I was wondering just how hard would it be to weld up a box and use it as an air tank?
There is a nice pocket of space in the fender behind the cardboard walls in the interior. I was thinking of building a box that would take up all that space efficently and using it for air storage. Any thoughts?

 

yuo could do it, but will need a drain at the bottom and also a need to severly coat the inside to resist rust... also your welds would basically have to be perfect......

 

There's a reason most air tanks are round. Try blowing up a paper sack, what shape does it try to become? Round. That's what'll happen to a square tank unless it has seriously thick walls. You don't want to mess around with pressure vessals, plus air tanks are cheap to buy.

 

ah, good point, didnt even think of that.... even custom bumpers that hold air have round tubes.....

 

My air tank is square, not coated inside and doesn't have a drain..

Guess I just don't follow anybodys rules

 

Yeah, but how thick is the steel? Much thicker than a store bought round one, I bet.

I mean, yeah, lots of people use square sliders or bumpers for air tanks, but they're using 3/16 to 1/4 tubing and usually it's 2x2 stuff. One wall of a 10"x10"x10" cube has an area of 100". At 100psi that's 10000 lbs of force exerted on 1 wall. Plus, you have to take into consideration fatigue and corrosion.

 

Hi Chris,

I'm breathing life back into an ancient thread. I've heard a lot of talk about making custom tanks but no one that I know of has done it.

I also thought this would be a cool thing. I think I have a basic understanding of how to design a tank but please (anyone) post up and let me know if I made any errors. From what I read there is a real danger of serious injury or fatal explosion with air tanks as large as what we might need to fill four 33" tires. I'd rather be wrong than maimed or dead.


Around 1.5 cu ft tank (about 10 gallons) or so is needed to air up four 33" tires at 130 psi.

Basically the bigger the tank the thicker the walls need to be? A tank that would fit under the rear where the spare tire normally lives could be as big as 9" x 12" x 36" that's about 2.25 cu ft or 3888 cu in. This is making a rectangular tank that does not have to go around the vapor system in the spare tire area that was new to the 2001 and 2002 4Runner models.


To make it easy to figure out the wall thickness I converted the equivalent volume to a cylinder form equaling a vessel 36 inches long with a radius of 10.5 inches.

The below formula calculates the wall stress for the cylindrical section, the stress in the hoop direction (in a circle around the wall).


t = (p x r) / sigma

Where:
p = pressure in psi
r = radius of the cylinder
t = thickness of a wall
Sigma = the tensile strength of the material

Mild steel has a tensile strength of about 55,000psi
Stainless 304 has a tensile strength of about 80,000psi

This figures out the the bursting thickness. So we should at least quadruple (4x) the margin of

safety.

Plug in the numbers using stainless steel and you get:

t = (130psi x 10.5) / 80000
t = 0.017 inches thick x 4
A safe thickness is 0.068"


That makes the wall thickness 1/16" thick (0.0625" = 1/16th inch) thick if we made it out of stainless 304 if it's mild steel then it would need to be 0.099" or 3/32" inches thick.

I'm assuming stainless steel is the way to go instead of mild steel because of the problem with condensation forming in the tank and rusting the tank from the inside out until it potentially explodes.

To help with ideas for other tank sizes keep in mind the larger the tank the thicker the walls need to be?

For example:

A 10" cube tank filled to 130 psi will have 13000pounds of push on each tank wall.
A 1" cube at 130 psi would have 130 pounds on each inch of the tank wall.

BTW the formula to figure the pressure at the ends (the AXIAL pressure) is:

t = (p x r) /2 x Sigma

Can anybody verify my calcs? 1/16th inch stainless 304 seems pretty thin for a tank even if the

problem of preventing the sides from deforming is figured out.