change the own frequencies of a structure

[Jack_Landis]

Good evening,

I have recently started working on a structure called vibrations from rotating components. using ansys I obtained the frequencies and the proper ways to vibrate of such structure. I would like the first frequency to be higher than the "forcing" vibration frequency. do you have any suggestions to get such a result?

the structure consists of two sides on which are mounted rollers that rotate at high speed. my first proper frequency is 9 hz while the rotation frequency of the rollers is 17 hz. if you had even only dispensers with tips or useful things I would be very grateful!

See you soon.

 

[PaoloColombani]

9 hz means that the first harmonica is dangerously close to the forcible.
the increase of the own frequency is obtained by increasing the stiffness/mass ratio (nervature), but it does not necessarily need to increase the frequency, depending on the cases it can be useful to lower it with the addition of a dampener.

 

[meccanicamg]

the proper frequency of a structure varies by varying stiffness by making countervents and ribs and applying dampeners in elastomer. They are normally taught at university and then when you go to companies there is no one who does the foundations for machine tools with under rubber or things like that....because it costs....because"we always did so" and so.. .then the wrong time they see lofts of 200 meters process lines that do like swing.

 

[Jack_Landis]

Yes, in fact in one something I did, so much math and no practical advice. ..luckily in this company the first time to make a car the checks decided to make them!

but so if the car works at 17 hz and the first two modalities are 9 and 26 can not be quiet? What safety margin should I take seconds? Unfortunately also looking online I found it difficult to have practical advice. . .

Thank you!

 

[PaoloColombani]

26 hz? Sorry, I was considering the monoaxial case. then you are almost out of danger, even if the ideal condition of project you have when the frequencies are very far away from each other. I said "almost" because all this is based on the certainty that the only forceful frequency is that of project and also that the structure that will be realized will behave just like the one you have simulated. So what to do? the answer is not unique, depends on the application. For example, if you are designing a counter tool it may be very economical to solve any problem by stiffening the structure, while if you design a bridge or a skyscraper it may be cheaper to design a proper damping system.

 

[Hope]

to give you more precise answers, at your own frequencies, you must associate the respective ways of vibrating.
if you attach a scheme of the system, direction of loads and modal forms we can tell you more.

 

[antonio_sc]

hi, they have already answered you in an exhaustive way.. of theory there is too much.. one thing that explained to me at the university is that if you pass quickly for those 9h until 17 you should not necessarily be in a situation of danger. ..because the system does not have time to vibrate in the same way that it would tend to do so in a natural way (enter resonance)...then there would be to evaluate the amplitude of such vibrations and check if compatible with the characteristics of the system.. .

 

[Hope]

hi, they have already answered you exhaustively

no one has answered exhaustively because without precise information you cannot give precise answers.

compatibly with the purposes of the forum, there are users able to enter the detail of the answers if put in conditions to do so.

 

[PaoloColombani]

I realized that my extreme synthesis produced an ambiguous phrase: “... depending on cases it can be useful to lower it with the addition of a dampener” could make you think that a dampener serves to lower the frequency, which is actually not so. rather I meant that in this case it intervenes with the addition of a dampener.

 

[antonio_sc]

no one has answered exhaustively because without precise information you cannot give precise answers.

compatibly with the purposes of the forum, there are users able to enter the detail of the answers if put in conditions to do so.

However, the question concerns 'advices and suggestions', which I think has been given to me when you enter the specific case and give us all the information even better

 

[Salcip]

Good evening,

I have recently started working on a structure called vibrations from rotating components. using ansys I obtained the frequencies and the proper ways to vibrate of such structure. I would like the first frequency to be higher than the "forcing" vibration frequency. do you have any suggestions to get such a result?

the structure consists of two sides on which are mounted rollers that rotate at high speed. my first proper frequency is 9 hz while the rotation frequency of the rollers is 17 hz. if you had even only dispensers with tips or useful things I would be very grateful!

See you soon.

Hi.
I intervene to give a theoretical opinion since I recently studied vibrational phenomena at university.
with its own frequencies of 9 and 26 hz you are not very quiet but you are still decently distant from the resonances.
the aspects to be evaluated however are also others:
1)I assume that the rollers are not powered 24/7 so you have to understand how much time you take from speed nothing until you reach the regimen condition. excessive slowness could be caused by breaking, so we should intervene on the structure
2)The structure must simply resist vibrations or is it important that you vibrate as little as possible?

According to the case, the procedure for intervention can be essentially two. stiffen the system or insert damping elements

 

[Jack_Landis]

Bye!

thank you for all the answers. in the end we decided to make more thick the structure and use of stiffenings so as to bring the first frequency (which has an associated transverse flexional way than the structure) beyond the 20 hz and to stay calm.

In fact, however, more than the resonance to worry about it was that it vibrated as little as possible, and we assumed that remaining away from the modal frequencies also reduced the vibration of the machine. can be corrected as reasoning?

 

[Salcip]

Bye!

thank you for all the answers. in the end we decided to make more thick the structure and use of stiffenings so as to bring the first frequency (which has an associated transverse flexional way than the structure) beyond the 20 hz and to stay calm.

In fact, however, more than the resonance to worry about it was that it vibrated as little as possible, and we assumed that remaining away from the modal frequencies also reduced the vibration of the machine. can be corrected as reasoning?

hi, then the idea of raising natural frequencies is not evil but is not even optimal. the optimal case you have when the latter are lower than the force (I attach a graph: blue line).
in practice this type of problems are studied by means of a parameter called “transmissibility coefficient” which is linked to the frequency value and which represents the relationship between the oscillation amplitude of the force and the oscillation width of the structure.
I explain briefly what you read from the chart:
1)for frequency values between 0 and the first natural frequency: the ratio starts from 1 and tends to increase (this means that the structure oscillates with a slightly higher amplitude than that of the force and the closer we approached this frequency the more the structure oscillates in a very wide way)
2) between the two natural frequencies there is a slight decrease and a new peak in the presence of resonance.
3) after the second natural frequency there is a decrease with values even lower than the unit. which means that the structure oscillates less than the rollers.

Now, if the roller vibration is acceptable I would say that the situation in which you are located is safe enough (at most, if possible I would try to increase the values of the two own frequencies a little more)
If, on the other hand, that love of oscillation is unacceptable, it is necessary to lower the natural frequencies. I'm not sure but I think a maximum natural frequency/frequency=1.5 ratio is enough.
to do this I know that more than aiming to stiffen the system just increase its mass

 

[Jack_Landis]

mmmmm...but is this not true for damping systems? like car shock absorbers?

 

[Salcip]

mmmmm...but is this not true for damping systems? like car shock absorbers?

that curve is derived from a model without damping. with damping its only differences are that at its own frequencies the transmissibility tends, theoretically, to inifintium while by increasing damping values, to increase the frequency the transmissibility approaches 1 (so paradoxically the situation worsens compared to the case without damping). so in case of damping the solution you adopted is the most convenient.
for other clarifications I leave you this link from which I took the photo. http://pcfarina.eng.unipr.it/public/acoustics-course/penn-state-course/12_vibration.pdffrom page 12-14 there is explained everything. do not be fooled by pattern design, before calculations is specified that you work in the absence of damping