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ACME thread design based on internal pressure

ACME thread design based on internal pressure

ACME thread design based on internal pressure

(OP)
I am trying to calculate the necessary length of thread engagement for an ACME thread on a pipe (and pipe cap)based on the internal pressure.

I have calculated for the Stress Area At for a 1.5-4 ACME-2G from the Machinery Handbook 26th, pg 1794. and then inputed it in the Length of engagement formula Le on pg 1490.

It gives me a value but isn't this based on the minimum length of engagement so a BOLT breaks before the thread strips? How do you correlate this to a pipe with internal pressure?

If I calculate the force on the pipe cap wall, F=PxA where the internal pressure is 50000psi and the area exposed to the internal pressure is .506in^2. This gives me 25300 lbf.

Now, if I use the Load to Break formula on pg 1491. P=S*At where my Ultimate Tensile Strength is 195000psi and At for my ACME is 1.266 it gives me 246903 lbf. Does this mean that it would take 246903 lbf to strip off the threads? But because the force on the cap is 25300 lbf I have a safety factor of almost 10 times?!

I am using a 1.5-4 ACME-2G on a pipe with a .803 I.D.
My material yield is 150000psi.,
Ultimate Tensile is 195000psi,
Radial stress is 57400psi
and hoop is 165600psi.

I was advised (by Cockroach) to check the normal stress under the root of the external thread...does this mean by calculating the area of the of the crosssection of the pipe (minus the threads) A=pi(R^2-r^2)...pi(.598^2-.4025^2)=.618 where .598 is the radius of the Minimum Minor Diamter?

Then calculating Stress=P/A where P is the 50000psi and A is .618, which gives stress=80912psi...and compare this with the yield of the material?

I tried for the past two days looking into engineering books/websites, etc.. but cannot seem to find any clear method to calculate the thread length of engagement of a given thread on a pipe. Anyone know of a book or website that can clearly show you how to do this and/or is my approach above correct?!

So close...help!....

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