Maintenance
Possible Causes of Vibration
The actual causes of vibration may change considerably. Some of the most common are as follows.
1–– Unbalance of one or more blades: the vibration caused by blade imbalance occurs on the tip path plane with a frequency equal to the fan RPM and at an amplitude which is dependent on the degree of imbalance and the square of the rotational speed.
2––Blade pitch angle not included in the ± 0.5° tolerance: this con- dition causes vibration outside the tip path plane at a frequency equal to the fan RPM and at an amplitude which is dependent on the square of the rotational speed.
3–– Blades too close to supports (periodic aerodynamic turbulence): this condition is characterized by vibration outside the tip path plane at a frequency equal to the product of the number of fan blades and RPM. The amplitude depends upon the extent of the aerodynamic turbulence.
4––Resonance between one of the possible forcing frequencies of the fan and one or more of the vibration modes of the structure on which it is installed. The main forcing frequencies generated by the fan, normally correspond to the following frequencies:
-Fan RPM
-The product of fan RPM and the number of blades
-The product of fan RPM and the number of structural supports capable of generating aerodynamic turbulence (if they are arranged in an axial-geometric fashion).
5–– Vibration transmitted by the structure on which the fan is installed: the frequencies of such vibration depend on both the external forcing frequencies and the resonant frequencies of the structure.
6––Resonance of the blades with one of the possible forcing fre- quencies; in the vast majority of cases the vibration occurs outside of the tip path plane.
7––Misalignment of the drive shaft: this generates vibration with a frequency that is once or twice the RPM.
8––Loosening of blade and/or speed reducer fixing bolts. The behavior of the rotor under these circumstances is totally unpredictable, as it depends upon the extent and location of the loosening.
