the detail is in the PDF file

Air_Fan_Cooler_Spectrum.pdf (35 KB, 117 downloads)

Hi Fadl

Belt frequency
= 3.142 X Pulley RPM X Pitch Diameter / Belt length

Timing belt frequency
= Belt frequency X #belt teeth
= Pulley Rpm X #pulley teeth

Regards,

Belt Freq= (3.142 X Pulley Rpm X Pitch Dia) / Belt Length

Timing Belt Freq= Belt Freq X # Belt Teeth
= Pulley Rpm X Pulley Teeth

Sorry,

James beat me to the post

Taking your numbers I see that the motor shaft speed is higher than fan blade-rate (7x fan speed). Now assume the higher of the two frequencies you marked is motor speed (24.6 Hz), then the other frequency is 24.6 x (36/264) x 7 = 23.48 Hz. The calculated lower frequency (7x fan speed) matches your marked frequency. I conclude that you did not have an accurate motor speed to start with, and the lower frequency is fan blade-rate and higher frequency is motor speed. No mystery here!

Walt

thank you for your interest, I know that the motor 1X and the FPB frequency are very close to my calculation and maybe there is some slipping between the actual motor speed and the theoretical speed, but I thought about the belt because the belt have a noisy behavior and I cannot find the symptoms of the belt fault in the spectrum

Your fan 1x correlates with Walt's comments so your motor speed must be 1476 rpm (lightly loaded?).

If you are looking for noise related to the belt teeth would you not need to look out around 885 Hz?

so i need to ask why the motor 1X apper in the fan drive end spectrum,what does it mean??!!

if the Fan blade Pass frequency exisit does it mean i have a problem with the airodynamic of the fan
(the blade clearance in the acceptable limits)

If you have 1xRPM motor significant on the fan one normal reason I have seen is that the pulley(s) are worn oval and not round, the other is that you have a resonance of the fan (mounting) that is excited/triggered by the 1xRPM motor.
You normally always have some blade pass if hi it may be the filters or valves before or after the fan makes it run outside the spec. or just as common, more airflow was needed and the speed was turned up by swapping ratio and the fan is operated outside spec or there is a less than desired installation having 90 degree turns in piping just before or after etc. Just some normal examples. Take a look at the thing with a open mind. OLI