NCCCO Sling Types and Load Distribution 2 — Questions and Answers

Question 1: What is the D/d ratio for a wire rope sling and why is it important?

It is the ratio of drum diameter to rope diameter, used for hoist drum sizing
It is the ratio of the bend radius (D) around a hook, shackle, or fitting to the rope diameter (d), which determines the efficiency loss — smaller D/d ratios cause greater bending stress and capacity reduction (Correct answer)
It is a quality control ratio used in wire rope manufacturing
It applies only to synthetic web slings, not wire rope

Correct answer: It is the ratio of the bend radius (D) around a hook, shackle, or fitting to the rope diameter (d), which determines the efficiency loss — smaller D/d ratios cause greater bending stress and capacity reduction

The D/d ratio is the diameter of the bend (fitting or object the sling wraps around) divided by the rope diameter. Small D/d ratios create sharp bends that reduce wire rope sling capacity by a rated derating factor.

When a wire rope sling bends around a hook, shackle pin, or structural member, the outer wires must travel a longer path than the inner wires, causing differential wire stress. The tighter the bend (smaller D), the greater this differential stress. Wire rope manufacturers publish D/d ratio efficiency tables showing how much the rated capacity must be derated for different bend diameters. For example: at D/d equal to 1 (the sling folds completely back), efficiency may be only 50 percent; at D/d equal to 10, efficiency is approximately 90 percent; at D/d equal to 25 or higher, efficiency approaches 100 percent. When connecting a wire rope sling to a hook with a pin smaller than the rated ratio, a shackle with an appropriate-sized pin must be used to achieve adequate D/d ratio.

Question 2: A rigging crew is using a two-leg bridle sling to lift a horizontal beam. The horizontal angle of each sling leg from vertical is 30 degrees. What effect does this angle have on the tension in each sling leg?

Each sling leg carries exactly half the load weight regardless of angle
The tension in each sling leg is greater than half the load weight — the more horizontal the angle, the greater the tension in each leg (Correct answer)
Sling tension decreases as the horizontal angle increases
Angle has no effect on sling tension as long as the load is level

Correct answer: The tension in each sling leg is greater than half the load weight — the more horizontal the angle, the greater the tension in each leg

In a bridle sling, as the sling legs angle out from vertical, the tension in each leg increases. At 30 degrees from vertical (60 degrees from horizontal), each leg carries approximately 57.7% of the total load weight.

The tension in each sling leg of a two-leg bridle can be calculated as: T = (Load divided by 2) divided by cos(vertical angle) = (Load divided by 2) times the sling tension factor from ASME B30.9 tables. At 30 degrees from vertical: T = (Load/2) divided by cos(30) = (Load/2) divided by 0.866 = 0.577 times Load. So each leg carries 57.7 percent of the total load — more than the 50 percent if the slings were perfectly vertical. At 45 degrees from vertical each leg carries 70.7 percent. At 60 degrees from vertical each leg carries 100 percent of the load weight. This is why ASME B30.9 prohibits sling angles less than 30 degrees from the horizontal (60 degrees from vertical).

Question 3: Under ASME B30.9, what is the minimum allowable horizontal sling angle for a wire rope sling in a two-leg bridle configuration?

15 degrees from horizontal
30 degrees from horizontal (Correct answer)
45 degrees from horizontal
60 degrees from horizontal

Correct answer: 30 degrees from horizontal

ASME B30.9 establishes 30 degrees from horizontal as the minimum sling angle for wire rope slings in bridle configurations. Below this angle, the increased tension in the slings creates a risk of overload.

ASME B30.9 establishes minimum sling angles to prevent overloading from excessive angular tension. At 30 degrees from horizontal (60 degrees from vertical), the tension factor is 2.0 — each leg carries twice the tension it would if vertical. The minimum 30-degree horizontal angle is a practical safety limit reflecting that: below 30 degrees, small angle measurement errors create large errors in calculated tension; below 30 degrees, the sling tension multiplier increases very rapidly; and achieving exactly the calculated angle in the field is difficult, so a minimum angle buffer is needed. Riggers must use a sling angle chart or calculator to verify that the angle for their specific rigging geometry meets this minimum before making any lift.

