Finding vertical CG of a large box structure
Finding vertical CG of a large box structure
(OP)
We have mfg a large box structure that we need to find the vertical center of gravity. We just shipped it out and have access to it on the other side of town. What I am looking for would be a way to to estimate the vertical center of gravity location on it. The base of it is structural so we can lift it or weigh it with scales no problem, but the walls and roof are not structural enough to tip it or anything like that.
Any help greatly appreciated.
Any help greatly appreciated.





RE: Finding vertical CG of a large box structure
Pick it up, place a spacer between thing and scales (creating an angle (a) of the base) at two adjacent corners and weigh again.
Calculate the amount the horizontal CG moved.
Vertical height of CG = Tan (a) / horizontal CG moved distance
RE: Finding vertical CG of a large box structure
RE: Finding vertical CG of a large box structure
if you can,
1) weight the box on it's own (standing up), one number is sufficient;
2) weight the crate on it's own (laying down, horizontally), at least two numbers (from both ends) so you can find the CG of the crate (between the two ends);
3) pack up the box in the crate (so the box is supported);
4) lay the crate+box on it's side and weigh each corner, it's probably good enough to weigh the corners one at a time, so long as you don't move the crate;
5) now you can find the CG of the crate+box, and then the CG of the box (by removing the crate).
Quando Omni Flunkus Moritati
RE: Finding vertical CG of a large box structure
Is this box on a trailer or a railcar, and the hauler wants to know the vert. CG? Calc. the CG and then ballast the load at deck height to bring the CG down below their limit.
MintJ’s method should work, it is technically correct. But, for small angles (a) of 5, 10 maybe even 15̊, the tangent and the sine of the angle (a) are so close, in value, that you may have trouble weighing accurately enough to do a meaningful calc., by his method.
Since we are not really playing 20 questions here, why don’t you tell us what the box is; its dimensions, weight, some of its construction details, etc. etc. so we might imagine how it could be lifted and tipped, etc.
RE: Finding vertical CG of a large box structure
Its getting shipped to northern canada and it just left our facility to travel across town for integration on one of the Kiewit chassis's, we get a call from the customer that we need to reveal the vertical CG so that paperwork can be submitted when it crosses the border. As long as we do our due diligence of trying to determine the CG (and add a couple inches to the found value for a SF) will allow it to cross the border.
If it had all been drawn in solidworks I would just go with what solidworks says, but we have a designer on staff how does a lot of this stuff in key creator (that's a whole other story).
RE: Finding vertical CG of a large box structure
My method should be workable.
RE: Finding vertical CG of a large box structure
RE: Finding vertical CG of a large box structure
Just rotate the box about one edge until the CG is just above the pivot point, Record the rotation angle a. Now do the same at the opposite edge and record the angle b. If the distance between the rotating edges is W then
h=W/[(tan(a)+tan(b)]
RE: Finding vertical CG of a large box structure
RE: Finding vertical CG of a large box structure
"Vertical height of CG = Tan (a) / horizontal CG moved distance"
Nitpicking,I get
Vertical height of CG = sin(a) / horizontal CG moved distance
For small angles they are close
RE: Finding vertical CG of a large box structure
I shouldn't do trig in my head without drawing a picture.
RE: Finding vertical CG of a large box structure
RE: Finding vertical CG of a large box structure
RE: Finding vertical CG of a large box structure
RE: Finding vertical CG of a large box structure
I redid the height calculation for Mint's solution and now find that the
vertical height=y1=[x1cos(a)-x2]/sin(a), not [x1-x2]/sin(a) as I previously posted (inverted)
x1= original position x coordinate of CG referenced from rotating edge
y1= original position= vertical height CG
x2 = x coordinate of CG rotated
x1-x2= moved x distance of CG
This is, in general, a significant change since x1cos(a) is less than x1 and the numerator may be small, so any error in either numerator term could be important.
FYI, the formulation easily derives from rotating the CG vector,viz
cos(a) -sin(a) x1
sin(a) cos(a) times the vector y1
y1=vertical height of CG
RE: Finding vertical CG of a large box structure
RE: Finding vertical CG of a large box structure
that is exactly what I am saying.
-have designer use key creator to determine the CG of each of the installed components; 2 days which is $1600
-get the CG for the mfg structure from mexico; I can't even guess how many days this would take
-the alternative; take two guys with four wheel scales, blocks of wood, digial camera, and tape measure; 1 day lead time and cost is about $500; its effective and its gets it done cost effectively
Which one would you choose?
RE: Finding vertical CG of a large box structure
that is exactly what I am saying.
-have designer use key creator to determine the CG of each of the installed components; 2 days which is $1600
-get the CG for the mfg structure from mexico; I can't even guess how many days this would take
-the alternative; take two guys with four wheel scales, blocks of wood, digial camera, and tape measure; 1 day lead time and cost is about $500; its effective and its gets it done quickly
Which one would you choose?
RE: Finding vertical CG of a large box structure
And how sure are you that the equation that you need to perform this task is accurate? After all, you got 3 answers for that and why would you use any one of them without proof? What I am saying,in general, it is very dangerous to rest a design (or calculation in this case) on the results of forum contributors who are giving you their honest opinion. But is that sufficient ?
