hoop stress in tapered threads-fittings are cracking
hoop stress in tapered threads-fittings are cracking
(OP)
Would like some help finding link to equations to calculate the hoop stresses in the female side of NPTF/NPTF 1 1/4" connection. The material is brass. The current best guide from Parker is to use the TFFT. ... for 1 1/4" the TFFT is 1 1/2 turns to 2 1/2 turns. Some fittings may have been over torqued, i.e. the TFFT was not followed. How can I get an idea how much stress was put into the fittings before they failed? The failure was a straight crack running along the outside of the female fitting parallel to the flow direction. I think there should be some idea how much hoop stress is in a brass fitting when assembled e.g. 2 turns from finger tight (TFFT). I'm looking for that type of equation.





RE: hoop stress in tapered threads-fittings are cracking
If you want to estimate how long a pipe they had to slip over the wrench, then, yes, you'll need to do some math.
Mike Halloran
Pembroke Pines, FL, USA
RE: hoop stress in tapered threads-fittings are cracking
Thanks. We need to ask fitting supplier what the exact alloy of the brass is, we don't know yet. I think we want to get comfortable that if we reinstall new parts to replace cracked parts that we are not near over stressing the material. For example if 2.5 turns past finger tight gives 15kpsi and the brass is good for 55kpsi..then we are feeling that fittings will not crack due to installation. We are not sure how far they were put on 3 turns or 4 turns??? Some large tools were used and the operators, not our company, were given instructions something like get it as tight as you can....
After some more thinking the fitting supplier should know exactly the stress as a funtion of turns past finger tight so we will ask that tomorrow.
Appriciate your interest....
-Henry
RE: hoop stress in tapered threads-fittings are cracking
I just had an idea..
I might be able to estimate the hoop stress because it is basically an interference of two hoops...
I know the interference addition per turn because the threads are tapered I think 3/4" per foot or something like that...
Does that make sense?
-Henry
RE: hoop stress in tapered threads-fittings are cracking
-h
RE: hoop stress in tapered threads-fittings are cracking
You need to give the ultimate assemblers a specific procedure, e.g. Parker's,
...
or use push-on fittings,
...
or cut their wrenches in half.
Mike Halloran
Pembroke Pines, FL, USA
RE: hoop stress in tapered threads-fittings are cracking
Check your ANSI spec. for this fitting. 2 turns past hand tight is a good place to start. If your threads are to spec.you should have 2~3 showing when tight.
RE: hoop stress in tapered threads-fittings are cracking
Take a sample of fittings to failure. Try a sample by going with the manufacturers recommendation. Try some samples with various thread dressings to see the difference. You will then have a feel for what the torque should be,and be able to come up with a standard; which will probably be far better than trying to calculate as you still seem to have some unknown variables affecting the outcome. Manufacturing tolerance and quality control on this type of fitting probably has a fair range so calculation would only be an estimate anyhow.
By knowing the failure torque you know the factor of safety.
The hardest part is then training your gorilla's to torque them correctly and making sure they do it.
Tighten till you feel the threads strip then half a turn back to lock it ...