Hello All,

Recently I did a FFT analysis of a roots blower(2 lobes rotor) and I found at the 4X RPM there was a big peak at the timing gear side bearings. Also from the time wave I can see some peaks and the peaks reached 20G. So I made the decision to change the timing gear set. After that the vibration signature was collected and compared with the old one. It improved a lot.

I just wonder what caused the peaks at the 4X RPM as from the timewave the peaks seems to appear at 2X RPM.

Thanks a lot,

Tony

The "blade/lobe" pass frequency is 4 X running speed that is where the 4 x is coming from. Any 2 x would indicate a rub of rotor lobes. This would occur is shaft/housing fits were loose thus allowing play in the system. These blowers typically have very tight tolerances and timed gears. If anything becomes too loose the rotors will crash into each other.
What did the failied/worn parts look like. How much did the vibration actually go down? From 20g's to what?

These blowers are also very sensitive to intake filter condition. A dirty filter can internal temperatures to shoot up. Thermal expansion causes rotor/rotor rubs and rotor/housing rubs.