Dynamic absorber presentation - for comment 1

[electricpete]

Below is link to a draft of a presentation on dynamic absorbers that I plan to provide to the EPRI Large Motor group this Tuesday.

http://home.comcast.net/~electricpete/PSLEMUG10/SCHIMPF_DA_R8.PPT

It is 5MB, around 60 slides. It includes three areas of calculations:
1 - sizing the absorber to give right resonant frequency
2 - sizing the absorber to achieve frequency separation
3 - estimating stresses in absorber.

Any comments, suggestions, questions, typo corrections etc are welcome.
Any by Monday night would be especially appreciated ;-)

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[FeX32]

I briefly skimmed it. If I have time I will look again in more detail.
Seems pretty informative and you obviously did a lot of work and are very versed in the subject.
I have one comment.
What if someone asked you how long the absorber will last under use? Have you estimated a fatigue life? (probably you thought of this)


Fe

 

[GregLocock]

One use of TMDs that you don't mention is to explicitly add damping to a lightly damped system. If you damp the TMD then every time it tries to move it absorbs energy.

This is very useful.

The warning about TMDs with identical frequencies for 2 orthogonal directions is an interesting one. Every crankshaft bending damper has precisely that property so I can't believe it is a rule. The difficulty is that the mode shape 'rotates' between the two directions.

As a first step tuning the resonce of the TMD to the problem frequency is not a bad idea, but you'll discover that the resonance when isntalled on the machine is different due to different boundary conditions. It still should be within fine tuning range though.

You also don't mention undertuning vs overtuning.

In general TMDs work best when they are very lightly damped.

Mass ratio is difficult to determine, the frequency split is governed by the ratio of the TMD effective mass at the mounting point to the machine effective mass at the mounting point, 10% is none too much.

However the machine's effective mass is often only 10% of its actual mass.



Cheers

Greg Locock

I rarely exceed 1.79 x 10^12 furlongs per fortnight

 

[desertfox]

hi electricpete

Its clear you have put a lot of work into this presentation and it looks pretty comprehensive.
My only question is the knowledge level of the people your presenting this too.
I just wondered if it might be worth giving a brief general overview of what one of these dampers actually do and the various types before getting into the real technical stuff.
Of course if their knowledge is already beyond that then there is no point.
Just my opinion, anyway goodluck

desertfox

 

[electricpete]

Thanks for the comments.

As far as fatigue life, I would hope to keep below the endurance limit.

Our applications of interest (power plants) are primarily fixed speed.

As far as overtuning/undertuning, I think I understand the implications looking at slide 17. If w1/w2 = 1.5 (brown curve) and we tune to the w2 = wexciting, then wexciting is very close to lower resonant frequency of the composite system.... so instead we tune w2 a little lower than wexciting to increase that margin (tradeoff between going for the zero and providing margin in case things change).

In general TMDs work best when they are very lightly damped.

Does that mean they work best when we have the damping as low as possible? Or are you suggesting that a small amount of damping on absorber itself is good? I could read it eitherway.

So you suggest a target mass ratio m2/m1 = 1/10... but considering m1 is an effective mass which can be much lower than actual machine mass. I imagine the only good way to find the effective mass m1 is to use an instrumented hammer?


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[electricpete]

Thanks desertfox. I think unfortunately you're rihgt..a good portion of my audience will not have a good background in vibration theory. I do plan to spend a lot of time on slide 16 which hammers home a basic point: The resonant frequency of the beam in it's cantilever configuration is the "zero" of the displacement/force transfer function in it's dynamic absorber configuraiton.

But I have a feeling you are right.
I should follow some basic simple rules:
tell em what you're going to tell em
tell em
tell em what you told em

Another unfortunate part - my presentation is only scheduled for 45 minutes. I have to do some thinking about what to leave out and how to present what's left. But at any rate I think it's good to have all the detailed stuff recorded so they can look at it later.

