Damping vs. Isolation

[izax1]

I need some explanation of the terms damping and isolation. Is it correct to define damping as reducing the response at the resonant frequency. while isolation is defined as reducing response due to shifting of the frequency away from the resonant frequency.

I am a little confused, because what we normally call vibration dampers (rubber, wire etc.) does not really reduce the response at the frequency, but rather shifts the frequency into the isolation area. So, is "vibration damper" the correct term to use for such a device?

Thanks for any clearifications.

 

[electricpete]

Is it correct to define damping as reducing the response at the resonant frequency.

I think that is roughly correct. Damping corresponds to energy absorption which reduces the magnitude of a resonant peak.

Isolation is typically an attempt to prevent vibration from transmitting through some path which may be accomplished by adjusting resonant frequency of the mounting. For example often large fans are mounted on a spongy spring isolation mount which has resonant frequency far below fan speed. This tends to reduce transmission of fan vibration to the base.

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[izax1]

So for the sake of argument (I got this question from a colleuge)when we put a fan on rubber mounts, they are not vibration dampers, but vibration isolators?

I dont think very many suppliers of rubber mounts know this.

 

[alexit]

The above mounts are both, depending on your frame of reference. From the fan's point of view they are damping the vibration from the imbalance, but from the table's they are isolating the vibration.

Alex

 

[MachineryWatch]

Marketing departments, who are usually responsible for determining product names, are not likely to try to change improper popular usage of technical terms. It is easier to sell people "dampers" if that is what people are asking for, even though the technically correct term for what their product does is "isolate".

Skip Hartman

http://www.machinerywatch.com

 

[electricpete]

I was referrring to a fan configuration that we have where the fan is mounted on metal springs. Rubber may have some damping as well.

http://www.fabreeka.com

has a ton of on-line info on this subject.

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[EnglishMuffin]

izax1 : 1. Damping normally has an effect over a much wider range of frequencies than just the resonant frequency, although it will have a greater effect there.
2. In addition to isolators, whose primary functional intent is to isolate something from vibration rather than to alter frequency, there is a device called a vibration absorber, whose function is to intentionally alter the frequency response of a structure such that it has minimal response at some problem excitation frequency. Such a device does not necessarily involve damping, but there is a variant known as a damped vibration absorber which does introduce additional damping. Some people refer to damped absorbers as dampers, which may or may not apply, either intentionally or accidentally, to some of the devices you refer to.

 

[MikeyP]

At the most basic level: Damping removes vibrational energy from the system by converting it to heat energy. Isolation attempts to prevent energy from reaching the isolated component. At particular frequencies, this can be achieved without damping.

Rubber mounts are a combination of the two. Their stiffness forms part of the isolation process and their damping removes some vibrational energy.

M

--
Dr Michael F Platten

 

[izax1]

Thanks for your input. You have confirmed my suspicion. When we say dampers (for rubber mounts etc.), we are mostly talking about isolators.

 

[Tmoose]

"When we say dampers (for rubber mounts etc.), we are mostly talking about isolators."

About the only time I want any damping around is when a resonance is being excited.

If the mounted resonant frequencies are low, like 1/3 the forcing frequency, yes. Then isolation can occur, and any "damping" provided by the rubber (which would tend to increase the fan vibration passed to the floor) is of little consequence.
If the fan must start and stop or run at wicked low speed, having some damping to reduce the vibration as the fan passes thru the mounted natural frequencies.

Hydraulic engine mounts work that way. At high engine speeds damping would just get in the way. Its good They provide damping to limit amplitude at low engine speeds or maybe when the mounted powertrain gets "bump tested" jumping over railroad crossings.

 

[SteveDickson]

Firstly, damping removes vibrational energy from a system by converting it into heat energy whereas isolation attempts to prevent energy from reaching the isolated component by temporarily storing it and releasing it again once the impulse has past.

