Choker Hoist FEA

[vex]

So I have a cruciform type structure that needs to be used a hoist point. What we've come up with is a choker arrangement (think slip knot) that slings over the cruciform. We've got the loads required and direction that it will be applied to but, and this is where I have a little issue, is breaking up the geometry to accurately model the line.

See:

http://liftechniques.com/images/lifting_img2.jpg

and

http://www.anselljones.com/includes/images/mooring/cruciform.jpg

So since the vertical portion of the cruciform is larger than the horizontal how should I slice the geometry to better model a sling being pulled in a specified direction using a remote force?

[kmeldfreyssinet]

Hi,

I have to say that it is not clear for me what exactly is your problem since you post only some general sketches and picture.

But: Depending on what kind of analysis you expect you have basically two options:

1) establish areas that are directly subject to load coming form sling and apply loads to them - this I expect will give you only general idea of ow load is applied, and local effects are hard to analyse.

2) model your element and sling and apply load to sling. This iif set up properly should give you better results.

Can you post screen shot from your model and some more clear description of particular problem?

Cris.

[vex]

Sorry for the bad description of the problem. I'm trying to show the FEA of the cruciform and not the line. I've modled the portion of the sling going to the lift point so I know the tension vector of the line and the component pull of the various portions. If I could I would model the sling geometry and apply the tension load directly, however I don't know where the choker will find equilibrium (ie where the knot will rest--or the forces balance the tension).

So to get around that I divided the cruciform up into various sections (as can be seen in the screen capture). I then took the components of the tension and applied them to the general areas of action. This doesn't account for the sinching effect of the line applying pressure on the various components. I'm curious if there's a better way to model this to get slightly more accurate results. Thoughts?

[kmeldfreyssinet]

Hi,

do you need this cruciform to be used only for one vessel? because this would be strange for me.

For tension component perpendicular to axis of bigger pipe you can try using bearing load. This in theory should give more reasonable distribution of load than simple force applied to curved surface.

Other thing from my nautical experience is that there are two extreme situations: 1) knot is directly in the cruciform, 2) two ropes go practically parallel around so they are tangent to cruciform. So if you would run calc. for both you should get extremes, but would need to model sling.

also it is possible in real life that direction of sling is "random" so this horizontal arms can take different portions of force.

So depending on what you really need to do, there are probably different techniques that can be used. Could you than describe your needs in terms of result you need.

Cris.

[swalton]

Could you treat the sling as a band brake?  I did some quick searches for "band brake force distribution" and found this paper: http://www.om.ugal.ro/AnnalsFasc8Tribology/pdf/2003/I-ANALE-25-DRUMEANU.pdf  that looks like it might give you an equation for the contact pressure of the band(or sling).

You might have to add a moment load at your contact patches to get the full load case, because IV's FEA package can't do friction.

[vex]

kmeldfreyssinet, I think I understand what you're suggesting; however I would have to discretize the areas to apply the bearing load on the vertical and horizontal member. I would also have to figure out the magnitude of the bearing stress for each individual area, which although possibly, might not be the most efficient use of time. I think I'm after the worst case scenario to ensure sufficient material strength in the cruciform.

Based on 1) do you mean the knot will be directly on the cruciform? I can see that being the nominal case in this situation but I don't know how I could go about proving it. I suppose it would be a force balance with the addition of friction (though since this is an aramid fiber line could be considered nill).

As for 2), the rigger may very well choose to lift that way... and in all honesty I wish I had access to a rigger's knowledge/help as I think they would be able to tell me the better direction and alignment for the slings.

swalton: it appears the link you posted doesn't work for me. I get a 404.

[JDMather]

What is your goal for this analysis?

[swalton]

Weird. I get a 404 when I try to open the link from here too. If I cut and paste the link from the Firefox address bar to IE, it works.

Here is a new copy, I'll check it to see if it works.

http://www.om.ugal.ro/AnnalsFasc8Tribology/pdf/2003/I-ANALE-25-DRUMEANU.pdf

Anyway, the band brake idea was to get a pressure/force distrubution in the contact patch betwee the sling and the lifted object.

[vex]

JDMather, the goal is to ensure that the structure, as designed, will meet the spec requirements for factor of safety (Ultimate) when placed under a a lift criteria. As I've run the FEA it meets that requirement, but I'm not very pleased with the application of the forces and as such desire to have a more robust/accurate representation of a worst-case scenario for a choker type sling lift.

Swalton, I got the link to work by copying and pasting directly in to the address bar. I'll check it out momentarily and see if I can apply the information (a cursory look makes me believe I can).

[kmeldfreyssinet]

Hi,

is that roughtly what you are after?

Cris.

 I wanted to post dwfx but did not let me.

[vex]

It might be. But I would need to see how the geometry is divided and how the forces are applied to see if I can apply it.

[kmeldfreyssinet]

Hi,

I have made a sling.

Curently it is only a rought estimation. You can try using this technique.

I attachment I post model files. There is also stress analysis setup, but no results (way to big file).

Cris.

[vex]

Ah, I see. Unfortunately that arrangement will not help me as it's not a choker type hoist (providing a pressure acting on the vertical member while still providing the forces in 3 component directions).

[kmeldfreyssinet]

Hi,

this is what I am not able to image. Could you post some sketch on what are conditions for your cruciform? What kind of operation you need?

If this sling would go directly vertical FEA still would produce pressure on this bigger tube.

If you define sling of such a material and geometry that it will not be so stiff for bending you should get reasonable results.

If you post all details and description how operation of lifting is performed I can look in to it this particular case.

What I did is just a first example to show principle.

Cris.