Role of Mounts In NVH

[vibrostics]

I would like to know the Role of the Mounts in NVH. I do understand them as the members, which isolates two systems from being coupled. Other then this what are the NVH criteria’s which drive the mount stiffness tuning? Please explain..

Thanks,

 

[GregLocock]

Softer ones give more attenuation than stiffer ones. Except at resonance.

Mounts with less damping tend to give better attenuation except at resonance.

However the above two rules of thumb ignore the crucial aspect of resonances, as in your other thread.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[vibrostics]

How the resonances are addressed and how to zero on the stiffness and damping values?

 

[jeyaselvan]

There are two things. The machine on coastup has to pass through the mounted resonance frequency and hence damping is also important to avaoid failure.

The mounts do not isolate (rather amplify) at the mounted resonant frequency, resonance of the source , receiver and the internal resonance of the mounts themselves. Hence while using mounts for noise control through vibration control we should consider these in mind.

In practical applications, we may not achieve the transmissibility characteristics of the SDOF model whihc is provided by mount manufacturers.

 

[vibrostics]

what are the thumb rules to start with selection of the mounts?

 

[GregLocock]

1) Copy someone elses design.

2) figure out the main vibration frequency problems you are going to have, and then select your suspension frequency appropriately.

3) you are unlikely to get much more than 25 dB attenuation across a rubber mount.



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[jeyaselvan]

Recently, I had a problem ending up in vibration amplification in a motor-coupling-compressor drive train (weighing 3500kg) with AVMs. We tried to change the stiffness of a couple of mounts by mounting rigidly to the base struture and the vibration levels came down to a larger extent. I am working on it to find whether we were close to the mounted resonant frequency in other directions.(especially axial in my case)

Hence we need to have an eye on the natural frequencies in the translational and rotational mounted frequencies as well. Normally, we tend to select mounts to isolate vibrations in vertical directions only.(the usual SDOF transmissibility curve provided by manufacturer) If your exitations (forces or moments) happens to be in other directions also, beware of it.

 

[Warpspeed]

The ratio of sprung to unsprung mass comes into this as well.

 

[Rob45]

Vibrostics:
The questions you're asking are so basic as to be answered very well in virtually any mounts handbook (e.g., Barry Controls, Lord, etc.), or any elementary text on vibration.

 

[vibrostics]

Thnak you Rob45 for proving the name of the handbooks.