Minimum length of tapped hole for bolt strength

[bugbus]

How long does a tapped hole need to be to develop the full strength of a bolt?

Below is a detail for a steel bridge bearing with tapped holes in the attachment plates. The length of the tapped holes appear to be about the same length or even slightly shorter than the bolt diameters. These bolts are subject mainly to shear force, with some tension. These are high strength bolts, Grade 8.8 (830 MPa / 120 ksi).

Does this change if we are considering Grade 4.6 (400 MPa / 58 ksi) or Grade 8.8 (830 MPa / 120 ksi) bolts?

Does it also change if the bolts are mainly in tension or shear?

 

[Lomarandil]

That's probably a good question for a mechanical engineer, and I'd imagine is very dependent on the base material being tapped.

Given that heavy hex nuts typically have a thickness/depth equal to the bolt diameter, I would expect that threads one bolt diameter deep into unhardened material (and potentially with looser tolerances) would not develop the full strength of the bolt.


----
just call me Lo.

 

[SnTMan]

gusmurr, this subject is discussed endlessly here. Have you tried a site search? It is covered thoroughly in Machinerys' Handbook and many other sources as well.

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[3DDave]

It depends on the strength and elastic modulus of the material the bolt is going into, but typically between 4 and 7 full threads are what carries the load, so the bolt may be fine and strip out the base material**. Typically the stretch of the fastener prevents the load from reaching any farther. Keep in mind the loss of full engagement by countersink and by the lead in on the bolt threads which are not full threads.

Here's a previous, umm, thread on the subject

https://www.eng-tips.com/viewthread.cfm?qid=35222.

** the base material has the advantage as the shear area of the thread is at a larger diameter than on the fastener, so a decrease in base material strength won't necessarily result in the fastener pulling out - that's why one needs to know the strength.

 

[dik]

Corypad... there's a name I haven't seen for a while... Is he OK?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[desertfox]

Hi gusmurr

Yes changing the bolt grade will impact on what the bolt can take in both shear and tension, the depth of the tapped hole will also be affected by changes to the bolt grade but because you are reducing the bolt grade you will be limited to what you can tighten the bolts too, so I would expect less stress IN The tapped holes because you cannot load the bolts with an inferior material grade to the same level as an 8.8 bolt. Have you a spefic question regarding the arrangement.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[Agent666]

Heres another thread where this was discussed many moons ago that I had bookmarked. A few solutions outlined in the thread. Refer in particular to the link posted by Bridgebuster.

https://www.eng-tips.com/viewthread.cfm?qid=299014

https://engineervsheep.com

 

[drawoh]

gusmurr,

Look at how thick nuts are. That probably is the amount of thread you absolutely must have.

Ask yourself a question. How much does your bolt with the male thread cost? How many orders of magnitude more expensive is your component with the female thread?

--
JHG

 

[StrucPEng]

I have taken to using the technical bulletin put out by the Industrial Fasteners Institute. They sell the tech bulletin but you can also google, "Industrial Fasteners Institute calculating thread strength" and there are versions available in PDF.

Provides a step by step method for determining the capacity of a threaded hole. I have crossed it with various codes (ADM, AISC, CISC) and the logic is all there, although I would argue you should be using a higher safety factor than is shown in the bulletin but I will leave that up to your judgement.

 

[r6155]

Never use a bolt for a tapped hole, use stud.

Regards

 

[Ron]

Agree with drawoh.....get the proper nut for the bolt grade/strength. Tap length should be the nut height plus a thread or two beyond.

 

[BJI]

General rule of thumb has always been equal to the bolt/stud diameter, which for most applications is close to the nut height anyway. About 60% of the load is carried by the first 3 threads, so you can usually get away with a smaller thickness than the bolt diameter if necessary. If you aimed for a minimum of 5 threads engaged this would be pretty safe in most cases. You could factor this based on the ratio of yield strengths between the fastener and plate if concerned.

 

[dik]

drawoh is a good start... if you need less thickness, you will have to go with a stronger material for both the backing as well the the bolt. It would also be prudent to do a couple of compression/tension tests on the assembly just to see how strong the final product is.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[BJI]

The shear area for the stud/bolt is less than the nut/plate, so it is likely too conservative to scale it up but the full difference of the relative strengths. Nuts are usually made from a medium carbon steel while the high strength bolting is low alloy like chromoly or equiv.