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Learning Center | Rigging | Fundamentals

A load does not care where it looks balanced. It responds to where its weight is actually concentrated.

That distinction becomes critical during crane lifts. A piece of equipment may appear symmetrical from the outside while containing a heavy motor, gearbox, vessel head, internal assembly, or other component that shifts its center of gravity away from the geometric center. If the rigging arrangement does not account for that weight distribution, the load can rotate or tilt as it comes free.

For riggers, knowing how center of gravity (COG) affects a suspended load is one of the foundations of safe, controlled lifting.

1. What Is Center of Gravity?

The center of gravity is the point through which the total weight of an object can be considered to act.

For a simple object made from material of uniform density, the COG may be close to its geometric center.

Industrial equipment is rarely that simple.

Consider a skid containing:

Motor → Gearbox → Pump

The skid itself may be rectangular, but the motor and gearbox could make one end considerably heavier.

The center of the skid is therefore not necessarily the center of gravity.

That difference matters when determining how the load will behave when suspended.

2. The Hook Wants to Be Above the Center of Gravity

One of the most important concepts in basic rigging is that a freely suspended load tends to seek a position where its center of gravity is below the point of suspension.

Imagine lifting a rectangular piece of equipment from a point located away from its COG.

As the crane takes the weight, the load may begin to rotate or tilt until the COG moves toward a stable position beneath the suspension point.

That movement can surprise workers who assumed the load would remain level.

The heavier and more irregular the load, the more important its weight distribution becomes.

3. Geometric Center Is Not the Same as Center of Gravity

This is a common misunderstanding.

Suppose you have a 20-foot equipment skid.

The halfway point is:

20 ÷ 2 = 10 feet

That gives you the geometric center of the skid.

It does not automatically tell you the center of gravity.

If one end contains significantly heavier machinery, the COG shifts toward that end.

A rigger therefore cannot simply measure halfway across every load and assume that is where the load balances.

4. Why Some Loads Tilt

Consider a load supported by two sling legs.

If the weight is distributed unevenly, the sling legs may not carry equal portions of the load.

As tension develops, the load can rotate until the forces reach equilibrium.

The tilt tells you something important about the relationship between the suspension arrangement and the load’s center of gravity.

Unexpected movement should never be treated casually.

5. Heavy Components Move the COG

Industrial loads often contain components with very different weights.

A compressor skid might contain a large electric motor at one end. A heat exchanger may have channel heads, bonnets, or attachments that influence weight distribution. A fabricated module might contain structural steel, piping, valves, equipment, cable tray, and platforms concentrated in different areas.

A vessel may contain internals or attachments that make its weight distribution different from what its external shape suggests.

The COG moves toward the heavier concentration of mass.

That is why engineered lifting information, equipment drawings, manufacturer data, lift plans, and verified weight information are so important.

6. Center of Gravity Exists in Three Dimensions

COG isn’t only left versus right.

It can also be:

Forward or backward

Left or right

High or low

A load with a high center of gravity may behave differently from one with most of its mass concentrated near the bottom.

This becomes especially important with tall equipment, vessels, modules, transformers, machinery, and loads that must be rotated or upended.

Rigging is three-dimensional.

The COG must be considered that way.

7. The Crane Hook Is Only Part of the System

A suspended load may involve much more than a hook and two slings.

A lift could include:

Crane Hook

↓

Master Link

↓

Slings

↓

Shackles

↓

Lift Points

↓

Load

Other lifts may use spreader beams, lifting beams, equalizers, engineered lifting devices, or specialized rigging arrangements.

Every component influences how forces travel through the lifting system.

The rigging configuration must therefore follow the approved lift plan and applicable rigging requirements rather than being improvised based solely on appearance.

8. Sling Length Can Affect Load Position

When multiple sling legs are used, their geometry influences the suspended load.

If the approved arrangement uses different sling lengths or adjustable rigging, those differences may be necessary to accommodate the load geometry and COG.

Changing sling lengths without knowing the lift design can change how the load hangs and how forces are distributed.

This is especially important when equipment must remain level for installation.

The goal is not merely to get the load off the ground.

The goal is to control it throughout the entire lift.

9. Sling Tensions May Not Be Equal

Another dangerous assumption is:

“There are two slings, so each one carries half the load.”

That is not automatically true.

Load distribution depends on factors including the location of the center of gravity, sling geometry, attachment points, sling angles, and the actual rigging arrangement.

A heavier side of an off-center load can place more demand on one portion of the rigging system.

This is one reason rigging calculations and engineered lift planning become increasingly important as loads become heavier, more complex, or less predictable.

10. Sling Angle Still Matters

Center of gravity is only one part of the problem.

Sling angle also affects sling tension.

