Galling in large buttress threads 4

[HotStab]

Guys,
For the second time I have faced problems with large buttress threads. In the first event the thread was more than 25”ND and the parts to be connected were heavy (more than 10 kips). Nevertheless, the recent galling problem involved a bigger thread pair (28” ND) but lighter pieces (1200 lbf).
Lubrication was used, the assembly was careful, no side load applied. Even so, the pieces were seized.
The thread profiles were slightly different. The heavier thread had a 35 degree back side angle. The lighter one has a 35 degree. In both cases the materials were low alloy steels (4130, 4140 and 8630).
Did anybody have similar problems with this kind of thread?
Thanks, HCKS.

 

[MikeHalloran]

Lots of things can go wrong at that size.
I'd start by looking for pitch errors, between turns, and even within a single turn.



Mike Halloran
Pembroke Pines, FL, USA

 

[CoryPad]

How about surface roughness? What was the surface hardness for the mating parts? What was the lubricant?

 

[hydtools]

Any heat treat? Hardness?

Ted

 

[tbuelna]

HCKS-

I would agree with MikeHalloran. Screw threads at that large of a diameter tend to suffer from pitch errors and runout. The galling is likely more due to interference in the thread engagement, rather than axial loads due to component masses.

As a last resort, you might try adding a sight crown profile to the working flanks of the external threads. This will help keep the external thread flank point of contact consistent and centered on the mating internal thread flank. A crowned thread profile will also reduce the load capacity somewhat, but it does not sound like this will be an issue in your particular case.

Hope that helps.
Terry

 

[Screwman1]

I would look at the actual thread dimensions themselves: who did the actual dimensional checks on things like Pitch diameter? I have commonly seen large sized threads that are outside of tolerance but simply because they are large, it was assumed that they were OK. The two prime potential problems in this one are likely the flank angles and the pitch diameter.

 

[HotStab]

In the first event with the heavier pieces, the galling occurred despite the use of grease (Alco EP 73 plus). After a tough battle, heating and loading, the pieces were unlocked and disassembled. The threads were refurbished and reassembled with a special procedure. A solid lubricant (Molycote spray) was applied and a weight compensator was used to neutralize the axial load on the thread.
The buttress threads have a terrific load capacity on its work direction (flat to flat). But seems that their performance in the opposite direction (sloping x sloping flanks) is subtle, they prone to seize during assembly.
According to our conclusions, in the first event the villain were the heavy weight discharged directly on the thread flanks. The result after one turn it galled.
After the thread repair, the weight relief and lubrication led to good results.
In the second case, the shallower flank angle (45°) would be worst, but, regarding the much lighter pieces, the assembly did not cause big concerns at first. It surprises us that the same problem occurred in the preassembly at the very first turns with pieces lubricated with oil. The external piece shall be sacrificed - nothing could disengage the threads this time.

Ted and Mike:
By now, the hypotheses of some possible mistakes in thread milling (thread pitch, profile geometry or surface finish) still are on hold. They are very likely causes. However, considering the previous problem and the lessons we have learned from it, the load on the sloping flank is a possibility that I would like to assess.
The questions are:
• Is the sloping flank angle a galling drive for large threads?
• Are buttress threads more prone to galling in assembling than stub ACME threads?
I am inclined to answer yes for both questions. It would not mean that buttress threads are not useful and must be discharged, but may be they are more sensitive, shall be used with prudence and would require additional care on installation procedure, lubrication, coating, etc.

Terry:
If I understood your proposal of a sight crown profile, a negative angle at the work flank would help to centralize the threads, however the problem have occurred at the installation with load on the opposite flank. In this case, a negative angle would not be helpful. Am I right?

Thank you all, HCKS.

 

[Cockroach]

I highly doubt that machining would be an issue in regard to thread geometry, shops use an insert. Most likely if one of the threads, either male or female, would be shallow, they would be difficult to assemble or at least require larger torque values. Take a look at the flanks of the threading and ensure that the rag edge has been removed. This should be an automatic if buddy is using the correct insert, but it is worth an effort. Also, check the pitch diameter of the male (pin) end using a three wire measure over wires. The pin can then be used to gauge the box over the ways of the machine as a quick method to ascertain your problem. I would also emery the box and pin, just out of habit.

