Bolt load capacity after tightening 15

[Nordic8]

Hi all,

I hope its not a stupid question, or if it is then maybe someone can point towards that "simple and obvious" answer that I'm failing to see here! Here goes:

I haven't got much experience with bolted joints, so far simple guidelines such as VDI2230 have helped me out just fine.

However, now I was trying to use VDI2230 for determining the tightening torques for a bolted flange and came across something that I fail to make sense of.

I'll give you a simplified description. It's a pipe with a flange, and an end cap bolted onto it to seal it off. Lets say it has 4 bolts that are evenly spread along the diameter, or 90degrees apart from each other. The holes in the flanges are not threaded, just bolts through and and nuts on the other side.

I'll use the VDI2230 method for calculating these bolts, following the procedure I found in a Würth publication.

Based on the diameter of the pipe and maximum working pressure I have determined that the maximum force on the cap will be 9.2 kN, that makes 2.3 kN per each of the 4 bolts. Therefore the axial operating force Fa = 2.3 kN.

As I have no shear forces to consider, the assembly preload force Fm = Fa = 2.3 kN. This will be my starting point.

Let's say I want to use grade 8.8 bolts. Looking at the VDI2230 table, the next closest load there is 2.5 kN -> size M4 is specified for 8.8 bolts.
Add one step for static concentric load -> 4.0 kN -> M5
Add one step for tightening with a torque wrench -> 6.3 kN -> M6

So the bolts that I need are grade 8.8 size M6.

I will estimate the coefficient of friction to be 0.11. The next closest step in the guidelines I have is 0.10, so lets got with that.

Based on all this, VDI2230 advises me that the tightening torque would be Ma = 9.0 Nm, which would create a 10.4 kN preload force in the bolt. It is said to utilize 90% of the screws yield strength.

Based on my calculation, the breaking load of a M6 8.8 coarse thread bolt at yield strength (640 MPa) is 11.29 kN.
10.4 / 11.29 = 92%, so that's just about right.

But... the assembly is not pressurized when I'm tightening those bolts. By just tightening them, I've already used up 92% of the bolts' reserve, not much left for when I will actually pressurize it.

If i will now calculate how much pressure this assembly can take after tightening the bolts, then I can only use the reserve left in the bolts (8 percent to yield strength) as in my calculation? That's not much...

Or lets give you another really simple example just to illustrate the point I'm trying to make here:
Lets say you have an M6 eye bolt, you insert it through a hole in a rigid steel plate in the ceiling, and screw a nut on the other side. If the nut is not tightened, then you can hang 11.29 kN load on your eye bolt before it breaks, but after you've tightened the nut according to VDI2230, you can only hang 11.29 kN x 8% = 0.9 kN off it before it snaps!.

I'm kinda confused here. What am I missing? All the example calculations I have seen for pressurized flanges, lids, end caps on pressure vessels etc just use the full capacity of the bolts in their strength calculations. But how can I do that if I've already used up 9/10 of it???

Could I tighten the nuts to a lesser torque than specified? But wouldn't the flange then leak, and would the bolts rattle loose / become undone too easily?

Thanks for any feedback (or for proving me stupid, it that happens to be the case

J.

 

[baoancnc]

Hello
The question of Bolt load capacity after tightening is not simple and it is really helpful. Thanks to this topic, I have learned a lot more useful information

 

[goutam_freelance]

The yield factor of safety is given by

As can be seen n_o reduced with increasing P.
So you need to know your P and design bolt such that n_o is never less than 1.
So there is no alternative for a proper design.
By the way if your load is variable or alternating you need to consider fatigue criteria instead of yield criteria.


Engineers, think what we have done to the environment !

https://www.linkedin.com/in/goutam-das-59743b30/

 

[dik]

Confirmed what I thought I knew, and added a little frosting on top... thanks gentlement/ladies (I never know which).

