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• Gearbox fault diagnosis using BFNCC is proposed for nonstationary condition. • The method can remove the effect of the structure transfer paths. • The proposed method is robust to the location of measuring points. • Method shows potential for a machinery with complicated structure transfer paths. The main challenges in gearbox condition monitoring and fault diagnosis are the heavy noises caused by harsh operating environments and the complicated nonstationary condition during the runtime. It is extremely difficult and even impossible to obtain sensitive fault characteristics through direct measurements. Thus, it is of great significance of fault characteristic inversion based on the structure vibration transfer paths for complicated mechanical systems fault diagnosis. This paper proposes a gearbox fault diagnosis method under nonstationary condition through using nonlinear chirp components extracted from bearing force (BFNCC). In the proposed method, the bearing dynamic forces are first identified based on the vibration transfer function matrix, and the intrinsic signal components are then decomposed through a nonlinear chirp mode decomposition method. The proposed method improved the quality of vibration signals by removing the effect of the structure vibration transfer paths. In comparison with order tracking, the BFNCC is capable of clearly reflecting the magnitude increase phenomenon under gear fault condition. Numerical simulation and experimental studies show that the proposed method is effective for gear fault diagnosis under nonstationary condition. The proposed method is demonstrated to be superior to the existing approaches and robust to the location of measuring points, and thus shows potential in machinery fault diagnosis under the interference of complicated structure transfer paths. [ABSTRACT FROM AUTHOR]