Internal Thread Stripping - Revisited....
Internal Thread Stripping - Revisited....
(OP)
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
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





RE: Internal Thread Stripping - Revisited....
So I'll guess-you have soft 304 or 316 SS, and enough threads (at least one full diameter). Your torque is low, so that leaves galling as the number 1, 2, 3, 4, and 5 top guesses!
Try a thread anti-seize compound, or use hardened heli-coil inserts or something like that.
RE: Internal Thread Stripping - Revisited....
Material Type is 420 stainless, HRc38 and more than one full diameter of thread engagement. Does this change anything? Ni Anti-Seize was being used "once-in-a-while."
Thanks again,
Mike
RE: Internal Thread Stripping - Revisited....
I had on case where a front wheel came off a security guards truck on his way home. He though some union people were trying to kill him-was pretty shook up.
I was investigating, and found out he had just rotated the tires/wheels around, doing it himself. He even had looked up the wheel-nut torque. Swore he had used a torque wrench.
Everything pointed to insufficient torque-all of the other nuts were very loose. I asked him to bring the torque wrench to the shop where we were examining/testing the truck. He called his wife who immediately brought it over. Problem was immediately obvious-he used a small wrench calibrated in in/lbs.!!!
I tried hard to keep some of the sarcasm out of my report!
RE: Internal Thread Stripping - Revisited....
http://www.cartech.com/
Click on Technical Information then Alloy Name and enter Gall-Tough
RE: Internal Thread Stripping - Revisited....
Mike
RE: Internal Thread Stripping - Revisited....
One more piece of information: The screws are used to load a ring that presses a piece of sheet metal. The sheet metal is stretched about 1300µm. It is the force of the tension screws that provides the stretch.
I checked on the use of heli-coil replacement threads. It seems that we are having trouble with them also. Maintenance believes the heli-coil failure is due to being used in through vs. blind holes.
About two years ago the number of set screws were reduced from 60 to 30. So, we installed one of the rings using 60 screws. The thought here was that by reducing the torque, we could extend the life of the threads. I was somewhat suprised that the final torque readings were only reduced by about 20% from 100in-lbs to 80 in-lbs. Does this make sense? It seems that it should be closer to a 50% in torque?
The internal threads have been used over 1300 times. With approximately the same load being applied each time. The screws are replaced every 5th installation. Over time, will threads just wear out? Especially considering WD40 or no lubricant was used for about 80% of the current thread life?
Thanks again for your help.
Mike
RE: Internal Thread Stripping - Revisited....
RE: Internal Thread Stripping - Revisited....
people and see if you should be using
some special compound if you are centain
that they are not failing by gauling as
was suggested earler. You would always
use the same torque on the screws and
would increase your safety factor. Is the
problem how you tighten the screws, ie. do
you tighten 3 at 120 degrees and then
move one 180 from the first and then 120
degrees etc. If you progressively just
go around the ring, you are imposing different
forces on the shaft or an imbalance.
RE: Internal Thread Stripping - Revisited....
I will contact Loctite on Monday. Thanks for the help again.
Do you think increasing the number of screws will help solve the problem, or is the torque already so low that reducing it further will not help much?
Thanks,
Mike
RE: Internal Thread Stripping - Revisited....
are doing. Are the setscrews in an
external collar locking an inner ring
as your first implied? If so, why are
you stretching an outer ring? Are you
trying this method to expand the outer
ring? Using the crisscross method of tightening,
you should only have to torque in 1/3
increments to reach the final torque value.
For example if your final torque was 36 ft-lbs
torque the first sequence to 12 ft-lbs,
the second to 24 ft-lbs, and then 36 ft-lbs
for the last time around again using a criss
cross pattern.
RE: Internal Thread Stripping - Revisited....
These are set screws for tensioning an ID saw blade. I was being cautious, because I am not sure what the company rules are as far as talking about specific processes. But, at the same time we need to try and solve this problem.
The blades are tensioned by turning the set screws and measuring the runout of the blade. When the specified runout is reached, they proceed to the next screw in series.
We stretch the blade about 1250µm initially, and then after several cuts, retension in 50µm increments. This continues over the life of the blade. The blade is only about 120µm thick.
The blades are clamped in the main "ring", and inside the main ring, is a tension ring. The set screws go through the main ring, and contact the tension ring. The tension ring is the pressed into the blade, and the blade is "stretched."
I apologize for being vague before...
Does it make sense that measured torque values would only fall by 20% using 2X the number of screws?
Would reducing torque by 20in-lbs, contribute much to thread life extension?
RE: Internal Thread Stripping - Revisited....
Probably would help, even tho your orig. torque is low. But you are using set screws, so nearly all of the friction generated by that torque goes into the threads. With hex-head bolts, up to 90% of the friction is right under the bolt head-relatively little on the threads. You lose a little bit to the contact point, but the diameter is probably small.
