Calculating Sling Angles: A Guide for Safe Lifting

Lifting slings are critical components of any rigging system. When it comes to choosing the right sling for your lifting needs, there are many factors to take into consideration such as working load limits, materials, number of legs, and hardware.

But a commonly overlooked factor in choosing the right lifting sling involves its rated capacities at different angles.

Sling angles form between the horizontal plane of your load and the sling, significantly impacting the load's stability and the stress exerted on the sling. Understanding the impact of sling angles on calculating the accurate capacity for your lifting device ensures the safety and stability of your lifted load.

This guide will help you understand the importance of sling angles, how to use them in determining the right capacity lifting sling, and best practices to follow.

What is a Sling Angle?

A sling angle is the area formed between the leg of a lifting sling and the horizontal plane of your load. As the sling angle decreases, the tension on the sling increases. This is because smaller angles require more force to lift the load vertically, putting more stress on the sling.

• READ: Tension vs. Compression Forces

When using a sling for a vertical lifting task, designated and qualified individuals use sling angles to calculate the appropriate capacity needed for the sling to lift the load safely and efficiently. Using slings beyond their limits can lead to overloading, causing sling failure, potential accidents, and deadly situations. Therefore, understanding how sling angles help with finding the right capacity is essential for ensuring the safety of personnel and the stability of the lifted load.

How to Use Sling Angles to Calculate Sling Capacity

The goal behind calculating sling angles is to determine the pounds per force (lb/f) on each leg when in a lifting position to ensure your chosen lifting sling can withstand the added tension. There are many factors to consider such as length of the sling leg (L), height of the load bearing points (H), load weight (LW), and load tension (TF). However, the key to calculating the appropriate capacity required for your lifting sling starts with your sling angle.

Calculating Sling Angles

To calculate the angle between your sling and your load, you need to use the arcsin trigonometric function for precise measurement, as shown below:

Sling Angle (θ) = arcsin(H/L)

However, many designated personnel find their sling angles using devices such as digital inclinometers or angle finders. These tools often have magnetic bases or clips to attach securely to the sling, where they find the precise measurement of the angle. Other ways of calculating the sling angle include using a protractor or a sling angle chart provided by your sling's manufacturer. By cross-referencing the sling's length and vertical height with the corresponding angle, you can find your measurement quickly and easily. (NOTE: These do not account for non-standard conditions, and may not account for all variables involved in your lift. Consult your sling manufacturer for any questions on these variables.)

EX: If sling length (L) measures 11 ft. long, and sling height (H) measures 10 ft. long, then arcsin(10/11) = ~65.38°.

Finding the Load Factor

Once you found your sling angle, then you can determine what capacity you need for your lifting sling. To do this, you need to find the corresponding load factor of your angle. The load factor is a multiplier used to calculate the increased tension in each leg of the sling as the sling angle decreases from vertical. It reflects how much the tension in the sling increases due to the angle of the lift.

• BLOG: Tensile Strength for Lifting & Rigging Equipment

In a single-leg lifting sling where the angle is 90° to the horizontal plane of the load, the load factor equals 1, meaning no additional tension needs to be considered for this lifting setup. However, if the angle decreases to 60° instead, then the load factor equals 1.154, indicating tension is making the load weight heavier than it actually is. 

EX: If you hoist a load with a three-legged lifting sling forms a 60° angle with the horizontal plane of your load, which weighs 15,000 lbs, then the minimum required vertical capacity of your sling = 15,000 x 1.154 = 17,310 lbs.

Additionally, another factor to consider is the reduction factor. This value represents the decrease in the effective capacity of a sling as the sling angle decreases. It shows how much the rated capacity of the sling should be reduced to due to the angle. Using the example above, if the slings angle is 60°, then the lifting sling is reduced to .866, or 86.6%, of its total effective capacity.

Many sling manufacturers include calculated charts for a load's increased tension and reduced capacity, as shown below:

Most sling manufacturers include these values for a range of sling angles, typically in increments of 5, starting at 90° and decreasing down to 30°. For safe lifting, it's generally recommended to keep sling angles above 30 degrees. 

Dividing Capacity by Sling Legs

Lastly, if you want to find the capacity for each sling leg, then divide the load weight (LW) by the number of sling legs and multiply it by the tension factor (TF). 

EX: If LW = 1,000 lbs. and your lifting setup involves two sling legs each at a 45° angle, then 1,000 / 2 x 1.414 = 707 lbs. per sling leg.

No matter how many sling legs are included, your lifting device should be positioned in the center of your load for proper weight distribution. Not only is this a safe lifting and rigging practice, but it makes calculating sling leg capacity much more accurate.

• LEARN: How Many Legs Do I Need for My Lifting Sling?
  

Sling Angles for Other Hitches

Sling angles affect all types of lifting hitches, reducing rated capacities in different ways. 

Basket Hitch

In a basket hitch, the sling is passed under the load with both ends attached to the lifting device. This setup effectively cradles the load, distributing the weight across both legs of the sling. A vertical basket hitch is often used when a balanced, stable lift is required.

When two lifting points are present in a basket hitch, the sling angles equal 90° to the horizontal plane of the load, with each leg supporting half of the load weight. However, with one lifting point, sling angles decrease, affecting the tension factor and increasing the rated capacities for the lifting sling.

Choker Hitch

In a choker hitch, the sling is wrapped around the load, with one end passing through the other to create a tightening loop around the load. This setup is commonly used for lifting cylindrical objects or loads that might slip if lifted with a straight or basket hitch. 

Sling angles play a significant role in the lifting device's rated capacity for a choker hitch. Because of the nature of the hitch, there is more load compression and friction at the choke point, adding more stress on the sling. Therefore, measuring sling angles is crucial when determining the right capacity for your lifting sling.

When measuring sling angles for choker hitches, the plane does not align with the horizontal surface of the load. Instead, the plane aligns with the choked sling leg, as shown in the graphic. The sling angle forms between the plane and the lifted sling leg, otherwise known as the angle of the choke. As the angle decreases, so does its rated capacity to lift the load.

To determine the minimum rated capacity for your choker hitch, the American Society of Mechanical Engineers (ASME) and the Web Sling & Tie Down Association (WSTDA) put together charts where a specific range of sling angles correspond to the appropriate reduction factor.

Remember, this is the reduction of the choker hitch capacity. When calculating the reduced capacity of your lifting sling, you need to use this value instead of other hitch capacities for proper calculation.

EX: If a lifting sling has a choker hitch capacity of 8,500 lbs., and my vertical lift has a choke angle of 102°, then my reduced capacity equals 7,395 lbs. (8,500 x .87).

Best Practices for Lifting Slings

For any lifting operation, you'll want to keep in mind the following measures to ensure safety, efficiency, and longevity of your equipment:

• Select the Right Sling: Choose the appropriate material for your lifting sling based on the load's weight and shape. For example, synthetic slings work best for delicate loads, while wire rope slings work better for heavy, rough loads.
• Follow Manufacturer Guidelines: Adhere to the manufacturer's recommendations for inspection intervals and criteria for retiring slings from service.
• Check Rigging Hardware: Inspect hooks, shackles and other rigging hardware for wear, deformation, or damage. Ensure all components are properly rated for the load.
• Ensure Proper Sling Usage: Inspect the environment of your lifting job to keep your sling and cargo protected. Avoid sharp angles and edges, maintain proper sling angles throughout the lift, and keep your load balanced to prevent shifting, which can lead to dangerous situations.

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