pressure rating threaded parts

[wellman]

I am trying to find out the pressure a screw together bowl will hold in a vertical pump. The bowls are made of cast iron. one is a 5" bowl with a thread of 4.750 major dia and is 12 thread. The other is a 6" bowl with a thread major dia of 5.093 and is also 12 thread. the 6" bowl has 1" of thread before the shoulders butt and the 5" has 5/8" of thread. Does anyone have a source or know how to calculate the pressure rating for these thraeds.

 

[Screwman]

Check out a copy of Bickfords "Design and Behavior of Bolted Joints". He has the formulas for cases where the females threads have higher, lower, and equal shear strength compared to the male threads. The results seem to come in pretty close (well within normal variation) to our test results.
Cast iron threads scare the heck out of me under pressure. The loading on the threads is primarily shear and iron doesn't have the greatest shear characteristics in the world.
Run the numbers and give yourself a healthy safety margin.

 

[MJCronin]

wellman,

For unique geometries, I believe that the best course of action is to make 5 - 7 of these and test to detruction....Take the average unlimate pressure of the group and divide by four or five.....that would be my design pressure for a production run of these......you may find that testing of ( cast iron )assembies is cheaper than analysis.

CI is crappy stuff to try to do any kind of detailed stress evaluation upon.....Empirical results can carry a lot of weight.

My opinion only

MJC

 

[Cockroach]

Wellman, you need much more information. Giving the nominal thread specifications are useless without reference to the tooth geometry. For example, 4 3/4 - 12 and is 5/8 inches long, so what? Acme, Stub Acme, Unified National (Coarse, Fine, Extra Fine, Special) would be required. Same applies for the 5 3/32 - 12, 1.0 inch long thread you have mentioned.

Also give the rating of the pump. You need to load the pipe system in some capacity to compute thread performance since this is handled as a boundry valued problem.

It would be nice to have the casting specification, material and grade are used to estimate factor of safety and other related information.

Once with the threading dimensions computed give the tooth profile(s), the loading will give the forces on the mating flanks and one could very easily estimate the factor of safety based on material performance.

...and there you have it....

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[wellman]

Cockroach,

I have been going through some older work and revisiting this problem. The thrad in question is Unified National. The rating I am currently using is 652 psi. The bowl is made of class 30 gray iron. The major dia of the thread is 5.089" 12 tpi 1" long PD 5.039 minor dia of 4.995. I have loaded the system with this 652 psi and found the resultant force at the mating flanges on the bowls I think. I am a little confused how to continue to the threads rating.

Thanks

 

[johnchrc]

Maybe I don't understand your problem but it seems you must have a seal somewhere for the UN thread to hold pressure.

What is the seal dia and location? From that, calculate axial loads on joint and hoop stress due to internal pressure. The value really depends on the placement of the seal relative the threaded connection. For instance, if you seal on the nose of the pin, then the burst is effectively the same as the wall the connection.

Use pressurized cylinder theory to find pressure at max. allow. stress.

If you just need to find the max allowable tensile load applied by pressure to prevent failure of the two threads, look at Machinery's Handbook or Shigley's design book. They have formulas for stress in threads from an applied load. You would check shear, tensile across thinnest wall (thrd relief if applicable), hoop stress from radial load component (the size depends on thread angle), and bearing stress on face of thread.