3.2 Working Load Limit (WLL) & Hardware Inspection

Working Load Limit (WLL) and Design Safety Factors

All rigging hardware and hoisting components are assigned a Working Load Limit (WLL), defined as the maximum static load that the component is certified to support under normal operating conditions. The WLL is established by applying a designated safety factor (also known as a design factor) to the component's Minimum Breaking Strength (MBS):

$\text{WLL} = \frac{\text{Minimum Breaking Strength (MBS)}}{\text{Safety Factor}}$

Standard Safety Factors in Industrial Rigging

• 5:1 Safety Factor: Standard for general rigging hardware, wire rope slings, synthetic web slings, Grade 80/100 alloy chain slings, shackles, hooks, and swivels. For example, a shackle with an MBS of 50,000 lbs has a WLL of 10,000 lbs (50,000 / 5).
• 10:1 Safety Factor: Mandatory for equipment used to hoist personnel, such as crane-suspended work platforms, personnel baskets, and man-baskets.

Dynamic Loading and Shock Loads

Safety factors account for minor dynamic forces, but they do not permit overloading. Rapid acceleration, sudden braking, or load dropping introduces shock loading, which multiplies instantaneous forces acting on rigging hardware. Stopping a falling load abruptly can double or triple the effective weight, instantly exceeding the WLL and risking catastrophic structural failure.

Rigging Shackles: Selection, Application, and Angular De-Rating

Shackles are the most common connecting hardware in industrial rigging. They are manufactured from forged alloy or carbon steel and consist of a bow/body and a removable pin.

Shackle Types

1. Screw Pin Shackles: Feature a threaded pin that screws directly into the shackle body. Ideal for temporary rigging tasks. Mandatory Rule: Screw pins must be tightened until the shoulder seats flush against the shackle body. If a screw pin shackle is used in a choke hitch or subject to vibration where the pin could unscrew, it must be moused by wrapping stainless steel or copper wire through the eye of the pin and around the shackle leg.
2. Bolt-Type Anchor/Chain Shackles: Feature a unthreaded bolt pin secured with a nut and a cotter pin. Required for permanent or long-term installations, or where load movement could rotate the pin.
3. Anchor (Bow) Shackles vs. Chain (D-Type) Shackles: Anchor shackles have a rounded bow shape that permits multi-leg sling connections or angular loading. Chain (D-type) shackles have straight parallel sides and are strictly intended for straight in-line tension.

Angular Loading De-Rating of Shackles

When rigging slings pull on an anchor shackle pin at an angle relative to the center line of the shackle bow, the rated capacity (WLL) of the shackle must be de-rated in accordance with manufacturer specifications:

Prohibited Practice: Shackles must never be loaded at angles exceeding 90° relative to the centerline. Chain (D-type) shackles must never be side-loaded.

Eyebolts: Types, Installation, and Angular Loading Rules

Eyebolts provide threaded attachment points on machinery, gearboxes, and motors. Using the wrong eyebolt or installing it incorrectly is a frequent cause of rigging accidents.

Plain Shank (Shoulderless) vs. Shouldered Eyebolts

• Plain Shank (Shoulderless) Eyebolts: Feature a smooth threaded shank without a shoulder. Strict Limitation: Shoulderless eyebolts are certified ONLY for 0° straight vertical inline lifts. They must NEVER be subjected to angular loading. Any off-axis pull creates severe bending stress on the threaded shank, causing it to snap off at the first exposed thread.
• Shouldered Eyebolts: Feature a machined shoulder between the eye and the shank. Shouldered eyebolts are designed for vertical and angular lifting up to 45° (and up to 90° with extreme capacity reduction).

Mandatory Installation Criteria for Shouldered Eyebolts

1. The shoulder must be tapped down to seat 100% flush against the surface of the load. If a gap exists, bending forces will fracture the shank.
2. The tapped receiving hole must be clean, unstripped, and deep enough so the shank bottoms out or seats fully.
3. If the pull angle is not aligned with the plane of the eye, steel washer shims (up to 1/2 thread thickness) may be placed under the shoulder so that when tightened flush, the eye aligns with the sling direction.

Eyebolt Angular De-Rating Table

For example, a shouldered eyebolt rated for 10,000 lbs in a vertical lift is safe for only 3,000 lbs (10,000 × 0.30) when pulled at a 45° angle.

Hardware Inspection and Wire Rope Discard Criteria

Before every lift, millwrights must conduct a thorough visual inspection of all rigging hardware and slings. Damaged components must be tagged out and destroyed immediately.

Hook Inspection and Discard Criteria

Hooks must be removed from service if any of the following rejection conditions are present:

• Throat Opening: Any expansion of the hook throat opening exceeding 5% of the original factory measurement (or 1/4 inch / 6 mm).
• Hook Twist: Any twist exceeding 10° from the plane of the unbent hook.
• Saddle/Bowl Wear: Wear in the load-bearing saddle exceeding 10% of original stock dimension.
• Safety Latch: Missing, bent, or non-functioning safety latch.
• Cracks or Gouges: Any visible crack, nick, or gouge. Grinding out nicks is prohibited unless approved by the manufacturer.

Wire Rope Discard Criteria (ASME B30.5 / CSA Z150)

Running wire ropes (ropes moving over sheaves or drums) must be removed from service immediately if any of the following criteria are met:

1. Broken Wires (Running Ropes): 6 randomly distributed broken wires in one rope lay length, OR 3 broken wires in one strand in one lay length. (A lay length is the linear distance along the rope in which one strand makes a complete 360° spiral turn around the core).
2. Broken Wires (Standing Ropes/Pendants): 3 broken wires in one lay length, or 2 broken wires at an end termination.
3. Diameter Reduction: Reduction in nominal rope diameter exceeding 5% due to internal core degradation or wire wear.
4. Core Protrusion & Deformation: Protrusion of the main fiber/steel core between outer strands.
5. Birdcaging and Kinking: Severe strand displacement ("birdcaging") resulting from sudden release of tension, or permanent sharp bends ("kinks").
6. Heat or Electrical Arc Damage: Discoloration or wire fusion caused by contact with welding torches or live electrical lines.