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Old 9th March 2008, 11:45 AM   #1
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Default DIY spring suspension feet for Chassis?

Hi gurus,

Does anyone know how to make feet for a chassis which are audio isolating? I am thinking of the Solid Tech brand.

I am trying to source some thick anodized aluminium tubing, then I thought I could use a tube cutter for nice cuts; though I don't have access to a machine shop, the final finish especially on the edges would be difficult.

Three small springs drilled to the inner rim, and finally a small foot on the inside as seen in the photo.

Or maybe someone knows of a source for said feet but without a heavy price tag...
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Old 9th March 2008, 04:14 PM   #2
rabbitz is offline rabbitz  Australia
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Half squash balls work well isolating equipment. Not as pretty but cheap.
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Old 9th March 2008, 07:44 PM   #3
oshifis is offline oshifis  Hungary
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I have an idea but I did not come to the realization yet: using two neodymium disc magnets that push each other. They should be fixed in two telescopic aluminium or brass tubes. The tubes should fit tight but with minimal friction. I measured the pushing force between two 10 mm diameter, 5 mm thick magnets by a kitchen scale, it is about 2 kg weight at 2-3 mm distance (measuring the attraction force would have been difficult). Four legs could hold 8 kg weight.
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Old 9th March 2008, 08:00 PM   #4
poynton is offline poynton  United Kingdom
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Quote:
Originally posted by oshifis
I have an idea but I did not come to the realization yet: using two neodymium disc magnets that push each other. They should be fixed in two telescopic aluminium or brass tubes. The tubes should fit tight but with minimal friction. I measured the pushing force between two 10 mm diameter, 5 mm thick magnets by a kitchen scale, it is about 2 kg weight at 2-3 mm distance (measuring the attraction force would have been difficult). Four legs could hold 8 kg weight.
Good idea in principle.

Except

There will still be contact and friction between the magnets and each leg.
So there will still be noise & vibration transmission.

Andy
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Old 9th March 2008, 08:25 PM   #5
dnsey is offline dnsey  United Kingdom
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You could, of course, eclose the 'fixed' and 'chassis' magnets within, say, a foam tube, which would serve to keep them aligned without transmitting any significant (HF) vibration. The weight would of course be carried by the repulsive force.
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Old 9th March 2008, 08:25 PM   #6
gootee is offline gootee  United States
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(Not that I know why any of this might be necessary, but...)

Yes, even if you used some more magnets, so the telescoping tubes wouldn't even touch each other, there would still be transmission of motion from the base.

And, in fact, it might be almost a worst-case for airborne pressure waves affecting the motion of the 'suspended' unit. Spring suspensions would suffer similarly. There would also be some nasty resonant modes, in either case.

The solution would appear to be adding damping (like small 'shock absorber' cylinders, or just the right amount of friction), and maybe some mass. Additional damping would also be needed for both horizontal modes. Sounds 'messy'. But the filter equations would be straightforward.

Either that or create an active-feedback motion-cancelation mechanical-stabilization system. That might be a fun project, using some MEMS accelerometers, and maybe some speaker motors as actuators, plus some feedback-control amplifier systems.

Any type of vertically-oriented 'suspension' system seems like it would create many more problems than it would solve, if it solved any at all.

And it seems much simpler, and probably much better as well, to just use a very massive base, to rest the unit on, or to which to firmly attach it, unless you need protection during earthquakes as well.
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Old 10th March 2008, 01:25 PM   #7
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If you suspend something from a spring or springs, say a bungee cord, or you support it on a spring suspension, the resonant frequency depends only on the amount of deflection between unloaded and loaded conditions. All the other terms drop out. So if you set your turntable on some sort of spring suspension (and we have to assume linear springs here), the resonant frequency can be calculated from a single number- how far it compresses the springs.

The formula is Frequency=0.15915*SQRT(386.22/Deflection in inches). "SQRT" is the Excel square root function.

So, if your suspension compressed 1", the resonant frequency of the system will be 3.128Hz.

I just find it intriguing that the result comes from such a simple measurement. Naturally some damping is needed, lest things get out of control.

Do some research on "minus k" and "zero length springs". There are some patents to watch out for, but the technique saves a lot of space. Useful to anybody? I dunno.
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Old 10th March 2008, 03:27 PM   #8
poynton is offline poynton  United Kingdom
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Quote:
Originally posted by Conrad Hoffman


Do some research on "minus k" and "zero length springs". There are some patents to watch out for, but the technique saves a lot of space. Useful to anybody? I dunno.


I am sitting right next to a piece of equipment based on the zero-length spring. So it is useful in that it provides me with employment!!

Most gravity-measuring meters use the zls. There's even a company called ZLS


Andy
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Old 10th March 2008, 04:00 PM   #9
gootee is offline gootee  United States
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Quote:
Originally posted by Conrad Hoffman
If you suspend something from a spring or springs, say a bungee cord, or you support it on a spring suspension, the resonant frequency depends only on the amount of deflection between unloaded and loaded conditions. All the other terms drop out. So if you set your turntable on some sort of spring suspension (and we have to assume linear springs here), the resonant frequency can be calculated from a single number- how far it compresses the springs.

The formula is Frequency=0.15915*SQRT(386.22/Deflection in inches). "SQRT" is the Excel square root function.

So, if your suspension compressed 1", the resonant frequency of the system will be 3.128Hz.

I just find it intriguing that the result comes from such a simple measurement. Naturally some damping is needed, lest things get out of control.

Do some research on "minus k" and "zero length springs". There are some patents to watch out for, but the technique saves a lot of space. Useful to anybody? I dunno.
Hi Conrad,

That IS interesting. I seem to vaguely recall having something about that pointed out to us (re the other terms dropping out), in Physics class, during my first year as an EE undergrad.

I guess I was thinking more about what might happen at higher, non-resonant freqencies, and was imagining that having something so 'loosely' suspended might make it much more vulnerable to having vibration induced in it.

Then again, I am still not sure what the goal is, here.
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Old 11th March 2008, 01:41 AM   #10
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i was working at a stereo shop in NC back in the 70's and we tried to make a magnetic isolation table for turntables. nice idea, except that magenetic fields are "lossless" springs, and the mass of the table against the lossless spring makes a very high Q resonance. you may want to add an aluminum vane as a damping element (aluminum is an antimagnetic material). since we didn't think of this at the time, we ended up with a turntable stand that oscillated uncontrollably at something like 5 hertz, and so was not such a great idea.
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