Rigging Formulas Free
admin 10.10.2019
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.
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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.RE: Rigging CG calculations in Excel.
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.
Free Rigging Formulas
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. RE: Rigging CG calculations in Excel (Mechanical) 26 Apr 10 00:40. 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.RE: Rigging CG calculations in Excel (Mechanical) 26 Apr 10 07:20. 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. RE: Rigging CG calculations in Excel. 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. 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.
Definition Of Free Rigging
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.RE: Rigging CG calculations in Excel (Structural). 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.
RE: Rigging CG calculations in Excel (Structural) 28 Apr 10 01:30. 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.RE: Rigging CG calculations in Excel (Mechanical) 2 May 10 18:23.
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).
I assume your angle represents and angle from vertical.Call you angles alpha and beta.Call the associated angles T1 and T2.Call the weight W (44,000 lbf)Draw free body diagram.From the vertical forces, we get equation 1:T1.cos(alpha) + T2.cos(beta) = WFrom the horizontal forces, we get equation 2:T1.sin(alpha) + T2.sin(beta) = 0Two equations is 2 unknowns. You can solve them by hand. Or else use excel to solve them using one of the techniques on attached spreadsheet (discussed a little bit on the thread on simutaneous equations)Eng-tips forums: The best place on the web for engineering discussions.RE: Excel to solve rigging problem (Electrical) 7 Sep 08 12:07. Your angles are angles from the horizontal. Mine were angles from the vertical. So put the following into the above equationsalpha = 90 - 34beta = 90 - 57Also, I assume there are two individual slings are individually attached to the 'hook' (or whatever the point of attachment at the top), rather than a single sling allowed to slide through the hook.Under those assumptions, result should be as I have given above.Can you try drawing a free body diagrams showin all the forces acting on the lifted piece? Your approach might work - I haven't looked at it closely.There was an assumption built into my solution that this thing is in static equilibrium with the piece level and lifted.
After all, most lifts are arranged to keep the piece level. From the drawing, it looks like the piece would tilt CCW. In that case, everything shifts relative to vertical (and therefore relative to the weight vector).
Equation 1 needs to be modified by multiplying W by cos(Phi) where Phi is tilt angle from vertical.An additional equation is required to solve for that new variable Phi. That new equation would come from setting the moments about the center of gravity equal to 0.At least that's the way I look at it. That's all I've got time for now. Maybe someone else will jump inEng-tips forums: The best place on the web for engineering discussions. RE: Excel to solve rigging problem (Industrial). Thanks electicpete.The load is in equilibrium and the sling angles are those measured while hanging.
It has a much heavier mass on one side and the lifting eyes are not on a horizontal plane. My sketch shows only half of the rigging and connects to an I beam.
There is an identical sling set 4 feet in from the first. That beam is then raised by 2 hydraulic posts. The total load is estimated to be 40 Tonnes.However the configuration is just a sample and I would like to make a spreadsheet that would easily resolve future lifts/ issues.To me the problem is fundamentally the math behind a common tug-of war type of problem.20 kids from grade 8 have challenged kids from grade 5 and 6.
Each grade has its own rope tied at a common spot. Grade 5 can only pull at 56 degrees, grade 6 can only pull at 33 degrees.(from the pull of the grade 8 line)How many kids does grade 5 need and how many kids does grade 6 need to exactly equal the pull of the grade 8's.
(Each kid has the same pulling power)RE: Excel to solve rigging problem (Aerospace) 7 Sep 08 23:46. I played with my rigging problem some more and took a different tact. I would appreciate anyone looking at this to tell me my logic is right / wrong?? Seems to work though.To recap.
I know the weight of a load and when lifted I can measure the angles of each of the two slings to the horizontal.The question was. What is the tension in each leg and what is the horizontal pull of each leg.My reasoning went along these lines. The vertical pull at the end of each sling is 1/2 the total weight. Knowing this and the angle I can now use c=a/2/sinA to determine the tension and b=a2.cotA to determine the horizontal pull. As my Excel does not seem to have a 'cot' function I ended up using b=a/2.1/tanAelectricpete. Your solution may have worked had I not erred in using your formula.
Seems that one needs to convert degrees to radians when using the trig functions.Included is my spreadsheet rigging1.xls if anyone cares to check its validityThanks all.RE: Excel to solve rigging problem (Mechanical) 14 Sep 08 21:09.