C of G Allowance

[UGMENTALCASE]

Morning all,

I am working on a fixture which when lifted by a crane can either be empty or have a weight one end. Now I use CAD software so it calculates where the C of G will be, so in playing about with it I've got it pretty good about the centre of the fixture. My question, is there some sort of tolerance/allowance either side of the C of G where you are still good to lift something before it starts to dramatically tip.

Obviously you want it bang on but for example the tool I have now the customer isn't going to want to load on weights to balance it out depending on what lift he/she is doing. So I need to some how balance the difference between the two C of G's.

Say you take the see-saw of a decent length, 10 kg one side 10 kg the other. Pivot bang in the centre, it's balanced. You add 1 kg to one end, it will tip yes but by what angle? This way you can work out what is an acceptable angle for the lift to tip by, and to what distance you would need to move the pivot in order to balance it out, therefore understanding what tolerance/allowance you can have between two C of G's before the lift becomes unstable. This tool is about 700mm wide, and locates on a central bung (hole on to shaft) a couple of degrees out on a lift won't be an issue, someone lowering it can easily adjust that by helping it in. 5-10 degrees I would say is slightly more unacceptable.

Am I talking rubbish? I'm not sure....

Thanks in advance.

 

[KootK]

I would expect the payload to rotate until it reaches a position in which the COG is aligned with the lifting point. If that's a problem, multiple lift points might be the way to go. In your see-saw example, I would expect the see-saw to rotate all the way to vertical.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Lomarandil]

Bingo. Depending on the geometry of your tool and lifting arrangement, when you're figuring out the rotation to line the COG up with the lifting point, you may also need to consider where the COG is in 2D/3D, not just along the length of your "seesaw". If the lifting points are above the COG (for a horizontal object prior to lifting), the rotation required for equilibrium will be less and you avoid the possibility of total instability (like the seesaw going to vertical).

 

[UGMENTALCASE]

Hi thanks for the replies, probably should've put this image on in the first place!

Ok so that's the rough shape of the fixture. We have the fixture centre line, the weight which comes on and off, the c/w (counter weight) and the two c of g positions, loaded and unloaded fixture. The only thing that changes on the weight that comes on and off.

The lifting point is where it is, it's on a welded plate on the top, and there isn't anywhere else to move it to, and no I didn't design it that way

So we have the 2 C of G positions. The loaded fixture is nearer the lifting point, where as the empty fixture is further away. (I've measured to the centre line of the fixture I don't know why) what I was getting at was I doubt I'll ever get them both bang on under the lifting point, so they will sit off the lifting point by an amount. How can I work out how much the fixture would rotate by if I was to lift with a loaded fixture and an unloaded fixture.
So if I have the c of g right in line with the lifting point, it's a straight lift. If I have it 8mm to the left, what angle would it lift at? I personally can't see that if I'm out by a couple of mm the fixture would tip up to vertical, it would just rotate a small amount and rest, don't get me wrong there is a point where it would tip vertical.

Cheers

 

[rb1957]

Don't lift it from a single point ! It'll be much more stable if you lift from four (or three) attach points.

If you lift from a single point, the body will rotate until the CG is under the lift point; statics, FBD. clearly setting the body back down to the ground will be somewhat "difficult".

another day in paradise, or is paradise one day closer ?

 

[dhengr]

UGMENTALCASE:
You are kinda over thinking this problem. Draw your fixture to a reasonably large scale, so you can scale things fairly accurately. Do this about each elevation. Your lifting point is at a fixed location on the fixture, and is it a lifting eye and single part sling, or what? If you then rotate your scaled drawings until any C.G. is directly (vertically) under the lifting eye, that is the position the fixture will assume when lifted. And, you can measure (scale) the angle and any dimensions to important points on the fixture off of your sketches. Most riggers would consider this good enough. When you start worrying about the C.G. being correct + or - a couple of mm, you better be sure to brush the dust off the fixture before you lift it.

BTW, your sketch shows the two C.G’s. and the general arrangement fairly well. But, you would be surprised at how important some weights (loads), dimensions, and a reasonably well proportioned drawing are to an experienced engineer as he first looks at the problem and makes some initial judgements and comments and directions of action.