As a rule, I ALWAYS case harden threads by liquid nitration, specify that liquid quenching is not allowable during the liquid nitration process. I also state that it is acceptable to leave the oil on the piece following finalization of the hardening process. The hardened threaded surfaces make pick-up and galling much less likely AND removal of free iron from the surface of threading ensures that binding by rusting is mitigated adding to longevity of the connection. Make-up, monitor your torque input values, they should be fairly free spinning to the last few threads. Noting that property alone will dictate the state of machining quality. Finally, I strongly recommend a copper based lubricant such as Copper Coat. The lubricity of threads from the liquid nitriding process and copper coat are actually well suited for eachother.

Been liquid nitrating threads for just about twenty-five years, haven't had an issue yet due to engineering! Best of luck with it.

Regards,
Cockroach

 

[Cockroach]

What is your Buttress thread specification. I will run the numbers for you and let you know your geometry. Forgot to mention that in the text above. Is this American Buttress 7/45 degree or a modified Buttress?

You question above also, Buttress behave as Acme threads and are very similar in stress mechanic performance. They do approach a Stub Acme case when the pitches become finer over 12 or 14 TPI. But you can figure that out just cranking out the mathematics.

Let us know your XX.XXX - XX Buttress-XG thread specification, I assume single start, right hand.

Regards,
Cockroach

 

[Screwman1]

You should try to have one part harder than the other. Threads with the same hardness are prone to galling on assembly. Since you said that you reduced the axial loading on the parts during assembly and didn't even get them together before galling, the only possible cause is geometry and that will be related to improper mfg. tolerances.

 

[MikeHalloran]

Shops use standard inserts for turning standard V-threads.

However, we seem to be talking about milling/whirling buttress threads, which requires uncommon cutter shapes.
... perhaps two separate cutters, because I'm not convinced that one can generate the desired internal and external threads with the exact same cutter.
... or maybe you could use the same cutter, but I think you'd need to whirl the internal thread twice in order to generate the proper axial clearance.

There are probably good reasons for not revealing any machining details here, but I think you need to go over the cutter design and the machining process with all the stakeholders, in excruciating detail.



Mike Halloran
Pembroke Pines, FL, USA

 

[Cockroach]

I recently cut 4.375-12 Buttress-2G threading, single stard, American Buttress 7/45 degree using an insert from a shop supplier. This was cut on a manual, I had the guys measure the pitch diameter over wires. Following liquid nitration, the prototype went together without incident and was tested to 10,590 psi using a glycol water medium, and charted for fifteen minutes. The third party inspector API certified the assembly.

Typical, nothing out of the ordinary. So I am confident and stand behind my statements.

Regards,
Cockroach

 

[HotStab]

They are standard buttress threads 7°/45° per ANSI B1.9 with 3 TPI, class 2A/B.
Unfortunately, the threads did not receive appropriate coating (just phosphate) and neither hardness treatment.

 

[tbuelna]

HotStab-

You did not mention what the engaged thread length was. But if you do a simple P/A calculation of your buttress thread joint, I'm sure you'll find that the calculated contact stress is extremely low. With such a large thread diameter, the total contact area is quite large. In fact, the contact stress from a simple P/A calculation would likely be well below the galling limit of annealed alloy steels. If so, this would indicate that the thread flanks are edge loading, and/or not load sharing properly from pitch-to-pitch. The flank crowning I suggested would help with the edge loading problem. As for load sharing from pitch-to-pitch, this can be addressed by improving the pitch accuracy of your threads.

Also, if you have a long thread engagement, the threads closest to the load will naturally be more highly stressed. You can address this problem by using a carefully controlled variation in pitch along the length of the threads. This is easier to do with thread milling of the internal threads. Most CAM programs now have a feature that allows this to be done on any good 3-axis mill.

Good luck to you.
Terry

 

[mfgenggear]

I agree with every one above. all are very good suggestions.
thanks every one for sharing.

I see multiple issues!!
a) have gauges made to verify acceptance. go & no go, this will verify size & form. if budget allows.
pins will not verify form. Pins are used for one or two parts. then manually verifying form."a mating part will work"
use bluing to verify the contact points of each mating parts.
b) Hardness is a factor to prevent any fatigue issues. I like cockroaches suggestion.
c lubrication ? I would suggest discussing with sales engineers.
d) ground threads are more exact then single pointing. if exact form is required then it should be ground.

just from my experience, any parts with axial movement will gull if not heat treated. I also suggest case hardening.
how many cycles?

HTH

Mfgenggear
if it can be built it can be calculated.
if it can be calculated it can be built.

 

[HotStab]

Guys,
I really appreciate your comments. They did not change my basic concerns yet. However, it seems that this problem may have more than just one root cause and even the type of thread might not be a relevant factor at all. To be sure about it, a dimensional assessment of the threads shall be addressed.
Thank you all, HS.