Question 4: What is the difference between a vertical hitch, a choker hitch, and a basket hitch for a single sling?

They are three different names for the same rigging configuration
Vertical hitch suspends the load from the end of the sling; choker hitch wraps around the load and passes through itself, reducing capacity; basket hitch wraps under the load with both ends attached to the hook, increasing capacity (Correct answer)
Choker hitches are the strongest; basket hitches are the weakest
Vertical hitches are only used with chain slings; choker and basket are for synthetic slings

Correct answer: Vertical hitch suspends the load from the end of the sling; choker hitch wraps around the load and passes through itself, reducing capacity; basket hitch wraps under the load with both ends attached to the hook, increasing capacity

These three hitches have different geometries and capacity ratings: Vertical hitch equals 100% capacity; Choker hitch equals approximately 75-80% (due to the bend at the choke point); Basket hitch equals approximately 200% (load shared across two legs).

The three basic hitch types have fundamentally different rigging geometries: Vertical hitch — a single sling leg attached at each end with the load hanging vertically, capacity = 100% of the sling's rated capacity. Choker hitch — one end passed through the other (or through a ring) and tightened around the load, capacity approximately 75 to 80% of the vertical hitch capacity (the exact factor depends on the choke angle). Basket hitch — sling wrapped under the load with both ends connected to the hook, the load is shared between two sling legs giving approximately 200% of the single-leg capacity when the sling legs are vertical. These factors are typically printed on sling tags for quick reference and are listed in the ASME B30.9 standard tables.

Question 5: A synthetic web sling has a working load limit (WLL) of 10,000 lbs in a vertical hitch. What is the approximate WLL in a basket hitch with a 45-degree horizontal sling angle?

10,000 lbs
14,140 lbs (Correct answer)
20,000 lbs
7,070 lbs

Correct answer: 14,140 lbs

Basket hitch WLL = 2 x Vertical WLL x sin(horizontal angle). At 45 degrees: WLL = 2 x 10,000 x sin(45) = 2 x 10,000 x 0.707 = 14,140 lbs.

For a basket hitch with angled legs, the capacity calculation combines the two-leg capacity with the angle reduction: WLL (basket angled) = 2 times Single-leg Vertical WLL times sin(horizontal sling angle). The formula uses sine of the horizontal angle (equivalent to cosine of the vertical angle). At 45 degree horizontal angle: WLL = 2 x 10,000 x sin(45) = 2 x 10,000 x 0.707 = 14,140 lbs. If the basket were perfectly vertical (90 degree horizontal angle): WLL = 2 x 10,000 x 1.0 = 20,000 lbs (full double capacity). The 14,140 lbs result is exactly 70.7 percent of the full vertical basket capacity, reflecting the angle reduction. This calculation is the basis for the ASME B30.9 sling angle tables that riggers use in the field.

Question 6: What is the concern with side loading on a shackle pin, and how does it affect the shackle's working load limit?

Both bow and pin have the same capacity — they are rated as a unit
Side loading on the pin can reduce shackle capacity by up to 30% or more because it creates a bending moment the pin is not designed to resist (Correct answer)
The bow is always the weakest part of any shackle
Pin capacity is always higher than bow capacity, making pin selection irrelevant

Correct answer: Side loading on the pin can reduce shackle capacity by up to 30% or more because it creates a bending moment the pin is not designed to resist

A shackle's WLL is established for in-line loading. Side loading on the pin (load not centered on the bow) creates a bending moment on the pin that significantly reduces the assembly capacity and must be avoided.

Shackle working load limits are established for in-line loading, meaning the load is applied along the centerline of the bow with the pin perpendicular to the load axis. Side loading occurs when the sling or load applies force at an angle to the bow's centerline, creating a moment (bending force) on the pin. ASME B30.26 notes that shackle capacities can be significantly reduced by side loading. To minimize side loading: use screw-pin or bolt-nut-cotter shackles (not snaplinks or trigger-gate hooks) for heavy loads, ensure only one sling eye is in the shackle bowl, and use a master link or pear-link at the crane hook when multiple slings are used. Multiple-sling connections on one shackle pin create side loading and should be analyzed.

NCCCO Sling Types and Load Distribution 2

What is the D/d ratio for a wire rope sling and why is it important?