Percentage Isolation Table
The table indicates the vibration isolation that can be reached, known as the external excitation frequency (linked to the operation of the machine) and the natural frequency of the suspension on vibration dampers.
The purpose of vibration isolation is to reduce the energy transmitted from the machinery to the surrounding environment. The index of effectiveness of this insulation is given by the transmissibility: it represents the ratio between the force (or acceleration) transmitted to the foundation and the force (or acceleration) produced by the machine. The lower this index is, the greater will be the effectiveness of the vibration isolation.
The ratio between the natural frequency of the suspended system (fn) and the frequency of the vibration to be isolated, called the excitation frequency of the system (fd), has a predominant influence on the transmissibility value.
Another determining factor is the damping D expressed as a ratio to the critical damping. Basically, D is a percentage dependent on the ratio between the energy absorbed and dissipated in the form of heat by the anti-vibration mount and the energy transmitted.
Therefore, in a perfectly elastic body, D will be equal to zero, while for D equal to 1, the body will only perform one oscillation and then stop. As an order of magnitude it can be considered that in rubber supports D it varies from 3 to 8% (depending on the characteristics of the compound and the operating temperature), while, for steel springs D it can be considered negligible. Percent isolation is inverse of transmissibility.
The transmission is therefore expressed by:
which for negligible damping values is reduced to: