Bolted Joint Stress Calculation 2

[niravpshah]

Dear Experts,

I have been struggling with this question since last couple of days. I have bolted joint connection which I want to analyze.

I have cantilever type of configuration for my flange connection. And the bolt carries all the dead weight load, pressure load and thermal expansion load. We have got the “in-house” spreadsheet which calculates,

Bending Stress,
Tensile Stress,
Shear Stress

&
Separation Margin.

Unfortunately, the developer of the spreadsheet is not available. But from what I understand we have got pretty good experience with the designs made by this spreadsheet.

The criteria it uses for the calculation is comparing all the stresses with 0.02% yield value and if the ratio of the stress to allowable (0.02%) is less than 1 then the design is ok.

e.g

Bending Stress – 80%
Tensile Stress, - 90%
Shear Stress- 60%

And the separation margin has to be higher than 15% for the design acceptance.

My question is,

1) Don’t we have to combine the loading and compare it to allowable stress? The scenario I ran gave me above numbers. What if have find say, Von misses stress, chance are that it will exceed the allowable..

2) I haven’t yet gone into detail to find out the logic of the spreadsheet, but in general for bolted joint, do we compare stresses individually to allowable? Say, shear will be carried by threads..

I am working onto find the theory used for the calculation in the spreadsheet, it’s taking lot of time since its pretty cumbersome. I am looking for general answers for bolted joint calculation. I would really appreciate your most valuable response. Does anyone know of good reference material for the subject?

Thanks again,
Nirav

 

[Barry1961]

http://www.team.net/sol/tech/torque.html

This gives some general bolt ratings.

There is a fairly good and easy explanation of bolt strength in the machinery handbook.

Barry1961

 

[niravpshah]

Barry1961, Thanks for your posting.

Could anyone answer the questions regarding the questions about design acceptance criteria?

 

[Cockroach]

Niravpshah, in your problem explanation at line 9 you say: "...and if the ratio of stress to allowable is LESS THAN 1 then the design is okay."

Unless I have misunderstood something, you are speaking of "factor of safety". This is the ratio of allowable stress (or yield stress of the material) to actual stress present under loading. The factor of safety would therefore generate a number GREATER THAN unity if the design is acceptable.

I think I got it right because you mention "...comparing all the stresses with the 0.02% yeild value..." or the percentage offset method. Typically this is 0.2% which is a strain of 0.002 (deformation to initial length). I would suggest 0.02% is a little on the tight side.

Just wondering, perhaps I have misunderstood you.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[XELR8]

Send me your information and I will calculate your bolt stress.

I need loads, temperatures, pressure, flange size, materials, and any other information you feel is relative.

I would compare your stresses to 40% yield value for shear, 66% yield for bending and allowable tension at temperature for tension.

 

[flamby]

Using a spreadsheet that you don't understand and getting the calculations from someone anonymous over internet... If it was so easy, all of us have been already out of work...

Nirav,

The best thing you can do for YOURSELF is to write your own spreadsheet from the basics. Why your spreadsheet can not be better than what you have right now?

You are right about combining the stresses. Workout the Principal stress and compare it with the allowable.

Ciao.

 

[wingbox]

you got to be careful

answer these questions

1- what is the preload on the bolt?

2- Is it permissible to use friction?

3- How the shear load is transfered? through the pin or visavis friction?

4- Can you mix the Tension of Preload with the tension of
Bending?

5- does preload tension relieve bending?

6- can one combine shear tension bending ? or bending is treated independently?

 

[MarkEIT]

Cockroach, I know you have it right, but so does niravpshah. "...and if the ratio of stress to allowable is LESS THAN 1 then the design is okay." ... stress/ stress allowable is 1/(FofS), which would require a value less than '1' to be safe. However, I know what you're saying. Usually people work off of FofS, not 1/(FofS).

I'd also like to mention to niravpshah, that having a value less than 1 does not make it a safe design. If you were to calculate the ratio you mentioned to be 0.995 (FofS ~ 1.005) I would not call it safe. Depending on the industry and application, many different factors of safety are permitted, but typically you're looking for an FofS greater than 1.5 as a basic guideline, and up to 4 or more for critical components, especially when human life or huge loss of capital is the outcome of failure.