I'm doing a resistance study on connections between two parts in solidworks simulation. But the result doesn't seem strange to me:
With class 8.8 screws I get a safety factor of 1.8 compared to its elastic limit (which is not much)
Then, no matter if I put 10.9 or 12.9, by adapting the resistance/elastic limit/torque parameters, I get the same result :/
Basically, the more the class of the screw increases (and therefore the elastic limit), the greater the torque to be applied for tightening, so the strength of the screw doesn't really increase?
Is there anything I didn't consider? What do you think?
If the resistance of the screw increases: the applied force increases over a constant section so the stress increases, it is the safety coef that is constant, which is consistent.
While waiting for the more detailed answer from @Zozo_mpour simulation pro, I have the impression that there is a wolf in your study, can you detail the exact objective of your study?
We had a class 8.8 screw breakage on an installed equipment, I am trying to see if the replacement of these screws in higher classes would be enough. It's a piece of equipment that undergoes regular starts, I think the breakage is due to the fatigue of the bindings.
OK, so I think your simulation is misdirected I'm not a simulation specialist, so I couldn't guide you on the software, but from a mechanical point of view, it's not increasing the tightening torque that will solve the problem. What you need to do is determine the force that is exerted on your screw and perhaps look at the mounting method (use of threadlock or other anti-loosening device).
It would be necessary to provide details on the conditions of use (numerous starts??? what about inertia and backlash) and breakage and specify how the thread or nut behaved and also the screw (what type, BTR or other), also the conditions of vibration of the part fixed by the screws in question.
In any case, if the part is not in question and only the screw is studied, it is not a simulation in Solidworks. I usually say on this forum among other things that the standards of the screws are known and the conditions ultra-detailed by all the suppliers so if it breaks it means that the elements that presided over the choice have not all been taken into account.
@Stefbeno dealt with a similar subject a few days ago and the solutions he has chosen are to be looked at closely AMHA because there are similarities with your problem.
Can you post your simulation so that at least look at the parameters used.
I just looked at your file and the conditions used for the simulation.
The parameters used for the simulation are incomplete and AMHA will not give anything anyway.
Looking at the big picture (post an image so that our colleagues understand what we are talking about) I would say that because there are three slotted holes (19 mm wide for M12) with large clearances, I would gladly bet on a radial shear of one or more screws.
You don't say anything about the washers and I would like to see their head after the screw breaks.
What I would do to prevent this from happening again, I would put a 6 or 8mm plate in one piece under the screw heads with 12.6 mm holes, knowing that the screw spacing of the motor is known and immutable. In addition, to avoid any loosening (which is the cause of shear) I would put special anti-loosening washers (look at Emile Maurin's reference 74992DP 12 ). I would ensure torque wrench tightening with a suitable tightening according to the screw supplier's recommendations.
For the record, a screw should never work in shear, so at worst, if you use 12.9 screws, you will at least get rid of the shear, but will not solve the fact of unscrewing, or other construction anomalies.
Conclusion : slightly modify the design with a plate and good washers.
If I looked closely at the geared motor, there is a hollow shaft outlet, which means that you have a shaft and that if this shaft is greater than 60 cm, then you have a formidable lever, so no wonder that your screws, in case of loosening, end up working in shear, which is strictly forbidden by the state of the art. Eventually, say more about what happens on the shaft at the output of the geared motor, I give you my ticket that there is no one by the slightest elastic coupling.
What would be missing elements in the simulation? Not having had full training, it would help me to improve myself.
What is planned for the attachment of the torque arm:
Torque arm elements: 2 M12 screws, 4 M12 flat washers, 2 grower washers and 2 nuts.
For the motor: 4 M12 screws, 4 M12 flat washers, 4 grower washers.
The screw that broke is the one identified by the annotation on the torque arm:
Here I am trying to solve the problem without redesigning the arm for now. If I can't find anything else, maybe the implementation of a soft starter would limit the problem?
Indeed, the shaft measures at least 600mm.... and drives a conveyor belt.
You pursue your idea of simulation, which is not the problem. With the additional explanations I realize that it is even more logical that it breaks.
Especially with crower washers! I've never seen that in people who do metal framing or heavy mechanics.
My diagnosis on the shear is even more likely, because you have a very high torque and a rotation between the two offending screws. The only solution without making a big modification is to put 2 to 4 cotter pins in the horizontal U to eliminate the rotation between the two parts held by these two screws or to put two more bolt screws on the U. Simple and effective !
I maintain my point of view to further inform.
That your breakage problem is not due to the size of the screws and the tensile forces, but to the forces generated by the conveyor belt (which I had to imagine and which you confirmed). Since you don't have any information on the displacement of the shaft in the three dimensions and the real torque at the output of the geared motor, nor the weight of the conveyor belt, nor its coefficient of friction when empty and under load, you can't do a serious simulation. Occasionally, look at the opposite side of the engine how the shaft bearing behaves, you might be in for surprises. Also we have no information on the actual attachment of the horizontal U on which everything is fixed and which is certainly a part of the carrier frame itself. To make the slightest simulation you need to have all the elements and moreover it is not a simulation problem, but a simple banal mechanical problem in a way. There is nothing to find or prove about screws that are made to work in shear, which is totally forbidden.
No, I don't persist in my idea of simulation, I was asking what was missing for it to be correct so that I could understand and improve with this module of Solidworks. (for my experience)
Thank you very much for your help and your solutions, I will follow your advice.
