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..