Nuts and Bolts....

[StaticFish]

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
~~Bold are the ones who come over the line to fall~~

 

[plasgears]

Also, the softer nut can be upset to act as a locking element. Further, how would you like to be confronted with soft studs in your car wheels? They are hard, and the nuts are softer and replaceable. The softer nut will not tend to gall against a harder stud.

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.

 

[StaticFish]

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~~

 

[desertfox]

Hi Staticfish

I have responded to this thread in the structural forum.

 

[StaticFish]

So does all of this mean that if a bolt is going to fail by shear then you'd want the bolt stiffer, and if it's going to fail in tension then you'd want it softer?
~~Bold are the ones who come over the line to fall~~

 

[boo1]

When specifying nuts, washers and bolts it must always be ensured that the appropriate grade of washer or nut is matched to the bolt grade. See ASTM A563 for suitable nuts by grade. Proof strength of a standard nut generally is greater the the fastener. One would pefer the bolt failure to the nut failure.

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.

 

[IRstuff]

But I guess the question is why one wants the bolt to fail. Is it because you'd wind up with bigger pieces, hence making it easier to find them afterwards? TTFN

 

[CoryPad]

boo1's comments are industry standard - bolt failure is preferred.

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.

 

[StaticFish]

A bolt or nut can begin to strip but still hold it's integrity. In a case where failure of equipment can become disasterous, or points of high wear/fatigue, these bolts should be checked on a regular basis. What it boils down to is that 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. A nut stripping can be seen by directly looking at the bolt/nut combination or by the fact that parts are not holding together as well as they should (gaps, vibrations, etc)
~~Bold are the ones who come over the line to fall~~

 

[Metalguy]

StaticFish,

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.

 

[CoryPad]

StaticFish wrote:

"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.