Stretch Magic strength and elastic constant

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Felger Carbon
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Joined: Thu Dec 15, 2005 11:06 am
Location: Klamath Falls, OR

Stretch Magic strength and elastic constant

Post by Felger Carbon » Tue Apr 25, 2006 12:50 pm

I made a short test length of 1mm dia Stretch Magic. Tied a loop at each end, but this was a one-strand length. Filled a gallon jug with water; 8lb dead weight. Used a shoelace looped through the jug handle and one of the SM loops four times and tied it. Stuck a screwdriver through the other loop of the SM. Slowly raised the screwdriver, stretching the SM. Finally, the SM (after stretching considerably) lifted the 8lb jug off the floor. I held it for a while, but got tired before the SM did. :)

I have yet to break SM, although I've seriously tried several times. Now we know that one strand of 1mm dia will lift 8lb. So the 8 strands of 1.8mm that I have supporting a P80 and 80mm fan, ~23oz, is capable of supporting 8lb * 1.8^2 * 8strands = 207lb, and I have a safety factor of ~14,300%!

I'm not worried about the P80 suspension breaking. :D

How much stretch is there when suspending an HDD using Stretch Magic? This question solved by a high-precision scientific experiment:

% stretch (1.8mm SM) = 1.5 * W / N

% stretch (1.5mm SM) = 2.16 * W / N

% stretch (1.0mm SM) = 4.86 * W / N

Where SM = Stretch Magic, W = the suspended weight in ounces, and N = the number of strands.

I filled an empty gallon jug with water about a third full. Didn't weigh it. Cut off an 8 or 10 foot piece of 1mm Stretch Magic (SM). Didn't measure it. Tied the SM ends to make a long "rubber" band.

Passed the band thru the jug's handles and looped the ends over a screwdriver; held the screwdriver up while I picked the jug up and lowered it to the floor. yeh, the SM was stretching nicely, in fact I had to lower the screwdriver to let the jug reach the floor.

Held the screwdriver in my left hand and picked up a metal measuring tape with my right hand. Lifted the screwdriver until all the slack was removed from the SM but no more. Measured the 4-strand loop's length: 21". Lifted the jug completely off the floor and measured the loop length: 33". So the weight of the jug stretched 4 strands of 1mm SM 57%. Weighed the jug on my pretty accurate Royal EX5 5lb digital scale: 47 oz even.

Voila! A 47oz weight stretches 4ea 1mm strands of SM 57%. [A one-loop "rubber band" is two strands.]

Right now I have an HDD suspended by one-loop "rubber bands" of SM, one loop on each corner. That's 8 strands, and I'm using 1.8mm SM. One strand of 1.8mm SM = 3.24 strands of 1mm, so 8 x 3.24 divided by 4 is 6.48 times the cross-sectional area of 4 strands of 1mm SM. So it would take 47oz x 6.48 = 305oz to stretch my suspension 57%. Or, if you prefer, 47oz would stretch my suspension by 8.8%.

Since I'm suspending a 20oz drive and an 80mm fan, I'm suspending ~23oz. So my suspension is stretching 4.3%.

The 3 largest sizes of SM are 1, 1.5, and 1.8mm diameter.

% stretch = W * K / N where W = the weight in ounces, K is a constant that depends on the diameter of the SM, and N is the number of strands. K is obviously higher for the smaller diameter SMs.

Solving for K1.8, we have K = %s * N / W, or K = 4.3 * 8 / 23, or K1.8 = 1.5 hence K1.5 is 2.16 and K1 is 4.86.

% stretch (1.8mm SM) = 1.5 * W / N

% stretch (1.5mm SM) = 2.16 * W / N

% stretch (1.0mm SM) = 4.86 * W / N


suspending a 47oz jug by 4 strands of 1.0mm SM:
stretch = 4.86 * 47 / 4 = 57% (check)

suspending a 23oz weight by 8 strands of 1.8mm SM:
stretch = 1.5 * 23 / 8 = 4.3% (double check)

suspending one 20oz HDD by a loop at each corner of 1.5mm SM: stretch = 2.16 * 20 / 8 = 5.4%

So, if you suspend a 20oz SP0822N using four 3 inch "bands" of 1.5mm SM, they will stretch 5.4% or 0.162" - about a sixth of an inch. If you had used 1mm, the stretch would have been .365", over a third of an inch.

(OK, so the initial experiment doesn't justify carrying the calculation to three decimal places. Sue me. Or perform the experiment yourself with more precision, and hope there's no variance in the diameter or elasticity of SM.) :D

edit: added strength data
Last edited by Felger Carbon on Mon May 01, 2006 6:07 am, edited 1 time in total.

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Location: No.Calif, USA

Post by epiphane » Thu Apr 27, 2006 9:21 am

Damn...I wish I had you sitting next to me in math class oh so long ago :D


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Joined: Thu Mar 23, 2006 4:56 am
Location: Sweden

Post by krille » Thu Apr 27, 2006 10:23 am

epiphane wrote:Damn...I wish I had you sitting next to me in math class oh so long ago :D

Word. 8)

~ Kris

Posts: 80
Joined: Mon Apr 24, 2006 10:34 pm
Location: Toowoomba, Queensland, Australia

Post by Shobai » Thu Apr 27, 2006 1:41 pm

as an aside, is stretch magic available in Oz-land? if so, under stretch magic, or another name?

thanks guys.

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