Modeling and analysis of a torsional magnetic negative stiffness structure for flexible vertical low-frequency vibration isolation

[Display omitted] •A quasi-zero stiffness (QZS) isolator with flexible adjustability is proposed.•Torsional magnetic negative stiffness (TMNS) is investigated and utilized.•One or two QZS regions and loaded mass of proposed model are adjustable.•The stiffness nonlinearity near QZS point can be adjusted and decreased.•The mass of TMNS can act as a nonlinear inertia in proposed QZS isolator. This paper proposes a novel quasi-zero stiffness (QZS) isolator with flexible adjustability, which consists of torsional magnetic negative stiffness structure (TMNS) and linear vertical spring. The TMNS includes inner and outer radially magnetized tile permanent magnets (PM), which can generate periodic torsional negative stiffness, and provide vertical negative stiffness for the system under the action of angle-displacement conversion. The proposed model possesses the ability to adjust one or two QZS regions, different carrying loads, and stiffness nonlinearity near the QZS point. Considering the nonlinear inertia caused by the mass of TMNS, the motion equation of proposed system is established. The system response under displacement and force excitation is solved based on the harmonic balance method (HBM). The effects of excitation amplitude, damping ratio, and mass ratio on the system transmissibility are analyzed. The nonlinear inertia can suppress the resonance response. When excitation amplitude is large or damping ratio is small, the system transmissibility will exhibit jumping phenomenon. The advantages of proposed negative stiffness mechanism are analyzed and compared with the typical QZS isolator. When the initial angle is much less than 90°, the proposed QZS isolator can achieve weaker stiffness nonlinearity near the QZS point by adjusting TMNS parameters, and has better vibration isolation performance under small excitation amplitude, especially for displacement excitation. Moreover, an experimental prototype is manufactured and tested. The experiment results show that the proposed QZS isolator has a lower initial isolation frequency than linear system, proving the validity of proposed system for low-frequency vibration isolation.