A load can look perfectly balanced while it is sitting on the ground.
Then the crane operator comes up on the rigging.
The load breaks free.
And suddenly one end drops.
That movement isn’t random.
The load is trying to position its center of gravity directly beneath the hook.
For riggers, understanding center of gravity—or COG—is one of the most important principles behind lifting safely and controlling a load. It affects sling placement, pick-point selection, load stability, sling tension, crane behavior, and whether a load remains level after it leaves its supports.
A rigger doesn’t need to be an engineer to understand the basic physics.
But every rigger should understand this rule:
The hook wants to end up over the load’s center of gravity.
If it isn’t, the load is probably going to move.
For more practical education across rigging, ironworking, electrical, millwright work, welding, and industrial construction, visit the Næxon Learning Center.
What Is Center of Gravity?
The center of gravity is the point where the weight of an object can be considered concentrated for purposes of balance.
For a simple object with uniform material and symmetrical geometry, the center of gravity may be close to its geometric center.
Consider a uniform rectangular steel plate.
If:
- Thickness is uniform
- Material is uniform
- Shape is symmetrical
the COG will normally be near the physical center.
But industrial loads are rarely that convenient.
A fabricated skid might have:
A 2,000-pound motor on one side.
A 700-pound pump on the other.
Structural steel underneath.
Piping on top.
A control panel at one end.
Now the geometric center of the skid tells you very little about where the actual center of gravity is.
The heavy components pull the COG toward them.
The Rule Every Rigger Should Know
During a freely suspended lift:
The center of gravity seeks a position below the hook.
That one concept explains a huge amount of load behavior.
Imagine the COG is here:
● COG
but your hook is several feet to the left.
As the load leaves the ground, gravity creates a turning moment.
The load rotates until the COG moves toward a stable position beneath the hook.
That’s why a load can suddenly:
tilt
roll
rotate
or
drop one end
as soon as it becomes suspended.
The load isn’t “acting weird.”
It’s following physics.
Why a Load Can Look Balanced on the Ground
Before the pick, the load isn’t necessarily being supported through the crane hook.
It may be resting on:
cribbing
dunnage
a trailer
steel supports
the ground
equipment foundations
or several different support points.
Those supports can hide an off-center COG.
The load appears stable because the ground is holding it.
Once the crane takes the full weight, those supports disappear from the equation.
Now the rigging system must support the entire load.
That is when the true relationship between the:
hook
pick points
and
center of gravity
becomes obvious.
Why One End Drops
Imagine lifting a fabricated skid.
The skid is 20 feet long.
Visually, you place the hook over the 10-foot midpoint.
That seems logical.
But most of the heavy equipment is concentrated toward one end.
The actual COG may be around 7 feet from that heavy end rather than at the 10-foot geometric center.
When the skid leaves the ground, the heavy end wants to rotate downward.
The lighter end rises.
Eventually, the COG positions itself beneath the hook.
Now the skid hangs at an angle.
The problem wasn’t necessarily the crane.
The problem was assuming:
geometric center = center of gravity.
Geometric Center vs. Center of Gravity
These terms should not be confused.
Geometric Center
The physical center based on the object’s dimensions.
Center of Gravity
The balance point based on how the object’s weight is distributed.
For a uniform steel beam, they may be nearly the same.
For an irregular industrial assembly, they can be dramatically different.
Consider a large vessel with a heavy nozzle, internal components, platforms, ladders, and attachments.
The outside shell may look symmetrical.
The total weight distribution may not be.
That’s why lift plans for critical or complicated loads can contain engineered COG information.
A Simple Example
Imagine a 10-foot beam.
If the beam has uniform cross-section and material throughout its length, its center of gravity will be approximately:
5 feet from either end.
Pick it directly over that point and it can hang level.
Now weld a large steel plate to one end.
The beam is still 10 feet long.
Its geometric center is still 5 feet.
But the COG has moved toward the added plate.
If you continue picking from the original midpoint, the plate end will tend to drop.
The dimensions didn’t change.
The weight distribution did.
