Preload in bolt used in weaker tapped hole 1

[SiggiN]

Hi all!

Do you guys know of any method (codes, handbook etc.) that lets you calculate the required preload for a bolt used in tapped holes of weaker material than the bolt itself?

My specific case: A4-80 bolt (yield = 600MPa) bolted in tapped hole of 316L steel (yield = 220MPa)

I did see a method in VDI2230, but could not determine if this code is applicable to stainless steel? (as far as i could tell it is meant for 8.8 and 10.9 quality?)

(I'm assuming typical torque tables are ment for bolt and nut of same material?)

Regards
Siggi

 

[desertfox]

Hi SiggIN

Have a look at these sites

https://mechanicalc.com/reference/bolted-joint-analysis.

https://roymech.org/Useful_Tables/Screws/Thread_Calcs.html

However preload really should be decided by the forces acting on the joint as well as the materials involved but you are right to consider the weaker materials when it is a tapped hole or even when the weaker material is clamped between a nut and bolt head.

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

 

[SwinnyGG]

Bolts are made from high strength steel alloys- grade 8/10.9/12.9 bolts are almost always harder/stronger than what they're screwed into.

Calculate the required preload for joint performance before you worry about the tapped material; just because the threaded flange is lower yield than the bolt doesn't mean much.

Calculate the preload you need -> select a bolt size that gives you that preload at the right % of yield -> look at the threaded part and determine if you have sufficient safety factor in the thread -> if not, evaluate how to strengthen the thread (longer thread, threaded insert, etc etc.

 

[drawoh]

SiggiN,

If your tapped hole is in material softer than your screw, you need long enough thread engagement, such that the bolt will fail in tension.

--
JHG

 

[dhengr]

SiggiN:
The previous three posts are really good and informative posts, if you read them thoroughly, carefully, maybe between the lines a bit. Desertfox’s two refs. are very good also. They are mostly just good common sense, engineering thinking and reasoning to solve a problem. You probably won’t find a code section/paragraph/formula for every possible engineering condition in the entire universe. Sometimes you just have to think like a real engineer and reason through your problem using “rational analysis in accordance with well-established principles of mechanics.” That verbiage is in many codes. Get your old textbooks out, particularly Engineering Mechanics, Strength of Materials, Theory of Elasticity, various Struct./Mech. design and analysis texts, and the like and do a little self-study. When you learn it that way, you will retain it much better.

 

[MegaStructures]

Reviving old thread, but isn't there a problem with just extending the tapped hole depth if material is too weak for the bolt pre-load? I seem to recall Roarks or Machinery Design Handbook saying ~80% of the load went to the first couple threads.

“The most successful people in life are the ones who ask questions. They’re always learning. They’re always growing. They’re always pushing.” Robert Kiyosaki

 

[SwinnyGG]

Reviving old thread, but isn't there a problem with just extending the tapped hole depth if material is too weak for the bolt pre-load?

There can be... the answer is really 'it depends'.

There are other paths forward, like threaded inserts or changing to multiple smaller fasteners which have smaller individual loads which are frequently used in the design of machinery.

 

[dik]

I was familiar with Roymech... the first link is also excellent... thanks for the info...

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

-Dik

 

[drawoh]

MegaStructures,

For complete failure, all the threads have to fail.

This does not make long threads in soft material good design practise, but if you don't plan on taking the thing apart...

--
JHG