Millwright Certification Rigging Techniques 1

Question 1

What does the D/d ratio refer to in wire rope rigging, and why is it significant?

Accepted Answer

The ratio of the sheave or drum diameter to the wire rope diameter; a low D/d ratio causes excessive bending stress and accelerates rope fatigue

Answer

The D/d ratio compares the diameter of the sheave or drum (D) to the diameter of the wire rope (d). When this ratio is too low, the rope is forced to bend sharply, creating high bending stresses in individual wires and significantly shortening rope life. Most standards recommend a minimum D/d ratio to preserve rope capacity and fatigue life.

Question 2

Compared to a vertical (straight) hitch, a choker hitch reduces the rated capacity of a wire rope sling to approximately:

Accepted Answer

75% — the choke angle creates a stress concentration that reduces the effective load rating

Answer

A choker hitch reduces a sling's rated capacity to approximately 75–80% of its vertical hitch rating. The choke point creates a stress concentration where the sling wraps back on itself, reducing the effective load-bearing cross-section. Riggers must account for this derating when selecting sling size.

Question 3

A wire rope sling must be removed from service when the number of broken wires in any one rope lay reaches:

Accepted Answer

10% of the total wires

Answer

Industry standards (ASME B30.9) require removal from service when 10% or more of the wires in any rope lay are broken. Broken wires indicate the rope has reached its fatigue life; the remaining intact wires must carry a disproportionately higher share of the load, greatly increasing the risk of sudden failure.

Question 4

What is the primary purpose of using corner protectors or edge guards when rigging a load with synthetic web slings?

Accepted Answer

To protect the sling from abrasion and cutting at sharp edges, which can drastically reduce its rated capacity

Answer

Synthetic web slings are highly susceptible to damage from sharp edges. A cut or abrasion can reduce the sling's capacity to a fraction of its rating without being visually obvious from the outside. Corner protectors or softeners spread the load over a larger area and protect the sling material, ensuring the rated capacity is maintained.

Question 5

Before making any crane lift, the rigger must identify the load's center of gravity (CG). What is the consequence of positioning the crane hook directly above a point that is NOT the CG?

Accepted Answer

The load will tip or swing as soon as it is lifted, creating an uncontrolled and hazardous condition

Answer

The center of gravity is the single point where the load's entire weight effectively acts. If the hook is not directly above the CG, gravity will rotate the load until the CG is directly below the hook. This sudden tilting or swinging can injure workers, damage the load, or cause a dropped-load incident.

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Millwright Certification Rigging Techniques 1