When should I use inertia blocks with the vibration isolators?
There are three major reasons for including an inertia mass with the isolation. One is that it lowers the center of gravity of the system. For a tall narrow unit, this could make the installation more stable in a seismic event.
Secondly, some pieces of equipment have large unbalanced vibratory forces. An example might be large water pumps or large medical vacuum pumps. In the case of one of these applications, the large unbalanced forces could cause larger than normal amplitudes, or physical movement, of the springs under normal conditions. Large movements can cause undue stresses to flex connectors and piping connections. The inertia block adds mass to the system so that the same amount of force has to move a much greater mass, thus minimizing the amplitude of movement.
The third one is that some equipment manufacturers mandate a common rigid surface to minimize or eliminate flex between two components that are connected by a flex drive.