Nuts and Bolts....
Nuts and Bolts....
(OP)
Hi
I'm currently in a debate with a co-worker about the hardness of nuts in bolts. I was taught (and found the reference in my old machine design textbook) that nuts should be made from a softer material for a number of reasons:
First to yield - maintaining integrity of bolt
First to yield/compress - first threads deflect, transferring some of the load to the following threads, more evenly distributing forces through each, smaller chance of stripping/crushing threads
His side of the story is that you want the bolt to fail first, but doesn't really back up his reasoning.
If a bolt is softer when it is pretensioned, the bolt threads furthest from the end will begin to deflect and the nut loses contact with the bolt. A much greater force is transferred to the remaining threads in the bolt, which inturn deflect and it is a domino effect.
Any insight would be very helpful.
SF
I'm currently in a debate with a co-worker about the hardness of nuts in bolts. I was taught (and found the reference in my old machine design textbook) that nuts should be made from a softer material for a number of reasons:
First to yield - maintaining integrity of bolt
First to yield/compress - first threads deflect, transferring some of the load to the following threads, more evenly distributing forces through each, smaller chance of stripping/crushing threads
His side of the story is that you want the bolt to fail first, but doesn't really back up his reasoning.
If a bolt is softer when it is pretensioned, the bolt threads furthest from the end will begin to deflect and the nut loses contact with the bolt. A much greater force is transferred to the remaining threads in the bolt, which inturn deflect and it is a domino effect.
Any insight would be very helpful.
SF
~~Bold are the ones who come over the line to fall~~





RE: Nuts and Bolts....
Another intereaction is an acme nut running into stall. It works best when the nut runs up against a harder element. Instead of galling/fretting you have polishing of the nut.
RE: Nuts and Bolts....
That's a good point you bring up.. I'll fuel the fire with it :)
~~Bold are the ones who come over the line to fall~~
RE: Nuts and Bolts....
I have responded to this thread in the structural forum.
RE: Nuts and Bolts....
~~Bold are the ones who come over the line to fall~~
RE: Nuts and Bolts....
The area of the nuts are much greater. Nut thickness standards have been drawn up on the basis that the bolt will always sustain tensile fracture before the nut or bolt thread will strip. If the bolt breaks on tightening, it is obvious that a replacement is required. If the thread stripping mode can occur, assemblies may enter into service which are partially failed, this may have disastrous consequences. Hence, the potential of thread stripping of both the internal and external threads must be avoided if a reliable design is to be achieved.
RE: Nuts and Bolts....
TTFN
RE: Nuts and Bolts....
IRstuff,
No, it isn't because bigger pieces are easy to find. The reason to prefer bolt fracture is that it is an obvious failure, which will require immediate corrective action. A bolt or nut can begin to strip without notice, and then be compromised when the joint is loaded during service. A delayed fracture may occur, with potentially disastrous consenquences.
RE: Nuts and Bolts....
~~Bold are the ones who come over the line to fall~~
RE: Nuts and Bolts....
Some machinery USED to be designed so that one had to keep checking bolt tightness during operation. But now we know better. We determine the highest "normal" loads on the bolts, use washers/hardened washers to keep the head from imbedding into the bolted material, and apply a preload that is somewhat higher than the service load. We make sure that a STRUCTURAL joint is truly a "rigid" connection, that there won't be a corrosion/temp. problem, and the bolts STAY tight.
Ever wonder why 99.9999% of connecting rod bolts stay tight without Loctite, lock wire, lockwashers or anything else?
As long as the nut withstands the applied preload, it's hard enough for the job.
RE: Nuts and Bolts....
"if something fails, it fails.. it doesn't matter which it is, but I think that a bolt failure is more instant and can not be forseen as easliy. "
This is completely untrue. I recommend you read some fastening references, such as those mentioned in Thread725-32122.
Metalguy,
Excellent point regarding con-rod bolts. I don't know why anyone would waste their time with safety wire (what a misnomer that is), etc.