Hardness in design? 1

[dummiboi]

Hi all,

I'm an engineer fresh out of college and I'm doing some design work.

I need to figure out if the ABMA standard for locknuts is sufficient for my application of it or if I need a hardened nut.

The standard that this locknut is going by is that it has to have a tensile strength of atleast 25000PSI and Rockwell B hardness of 82.

In my application I will be tightening the nut down to 13.5 ft-lbs and applying a 1000 lb load to it.

I know that with the given tensile stress it should be able to handle about 3000 lbs of load (calculated from the futek bolt calculator). But what about the hardness? How will that affect the performance?

What I'm worried about is the threads in the nut deforming and eventually stripping from the ammount of load.

Thanks much!

 

[MikeHalloran]

The threads in a nut are _supposed_ to deform, slightly, so as to distribute the load among all the threads.

If the nut is too hard, all the load will be carried by one thread, which will shear, and then the next thread will take up the load, etc, and the nut will strip by pulling a nice triangular wire out of it.

I swear that used to be in engineering textbooks.



Mike Halloran
Pembroke Pines, FL, USA

 

[dummiboi]

Thanks Mike. I am aware that there is some deformation on the threads in a nut, my concern is what is too soft?

Are there any equations or rules of thumb to go by in determining what hardness should be used with a certain load/application?

Thanks again.

 

[CoryPad]

There are equations relating force, area, and stress. The force applied to the nut divided by the shear area equals the applied stress. This stress should be less than the proof strength of the nut. Hardness is used as a substitute for nut proof strength.

If you don't have experience with bolted joints, you can educate yourself with some of the free information on the Internet:

faq725-215

or from better sources:

faq725-600

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[dummiboi]

Awesome, thanks Cory

 

[Tmoose]

The Fafnir/Torrington and Barden FAG precision bearing catalogs have torque specs for standard bearing nuts.
Typically for regular bearing types the bearing balls or races will complain long before the nut. A nice detail is to have a shaft shoulder positioned to handle the usual or heavier thrust load