Do Spreader frame effect CG

[teddyt11]

Hello,

Not sure if this is a stupid question but I've been staring at this for a while.

I have a big cabinet with 6 hooks on top that are not evenly spaced. I am designing a spreader frame so that there will be cables going vertically to the frame and then from 4 positions on top to a lifting device. The center of gravity is about 50 inches along the length and in the center of the width.

Will the frame effect the position of the Center of Gravity if it is not symmetrical around the CG?

Thanks so much!

 

[WinelandV]

Yes, it can, depending on the relative masses of the cabinet to the rigging. It should be fairly straightforward to account for, though. How level do you need the cabinet to be when you set it?

 

[teddyt11]

Hm ok, the mass of the cabinet is about 7 kips and the frame is most likely going to be w8x12 beams so I doubt it will effect much just was not sure.

The cabinet cant tilt more than a 7° incline.

 

[rb1957]

how heavy is the spreader ? .. that'd be it's effect on the CG

so you've got 6 loads and 4 reactions ... you shouldn't say 6 equal loads (since the 6 are not symmeric about the CG); likewise you don't have 4 equal reactions ... i'd determine the reactions as inversly proportional to their distance from the CG.

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

 

[teddyt11]

hmmm ok. Sorry didn't mean to make it sounds like they were even. But ok. So my first thought was to have the spreader just extend over the cabinet so that the CG of the frame would be the same of the cabinet and I would not have to worry about it.

Then I would just solve for reactions and design the frame beams. How does that sounds?

By the way thanks so much

 

[rb1957]

yes, even or not is the same. the CG is where it is reacted by 6 reactions into the spreader and 4 reactions into the wall.

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

 

[dhengr]

Teddyt11:
Show us some sketches of what you have, reasonably proportioned, and with dimensions and weights, including CG locations in all three views (plan, side and end views). What is this cabinet and what are the limits and weights on its six lifting points? Show their locations and orientations, with dimensions. We can’t see it from here, and if you look at your description again, and imagine you don’t know any more about the cabinet than we do, you’ll see that you haven’t given much meaningful detail or info. Can you lift from the four corners, and forget the two inner hook points? Trying to lift something like that from six points is very difficult to work out. What happens if a couple of the cables aren’t of perfect length? Everything below the main hook must theoretically be included in your lifting calcs., and the total CG (wire rope, hardware, lifting frame, and cabinet, etc.) will end up right below the hook. The whole lifted system will just swing into position, as cable lengths and stretch, and cabinet tilting allows. Usually, the lifting hardware is a small percentage of the total and is ignored, but sometimes it can’t/shouldn’t be. If the 7̊ tilt max. is really critical, you will likely have to provide for some cable length adjustment to make this work out. Why not just make a lifting frame which pins/bolts to the six lifting points on the cabinet, if you must use them all. Then lift from the top of that frame from four points. Then you can locate your top lifting points so the cabinet CG ends up under the hook. Your four lifting lugs should be centered about the cabinet CG lengthwise. You could lift from two lifting lugs centered on the cabinet CG lengthwise and provide two lighter and adjustable length snubbing cables to the ends of the lifting frame, just to limit tipping. Many times when you design for four lifting points, you design each for 1/3rd of the load, because invariably one of the cables will not be carrying its full load.