Question about bolt depth

 

How long should a bolt go into an internal thread?

 

 

Some say “It’s okay if a bolt goes 3 thread pitches.”

But others say “Bolt depth should be at least 1.5 times of the bolt diameter.”

There may be other opinions. Which opinion is correct? 3 thread pitches theory? 1.5 bolt diameter theory? Or another theory? And why?

 

Actually, there is no correct answer. Bolt depth should be decided depending on the situation. However, since there are some opinions like the above, I’ll let you know the reasons behind the opinions in this article.

 

 

3 thread pitches theory

 

By the way, there are two ways of how a thread gets broken like below.

 

 

When a bolt goes into an internal thread deeply, the bolt has a tensile force and then “tensile failure” occurs. On the other hand, if a bolt doesn’t go into an internal thread deeply, the internal thread can’t resist a shear force and then “shear failure” occurs.

 

There is an opinion that says “If bolt depth is longer than a certain length, the internal thread will not get broken because the bolt itself gets broken earlier than the internal thread gets broken. That’s why to know the length of when shear failure shifts to tensile failure is important. It should be the minimum length that bolt depth can have.”

 

So, I roughly estimated the minimum length that doesn’t cause shear failure.

 

 

According to the rough estimation, if bolt length is 0.4 times longer than the bolt diameter, shear failure will not occur.

 

The length of 0.4 times of a bolt diameter is almost the same as the length of 3 threads pitches. I guess that this is the reason behind “3 thread pitches theory”.

 

 

 

Nut height theory

 

Is it no problem if a bolt goes 0.4 times of the bolt diameter?

It will cause a problem because the theory doesn’t have a margin of strength as you have already known through the rough estimation I showed. And also, it is just roughly estimated in the first place.

Considering safety a little, you should at least double 0.4.

 

Actually, 0.8 times of a bolt diameter is almost equal to the height of the nut.

This may be the reason behind “Nut height theory”.

 

 

 

1.5 bolt diameter theory

 

Can we conclude that “Nut height theory” is correct?

We can’t say for sure because it’s not that a nut guarantees like “the thread will never get broken”.

 

Because of that, “1.5 bolt diameter theory” appears.

1.5 times of a bolt diameter is approximately equal to double length of the height of a nut. In other words, the opinion says “A bolt should be inserted at least double length of a nut.”

 

 

If there are no concerns about the strength of a thread, the idea of inserting a bolt at least 1.5 times of the bolt diameter is widely adopted.

 

When shouldn’t we make use of “1.5 theory”? For instance, we must not use “1.5 theory” when bolts support main forces of a machine because the bolts are supposed to support a heavy load. And also, we may not be able to make use of “1.5 theory” when the forces applied to the bolts are repeated load because we have to consider metal fatigue. In addition to that, when a material that has threads is weak, we must not make use of “1.5 theory”.