Abrupt decrease in vibration on runup

[JulianH]

On a high speed, well balanced rotor, when running up in speed, there is a speed where vibrations start to increase significantly. At this point, increasing the power does not increase rpm very much, but does increase vibration. After considerable increase in power and subsequent slight increase in speed, the vibration in the system suddenly drops to a level about the level before the vibration increase started. There are several resonances involved within the speed range with the increase of vibration. During the high vibration band, and only during that band, a sub-order resonance that can be explained is apparent, but also a second sub-order resonance that is not explained appears. The rotor is supported by two ball bearings, one is rigid, and one is floating in all directions. An axial mode is probably at play here as well as possible excitation of a structure resonance excited by bearing cage frequency. I have seen data similar to this before, and I do not expect simple unbalance to be the full explanation here. Is this a well known phenomena? What Google search terms would lead to info on this?
Thanks.JH

 

[SomptingGuy]

What's the run-down behaviour?

- Steve

 

[JulianH]

No data from a controled rundown; unpowered coast down shows high vibes in the same frequency range, but the rate of speed change is so fast, it is difficult to see what is really happening.

 

[MikeHalloran]

It sure sounds like it's got a "critical speed" below the operating speed, which I think is fairly common in 'high speed' machinery. Come to think of it, that might be a usable definition of 'high speed' for rotating machinery.

While near the critical speed, the assembly is flexing, possibly a lot, and dissipating extra energy, some as vibration, by doing so.

Do not attempt to force it into operation in that critical speed range; it clearly doesn't like doing so.

Read up on 'critical speed'.




Mike Halloran
Pembroke Pines, FL, USA

 

[Spud6414]

Critical speed. At certian RPM's some equipment, well all equipment has the tendency to flex and stretch causing it to vibrate. In a rotating equipment run up, if you are using VFD's or some other method of governing speed, one needs to push through known critical speeds quickly. Do not rest runup at a critical speed, it will cause damage to the equipment. It does not like being there.

http://www.pump-zone.com/pumps/pumps/resonant-frequency-and-critical-speed.html

Here is a good article on it.

Spud says; Meticulous effort and willingness to learn are honorable traits.

 

[Spud6414]

Here is some more info that may be of use.

http://centrifugalpump.org/bulletinboard_02.html

Spud says; Meticulous effort and willingness to learn are honorable traits.

 

[electricpete]

During acceleration of a shaker motor through a critical speed, it also slows down and draws more power from the motor similar to what you describe. The reason I guess is that the motor has to add the energy of vibration into the system.

=====================================
(2B)+(2B)' ?

 

[JulianH]

Thanks for everyone's comments. I think there is clearly a radial shaft resonance in the high vibration area, but I still don't understand the sub-synchronous vibration that occurs at the same time. I am wondering if the rubber mounting of the outer race of the bearing is at play here.

 

[GregLocock]

More google search terms: half speed whirl and oil film whirl.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[MikeyP]

Your description sounds remarkably like the "shock jump" that you get when doing an upward speed sweep through the resonance of a system with a hardening stiffness non-linearity. Never done it on a rotating system, but I have seen it plenty of times in other vibration tests.
The test of this hypothesis would be to see what happens in a run down...

M

--
Dr Michael F Platten

 

[BrianE22]

When you say "sub-order", is that 1/2 or 1/4 or ? of the resonant frequency? What is the explanation for the one you understand?