Internal Thread Stripping - Revisited.... 1

[MikeLiljenquist]

We have a stainless steel ring, that has 30 threaded through holes. Into these holes we place set screws, and the set screws are used to apply a load to an inner ring...used for clamping another machine part.

The ring is stainless, and the set screws are nickel plated. These screws are replaced about two times per week. For the past year we have seen a problem with the internal stainless threads stripping. The screws are 3/8" Diameter. We are applying a maximum of 160in-lbs of torque each time we install these.

So, my questions are:

1. Is there a limit to how many times you can use a threaded hole? Fatigue?
2. Could reducing the torque, by doubling the number of screws solve the problem?
3. Would a different plating or screw material help solve the problem?
4. Any tips on how to solve this problem would be appreciated.

Thanks,

Mike

 

[Grunchy]

Wait a minute.. just exactly what is the failure mode here. Is it stripping when you remove and re-tension the bolts after about 1,300 times, or are the bolts falling out during sawing, or what?

If you don't have bolts falling out when sawing then your thread lock solution, whatever it is, is working fine for you. If you don't have bolts freezing in the tapped holes then whatever lubrication you are using to prevent galling is working fine too. If you have a thread-stripping problem because you are inserting and removing bolts 1,300 times in a row, then I'm afraid you've got a fatigue problem caused by (1) over-tightening which (2) causes the first couple of threads to yield, which will subsequently (3) relax and transfer load to the next set of threads until ultimately (4) all the threads have yielded, fatigued, and will simply shear through.

If you indeed have a thread-stripping problem caused by too many assembly/disassembly cycles, which is what it sounds like to me, then I just cannot see how adding more loctite, lockwashers, preload, etc. is going to fix it.

If anything, I would recommend using a LOWER tightening torque to prevent overloading of the threads in the first place, as long as the bolts don't back out then you're ahead of the game. As I said, a better bet is to investigate something like spiralock which distributes load over many more threads than the typical UNC/UNF threadform.

 

[DaleJ]

If it is the thread that is failing, I would also recommend using a roll form tap over a cutting tap. When a threaded hole is cold formed using a roll form type of tap, the material strength & properties are not removed by this operation. Cold forging or forming the threads make these types of threads much stronger than threads that have been produced with a cutting tap.

If you are looking for something to replace problem inserts, you may also want to look at using a friction drill tool to produce the hole and then tap using a roll form tap.

 

[kalvin]

I agree rolling the threads will increase the strength. It will make the person who is tapping the holes very happy because the tool life for cold forming taps is about 10 tmes higher. As for Spiralock, it is reuseable and according to NASA the torque tension relationship improves with each assembly and disassembly ( their testing was 60 times minimum). Also Spiralock makes wire inserts with their threadform on them and they install the same as Heli-coil.

 

[strokersix]

I would be very careful with the thread forming taps in 420 SS. It might be worth trying an undersize thread cutting tap and then final sizing with thread forming.

Is there any possibility of a different thread form? Acme? A different pressure angle would be less likely to gall.

I still say the simplest change to try is solid and replaceable inserts with threads inside and outside, provided there is room for it.

Mike

 

[MikeLiljenquist]

Thanks again everybody for your input regarding this issue.

At this point we have implemented the use of Saf-T-Eze Ni Anti-Seize. A small amount is applied to the first three threads, and threaded into the hole. We tested three different lubricant types; WD40, Multi-purpose Grease, and Anti Seize. For each run, the same operator and tensioning method was used. Anti-Seize required the least amount of torque to tension, approximately 20% less than the other two. Approximately the same amount of torque required using double the number of fasteners...

Today, we had another heli-coil come out with the tension screw when it was removed. It appears that heli-coils may not be suitable for repeated uses??

I looked at the internal threads for the remaining fasteners. On several holes, the second thread from the bottom is split and coming apart. I am not 100% sure what this type of defect is called, is this fretting?

Inspection of the fasteners, these have been used two times, revealed gouges, pitting, and some corrosion. I am talking to the vendor today, to follow up on what the base fastener material is, plating allowance, and finish.

I am looking for the types of inserts you referred me to in the above, m10x1 appears to be a problem and I have not been able to locate.

Thread engagement length may also be a factor after all. It appears that we have a maximum of one diameter of thread engagement available. This will be less if we go to solid inserts, because of the countersink.

Would it make sense to go to a smaller diameter tension screw?

Thanks for your help. If more information would be helpfull, or if I am doing something stupid....just let me know!!!

Regards,

Mike

 

[bhaden]

Have you tried coating the screws with other materials? Look at

http://www.flexbrite.com

for ideas.