About Lifting and Lifting Rings

Simulation
1

Hi all

I want to carry out a lifting study in solidworks for a part on which another is placed. I also distributed the mass of the upper part on its 4 supports, and added 4 welding lifting rings made in Norelem 3D.

My concern comes from the consideration of these lifting rings. Within the design office, it is considered that these rings are not recalculated, because they are already sized by the manufacturer, there is no need for them to go through the calculation box. As a result, I don't really know how to apply the load... an equivalent of the force of each pulling strand on the contact surface of the weld? A force from a distance? For me it's not good because we don't take into account that the weld will be both in compression at the top and in tension at the bottom... Anyway, all this to say that I don't know if I can trust the "make rigid" function that I applied to each ring. Does this impact the displacement of the latter during the calculation? I have a slightly strange visual at the result when I do this...

If you have a better method to recommend I'm all for it.

For the moment, the method of the person I replace at short notice was to apply forces or masses at a distance at the level of contacts with other elements, and to block all or part of the model with plane supports to prevent it from going into "great displacements" during a lifting in calculation (imagining an old drawer I was told). But it seems to me a little... wobbly.


fer_levage.zip
2

Hello

Concerning the lifting rings, indeed the CMU is often notified on it. So at this level, there are no worries. However, the rings are welded vertically! Is this normal?

3

Hello

I think the problem of non-symmetry that you see comes from the fact that your structure is not immobilized. The 4 cables (straps, chains?) being parallel, it is free to oscillate from left to right, back and forth, or to pivot around its vertical axis of symmetry.
In real life, he won't get far, but in the SolidWorks solver, it introduces mobility that he doesn't know how to manage very well. It also rejects the calculation with the default solver...

The solution of the colleague you are replacing is the simplest here: add imposed movement constraints that will immobilize the 3 movements. We simply need to ensure that these trips have no effect on the distribution of efforts. It will be enough to check after calculation that they are zero.

A proposal:
- Impose on the "front/left" vertex a zero displacement in the horizontal plane (X and Z of the general coordinate system), which eliminates two mobilities;
- impose on the "back/left" vertex a zero displacement following X of the general coordinate system, which removes the last mobility.

You rerun the calculation, and notice three changes:
- the default solver works;
- the movements are symmetrical;
- the forces are distributed identically in the cables, and the last "supports" introduced are ineffective, almost nil.

A word on the limit of validity of the model: the fact of using 4 cables gives the structure a hyperstatic character (d°1). By imposing zero vertical displacement for the 4 rings, you have implicitly assumed that the voltages are identical in the 4 cables (centered load). However, the lifting system, through its mobility or elasticity, contributes to the distribution of forces, as does the cables themselves.

In the worst case, one can imagine that one of the cables is relaxed. In this situation, the diagonally opposite cable is also relaxed, and the load will be fully supported by the two cables of the other diagonal, always assuming a perfectly centered load.
This situation can easily be simulated by removing the zero displacement in one of the rings (at point 4).

The simulation shows a displacement of 10 mm at point 4, zero load on the ring at point 2, and a higher stress state than with the 4-cable solution identically loaded (118 MPa instead of 54 MPa).
Intermediate situations could be studied by placing an elastic support at the level of one of the cables. But with what stiffness?

In conclusion...
We would have to know more about the lifting system to go further...
Although it is "rigid", a 3-cable solution has a virtue: isostatism, which makes it possible to know the forces in the cables, and to get closer to an equal distribution by choosing an appropriate arrangement.
Note that the problem is of the same nature for the charge, represented by a distributed mass, also distributed over 4 zones...

Kind regards


levage03.png
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4

I'll test it tomorrow, a huge thank you;)

5

So to be sure:

Can I keep the 4 displacements imposed on my rings? What about the exclusion of rings from the calculation? I take note for the lift on 2 strands. Thank you very much

6

Hello Clément,

This is a model, so only you decide which assumptions are suitable or not... You can keep your 4 imposed displacements, keeping in mind that you have modeled the most favorable situation from the point of view of the load in the cables (70% of the WLL of the ring), and probably the constraints in the frame.
Lifting with 3 cables (actually 2) gives you the most stressed model (with a centered load). Probably the one that should be taken into account for safety reasons: force on a 13.6 kN ring for a 10 kN WLC. It's starting to raise questions...

To answer your doubts, you should know more about the lifting system, in order to make sure that the model is a good image of reality:
- the parallel cables (suggested by the axes, but are they?) suggest that there is a rudder system in the upper part: true or false. If they are converging towards a single centered hook, it is necessary to model in such a way as to incline the forces...
- Are they cables or chains, rather rigid, or more deformable straps. Are the attachments neat or "rustic", with or without adjustment of the lengths by tensioner(s)?

In the absence of a simulation that is 100% in line with reality (which one?), the analysis of the results must validate the construction in the most unfavorable situation. In this study, the frame seems generously sized, it is more the rings that deserve attention.
They are part of the calculation with the assumption of a perfect embedding in the structure. I think that we should not rely on the simulation as far as they are concerned: unsuitable mesh, questionable loading mode, welding modeling...
For them as well as for cables, it is  better to trust the manufacturers, based on the CMU they give for the choice of their components.

Have a good weekend.
 

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