Rigging CG calculations in Excel 2

[svtolman]

There was a question in a thread that is now closed but there was no solution to the Rigging question that was asked. Itwas what is the tension on a sling with the load and angle known. I have made this simple spread sheet to calc it. You can input the Weight and angles and it will calculate the tensions.

On another note I am trying to figure out the equation to adjust the vertical slings on a 4 point pick with 3 spreader bars. I have been using a CG calculation that adjusts the Y part of the upper spreader bars to move the crane hook over the center of gravity. What I am trying to figure out is how to make it so I adjust the 4 lower vertical sligs by adding shackles to come up with the CG adjustment.

The center of gravity is a known factor. This is for hoisting equipment where the load on each sling is calculated from the vendor which in turn gives an accurate center of Gravity. I have been using excel to generate lift plans for 6 years and I would like help for this equation. If possible can we leave out the beta's and alpha lingo since all it requires is simple math equations. The idea is so people actually doing the work can understand how to figure the new sling length.

So to get started here is the idea. If the CG was perfect between 4 pick points then all slings would be the same length (lets say 20'). If it moved to one side horizontally then the 2 slings on the heavy side would have to stay the nominal length while the 2 on the light side would have to get longer (20' + 1ea 25ton shackle). The idea is to lengthen the slings with the use of a shackle or 2 on one or 2 slings while the other ones stay the original length. This can be compounded if the CG moves in both horizontal directions.

If you have a solution please advise and I will try to implement and share the file as I generate it. Appreciate the help.

http://files.engineering.com/getfil...1-519a8108ea9d&file=Simple_Sling_tension.xlsx

 

[desertfox]

Hi svtolman

I think I understand what you're trying to do, ie get a formula that calculates the tension in a four point lift with slings going to a single crane hook, which is situated over the centre of gravity of the item you're lifting and in addition calculate the length of the slings, assuming the centre of gravity is asymmetrical in two planes.
If this is the case I think it can be done using the displacement method and compatability of the wire ropes or slings.
I'll post back when I have looked into it a bit more.
Oh just one other thing what do you mean by the 4 point pick with only 3 spreader bars?

Regards

desertfox

 

[svtolman]

Thanks Desertfox for the reply. As far as calculating the sling length or tension with the use of a 4 way that is a differnet situation. What I am tlalking about is picking up a skid or module based structure. So imagine a box that is 40 feet long 15 feet wide and 15 feet tall. You will have a 3 spreader bar configuration to pick it. The upper bar will have 2 slings on it and be 40 feet long to give the long spread. Off the bottom of that upper bar will be 2 15 foot bars, one on each end. These will attach with 2 slings off the top to the bottom of each end of the 40 feet bar. Off of the bottom of these 15 foot bars will be vertical slings that will attach to your trunions or lifting eyes located typically at the bottom of the skid or module.

So if the center of gravity is not perfectly centerd between all of these 4 pick points then there has to be adjustment made. This can be done to the upper slings off of the spreader bars to move the hoisting hook over the center of gravity so the load is picked level or it can be done with the 4 vertical slings that attach to the picking eyes from the bottom of the lower 2 spreader bars. I know how to figure the upper sling adjustments but this is rarely done since if it was not correct then the whole assembly of lifting equipment would be lowered to the ground. More often there are shackles added to the lighter sides of the slings that attach to the lifting eyes causing them to be longer.

I am trying to figure out a way to calcualte the length of the longer slings to hoist the load level.

 

[desertfox]

Hi svtolman

Okay looks like I was slightly off with my thinking, can you put a sketch up so I can see exactly what you mean, I still think it can be worked out with sling displacement and compatability.

desertfox

 

[svtolman]

Here is a picture of the spreader bar arrangement. The back crane is using the 3 spreader bars. The lowest slings that attach to the load will have shackles added depending on where the CG of the load is, this allows the load to pick level as well as not damage the structural integrity of the equipment being lifted. I am trying to figure out how to calculate the lower sling lengths knowing the center of gravity. I am not sure what sling displacement and compatability is that you reference. I am new to forums but am starting to use them to find information as well as provide information that I may have for others. Thanks for the help.

http://files.engineering.com/getfil...-4d8a-b437-c8556adbb99f&file=20-11-09_038.jpg

 

[desertfox]

Hi svtolman

Thanks for the picture, okay I think in order to calculate the lengths of the slings and in order to keep the lift straight you need to calculate the tension in the sling first.
The reason for that is that if you imagine the CofG is not symmetrical, so the slings come to an apex at different angles, however the vertical dispacement of each angled sling as to be the same, in order to lift straight up, but the actual strain in each of the slings will be different but the vertical displacement of each sling will be the same as previously stated.
This is what I mean about compatability and sling displacement or strecth if you prefer.
I'll try and upload an example in the next day or so.

desertfox

 

[svtolman]

I have done many calculations figuring the tensions on the slings and the upper angles of spreader bars. Maybe it is required to figurethe lower slings as well so I will work on that. The main thing about the lower slings is that they will be in the vertical position and no angle on them so there should be minimal tension from a diagonal pull unless the spreader bars are not the exact length to the picking eyes. For information most arrangements like you see in the picture require no more then 5deg from vertical on the sling to lifting eye connection.

The primary factor is that if your nominal length is lets say 20 foot slings and the CG was to move only in the long plane then 2 slings would stay at 20ft and the other 2 would have to be lets say 21 ft to compensate.

