Noise from shaft of centrif fan.

[remp]

I have a York split a/c unit with a twin centrif. fan forward curved delivering air into a large room.

The reutrn air is via a return air fan and ductowrk from the room to a return air plenum box on the back of the ac unit. The return air fan is a plug centrif on a frequency inverter.

Sometimes the supply air fan in the York a/c unit start to make a squeeking noise, as if its coming from the shaft.

I reduced the speed of the return air fan on the feq. invertor to reduce the pressure at the back of the a/c unit from positive to slightly negative and the noise goes away. However when I increased the speed earlier on the return air fan it also coused the noise to go away.....what is going on???
Is having too much positive pressure on the back of the York unit fans causing the noise?

Please help..1!!!

 

[MintJulep]

Either an imbalance vibration that just happens to be resonant at some specific frequencies, or an impending bearing failure.

Does it squeak at half the squeaky speed?

 

[DrRTU]

It sounds like you have the 20 to 50 ton “L or K”AHU in a CV application. Check the belts and alignment of the York AHU fan motor. How are you controlling the OA / RTN flows? I find it odd that you have a RTN drive on this application. It sounds like you are loading and unloading the supply fan. Have you looked for supply fans amps swinging up and down? Check the long fan shaft in the center for runout.

 

[Artisi]

It could be axial loading on a bearing, but I can't offer any advice on what to do about it