By keep asking “what happened to the part that was connected between normal and fault?” The basic problem of fault diagnosis is put forward. Based on the parameter analysis, the most basic problem of fault diagnosis is pointed to the generalized stiffness looseness through the congenital underlying logic presupposition of the fault. The relationship between generalized looseness, part fault and system fault is analyzed by analytic method, which further proves that looseness fault is the most basic problem in fault diagnosis.Finally, the essential priority of looseness fault in fault diagnosis is expounded.The essential priority of loosening fault in fault diagnosis explains the basic problem of fault diagnosis and provides a scientific basis for the systematic development of fault diagnosis.

Abstract In view of the limitation of the numerical method for coupling faults of rotating machinery, a modeling method for coupling faults of rotor, Deformation-Motion Vector Analysis Method, is purposed in order to explain the fault mechanism more thoroughly and universally. Firstly, the “Deformation-Motion” is defined by fault and the relationship between fault deformation and periodic signal is established by Fourier series. Secondly, the “Vector Analysis” analyzes the deformation-motion in the dynamic coordinate system, and establishes the relationship between the fault deformation-motion and the periodic excitation force by using the dynamic and static coordinate systems. Thirdly, the analytical solution is obtained by harmonic balance method, and the coriolis acceleration term is first found in the analytical solution of rotating machinery failure. Finally, the validity of the modeling method and the correctness of the analytical solution are verified by engineering practice.

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