Ultra-low frequency vibration isolation based on a symmetric-converging series horizontal-spring quasi-zero-stiffness mechanism

Low-frequency vibrations, especially those in the ultra-low frequency range below 1 Hz, are difficult to suppress due to their long wavelengths, slow attenuation, and strong penetration. To address this challenge, a novel quasi-zero-stiffness (QZS) isolation mechanism is proposed. The mechanism consists of a vertical supporting spring and three pre-tensioned horizontal springs arranged with one at the center and two in series at the base. The restoring force is analytically derived to identify the parameter conditions required for QZS behavior. Stiffness–displacement analysis confirms that the mechanism achieves an ultra-broad QZS region. A dynamic model based on a Duffing-type equation is further developed to obtain the amplitude–frequency characteristics and displacement transmissibility, and the effects of key structural parameters on isolation performance are examined. Numerical simulations using practical parameters show that the mechanism achieves an initial isolation frequency below 1 Hz and reduces this starting isolation frequency by approximately 74.6% compared with a conventional linear isolator. The results demonstrate the potential of the proposed mechanism for ultra-low frequency and broadband vibration isolation.