This paper presents an approach for the force analysis of an open TBM gripping–thrusting–regripping mechanism, which is a special parallel mechanism driven by hydraulic actuators and constrained by rocky surroundings. The static equilibrium equations of the cutterhead–mainbeam–saddle subassembly are formulated first by exploring the reaction forces in the cross pin situated between the saddle and the gripper cylinder. This is followed by formulating the static equilibrium equations of the inner closed loops formed by the above subassembly, the torque, and gripper cylinders. Consequently, the linear map between the externally applied wrench imposed on the shield and the equivalent thrust forces of the cylinders is developed. The functionality of the force model developed is twofold, i.e., it can be used either to estimate the thrust forces of the cylinders required to resist against the tunneling loads, or to predict the tunneling loads using the measured thrust forces of these cylinders, thus providing important theoretical basis for the design and control of the mechanism.
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