: As the vibration signal of helical gears is nonlinear and nonstationary, it is very difficult to diagnose their faults based on the vibration signal analysis and processing, particularly when the gears rotate at low speed. In this paper, an applicable fault diagnosis approach is proposed based on Hilbert demodulation and EMD (empirical mode decomposition). Firstly, the modulated signals are extracted through Hilbert envelope demodulationand low pass filter. Furthermore, EMD is used to decompose the multi-component demodulated signal into a series of intrinsic mode functions (IMFs) whose instantaneous frequencies have a physically meaningful characterization of the original signal. Finally, the fault features of low-speed helical gear are obtained by the spectrum analysis to each IMF. The experiments of tooth broken fault diagnosis show that this method is more effective than traditional Hilbert demodulation analysis.

Abstract: In order to get PTFE(polyfluortetraethylene) composites of good friction and wear properties used in vacuum and low temperature conditions, do the tribological experiments of GF (glass fibre)/PTFE composites under vacuum and low-temperature environment. The results show that: with the increase of the GF size, the friction coefficient increases, with the increase of GF content, the friction coefficient increases.

Abstract: Mechanical seal plays a very important role in rotating machinery for space exploration, but it is very difficult to estimate its friction performance by simulative experiment. In this paper, a experimental system for measuring friction properties of mechanical seal materials used in spacecraft is developed. It is able to provide some adjustable parameters in the experiment, such as environment temperature -60°C~60°C, load 100N~300N, rotational speed of 30r/min~110r/min and atmospheric pressure and high vacuum. Friction coefficients of babbitt graphite M120B and hard alloy YG6 are obtained by the test rig and the experimental results testify that the device can conduct simulative experiment effectively.

Abstract: In order to investigate the material and lubrication of the moving parts of a space probe, a sliding frication tester was developed under vacuum and cryogenic environment. The tester includes the following four parts: driving system, loading system, measuring system and refrigerating system. A computer program was also developed to acquire and record the experimental data. Friction coefficients of component pairs of material GCr15-GCr15 were tested in different environments on the tester. The results show that in terms of the friction coefficient and life of the friction pair under vacuum and low temperature environment, the pairs with vacuum grease are far better than that without grease. The tester meets the design requirements.

Abstract: The apparent viscosity of aircraft grease with different nano-particles content, temperature and shear rates were studied using a rotational viscometer. The rheological properties of two types of aircraft grease, the basic grease and the one with nano-particles additives, were investigated using a rheometer. The results indicated that the apparent viscosity increases with the increase of nanoparticle concentration with the given ratio of nano-particles added. It was also found that the grease with and without the nano-particles both have yield stresses and clear shear-thinning properties. The shear-thinning phenomenon of the grease containing nano-particles is more evident than that of the basic grease. The experimental results also reveal that the rheological characteristics of both types of grease fall in Herschel-Bulkley class, and the nano-particles have a significant influence on the rheological parameters. At the end, the rheology mechanism was discussed based on the entanglement and orientation theories.