Analysis and design of an adjustable stiffness three-axis horizontal vibration isolator using elastic columns and a string in tension

• A vibration isolator with adjustable quasi-zero-stiffness along 3 axes is designed. • All natural frequencies are tuned by just varying the tension in the central string. • Implicit and explicit analytical, and finite element models are formed. • Optimal sizing enables all horizontal modes to be very close to each other. • Large isolation bandwidth is obtained for a wide range of payload masses/inertias. The aim of this study is to design a horizontal vibration isolation system with adjustable quasi-zero-stiffness along three axes (translational stiffness in the horizontal x and y axes and torsional stiffness about the vertical z -axis). For this purpose, a string is placed in the central axis of the system, and a string tensioning mechanism is developed to tune the horizontal (lateral) stiffness of the elastic columns that support the top platform on which a payload is placed. By increasing/decreasing the tension force in the string for smaller/larger payloads, axial compressive forces on all the elastic columns can be varied simultaneously, which in turn enables to adjust translational and torsional natural frequencies of the system. Various parametric studies are conducted by using normalized length and force quantities such as the ratio of string length to column length, and the ratio of critical buckling force due to a tension force directed towards a fixed point to critical buckling force due to an external vertical load. Robustness of the system against eccentric payload placement is quantified. Moreover, transmissibility of the system is calculated for translational and torsional excitations. Implicit and explicit analytical, and finite element models demonstrate that by selecting the parameters of the system appropriately, the lower limit of the isolation bandwidth for both translational and torsional excitations can be kept below 1 Hz, and the upper limit can be kept above 250 Hz even if the payload mass and its mass moment of inertia change in a wide range. Thus, the system is capable of achieving a very large isolation bandwidth at low frequencies.