Cotter pins or safety wire are definitely the best choices for keeping nuts from vibrating off. Using pre-drilled bolts only works however, when you know the exact length of the bolt, and getting bolts drilled at the length YOU need may not be possible. That's why you normally have to drill bolts yourself when using safety wire.
What about locktite? The red stuff is pretty strong as well. Or, nyloc nuts. I've seen nyloc nuts in grade 8, but don't know about 9.
Actually, I'm not at all familiar with grade 9. I do know that grade 5 bolts are not heat treated, but grade 8 are.
Any welding (or even brazing), will kill the performance of a heat treated bolt. Its just a bad idea, and something to be avoided at all costs.
From the fact that this was welded in the past, I am assuming that easy removal isn't an important factor, so I would go with locktite over nyloc.
The red stuff is so strong, that in trying to remove the nut, you usually end up twisting the bolt apart before the nut comes off, unless you apply a good deal of heat first.
Because of that, unless you need resistance to direct torque while operating above 200F, the red threadlocker is just about as strong as a weld on a grade 2 bolt.
chet , rlitman , very good response . i might also add because we use them , we use drill bits for drilling safes you can drill thru ball bearings so should be able to drill thru most bolts that you can get . good luck . willie
for grade 8 and above stover nuts or crush nuts (same thing different name, there are also different names) and red loctite, make sure the bolts are cleaned off with brake clean or some other solvent that leaves no residue and make sure they are completely dry. you are good to go. expect to pay quite a bit for grade nine. but i am sure the problem on your desk has an elevated cost as well.
darren
If all of the above mentioned products & procedures fail [I think the self locking nuts and red [PERMANANT] Locktight will work] fail, and You have no alternative to welding the nuts, AND ONLY AS A LAST RESORT I suggest You put more nuts on the exposed threads of the bolt untill You get within 2 threads the end, torque then all to spec, and weld only the extreme bolt end 1 tack at a time never allowing that last nut to become too hot to touch with a bare hand.
Thanks for all of your advice!
My problem is I need to find out why they snapped. I understand that they became more brittle with the heat from welding however my boss & upper management will not accept that answer unfortunately. They need proof and for me to show them with numbers or calculations with legitimate specifications. I believe I was given this project because we may have done this on several peices of equipment (not sure) and they are wondering the amount of stress they can take. If it is shown that this will happen to most of the time it is probably worth changing. It is easier safe than sorry but not in a manufacturing world when time is money as everyone knows...
A simple test could be to use a much lower grade bolt and use the same welding of the nut. A softer bolt would be less likely to snap and the weld may induce just enough heat treat to reach the same performance needed. I understand the upper ups will probably not even want to entertain this idea. Just a thought.
Another thought, looks to me like the tack weld may have drawn the nut enough to cause a less than tight condition at the point of contact with what ever it was threaded into. May not be the issue that is being considered. That is if it was tacked after tightening, which is almost obvious. A loose bolt fastened in that manner is more prone to vibration cracking as the shear plane is right under the nut.
I am thinking all this since it did not snap at the weld point but rather under the nut.
One other thing, I though heating a hardened bolt with improper cooling would soften it not make it harder.
Mooseye: With regard to weather welding will make the bolt softer or more brittle depends on the type of material the bolt is made from, and how fast it cools through the criticle temperature. Welding a part on a large assembly can cause the HAZ to cool quickly enough [due to the large mass of cold material] to form martinsite, the hard & brittle stage. If the part cools slowly enough it may be softened as it has been heated above the temperature it was tempered at.
So if what you want to do is pove welding weakens bolts, why not take some bolts and test them?
It seems that you could put unwelded and welded bolts into a tension testing machine and see what they break at. Naturally big bolts would need a big machine but you could start out with smaller diameters to start and it would be relatively easy to do.
Dave's response reminds me of a memo from caterpillar once that said the only acceptable locking application for high strength severe duty bolt/nut application was double nutting the bolt.
also you did not mention if any washers were in play in the connection.
i would check caterpillar as they are one of the best bolt manufacturer's in the world.
darren
High Strength Bolts are heat treated...this is partially where they get their strength. By welding the heat from welding has altered the heat treating. Call any bolt mfg and askl them if they recommend welding on high strength bolts. Alternative methods of fastening with welding is suggested. If you only have access top the bolt before fabrication, find a way to trap the bolt head with plates or something to retain the bolt in the assembly.
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