Frequently Asked Questions :

  QUESTION 1 : 

 HOW TO DECIDE LIFTING CAPACITY OF A SLING ?

1. Know Your Wire Rope’s Breaking Strength

Every sling is manufactured with a specific breaking strength. This value depends on factors such as:

• Diameter

• Construction type (e.g., 6×19, 6×37, etc.)

• Material grade

For example, a given wire rope might have a documented breaking strength (BS) of 10,000 lbs. Manufacturers typically supply this number on a certificate or in a technical data sheet.

2. Apply the Appropriate Safety Factor (Factor of Safety, FS)

Due to the dynamic loading and inherent uncertainties in lifting operations, a safety factor is applied to the breaking strength. For wire rope slings:

• A common safety factor is 5:1.

The basic equation is:

> SWL = BS / FS

For instance, if BS = 10,000 lbs and FS = 5, then:

> SWL = 10,000 lbs / 5 = 2,000 lbs

This number represents the maximum load the sling should safely support under ideal conditions.

3. Consider Sling Configuration and Geometry

The configuration in which you deploy the sling significantly affects its effective capacity. The two main points are:

A. Number of Legs:

• Single-leg vs. Multi-leg:

  • In a single vertical leg, the SWL is directly the calculated value above.

  • In multi-leg (ladder, basket, or choker) configurations, the load is shared among the legs. However, this sharing isn’t always a simple division by the number of legs—especially if the sling is not perfectly vertical.

B. Sling Angles:

• When sling legs deviate from the vertical, the effective load on each leg increases.

• A simplified way to look at it for a two-leg sling is to note that each leg experiences a pull of the load divided by the cosine of half the angle between the legs. Visualize it like this:

           / \
          /   \
         /     \
    Leg1/       \Leg2
       /         \
      -------------
         Load

• If each leg makes an angle (θ) with the vertical, the effective load on each leg (EL) becomes approximately:

> EL ≈ (Total Load ÷ 2) / cos(θ)

For instance, if the legs are angled at 30° from vertical (cos 30° ≈ 0.866), each leg “feels” about 1/0.866 ≈ 1.15 times the half-load. The table below gives a rough idea:

Important: For a multi-leg configuration, you must ensure that none of the individual legs is overloaded beyond its capacity after accounting for the angle effect. Often, manufacturers provide capacity charts for various sling configurations.

4. Evaluate the Effect of Hardware and Fittings

The entire lifting assembly is only as strong as its weakest link. Consider:

• Thimbles, shackles, and hooks: These components have their own ratings. The overall capacity of the sling must not exceed the lowest rated element in the lifting assembly.

• Attachment methods: Swaging or other termination methods can influence the effective capacity. Ensure that terminations are in good condition and installed per manufacturer instructions.

5. Consider Service Conditions and Inspection Findings

Even if the theoretical capacity is high, practical considerations can reduce it:

• Wear and Damage: Kinks, abrasions, corrosion, or internal wire breakage (often detected during regular inspections) lower the effective capacity.

• Bending and Abrasion: Repeated bending or friction over pulleys, sheaves, or sharp edges can weaken the rope over time.

It is essential to incorporate any deductions for wear or uncertainty in the operating environment, often by using a more conservative safety factor or limiting the load further.

6. Consult Standards and Manufacturer Guidelines

Always refer to applicable codes and standards such as:

• OSHA Standards

• ASME B30.9 (for slings)

• Local or industry-specific guidelines

These documents often provide detailed formulas, charts, and procedures for calculating safe working loads, including considerations for sling angles and multi-leg configurations. Manufacturer-supplied literature is equally crucial.