The Hook and the COG Create a Vertical Line
For a stable freely suspended load, think about drawing an imaginary vertical line:
HOOK
↓
COG
The closer the hook is horizontally positioned over the COG, the less tendency the load has to rotate because of an offset between them.
If instead you have:
HOOK
↓ **COG**
the load has a reason to rotate.
Once suspended, gravity works to bring those two into vertical alignment.
This is one of the most useful mental pictures in rigging.
What Is a Moment?
To understand why the load rotates, you need one more concept:
Moment
A moment is the turning effect created by a force acting at a distance from a point.
A simplified relationship is:
Moment = Force × Distance
or:
M = F × D
Rigging calculations frequently use weight as the force.
For example:
A 1,000-pound load located 4 feet from a reference point creates:
1,000 lb × 4 ft = 4,000 lb-ft
of moment about that point.
Now imagine another weight on the opposite side.
If the moments balance, the system can balance.
This gives us a way to calculate the center of gravity of loads made from several components.
Calculating Center of Gravity
Suppose a skid contains two major pieces of equipment.
Component A
Weight:
4,000 lb
Location:
3 ft from the left end
Component B
Weight:
2,000 lb
Location:
9 ft from the left end
First calculate the moments.
Component A:
4,000 × 3 = 12,000 lb-ft
Component B:
2,000 × 9 = 18,000 lb-ft
Total moment:
12,000 + 18,000 = 30,000 lb-ft
Total weight:
4,000 + 2,000 = 6,000 lb
Now:
COG = Total Moment ÷ Total Weight
COG = 30,000 ÷ 6,000
COG = 5 ft
The combined center of gravity is approximately:
5 feet from the left reference point.
Even though one component sits at 3 feet and the other at 9 feet, their different weights move the combined COG.
The General COG Formula
For multiple components along one axis:
COG = Σ(W × D) ÷ ΣW
Where:
W = component weight
D = distance from the chosen reference point
Σ = total of all values
In plain language:
- Multiply each component’s weight by its distance.
- Add all those moments together.
- Add all the weights together.
- Divide total moment by total weight.
That gives you the combined center of gravity along that axis.
Center of Gravity Exists in Three Dimensions
A major mistake is thinking only about left and right.
The COG exists in:
Length
Width
and
Height
A load can therefore be balanced lengthwise and still want to roll sideways.
It can also have a high center of gravity that makes it less stable during certain phases of handling.
For complicated equipment, riggers need to consider:
longitudinal COG
transverse COG
and
vertical COG.
This becomes particularly important with vessels, machinery skids, transformers, structural assemblies, and irregular equipment.
Why a High Center of Gravity Matters
Imagine lifting a tall piece of equipment.
Most of its weight is concentrated near the top.
That gives it a relatively high COG.
Depending on the rigging arrangement, this can make the load more prone to:
rolling
tipping
or
rotating
than a load with most of its weight near the bottom.
This is one reason pick-point elevation matters.
If the effective suspension points and rigging geometry do not provide appropriate stability relative to the COG, the load may behave unexpectedly.
Pick Points Above the COG
In many stable lifting arrangements, the effective suspension points are located above the load’s center of gravity.
This helps create a naturally stable suspended condition.
Think of a pendulum.
The weight hangs below the suspension point.
Now imagine trying to balance the weight above the suspension point.
The system becomes much less stable.
This basic principle is extremely important when planning lifts of:
vertical vessels
equipment skids
fabricated assemblies
structural components
and other irregular loads.
Sling Placement Changes Load Behavior
Suppose you’re using a two-leg bridle.
One sling attaches near each end of the load.
If the COG is centered between those pick points, the load may hang level and the reactions at the pick points may be relatively balanced, subject to the rigging geometry.
Move the COG toward one end and things change.
The sling closer to the heavy end may carry more of the load.
The other sling may carry less.
This means:
Equal-looking sling legs do not automatically mean equal load.
That’s a critical rigging principle.
A Two-Point Pick Does Not Automatically Split the Weight 50/50
Suppose a 10,000-pound load is supported at two points.
It is tempting to say:
10,000 ÷ 2 = 5,000 lb per point.
That is only true under the appropriate symmetrical loading conditions.