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[desertfox]

hi electricpete

Well if they are new to this I would drop out some of the detailed maths and concentrate on getting the basic principle across, you might be able to do another presentation at a later date giving a more detailed view.

regards

desertfox

 

[GregLocock]

Yes I'd drop all of the equations and much of the stress analysis.

I meant just a little discrete damping, if you add too much the thing doesn't really get going. As you can probably tell that is based on practical experience not any great theoretical basis.

Yes, use an instrumented hammer to get the modal mass.

One thing you don't explicitly mention - always try to mount the TMD where the vibration is strongest, and in the strongest direction.

Cheers

Greg Locock

I rarely exceed 1.79 x 10^12 furlongs per fortnight

 

[rob768]

We just installed a similar set on the deckhouse of a large vessel. works like a charm lowering overall vibration levels from some 8 mm/s to 2.5 mm/s.
With respect to the fatigue life: a particular area of interest should be the lower part where the plates (the springs in your system) are clamped together. You will get fretting corrosion here due to the breathing movement of the plates which will very strongly attribute to fatigue initiation. Furthermore, I am intrigued as to why you did not choose for the option of simply improving the frame. It is a generator set with usually only one significant disturbing frequency (assuming engine orders are well above criticla speed, which is normally the case). I would opt for some improvement to the frame to operate away from resonance.

 

[IRstuff]

Looks like a fancier version of something someone at work codged to deal with vibrations on an overhead projector. I think he used a rod with washers as the adjustable weights.

Overall, there are too many slides for your stated time; does 45 minutes include question/answer? Removal of the bulk of the math might do it. Depends on what type of talker you are. I can do about 60 slides/hr, but I talk fast and expect the audience to do the bulk of the reading. A few of the slides seem extremely verbose (e.g., slide 12), but I don't know what audience you're addressing. There also seems to be lots of abbreviations that are not obviously defined, again, that's audience dependent.

TTFN

FAQ731-376

 

[electricpete]

IRStuff - the presentation was yesterday, thanks anyway.

Rob - Regarding fretting - I would like to ask what do you forseee is the the source of relative movement you envision... vibration or thermal cycles?

As far as choice of stiffening vs dynamic absorber, there may be pro's and cons. I can see two advantages for dynamic absorber over stiffening in this particular situation where cracking of machine foundation is supsect:
1 - The dynamic absorber reduces forces transmitted to vibration -> may act to prevent/slow deterioration of the foundation.
2 - If the foundation should shift and change it's effective mass, the machine resonance may change and may land on top of a stiffened resonaonce. However the dynamic absorber tuning is not dependent upon that... depends only on the absorber and stiffness of mounting to the machine.

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[rob768]

if you look at slide 8, in between items 1 (the plate acting as the "spring) and item 4 (the spacer bar), you will get relative movement due to vibration. you would be surprised as to the level of vibration such a damper might experience.

The thing with the adsorber is that it is a way of supressing a problem. I my opinion, if you have a choice, cure the problem, not suppress its result, because you will need additional care and maintenance on the damper now.

 

[IRstuff]

There are lots of situations where it's neither feasible or possible to "correctly" fix a problem. The previously mentioned overhead projector is one such situation. There are no available hard-mount surfaces due to the hung ceiling and service plenum above. The projector is mounted on a ceiling beam that gets lots of vibrations and the damper is actually the cleanest and easiest solution.

TTFN

FAQ731-376

 

[Rob45]

Pete:
I like that presentation; it is simple, covers the bases.
It occurs to me that as a safety feature, overtravel stops could be devised to limit travel of the damper mass in the event something changes in the mg set itself, thus preventing a fatigue failure of the TMD?
Your presentation suggests a question that I will present as a separate post.

Best regards,
- R

 

[rob768]

The overtravel devise might result in impacts at ends. Don't think you want that to happen as it will results in both noise and vibrations. Adding damping to control the motion is an option, but that would lead to a decrese in functionality.