Strange vibration problem
Strange vibration problem
(OP)
I'm working with two similar machines showing massively excessive vibration (50mils vs. 2.7mil limit). It's a VFD-reduced double-reduction gear drive mounted on a tank.
Regardless of motor speed, the vibration is showing very strongly at 1x motor speed. 2x, 3x, 4x are measureable but small. There are no resonances near the motor speeds that show max amplitude. We realigned coupling alignment with a laser alignment tool. Motor ran smoothly when decoupled (and unloaded). We improved the stiffness of the motor-gearbox connection.
I have heard of cases where flimsy mounting will cause normal excitation sources to result in excessive vibration, but I'm puzzled why 1x motor speed is the *only* vibration showing and why it's so intense. While I can see a stiffer mount reducing amplitude, intuitively it seems like this is not going after the source of the vibration energy - true?
Has anyone else seen such intense response at motor rpm due to insufficient mounting stiffness? Insights appreciated.
Regardless of motor speed, the vibration is showing very strongly at 1x motor speed. 2x, 3x, 4x are measureable but small. There are no resonances near the motor speeds that show max amplitude. We realigned coupling alignment with a laser alignment tool. Motor ran smoothly when decoupled (and unloaded). We improved the stiffness of the motor-gearbox connection.
I have heard of cases where flimsy mounting will cause normal excitation sources to result in excessive vibration, but I'm puzzled why 1x motor speed is the *only* vibration showing and why it's so intense. While I can see a stiffer mount reducing amplitude, intuitively it seems like this is not going after the source of the vibration energy - true?
Has anyone else seen such intense response at motor rpm due to insufficient mounting stiffness? Insights appreciated.





RE: Strange vibration problem
What type of coupling? The rubber element types are inherently bent, forming an offset crank between the driving and driven machines.
I'd rig a dial indicator up comparing "ground" to the motor motion and turn the motor by hand. If it's cranking 50 MILS statically, it's not going to get any better at speed.
Disc couplings tightened up in the misaligned state cause a similar cranking action. Escess coupling capacity makes it worse.
If the mounting is flexible the 1X amplitude (in MILS or mm) will be pretty constant from 1 rpm on up. Add some resonance, and some speeds will have tasty 10 X higher vibration.
"Tank mounted" sounds suspiciously flexible to me at the start.
RE: Strange vibration problem
The motor is a scoop mount. No isolation mounts right now. I've done some reading and the spectral plots are similar to those of soft foot, but I'm not convinced that a scoop would have the rigidity to pull a motor frame into misalignment. (We'll check for soft foot anyway)
The vibration is similar on various parts of the motor and motor scoop. It has a vertical support, and adding more bracing hasn't done much. We didn't check phase - I don't have experience yet with dual probes. (vib unit is a CSI 21/20, if it even has that capability)
Coupling is a new Lovejoy C276 Hytrel jaw type. It was laser aligned to .004" parallel, but it wasn't verified using another method.
I do suspect that the tank mounting is part of the issue. The tank feet are load sensors at the bottom of the tank. Tank stands maybe 10-20' tall. Even if the tank flexibility plays a major role, I would prefer to eliminate any/all possible sources first. Could the tank flexibility alone cause this kind of vibration at 1x?
RE: Strange vibration problem
Cheers
Greg Locock
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Strange vibration problem
Dave
RE: Strange vibration problem
Cheers
Greg Locock
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Strange vibration problem
If you are using proximity probes, you could be looking at glitch or runout rather than true vibration
RE: Strange vibration problem
The probe is a magnetic accelerometer. I don't have the details handy but we've used this probe extensively on many machines. And it's quite obvious the unit is operating with destructive levels of vibration.
Our last set of data looks like this, at the NDE of the motor (worst case):
Motor speed Vibration
(rpm) (pk-pk, mils)
270 1.38
865 12.0
1077 50.12
1494 13.9
Prior to adding some supports to the system:
427 4.82
851 70.2
1486 16.49
Are you suggesting we take readings at more motor speeds, or do a capture during coast-down?
Thanks everyone
RE: Strange vibration problem
Yes, I'd do a coastdown waterfall plot. I suggets you do it with and without the gearbox coupled, just to make sure that it isn't a problem with the motor
Increasing the stiffness by a further factor of two or so might well be a challenge, the trouble is you are going to make things worse before they get better, ie you'll be pushng the resonance up in speed where there is more energy.
Sorting the forcing out is likely to be a much more satisfactory approach.
Cheers
Greg Locock
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Strange vibration problem
RE: Strange vibration problem
Admittedly that theory doesn't square with your observation that the vibration is the same (magnitude?) at all locations and directions.
Cheers
Greg Locock
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.