bolts: connection for friction and cutting

[Atheneus]

Good morning to all, I am studying bolted joints and in particular the comparison between friction links and cut links.
from what I understand, when the friction between the connected plates exceeds the strength with which the plates are drawn then is the connection is friction and therefore the bolt does not work to cut.

what is not clear to me is how to move in the verification of the plates: On the one hand I have the traction effort deriving from the force with which they are pulled but then I also have the compression in the direction of the bolt axis deriving from the preload of tightening, so to check them I should use a resistance criterion, right?

when the link works in cut, I have to do the bolt cut check but from my notes I have marked that I have to check the cut plates too: But if you force the "tira" shouldn't press them to traction? Why should I check them out?

I thank you in advance and apologize if the questions may be trivial but on the net I found nothing that helped me understand the difference between these two links.

 

[biz]

In theory the screws should not work to cut, only to friction. However, it is true that it also occurs when cutting. when you make a verification of a bolted joint consider this as if it were a unique body and the traction, cutting, twisting and bending check.
then if you have to follow the various regulations is not very different but something changes. What do you mean you have to check the cut plates? typically you do a refractive verification and a non-detachment verification of the flanges for example.

 

[Atheneus]

In theory the screws should not work to cut, only to friction. However, it is true that it also occurs when cutting. when you make a verification of a bolted joint consider this as if it were a unique body and the traction, cutting, twisting and bending check.
then if you have to follow the various regulations is not very different but something changes. What do you mean you have to check the cut plates? typically you do a refractive verification and a non-detachment verification of the flanges for example.

by doing some research on the net I had found that several dispensers highlighted the possibility that the flanges could go to meet yielding or cutting or traction, so I wondered how it was possible that a force that "draws" the plates should be considered as a cutting force.

 

[mastraa]

Hi.

as first advice, although no longer in force, you may look at cnr10011 that usually from an explanation very similar to that is provided at a universal level.


for the rest you can have cut and friction joints. Of course the second ones are preferable because cutting a screw really keeps little.
cutting verification:

• divide the force by the number of screws acting, you get the cutting tension agent on each of the screws. In case there was also a traction force on the vine you must also find the traction. the verification is: [math]\big( \frac {\sigma_b}{\sigma_adm})^2+\big( \frac {\tau_b}{\tau_adm})^2\le1[/math]
• check to punch the material, find the 'weight' path between the various holes and check the traction of the net section
• hole recroaching test: where the resistant section is given by the diameter for the thickness of the plate.

in case you have joints for friction first you have to calculate the strength of static friction you need. knowing the coefficient of friction revenues the total preload you need. divide it by the number of screws you want/you can put and then revenues the section of the screw you need according to the maximum preload voltage that can accept the given diameter.
If you have a force that causes a moment on the joint you must then check on the most disadvantaged screw (i.e. the most distant from the 'pin' point).


ps: from your last message I feel like I'm not making you clear how a bolted joint works. you have two (or more) plates that can have a stress that makes them flow or trying to divide them away from each other.
In the second case you only need a normal force to the screw axis and there is no doubt.
in the case of the running plate you need a force that you can get or using the cutting screws (such as ' obstacle' to movement) or using friction between the plates or screw and plate (preferable method).

Andrea

 

[Atheneus]

Hi.

as first advice, although no longer in force, you may look at cnr10011 that usually from an explanation very similar to that is provided at a universal level.


for the rest you can have cut and friction joints. Of course the second ones are preferable because cutting a screw really keeps little.
cutting verification:

• divide the force by the number of screws acting, you get the cutting tension agent on each of the screws. In case there was also a traction force on the vine you must also find the traction. the verification is: [math]\big( \frac {\sigma_b}{\sigma_adm})^2+\big( \frac {\tau_b}{\tau_adm})^2\le1[/math]
• check to punch the material, find the 'weight' path between the various holes and check the traction of the net section
• hole recroaching test: where the resistant section is given by the diameter for the thickness of the plate.

in case you have joints for friction first you have to calculate the strength of static friction you need. knowing the coefficient of friction revenues the total preload you need. divide it by the number of screws you want/you can put and then revenues the section of the screw you need according to the maximum preload voltage that can accept the given diameter.
If you have a force that causes a moment on the joint you must then check on the most disadvantaged screw (i.e. the most distant from the 'pin' point).


ps: from your last message I feel like I'm not making you clear how a bolted joint works. you have two (or more) plates that can have a stress that makes them flow or trying to divide them away from each other.
In the second case you only need a normal force to the screw axis and there is no doubt.
in the case of the running plate you need a force that you can get or using the cutting screws (such as ' obstacle' to movement) or using friction between the plates or screw and plate (preferable method).

Andrea

thank you very much for the answer gorea and for the link to the norm! As for your ps, the doubt has arisen precisely because today I went from studying the case in which the plates are removed to the case in which they would be flowed.
in case the plates were removed I had the preload of the bolt to compress them and to make sure that, considering the force that removes the plates, I remain a residual force not nothing.
when I go to consider the case where the plates are flowed, do I still have to consider the preload compression of the screws to be considered in the verification of the flanges?

 

[mastraa]

thank you very much for the answer gorea and for the link to the norm! As for your ps, the doubt has arisen precisely because today I went from studying the case in which the plates are removed to the case in which they would be flowed.
in case the plates were removed I had the preload of the bolt to compress them and to make sure that, considering the force that removes the plates, I remain a residual force not nothing.
when I go to consider the case where the plates are flowed, do I still have to consider the preload compression of the screws to be considered in the verification of the flanges?

atheneus,

read the previous answer well and see that the answer is there. also in the norm that I linked you or in all the dispenses: depends! If you have a friction joint yes, but if you have a cut joint no. the cut joint is when you do not have enough tightening to ensure friction, the plates run until the holes surfaces lean on the screws. at that point the sliding is prevented by the screws that work to cut.

 

[biz]

at that point the sliding is prevented by the screws that work to cut.

should not happen

 

[mastraa]

should not happen

it was to explain what the cutting situation (he couldn't understand the difference) and the only example that came to me is this.
rightly it is good to clarify that they are two different functions, if you make a friction joint the friction must be calculated so that you do not run!
and clearly, as mentioned above, the cut joint does not find it very useful because it exploits little the resistance of the screw.


Andrea

 

[meccanicamg]

This topic has also been discussed many times on the forum so just look.
we have the standard references that clarify the procedure:
- one cnr 10011 withdrawn but for very used school
- uni en 1993-1-8 said eurocode 3....explains current bolting for structures
- ntc 2018 that are the technical norms for constructions in Italy....new and in force.

the preloaded screw so much or little generates a force that keeps the plates united. if the force is perpendicular to the bolt axis or holds for friction or could yield for cutting a broken friction.