If the COG is closer to one support point, that point carries more load.
This is essentially the same physics as a beam supported at two locations.
The load distribution depends on:
total weight
COG location
and
distance between supports.
Example: Unequal Pick-Point Loading
Imagine:
Total load = 10,000 lb
Distance between pick points = 10 ft
COG is:
3 ft from the left pick point
and therefore:
7 ft from the right pick point
The reaction at the right side can be found from moments about the left point:
Right Reaction × 10 = 10,000 × 3
So:
Right Reaction = 3,000 lb
The left reaction carries the remainder:
10,000 – 3,000 = 7,000 lb
So the pick-point loads are approximately:
Left = 7,000 lb
Right = 3,000 lb
Not:
5,000 / 5,000
That difference can completely change sling selection and rigging requirements.
The Heavy Side Carries More
Notice something important from the example.
The COG was closer to the left pick point.
The left pick point carried:
7,000 lb
The farther right point carried:
3,000 lb.
A useful rule is:
The support closer to the center of gravity generally carries more of the load.
This becomes extremely important when using:
multiple slings
lifting lugs
trunnions
spreaders
lifting beams
and engineered pick points.
Sling Angle Is a Separate Problem
COG determines how the weight is distributed.
But sling angle determines the tension inside the sling legs.
Those two effects can combine.
Suppose one pick point is already carrying more weight because the COG is offset.
Now place that sling at a shallow angle.
The sling tension can become considerably greater than the vertical reaction alone.
This is why rigging calculations cannot stop at:
“The load weighs 10,000 pounds.”
You may also need to know:
Where is the COG?
How is the load divided between pick points?
What are the sling angles?
What are the capacities of the rigging components?
Næxon’s Learning Center includes additional field education on sling angles, load handling, industrial math, and other topics used by riggers and construction crews.
Why Sling Tension Increases as the Angle Gets Flatter
For a symmetrical two-leg lift, when sling angle is measured from the horizontal, a simplified relationship is:
Sling Tension = Load ÷ (2 × sin θ)
where:
θ = sling angle from horizontal
As the sling becomes more horizontal, the tension increases.
For a 10,000-pound centered load:
90° from horizontal
Each vertical leg theoretically carries:
5,000 lb
60°
Each leg carries approximately:
5,774 lb
45°
Each leg carries approximately:
7,071 lb
30°
Each leg carries approximately:
10,000 lb
At only 30°, each sling leg can experience tension approximately equal to the entire load in this simplified symmetrical example.
Combine poor sling angle with an offset COG and the loading can become even more unfavorable.
Why Trial Lifts Matter
A controlled trial lift can reveal information before the load is raised to working height.
The idea is to take enough load to verify how the rigging behaves while keeping the load as low as practical and following the approved lift procedure.
As the rigging becomes loaded, riggers watch for:
Unexpected tilt
Rigging movement
Load shifting
Sling sliding
Unequal loading
Pick-point problems
Unexpected deformation
Interference
If the load begins behaving differently from the lift plan, the answer isn’t:
“Keep going and see if it straightens out.”
The load should be returned to a safe condition and the rigging arrangement reassessed.
Never Put Yourself Under the Load to See Why It Is Crooked
This should be obvious, but jobsite familiarity can make people complacent.
If a suspended load is hanging crooked, don’t walk underneath it to inspect the problem.
Don’t put hands, feet, or other body parts where movement could trap or crush you.
A load that is already demonstrating instability deserves more distance, not less.
The rigger needs to control the load from an appropriate safe position according to the lift plan and site requirements.
Tag Lines Do Not Fix a Bad Center of Gravity
Tag lines can help control rotation and orientation when properly used.
They do not correct an improperly rigged load.
If a 20,000-pound assembly wants to rotate because the hook isn’t properly positioned relative to the COG, a worker should not be expected to overcome that fundamental imbalance by pulling harder on a tag line.
The rigging arrangement needs to control the load mechanically.
Human strength is not a substitute for correct rigging geometry.
Why Loads Sometimes Rotate After Clearing an Obstruction
Imagine a long structural assembly being lifted from a congested area.
