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Questions on ACME stub thread design

Enginerd607

Plastic
Joined
Mar 16, 2022
Im not a thread expert, but was asked to weigh in on a non-standard ACME stub thread design for joining two sections of a downhole tool.

Proposed design is 1.5625in nominal diam ACME 2G stub thread. 4 TPI but with 4 starts, so effectively 16 TPI. Which is a little confusing when using online thread calculators, but OK.

Material will be BeCu or some non-magnetic SS. Total length of engaged thread is about 0.5625in.

Sketching this in CAD with the tolerances given, it seems like in the worst tolerance stackup case there could be only about 0.007in of height overlap between male and female threads (min major dia/2 - max minor dia/2). Thread height is on the order of 0.032in.

Roughnecks will tighten this up by standing on a long pipe wrench, so would need to handle at least 400 ft-lbs of tightening torque and Im not sure that would be sufficient.

Hard to find torque or axial load strength predictions for non-standard threads given the variables involved. I have the ability to do FEA but dont have experience with thread studies so would likely screw it up (pun intended).

Can anyone weigh in with some experience on whether this thread design sounds OK or not? Thanks.
 

4GSR

Diamond
Joined
Jan 25, 2005
Location
Victoria, Texas, USA
Okay, first of all, does this tool have to require field make up on this connection? or can this be made up in the shop before it goes to the field?

The way the oilfield works, If it has to be made up out in the field, it better have a rotary shouldered connection or tubing connection designed to handle five 300 lb "Bubbas" on a six foot cheater humping it to tight. Your connection will not survive the first time being made up in the field. I promise you that. As designed. Not to bust your bubble here just saying.

I hardly use any 16 pitch threads on anything going down hole in the stuff I deal with. 12-pitch is about the finest I go with in new design. Most of the time, I use 8-pitch Stub Acme. As for calculations to determine maximum torque makeup and stresses, find you a copy of the API 7-2 specification and find the formula for make up torque on rotary shouldered connections. I've used this for basis for calculating torque when shoulder/face contact areas are used. It's fairly accurate.

I know you are trying to design a thread for quick makup and breakout in a minimum space using standard thread profiles. This is going to be one of those "specials" you are going to have to design. Suggest using 6-pitch Stub Acme for your basis, and truncate the height to fit your envelope and go from there.

I would also write up a technical document detailing with pictures where to make up and how much torque torque to apply. 400 ft-lbs is really not much on weight on the end of a 36" pipe wrench. About 125 lbs! But Bubba must be instructed how much torque to apply. The entire rig crew needs to know this. Once they know this, and follow thru, your thread should work fine. Be sure to apply never-seige compound to the threads of your assembly. Use the proper one for BeCo you are using! Ken
 

Enginerd607

Plastic
Joined
Mar 16, 2022
Thank you 4GSR. The thread design was actually done by an R&D partner and we are questioning whether it is sufficient. So dont worry about bursting bubbles.

They are going for maximum axial motion for minimal rotation as you noted. Also constrained by material OD and ID packaging.

I was told that this needed to be field serviceable and no matter what spec we put on it, it was a matter of time before Bubba got to it with a 2 ft pipe wrench. I see those assumptions as negotiable, but that's what I am working with at the moment.

My main question is: Does 0.007in of thread overlap (radial) sound sufficient? This is the worst-case admittedly, but still seems like a failure waiting to happen.

I will check out the API 7-2 spec that you suggest. Thanks.
 

BoxcarPete

Stainless
Joined
Nov 30, 2018
Location
Michigan, USA
Thank you 4GSR. The thread design was actually done by an R&D partner and we are questioning whether it is sufficient. So dont worry about bursting bubbles.

They are going for maximum axial motion for minimal rotation as you noted. Also constrained by material OD and ID packaging.

I was told that this needed to be field serviceable and no matter what spec we put on it, it was a matter of time before Bubba got to it with a 2 ft pipe wrench. I see those assumptions as negotiable, but that's what I am working with at the moment.

My main question is: Does 0.007in of thread overlap (radial) sound sufficient? This is the worst-case admittedly, but still seems like a failure waiting to happen.

I will check out the API 7-2 spec that you suggest. Thanks.

Normally, thread failure calculations are done based on the shear strength of the weaker of the two materials. If you are looking at the last .007" of a .032" tooth profile, you are not in shear world, you are in agony city.
 

Trueturning

Diamond
Joined
Jul 2, 2019
I have cut them before. It is neat you have a multi start. I like those. Anyway I wanted to say that it is true a lot of times that in the field they will apply proper force like you describe. I have never concerned myself in the least about design as those Guys are very knowledgeable. There are many features in oil well types of threads and other neat features which are not seen except when doing these kinds of parts. (Oil Field and well)

They likely know in the field to apply less pressure actually. They do often put muscle into things a lot of things when it resists them. Better than being down get better stuff later. Blows off some steam that way.

Multi start threads are useful they allow several points to engage the thread. They are a very strong thread even being called a micro. I think it replaces other former threads which had problems. I never saw one until around 2000. But hey I did not get out much. Several points are better than just one especially when the only thing you have is one point of engagement and that got dinged up.
 

Enginerd607

Plastic
Joined
Mar 16, 2022
Thanks. I did purchase the API 7-2 spec as 4GSR suggested. I guess the relevant calculations are in the Annex E under seal face load calculation?
 

4GSR

Diamond
Joined
Jan 25, 2005
Location
Victoria, Texas, USA
Engine607,

Keep in mind, that your threaded connection needs to stay concentric to each other as close as you can make it. Thus, eliminating all slop as you can possibly remove and still be able to machine the parts. Most of us in down hole tool business use a 5-5-5 method in connections and sealing members. May require you to go with 3-3-3 method for fits. Doing this along with tightening the OD tolerance of the male thread and likewise for the female thread should allow you to have a maximum amount of radial engagement between threads. This will help the thread in bearing allowing more torque to be applied. Tighten up the pitch diameters between the threads and remove some of the clearance between the threads, too. Do not specify a 2G fit. Thats way too sloppy! Specify either 3G or 3C, tighter fit for the threads. Hope this helps. Ken
 








 
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