Are hardned nuts necessary?

[stuckmotor]

I don't think I've ever seen a bolt pull the thread out of a nut so, is there an advantage of putting a hardened nut on a hardened bolt? Is it OK to save a few cents and use the mild steel nuts?

 

[greasemonkeyok]

I guess it would make a difference of whether the stress was perpendicular to the length of the bolt and the nut simply holds the bolt in the hole (like a lynch pin), or whether the primary stress was along the axis of the bolt pushing against the nut.

 

[Tractor Seabee]

Sounds like a nickel/dime routine to me. The bolt is worth many times the cost of the nut. Simple, if you need a graded bolt for the application, why would you compromise the assembly with an ungraded nut. You accomplish the same result using ungraded parts to start with.

Ron

 

[DFB]

You need to use a correct grade nut to achieve proper tightening. Torque values vary based on grade strength. A lesser value nut has a higher potential for thread stripping. For tightening value matched bolts and nuts the bolt shank is engineered to shear before the thread strength gives out.

Not that it cant be done especially for some non critical applications, but like it been already been pointed that you would not be getting full value out of using the higher grade of bolt, the threads of the lower grade nut being the weakest point.

 

[Leejohn]

Try it on a rod bolt in a engine, that well give you the answer.

 

[crazyal]

I use them all the time on the sheer bolts on my snow blower. I can get grade 2 lock nuts easy as well as the grade 5 bolts at the local hardware store vs the grade 5 lock nut that I can only get from a tractor dealer. Since it's strictly a sheering action the nut plays no roll. In other applications you could find out the hard way that the threads in a nut will pull out before the bolt breaks.

 

[hosspuller]

DFB has given you a good answer. Full tensile loading needs an equal nut unless you've got more than 1 1/2 diameters of thread engagement. example 1 inch grade 5 bolt should have at least 1 1/2 inch of threads engaged. Problem with higher strength bolts. The first thread carries much of the load. If it fails, the second thread is overloaded, it then fails and the third thread is overloaded in a cascade failure.

 

[stuckmotor]

Thanks to everyone for your answers.

 

[Zebrafive]

Hardened nuts will not "round" as easily as lesser grade nuts. If that is a consideration.

 

[Soundguy]

i agree.. i prefer the same grade nut as the fastener i'm working on.. even if only for prevention of rounding.. but especially to prevent pull out for axial loads..

 

[SPYDERLK]

I don't think I've ever seen a bolt pull the thread out of a nut so, is there an advantage of putting a hardened nut on a hardened bolt? Is it OK to save a few cents and use the mild steel nuts?

There is a good reason for what you have noticed. Think of the thread roots in the bolt vs the nut. The nuts threads make a longer path and hence provide more shear area per turn than do those of the bolt. Forget stripping ... Once youve got a nut as tall as the minor diameter of the bolt you have a nut that will pull on the bolt til it snaps. If you use an equal grade nut you are more likely to snap the bolt a little sooner because the bolt stretches elastically during tension. If a nut of equal strength steel to the bolt is used, the elastic tension/stretch of the bolt forces huge stress on the ~1st thread of engagement. The nut thread, have superior area, is stronger, more rigid, and can damage the bolt here. - Very slight damage to that 1st bolt thread but much more danger caused by the abrupt stress riser in the root of the thread/minor bolt diameter. This is where you see a bolt break.
,,, OTOH, use of a nut the next grade down acts to spare the bolt stress by allowing its elastic stretch to go a little further into the nut. -The 1st threads in the nut displace plastically very slightly to accomodate this, giving more equal thread loading/sharing within the entire engagement. This works great for initial and repeated assembly with the condition that the nut load face is always the same. This complied with you will find that repeated full [bolt grade] torque fastening with a slightly softer nut will endure through more tightenings than if a hard nut is used. I have seen this occur on a puller I devised for a special task

I use Gr5 nuts on Gr8 bolts. Std nuts on Gr5 bolts.
larry

 

[Soundguy]

i usually change nuts at least, if not most fasteners on critical items if I can. no need to save a buck and have soemthing hundreds of dollars fly apart ( think mowers.. etc.. ) head studs / nuts.. etc.