huangyhg

4 leg sling
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.
it will tilt to a position where the cg will be directly below the hook.
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).
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.
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
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.
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.)
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.
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.
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).
ballast ! ... what a good idea wmacg !
(a flash of the blindingly obvious ?)

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