Shear stress or more

[somebodyfromworld]

Hi there
I've got a task where i need to use manual calculations with this setup (see attach.)
there is a cylinder with two internal grooves (yellow part). 5 Dogs are equally spaced (72deg.) inside that cylinder. They are being pushed by the Force against the grooves in the cylinder. What stresses i have to calculate to make sure the construction is stable with applied Force. And what equations I need to use.
Kind Regards, S.

 

[TGS4]

This is a very odd configuration. Is it for a school assignment?

 

[somebodyfromworld]

no it's for my project, i usually do elastic-plastic analysis according to ASME VIII Div.2 using ansys, but the client is asking me to prepare manual calculation in parallel with FEA. And I'm not sure what stresses in this particular case i have to calculate.

 

[TGS4]

What would you be looking at in your FEA?

BTW, elastic-plastic analysis ( the factored loads part) doesn't do contact well. How were you planning on managing that for your FEA?

 

[somebodyfromworld]

Stress/Strain curve using nonlinear analysis but the question is not regarding FEA,
I need to prepare calculation thru the stress equations manually how we did at school. But what stresses i have to use i'm not sure.

 

[TGS4]

I understand that you need to prepare manual calculations. You should be doing that for every FEA that you perform to demonstrate that the FEA is providing appropriate results.

In the context of the manual calculations, however, you asked what "stress" you should be looking at. I replied that it should be the same thing that you look at in your FEA. That you don't know what you would be using as a criteria in your FEA gives me very little confidence that you will be able to perform any calculations (manual of FEA) with any competence. If you don't know how you would calculate this manually, then how do you know that your FEA is correct?

You are aware that the LRFD approach in VIII-2 doesn't handle contact well/appropriately, right?

 

[somebodyfromworld]

As I wrote above i'm following this method protection against elastic plastic collapse where the stress/strain curve matters only, and the acceptance criteria (accord. ASME VIII Div2) is a converged result. If i'd doing any elastic method using FEA i will face the same question as I asked here. Because i don't know what stresses i should be looking at if i would use elastic method in FEA.
I guess is only shear stress, maybe a compression stress from 45deg angle, but someone is mentioned bearing stress as well. Is there any kind of stresses i should be looking at?

 

[TGS4]

It is a good thing that your client has asked for complementary manual calculations. If you don't know how to tackle a problem at least 80% using basic engineering techniques/first principles, then you probably shouldn't be using FEA, period.

Btw, I am extremely familiar with those ASME rules you refer to, as I have been an active and contributing member of the ASME Committee responsible for those rules.

 

[somebodyfromworld]

it's good to hear that you were apart of building that standard then I found a right person to help me with my issue. Could you suggest me what stresses i should calculate there?

 

[dhengr]

SergeyZh:
From your sketches and verbal description so far, it is pretty hard to know what you’ve really got there. Some better sketches with good proportions, dimensions, sizes, thicknesses, several views, forces, etc. would go a long way to explaining your problem. What’s the cylinder dia., the dog teeth sizes and thicknesses? How are the dogs attached, and to what? I share TGS4's concern for your lack of first principles and fundamental engineering understanding of Strength of Materials. You say... “I need to prepare calculation thru the stress equations manually how we did at school.” What school, and did you actually do those homework problems, that semester? Have you looked in your Strength of Materials or Machine Design text books before asking for help here. You will learn more if you have to dig, study and think a little bit, as opposed to being spoon fed here on E-Tips. A few questions for you to ponder and come back to us with your answers. What’s the shear stress in the dog teeth? What’s the bearing stress btwn. the dog teeth and the cylinder grooves? What’s the preferred angle for the teeth and groves, and clearance btwn. them? How many of these dogs and their teeth act in unison to take the total torque loading? Can either the dog teeth or the cylinder flex out of their normal shapes and positions under these loads? Show and explain your answers to the above questions, and we’ll critique them.

Try to keep in mind, that a computer program does not an engineer make. And, if you don’t know how, have the experience, judgement and knowledge, to start to approach and define and refine your problem long-hand; for the life of me, I don’t know how you know how to model it for input to a FEA program. Those programs don’t do the thinkin for you, they just do the finer grunt labor of a more detailed analysis once you’ve modeled your problem properly. Finally, I can’ t help but think that elastic-plastic analysis has really not improved our final structures much, or our understanding of these structures, for all the added number manipulations, when only a few percent of the total engineering population even begins to understand the concept and those number manipulations.

 

[chicopee]

I am still trying to figure out what he has in the two sketches. I agree with dhengr that the sketched are deplorable. Proper sketching requires top, front, end and section views or an isometric view with a cut section for this post.

 

[somebodyfromworld]

It seems the opinion here has built that I'm nooblet . I just wanted to confirm what I'm going to calculate is correct. Here is more detailed screen shots. Am I right with the last screen shot?

 

[tbuelna]

The arrangement you show is very complicated. Have you considered factors such as tolerance stack-up between the mating parts? Under some conditions it would be quite possible that only 1 or 2 of the dog rings would carry the load. Or that the two threads of each dog sector will not share the load equally. Have you accounted for the end moment friction produced on the dog ring sectors when the thread joint is tightened?

 

[maripali]

What is holdong the five dogs in their place. What if the dogs will thrust to one side of the ring?
Also check the contact pressure form F2 when F3=0 and F3 when F2=0

 

[IFRs]

Sometime I make a small clay or cardboard model to visualize the forces and movements.
Where there is force or movement there is stress or strain.
There may be hoop tension in the outer section to be considered given the sloped portions...

 

[somebodyfromworld]

IFRs
Could you please draw the area on my sketches where possibly the hoop stress may occur.