While one end is still touching cribbing, a structure, or another surface, that contact point is helping resist rotation.
Then the load clears.
Suddenly that external support disappears.
The load rotates.
Workers sometimes describe this as:
“It shifted when it came free.”
That’s exactly what happened.
The support conditions changed.
Now the crane and rigging are carrying the load freely, allowing the COG to move beneath the hook.
This is another reason to anticipate what will happen at the moment the load becomes fully suspended.
Fluid Can Move the Center of Gravity
This is especially important when lifting tanks, vessels, drums, and equipment that may contain liquid.
If liquid is present, it can move.
That movement can shift the COG during the lift.
This phenomenon is related to free-surface effects and moving internal contents.
A vessel that seems stable in one orientation may behave differently when tilted.
Whenever the contents of equipment are uncertain, that uncertainty must be addressed during lift planning.
Never casually assume:
“It’s probably empty.”
Know what you’re lifting.
Loose Material Can Shift Too
Liquid isn’t the only concern.
Loads may contain:
sand
gravel
catalyst
scrap
tools
internal components
or other material capable of moving.
A load can therefore have a COG that changes during the lift.
That’s much more dangerous than simply having a known off-center COG.
The load may begin stable and then suddenly shift as its contents move.
The Crane Hook Is Part of the Geometry
Riggers sometimes focus entirely on sling placement.
But hook position matters just as much.
If the crane hook is not positioned appropriately before taking the load, the rigging can pull sideways as tension develops.
That can cause:
load movement
rigging movement
side loading
unexpected rotation
or
dragging before lift-off.
Before lifting, the crane should be positioned according to the lift plan so the load can be taken as intended rather than being dragged into position by the hoist line.
Side Loading Is a Warning Sign
Rigging components are designed for specific loading conditions.
Hooks, shackles, lifting lugs, eyebolts, and other components may have significant capacity reductions or restrictions when loaded incorrectly.
An offset COG can contribute to unexpected side loading if the rigging arrangement was designed under the assumption that the load would hang differently.
This is why rigging is a complete system.
You cannot evaluate:
only the sling
or
only the shackle
or
only the crane.
Every component has to work together.
Spreaders and Lifting Beams
For certain loads, spreader beams or lifting beams can help control rigging geometry.
They may:
Maintain sling spacing
Reduce certain sling angles
Control load distribution
Keep slings away from equipment
Provide engineered pick locations
But using a spreader doesn’t eliminate the need to know the COG.
The lifting device itself also has:
weight
and
a center of gravity.
Its weight must be included when determining total load on the crane hook.
Don’t Forget the Rigging Weight
Suppose equipment weighs:
48,000 lb.
The lifting beam weighs:
4,000 lb.
Slings, shackles, and other rigging weigh another:
1,000 lb.
The crane isn’t lifting only 48,000 pounds.
The hook load is approximately:
53,000 lb
before considering other factors required by the lift plan.
For large industrial lifts, rigging weight can become substantial.
Everything hanging below the hook counts.
How to Estimate an Unknown COG
Not every load arrives with an engineered drawing showing:
COG HERE.
For relatively straightforward loads, experienced riggers may use:
Known component weights
Dimensions
manufacturer information
fabrication drawings
load drawings
previous lift data
and
controlled trial lifts
to determine or verify the COG.
For complicated, high-consequence, or critical lifts, the COG may require engineering analysis.
The key rule is:
Don’t invent certainty where you don’t have it.
If the weight distribution is unknown, treat it as unknown until it is properly established.
Manufacturer Lift Points Can Tell You Something
Heavy equipment often comes with designated lifting lugs or lifting instructions.
Those locations were generally selected with the equipment’s weight distribution and structural design in mind.
Don’t automatically assume any convenient hole, nozzle, handrail, structural member, or bracket is a lifting point.
A component strong enough to support itself during operation is not necessarily designed to carry the entire equipment weight during lifting.
Use approved lifting points and procedures.
Why Equipment Nozzles Are Not Rigging Points
On industrial equipment, pipe nozzles can look extremely substantial.
