Is a tuned damper effective in forced vibration? 5

[GMarsh]

Hi,

I have a structure on which a tuned damper is mounted, tuned for one of the resonant modes of structure. Now this structure is also being subjected to a forced vibration.

Now I am interested to know if the tuned damper will have effect on minimising forced vibration (because of added mass or some other effect)? If yes, how does the effect vary with amplitude of forced vibration?

I am also interested in simulating this through FE ? Any idea how to apply forced vibration with known frequency and amplitude in FE?

Thank you

Regards
Geoff

 

[GMarsh]

Hi Mike, firstly there is no regenerative chatter. So now sweet spot requirement. I made sure that spindle speed also doesn't cause any chatter. If it is needed I have Metalmax kit with me.

Immersing the whole cylinder in coolant is good but not feasible as I have some instrumentation there to measure cutting force, etc.

 

[Tmoose]

Harris" Shock and Vibration Handbook has a whole section on machine tool vibration. My copy is in a box someplace, but I think it had some Discussion of "impact dampers" and other curiosities that could be applied to tooling, although maybe not easily to rotating tools.

First tact often was to evaluate stiffness of the tooling, spindle, spindle mounting, workpiece holding with the intent of correcting weak spots, of which there often were many

There was a lot of work going on in the 90s and early 2000s by GM and Ford and FAG bearing.(Probably still is) Some of the info was available then. There were lots of theories, some confusing to me and over my head. Using vibration feedback to trigger speed changes was catching on.
Here is an article on instability with some machine tool content from Sound and Vibration in 1997. Those years are not available on the S&V website.

http://www.xcitesystems.com/pdf/UnderstandingSolvingMechanicalInstabilities.pdf

 

[GregLocock]

Before you dismiss putting a TMD on the cutter we fit torsional and bending dampers to crankshaft pulleys, it may be easier than you think.

Cheers

Greg Locock


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

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

 

[GMarsh]

Hi Tmoose, thank you for providing me with the article.

As I said earlier there is no instability in the process. Only strong forced vibration.

I checked the book which you said. It has short discussion on minimising impulsive forces on machine tool structure for which they suggested improvement in stiffness. And in the place where they discussed about dampers there is no discussion about forced vibration. For tool they suggested variable pitch cutting teeth. No more inputs.

As discussed earlier, I wish to design a tuned damper. But not really sure about its efficacy. But will give it a try. But I am just looking to understand the dynamic characteristics of a thin wall casing - is it a stiffness controlled system or mass controlled system in the presence of a forced vibration.

Thank you.

 

[electricpete]

But I am just looking to understand the dynamic characteristics of a thin wall casing - is it a stiffness controlled system or mass controlled system in the presence of a forced vibration.

I'm not sure about where you are headed with that question. I'd like to clarify the terminology. The way I've heard those terms mass-controlled and spring-controlled is for a system that can be represented as a SDOF system. If the exciting frequency is far below the SDOF system resonance, it is spring controlled. If exciting frequency is far above resonance, it is mass controlled. I guess the extension of that concept to a continuous or multi-degree of system would look at the closest resonance to the exciting frequency (which generally has the most effect of the vibration of the forced system *). If the exciting frequency is far below the closest resonance, it is spring controlled....If far above the closest resonance it is mass controlled. If excitation is between resonances (and it is not clear that one of them has much more effect on the response than the other), then the term might not be applicable. So, I guess if you wanted to answer the question you'd have to try to identify the workpiece modes that are or can be excited by the tool vibration and compare their frequencies to your exciting frequency (which we assume is a tool resonant frequency). Sounds like a challenging task. And I'm probably missing the point because I never understood the purpose of your question.

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

 

[GMarsh]

electricpete - many thanks for clarifying to my confusing question. You guessed right. My forced vibration frequency is between two resonant frequencies. So I don't know how to define this - stiffness or mass based because it changes with respect to the frequency I consider. Hence that question. But now I understood that the definition indeed varies w.r.t. the frequency we look at this forced vibration.

In any case it looks imperative to improve upon stiffness and mass if the forced vibration frequency is not lying close to one of the resonances. Now can I use this as argument to design a tuned damper with higher mass ratio - higher than the traditional 5% (as recommended in some books).

Thank you.

 

[Tmoose]

Hi Geoff,

Dec 15 - "I have not built this structure. A model similar to this is in service. My interest is to find out the effect of tuned damper in presence of forced vibration and if possible simulate it."

Dec 20 - "As I said earlier there is no instability in the process. Only strong forced vibration."

Dec 20 - "there is no regenerative chatter."

What issues does the in-service unit have?
Are the "force" measurements from strain gage?
Do you process the signal and "see" vibration at tooth pass frequency?

regards,

Dan T

 

[GMarsh]

Hi Dan,

I work on a research project on damping machining vibrations. Presently the component which the industry gave is being machined at reduced parameters due to forced vibrations and hence consequent problems such as heavy vibrations, noise, etc. My task is to design some damping solution. Now had there been any regenerative chatter I would have used my Metalmax kit and solved it straight away.

As part of my project I use Kistler dynamometer (piezoelectric) to measure cutting forces. Tooth passing frequency and its harmonics we see of course. But it is significantly modulated at tool's resonant frequency. So for all practical purposes it can be taken as tool resonant mode only.

I usually prefer to post the problem in its scientific relevance rather than as solving an industrial problem. Hence I didn't give this background at the start.

But I learnt some good things about tuned damper design through this discussion. Thanks mainly to electricpete, Denial, and Greg.

Thank you.

 

[Denial]

This is not relevant to GMarsh's problem, but might be of interest to some other people following this thread.

I mentioned above a spreadsheet I developed some years ago to analyse two-degree-of-freedom problems. Inspired by this discussion I have dug up that spreadsheet, and put some time into improving its clarity and making it a bit more robust. It is now available for download from my web site (whose URL I have already given above in a post on 19Dec12@05:33).