This study deals with the vibration transmissibility of a vibration isolation device, which is composed of frictional damping and nonlinear softening springs, when its base is harmonically excited. The SCAP method, a type of averaging method, is employed to obtain steady-state responses. The vibration characteristics due to excitation of the base are investigated through the analysis of displacement transmissibility in the steady-state response. In this process, displacement transmissibility for design parameters is analyzed, and the stability of the response is also investigated. The vibration isolation effect due to frictional damping is found to be more effective in the case of the softening spring than in the case of the hardening spring. Additionally, the pattern of the jump phenomenon observed during frequency sweeping, both upward and downward, has been identified.

The customer of massage chair is expanding day by day from middle age to all ages. In 2018, the market size was 700 billion KRW, an increase of 30 times over 10 years. However, most related SMEs suffer from excessive competition by the market monopoly of some major companies. In this situation, in order for a related company to survive, it is necessary to steadily research and develop new products. Founded in 2009, company L produces massage chairs for health and relaxation of customers. L’s products use a sound wave vibration module that is favorable for human body, unlike other products that use vibration motor type. However, frequent breakdowns of massage chair due to the vulnerability of plate (leaf) springs, which play an important role in sound wave vibration modules, made sap its competitiveness. In this paper, we propose a method to design desirable plate spring structure by sequentially experimenting with five different plate springs. The results of this study are expected to contribute to improve the quality of plate spring and the reliability of sound wave vibration module. In the future, it is necessary to find a way to use it in the development of foot massage or scalp management device as well as continuous research to find optimal plate spring structure through various analysis.