A large flanged nozzle may appear stronger than a small lifting lug.
That doesn’t make it a lifting point.
The nozzle was designed for its intended mechanical and process loads—not automatically for lifting the equipment.
Improvised lifting from unapproved locations can damage:
nozzles
shells
flanges
welds
internals
and
equipment alignment.
This is one of many areas where rigging knowledge overlaps with piping, millwright, boilermaker, and mechanical work. The Næxon Learning Center is designed around those cross-trade connections.
Center of Gravity and Crane Capacity
COG also affects where the crane ultimately carries the load.
Crane capacity depends heavily on factors such as:
load radius
boom configuration
counterweight
outrigger configuration
boom angle
crane setup
and the applicable load chart.
If a load rotates or shifts unexpectedly, its relationship to the crane can change.
That’s one reason accurate load information and controlled movement matter.
The next major concept in this series—load radius—explains why crane capacity can fall dramatically as a load moves farther away from the crane.
Common Center-of-Gravity Mistakes
Mistake 1: Picking from the physical center
Physical center does not necessarily equal COG.
Mistake 2: Assuming two slings split the weight equally
They only do under the appropriate load geometry.
Mistake 3: Ignoring equipment attached to the load
Motors, gearboxes, valves, platforms, and other components shift the COG.
Mistake 4: Ignoring vertical COG
A load can be balanced left-to-right and still roll.
Mistake 5: Trying to control bad rigging with tag lines
A tag line is not a substitute for proper rigging.
Mistake 6: Continuing when the trial lift looks wrong
Unexpected movement is information.
Use it.
Mistake 7: Forgetting the contents
Liquid or loose material can shift the COG during the lift.
Mistake 8: Forgetting rigging weight
The crane carries everything below the hook.
A Simple Field Example
Imagine lifting a pump skid.
Total skid weight:
12,000 lb
There are two pick points spaced:
12 ft apart.
The calculated COG is:
4 ft from the left pick point.
Calculate the right reaction:
R-right × 12 = 12,000 × 4
R-right = 4,000 lb
The left reaction therefore equals:
12,000 – 4,000 = 8,000 lb
So:
Left pick point ≈ 8,000 lb
Right pick point ≈ 4,000 lb
Even before considering sling angle, one side carries twice the vertical load of the other.
Now imagine both slings were selected under the assumption that each carried:
6,000 lb.
That assumption could create a serious problem.
This is why understanding the COG isn’t academic rigging theory.
It directly affects component loading.
The Three Questions to Ask Before the Pick
Before lifting an irregular load, think about three basic questions:
1. How much does it weigh?
You need a reliable load weight.
2. Where is the center of gravity?
You need to understand how that weight is distributed.
3. Where is the hook relative to the COG?
That relationship tells you a great deal about how the load will behave when it becomes suspended.
Then come the rest of the rigging questions:
Sling capacity.
Sling angle.
Shackle capacity.
Pick-point capacity.
Crane capacity.
Radius.
Clearances.
Travel path.
Environmental conditions.
Communication.
And the lift plan.
The Simplest Way to Remember Center of Gravity
Picture this:
HOOK
↓
● COG
That is what a freely suspended load wants.
If instead you begin with:
HOOK ● COG
expect movement as the load becomes suspended.
The farther the horizontal offset—and the more complicated the load—the more important proper lift planning becomes.
A Good Rigger Watches the Load Before It Leaves the Ground
Rigging isn’t simply connecting slings and giving the operator an up signal.
A good rigger looks at the load and starts asking questions.
Where is the weight?
Which side is heavy?
Where are the approved pick points?
Where will the hook be?
What happens when this end clears the cribbing?
Could anything inside move?
Are the sling legs actually carrying equal load?
What happens if the load rotates?
That kind of thinking separates simply attaching rigging from understanding how a lift works.
Because the most important moment of many lifts happens within the first few inches.
The load leaves its supports.
Gravity takes over.
And the center of gravity tells you exactly where the load wants to go.
Continue learning practical rigging, crane, ironworker, millwright, electrical, welding, boilermaker, and industrial-construction fundamentals in the Næxon Learning Center.