As sling legs become flatter relative to horizontal, the tension required in the sling legs increases.

Therefore, riggers must consider both:

Where is the COG?

and

What forces will this rigging geometry create?

A load may weigh the same regardless of the rigging arrangement, but the forces acting on individual rigging components can change substantially.

Always use the applicable engineered calculations, rated-capacity information, manufacturer requirements, and approved lift plan rather than estimating sling capacity from appearance.

11. What Happens During a Test Lift?

A controlled initial lift can provide important information about how the load is responding.

When permitted by the lift plan and site procedure, the load may initially be raised only enough to verify its behavior before continuing.

Qualified personnel can evaluate whether the load is responding as expected, whether the rigging is properly seated, and whether the planned load orientation is being maintained.

If the load unexpectedly tilts, shifts, binds, or behaves differently from the approved plan, the answer is not to simply keep lifting.

The condition needs to be evaluated.

12. Never Try to Stop a Moving Load With Your Body

A suspended industrial load can contain enormous energy.

If a load begins rotating or tilting unexpectedly, workers must not put themselves into a pinch point or attempt to physically overpower it.

Load-control methods such as tag lines may be used where appropriate and where required by the lift plan and site procedures, but workers still need to remain clear of hazardous positions.

Never place yourself under a suspended load, between the load and a fixed object, inside an uncontrolled pinch point, or anywhere unexpected rotation could trap you.

A few inches of load movement can become extremely dangerous when thousands of pounds are involved.

13. Center of Gravity Can Change During the Job

This is an advanced concept worth learning early.

The COG of a load isn’t always permanently fixed.

Imagine lifting equipment that contains liquid.

If the liquid can move inside the load, its movement can change the weight distribution.

The same principle can apply to loose internal components, shifting material, partially filled vessels, or equipment whose configuration changes during lifting.

A load that behaves predictably at one orientation may behave differently as it is rotated.

That possibility must be considered during lift planning.

14. Upending Equipment Makes COG Especially Important

Some industrial lifts involve rotating equipment from horizontal to vertical.

Large vessels are a common example.

During an engineered upending operation, the relationship between the equipment, lifting points, cranes, rigging, and center of gravity changes continuously as the load rotates.

These are not lifts that should be improvised in the field.

They require proper engineering, planning, equipment selection, communication, and execution by qualified personnel.

Knowing basic COG principles helps workers recognize why these lift plans are so specific.

Common Mistakes

One common mistake is assuming the physical center of a load is automatically its center of gravity.

Another is assuming identical sling legs must carry identical loads.

Workers may also underestimate how quickly a suspended load can rotate when it leaves its support, particularly when the actual COG differs from what was expected.

The most dangerous mistake is trying to correct unexpected load movement with manpower while the load remains suspended.

When a lift behaves differently from the plan, stop and reassess according to the site’s lifting procedure.

Field Rule

The load will seek balance beneath the suspension point—not where you hoped it would hang.

Before lifting, know the weight, center of gravity, lifting points, rigging geometry, equipment capacities, and expected load behavior.

For complex or critical lifts, those details belong in an engineered or otherwise approved lift plan as required by the facility.

Knowledge Check

1. What is the center of gravity?

The point through which the total weight of an object can be considered to act.

2. Is the geometric center always the center of gravity?

No. Uneven weight distribution can move the COG away from the geometric center.

3. What position does a freely suspended load tend to seek?

A stable position with its center of gravity below the point of suspension.

4. Do two sling legs automatically carry 50% of the load each?

No. Load distribution depends on COG location, attachment points, sling geometry, and other factors.

5. Why can an industrial skid have an off-center COG?

Heavy components such as motors, gearboxes, pumps, or other equipment may be concentrated on one side.

Practical Exercise

Imagine a 12,000-pound equipment skid.

From the outside, the skid looks rectangular and symmetrical. However, a large motor and gearbox are installed near one end.

Two lifting points are positioned symmetrically around the physical center of the skid.

Before the lift, ask yourself:

Would you automatically assume each side of the rigging carries 6,000 pounds?

No.

The motor and gearbox shift the center of gravity toward the heavier end. The actual load distribution must be determined using the appropriate lifting information and approved method.

That is the difference between simply connecting rigging and knowing how the load will behave.

Continue Learning

Center of gravity leads naturally into more advanced rigging topics including sling angles, load distribution, bridle hitches, shackles, spreader beams, lifting beams, crane load charts, tag-line control, critical lifts, tailing operations, and calculating sling tension.

Continue through the Næxon Learning Center for practical training across rigging, crane operations, welding, electrical, millwright work, instrumentation, ironwork, scaffolding, operations, and the other crafts that build and maintain industrial facilities.