Isolation Design for Fully Flexible Systems

Abstract

It is commonly suggested that vibration isolators should be designed with a low-frequency mount, that is, the isolator frequency is much less than the first mode of the system being isolated. This is a direct result of the single-degree-of-freedom (SDOF) isolator model which states that the best performing isolator is the one that has the lowest natural frequency. When flexibility is included in the isolator design, the low-frequency mount design still performs best in an overall sense. However, with system flexibility being modeled, the isolator can now be designed between the modes of the flexible system. These types of isolator mounts tend to have a coupling effect, that is, the frequencies around the isolator mount have such a strong interaction between each other that when isolator damping is present, multiple system modes are attenuated. Therefore, this paper fully explores the choice of isolator mount frequency and isolator placement for fully flexible systems. Results show that a low-frequency mount design considering base and equipment flexibility, will have the first natural frequency shift by as much as 15.6%. For a mid-frequency mount design, the shift of the first three modes can be as high as 34.9%, 26.6% and 11.3%, respectively. Also, when base and system flexibility is considered, isolator placement becomes a critical issue. There can be as much as 16% difference in the first mode for a low-frequency mount design and as high as 25% difference for a mid-frequency mount design.