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The design of hydraulic fittings is relatively straight forward. The first
thing you have to understand are the terms used. In particular are "Rated
pressure", "Yield Pressure" and "Burst pressure". Burst pressure is the
point where something ruptures, yield pressure produces a stress in the
component greater than the yield strength but less than the tensile
strength. Rated pressure is generally 1/3 of the burst pressure for hoses
and 1/3 the yield pressure for metal components (safety factor of 3 and
a margin of safety of 2).
Calculating the stress in a round pipe is easy. The formula is Pressure X Inside Radius/ Wall Thickness. So a piece of 1/4" x 0.035 tubing
at 1500psi would have a stress of (1500 X 0.090/.035)= 3857psi.
Dead soft aluminum tubing has a a yield strength of around 12ksi (1ksi =
1000psi) and a third of 12000 is 4000psi, the rated pressure of this tube
in aluminum.
As you can see by the formula, small diameter tubes with thick walls can
resist high pressures with low stress levels. This is why the "fish tank
tubing" works in the brake systems. Of course it does not take into
account aging, abuse or heat, three things brakes are good at.
For rectangular section manifolds the variation in wall thickness tends
to concentrate the stresses in the thin areas therefore, as a rule, I
multiply safety factor by 1.5, from 3 to 4.5, and use the thinnest portion
of the wall for T.
So lets say you drive down to Ace and buy a chunk 1 X 1 square stock of
what looks like aluminum to build your manifold. You decide to use 3/8
pipe fittings so you drill some 37/64 (0.578) holes for the tap. One of
the holes is 0.050 off center, oops. How much pressure will your manifold
endure?
S = P X R / T therefore P = S X T / R
S = 4000 / 1.5 = 2666
R = (0.578 / 2) = 0.289
T = ((1.00 - 0.578) / 2) - 0.050 = 0.161
P= 2666 X 0.161 / 0.289 = 1485 psi
I like monolithic manifolds because I hate leaks and the "fitting farm"
manifolds leak, and they are expensive. I would also use a piece of "known
alloy and temper" material. Hey Billy Bob, git me a chunk of that silvery
stuf with the white spots on it from under the tahr of yur Trans Am.
Brent Regan
[Since I want to use 1/8NPT in my piece of 1^2 stock, and to do the entire
assembly out of 6061T6, based on the formulae you've provided it looks
like I'll be more than ok. Thanks for the info. (I hate leaks too!) <Marv> ]
LML homepage: http://www.olsusa.com/Users/Mkaye/maillist.html
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