ASTM A193 GR B7 bolts keep breaking 4

[mikejeffers]

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.

 

[btrueblood]

Oh. Duh. I was going to try and tell the OP to use a bolt extension method for checking preload, rather than torque wrench or turn-of-nut methods. But, when I went to calculate the bolt stretch, I realized I'd overlooked that the bolts are turned down from 1.312 OD to 1.125 on the ends. Presumably the majority of the bolt length is at the larger OD. This significantly affects the K-factors of the bolt joint, and I think may be affecting the bolt preload if there is any differential expansion (due to heat) between the gear halves and the bolts... Are the bull gears steel or cast iron?

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?

 

[mfgenggear]

Please see this link below
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.