Center of gravity, by definition, is a point from which the weight of a body or system may be considered to act.
In rigging, the center of gravity is the point within a load where the object balances evenly in all directions. You may also hear it said that the center of gravity is the point at which all the load’s weight is concentrated. Knowing the location of the center of gravity of a load is an important part of safe rigging since a load is most stable when it is lifted directly above the point of the center of gravity.
Sometimes, finding a load’s center of gravity is pretty straightforward. For objects that are uniform in shape and composition, such as a single piece of pipe, the center of gravity is generally in the middle of the object. Of course, even when you are confident you have set your pick point above the center of gravity, it’s still recommended that you do a quick test before performing the lift. This involves lifting the load a few inches at a time to ensure it retains stability. If the load begins swinging freely, you may need to set it back down and re-rig it before testing again. You can read more about testing a load here.
Determining a load’s center of gravity for oddly configured objects will require a little more work. You could opt for a trial-and-error method where you rig the load at the point you estimate the center of gravity to be at and then perform the aforementioned test run until you can lift the load securely, but this can take a substantial amount of time.
A more efficient way may be to calculate the center of gravity of a load using a simple mathematical formula. Although lengthy explanations may make this sound like rocket science, it’s actually quite simple. Below, we’re going to break it down step by step. Once you know the steps, you can easily determine a load’s center of gravity in minutes using a couple of load links, a tape measure and your phone’s calculator. Note that in this example, we are finding the east / west CG. These steps can be repeated to find the north / south CG.
For determining the center of gravity for objects that are complex or not solid, you will need to begin by attaching a load link to each end of the load. Take the measurements of the load on the left and on the right sides (WL and WR in our example). Then combine both to calculate the total weight (TW).
Here’s what the formula should look like at this point, using randomly generated numbers from our example.
WL (LEFT End Weight) = 3,500 lbs
WR (RIGHT End Weight) = 8,650 lbs
TW (Total Weight) = 12,150 lbs
Keep in mind that the center of gravity will be closer to the heaviest end of the load. This is the right end in our example. The next step in determining the center of gravity is to take the heavier end weight and divide it by the total weight.
(WR / TW) = X
8,650 / 12,150 = 0.71
This tells us the center of gravity is .71 times the distance between the two pick points, toward the heavier end or to the right in this example. *If your load is heavier on the left end, simply replace WR with WL in each place we’ve written this formula. You only need to calculate one side to locate the CG. *
For the final step, measure the distance across the two pick points. This is called SPAN, and in our example, it is calculated at 126 inches. Now you will take our previous calculation and multiply it by the SPAN, as shown below step by step.
(WR/TW) x SPAN
(8,650 / 12,150) x 126”
(0.71) x 126” = 89”
You have now found that the center of gravity is 89 inches from the left in our example since the right was the heavier end. Now you can rig the load around the center of gravity, and as usual, perform a quick test load to ensure the load is balanced.
Note that you can also use tenths of a foot or any other metric unit in place of inches. The formula will work the same with all these units of measurement, provided all units of measure being used are the same. The complete formula is laid out like this. Which one you use will depend on if your load is heavier on the left or right side.
(WL / TW) x SPAN = CG
(WR / TW) x SPAN = CG
For more rigging tips, check out our post on 5 Rules You Should Know About Rigging Safety.
If you still have questions regarding how to calculate a load’s center of gravity, please reach out to your local LGH representative or give us a call at 800-878-7305 to speak with one of our rental specialists. We can assist you in finding the center of gravity for complex loads where the CG needs to be located horizontally, as outlined here, as well as along the y and z axis for 3-dimensional / box type loads.
Information and Illustration, copyright by Jerry Klinke, all rights reserved.