Bags of sand instead of cement in table?

On: Fri, Aug 03, 01 01:40:02 PM

Michael S wrote:

I was just wondering if anyone has tried bagging up sand in thick bags possibly innertubes for an isolation table. The bags of sand would replace concrete tiles or slabs making the table cheaper. Or is concrete used for its stability not just weight? Let me know if you think this might work. Thanks in advance.


Brian - Fri, Aug 03, 01 02:15:34 PM

There's 2 things to consider: 1) your isolation table needs to dampen external vibrations. 2) your isolation table needs to dampen internal vibrations. I'm currently building an isolation table, but I'm using both steel and sand, plus inner tubes. The steel sits on top of the sand (2" thick at 5' x 3') The sand and inner tubes will damped any external vibrations from the floor, but the steel will dampen or at least keep steady all vibrations generated on the table itself. If the steel plate moves, all of the components will move with it and thus you're okay. Using just sand is fine, but you have to let all of your components settle in the sand for quite some time. By using steel, you don't have to worry about that. -Brian

Michael S - Fri, Aug 03, 01 07:21:54 PM

I plan on using the sand as weight on top of a large innertube but below a 4'x4'x1/4" piece of steel. I was told that the 1/4" plate would not provide enough weight by itself. I figured it might be a cheaper alternative to concret blocks. My setup is on a concret floor in my garage.

Brian - Fri, Aug 03, 01 09:30:59 PM

Michael, that's pretty much exactly what I'm doing too. I did some pretty lengthy calculations to get the weight correct. My 3'x5' steel top weighs 150lbs. To cover the table top with 2" of sand will require250lbs of sand. That plus lumber is roughly 500lbs. My 8" rim inner tubes can comfortably hold 140lbs before they get too "stiff" from over pressure, so 500lbs / 4 = 125lbs which is will within range for the inner tubes to still function. -Brian

Colin - Sat, Aug 04, 01 10:12:17 AM

Stiffness is a very important criterion as well. Holography benches are often built on a 12" thick granite block because it is very stiff. It is actually easier to isolate a light stiff table from external vibrations than a heavy stiff table because of the natural resonant frequencies involved. I have a 3'x5'x1/2" steel plate that is very well isolated above 15hz and very well damped below 15hz. It is resting on cinder block on my concrete floor in the garage. With an interferometer I can measure the flex in my table from changing my weight from foot to the other! I have to stay very still during my exposures. I am redesigning my table after a long conversation with Jonathan H. After some research I decided a reasonable stiffness would be a deflection of .00006 mm with the addition of .5 Kg of mass. Since I don't have a lot of money and need to move my table someday I am designing a composite structure. Take a look at: 8.asp This is my design ideal. I can't manufacture a honeycomb core So I am looking into alternative cores.

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