Research on the three degrees of freedom hysteretic mechanical model of floating rafts

A floating raft is a complicated multilayer vibration isolation system, which is often simplified as a two-degree-of-freedom viscous damping system in analysis. In order to research the influences of the hysteretic damping and the raft flexibility on the vibration isolation effect, a three-degree of freedom hysteretic mechanical model of the floating raft is established, and the impedance method is adopted to deduce the model of the force transmissibility of the floating raft. The analytic calculation methods of force transmissibility are given aiming at the low-frequency range, the resonance range, and the high-frequency range, respectively. Finally, the influence rules of damping, mass, and stiffness on force transmissibility are analyzed aiming at the value range of parameters of most floating rafts. The main analysis conclusions are listed as follows: the analytic calculation equation for resonance frequency and resonance peak is accurate; the vibration isolation effect of the high frequency range is determined by the entire floating raft, the unconstrained raft, the inherent frequency of the equipment, the mass ratio between the raft and the equipment only; the hysteretic damping can reduce the force transmissibility at the place of the resonance frequency of the floating raft, which shall be maximized; the increase in the mass of the raft will reduce the force transmissibility during the frequency range above the inherent frequency of the equipment, but it will reduce the marginal benefit; the reduction in the stiffness of the isolator or the raft will reduce the force transmissibility during the frequency range above the inherent frequency of the equipment, and will increase the displacement of the equipment; with limited displacement, it is better to design the isolator and the raft according to the principle of equal stiffness.