4 leg sling

[vscid]

If I am using 4-leg wire rope slings to lift a load whose CG is offset from the pick point, would my load remain perfectly horizontal or would it tilt?
Thnks.

 

[WmacG]

It will tilt to a position where the CG will be directly below the hook.

 

[rb1957]

if the load and the lift aren't in-line then there's a couple to be considered.

either, one end reacts this moment, or the load will shift to align itself with the lift.

most likely (?), you'll be lifting with a crane hook (not much moement resistance there) so most likely the moment reaction will be where you pick up the load. your post implies a four point pick-up, so i think the load will redistribute itself internally (so that the four slings will react an equal amount of the load (assuming they're equally distributed about the lift point).

 

[baw1]

I agree load will tilt to align CG of load to directly below the hook for a single hook lift. There is no moment capacity in the cable supporting the hook.

Additionally, it is quite typical to size the slings for a four point lift, for a three point lift due to the possiblity of the load not equally distributing to the fourth sling. Something to consider at least.

 

[desertfox]

Hi vscid

agree with others it will tilt, however you can shorten the slings on one side to compensate and bring the thing your lifting level, in doing so though you increase the tension
in the slings you have shortened so you need to look at the lifting capacity of those slings.

regards

desertfox

 

[rb1957]

well, i'm guess i'm in the minority (i still think that the lift Could be un-tilted if the the lifting structure could absorb the off-set couple)

but why not lift above the CG ? ... if you don't know exactly where this is, do as desertfox suggests, and shorten some slings to move the lift point.

 

[vscid]

I cannot lift directly above the CG due to certain space constraints. I guess I will shorten the slings and size the slings so that load will be carried by only 2 slings (which are the shorter ones.)

 

[rb1957]

IMHO that's not going to work for you.

i think the only thing that changing the length of the slings does for you is move the lift to be aligned with the load; which you say you can't do.

how does the lift structure pick up of the load? i still think that if this attachment can react the off-set couple then the structure won't tilt, that the free body of the lift structure is the lift load, the weight, and a moment opposing the off-set couple.

 

[WmacG]

You may be able to add or remove weight to the lifted item to get the CG where you need it.

You may be able to use a spreader frame to allow the slings to clear an obstruction over the CG.

You may be able to use two cranes.

You may be able to tolerate lifting the object in a tilted postion.

The only external forces acting on the lifted object are gravity and the tension in the crane cable. Any misalignment will cause the object to rotate to a point of equilibrium where these two forces are aligned.

 

[rb1957]

i've been thinking alitle more about this. assume the lifting sling is an isosceles triangle (so the load in each sling is the same, equilibrium at the lift point). this means that each sling puts the same load into the spreader bar (the base of the lift structure). assume the load is attached at two places to the lift structure. for this attachment to react the off-set moment, the load is unevenly reacted by the two attachments, which contradicts the equal load for the slings; so the load will tilt.

changing the lengths of the slings, distorting the lift structure resolves the off-set couple by mobving the lift point to be above the load.

another way is to incline the lift line (so that the line of action passes through the load point), and you'll need to provide a lateral force (onto the spreader bar) to offset the lateral component of the lift. you could do this with a fixed wall and a roller.

these are probably the only? ways to resolve the two-force loading (considering the lift and the weight).

 

[rb1957]

ballast ! ... what a good idea WMacG !

(a flash of the blindingly obvious ?)

 

[vscid]

Is there a relation to calculate the reduction i would need for 2 of my slings? How about using turnbuckles in all my slings?

 

[desertfox]

Hi vscid

Yes there is but you need to know were the lifting points are in relation to the centre of gravity of the component your lifting.
A diagram of your lift points relative to the centre of gravity and the angle of slings or position of hook relative to the centre of gravity will help us assist you.

regards

desertfox

 

[vscid]

desertfox,

heres a pdf showing the frame and the payload.
The CG is 7 in left of the frame axis, as can be seen in the pic.

thanks.

 

[unclesyd]

I think you will find the answer to your problem with one of the following devices.

http://www.constructionlifters.com/shopcl/C_ModelDetail.asp?idCategory=379#oper

http://americanriggers.thomasnet.co...ustable-bail-lifting-beam-model-24?&forward=1

http://www.constructionlifters.com/shopcl/C_CategoryModel.asp?idCategory=553

 

[desertfox]

Hi vscid

Looking at your pictures you posted their seems to be a metal bar with holes in that the device your lifting is attached to,why can't you just move the bar along so that the centre of gravity is under the central lift point?
I'll have a look at the maths for shortening the slings in the meantime have a look at this site it may help

http://www.osh.govt.nz/order/catalogue/pdf/rigging-ac.pdf

regards

desertfox

 

[rb1957]

do the "rollers" at the ends of the horizontal bars contact anything (substantial)?

if you're going to lift this thing, i can't see why you can't pick up 7" away (above the CG) ... but then you know the situation better. of is the idae to take the weight (lift a very small amount) then translate horizontally ??

 

[vscid]

desertfox,rb1957,
I can't do what you said, because the load needs to be exactly at the position where it is in the pic.
Yes, the rollers align with rails inside a big cylindrical vessel.Hence, I cannot move the load anywhere else on that bar.

 

[rb1957]

do the rollers contact something (that could react load) ?

this is a loadpath for reacting the offset moment between the CG of the load and the lift point.

 

[desertfox]

Hi vscid

Well I calculated that the two slings nearest the centre of gravity will take about 2/3 of the total load and the other pair taking the remaining 1/3.
The extension therefore of the slings are in the ratio of 2 to 1 but the actual extension on the slings will depend on their modulus of elasticity.
I am beginning to think your best bet would be to fit a tag line so you can manipulate manually during lifting.
Have a look on page 63 of the link I posted earlier.

regards

desertfox