Fans do often get uneven coating of particles on the blades, especially if the air or gas the fan is spinning around has a high particle density. The uneven coatings result in unbalance. If a blade is deformed, cracked or broken the peak level of the blade passage frequency will increase. If the number of blades is big there sometimes exist side bands to the blade pass frequency.
Blade pass frequency, bpf = The number of blades multiplied with the rotational speed
The blade pass frequency is usually not considered a problem. Large bpf-amplitudes can however be generated in a pump if the gap between rotating blades and stationary mouth pieces is not kept even all the way around.
Bpf can also coincide with the natural frequency of a system and thereby causing resonance.
High bpf-amplitudes can also appear due to sudden turns in pipes, obstacles hindering the flow or if the pump/fan rotor is eccentric placed in the pump-/fan house. A common cause of high bpf-amplitudes is consequently problem with the flow.
|Source of vibration||Exciting frequency||Dominating direction||Amplitude||Spectral characteristic||Comments|
|Fan house problem with clearance between wheel and hose.||Blade passage = X times number of blades||radial, axial, tangential||constant||narrow band||Sometimes include multiples of blade passage|
|Unbalance||1 X||radial, tangential||constant||narrow band|
|Fan Pitch Problem||1 X||axial||constant||narrow band|
|Uneven flow||Blade passage||radial, tangential||narrow band||smalband|