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Under variable working conditions, the components of gear fault vibration signals had complex time-frequency characteristics that crossing multiple time scales or overlapping time-frequency domains. The traditional decomposition method could not decompose this kind of signals based on local time-scale characteristics. A new MMD method was introduced. The MMD method innovatively regarded the mono-component signals as a linear combination of eigenvectors with different weight coefficients. By iteratively optimizing the weight coefficients, the corresponding component signals were obtained. The problems that large amount of data reduced the decomposition efficiency when MMD analyzed the actual vibration signals with high sampling rate was solved. And the MMD method was applied to analyse gear fault vibration signals under variable rotating speed working conditions.The results show that the method may effectively decompose to the overlapping fault component signals in time-frequency domain, and has obvious advantages compare to the traditional method based on time-scale characteristics.Combined with order analysis, the gear fault feature information may be extracted clearly and accurately. [ABSTRACT FROM AUTHOR]

At present, there are two main ideas for gear fault diagnosis. One is the model-based gear dynamic analysis; the other is signal-based gear vibration diagnosis. In this paper, a method for fault feature analysis of gear crack is presented, which combines the advantages of dynamic modeling and signal processing. Firstly, a new time-frequency analysis method called local oscillatory-characteristic decomposition (LOD) is proposed, which has the attractive feature of extracting fault characteristic efficiently and accurately. Secondly, an analytical-finite element (analytical-FE) method which is called assist-stress intensity factor (assist-SIF) gear contact model, is put forward to calculate the time-varying mesh stiffness (TVMS) under different crack states. Based on the dynamic model of the gear system with 6 degrees of freedom, the dynamic simulation response was obtained for different tooth crack depths. For the dynamic model, the corresponding relation between the characteristic parameters and the degree of the tooth crack is established under a specific condition. On the basis of the methods mentioned above, a novel gear tooth root crack diagnosis method which combines the LOD with the analytical-FE is proposed. Furthermore, empirical mode decomposition (EMD) and ensemble empirical mode decomposition (EEMD) are contrasted with the LOD by gear crack fault vibration signals. The analysis results indicate that the proposed method performs effectively and feasibility for the tooth crack stiffness calculation and the gear tooth crack fault diagnosis. [ABSTRACT FROM AUTHOR]

• A novel estimation method of friction coefficient is proposed. • Proposed method can estimates the gear fault mechanism from strain signal. • The influence of friction coefficient on gear pitting vibration system is investigated. • The effectiveness of strain measurement in studying the effect of the gear pitting fault is validated. In present, the model-based gear dynamics is the main method for gear pitting fault diagnosis. However, most of gear dynamic models use a constant friction coefficient or friction coefficient obtained by calculation method that contains many simplifications and assumptions. Especially when a gear has fault, previous studies lack calculation of friction coefficient. In this paper, a new method is proposed to estimate the friction coefficient of gear pairs with different pitting. Based on the strain signal of actual gear pitting damage, the proposed method combines the finite element method and optimization method to obtain the friction coefficient at the different gear pitting level. Furthermore, by using calculated friction coefficient to obtain dynamic responses of different gear pitting states, the effects in dynamic responses of different types of friction coefficient are compared and analyzed. [ABSTRACT FROM AUTHOR]

• A novel method avoids drawback of AM for cracked gear, keeping higher accuracy. • The strain measurement technique is introduced to obtain practical TVMS. • The model parameter can be inversely reflected by using strain signal. • The effect of crack depth on vibration feature of the gear system is reflected. In this paper, a novel analytical-FE method which overcomes the drawbacks of the analytical method (AM) and finite element method (FEM) is proposed to evaluate time-varying meshing stiffness (TVMS) under the different gear crack status. In this method, the strain measurement technique is introduced. As a modern measurement technique, the proposed method can provide the strain signal of the cracked gear which can be used to modify the FE model parameter to obtain the engineering practical TVMS. In addition, it provides a new research idea that the model parameter can be inversely reflected by using engineering practical signal. The analysis result of dynamic response show that the proposed method can reflect the effect of various crack depths on the vibration features of the gear system, and the validity of the proposed method is verified by the vibration signal analysis of experimental rig. [ABSTRACT FROM AUTHOR]