Thanks for the input and I am sure we will come up with the solution if I am ableto word it so you can picture what I am looking for.

 

[dhengr]

svtolman:
You may be kinda beating a dead horse here. I’m sure an exact sling force, length, elongation, (the latter two are desertfox’s compatibility items) etc. can be calculated, but the real world keeps getting in the way. The vendor gives you a weight and CG, within what error limits or tolerances? They should expect some lateral load input into their lifting lugs and skid structure due to field lifting (your 5° or less from vertical, maybe should or could be 10°), unless they furnish the rigging equipment to prevent this. Ask them what this limit is, and if they expect it to be zero, they should furnish the spreaders and slings tuned to their exact CG’s and out of level lifting criteria. Furthermore, you have a limited number of fairly standard length slings, not an infinite number of lengths and diameters. Finally, you say your easiest length adjustment method is with 25t shackles, that’s 7" length increments, and not a particularly good treatment of shackle pins. You know or should analyze which sets of slings are most economical to get in various lengths and increments of lengths. I suppose the shorter sets (approx. 4' to 8' long) are least expensive because of the req’rd. length and cable size.

Otherwise, I think this is kind of an iterative process where you get your field guys in the right ball park and then offer them some fine tuning means if they really need it. Don’t forget, everything here has a F. S. of 4 or 5, in part be cause absolute fine tuning is not ver likely in the field. The venders lifting lugs and skid structure in the immediate vicinity of the lugs should be expected to meet a similar criteria. I would look at the side and end elevations of the skid structure, with the lifting lugs and CG located; then you said it, ‘the lighter side slings must be lengthened’ to swing the CG under the hook. This is just geometry and trigonometry and structural statics, isn’t it? Now, you make a judgment about which of your std. sling lengths and a number of shackles, for your needed length. Finally, you check stress and elongation and make adjustments accordingly. In the photo you provided, the end view involves the 4 vert. short slings at their lifting lugs, and the 4 inclined slings above the lower two spreader beams. In the office you can make adjustments in either sets of slings. In the field I would have final adjustments below the spreader beams or at the vendors lifting lugs. The side view involves the upper two slings, just below the hook, and could involve equal length changes in sets of slings below the upper spreader, on the left or the right.

After the first iteration or so the geometry, forces and elongation won’t change enough to influence your final adjustments. But, of course, your final calcs. should show the final configuration of the entire lifting system. What you are trying to do isn’t really worth it, because you aren’t going to buy two 3" longer slings for one lift. If you are going to use shackles the way you suggest, I would make some doublers to fit over the pins, in sets of two. These would be made from mech. tubing or round bar stock: and have a length of 2.75"+; an I.D. of 2.062"+; and O.D’s of 3.5, 4.5, 5.5", etc. Given your calced. stretched length of the various sling sets, this would give you adjustments of .5 and 1" in sling length under load.

 

[svtolman]

Thanks Dhenger for the input. You are correct on all instances. The shackles do in fact only give minor adjustments but with loads like this that is all that is usually needed. The rigging contractor or ourselves usually have a wide variety of spreader bars and rigging. So we would have 18' and 20' lower slings and also 8' , 10', and 12'. What I ahve done is build sheets that will calc the sling lengths when the CG is moved. Of course if it shows that one sling needs to be 2" longer then we would just go with it since it is not out of level much. This is like you said just some geometry and Trig. Just trying to see what kind of knowledge and information is out there on this matter. Once again thanks for the good points and appreciate the comments.

 

[dhengr]

svtolman:
In my last paragraph above, re: shackle pin doublers; I actually meant the doublers for another reason, but length adjustment could be an added feature. The shackle pins are not really intended to be loaded pin on pin or pin on shackle, the way you would be loading them, and I wasn’t specific enough on this being the main reason for the doublers. The bearing stresses can be very high when you use them your way, causing pin denting and yielding in bearing; also pin bending stresses can be very high, with potential yielding, etc. I would want to distribute the point loading and protect the pins if I used the shackles this way. Slings or holes in lifting lugs treat the shackle pins much more gently.

Maybe you’re trying to hard to use a spread sheet to do the first iteration. I would just do a scale drawing of the two elevations and start inserting, superimposing, my standard slings and spreaders below the hook and above the lifting lugs. If you have access to a CAD program I’ll bet you could lay this out, and fiddle with it, and the program would keep giving you your new unknown lengths ever time you moved the end of a std. sling or end of a std. spreader.

 

[desertfox]

Hi svtolman

I have been working on your problem and I must confess I've
hit a wall.
I calculated the tension in four slings all vertical with an offset centre of gravity in one plane only,but now I cannot see for the life of me how I can adjust the slings and keep them vertical and keep the device I am lifting horizontal.
Have you any calculated examples of a four point lift with an offset centre of gravity?

desertfox

 

[BwB75]

If you can determine which way the load is leaning on a test pick, I would use chainfalls (assuming your calculated loads are reasonable) on the side it is leaning to using the fixed length slings on the 'short' side to make load adjustments instead of trying to use extra shackles and alot of other equipment that you will have to keep changing out to get it right. This may be required above and/or below spreaders. If you aren't quite sure of the tensions, include some load cells if necessary. As for a spreadsheet...

To an earlier poster: there is no issue putting pin to shackle, or pin to hook, only pin to pin is a no-no (speaking with regards to the Crosby product line, can't say for other manufacturers as I don't use them). Load just needs to be reasonably centred on pin (and cover at least 50% of it preferably).