Threaded Cup Difficult to Remove

[mjgarrin]

Hello,

I have two cups from two different suppliers. The design of the cups call for an external thread M85X1.50 with a 6g tolerance. The hardness value of both cups were checked around 30-36 HRC. There are a few differences in the way the cups were made:

Supplier A Cup:
Made from a casting
Hardness value towards lower end of spec

Supplier B Cup:
Made from a billet
Hardness values slightly higher than cast piece
Threads cut using single point cutting

The female component has the same thread with 6H tolerance. The component is made from a casting, and the material of the female component has a hardness value of around 23-32 HRC.

After both cups were used in the mating component, they were disassembled for inspection. We noticed that after testing, the billet material cup will un-thread fine about half way, then it gets stuck and needs some force to remove. Threads themselves look fine, and do not appear to be damaged. The cast cup has no issues, and did not get stuck during removal.

Can anyone explain why the billet cup gets stuck halfway during removal?
Would opening the tolerance on the threads help in any way?
Does the cast cup in cast mating component have a benefit?

Any thoughts would be appreciated.

Thank you,
mjgarrin

 

[CoryPad]

What are the exact materials? Heat treatment? Coatings? Lubricant? That will help answer your question.

 

[mjgarrin]

The material is 4140 Q and T. No coatings are used, no lubricants.

 

[CoryPad]

The castings are 4140? Are the threads in the as-cast condition?

 

[tbuelna]

An M85x1.5 thread is quite large in diameter and can be difficult to produce accurately. While you specified 6g/6h tolerances for the mating parts, did you check both threads separately to see if they were within tolerance? It is usually best to mill or grind a thread of this diameter, rather than single-point turning. PD runout is usually the most difficult thing to control with large diameter, small pitch threads.

The actual hardness of the material should not present a problem, but the heat treat process used might not provide sufficient dimensional stability in the finished parts.

Also, what is the length of the thread engagement? If it is more than about 5p, accumulation of pitch spacing errors can be a problem.

 

[3DDave]

Very small changes in shape will produce relatively large changes in torque. One of these threads could be free-running and the other sticks a little for a huge ratio between them. It sounds like one of them has same adverse taper as does the hole it is threaded into, reducing the clearance and possibly creating interference. As the parts thread together the taper acts to increase the clearance. If the cup is threaded in open-end first then the cup might be out-of round and as the cup comes out it has fewer threads to act on, increasing the binding force.

Anything else might be surface finish related.

 

[mjgarrin]

Thanks all, the thread length is roughly 28mm. You make a good point, I should check the thread tolerance and make sure its accurate. Should the tolerance on the internal and external thread be the same? Is there any advantages or disadvantages to say, using a 6g on the external, and a 4H tolerance on the internal?

Thanks again,

 

[CoryPad]

Are the threads produced before or after heat treatment? If before, then heat treat distortion could be a contributor to this problem. Is the cast part 4140 Q&T?

Making the internal thread tolerance class 4H likely will make your problem worse since you still have zero deviation associated with the H tolerance position but the 4 tolerance grade has a smaller range than the 6 tolerance grade.

 

[racookpe1978]

Very, very difficult to cast threaded parts - the shrinkage and distortion alone as the metal cools down from molten to final temperature mean every different part is going to be a slightly different shape and contour between every thread.

Heat treatment makes each distortion in each thread from the original mold, then the shrunk part, then the heat treatment movement and re-contraction even worse.

 

[tbuelna]

Thanks all, the thread length is roughly 28mm

I'd say that's your most likely problem. An engaged thread length of 18p is way too many threads. That many engaged threads is probably creating substantial interference. Try reducing your engaged thread length to something closer to 6p.

 

[3DDave]

Typical thread tolerances are deviations to one side of nominal. That is, if the threads are made perfectly, there should never be interference between them. Instead the tolerance will specify a minimum clearance with nominal and a maximum clearance from nominal; a tight tolerance thread should engage a loose tolerance thread or another tight tolerance thread without interference.

I suspect most threads are checked over small distances, so that increased engagement allows lack of straightness of the pitch diameter to result in interference.

It is still odd that one becomes more difficult to remove as the engagement increases, hence my thought about taper.

 

[3DDave]

TB - I see that this is what you are talking about:

http://www.boltscience.com/pages/screw8.htm

covering accumulation in mismatch in thread pitch.

 

[tbuelna]

3DDave- I once had a custom thin-section large thread diameter bearing locknut made that intentionally had the threads milled with a gradual increase in pitch to provide a locking function. It worked very well, but the engaged thread length was only around 5p.