4 leg sling
4 leg sling
(OP)
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.
Thnks.
When was the last time you drove down the highway without seeing a commercial truck hauling goods?
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RE: 4 leg sling
RE: 4 leg sling
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).
RE: 4 leg sling
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.
RE: 4 leg sling
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
RE: 4 leg sling
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.
RE: 4 leg sling
RE: 4 leg sling
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.
RE: 4 leg sling
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.
RE: 4 leg sling
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).
RE: 4 leg sling
(a flash of the blindingly obvious ?)
RE: 4 leg sling
RE: 4 leg sling
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
RE: 4 leg sling
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.
RE: 4 leg sling
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RE: 4 leg sling
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://
regards
desertfox
RE: 4 leg sling
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 ??
RE: 4 leg sling
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.
RE: 4 leg sling
this is a loadpath for reacting the offset moment between the CG of the load and the lift point.
RE: 4 leg sling
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
RE: 4 leg sling