Resilient Mounts - Materials Science help please

[gingertube]

I am designing a Parallel Push Pull 300B Amp using some fairly expensive components such as Plitron Power and Output Trannies.

I had an idea of mounting the main tube circuitry on a sub-chassis isolated from the main chassis via resilient mounts, so started to look at the basic theory.

I was quickly apparent that if you don’t know what you are doing (and at this stage I don’t) it is quite easy to make the problem of coupled mechanical vibration worse rather than better.

Given:
fe = excitation frequency
fn = natural frequency (resonant frequency) of the resilient mount

If fe/fn < root 2 then you get amplification of the vibrations rather than attenuation

For good attenuation you need fe/fn > root 2 (much greater is better).

Now here is where I need help:
1) Can I assume that the mechanical excitations are mainly from the power transformer and excitation frequency will basically be 50Hz and 100Hz

2) When I start looking at the various resilient mounts (Ettinger Rubber Resilient Mounts is what I looked at) they do not generally give a figure for the natural frequency but simply give a value of the Shore Hardness of the rubber material (options of 43, 57 or 68).

3) Can I conclude anything useful (such as resonant frequency) from the Shore Hardness values. Is there a table for various materials cross referencing these things?

Any guidance gratefully received.

Cheers,
Ian

[analog_sa]

Originally Posted by gingertube

1) Can I assume that the mechanical excitations are mainly from the power transformer and excitation frequency will basically be 50Hz and 100Hz

2) When I start looking at the various resilient mounts (Ettinger Rubber Resilient Mounts is what I looked at) they do not generally give a figure for the natural frequency but simply give a value of the Shore Hardness of the rubber material (options of 43, 57 or 68).

1. I would be more worried about air borne vibration at higher frequencies and structural vibration at very low frequencies. Suppression will depend upon the amp + support structure and not just the tube mount.

2. They cannot have a "natural frequency" given as it is a function of the carried mass as well. Not sure if calculating this will be of much help anyway. Successful vibration control is the result of a combination of treatments, usually isolation and damping and certainly a lot of listening.

[SY]

The durometer will give you an idea of the spring constant (translating that to n/cm or anything useful is not straightforward!) but will tell you nothing about the loss (in this context, a high loss is a good thing). I'd go as soft as I could find, then use mass and damping to empirically optimize.

For a low level circuit (e.g., a phono stage), this is FAR more critical. If you have significant microphonics in a power amp, you've done something terribly wrong.

[revintage]

Hey Ian,
I suggest you experiment and test some home-cooked suspensions. The commercially available ones I´ve found seems to be far to hard. I am just about to try a Casco/Nobel silicone rubber for glass. It is the softest one I´ve found and is said to be 16 Shore A. My aim is to use them for a really low level DHT-preamp PCB.

[Ian444]

I have seen shock mounts used often in small helicopters to isolate avionics black boxes from the airframe, they are rated in weight ranges for the mass they will support, anything from 1/2 kg to a few kg. I have not seen them at all in larger aircraft. I don't know what's inside but they feel "dead" and have around 1" travel. Be interesting to dissect one. I could have given you some when I worked on choppers, but that was quite a few years ago. It's possible light aircraft use them too. Might be something to look at. Since aircraft are continually being modified to keep up with Federal aviation standards, there should be quite a few racks with shock mounts sitting around gathering dust that will never be used again.

[pedroskova]

from a quick google of "isolation grommets pcb"

Gelmec :: Soft Silicone Gel Grommets

[revintage]

isolation grommets pcb

Not the first search-phrase that came to me. Thanks a lot, great site! Hopefully they sell small quantities too.

[Richard Ellis]

What was that stuff used in speakercone surrounds?....Was it called 'Nomex'??? Can't recall too clearly.......I must reread/scan my textbooks thru & thru. I think that was the name. Surrounds must damp out any & all frequencies to be encountered, lest they reradiate back into the cone.

_______________________________________________Ric k.........

[boywonder]

Lord Corporation is a big player in the vibration isolation business. They have mounts from tiny to gigantic in lots of form factors.

Vibration, Shock & Motion Control Products

In my experience, the low profile "motor mount" style isolators give better isolation than the grommet style isolators.

http://www.lord.com/Home/ProductsSer...6/Default.aspx

[Richard Ellis]

"Norsorex" a trademarked material used in surrounds.
Perhaps a small quantity of this stuff will damp out those pesky vibrations.

__________________________________________________ _Rick....