help fem

[boscar]

Bye to all,
I am a student of meccaic ing and I am currently doing an internship.
I was given the following task:

check if the pin (v attached) holds the force f (see diagram).

Now, the problem is that here they do not use nessum fem (also I am not yet able to use one), so the various details come, say in a few words, dimensioned to the eye and so much experience!

I would like if someone could do me a static and dynamic analysis of the particular (in three points where the diameter changes) to be able to compare the result of the fem with the one calculated by me!

the points where I performed the calculations are represented in the annex. in practice where the diameter changes.

the problem data are as follows:

Pin material: aisi 304
position of force: on the axis of the pin, re-entry of 2.25 mm on the non-threaded part

static analysis
value of force f = 175 n

dynamic analysis
f = 175 n
force acts 12 times a minute.
The duration of the action is 2

here are my results:

static analysis
(I found two values for r0.2 of theaisi304, I'm pretty sure that the second is the most reliable one! ! )

- with r0,2 = 190 n/mm2
voltage point 1 = 94 n/mm2 - > breaking safety factor = 2
voltage point 2 = 154 n/mm2 - > breaking safety factor = 1.2
voltage point 3 = 55 n/mm2 - > breaking safety factor = 3.5

- with r0,2 = 400 n/mm2
voltage point 1 = 94 n/mm2 - > breaking safety factor = 4.25
voltage point 2 = 154 n/mm2 - > breaking safety factor = 2.6
voltage point 3 = 55n/mm2 - > breaking safety factor = 7.2

dynamic analysis
voltage point 1 = 184 n/mm2 - > breaking safety factor = 6
voltage point 2 = 165 n/mm2 - > breaking safety factor = 3
voltage point 3 = 211 n/mm2 - > breaking safety factor = 12

I thank everyone who is available for this small analysis!:finger:

 

[gerod]

for dynamic analysis do you mean fatigue?

I think this is a problem that solves with 4 counts on the cross without fem.
It's a pin, it's a simple geometry.
Take the theory of construction science!
for fatigue on books find various coeffcients to use for carvings and much more.

I think you're doing this first as I refuse to do it with the fem, too trivial.

Note: learn how to deal with the hand before putting on the fems that often give strangeness!
Hi.

 

[cacciatorino]

In addition to agreeing with gerod, I ask you two questions:

1) Have you considered the tension state present in the pin due to the precarious tightening? In principle you can apply the overlap of the effects between tensions due to preload and tensions due to the applied loads.

2) Have you considered the cutting factor (which however for ductile materials is not significant)?

Maybe more than fem would be interesting that you explain to us how you got those results, it would be a more constructive comparison.

Bye.

 

[Fulvio Romano]

[...]check if the pin (v attached) holds the force f (see diagram).

Now, the problem is that here they do not use nessum fem (also I am not yet able to use one), so the various details come, say in a few words, dimensioned to the eye and so much experience![...]

just out of curiosity... to check the rest at the supermarket, use excel? Or maybe matlab... you never know?

I think it's a pretty trivial continuum, one of those that with the fem becomes almost incomparable.

You have a set of carvings. The theory of mechanics tells you how to deal with them, but you will never manage to mesharli as you must. then, what is dynamic analysis? The result of fatigue analysis doesn't look like what you posted.

I would recommend, calculate the moment, calculate the cut. apply complete von mises on the most stressed section. then found the cut coefficient and multiplies by the found sigma.
What values do you reach? Do you have a security factor of at least 1.5?

 

[boscar]

with all respect your post seems slightly arrogant!

if you read carefully I have already put the results from me found.

the values that I posted are the stresses I calculated in the critical points, (in the static case where the force acts consistently and in the case of fatigue where the force acts intermittently), which then compares with the values of the material allow me to find the safety factor.

 

[boscar]

by dynamic analysis I mean fatigue.

Now I think I do this, I'll show you the procedure and tell me what you think... .

 

[gerod]

with all respect your post seems slightly arrogant!

if you read carefully I have already put the results from me found.

Good, we don't make bum!! !
you are new and you need to know that the forum is not a mutual aid company.
Here we also give a hand but do not do the jobs for others (among other things aggratis), so, when asked, you ask yourself also to be positive!
I don't think you'll find someone who makes you a fem calculation on two feet. Most of us users work and put their time to help.
the answers that have been given are very exhausting and I do not think they are offensive, indeed, I see it as a joke to make you understand that simple problems must remain such (you will experience it on the field).
Indi... accept advice.
Thank you.