Isolation is a reduction in the ability of a system to react to an excitation. This is achieved by the use of a flexible decoupling element between the equipment and mounting surface. It allows the inertia of the isolated component to oppose and thereby reduce the vibratory motion transmitted to the support. Damping removes energy from a system by converting kinetic energy into heat energy (in the case of conventional shock absorbers, for example)
Every isolator has a peak resonant frequency below which transmissibility is almost unity and above which attenuation occurs. However, should the natural frequency of an isolated system coincide with the forcing frequency then resonance will occur i.e. amplification of vibrations. In this case, damping is required to reduce the amplitude of the resonant response by removing energy from the system. The ideal isolator would have as little damping as possible in its isolation region and as much as possible at the systems natural frequency to reduce amplification at resonance – this is clearly not possible with standard damping systems.
If there is no resonance present in the important operating frequencies of a system then damping can actually make things worse as it leads to a loss of isolation efficiency. The damper will act somewhat like a strut and vibrations are then transmitted to the isolated element. For example rubber is not an ideal isolator as it not only has isolation properties but also some damping properties inherent to the material. This can increase the transmission of non-resonant vibrations.

http://www.reactec.com

 

[Rob45]

izax1: I'm sure you'll find that rubber mount suppliers perfectly understand the distinction between "damper" and isolator in the engineering sense, but they don't try to change the language used!

It's enough that they call them "dampers" and not "dampeners."

 

[izax1]

Rob45

You may be right, but I have come across supplier who at least communicate that they don't differ between the two. May be that is because they think the customer will not understand the difference.

The main problem though, is on the engineer's side, when they get the impression that rubber mounts (or any other device normally termed "damper") reduces the respons at the natural frequency, when they, in reality, shifts the frequency (to a lower value) That can generate wrong solutions to a design.

 

[Rob45]

Well let me amend my statement: American rubber compoanies understand the difference, as I'm sure those in the other major industrialized nations do: Australia, Britain, Germany, Italy, the Scandinavian countries; maybe your suppliers are not in that category. Simply making rubber doesn't require a very high degree of technological sophistication.

Keep in mind, as has been stated previously: rubber isolators will do both: provide isolation and, depending on the rubber compound, provide damping at resonance.

 

[frankandrews]

Damping is beneficial in a vibration isolator because it reduces the amplification at resonance. However, there is a trade-off, because damping also decreases the vibration isolation at higher frequencies.

If you send me your e-mail address, I can send you a paper that addresses this in lay-men's terms.

FAndrews@SVS-Engineering.com

Frank Andrews, PE
SVS-Engineering

 

[izax1]

Frankandrews

Thanks a lot. I don't like to consider myself a layman in this field, but any input will surely be of help.

What confuses me is when you say
"Damping is beneficial in a vibration isolator because it reduces the amplification at resonance"

But when you move the natural frequency into the isolation area, you do not have a resonance anymore.

I'll give you my email adress.

 

[Tmoose]

"you do not have a resonance anymore"

Are your questions of a general nature, or do you have a specific application that is causing some trouble?

Someday the machine will be shut off, and coast down, perhaps doing a nice leisurely swept sine analysis of the system with the vice president watching.
wiki-wiki-wiki-wiki-whack-whack-WHACK-WHACK-WHACK

If there are any lower frequency excitations present (belt frequency or some danged aerodynamic thing) they sometimes conspire with the VFD to hunt out and torture frequencies "safely" below 1X.

 

[lyngengr]

It is interesting to read this long list of posts and realize that most of the responders don't understand how isolators work. Some have hinted about the effects, but nobody really answered the initial question.

First, an isolator works by shifting the natural frequency of the supported mass. To be effective, the isolators must lower the spring-mass resonant frequency well below primary forcing frequencies. How far below the resonance is located determines the isolation effectiveness or efficiency. Note that there is NO mention of damping in this spring-mass system.

However, on start/stop, the spring-mass system will run through a fundamental resonance. In order to limit motion at the resonance, most isolators employ damping. Large amounts of damping will hurt the effectiveness of the isolation system, so a tradeoff is required.

Isolators constructed out of rubber or other natural energy dissipating materials have inherrent damping. It is not necessary to have the damping in order to isolate, but most manufacturers make their mounts from damping materials to cover all possible applications.

In answer to the original question, the correct terminology would probably be damped vibration isolators, since you can purchase isolators that have very little damping (such as spring towers).

 

[CESSNA1]

IZAX1: Damping are energy absorbers. They basically convert the energy to heat that is dissipated. The shock absorber on your car is a damper. An isolater reduces the transmission of the vibraton to another part of the structure. Vibration/noise isolators are often seen on heavy machinery such as in ships. If you look at a transmissbility curve in a vibrations book you will see the resonance peak at w=wn. This curve shows the motion of a structure as a function of the excitation. Damping reduces the magnitude of this peak. As you move out to larger w you will find that the vibration of the structure approaches zero at higher frequencies.

Regards
Dave