86 H. Xiuchang et al. Fig. 7.3 The comparison of tested FRFs and the synthesis results for floating raft system under unsymmetrical excitation. (a)–(d) Same as in Fig. 7.2 Fig. 7.4 The axial deformation (a) and the coupled axial/bending deformation (b) of the isolator 7.3.1 Experimental Set-Up to Identify the Rotational Mobility of Isolators The impedance of isolators has been studied extensively. While most effort is focused on translational DOFs. Literatures concerning the impedance of rotational DOFs are rare. The characterization method proposed by Kim and Singh can be applied to identify the rotational stiffness [13]. While there are two short backs, one is the characterization cannot take into account the static loading; the other is that the method cannot identify the impedance of the DOF corresponding to the torsional deformation of the isolator. Forrest [14] presented a method to measure the dynamics of isolators in free-free conditions and calculate the four-pole parameters. The natural frequencies, mode shapes and associated modal damping are applied to construct the FRFs in all DOFs, including coupling between different DOFs. For the same reason, the static load cannot be considered. As seen from the results in [15], the static load can influence the vibration transmission for troughs at higher frequencies. In what follows, an improved measurement set-up based on Lim and Singh that can consider the static load is proposed. The difference is the loading parts composed of several air springs, which support the system in a very low frequency simulating the free-free condition and provide static forces for the system by varying pressure in the air bags. The whole system is hung by the elastic cord. When exciting at point 1, the axial mobility can be identified; when exciting at point 2, the transverse mobility and the bending mobility can be identified (Fig. 7.5).
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