The gripper cylinder that provides braced force for Tunnel Boring Machine (TBM) might fail due to severe vibration when the TBM excavates in the tunnel.Early fault diagnosis of the gripper cylinder is important for the safety and efficiency of the whole tunneling project.In this paper,an online condition monitoring system based on the Empirical Mode Decomposition (EMD) method is established for fault diagnosis of the gripper cylinder while TBM is working.Firstly,the lumped mass parameter model of the gripper cylinder is established considering the influence of the variable stiffness at the rock interface,the equivalent stiffness of the oil,the seals,and the copper guide sleeve.The dynamic performance of the gripper cylinder is investigated to provide basis for its health condition evaluation.Then,the EMD method is applied to identify the characteristic frequencies of the gripper cylinder for fault diagnosis and a field test is used to verify the accuracy of the EMD method for detection of the characteristic frequencies.Furthermore,the contact stiffness at the interface between the barrel and the rod is calculated with Hertz theory and the relationship between the natural frequency and the stiffness varying with the health condition of the cylinder is simulated based on the dynamic model.The simulation shows that the characteristic frequencies decrease with the increasing clearance between the barrel and the rod,thus the defects could be indicated by monitoring the natural frequency.Finally,a health condition management system of the gripper cylinder based on the vibration signal and the EMD method is established,which could ensure the safety of TBM.