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

-Dik

 

[dik]

fox... the article by Instar on the 'design and analysis of bolted joints' is great. 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

 

[desertfox]

Hi r13

If you look at my example calculation,I have assumed the load is applied to the clamped plates which is actually a similar scenario which occurs with the pressure vessel, that Nordic8 is referring to.
The vessel is pressured after the bolts have been preloaded so this force is directed onto the end cap; provided the bolt preload is sufficient to prevent separation occurring, the following sequence takes place:- the compressive stress between the clamped flange faces is relieved as the faces now try to expand to their unloaded state under the action of the external force, the bolt also has to extend by the same amount to accommodate this expansion and maintain equilibrium. The forces both in the flange faces and the bolt which are generated by the external force are proportional to their relative stiffness. Referring to my example calculation you can see the stiffness for both the bolt and the clamped plates have been estimated but it is clear that the clamped plates have a much higher stiffness when compared to the bolt by almost a factor of 3. The clamped faces being much stiffer than the bolt and the main load path carries a bigger proportion of the external load when compared to the bolt and is the main load path, so the force in the clamped plates is 22.3KN and that in the bolt is 7.698KN

Deflection of the joint
Under external force = force in joint/ stiffness of joint = 22.3*10^3/ 4.696*10^6

= 4.748*10^-3 mm



Deflection of bolt. = force in bolt/ stiffness of bolt = 7.698*10^3/ 1.6214*10^-6

= 4.748*10^-3 mm

Equilibrium is maintained because the expansion of the joint equals the expansion of the bolt.

Now Nordic8 is correct because the bolt has to stretch to accommodate the expansion of the clamped flanges and therefore this results in additional bolt tension of 7.698KN adding to the preload of 40KN making the bolt tension 47.698KN.




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

 

[desertfox]

Hi dik,

You’re welcome

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

 

[desertfox]

Hi Nordic8

The calculation is only for a single bolt imagine if you put four bolts in the joint and tightened each one to 90% yield and then apply the 30KN external load, the external load now would be spread over four bolts. So to find the proportion of the external load in the bolt you would divide the external load by four and then repeat the calculation like my example. At the end of the day yes you would want the bolts to stay below yield although there is a school of thought that says tightening bolts into yield isn’t an issue but I would increase the number of bolts or size and run the calculation till the bolts were below yield.

https://www.boltscience.com/pages/glorimermorethoughtsonfasteningarticle.htm

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

 

[SSCon]

Mechanical engineers talking about bolted connections is nearly always a mess (I think it's because of Shigley's).

If doing a pressure retaining bolted joint you need to follow a pressure vessel/ piping code.

If doing a structural connection, refer to a structural code.

For structural connections the preload is not additive; as in preload does not effect the strength of the bolted joint. This is stated in the Research Council on Structural Connections' Specification for Structural Joints Using High-Strength Bolts. They provide an explanation with references.

https://boltcouncil.org/files/2014RCSCSpecification-withErrata.pdf#page=47

A pressure retaining connection is more complicated, with a gasket, a leak criteria and the potential for a temperature gradient. The best approach is of course to use a standard flange. If you must use a custom flange then you need to do calculations in accordance with a code such as EN 1591-1.

Any calculations which do not follow a recognised code should be ignored.

 

[desertfox]

Hi SScon

I agree with you about following the appropriate code but as you can see from the posts the OP was confusing preload and external load, so the thread went general specifically looking at bolted joints, preload, joint stiffness etc whilst we got the concept across. I assume that Nordic8 would be following some pressure vessel code for the project he was discussing.

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

 

[dik]

I've occasionally done reports for court where you reference code related issues as well as 'the real' issue.

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

-Dik

 

[r13]

SSCon has cleared my confusion. Thanks.

Desertfox, Thanks for your detailed explanation. I've archived this thread for future reading. Thanks.

 

[goutam_freelance]

A pressure retaining connection is more complicated, with a gasket, a leak criteria and the potential for a temperature gradient. The best approach is of course to use a standard flange. If you must use a custom flange then you need to do calculations in accordance with a code such as EN 1591-1.

A structural joint is no less complicated as the emphasis is on shear failure.

Engineers, think what we have done to the environment !

https://www.linkedin.com/in/goutam-das-59743b30/

 

[mfgenggear]

Mechanical engineers talking about bolted connections is nearly always a mess (I think it's because of Shigley's).

If doing a pressure retaining bolted joint you need to follow a pressure vessel/ piping code.

If doing a structural connection, refer to a structural code.

For structural connections the preload is not additive; as in preload does not effect the strength of the bolted joint. This is stated in the Research Council on Structural Connections' Specification for Structural Joints Using High-Strength Bolts. They provide an explanation with references.

https://boltcouncil.org/files/2014RCSCSpecificatio...

A pressure retaining connection is more complicated, with a gasket, a leak criteria and the potential for a temperature gradient. The best approach is of course to use a standard flange. If you must use a custom flange then you need to do calculations in accordance with a code such as EN 1591-1.

