Simple rubber vibration isolator - axial/radial displacement/stiffness

[Auwieje]

Hello,

I'm currently doing some research to remove noise from cabinets in a bus/coach. The cabinets are mounted directly above the frontwheels so have a lot of vibration input from the road through the suspension. The cabinet is mounted on a frame which is bolted on the wheel arch. To remove already some noise I am thinking to add some rubber pieces between the wheel arch and cabinet frame. However the cabinets shouldn't be able to move when you push against it, for example. The weight of the cabinets can vary between 30 - 50 kg.

- So basically will this work by adding some rubber pieces?
- And how can the axial and radial stiffness/displecement be calculated?

I'm pretty new to this topic, I find a lot of information about it, but is not exactly where I was looking for.

Thanks in advance

PS I hope the language was clear enough

 

[MikeHalloran]

A layer of rubber will reduce high frequency noise transmission somewhat, maybe even enough to measure, but it may not be effective unless you bond the rubber and remove any mounting/retaining bolts, or isolate the bolts with layers of rubber. ... which is sort of what commercial vibration mounts do.

Isolating any vibration requires that the cabinets must be able to move relative to everything else, and they have to move quite a lot to isolate lower frequencies.

If you really intend to get educated on NVH (Noise, Vibration, Harshness), you might as well buy a bookshelf now.
There is no lack of material, and some of it is not simple to grasp.
I don't want to recommend a particular 'Mechanical Vibrations' book, because all the ones I have studied were about equally impenetrable.


Mike Halloran
Pembroke Pines, FL, USA

 

[Auwieje]

The past few weeks I have gathered multiple mechanical vibrations, BSR books and research papers, they where all pretty impenetrable.
But the basics have become pretty clear now and tests have been done to measure the vibrations, the main short-term conclusion from all this was to add more damping.
So around the cabinet will be added more (felt)tape/foam, however I expect some more short-term solutions will be needed, that's how I came to isolating the vibrations.

So even if I would add commercial vibrations mounts, the cabinet will move relatively in the horizontal directions? They wouldn't have a high horizontal stiffness?

 

[MikeHalloran]

Most mounts are >relatively< stiff in a lateral direction, but that's not the whole picture.

If, for instance, you mount a box on four mounts attached to its bottom, that leaves the box's CG well above the mounts' plane, so the box will try to roll with lateral and axial accelerations of the bus. You then need to provide swing clearance so it doesn't go 'clunk' against whatever is next to it, or mount it so the isolators are on the same level as the CG, or provide Panhard bars or Watts linkages or anti-roll bars in two directions to limit its movements to the vertical axis only.

You also probably want to go easy on the foam if you offer a long-ish warranty on the bus. Many foams degrade quickly here in SoFla, even indoors in air-conditioned spaces and protected from sunlight.



Mike Halloran
Pembroke Pines, FL, USA

 

[Auwieje]

Well currently they 'clunk' already against other parts, which gives basically squeak and rattle noises. A bus is relatively not very stiff, so it can twist and bend a lot.
That's why already enough tapes should be added, so that parts connecting sometimes are less likely to produce noise. However for the long-term this won't be that great. Materialpairing should eventually be optimized a lot.
By adding more damping I was hoping the cabinets would experience less vibrations and thus less displacement. Unfortanely this won't work entirely then.
If on the frame four mounts are attached to the bottom and I attach two to the sides/horizontally (across from each other?), roll would already be less possible to occur?
Well this solution doesn't look that simple anymore.