Balloon with minimal displacement 1

[thewolf5589]

Hello--can anyone offer insight into designing a balloon--essentially a thin air mattress--that when fully inflated will minimize or even prevent displacement if a person, say 250 lbs, walked across it? What PSI might be required? Balloon materials and wall thickness have yet to be determined, so please feel free to suggest materials as well.

Any and all insights appreciated.

Thank you.

 

[btrueblood]

Prevent displacement? Sounds like you want concrete, not rubber.

How MUCH displacement are you willing to tolerate, and it what directions (vertical vs. lateral), and to what are you comparing (nobody I know ever walks across an air mattress deliberately) the displacements to. Describe the geometry a bit better. Air mattresses limit the "ballooning" effect by building in tie point or ribs...which are stress concentrations that will be the site of eventual tears. Foam rubber might make a better choice, or convoluted rubber mats. One of the best anti-fatigue mats I've ever seen are made by a company called SA-Tech, they have limited displacements (firm footing) under normal walking pressures, but have excellent impact absorbtion (you can drop an egg on them without it breaking).

 

[tom1953]

Sounds like a good college exam question

 

[btrueblood]

wups. Now they are called Smart Cells:

http://www.smartcellsusa.com

 

[thewolf5589]

Thanks, @btrueblood. Tie points & ribbing are great insight.

Minimal displacement tolerated but not to degree it feels like a bounce-house when people walk across it. It's important the shape be dynamic between two states (deflated and inflated) so I imagine air is my only option (the BEST medium would be the liquid metal terminator from Terminator 2 but I won't be responsible for skynet).

Bear with me...force exerted by a 250 lb man standing still is ~1110N. Assuming sz 12 shoes are worn, the pressure exerted is ~2.4 PSI when standing with both feet (and ~4.7 PSI when standing with one foot). Assuming the force is doubled from jumping, pressure exerted is ~4.7PSI with two feet and ~9.5PSI with one. So if balloon PSI > 9.5 PSI, there should be minimal displacement from a 250 lb man? I've read that inflatable raft floors should be ~10PSI.

Does this make sense? If so, any suggestions on materials, adhesives, and construction methods to make a prototype at home?

Thank you.

 

[Compositepro]

So how will your air mattress be different from the multitude that you can buy?

(hint. Filling with water will eliminate the compressiblity of the air. Then firmness will only be a function of the stretch in the fabric.)

 

[thewolf5589]

Hi Compositepro, not an air mattress but conceptually similar. It'll be a custom, asymmetrical shape with inconsistent surface height when inflated.

So you're saying if I choose a firm fabric--like a pvc shower pan liner--to build the balloon/airbag from and fill with water, it'll be more stable than the same balloon/airbag filled with 10PSI air? Please enlighten me!

Everyone else--I'm still hoping for input on the 1/14 18:57 post re calculation, prototype suggestions.

Thank you.

 

[btrueblood]

As cpro notes, the thickness of the material will change how the surface deflects, part of the weight of a person on an air mattress is borne by the deflection of the fabric/sheet. You could minimize those deflections with a stiffer bearing surface (e.g. a sheet of plywood on top of the air mattress, or a thick-ish layer of rubber).

 

[Compositepro]

The flexural stiffness of the surface is certainly a factor, but for fabrics and other films that is essentially zero. What is important here is the bulk modulus of the fluid in the bladder and the tensile modulus of the fabric because loads will cause a pressure change in the bladder which will result in a volume change of the bladder. The greater the volume change, the softer the bladder. A thin, stiff sheet will help considerably stiffen the surface by spreading the load and result in a much lower pressure change in the bladder.

 

[hendersdc]

You might consider looking into the construction of inflatable paddle boards as a good reference. They distribute loads nicely and pack down to much smaller than their inflated size.