 

[boscar]

Okay, get the lesson! !

Now I put you in the annex (I messed up writing it nicely) the calculations I made for the static case.

I have not considered the tension state present in the pin due to the precarious tightening because first I did not notice, second I do not know well how I can calculate it.

Tell me what you think

 

[boscar]

classic forget the annex....

 

[cacciatorino]

I have not considered the tension state present in the pin due to the precarious tightening because first I did not notice, second I do not know well how I can calculate it.

You must consider it, it is extremely important for points 2 and 3.
it would be so high to cancel the effect of the tension due to external loads, it depends on how the forces are deployed.
For example, in the flanges that close the pressure vessels, the preload of the screws is such that the sigma in the material generated by the preload is always higher than that due to pressure in the container. this allows to avoid dif atica stress in these screws even in the presence of pressure pulsations.

for the calculation of the voltage due to the screwing, I think on your machine construction book you will find all the necessary references.

 

[boscar]

I can't see how the force you tell me is acting.
is this the torque moment due to the screwing of the pin?

 

[pika]

but are you sure that the chart of the moment is what you calculated?

for calculations have you hypothesized a beam with two supports at the extremes of the violet surface?

 

[Fulvio Romano]

Hello boscar,
I'm sorry I seemed arrogant.
I don't have time to check the accounts, but so on two feet I would say that on a "trave" so short, the bending counts little.
Then you should consider:
- bending, butterfly tension, maximum "in the outskirts", sigmai type voltage
- the cut, jourawski says tau = t*s/i*b, on circular section you have the max tau at the center of 4/3 f/a
- traction, nothing easier. = f/a

At this point the sigma equivalent to von mises, something like (sigmai^2 + 3*tau^2)^0.5, I think (sigmai is the sum of bending and traction).

here you found your reference sigma.

Careful!
calculates the two sigma at the center of the section (maximum cutting, no bending) and in the periphery (zero cutting, maximum bending) and considers the maximum.

I hope I didn't forget anything, in case, hunter, throw an eye!

 

[boscar]

He's right, I'm wrong!
on the right side is framed, while on the left is free, so it is simply a straight!

 

[pika]

for the calculation of the voltage due to the screwing, I think on your machine construction book you will find all the necessary references.

I can't see how the force you tell me is acting.
is this the torque moment due to the screwing of the pin?

I believe that hunter refers to traction within the pin that is generated by tightening a bolt.
here however, being a prisoner, I think the torque moment applied is enough to hold the screwed pin, so it will be little relevant (...it is only a guess!).
But now I have a doubt... if the pin is screwed "very tight", can't it be assumed that the behavior is that of a beam stuck with length to the wall of the block and not until the beginning of the thread?

@cacciatorino: I went to recover the construction notes of machines but of torque momentum values I found nothing. do you not have tables that report such values (something very "practical")?

 

[Fulvio Romano]

I wouldn't consider torsion... only traction. . .

 

[cacciatorino]

classic forget the annex....

I'm sorry, but it seems to me that the moment you drew is completely wrong. How do you have a moment not null at the point of application of force if there is nothing left??? ? :confused
and however neglecting the tension due to the screwing introduces too big approximation that makes almost entirely inaccuracy your analysis.

It forgives the drawing made with paint but I did not fall underhand right now.

 

[pika]

I wouldn't consider torsion... only traction. . .

the traction on the pin in this case is generated by the twisting moment I hang (there are no other loads in play that generate a horizontal force), that is why I wondered if there were tables that indicate how close a screw is.
having at disposal the torque moment can be obtained the traction. I unfortunately do not have the sensitivity to say how tight/snatched that pin.

 

[pika]

It forgives the drawing made with paint but I did not fall underhand right now.

This is the right time chart!

 

[radio]

the traction on the pin in this case is generated by the twisting moment I hang (there are no other loads in play that generate a horizontal force), that is why I wondered if there were tables that indicate how close a screw is.
having at disposal the torque moment can be obtained the traction. I unfortunately do not have the sensitivity to say how tight/snatched that pin.

guard qui:
http://www.bossard.com/index.cfm?ap...mid=3075&app_fileid=7074&app_source=text_fileHowever, focus on the step of your thread, the materials and coatings of the screw and screw and the lubrication conditions at the coupling; errors in the evaluation of the friction coefficient lead to large variations between hypothesized and actual precarious.

n.b. if you want to know all about threaded links try to put your hands on vdi 2230, that I know is the most complete traction in the world; I can't go much further than the introduction.