DESIGN AND ANALYSIS OF AN X-STRUCTURED VIBRATION ISOLATION MOUNT (X-MOUNT) WITH WIDER QUASI-ZERO- STIFFNESS RANG

Increasing demands for high performance of vibration control drives the development of nonlinear vibration control devices to overcome the limitations of traditional linear vibration control methods. A novel and compact X-structured mount (X-mount) with an oblique spring is designed and investigated in this paper which demonstrates beneficial nonlinear stiffness and damping, tunable payload and resonant frequency property, and a special enlarged quasi-zero-stiffness region, which thus lead to excellent vibration isolation performance without instability or other strong nonlinear response such as jump phenomenon. The X-mount is designed, based on a bio-inspired X-shaped structure, and a special oblique spring is introduced which can effectively widen the working quasi-zero-stiffness range by employing negative stiffness due to a special contact stiffness. The oblique spring can also help to reduce the dynamic stiffness achieving an ultra-low natural frequency and broadband effective vibration isolation range. The mathematical modeling, theoretical analysis and experiment results show the high performance of this X-mount, potentially providing a promising solution for vibration control in many engineering practices. 4434 EURODYN 2020 XI International Conference on Structural Dynamics M. Papadrakakis, M. Fragiadakis, C. Papadimitriou (eds.) Athens, Greece, 23–26 November 2020 Available online at www.easdprocedia. org EASD Procedia EURODYN (2020) 4434-4440 ISSN:2311-9020 © 2020 The Authors. Published by EASD Procedia. Peer-review under responsibility of the Organizing Committee of EURODYN 2020. doi: 10.47964/1120.9360. 18452 INTRODUCTION Vibration generated by machines or other environmental sources are often troublesome and unavoidable. However, their influences on dynamical systems or structures can be suppressed through properly designed vibration isolators. To achieve an effective vibration suppression effect, the natural frequency of an isolation platform must be smaller compared with the excitation frequency. For the low-frequency vibration environment, this requirement can be achieved by using a soft spring/stiffness component, but this might sacrifice the static loading capacity of the isolation platform. To conquer this intractable problem, some quasi-zerostiffness (QZS) vibration isolators are developed to achieve a low dynamic frequency and maintain the static loading capacity simultaneously [1-11]. A traditional QZS isolator consists of three springs, where the negative stiffness provided by the horizontal springs is designed to offset the positive stiffness from the vertical spring, thus quasi-zero-stiffness is achieved. While, QZS isolators could lead to unstable problems, e.g., jump phenomenon, multi-stable [8, 11, 12]. Moreover, QZS isolators are limited in large vibration motion and very sensitive to the loading. Therefore, proper nonlinearity should be considered to avoid the strong nonlinear problems and the disadvantages mentioned above. For instance, the benefits of the applications of the geometric nonlinearity was summarized in Gatti et al [13] which may provide solutions for the problems above. The applications of the QZS in shock isolation can be found in [9, 14, 15]. The methods of realizing negative stiffness and QZS inlcude horizontal spring [7, 10, 11, 16], buckled beams [14] or magnetic components [1, 17] and structures [2]. Another important component for an isolation system is its damping. The limitation of the linear damping is that increasing the linear damping can suppress the resonant peak but will deteriorate the performance in higher frequencies. Therefore, nonlinear damping attracts attentions recently for the advantages. Among the nonlinear damping, the displacement-dependent damping shows advantages in both of the resonance and the high frequencies [18, 19]. Recently, a bio-inspired structure called X-shaped or limb-like structure (LLS) attracted many attentions due to the beneficially nonlinear stiffness and damping properties, and could be conveniently designed for many engineering applications [6, 12, 20-26], which presents an alternative and totally new approach for creating QZS or negative stiffness with only simple bio-inspired structure design. Based on this X-shaped structure, an innovative X-mount is thus proposed in this paper. This X-mount is optimally designed with a special oblique spring which can reduce the equivalent stiffness before contact and achieve a wider QZS range when the contact happens. This contact stiffness can contribute to remove/offset the negative stiffness after the zero-stiffness point, and the combination effect enhances the system isolation performance with an obviously enlarged QZS range. In addition, the obliqueness of this spring can be used for reducing the system stiffness further to achieve an ultra-low natural frequency and broadband effective isolation range. Experimental validation is conducted, together with theoretical analysis to verify the effectiveness of the proposed method. 1 MATHEMATICAL MODELLING 1.1 Static force The X-mount is presented in Fig. 1 with specification of the structural parameters. The structural parameters are defined as: L is the length of the rod, is the initial assembly angle, n is the number of the layer, M is the isolated mass and K is the stiffness of the spring. z-axis is the vertical direction and x-axis is the horizontal direction, is the rotational angle of the rod, is the relative displacement in the horizontal direction, is the total relative displacement in vertical direction. By changing , the initial angle between the oblique spring