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.

 

[hydtools]

If you can pick it up and get it swinging from a single hook approximating a simple pendulum use pendulum equation.

http://hyperphysics.phy-astr.gsu.edu/hbase/pend.html

Ted

 

[mackconsult]

We have already ball parked it at 30 to 32" from the truck frame top. We don't have the actual weight, so going though this exercise has validity in just that. I was going to use all the calculations and see how they come out.

 

[chicopee]

Looking at your web site, your ball park value looks about right. I guess your Artic lube body fits on a truck frames. Nice.

 

[racookpe1978]

I am going to disagree with any criticism of not getting the vertical CG correct - at least within 3- 6 inches of actual vertical position. Horizontal CG is vital also when lifting 17,000 (or 700 ls!) of an unknown load. The riggers will look at YOUR marking and use them. You can be off a little, but you can be wrong when somebody else's crane and people and trucks and forklifts are using YOUR painted mark for THEIR lifts.

Long truck trips through the wilderness need adequate CG info for loading and moving.

 

[racookpe1978]

By the way, don't you have it in 3D CAD? With proper and accurate internal eqpt loadout of the internal GG's of the all of the stuff inside, you should be able to the internals, the frames, the walls, and the floor/roof added together.

 

[mackconsult]

I don't think key creator has this capability. Its not parametric CAD application. What ever we come up with we are going to add a couple inches as a SF.

 

[rb1957]

could it be higher than the geometrical center ?

how much validation do you need ?

Quando Omni Flunkus Moritati

 

[mackconsult]

No it won't be higher than geometric center. The inside is full of lube/oil tanks that only come up about 2/3's of the height of the structure. I don't think we need much validation, the owner of the company has experience with this stuff and he is sure that the number we come up with will pass any requirements.

 

[rb1957]

so if you think the geometric center is reasonably conservative ... why fuss it any more ?

Quando Omni Flunkus Moritati

 

[mackconsult]

Its a good exercise, and documenting it gives the ability to determine this again on other projects.

 

[1gibson]

Do you have a crane with spring scale? Lift up one side (SLOWLY!) with the crane until the weight reading is below some threshold, 1,000lbs, 2,000lbs? Whatever you feel is safe, crane still supporting enough weight to ensure you don't topple the thing over. Treat that as the tipping point, measure the tilt angle, and use trig to get the vertical COG. It will be as conservative as your spring scale threshold.

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.

 

[rb1957]

procedure could easily be "use geometric center, if appropraite" and bill the customer the same ...

Quando Omni Flunkus Moritati

 

[1gibson]

Will keep that in mind next time an interesting problem comes up: "Why bother, put a conservative number down and move on." I would be interested in hearing the engineering justification for dry and wet COG being the same number though. Have a cup of coffee and maybe reconsider?

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.

 

[dhengr]

Mack:
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.

 

[hydtools]

Pendulum still sounds simpler. Suspend the box. Measure time for one swing cycle. Calculate distance from point of swing to cg. The rest is measuring distance from point of swing to bottom of box support structure and subtract calculated pendulum length.

Ted

 

[zekeman]

"Pendulum still sounds simpler. Suspend the box. Measure time for one swing cycle. Calculate distance from point of swing to cg. The rest is measuring distance from point of swing to bottom of box support structure and subtract calculated pendulum length".

Not a simple pendulum. You need I.

 

[hydtools]

Do not need I. Assume mass is concentrated at the center of gravity.

Ted

 

[1gibson]

I like the pendulum idea, and it does sound simpler, but maybe not for something this size. By the time you rig something up that is solid enough to let swing around safely, you've got a good amount of labor and material cost sunk in there. Then you still have to convince the shop that swinging a 17,000 lb box around is a good idea.

 

[zekeman]

"Do not need I. Assume mass is concentrated at the center of gravity."

With assumptions like that, I would take Rb's center of box as better.

 

[rb1957]

we could put the box into a fixture and apply horizontal load and adjust the position of the load until the box translates (ie doesn't rotate) ...

Quando Omni Flunkus Moritati