I try to size a spreader spreader used to lift a lifting platform (illustration below). Spreader spreaders connect two pairs of cables that do not have the same direction.
My problem: I can't find the right boundary conditions that are representative of the real conditions of use.
Working only on the spreaders, I have already tried:
1: On the upper holes: zero imposed displacements according to the direction of the upper cables; on the lower holes : load directed according to the lower cables
2: Exactly the opposite of the first attempt
Conclusion: the results of the two studies are completely different.
Do you have any ideas to help me solve this problem?
Thank you for your advice, this is indeed more interesting from a design point of view and I adopt it. On the other hand, I started the simulation again and it doesn't solve my problem.
@stefbeno
Can you elaborate on what you said? I'm not sure I understand what you said.
Yes I think I see what you mean, in fact do like the majority of hoists? The problem is that the distribution of the load on the board will not be the same and very rarely homogeneous and centered, it seems too risky to me but otherwise indeed it would have been much better.
I confess that I do not agree with you on this. In statics, we consider the system immobile and at height. At this moment we observe on the one hand the upper hole which is blocked in translation because of the hook and a force that pulls down on the lower holes (due to the mass of the plate + load).
It is true that in reality, the hook exerts the reaction force to the weight of the system, but for the simulation, we block the translation and if we look at the resultant at the level of the upper hole, we must indeed have the weight in the opposite direction.
If you want an alternative idea, I'll say that @MaD is only half wrong. The vertical components do indeed compensate for each other. On the other hand, the forces being in different directions, there remains a horizontal component on the upper hole.
So, with regard to the tube, you can keep a recess in the center only (symmetry condition) and apply only the horizontal load on the upper hole of the clevis. This can allow you to get rid of the parasitic effects of blockages on the screed.
For the sheet metal clevis at the end of the tube, generally it is largely oversized (and visually it seems to be the case). But if you want a more precise result, you have to add the tensile stress related to the vertical component. It is very easy to calculate by hand (sigma = F/S).
I think I see what you mean but I don't know if I was clear enough in saying "vertical direction": this one does not correspond to the vertical direction of my assembly. For the simu, I only work on the rudder pedal as a part and in this context, the vertical direction corresponds to the right one that goes through the center of the two holes.
In my opinion, this is what seems to correspond most closely to the reality of the placing under load. The boundary conditions must not be put in terms of the deformation that we imagine the system will have, they must above all represent the real conditions to which the system will be subjected.
@Chamade
Not blocking the lower holes would oversize the system. That being said, given the efforts I have (the chainring is made to lift 15 t so each hole takes at least 37,500 N), it will no longer be tube that I will have, it will be an IPN 200, 300... to resist such bending... My goal is also to optimize the mass of the equipment so as not to have an excessive total laden weight.
I should also tell you that I am working on the basic Solidworks version with the basic simulation module as well. I know that Solidworks is not the perfect software to do FEM (before I worked with Ansys) and I have a lot of trouble trusting the results it gives me.
I don't really see why your "tube" would be oversized if you remove the blockage in the bottom hole and only apply the horizontal component to the top one. The rudder pedal is balanced and all the vertical load passes through the end screeds. The tube alone therefore sees no vertical force (except its own weight if we want to be exact). You probably have a higher risk of buckling.
Also, in this case, if you don't trust the tool, you might as well do a check by hand or with RDM6.
Given the limitations of SW Express, I will only block the tube section for symmetry and apply the 2 forces.
Or to limit the effects of pure blocking, make a dividing line in the lower holes of the clevis and block only this face (instead of the complete hole).
Simulation Express will not allow you to get a correct result. This software requires you to have a recess (which allows it not to diverge and to make calculations quickly).
In my opinion, it is impossible to model your part correctly with a recess.
The least worst would surely be to put it on the symmetry plane and calculate your efforts and directions of application manually.
Conclusion: express simulation and just there to make you want to buy a premium license or a simulation license. The limitations of this 'demo' software are such that apart from a bending recessed beam you won't be able to calculate much
