3.1 Center of Gravity and Suspended Stability

Locate mass rather than appearance

The center of gravity (CG) is the point through which the resultant weight acts in the gravitational field used for ordinary rigging calculations. For a uniform, symmetrical body, it may coincide with the geometric center. For an assembled machine, hollow frame, vessel with internals or partly filled container, external symmetry does not establish its location.

In a freely suspended, static system supported from one hook, the resultant gravitational force acts beneath the suspension point. If the initial arrangement places that point away from the system CG, the load can translate or rotate as tension develops. The final attitude depends on the attachment geometry, sling lengths and restraints. Putting the hook above the estimated CG is necessary planning, but it does not by itself guarantee level attitude or equal sling forces.

Distinguish the CG of the payload from the CG of the complete suspended assembly. A heavy beam, asymmetric rigging or an offset handling device changes the assembly’s mass distribution. In a simple training model, rigging mass may be negligible or explicitly excluded. In an actual lift, the approved model must account for relevant masses and their positions.

Stable, neutral and unstable configurations

A simple body pivoting about a fixed horizontal suspension axis illustrates three conditions. If its CG lies below the axis, a small displacement raises the CG and gravity tends to return the body toward its resting orientation. If the CG lies on the axis, gravity provides no restoring moment in the ideal model. If the CG lies above the axis, a small displacement lowers the CG and gravity tends to increase the overturning movement.

These are model conditions, not a universal rule that any pick located below a CG makes every sling assembly instantly invert. A basket cradle, multiple restraints, a rigid handling frame or a complex suspension can behave differently. Evaluate the full support system and its possible rotation axes. A spreader changes force geometry but does not automatically prove a top-heavy load is stable within its lower slings.

Height matters independently of horizontal balance

A CG location along a skid tells how vertical reactions divide between two end supports. Its elevation tells how the body responds to tilt and lateral movement. Two machines can have the same total weight and longitudinal CG but different tipping resistance because one carries its heaviest mass higher.

For a body resting on supports, draw the support polygon through the effective support locations. The vertical projection of the CG must remain within the appropriate support region for static resistance to tipping, with the necessary planning margin. During acceleration, slope or rotation, additional forces change the stability assessment. An initial projection just inside the boundary is not a robust assurance of safe movement.

Consider a tall cabinet on two narrow skates. Its CG may be centered between them on a level floor, yet a turn, uneven joint or ramp changes the effective stability. Widening the approved support arrangement may help, but only if the cabinet base and handling equipment can accept that arrangement. Do not attach extra skates to cosmetic panels merely to enlarge a drawing’s triangle.

Confirm information through controlled verification

Manufacturer CG marks or drawings are useful only when they describe the actual configuration. A skid with an added transformer should not use the original bare-skid CG. Retained fluid, removed accessories and movable components require reconciliation. Mark the confirmed CG on the plan or load where practical so hook positioning uses a visible reference.

A trial lift verifies observed attitude and seating under the planned rigging. It does not replace a pre-lift CG determination. Raise only as necessary for the approved check, keep personnel clear and stop for unexpected tilt, migration or slack. Lower to stable supports before adjustments. Do not chase a shifting sling by hand beneath or beside a suspended load.

Interpreting a trial-lift result

Suppose a machine tilts toward its motor end as the slings take load. Possible causes include an off-center CG, inappropriate sling lengths, unequal pick elevations or a connection that is binding. The observation identifies a discrepancy; it does not establish a unique diagnosis. Return to support, compare the actual configuration and measurements with the plan, and revise the geometry deliberately.

A trial lift that looks level also does not prove equal sharing in every leg. A rigid four-point assembly can have different tensions while retaining level attitude. Use the applicable rated assembly, calculations and approved verification method. Load cells may assist when specified, but each reading must be interpreted as the force at its installed position.

The examination trap is to treat “balanced” as a synonym for “equal weight in all slings.” Static balance means forces and moments are in equilibrium. Unequal vertical shares can satisfy that condition when the CG is off center. The calculation chapters show how to derive those shares; this section establishes why locating and controlling the CG comes first.

Source: CCO Level II outline and reference materials.