Dominant Peak at 6 times RPM
Dominant Peak at 6 times RPM
(OP)
I have a problem with a dominant velocity peak at 6 times the rpm. What symptom does this indicate. This peak is on the NDE of a 400 kW Booster fan. The fan uses journal bearings. The DE of the fan shows 2 times rpm vibration but on the motor DE there is no indication of 2 times which would point to misalignment.
I need help with the 6 times component. And also whether taking measurements across the coupling should both have 2 times component to point to misalignment. Thanks.
I need help with the 6 times component. And also whether taking measurements across the coupling should both have 2 times component to point to misalignment. Thanks.





RE: Dominant Peak at 6 times RPM
Cheers
Greg Locock
SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Dominant Peak at 6 times RPM
On the discharge side however, there is an extra small mechanical (screw-like) support rod, type that looks like adjusting rods for accelerator linkages on old heavy trucks. No one has ever touched it though, even during shut-downs. I have attached a photo, dont know if it will be any help. Please let me know about the picture. I can always get another one.
Thanks GregLocock.
RE: Dominant Peak at 6 times RPM
Johnny Pellin
RE: Dominant Peak at 6 times RPM
I dont understand when you say "fan running pinched back at low flow"?
RE: Dominant Peak at 6 times RPM
Johnny Pellin
RE: Dominant Peak at 6 times RPM
Why do you think the sixth harmonic (300 hz) is dominant?
Cheers
Greg Locock
SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Dominant Peak at 6 times RPM
RE: Dominant Peak at 6 times RPM
On the Horizontal measurement you have a big spike at 50 Hz bigger again at 100 hz, and then a descending series of harmonics. No way would I describe 6th order as dominant.
The sixth order is just part of the harmonic series which I suspect is based on 1st order , 50 Hz.
Have you balanced the fan recently?
Cheers
Greg Locock
SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Dominant Peak at 6 times RPM
RE: Dominant Peak at 6 times RPM
RE: Dominant Peak at 6 times RPM
Where is the evidence that this is a 6th order vibration problem? Your graph in your first post does not show "domninant" 6th order vibration.
M
--
Dr Michael F Platten
RE: Dominant Peak at 6 times RPM
Brought it down from 35m/s2 to 4m/s2.
Unquote
If correct, your vibration units are in terms of acceleration, so you've balanced it from nothing to less than nothing.
The data you show doesn't indicate a problem: an overall amplitude of <6mm/sec pk is OK. Take any machine vibration spectrum and, look hard enough you will find things that cannot and do not need to be explained - they just are!!
Can you explain your concerns a little better?
RE: Dominant Peak at 6 times RPM
could be worse!! Could be me. ;)