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RE: Center of Gravity & Sling Loadings

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I don't have a good feel for what you mean by "trolley." Assuming you are
trying to pick up a fairly rigid object at four points, and the center of
gravity is within the polygon defined by the four pick points, then the load
should be stable. 

If you don't mind being conservative, you could successively analyze the
system four times with a different sling assumed slack each time. It's not
all that much math. A few of the models will fall out as unstable unless the
center of gravity lines up exactly with the intersection of the diagonals
between the corners of the polygon.

It won't be an accurate solution, but it will give you a load envelope that
should cover you. I suspect that one of the cables will carry very little
load in any case, so the results may not be that far off.

-----Original Message-----
From: Jeremy Kuhn [mailto:jkuhn(--nospam--at)americancrane.com]
Sent: Monday, August 12, 2002 5:18 AM
To: 'seaint(--nospam--at)seaint.org'
Subject: Center of Gravity & Sling Loadings


I have a trolley which I need to lift which has 4 lifting lugs on it...I
have determined the C.G. of the load and now I am in the process of
determining the different vertical load which each lug sees due to its
proximity to the C.G. and then I will determine the tension in the slings by
trigonometry.

Here is the question...I have come up with 2 different methods of
determining the vertical loads on each lug - both seem to be logical...but
results are very different ! Is there a correct way to determine vertical
loads at 4 different locations which are at unequal distances from the C.G.
? I looked through all the Riggers Handbooks in the office and came up
empty.

The first method I used (working in an XZ plane) was to divide the problem
into 2...two lugs had a distance of 30% of the length to the C.G. in the X
direction - so I assumed those two lugs saw 70% of the load - then one of
the lugs which saw 70% of the load had a distance of 40% of the length to
the C.G. in the Z direction so it got 60% of the 70% (42% of entire
load)...and so on for each of the other lugs..

The second method which I used was to take a straight line distance from
each lug to the C.G. and then determine the vertical load seen by assuming
that each vertical load would be inversely proportional to the distance from
the C.G. - little tough to explain the exact process.

Anyhow...any insight into an approved method would be appreciated ! Thanks !


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