Any calculations which do not follow a recognised code should be ignored.

amen brother, but it's important to understand the concept more than the calculation, then the calc's. . but yes the specification or code unless it does not cover it.
code is written by humans. god bless

 

[Nordic8]

Yeah, I know the load will get spread between a number of bolts. Just focusing on one bolt now to understand the theory.

I did a few calculations and actually in my case the "plate" is really thick and bolts are skinny, so the joint constant is only C = 0.07 (I found this online tool for quick calculation:

https://www.tribology-abc.com/calculators/e3_6h.htm)

Which means that when I tighten the bolts to the values given in VDI2230 tables, I wont be reaching the yield point.
But yes, otherwise I would have needed to specify a lower tightening torque or change the number and size of bolts. As it appears, you can play around with the joint constant - you'll get it to go down by using skinnier bolts (just need increase the number of the bolts - if that's possible in a given situation, of course).


And some other interesting info...
I also found this thread that dates exactly 20 years back:

https://www.eng-tips.com/viewthread.cfm?qid=1002

(The thread ID is 1002, whereas this thread is 477945... so it means the forum must have been only about a thousand threads old that time )

Anyway, I found this quote in that thread:

The bolt pre-load is applied when there is no additional load applied at the joint. When the external load is now applied, ideally, the compression at the pre-loaded surface is reduced by the amount of the external load. If the initial bolt pre-load is higher than the external load, then the bolt should not see any alternating stress when the external load is applied. This should produce a good factor of safety. Typically, the bolt pre-load should be about 150% to 200% of the anticipated external load.

And here's a tightening torque calculator that I found (I think I found that link in this forum, somewhere...):

http://www.online-iso-calculator.com/online-bolt-torque-calculator-metric-vdi-2330/index.php

 

[Nordic8]

The info in this thread is great!
I have a collection of resources now, I've saved some links from here and I'll bookmark this thread as well.

I've also set up a simple Excel file for myself which allows me to enter the bolt properties, suggested tightening torque values (VDI2230), joint constant and external loads and it will calculate the FOS against separation and FOS against yield. I reckon it'll come handy in the future!

 

[desertfox]

Hi Nordic8

You found some interesting stuff there. I agree with the quote you posted by Anthony in regard to the preload being 150 to 200% of the external load however the cycling stress bit is not quite in line with the calculation I provided. Consider my calculation with the 30KN external load we calculated that the bolt sees an increase of 7.69KN so if you divide that by the bolt area that’s gives you the increased stress, so an M16 has a stress area of about 156mm^2, therefore giving a stress increase in the bolt of 49N/mm^2 which in the greater scheme of things is about 19% of the stress due to preload however it is not zero as per the quote by Anthony.
I have archived the thread too, there is some good stuff here.

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

 

[r13]

You found some interesting stuff there. I agree with the quote you posted by Anthony in regard to the preload being 150 to 200% of the external load however the cycling stress bit is not quite in line with the calculation I provided. Consider my calculation with the 30KN external load we calculated that the bolt sees an increase of 7.69KN so if you divide that by the bolt area that’s gives you the increased stress, so an M16 has a stress area of about 156mm^2, therefore giving a stress increase in the bolt of 49N/mm^2 which in the greater scheme of things is about 19% of the stress due to preload however it is not zero as per the quote by Anthony.

Again, the people commented on that thread had split thoughts too.

 

[desertfox]

Hi r13

I went through the thread and all the posters seemed to agree that after the joint was preloaded any external force was shared between the bolt and clamped parts, it was only in this thread that dauwerda believed there was no load sharing and that the bolt was the main load path.

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

 

[goutam_freelance]

I think there is a confusion between mechanical and structural joints.
1.Bolts in mechanical joints take primarily tension load and used for leak tight joints which we are discussing here. Here both bolt and member share the load.
2. Structural joints primarily take shear loads where the bolt(and the member too) has to take the full external load while experiencing the preload.

Engineers, think what we have done to the environment !

https://www.linkedin.com/in/goutam-das-59743b30/

 

[r13]

desertfox,

I don't mean the load sharing, I was pointing to the effect of tension applied after the bolt was tightened. See requote below.

If the initial bolt pre-load is higher than the external load, then the bolt should not see any alternating stress when the external load is applied.

Actually I don't fully agree with Anthonr on the text in bold, IMO, the bolt preload is negate/changed by the external load, but you and OP seem do not agree either. So, for now, I think the different application and focus point between disciplines is the divider between us.