ISO 14694 vibration limits 1

[rQuestionEngineering]

Referring to ISO 14694 vibration limits , what is the definition of rigid mounting and flexible mounting ?

Rigid mounting means anything attached to concrete foundation ?
Flexible mounting means attached to steel frames / structure?

 

[petb]

No.
Flexible: First natural frequency of the system is well below the operating speed of the fan.
Rigid: First natural frequency of the system is well above the operating speed of the fan.

The rigidity of a foundation is a relative quantity. It must be considered in conjunction with the rigidity of the machine-bearing system. The ratio of bearing-housing vibration to foundation vibration is a characteristic quantity for the evaluation of foundation flexibility influences. A foundation may be rigid and of sufficient mass if the vibration amplitude of the foundation (in any direction) near the machine’s feet or base frame is less than 25 % of the maximum amplitude that is measured at the adjacent bearing housing in any direction.

Thus, dynamic characteristics determines support type, foundation material doesn't.