Acme Thread Stress - Drill String Connection
Acme Thread Stress - Drill String Connection
(OP)
Hi all,
I have been asked to design a 4 TPI Stub Acme thread for a drill string application. Most of my reference material seems to simply design based on a transverse shear area and bending, however, at the top root of the thread, there is a plane stress present that is comprised of the bending stress, an axial stress, and a torsional shear stress. Obviously this combined stress is significantly higher than either the transverse shear or the bending. Does anyone have any experience with this type of connection that could offer some insight as to why this plane stress appears to be...over-looked? Any reference material for this type of drill string connection would be greatly appreciated.
Thanks in advance.
I have been asked to design a 4 TPI Stub Acme thread for a drill string application. Most of my reference material seems to simply design based on a transverse shear area and bending, however, at the top root of the thread, there is a plane stress present that is comprised of the bending stress, an axial stress, and a torsional shear stress. Obviously this combined stress is significantly higher than either the transverse shear or the bending. Does anyone have any experience with this type of connection that could offer some insight as to why this plane stress appears to be...over-looked? Any reference material for this type of drill string connection would be greatly appreciated.
Thanks in advance.





RE: Acme Thread Stress - Drill String Connection
Ted
RE: Acme Thread Stress - Drill String Connection
In the drill string, axial load is usually the least concerning, torsion the greatest. For example, in the load application of a drilling driveshaft, 85% of stress is attributed to torson, 12% to bending and the balance to axial load. This has been my experience with a major energy service company providing drilling solutions to the industry. Several FEA model support the API specifications that HydTools speaks of, say API 7, 7G, 5CT for alternate tubulars and the various thread forms thereof.
The 4 TPI, 3/4 TPF Acme thread is a personal favorite, never let me down yet. Highly robust, easy to cut, easy to QC/QA. Hope this works out for you.
Regards,
Cockroach
RE: Acme Thread Stress - Drill String Connection
1. Do I simply need to check thread shear, tensile stress in the box and pin, bearing load on the faces that shoulder out, and torsional stress in the box and pin?
2. Or should I be checking bearing stress and bending stress in the threads and combining the appropriate stresses to obtain the Von mises stress at the thread root?
Any insight would be appreciated.
Heliakon
RE: Acme Thread Stress - Drill String Connection
Get both those specifications and write the mathematical model so you can compute each threading geometry. The equations are fully covered in that standard. It's really not a big calculation and definitely nothing to fret over! But you need to do the work.
Regards,
Cockroach
RE: Acme Thread Stress - Drill String Connection
Cheers
Heliakon
RE: Acme Thread Stress - Drill String Connection
I've seen some of your calcs. in the past and you usually lay things out very nicely, cleanly and clearly. I've run across a few API specs. in my past life, but basically don't know one from the other unless I've got them in hand. And, I agree with your statements about Acme thread strength and you general magnitudes of torsion, bending and tension stress in this joint. But, maybe a non-oilman's take on coupling stresses and loads would help Heliakon's understanding.
Once this joint is made up, it essentially acts as a continuous shaft, except for minor secondary loadings and stresses in the joint. The sum of the shear stresses/loadings in the threads equals the bearing stress/loads on the shoulders, plus or minus. These stresses and loads are quite high, as the material grade will allow, and essential make the shaft a continuous piece. For bending or tension to become significant issues they would have to overcome this high preload in making up the joint. Tensile loading is well distributed around the joint, but starts to counteract this preload, and in the extreme would start to open up the shoulder joint. Bending stress will vary across the shaft/pipe and will be additive or subtractive from the shoulder bearing stress or thread shear stress, depending upon which you are looking at or their radial location on the shaft. But, until these combined bending and tension stress start to overcome some significant percentage of the preload the joint remains tight (a continuous shaft) and bending or tension stresses are secondary stresses, but not to be ignored, either. Probably, experience and testing allows you or API to put numbers on what I called 'significant percentage.' In the structural business I would liken this problem to preloaded bolts in a tension or moment carrying structural joint.
Please flesh my thought out for Heliakon, as you see fit; or tell him I'm all wet and should mind my own business.
RE: Acme Thread Stress - Drill String Connection
Now dealing with this threaded connection makes sense.
Thanks all for your input.
Cheers
Heliakon