For some time now I've been having the same problem with bolt connectors in simulation,
and I can't find clear explanations.
I'm looking to check the strength of several bolts in an assembly.
I have the impression that the axial force due to the torque of my bolts is distorting my results.
What I don't understand is why the axial force of preload and the axial force due to a tensile force add up?
For me, as soon as the tensile force is lower than the prestressing force, on the one hand there is no risk of detachment, that's ok
but the force in the screw must remain equal to the prestressing force and not the sum of the two? In this case, it's better that I leave the tightening torque at 0.
I hope I'm clear enough and I hope that someone can point me in the right direction.
I agree with your analysis on the non-accumulation of charges. However, according to certain standards, it is accepted that 10 to 20% of the tensile force in the joint is also reflected in the prestressed screws.
If it is not possible to configure the software, it is indeed better to recover the raw forces and do an analytical validation on the side. By the way, in all the companies I went to, the calculation design office always checked the screws in an Excel file and not on the EF software.
Thank you, it's nice to finally find someone who shares my analysis.
As you say, from now on I would check with zero prestress for the resistance and I would manually compare with the prestressing force to check the non-detachment.
I'm a little late, but indeed only part of the "external" pulling effort is reflected in a screw subjected to pretension. This is called filtering: the part of the external force is passed on in a certain proportion (filtering coefficient) and Simulation is supposed to take this into account. It is therefore normal that the tensile force in your screw is greater than the pretension when you apply an external load. Indeed, the proportion received is supposed to be around ten % as long as there is no detachment as you rightly point out.
If you introduce a PT of 10,000N and an external force of 3000N, you should therefore obtain an effort of the order of 10,300N. Depending on the type of screw, and many other conditions it varies, but you can deduce the filtering coefficient associated with your screw connection yourself depending on the final result;)
This additional stress seems to me to be very important to take into account insofar as screws can be pre-stressed up to 85% of their yield strength by tightening alone. It depends on the tightening conditions (coefficient of friction at the threads and under the screw head + dispersion due to the clamping medium which can be very important). In fact, an uncontrolled or too large overvoltage can bring the screw to its limits.
So I advise you not to apply the torque but rather the pretension in the definition parameters of the bolt connector, if you know it. From experience it works quite well: for my part I always make 2 calculations, one with the maximum pretension and the other with the minimum pretension taking into account the different dispersions (following a standardized method). The bolt connector model is made in such a way that the axial pretension also generates the torsional torque: in the stress results it can be verified that the torsional stress generated in a bolt connector without additional external load corresponds to the torsional stress introduced by the tightening torque (thanks to the Kellermann relation).
So you can check:
- Mini pretension: so that your assembly doesn't slip under load
- maximum pretension: so that your screws don't break
Obviously the best thing is to start this by hand, since it gives you 2 equations that are generally quite simple. But when the geometry is too complex, the analytical calculation becomes relatively crude or even impossible since it is very difficult to take into account the deformation of the parts.
I would like to draw your attention to the fact that you have just unearthed a topic from 2017 that is considered by many to be resolved.
That said, you probably want to talk about "bolt pre-stressing".
I'm not going to paraphrase what was said by @peysette who strongly suggests not using the couple.
The advantage of the torque is that when we give its value (torque T) then SW simulation calculates the axial tension, which is as already said the pre-stress of the bolt by the formula F= T/(KxD)
For the record, D is the diameter of the bolt and K is the coefficient of friction. The value of K is quite complicated to calculate, however the value of K=0.2 is a good enough one for most common cases.
Thank you very much for the answer, but I don't understand yet sorry
The section pre-stresses on SW at the bolt connector, and using the Toolbox - Smart Fasteners function .
Is there either axial or torque, what do I do? since SW has already set a value in axial in Newton, I take this value by default? , or is it calculated internally by SW?
If not, what do I do?
Kind regards
PS: You give me the formula of F as a function of the couple T, but on SW needs one or the other, right? by both at the same time and (if we have F no need the T)
the question is also is the F you mention the value that SW displays in the prestressed part _axial ??