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Basic Vibration Damping and Isolation

  1. Dec 1, 2014 #1
    Hello,

    I am studying and trying to understand Powertrain / Engine mounts used in automotive such as these:

    http://images.gasgoo.com/MiMwMDRfMD...ve-chevrolet-traverse-gmc-acadia-25840458.jpg

    http://www.carid.com/images/westar/engine-parts/em-2838s.jpg


    It is really hard to even get terminology straight.

    Damping and Dampening both seem to refer to a means by which vibration energy is canceled out or absorbed.

    Isolation seems to refer to preventing energy or force transferring from one system and into another.

    And transmissibility seems to be a rating for how easily energy passes through a material or system, and it seems it is normally drawn over a spectrum of frequencies.

    Where there will be some point where vibration actually increases, like at resonance, and then tapers off sharply into fully damped "isolation".

    Resonant frequency or natural frequency seems to describe a frequency at which an object or system would tend to vibrate at. I get the impression that at a natural frequency the repeating motion of the vibration reinforces itself, whereas at other frequencies is damped out by losses.

    As far as automotive is concerned, the body of a vehicle apparently has a resonant frequency around 20 Hz. Which is apparently very close to the rate of combustion events for an idling engine.

    If the vibration from the engine isn't isolated, it causes the whole chassis/body to ring. I have studied one hydraulic mount with an "inertia track" orifice, but I am more interested in elastomer / rubber mounts.

    From what I can tell, rubber has a natural frequency which is far below the frequency of the engine, the molecules are very large and long with lots of space, and the molecules are flexible.

    So when vibration hits rubber it begins to deflect in a disorganized way, and because the vibration is faster, the rubber will not have settled and completed returning when the next wave hits. So as far as I can tell the vibration energy doesn't stack up, but works against itself.

    I think this is the mechanism for the suppression of vibration. And when I look at the pictures I linked to at the beginning, it seems like the shape and geometry of the parts are designed to allow some degree of "springing" deflection. The V shape would seem to make it non-linear in terms of stress/strain.

    Am I on the right track? Corrections and comments are appreciated,

    Thank you.
     
  2. jcsd
  3. Dec 1, 2014 #2
    Resonance is when an excitation frequency coincides with a natural frequency, giving rise to a large (or unbounded if no damping) response.

    Rubber is a "lossy" material meaning that when a piece of rubber is deformed, the energy input is larger than the energy recovered when the deformation is released. Rubber it self has no natural frequency; a natural frequency is a system property depending on the stiffness and inertia of the system.
     
  4. Dec 1, 2014 #3
    Is natural frequency an intrinsic property of all systems and objects or just more pronounced in some? To have a natural frequency wouldnt you need a system or object capable of storing and transmitting energy?

    In the case of rubber some of the energy is lost to the deformation, and that energy may be redirected against the next vibrational input. The loss factor describes the relationship. I think.

    The impression that I get is that things like metals conduct vinration energy very well with litle damping. Perhaps in the same way as when billiards are racked in the triangle on a pool table and struck a breaking blow.

    Rubber by comparison could be described as a disarray of bowling balls that scatters the blow randomly because the gaps are so large and the rubber molecules so heavy.

    This is the impression I get from exbaustive study. I think some energy is reflected back with elastomers but some is dissipated as waste heat. So it is a form of elastomeric hystereis.

    Thats just the impressio I get. Sorry cor typos - broken phone.

    Thanks
     
  5. Dec 1, 2014 #4
    l
    I hesitate to make a statement about all systems, because sure as I do, somebody will find an exception. I cannot think of any physical systems that don't have any sort of natural frequency, although I can think of some idealized systems that do not. For example, a coil spring, all alone possesses both mass and stiffness, and it will exhibit a natural frequency. An ideal dashpot, having no stiffness, has no natural frequency. Any real system will have some mass and some stiffness, so it should have a natural frequency.

    I have no idea what this means. What is the "next vibrational input?" How do you think this energy is "redirected"? Energy lost in a rubber or isomeric isolator is simply converted to heat in the isolator. An isolator that is called upon to absorb a lot of energy will heat up noticeably.
     
