Finding vertical CG of a large box structure 1

[mackconsult]

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.

 

[MintJulep]

Pick it up, weigh all four corners.

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

 

[mackconsult]

Thanks

 

[rb1957]

crate it up on a close fitting crate, so the box doesn't have to support itself.

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

 

[dhengr]

Mack:
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.

 

[mackconsult]

Okay then ..... let me try to explain. I work at CARCO industries as the engineering manager, and we just built an Arctic Lube Body.

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).

 

[MintJulep]

So it's big and not particularly delicate and the CG should be a fare distance above the floor.

My method should be workable.

 

[miningman]

By comparison to some of the stuff that goes into northern Canada these days, this is pretty small and I cant see Canada customs having a qualified P.Eng checking the accuracy of C of G calculations or anything similiar. Give them a number that looks about right and all should be OK

 

[zekeman]

I believe you can do it without weighing it.

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)]

 

[mackconsult]

BTW it weighs 17000 lbs and has pieces of it that go down 30", so its not something we can just tilt over on an edge.

 

[zekeman]

From Mint..
"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

 

[MintJulep]

Zeke is correct.

I shouldn't do trig in my head without drawing a picture.

 

[mackconsult]

Thanks guys. We are going out next week with wheel scales and will get it done.

 

[chicopee]

Estimate your CG from your blue print for Pete sake. The Canadian border people will just want a number for their paperwork

 

[mackconsult]

chicopee, I am sure we could do that ..... but the practice is good and establish me as a good engineering manager for this company. I am in my 3rd week and want to make a difference.

 

[zekeman]

My bad,
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

 

[chicopee]

Making a mountain out of a mole hill will not establish you as a good engineering manager for this company as it make more work for very little gain. A good engineering manager makes decisions based on the availability of material at hand without adding extra unecessary expenses. Proposal from the above responders will create extra expenses for little gain and when such resources can be used for other problem areas.

 

[mackconsult]

chicopee,

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?

 

[mackconsult]

chicopee,

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?

 

[zekeman]

"-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"

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 ?