Gauge for Thread Wear?

[Handy Man]

Hi! I'm in a situation with threads that are being torqued, and unthreaded, and then torqued again hundreds of times per day. While torqued they are required to lift thousands of pounds of load. We've experienced failures where the threads appear to have worn/galled to the point that they stripped out when lifting the load, which results in a serious failure. Some of the threads are tapered V-threads, others are straight square threads.

How can I inspect the threads for wear? So far I've considered:

1. Use a normal go/nogo thread gauge. I'm not sure if they will catch a reduction in the major diameter if the pitch remains OK.
2. Measure the OD with a micrometer (or using a ring gauge). I think this would work for the straight buttress threads, but it isn't feasible for the tapered V-threads.
3. Use a thread profile gauge and visually identify wear. This seems to be the best option, but it requires some skill and isn't as "idiot proof" as I would like.

Any recommendations?

 

[MRFMengineer]

What is the material?

Can the design be changed? It sounds like threaded fasteners might not be the best solution.

 

[Nidum]

Screw thread micrometer for the V threads . Probably standard anvils available for your thread sizes but easy enough to make specials . You can also improvise a vernier caliper version with DIY anvils if threads are big or you need quick readings .

Narrow anvils or ball probe micrometer for the square threads .

 

[Ranger Mike]

we run into this all the time on axle nuts..we have formula car and one huge 1" threaded axle per wheel.
The nut felt real mushy when it was torqued to 100 ft. lbs. This was a huge red light..

First thing to find out is ...
is the nut threads wearing out or is the axle thread being worn. We found in our case that the nut was becoming worn as was the axle. When we miked the thread on axle it was good until the portion that was constantly torqued. Plus a new nut was hard to turn on the axle until it hit that torque spot. If i remember correctly, it was a little more shiney ( appearance wise). A caliper can measure the OD of the axle but the ID of the nut is harder to check. You might be able to do it with precision plug gages.

 

[Handy Man]

Thanks for the input guys.

MRFM: The material is 110 ksi steel. A design change would be a great solution, but unfortunately isn't possible for this application. We are lowering tubing into a well, and must use the standard threads the tubing comes with.

Nidum: For the tapered V threads (similar to NPT, but 2-3" diameter), the diameter will change with where you measure the thread (because of the taper). Is there any accurate way to control for that with a micrometer?

Mike: Awesome example! In this case its only the "axle" side that's wearing. The "nut" side is a new part every time.

 

[JBA]

I'm curious as to the purpose for which you using the well tubing that requires the multiple daily tightening/loosening of the well tubing joints.

 

[Handy Man]

We are performing workover operations. Cleaning out old wells. We torque up a joint of pipe, rotate it and circulate fluid through it as we lower it into the well (often with a bit or other cleaning apparatus on the bottom of the pipe) Once that joint of pipe is in the well, we disconnect the connection, pick up another joint of pipe, torque it up and lower it into the well the same way. Repeat until we've reached the bottom of the well.

 

[Ketch22]

Tapered threads pose a difficult situation. If this is to be performed frequently I would suggest a combination of profile gauge and a Go/No go type gauge. By evaluating with the profile gauge in a couple locations it would be possible to see wear on either face of the threads. Then use a special Go/No go nut that could be threaded onto the end (to whatever reproducible torque you choose) the goal would be to measure the distance extended from the face. If you have uniform wear causing reduced wall thickness it will extending further due to deeper seating of the taper. For casing pipes ,which experience some rough handling, I would also check for out of round. This could be easily done by adding some bluing to the threads prior to screwing on the gauge. Distortion would show as uneven removal of the bluing.