RE: Finding vertical CG of a large box structure
http://hyperphysics.phy-astr.gsu.edu/hbase/pend.ht...
Ted
RE: Finding vertical CG of a large box structure
RE: Finding vertical CG of a large box structure
RE: Finding vertical CG of a large box structure
Long truck trips through the wilderness need adequate CG info for loading and moving.
RE: Finding vertical CG of a large box structure
RE: Finding vertical CG of a large box structure
RE: Finding vertical CG of a large box structure
how much validation do you need ?
Quando Omni Flunkus Moritati
RE: Finding vertical CG of a large box structure
RE: Finding vertical CG of a large box structure
Quando Omni Flunkus Moritati
RE: Finding vertical CG of a large box structure
RE: Finding vertical CG of a large box structure
If you wanted to further reduce the chance of tipping, and increase the accuracy, you could plan to use the spring scale weight as part of the calculation. Modified method of the "tilt and weigh 4 corners" mentioned earlier. If you've got X,000lbs on the crane, X,000 on the ground, know the horizontal COG, and can measure the tilt angle, from there it is physics 101.
I'd like to think that this is a pretty elegant solution. Depending on the details it works out as the best option for accuracy, man hours (only thing quicker is a WAG - wild ass guess), engineering justification, and safety. You could eliminate ALL assumptions and peg the vertical COG dead on if you wanted, but as already mentioned, probably not worth the trouble.
I'd pull one side up with a spring scale until I had an appreciable angle and a clear change in weight distribution, then plug it in.
Because he is the engineering manager, not the shipping manager. And if this works out well he can write a procedure, this could be a chargeable item for a customer - dry and wet COG's verified.
RE: Finding vertical CG of a large box structure
Quando Omni Flunkus Moritati
RE: Finding vertical CG of a large box structure
Neither of us knows the value this information might have to the end user. It goes on a truck, is vertical COG limiting any operating parameters? Does the end user need to confirm max incline it can climb, max safe speed, safe braking force, govt regulations if it needs to travel on a public road, blah blah blah, all the boring "no, because we can do what we've always done" questions that a silly engineer might ask. Sorry for being pedantic.
RE: Finding vertical CG of a large box structure
The two center sills in the floor framing of your lube body which will mate with the truck chassis sills, should be fairly husky structural members. Maybe you can weigh it and lift it from these. Weighing it on/at the four end points of these sills should give you a pretty good CG in plan. Accuracy in scale positioning and these measurements and in the four actual weight readings is important for good final calcs. and results. If you were to put the lube body up on two saw horses and on these two center sills, this should allow clearance for the parts that hang lower, and still allow you to roll the lube body. Then lift on one sill, and roll the body until all 17k is on the two scales on the other sill; then draw a plumb line (hang a plumb line) from that pivot point, up the ends of the body; do this same thing on the other sill, in the other direction, draw the plumb lines each end; where the two plumb lines cross on each end should be the height CG. Of course, this requires that you support the lube body so you can get to these balance points on one sill, without the body actually rolling over, out of control. This larger rotation will give you the most accurate results without any trig. calcs.
In my first post the point I was trying to make is that a larger angle of rotation will yield better calculated results because your weight readings and angle measurements are not so small and critical. I still think that with a small angle of rotation you may not be able to weigh accurately enough at the four corners to make an accurate height calc. A larger rotation would likely yield better results. Both plan location and height location of the CG will change btwn. dry and full wet weight conditions. Maybe you want to bracket these extremes.
I’m with 1gibson on this, this is not a trivial problem, or bit of info. But, only you know what degree of accuracy you need to achieve. maybe do this whole process several times, in opposite directions, or on different lube bodies to see that you get some consistency in your results. Something you can really hang your hat on. And, they type of equipment you have at that shop will determine how you actually do this.
RE: Finding vertical CG of a large box structure
Ted
RE: Finding vertical CG of a large box structure
Not a simple pendulum. You need I.
RE: Finding vertical CG of a large box structure
Ted
RE: Finding vertical CG of a large box structure
RE: Finding vertical CG of a large box structure
With assumptions like that, I would take Rb's center of box as better.
RE: Finding vertical CG of a large box structure
Quando Omni Flunkus Moritati
RE: Finding vertical CG of a large box structure
The process works by moving a known weight over a known distance, and measuring the resulting inclination. you have to pick the item up and sling it with a beam make sure that it can only rotate about the longitudinal direction. Then you have to make an apparatus mounted to the top (or bottom) where you can move the known weight a known distance. you might need a small leveling weight to ensure you start from a level position.
You measure the resulting angle with a pendulum or a digital inclinometer.
Then you use an equation that says GM= (known weight x known distance)/(tangent of the resultant angle x the weight of the entire object) . For an incline in air, the GM is the distance from the hinge point to the VCG.
See ASTM F1321 for the process to perform the inclining in water. Same principles apply for in air.
RE: Finding vertical CG of a large box structure
http://www.longacreracing.com/articles/art.asp?ART...