Engineering Question, for Simple Problem?

[Vette40th]

When threading a bolt into a nut, is there a standard number of threads a nut must be threaded into to get full strength when you torque it?
Growing up I was always told have 2 to 3 threads, full threads, coming out of the nut.
In the Navy, working QA/QA -SOC, some people said all you need is 3 internal threads of the nut to be threaded to work.
Without getting into crazy metals, formulas, which is truest with standard grade 8 bolts and nuts being torqued to 150 ft/lbs.

[ZipZap]

The torque/strain equations work only when all nut threads are engaged. Prior to that the stress on the threads is higher at same torque due to loss of friction.

There is no "standard" for less than full engagement, and anything beyond full engagement does not change anything except the ease with which full engagement is determined.

[Bruze]

Adding to Zippy's good points: If the bolt is hardened (or partially, like Allen bolts) and the nut is just mild steel then the nut will strip before the bolt.

Also if the bolt goes into an iron casting, like an engine block or something, then the thread depth should be somewhat longer than the diameter of the bolt. Meaning, the bolt should go into the casting a greater depth than its diameter.

[PLRX]

Van Steel, the company that fabricated the coilovers I had in my C4 The Ghost engineered all bolts to be half thread above the nut. Ensure you use loctite and torque to specs.

[ZipZap]

Originally Posted by Bruze

Also if the bolt goes into an iron casting, like an engine block or something, then the thread depth should be somewhat longer than the diameter of the bolt. Meaning, the bolt should go into the casting a greater depth than its diameter.

That gives you >6 threads for coarse pitch and >10 threads for fine. Probably a decent rule.

[TheHammer]

loctite

[Vette40th]

Originally Posted by TheHammer

loctite

Not allowed on this project.

[Rodnok1]

2 threads should be past the nut, the first thread is a starter thread. Longer is fine.
That's provided the nut is of proper thickness. Higher torques needs a thicker nut.

[higgyburners]

I like just give her full engagement ...no sense in short stroking her

[PLRX]

Originally Posted by Vette40th

Not allowed on this project.

[ZipZap]

Been so long since mechanics of materials class...

I decided to go look for an actual answer. The chart below is for SAE, but there's a metric one as well. I think it basically shows minimum thread engagement via linear dimension, given the thread pitch. So, any nut that you have in the application should be at least as thick as the minimum engagement length, and theoretically, you could partially thread to the minimum distance and get the values on the chart.

It does not show Grade 8/Grade 8, which should give a yield of about 130ksi and tensile of 150ksi. However, looking at the trend, using the low carbon steel (Grade 8 has more carbon and hardening) nut values would be safe.

[Tikiman]

The screw should fail before the thread strips. For this, it is necessary for the shear area of the threaded feature be at least two times the tensile area. Assuming that the threaded feature is of the identical material.

https://www.engineersedge.com/thread...engagement.htm