ASTM A193 GR B7 bolts keep breaking
ASTM A193 GR B7 bolts keep breaking
(OP)
Hi,
I'm new here. What a great resource.
I have a machine shop and we have been making some bull gear splice bolts for one of our customers. The material is 4142 G&P Q&T BHN 255/321 ASTM A193 GR B7.
These bolts keep breaking and I am looking for an alternate material. These studs are 1 3/16Ø by 8 1/2", turned on each end to 1 1/8" and threaded with a 12 pitch. The bolts have broken in various places but never on the bigger shank portion of the stud. Sometimes they will break at the end of the thread, but mostly they are breaking where the side of the nut meets the plates they are fastening. My customer has requested we use a material similar in tensile strength to a grade 5, even though I have told them that these bolts will break before the B7 bolts, but they insist on trying the grade 5 material, for which I am going to use 1045 material. The application sees some heat, but I'm guessing no more than 500 degrees. They started torquing the bolts at 650 in lbs, and have tryed reducing the torque to 500, but they are still breaking the bolts. There is obviously something wrong with the dryers, because they don't break on all of them. But they want to solve there problem with bolts that wont break. Any ideas on what material to try next? I would like something that is readilly available.
I'm new here. What a great resource.
I have a machine shop and we have been making some bull gear splice bolts for one of our customers. The material is 4142 G&P Q&T BHN 255/321 ASTM A193 GR B7.
These bolts keep breaking and I am looking for an alternate material. These studs are 1 3/16Ø by 8 1/2", turned on each end to 1 1/8" and threaded with a 12 pitch. The bolts have broken in various places but never on the bigger shank portion of the stud. Sometimes they will break at the end of the thread, but mostly they are breaking where the side of the nut meets the plates they are fastening. My customer has requested we use a material similar in tensile strength to a grade 5, even though I have told them that these bolts will break before the B7 bolts, but they insist on trying the grade 5 material, for which I am going to use 1045 material. The application sees some heat, but I'm guessing no more than 500 degrees. They started torquing the bolts at 650 in lbs, and have tryed reducing the torque to 500, but they are still breaking the bolts. There is obviously something wrong with the dryers, because they don't break on all of them. But they want to solve there problem with bolts that wont break. Any ideas on what material to try next? I would like something that is readilly available.





RE: ASTM A193 GR B7 bolts keep breaking
What was the tempering temperature of the stud stock?
Why are you using 12 pitch threads?
Fist off you are not tightening the bolts properly if it's breaking in the first thread of the nut. You studs are apparently failing from fatigue as the bolt hasn't the preload to carry it pass it's working load.
Gr B7 is a much better choice than Gr 5 if any temperature is involved.
RE: ASTM A193 GR B7 bolts keep breaking
Is a torque wrench being used?
How are the fasterns being assembled?
are the thread Pitch Diameters or Pitch correct?
to verify correct threads are go & nogo gages being used to verify size & pitch.
These studs should be torqued upto 100 ft lbs
650 in-lbs is only 54.1 ft-lbs
see this web site nice informatiom on material & tensile properties.
http://w
RE: ASTM A193 GR B7 bolts keep breaking
The customer says they changed from a coarse pitch to a fine pitch to gain strength at the root of the thread. (a 12TPI doesn't cut into the stud as deep as a 7TPI. We use thread wires to check the pitch diameter. They are buying grade 8 nuts from a local bolt shop like Fastenal. The temerature inside the dryer is about 500°, but where the bull gear bolts go they only see about 200°. They are sliding the stud through the hole, running a nut up one side of the stud, holding that side with a wrench, running the nut up the other side of the sted and using a torque wrench to tighten the nut to 680 ft lbs. They have tried different torque ranges, from 500 up to 680 ft lbs. Why do you say they should be torqued to 100 ft lbs, max? Maybe that's the problem right there. As for the temperature the steel was tempered at, I have no idea. We bought standard 4142 G&P Q&T BHN 255/321 ASTM A193 GR B7, from our steel supplier. We sent them some of the failed studs and they checked the material properties and said all was OK.
RE: ASTM A193 GR B7 bolts keep breaking
I recommend you contact a engineering consulting firm.
on the maximum torque values, depending on the material & hardness values required.
or maybe some one here can assist?
RE: ASTM A193 GR B7 bolts keep breaking
What is happening to the Gr 5 OEM bolt?
Why was the nut changed?
For situation such as yours a nut of equal or slightly lower hardness would be called out? An ASTM A194 2H would do the job.
Why was the torque of 680 ft lbs chosen?
This give a bolt stress of approximately 35,000 psi which is some what low. I would use a bolt stress of 50,000 psi which is equivalent to 1148 ft lbs torque.
After the completion of the initial tightening are the bolts checked again?
On our dryers when we change a gear the bolts are initially tightened and dryer is run for a period of time and the bolts are retorqued.
Can you or your customer post pictures of the fracture surface of the failed bolts?
Again I believe if your studs are failing in first thread in the nut, indicative of fatigue, means only one thing the bolts are not tight enough for the job. They are few other mitigating causes of bolt failure but most are caused by or work in conjunction with a loose bolted connection
RE: ASTM A193 GR B7 bolts keep breaking
I assume you are single pointing this thread?
Could the break be induced by a sharp V?
RE: ASTM A193 GR B7 bolts keep breaking
So I don't think they are retorquing after the studs equalize temperature. I tend to agree with unclesyd. They are not getting them tight enough. I think the nut was changed to a fine pitch (12tpi) in place of the coarse pitch (7 tpi) with the thinking that there is a smaller root diameter on the deeper thread, thus a smaller effective diameter, making the bolt weaker. Again, we are no longer using the grade 5, but we have gone to a 4142 G&P Q&T BHN 255/321 ASTM A193 GR B7. I'll try to get some pictures of the failed bolts and post them here. Thanks for all the help.
