bolt preload cbar

[marulanda]

Hello I have a problem that I can't solve I have tried in many ways, I give you help kindly because I don't know what to try anymore.
the problem and banal, I have two audiences one from 3mm and one from 2mm. are attacked by two bolt m5 8.8 nominal preload 6500n.
I have to apply the preload.
I tried to apply the preload with a thick thermal load and inserting contact between the surfaces of the audience (I used the bctable sensa friction obviously).
The plates are meshate tet10, the bolts are cbar and are attached to the rbe2 plates (hard links).
the problem is that the bolt contractes, and crushes the plates, generates a stress of enormous von mises in the same, ciraca 500mpa (admissible 230).
nastran use, patran for simulation.

How is it possible?
Did I do something wrong with modeling?

Thank you!

 

[PierArg]

...
Did I do something wrong with modeling?

Thank you!

I think you made something wrong with the forum approach.
I see you wrote the message at 02:52, I imagine taken from despair and without even presenting yourself... but what do you think the forum is? a vending machine for answers?

But does it seem normal if I knocked at your house at 3:00 a.m. without showing up, and wondering if I did well to simulate the pre-loaded bolt?

That said, apart from the thermal load of which I do not understand the meaning, you would need to better describe the problem and maybe even post an image.

 

[Onda]

Since the ramanzine is already rightly done I now try to give you a technical answer.

the system is correct. But it depends on how you put the rbes. If they take all the knots of the hole, you're wrong. in fact the material in a joint is compressed from above. Moreover, using a rbe2 do not allow expansion to the compressed material and find fictitious peaks.
You can do two tests:
- connects rbe2 only to the nodes of external surfaces
- use rbe 3 that do not add rigidity to the model and connect them in any case only to the external knots taking the diameter of the washer.

 

[PierArg]

hello wave,

In fact I also use rbe3 to simulate the undertes of the screws and the cabars for the stem.

As we are, I take advantage of this discussion to ask if you could clarify the distinction between rbe2 and rbe3.
From the mathematical point of view the difference I know, is that I care about the practical point of view:wink:

I do fem analysis only when there's need, so I'm not very hard on it.

from the notes I have (perdonate if I use non-precise terms) it turns out that:
- rbe2: the nodes to which the rbe2 element is connected are all dependent, while the central node is independent.
- rbe3: the nodes to which the rbe3 element is connected are all independent while the central node is dependent and its gdl are the average of the gdl of the independent nodes.

so it would seem like a supercazzola.

 

[Onda]

the rbe2 is an element that rigidly connects the knots according to the chosen gdl. for which a set of knots connected to each other by a rbe2 does not present any type of relative shift. introduces a fictitious stiffness in the structure and should be used with caution.

the rbe3 does not introduce any stiffness. As he works, it's a bit complicated, we say he interpolates the movements of the independent knots and determines from this the shift of the dependent node. is generally used to apply loads without introducing fictitious stiffness.

 

[marulanda]

I apologize if I seemed rude, and there was a moment of frustration , cmq I am an aerospace engineer recently sat down for qusto I ask advice, thank wave for the answer.
cmq with superficial rbe2 things improve, I have a doubt however to apply in this case the preload (6500 n) on a junction of two so thin plys even if of steel is not excessive?
a load so high does not give poles 2 regardless?
basically decrease the preload and readiness? (in this case at 2500n for example)
Thank you and excuse me again.

 

[Onda]

I apologize if I seemed rude, and there was a moment of frustration , cmq I am an aerospace engineer recently sat down for qusto I ask advice, thank wave for the answer.
cmq with superficial rbe2 things improve, I have a doubt however to apply in this case the preload (6500 n) on a junction of two so thin plys even if of steel is not excessive?
a load so high does not give poles 2 regardless?
basically decrease the preload and readiness? (in this case at 2500n for example)
Thank you and excuse me again.

impossible to answer . what load take the plates? How do they transfer it to bolts? What's the preload? .
What material are the plates? from the fem what maximum stress do you have?
However, if you want to assess the stress on the head a rbe2 is too rigid. better a rbe3 or also model the washers and apply the peat to the washers. so that stress on the studied piece is more real.

 

[marulanda]

then the bolt has fbu 700mpa while the audiences from 2 and 3 mm have fby=250 and fbu360mpa, I followed the advice I used a rbe3 that takes a surface of the plates equal to the washer in practice I have that with preload 2600mpa I just have the margin to apply the exercise load.

from what I have understood, permissible is thickness of the connected audiences influence in the applicable preload, but there is a way a formula or similar that allows to estimate the maximum preload applicable taking into account anke of the material of the plates? ?
or are they completely off the road? ? ?

Thank you.

 

[Onda]

there are the formulas you ask, just find a text on the unbearable links. For examplehttp://www1.unipa.it/giovanni.petrucci/disp/bulloni.pdfyou have to evaluate the stiffness of the pin and that of the plates and then find the correct point of intersection

in the attached pages find what you ask. of course the elastic module of the plates influences not the preload, but the slope of the straight of deformation.
In practice, fixed a precarious value, with more rigid or less thick materials, you have a lower deformation than you find with less rigid or thick materials.

but 2600mpa precarious are not too many?

 

[marulanda]

Thanks is just what I was looking for, having to deal with the aerospace since now, I approach for the first time with the problem of the precarious (rail sector), so I found myself in serious difficulty.

to clarify well from the pdf you sent. in practice the preload is calculated assuming a 'safe' value of the residual force on the connected elements once applied the maximum axial load of exercise p (e.g. fe=2p), and entering in the 18,19 of the file. ?

in this sense the classic formula f=k fuy at (frecarious, k=.75, fuy:admissible to snerv. bolt, at:sez. res), expresses the maximum precarious applicable to the bolt, but that applied should be designed case by case right?

Thank you very much!

 

[marulanda]

ps. previous post paralvo correction of 2600 n not 2600 mpa bolt m5, sorry

 

[Onda]

Thanks is just what I was looking for, having to deal with the aerospace since now, I approach for the first time with the problem of the precarious (rail sector), so I found myself in serious difficulty.

to clarify well from the pdf you sent. in practice the preload is calculated assuming a 'safe' value of the residual force on the connected elements once applied the maximum axial load of exercise p (e.g. fe=2p), and entering in the 18,19 of the file. ?

in this sense the classic formula f=k fuy at (frecarious, k=.75, fuy:admissible to snerv. bolt, at:sez. res), expresses the maximum precarious applicable to the bolt, but that applied should be designed case by case right?

Thank you very much!

depends.
Does your bolts work in traction? Cut? if they work to cut, will the load pass it by friction or on the pin itself?
what method do you use to establish preload? How precise are you?
If you were very precise in giving the precarious you can get to 90% yielding, if you work with traction or friction.. as it maximises friction between the two components and if instead works in traction avoid the detachment of the parts, but do not increase the load on the pins. the problem of working at 90% is to have the certainty of getting there without ripping the pin. as the preload depends on the lubrication of the fillet, enter a very aleatory field. so, unless you use systems that pull the pin with a hydraulic cylinder, then you tighten the nut and release the pin, so regardless of friction, you normally use a little lower preload values, which normally range from 70 to 80% of the pin yield.

here you find a lot of material on the unbearable links:
https://www.bossard.com/it/engineering/informazioni-tecniche.aspx