8.8 tensile strength

[engineerfin]

I have some 8.8 10mm metric bolts that will experience load in tension. Rated at 830mpa but I am thinking I should be more concerned with thread pullout then tensile. I Could not purchase 10.9 bolts so was wondering if these would work and what the calculation would be for the thread pullout.

 

[engineerfin]

See if my math works: 10mm bolt = .375 shear dia x pi x thread eng x material shear strength = .375 x 3.1415 x 1" x 2922psi = 3442psi thread pull out. Hard to believe.

 

[robyengIT]

result 3442 can't be psi but lbs

 

[HTURKAK]

http://rocksideltd.co.uk/PDFS/23-GRADE 8.8 BOLT & NUT (ISO).pdf

thread404-479056

thread404-370287

thread507-87318

 

[Tmoose]

What thread engagement do you have ?
Is your loading static or dynamic?
There may be a LOT more to think about than bolt tensile strength.
Improper installation or joint design will murder the best of bolts.

That site calculates a bunch of interesting things all at once, and bases it in actual "bolt" and "nut" bolted materials, calculates minimum engagement and etc.

http://www.boltplanet.com

 

[desertfox]

Hi engineerfin

I think you are looking for thread shear calculations in which case I think the calculation you have posted is incorrect because it is the load to shear the bolt or screw across its whole cross section. Thread shear calculates the shear area of the threads and equations can be found at this site.

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

What also needs to be considered is the shear strength of the female threaded portion as this might not have the same strength capacity as the bolt grade, which although the shear area of the female thread is larger than the shear area of the male thread I have seen tapped holes strip out due to a much lower material strength value, the design of the bolted joint also plays a part in how the threads are loaded due to the stiffness of the bolt and clamped components.

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

 

[drawoh]

engineerfin,

What material are you threading into? This kind of matters.

--
JHG

 

[engineerfin]

Its cast iron. Static, not dynamic and its tensile, not shear.

 

[desertfox]

Hi engineerfin
The formula I have put links too are for the shearing of threads failing due to tensile loading ie thread stripping. Depending on the cast iron grade that will probably fail long before the bolt itself.
If you are not interested in thread shear, then why are you using material shear strength in your second posting?
In the link in my previous post you should scroll down until you see the heading screw shear area calculations, this covers the threads being stripped due to tensile loading.

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

 

[drawoh]

engineerfin,

Let's be clear here.

[dt]Tensile force on a screw...[/dt]
[dd]Structurally, joint is being pulled directly apart. If the load exceeds the tightened force of the screw, the joint will separate, otherwise, it won't. The screw threads, male and female, are loaded in shear.[/dd]
[dt]Shear force on a screw...[/dt]
[dd]The screw is loaded in shear, possibly because it was not tightened down properly. The load on the threads is weird and complicated.[/dd]

--
JHG

 

[engineerfin]

Yes shear on threads. I thought someone was asking if the bolt itself was in shear. Its the bolted plate scenario, tensile in bolt direction. I have used multiple formulas and have come up with the same number. Is that in PSI and if thats the case do I multiply the area of the bolt x the psi. That would make more sense. Hard to believe a 3/8 bolt could lift a 3000lb car but 3000x.375 = 1125lb thats believable. Then you have all of the factors like the head of the bolt, surface area and preload on threads, right?

 

[desertfox]

Hi engineerfin

The area you are using for thread shear is not 0.375 x pi, if you look at the link I left in my first post you will see that the shear area of the threads is significantly less than 0.375 x pi which will yield a much lower strength value than you are currently calculating.
You also need the strength values of the cast iron because if it’s grey cast iron there is every chance that the thread in there will fail first.

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

 

[3DDave]

.375 * pi is the circumference - times the engagement is an area; at least that was in the original formula. The rough pitch diameter is .344. I suppose it depends on what one means by significant.

The more troubling aspect is the idea that there can be that much useful engagement.

 

[desertfox]

If I use the formula for the M10 male thread the shear area for the threads calculates to 494.69mm^2 and that is for an engaged thread length of 1” or 25.4mm.
Compare this with 0.375 * 1* pi= 1.17”^2 convert 1.17”sq to metric and it equals = 760mm^2.
I would say that a difference in area of over 260mm^2 was pretty significant.



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

 

[3DDave]

Sorry. "0.375 x pi" is not a shear area; which is what I was getting at.

If you mean that 10mm <> .375 inch, that is a different matter.

 

[engineerfin]

I used this calculator for the thread shear:

https://www.engineersedge.com/calculators/external-thread-shear-area.htm

 

[desertfox]

Hi engineerfin
Well it’s possible that there is an error in the calculator, can you post a snap shot of your values that you put in that calculator?
The formula the calculator is based on is the same as the one I found on Roy mech.
However that formula is nothing like the maths you posted see below

engineerfin (Automotive)(OP)27 Sep 21 23:28
See if my math works: 10mm bolt = .375 shear dia x pi x thread eng x material shear strength = .375 x 3.1415 x 1" x 2922psi = 3442psi thread pull out. Hard to believe.

Let’s try and bottom this thing once and for all please explain what it is you are bolting and give us the information about the cast iron and let’s get a conclusion.


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

 

[drawoh]

engineerfin,

For a screw of given strength, you need a minimum thread engagement if you want to fail in tension rather than shear. If you go to a higher strength bolt, you need more thread engagement. If you need to work this out accurately, you need some literature. John H. Bickford's books have been recommended repeatedly on this forum. I have here Handbook of Bolts and Bolted Joints, by Bickford, and Sayed Nassar. It does explain the shear strength of bolt threads.

For an extremely quick and dirty analysis, compare the major diameter of your bolt, with the thickness of your nut.

Consider the fact that almost certainly, the thing with the female threads is many orders of magnitude more expensive than the thing with the male threads.

--
JHG

 

[desertfox]

Hi engineerfin
I used that calculator you gave the link to and I get a similar area to my earlier calculation, the result was a shear area of 468mm^2 within 6% of my original calculation so not sure why it didn’t work for you.

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

 

[SwinnyGG]

See if my math works: 10mm bolt = .375 shear dia x pi x thread eng x material shear strength = .375 x 3.1415 x 1" x 2922psi = 3442psi thread pull out. Hard to believe.

The minimum proof load for an M10 bolt that's rated 8.8 is over 10,000 lb.... 3,000+ lb of thread capacity is not hard to believe at all.

However...

the 2922 psi you used in your calculation is a shear load value, not a shear stress value. I'm assuming you got that from a chart of 8.8 bolt ultimate load capacities. The usual published value is 12-13 kN, which is close to the value you used.

You need to use a stress value instead. Yield stress for an 8.8 bolt is roughly 90,000 psi. For shear you want to use 50% or so of the ultimate. So use 45,000 psi.

Also, the shear diameter of the threads is not the shank diameter of the bolt, it's smaller; and the area under shear is also not the full length. The threads along the pitch diameter have an area roughly half that of the bolt shank for the same length. This will affect the final answer a LOT.

At the end of the day, when calculated correctly, for an M10 8.8 screw with 25.4 (1") of engagement, thread ultimate strength is roughly 25,000 lb for the screw only. This will only be an actual limit if the strength of the material for the female thread has the same UTS as the bolt - which it probably doesn't.