2.2 Load Weight and Center of Gravity

Weight and CG in the Level I Scope

NCCCO defines Rigger Level I work around simple, repetitive lifts when critical load data are known. Two of the most important known values are:

1. Load weight — how heavy the object is (including contents, packaging, and attached appurtenances that will lift with it)
2. Center of gravity (CG) — the balance point of the load’s weight; the point through which the resultant weight acts

Level I does not mean you ignore weight. It means the exam and the job expect weight and CG to be provided or readily verifiable, not invented on the spot. Your skill is to confirm those values, understand how CG affects the hitch, and refuse the lift when the numbers are missing or untrustworthy.

Verify — Do Not Assume

"Known" weight still requires verification. Prefer independent, documented sources:

Verification habits that save careers:

• Match the identity of the load (serial, tag, description) to the document you are reading
• Confirm whether the number is net, gross, or shipping weight
• Add contents (oil, product, water, tools left inside) if the plate lists only empty weight
• Account for skids, cribbing, and temporary attachments that will ride with the load
• If two sources disagree, stop and resolve the conflict before selecting gear

Never guess weight

Guessing is not a Level I method. Statements like "it looks like about a ton" or "we lifted one like this last year" are not verification. On exam scenarios, any option that treats estimate-by-eye as acceptable is almost always wrong. If no reliable weight is available, the correct action is to obtain weight (weigh, get manufacturer data, engineered calculation by a qualified person)—not to pick a sling based on feel.

When Calculation Is Required: Level II Territory

Some situations require calculating weight or CG rather than reading a known value:

• Irregular fabrications with no plate or drawing weight
• Partially full vessels or mixed materials
• Multiple objects lifted as one assembly without a combined weight
• Offset machines where CG must be located from component weights and distances

NCCCO Rigger Level II expands into more complex rigging problems, including situations where load weight and CG are not simply provided. For Level I study, remember the boundary:

You may still see simple applied questions (for example, reading a stated weight and choosing adequate WLL). That is verification and application, not inventing missing data.

What Center of Gravity Means Under the Hook

The load will hang so that the CG is directly below the hook (along the line of support) once motion settles. Practical consequences:

• If attachment points and the hitch place the resultant above a CG that is centered between them, the load tends to hang level
• If the hitch resultant is not above the CG, the load tilts until the CG lines up under the support
• Tilting changes leg loads, edge contact, and clearance along the travel path

Balanced vs offset CG

Rule of thumb for multi-leg hitches: the leg(s) closer to the CG carry more of the load. An offset CG is not automatically unsafe, but it must be known so that:

• Attachment points and hitch type still control the load
• No single leg or fitting is overloaded
• The planned path still has clearance when the load hangs at its true attitude

How CG Affects Sling Angles and Stability

Sling included angles and horizontal angles depend on where the attachment points sit relative to the hook and to each other. CG location interacts with that geometry:

1. Level hang assumption breaks when CG is offset — measured sling lengths that look equal may still produce unequal tension.
2. Increasing horizontal distance between attachment points (or lowering the hook relative to the spread) changes angles and the load in each leg.
3. Choker or single-wrap arrangements on offset loads can roll or slip toward the heavy end if not controlled.
4. Stability suffers when the CG is high and the effective base (distance between support points) is narrow.

Level I candidates should be able to reason qualitatively:

• Steeper sling angles (closer to vertical) generally reduce tension for a given share of weight
• Flatter angles increase tension and horizontal forces at attachment points
• An offset CG can push one leg into a flatter effective share of load than the drawing suggests if the load tilts

Detailed angle-factor arithmetic is reinforced later in Execution domains; here the Scope lesson is: you cannot judge capacity or stability without knowing weight and where the CG sits relative to the lift points.

Worked Scenarios: Known Weight Verification

Scenario A — Nameplate vs contents

A pump skid nameplate lists 1,850 lb. The drawing notes the plate weight is dry. Field crew filled the oil reservoir (estimated 40 lb) and left a 25 lb toolbox welded to the skid for shipping. Verified lift weight is not 1,850 lb — it is at least 1,915 lb plus any remaining packaging. Selecting gear for 1,850 lb alone is incorrect. Exam takeaway: read what the number includes.

Scenario B — Two documents disagree

Bill of lading says 3,200 lb. Shop drawing says 2,750 lb. No scale is available on site. Correct action: do not average and do not pick the lower number for convenience. Stop, investigate (scale, manufacturer confirmation, engineering), and use a reliable figure. Conservative temporary practice on some sites is to plan with the higher credible value only after a competent person accepts that approach—but inventing a middle value is never correct.

Scenario C — Offset CG mark

A generator skid has a painted CG mark closer to the radiator end. Two vertical legs are planned at equal distances from the geometric center—not from the CG mark. Expected result: the radiator end hangs lower and the near-side leg carries more load. Correct Level I response: reposition attachment points (or use a configuration specified for that CG) so the support lines keep the load controlled and legs within capacity. Do not "see what happens" after hoist.

Scenario D — Scale check

A fabricator provides a certified weigh ticket of 4,100 lb for the exact assembly with shipping frame. Frame will be removed before the lift. Correct verification: obtain weight without the frame or weigh again after removal. Using 4,100 lb after removing a 600 lb frame overstates weight (capacity-wise safer but may confuse path/crane charts); using 4,100 lb when the frame stays on is correct only if the frame truly lifts with the load.

Practical Verification Sequence (Exam-Ready)

1. Identify the exact load to be lifted
2. Collect weight evidence (plate, drawing, ticket, scale)
3. Adjust for contents and attachments that ride along
4. Locate CG from marks, drawings, or provided lift data
5. Compare CG to planned attachment points
6. Confirm the load will hang in an attitude that keeps the path clear
7. If weight or CG cannot be verified, stop — do not guess

Linking Weight/CG to the Rest of Scope

Weight and CG feed every later decision:

• Hardware and sling selection depend on forces, not hope
• Attachment point choice must straddle or align with CG appropriately
• Path clearances depend on how the load will hang and tilt
• Special handling (fragile corners, liquids, long loads) often ties back to CG height and offset

Common Exam Traps

If you can verify weight, locate CG, and explain how an offset shifts leg load and hang attitude, you have the Level I Scope foundation for safe simple lifts—and for the weight/CG items on the written exam.