Millwright Certification Rigging Techniques 1 — Questions and Answers
Question 1: What does the D/d ratio refer to in wire rope rigging, and why is it significant?
- 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 (Correct answer)
- The ratio of the design factor to the working load limit; it determines how many lifts a sling can safely perform
- The ratio of dead weight to dynamic load; it accounts for shock loading during sudden starts or stops
- The ratio of drum diameter to fleet angle; it prevents the rope from piling up unevenly on the drum
Correct 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
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:
- 125% — the choke improves grip and increases the effective load rating
- 100% — hitch type does not affect the rated capacity of a sling
- 75% — the choke angle creates a stress concentration that reduces the effective load rating (Correct answer)
- 50% — a choker hitch should only be used for loads at half the sling's catalogue rating
Correct answer: 75% — the choke angle creates a stress concentration that reduces the effective load rating
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:
- 5% of the total wires
- 10% of the total wires (Correct answer)
- 25% of the total wires
- Any broken wire, regardless of percentage
Correct answer: 10% of the total wires
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?
- To increase friction between the sling and the load to prevent slipping
- To protect the sling from abrasion and cutting at sharp edges, which can drastically reduce its rated capacity (Correct answer)
- To help center the sling on the load for a more balanced lift
- To prevent heat transfer from hot loads into the sling material
Correct answer: To protect the sling from abrasion and cutting at sharp edges, which can drastically reduce its rated capacity
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?
- The lift will take longer because the load oscillates until it self-corrects beneath the hook
- The load will tip or swing as soon as it is lifted, creating an uncontrolled and hazardous condition (Correct answer)
- The crane's load cell will give an inaccurate reading, requiring a re-weigh on the ground
- There is no consequence as long as a tag line is attached to the load
Correct answer: The load will tip or swing as soon as it is lifted, creating an uncontrolled and hazardous condition
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.
Question 6: What is a 'proof load' test for rigging hardware, and at what multiple of the Working Load Limit (WLL) is it typically conducted?
- A visual inspection at 100% WLL performed before each use on a job site
- A non-destructive load test at 200% of the WLL to verify the hardware sustains the load without permanent deformation (Correct answer)
- A destructive pull test at 500% of the WLL used to establish the breaking strength for new hardware designs
- A field test at 50% of the WLL performed after any incident to confirm the hardware is still serviceable
Correct answer: A non-destructive load test at 200% of the WLL to verify the hardware sustains the load without permanent deformation
A proof load test is a non-destructive quality-assurance test applied by the manufacturer or a certified testing facility at twice (2×) the Working Load Limit. The hardware must hold the proof load without any permanent deformation, cracking, or failure. This confirms the hardware is fit for its rated service — it is not a routine field check.
Millwright Certification Rigging Techniques 1
What does the D/d ratio refer to in wire rope rigging, and why is it significant?