Center of Gravity and Suspension of Object

  • #1

Main Question or Discussion Point

Hello everyone,

I am trying to understand a center of gravity question regarding an object that is being lifted. I have a part that has a lifting circle that is used to put a hook through to hoist. The part needs to slide onto another part's surface that is about 30 degrees above the horizontal. For easier installation of the parts, I want to hoist it away from the center so that it is cocked to one side (about 30 degrees). I am having a hard time understanding how much to the right I need to place the lifting circle so that when it is lifted from a hoist directly above it, it is slanted at 30 degrees above the horizontal for easier installation. Please see the attached image for better understanding.
I was looking at tipping point of objects but I was not able to find all that I need.
Thank you in advance!

Shumon
 

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  • #2
DEvens
Education Advisor
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Possibly some careful experiments are the way to go here. But do be very careful when lifting things that are oddly shaped. Be alert for shifting loads, sudden changes of balance, etc.
 
  • #3
billy_joule
Science Advisor
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You want to position the hook so a line passes through the COG & hook 30deg from vertical.
 
  • #4
Baluncore
Science Advisor
2019 Award
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Experiment, hang the frame from a loop of rope. Adjust it until it is at 30°. The point of suspension and the CofG will then be in the line of the rope.

Alternatively evaluate the double integral to find the CofG in two dimensions.
Draw a vertical line through the CofG when rotated by 30°.
Suspend it from a point above the CofG on that vertical line.
 
  • #5
DaveC426913
Gold Member
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If you were to roughly estimate the location of the hoisting point, you could then then fine-tune it with a weight.

This method has several major advantages:
a] you don't have to get the hoisting point completely correct,
b] you can decide on the optimal angle after it's all constructed (when you're working with the real objects)
c] you can adapt the angle, in case the part changes, by simply changing the weight.

centre-of-gravity.png


The disadvantage, which you have not mentioned, is if/when the part needs to be removed regularly, for maintenance or what-have-you. In this case you'd need to re-weight it.
 
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  • #6
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This sort of thing can be quite dynamic as the Lifting eye essentially rotates about the CoG. It is however a fairly standard issue that riggers have to deal with. The method used in assembling large machine parts is to use two lifting eyes that are well apart and symmetrical about the CoG. From a central lift ring one Pendant is dropped that is of a fixed length. The other pendant is adjustable while carrying weight ( this would be a chain fall, Come along, or even a ratchet strap depending on actual load).

You will also need to be aware that as you lengthen or shorten the adjustable leg the load will also shift laterally by small amounts This is due to the rotation of the support point about the CoG. In all stable states the CoG will be directly below the suspension point until acted upon with large force to be out of position. Also to be aware of is force vectors, as the angle between the two pendants increases the apparent load does as well. By the time you reach 120 0 both legs are carrying the full weight of the object.

It is a simple solution to trying to mate pieces and in large, heavy, or cumbersome items dealing with "about 30 degrees" is often not close enough. This is one way to compensate for actual alignment issues.
 
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