  6. Dec 2, 2014 #5
    I was referring to this concept:

    http://www.easyflex.in/pdff/latest/Vibration Isolation Theory.pdf

    Vibration is a force and establishing an opposed force can effectively reduce its transmission. This is
    accomplished by incorporating a truly resilient material, which when subjected to a static load, deflects and
    by so doing establishes the natural frequency of the isolation system. When the natural frequency of the
    isolation system is lower than the operating or disturbing frequency of the supported machine, each cycle of
    vibratory force finds the resilient material in the returning phase of its cycle. The effectiveness of the isolation
    then, is a function of the distance of return travel remaining at the time of impact.

    This is best explained by visualizing each cycle as an individual blow. This blow drives the isolator into
    dynamic deflection. When the force of the blow is spent, the isolator starts its return at its own frequency.
    Since the frequency is slower than that of the blows, it is obvious the return will be only partial before next
    impact. Because the isolator possessed the energy with which to complete its return to equilibrium, the
    unaccomplished portion of travel represents the amount of opposed energy that will absorb the next impact.
    Therefore, the greater the ratio of disturbing to natural frequency the more efficient the isolation, subject to
    diminishing returns.


    What it seems to be saying to me is that the energy of disturbing vibrations is stored in the rubber and reflected back in a way that opposes it... But I am asking because I don't really understand yet, and there are probably 15-20 of these documents I've found and read through.

    Thanks,
     
  7. Dec 2, 2014 #6

    OldEngr63

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    Gold Member

    I have no idea where you found that description, but is seriously flawed, to the point that I would say it is simply false. It needs to be roundly ignored. Pay no attention anything in it; whoever wrote it is more confused than you are!

    Vibration is not a force. Force is a very specific concept, and vibration is not a force but a phenomenon.
    This is utter nonsense!!!
    The babblings of an idiot!!

    Pay no attention to this article; it is written by a person with no knowledge of the area.

    What is your objective here? To understand isolation?
     
  8. Dec 3, 2014 #7
    You have to take into account a lots of parameters while designing whatever machine using a combustion engine which transforms a vertical movement (explosions within the cylinders) into a rotational movement (outer engine axis). I assume it's based on the study on a combustion engine seeing the photos you uploaded. Because of the nature of the transformation, you will encounter vibrations. To be precise, what we call a exciting vibration. If the exciting vibration goes directly into the body (without being damping which is the process of decrease the amplitude of the vibration or even erase parts of the spectrum depending on the technology used), you can really put in some deep ****. The natural frequency is the frequency of a object (generally rigid) at which it enters in resonance when excited at this particular frequency. It can result in big movements of the object which may cause his partial or complete destruction. A typical example of it in mechanical engineering formation is the TACOMA NARROWS bridge.

    So what happened? Exactly what was trying to say OldEngr63. The wind excited the bridge (chocking against the wires supporting the structure in concrete and steel). It's the exciting vibration. Unfortunately, the natural frequency of the bridge wasn't taken into account in the mechanical design of the bridge nor means of damping were thought.
    The gale continued to blow, the exciting frequency of the gale reaches the natural frequency of the structure and made it collapse.

    SO! What's happening with a car ? The same scheme as the bridge. The engine is producing exciting vibrations and what we want to try to avoid is to excite the body. It can result otherwise in lost of control of the car then accidents, to put it in a nutshell IT CAN KILL SOMEBODY.
    The solution? Damping
    Damping is a means to transform the exciting vibration into heat in the example of the car. You have to damp the vibrations of the road on your wheels and from your engine. You also have into account others parameters such as the environment (heat, types of contact, impurities....) or the durability of the entire system (a car is supposed to be designed for a lifetime of 300 000 kilometres). The cost is also another factor of the designing process.

    As you can see, you cannot just got out in a crusade saying "this system is ****, there is a better solution". You have to put proof on the table and defend it.
    Actually, the cheaper solution for damping machines as developed as a washing machine is polymers plots. (Yeah that's right, look under yours). So, there are worlds of difference between the technology needed to produce a washing machine and a car. This difference results in completely different damping means.

    I can direct you to a good site to understand the BASICS of vibrations http://www.splung.com/content/sid/2/page/shm

    So please, next time. Be an engineer okay? You have to keep a critical mind about what you read. The theory you uploaded is really a piece of **** written by an illuminated.
     