RE: ASTM A193 GR B7 bolts keep breaking
RE: ASTM A193 GR B7 bolts keep breaking
680 ft lbs for a Gr 5 would be right for a lubricated, k = .15, fastener.
There are a few other possibilities like the nut landing areas, cleanliness of the mating parts, and looking the hot bolting aspect of the assembly.
RE: ASTM A193 GR B7 bolts keep breaking
RE: ASTM A193 GR B7 bolts keep breaking
Are the bolts coated? This will affect the friction coefficient (and the required tightening torque).
Using the simple torque-tension equation:
T = KDF
where
T = torque
K = nut factor (use 0.2 if you have no better data)
D = thread diameter
F = force
For a grade 5, the proof force is 74 000 lb. A conservative preload is 75 % of proof force, or 55 500 lb.
T = 0.2 · 1.1875 in · 55 500 lb
T = 13 200 in·lb or 1100 ft·lb
It appears your tightening torque (and preload) is insufficient, which cannot withstand the applied forces, which causes fatigue cracking and failure of the bolts.
Regards,
Cory
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RE: ASTM A193 GR B7 bolts keep breaking
I will have to agree with you that in respect to a B7 a bolt stress of 50,000 psi is probably low for this application. I would not normally go above 65,000 for a single point thread so this is probably why I was shy in that respect. I have seen fine threads loaded to a 90,000 bolt stress at which point I holler lets go more smaller bolts.
mikejeffers,
Yes you can go up on the preload without any problems other than tightening. At the higher torque values make sure you tighten the studs is stages.
Are you replacing the studs in set or one or two at a time?
Do you have room for a torque multiplier?
See if you can get picture of the fracture surface head on with oblique lighting if possible. Looking the existing pictures the nut is coming very close to the thread runout so make sure you check the relative location of the nut being tightened. The dead end doesn't really matter but there should be a little free running thread.
As stated above it looks like fatigue even with the poor pictures.
One other thing.
Do you have a good fit on threads?
And another.
Do you have a good landing area for the nuts?
It should square to bolt axis and smooth.
RE: ASTM A193 GR B7 bolts keep breaking
You may be right about sticking with a lower stress for the cut threads; but these are big bolts, and the finish ought to be inspectable. The Grade 5 fasteners I thought were plain old rolled-thread studs, not cut threads. I'd still torque the bejeezus out of them; worst case is they still fail, and your solution of more smaller bolts is the direction I would go too.
Mike,
Syd is being conservative, I'm probably being agressive (but I will routinely go through a lot of bolts, they are throw-away items to me...and I doubt you think so about the monsters you are machining).
But at least 3 or 4 of us here think the existing torque spec. is low by a factor of about 2.
RE: ASTM A193 GR B7 bolts keep breaking
Thanks everyone for your help.
RE: ASTM A193 GR B7 bolts keep breaking
The value for the B7 of 1148 ft lbs is a dry fastener and it needs to be reduced if lubricated.
Again I would emphasize that you need to check the location of the nuts in respect to the thread runout as in you pictures it looks very close. If your nut gets into this area while tightening your torque value goes out the window.
Keep us informed as we are making something like a WAG and if we can get a little feedback it could help us in the future turning a WAG into SWAG.
RE: ASTM A193 GR B7 bolts keep breaking
I reiterated the fact about the lube to my customer. We have also lengthened the threaded length and corresponding shoulder to give a little extra room for the nut to travel.
We picked up the nuts from our local supplier, and we have found that these are also a part of the problem. About half of the 64 nuts we picked up, 36 of them are bad. The faces are not square with the thread. Of the 36, they range in face runout from .015 to .060. This is unacceptable. And could be a major contibutor to the problem.
RE: ASTM A193 GR B7 bolts keep breaking
Getting the system squared up and with the proper preload a lot of your problems should go away.
I keep asking questions.
Does your gear use alignment pins at the split line?
The reason for this question is that our dryer gears and tires tend to have a lot of relative movement. Has nothing to do with our overloading same, so I've been told.
RE: ASTM A193 GR B7 bolts keep breaking
RE: ASTM A193 GR B7 bolts keep breaking
http://www.superbolt.com/
PS:
I forgot to mention that your studs show attention to detail especially at the translation from thread to body.
RE: ASTM A193 GR B7 bolts keep breaking
I.e. the bolt may be too stiff in its current configuration, and the bolt joint may lose a lot, maybe even all of its preload if things like nut/mating face irregularities or thermal expansion take place. I'm seeing a heavily shock-loaded environment, where the vibration may be playing a part in shakedown and loss of preload too; this too points to doing whatever you can to push up the preload and try to maintain it high. It may make sense to try putting 1-1/8 Grade B7 roll-threaded studs (thru some bushing material to fill the holes if the bolts are also used as alignment pins for the two halves), and torquing those studs to the full rated stress of B7 material (85000 psi) using a bolt extension method of checking preload.
And...how often do the bolts get checked and re-torqued?
RE: ASTM A193 GR B7 bolts keep breaking
appling the proper torque
http://www.zerofast.com/torque.htm
the author explains why some bolts break when appling torque. all due to the coefficient of friction.
I would recommend reading this article.
I was surprised. at the beginning of this topic I was way off on my est. torque value. Smart people on this board corrected my mistake. and I learned some Interesting facts.
Mike
from the picture you posted it appears to me also that your bolts possibly where lacking thread length. Thus possibly when torque caused failure.
either way I never saw such a good topic as this. :)
hope your issue has been resolved.