Bridgestone cabin mounts
Bridgestone's high performance cabin mount
The unique internal design of Bridgestone's viscous fluid type mount gives it a high damping characteristic. It is widely used as a cabin mount for construction machinery which need a high damping and heavy-duty performance.
Generally, steel springs consist of only the spring element as shown in the diagram below. On the other hand, Vibration Isolation Rubber consists of both a spring element but also a damping element as the corresponding diagram shows. Therefore, when using Vibration Isolation Rubber as an isolator, we can expect a damping effect which is absent in steel springs. This is one of the most compelling reasons to consider Vibration Isolation Rubber.
Steel spring
Rubber spring
Damping effect provided by Vibration Isolation Rubber
When we study the theoretical design of Vibration Isolation Rubber (VIR) mounts, we set the spring stiffness of the rubber to exceed the N/fo ratio by a factor of 1.4. (N:Hz = Targeted vibration isolation frequency, fo:Hz = Natural frequency which is defined by the supporting weight and the spring stiffness of the vibration isolation rubber).
However, when "N" corresponds exactly to the "fo", vibration will be amplified. This is commonly known as Resonance. Its magnification rate depends on the loss factor. (=tanC).
When we study the magnification rate at the point of resonance with the tan value, the resulting vibration transmission appears as a curve (See Figure 1.) When using steelsprings, tanδ becomes almost "0" and the magnification rate is unlimited. On the other hand, the tanδ value of common low grade rubber becomes 0.1 to 0.2, whereas using Bridgestone's specialized Vibration Isolation Rubber products like viscous fluid cabin mounts, the tanδ becomes 0.7 to 1.4.
According to Figure 1, as the tanδ value gets higher, the magnification rate at the point of resonance will decrease.
Figure 1. Magnification factor