  9. Dec 3, 2014 #8
    Conceptually this can all be broken down into mass spring damper systems.
    http://en.wikipedia.org/wiki/Harmonic_oscillator
    http://en.wikipedia.org/wiki/Damping

    Undamped natural frequency is proportional to sqrt(k/m)
    Where K is the spring (in this case mount) stiffness. M is the mass.




    We tend to use the term 'mode' when discussing this, rather than natural frequency.
    Modes have a frequency (ie the natural frequency), but they also have a shape. Which is transmissibility to vibration.
    http://upload.wikimedia.org/wikipedia/commons/0/07/Resonance.PNG

    Take the undamped example above you see the transmissibility increases as you head to the natural frequency of the system. In this case vibration is being amplified. Above 1.4x the natural frequency the system begins to attenuate the vibration. (i.e It is now isolating).

    When you go above a mode in terms of frequency (as above), the vibration will be out of phase. you can see this on Bode plots of vibration. There will be a 180 degree phase change at the natural frequency. Energy put in at frequencies above this will then start to be attenuated. Mounts and bushes operate conceptually similar to electronic filters in this respect.
    http://ctms.engin.umich.edu/CTMS/Co...lysis/html/Introduction_SystemAnalysis_11.png

    I'd recommend the MIT OCW lectures on this.




    Relating this back to your original question.

    20Hz will be above all rigid body modes of the body, and would typically be into the bending frequencies, and a 4cylinder engine idling at 600rpm (firing frequency 20Hz).

    Putting in energy right at a modal frequencies are something that you would desperately try to avoid. If vibration couldn't be solved in the mounts, i.e. you can't have them soft enough (mode below 20Hz) to attenuate the vibration into the body, you'd maybe try to raise the idle RPM. Which then separates the energy input and the problem mode.
     
    Last edited: Dec 3, 2014
  10. Dec 3, 2014 #9

    OldEngr63

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    Gold Member

    To the OP: I strongly suggest that you abandon the internet and get a good book on the subject. There are far too many ignorant, uninformed idiots writing on the internet. Two very good books would be
    Den Hartog, Mechanical Vibrations, 4th ed, Ch 2
    Timoshenko, Vibration Problems in Engineering, 3rd ed, Ch 1
    In this case, I think Den Hartog may be slightly better than Timoshenko for the questions you are posing.
     
  11. Dec 4, 2014 #10
    Hello,

    I greatly appreciate everyone's responses and instructions. Unfortunately work and home life has prevented me from studying for awhile so I have not been able to follow up on the research.

    I will look into the book OldEngr63 recommended. I intend to work with automotive machinery for the long term, so a deeper understanding of Noise, Vibration, and Harshness would be a great asset. After studying vibration for awhile, some things are really eye-catching when looking over a vehicle.

    I think I was too hasty, and I should start with basic things like "tuned mass damper" theory.

    I know about transmissibility graphs and the 180 degree phase shift at the natural frequency from steadily increasing amplification into isolation. If I've got that right anyway. I don't really understand what the relationship of material properties and vibration frequency is that shape that graph.

    I understand that when a material is deformed it takes some energy to accomplish, if the deformation is elastic, I think the structure or molecular bonding of the material causes it to rebound. How that relates to vibration damping, I would need to study. Spring steels have a great ability to deform or deflect within elastic limits for many cycles. A rubber band is similarly so.

    So I have a lot of work to do and many great resources have been provided for further studying, like the MIT OCW, and others.

    One thing that I am very curious about is the impact of engine and road / traction induced vibrations on the fatigue life of transmission components, I have never really seen much of any discussion of that as a problem in transmission engineering however.

    Many thanks to everyone,
     
  12. Dec 4, 2014 #11
    I think a key thing to point out.

    A material doesn't have a natural frequency. A system has a natural frequency. Based on the spring stiffness and the mass acting on it.

    Largely speaking the material is irrelevant to the concept. Conceptually all you need to know is the mass of the system m, stiffness k and damping c.

    The material is merely a means of applying the correct values of k and c in real life.
     
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