Machine TypeRated Power / Shaft HightSupport System Flexability (Foundation Type)
The Foundation Type;
Determine the relationship between the machine and foundation flexabilities ("Rigid" or "Flexable" foundation).
To consider any foundation to be "Rigid",
The lowest natural frequency of both machine & foundation must be higher than it's main excitation frequency (Forcing frequency of rotating machine) by at least 25%.
Another difinition:
It is the foundation that has sufficient stiffness that makes the structure resonance frequencies at least 25% above the resonance frequencies of the rotating equipment attached to it.
For Example;
Large & Miduim sized electric motors with low speeds, normally have Rigid Supports.
Turbo-generators & Compressors with power more than 10 MW, normally have Flexibile supports.