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Old 08-17-2008, 03:55 PM   #41
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Originally Posted by vfc View Post
Kurt, You need to read a book on physics or go back to school. ...
Trust me... I've read a few. I have a pretty successful career of "not understanding physics", so I'm not really too concerned about your suggestion or the fact that you think I'm personally driving tank prices up!

No need to lock the thread in my opinion, An t-iasg - I'm done with it!
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Old 08-18-2008, 03:23 PM   #42
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Dskidmore...

You mention the foam putting force on things like it's creating the force.
Technically the only real forces in the equasions are gravity, the strong and weak forces holding the atoms together and the electromagnetic forces holding the molecules/mateials together. I have no motivation to attempt to individually sum up the basic forces of the foam, the stand, the house, and the earth itself that make them able to hold up the aquarium when it is obvious by the fact that the tank is not accelerating that this force is exactly equal to that of gravity, which is much simpler to compute. So the foam is exerting a force up on the tank equal to the load the tank is exerting on the foam. This also happens to be equal to the amount of force, minus the weight of the foam, that the stand is exerting on the foam, and the foam is exerting on the stand. (And so on through the stand to the floor, to the rafters, to the foundation, to the ground, to the bedrock...) But that's all semantics.

I don't get your point at all. Physics problems are regularly simplified as much as possible to obtain the rough numbers. Like my treating the foam as a series of infinately small theorretical springs. We all know that foam is not a perfect spring, but the comparision will help us understand what is happening even if it is not a perfect model. If we required perfect models for everything, the science of physics would not exist, for it is too complex. Instead we build approximate models, and over-engineer in a safety factor.
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Old 08-18-2008, 03:42 PM   #43
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Originally Posted by Kurt_Nelson View Post
No need to lock the thread in my opinion, An t-iasg - I'm done with it!
Ah, but it was so interesting. I talked with my mother in law (a math teacher) about it, and the next evening I have reading time I'm going to pull out hubby's statics and dynamics book. I never got that far in engineering before switching majors, but I think with the textbook in front of me I could puzzle out in a few weeks what the adept student should be able to handle in their evening homework.
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Old 08-18-2008, 05:31 PM   #44
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Ok, here's the best I can do without the statics and dynamics book:

Fx = function describing the curve of the stand
k = spring constant
t = thickness of the foam
d = t-Fx = distortion of the spring at point x(assuming foam is compressed everywhere enough to touch the load everywhere. This is a minimal case for usefulness, a real case would have an additional constant compression, and therefore constant additonal force, across the whole contact surface. Because this other force is constant, we will ignore it for now.)
Fs = kd = spring force
Fs = k(t-Fx) = spring force at point x

W = total weight of load supported by foam

W = k(t-Fx) dx

I'm not going to solve it with real numbers right now as Fx is realistically complex, I have no real data for a sample k, and it's been a long time since I took calculus, but we can look at the relationships and make some conclusions.

We'd want W to be less than the weight of the tank, as if it was more than the weight of the tank then that means our tank would not compress the foam enough to provide support across the whole tank. We would still have an unfilled gap between the tank and stand.

As the tank weight begins to dwarf W, the tank is lowered closer to the stand, and will compress the foam by the same additional amount at all points, and therefore the spring force will also increase constantly across all points. The more the weight of the tank dwarfs W, the greater the percentage of the load is evenly distributed. The softer the foam, the more the weight of the tank will dwarf W.

There is a limit to how much the tank weight can dwarf W, at the point where the foam is compressed enough to deviate significantly from the ideal spring, reaching maximum useful compression. At this point it's just like setting the tank down without foam. A more resistant foam must be used if you reach this state.

There is a second limit in that the maximum (used) deflection of the foam should be less than the diffrence between the bottom of the tank trim and the bottom of the glass, to avoid the earlier mentioned situation where the foam starts transfering some load to the aquarium glass. This is probably the situaton that tank manufacturers worry about when warning against foam. A more resistant foam must be used if you reach this state.


The thickness of the foam should have it's diffrence between minimum and maximum compression minimally equal to the diffrence between minimum and max height of the supporting surface. Thicker is better in this case, but given the same material, a thicker foam is softer, and could bring you to one of the above limits.

So, in conclusion:

You want a foam that will compress at least a little bit everywhere, but not touch the bottom of the aquarium glass, or get completely squished. Within those limits, a softer foam will redistribute more weight than a firmer one.

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Old 08-19-2008, 10:16 AM   #45
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dskidmore,
Good work; the world needs geeks like you to take theories and separate them into facts or myths.

Quote:
So, in conclusion:

You want a foam that will compress at least a little bit everywhere, but not touch the bottom of the aquarium glass, or get completely squished. Within those limits, a softer foam will redistribute more weight than a firmer one.
That what I have been saying: foam selection is most important.

The perfect foam for a 75G tank is the 1/2" interlocking black foam exercise mat that Target sells. It comes with six 24" pieces that interlock like puzzle pieces. The foam compresses between 10-20% when you initially fill the tank and can offer stress relief for stand level variations of up to ~ 3/8". You can stack two pieces of 1/2" foam to handle larger variations.

If the foam is too stiff, it offers no stress relief. I have that situation on a tank now. I built the stand, and because I had an extra piece of 1" pink foam from a home project, I placed that under the tank. When I filled it up I had three spots along the frame where I could slide a plastic card between the tank and foam. After 8 months, the gaps are still there; the foam did not compress at all.
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Old 08-19-2008, 01:18 PM   #46
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I have a white foam with blue backing under my tank right now. It's slightly softer than the pink foam, but not much, and it has no rebound after the load has been lifted. My stand is hollow in the center, so there is little risk of pressure on the glass if I use a foam too soft, so I think when I next move, I will replace it with a softer foam.

I do need to use some sort of foam, as my stand is made from interlocking 4x4s and there is a little bump where the join is. The good news though is that they can hold the weight with no center brace and no noticable bowing.
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Old 08-19-2008, 06:18 PM   #47
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dskidmore,
Here's a better picture of the entire front of the tank. I put three small pieces of uncompressed foam on the stand for reference.

Here is a rough representation of the frame and force distribution with and without foam.
Attached Thumbnails
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Old 08-19-2008, 07:00 PM   #48
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I'm not going to check your exact numbers, I don't have the data, but assuming the foam is not totally crushed on the ends, and there are no gaps in the middle, that could be right. Of course the blue area is itself not evenly distributed weight either, the point load varies with the amount of compression at that point, which varies by the origional gap size.
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Old 08-19-2008, 07:06 PM   #49
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vcf the no foam pic, is that how the tank sits full of water on the stand, or just empty?
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Understanding the basics will let you make informed decisions on what advice to follow, and what advice is just plain hooey!
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Old 08-19-2008, 09:15 PM   #50
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SpeedEuphoria,
The first picture I took a few hours ago. The tank has been set up for close to 3 years.

If I did not have the foam under the tank, there would be a gap between the tank and stand in all the blue areas.

When I first put the empty tank on the foam/stand, there were no gaps. When I filled it up, the foam compressed evenly.
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