Since your thread friction is actually far higher than it would first appear (at least to me), it is very important to *always* use a good lube on them-something WD40 surely is not.
RE: Internal Thread Stripping - Revisited....
Second time around at 2/3 the torque value
Third time around at full torque value.
Assuming you are using a calibrated torque wrench
and tightening in a crisscross manner.
4 times around maximum at 1/4 intervals might be ok
as well. Is the collar so thin than you are getting
false value?
RE: Internal Thread Stripping - Revisited....
RE: Internal Thread Stripping - Revisited....
I agree with kalvin, check out spiralock. Regular threadform fasteners concentrate all their stress on the first few engaged threads even if the threads are 1d long, and what you wind up with is a fatigue failure that progresses through the tapped hole a few threads at a time. Eventual failure is guaranteed. Spiralock-type threads spread the load much more evenly over the threads, I bet you would at least double the life of those tapped holes.
RE: Internal Thread Stripping - Revisited....
It seems some of my posts go to limbo
and then appear a day or too later.
I think vibration is your biggest killer
and I would torque as high as possible so
the the load does not go from loaded to
unloaded. I wonder if double setscrews would
help. Are you certain that these do not loosen?
Without having a drawing and only relying on
your info. Maybe even stretching oftener would
ensure having load on the setscrews at all times.
Much of the other advice seems plausible as
well. The spiralock for one time fastening,
I like, but not certain how it works for multi
tightening. But certainly try all of the options
cause what you are doing now isn't working. I
assume you have several machines. What kind of
points are on the setscrews? Are you turning
the setscrews angularly or by torquing? Are you
replacing the setscrews after so many tightenings?
There are so many unknowns which make this interesting
and not simple. I am still trying to imagine if
each point in creating a temporary flat from screw
to screw to understand the difference in torque
when doubling the number of screws. Please keep
us posted what works. The fact you are stripping the
internal threads kind of implies that maybe you
should be using helicoils as the material seems
to be not hard enough to withstand the torque
forces. Is the blade getting hot and expanding
and loosening the torque values. The every two
weeks represents how many tightenings? Is the blade
acting as a borer or a debarking action? I am really
leaning toward the helicoil idea because of the cyclical
vibration. Have you used any of these before?
We use these on machine gun applications and seem
to hold up well in aluminum parts which are relatively
soft and see much vibration.
RE: Internal Thread Stripping - Revisited....
The set screws are m10x1, not the standard 1.5. The set screws we use have a non tapered flat tip, so the diameter is the same as the minor diameter I believe...Nickel Plated, I inspected several of the new screws and there are visible surface imperfections, pits, etc. Maybe this is also a contributor to our problems. Heat seems to be very minimal, and very localized.
Two weeks represents about 12 retensions of the blade.
I went to the Emhart Teknologies website to look up the helicoil info, and there is a Screw-Lock Helicoil that looks promising.
Our application is for use in thru-holes. Our maintenance department believes that this is why the helicoils do not appear to be effective. Currently, helicoils are only lasting approximately 2 tensionings.
Are there any good text/reference books on this subject?
As of today, we are using a small amount of anti-seize on all of the tension screws. Threaded holes are being cleaned with a nylon brush during blade changes.
Your help is greatly appreciated,
Mike
RE: Internal Thread Stripping - Revisited....
Hi There,
If as you have said you are getting Galling or
Thread Stripping then you should LOOK at the thread itself.
First is the thread in the Stainless Steel made by
Thread Cutting or Thread Forming.
Thread Cutting is making thread using a TAP.
Thread Forming is when thread is Formed using Form Tap.
The Thread made by the Forming method is superior in
Strength--- Surface Finish in comparison to cut thread
threads.
Also the Screws you use Should be looked at.
Have these made by Forming Method and if made from
a Material that can be Hardened and Tempered this will
if done properly will be VASTLY SUPERIOR to any standard
bolt no matter what Coating it Has which unless you like
Colours most coating (except for corrosive preventative)
has LITTLE ADVANTAGE when the BASE METAL UNDERNEATH IS
NOT SUITABLE for the job it is required.
Probably chat to some TOOL & DIE SHOPS and this is
standard practice for them as alot of this work is done
by these overhere in UK.
Using a lubricant is okay bur Basically your Threads must
be a good finish (flawless...no pitting... complete form...
not too coarse --for M10 a 1mm pitch is good as you stated)
and if both are approx 40--45 HRC in Hardness and must be
STRESS RELIEVED before Hardened (if this is done) you should get good result's.
Regard's
SBRA.
RE: Internal Thread Stripping - Revisited....
not working is keenserts which are
solid inserts and the ods are much larger
than helicoils which may be a plus.
RE: Internal Thread Stripping - Revisited....
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.
RE: Internal Thread Stripping - Revisited....
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.
RE: Internal Thread Stripping - Revisited....
RE: Internal Thread Stripping - Revisited....
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
RE: Internal Thread Stripping - Revisited....
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
RE: Internal Thread Stripping - Revisited....