Part 112 min

1.3 Center of gravity: level, stable and under the hook

A load that weighs exactly what you calculated can still roll over the moment it leaves the ground. Weight tells you whether the rigging is strong enough; the center of gravity tells you whether the load will hang the way you expect. Get it wrong and the load tips, slides in the slings, swings into something, or dumps — none of which is a strength failure, and all of which is a rigging failure.

1 What the center of gravity is, and the one rule that follows from it

The center of gravity is the single point where the whole weight of the load can be considered to act. Hang the load from directly above that point and it hangs level and still. Hang it from anywhere else and gravity rotates it until the center of gravity swings underneath the hook — which it will do, immediately, whether or not anyone is ready for it.

The rule: the hook goes directly above the center of gravity. Everything else in this chapter is a way of achieving that, or of coping with not having achieved it.

Two consequences are worth stating plainly. First, a load whose hook is off to one side does not lift crooked and stay crooked; it lifts and then swings, and the swing is fastest at the moment of pick, when people are closest. Second, the legs of a multi-leg sling stop sharing the load: the leg nearer the center of gravity takes more, sometimes much more, which is why the calculations in Part 3 all assume the center of gravity is under the hook and say so.

2 Finding it without instruments

Nobody hands you a center-of-gravity coordinate on a jobsite. You work it out from what the load is made of.

Symmetrical and uniform? Then it is in the geometric middle. A plate, a bar, a plain length of pipe, a full drum standing upright.
Made of parts of different density? The center of gravity is pulled toward the heavy part. A motor bolted at one end of a base frame moves it toward the motor; a gearbox does the same.
Hollow, or partly full? A tank half full of liquid has a center of gravity that moves while you lift, because the liquid slops. A partly loaded bin is the same problem in a slower form.
Long and flexible? A pipe or a beam sags between pick points, so the effective geometry changes under load. Two picks near the ends are usually better than one in the middle.
Unknown? Test it. Take the load a few inches off the ground, stop, and look. A level load with slings evenly tensioned tells you the hook is over the center of gravity. A load that tilts tells you which way to move.

That last one — the trial lift — is the most useful tool in the chapter, and it costs a minute. Lift just enough to take the weight, hold, look, set it back down and adjust. Never adjust rigging on a suspended load, and never let anyone get under it to look.

3 Three ways to get the hook over it

Adjusting for an off-center center of gravity
Method	How it works	Watch out for
Move the pick points	Attach nearer the heavy end so the hook ends up above the center of gravity.	The legs are now unequal in length and in tension. Each leg must still be within its own rating.
Unequal leg lengths	A shorter leg on the heavy side and a longer one on the light side put the hook where you want it while both legs stay attached at fixed lugs.	The shorter leg usually carries more. Never assume an even split.
Adjustable device	A spreader or lifting beam with movable attachment points, or a chain hoist in one leg, moves the balance point deliberately.	The device is part of the load: it has its own weight and its own rated capacity, and it belongs on the weight sheet.

What none of these do is make the load lighter or the legs equal. An off-center center of gravity is normally a tension distribution problem, and 1926.251(a)(2)(ii) still applies to the leg that ends up carrying the most. OSHA's sling guidance is explicit about where that goes: for multiple-leg slings used with nonsymmetrical loads, an analysis by a qualified person is to be performed to prevent overloading of any leg.

4 High center of gravity: the load that wants to roll

Position under the hook is only half the question. The other half is height — how far the center of gravity sits above the points where the slings grip.

When the center of gravity is below the attachment points, the load is stable: push it and it swings back. When the center of gravity is above the attachment points, the load is unstable in exactly the way a ladder standing on end is unstable. It will stay upright as long as nothing disturbs it and roll over as soon as something does — a gust, a snatch on the hoist, a tag line pulled too hard.

1
Grip high, not low. Slings around the top of a tall load put the attachment points above the center of gravity. A basket around the bottom of the same load does the opposite.
2
Widen the base of the sling geometry with a spreader, so the attachment points are wide as well as high.
3
Control it from the ground with tag lines, and keep everyone out of the direction it would roll.
4
Move it slowly. Acceleration is what wakes up an unstable load.

On the job: the classic version of this is a tall cabinet or a vertical vessel choked at its waist. The rigging is strong, the weight is right, the hook is centered — and the first time the boom swings, the load rotates through ninety degrees because the mass is above the grip. Nothing about the sling failed.

5 Loads that move while you lift them

Some center-of-gravity problems are not static, and they are the ones that surprise experienced crews.

Liquids. A partly filled tank has a center of gravity that shifts with the surface. Full or empty is stable; half is not. Where you can, lift it full or lift it empty.
Loose contents. Parts in a bin, spoil in a bucket, a stack of loose material. It all slides toward the low side once the load tilts, which makes the tilt worse.
Bundles that are not banded. Pipe and bar bundles redistribute as the bands slip. This is also a struck-by hazard, not only a balance one.
Loads that flex. A long, slender beam sags, and a sagging beam's ends rise relative to its middle, changing the sling angles as it comes off the ground.
Loads still attached to something. A single remaining anchor bolt turns the whole lift into a rotation about that bolt.

The countermeasure for all five is the same and it is not clever: secure the contents, band the bundle, verify the load is free, and do the trial lift with everyone clear.

6 The five-minute routine

1
Decide where the center of gravity is and say it out loud — "heavy end is the motor end." A stated assumption can be corrected; a private one cannot.
2
Set the pick points so the hook will be above it, using lugs where the manufacturer provided them.
3
Ask whether the center of gravity is above or below the grip, and if above, treat the load as unstable.
4
Confirm the load is free — nothing bolted, frozen, welded, tacked or bound.
5
Trial lift, stop, look from a safe distance, and adjust on the ground if it is not level.
6
Tag lines on, hands off once the load is in the air.

Key takeaways

The center of gravity always ends up under the hook — the only question is whether it gets there before or after the load swings.
It sits at the geometric middle only for a uniform, symmetrical load; heavy components pull it toward themselves.
An off-center center of gravity makes legs share unevenly; OSHA's guidance sends a nonsymmetrical multi-leg lift to a qualified person for analysis.
Above the attachment points, a load is unstable and will roll when disturbed. Grip high and wide.
Liquids, loose contents, slipping bundles and flexing beams move the center of gravity during the lift.
The trial lift — lift, stop, look, set down, adjust — costs a minute and answers the question directly.

Free educational content — not OSHA-authorized training, no certificate or card issued. Follow your employer's program and the standards cited.