Understanding WLL, SWL, and Breaking Strength
When working with rigging, lifting, or load-bearing equipment, terms like Working Load Limit (WLL), Safe Working Load (SWL), and Breaking Strength are critical for ensuring safety and selecting the right tools for the job. While these terms are often used interchangeably, they have distinct meanings and applications. Understanding their differences is essential for riggers, engineers, and anyone involved in lifting operations.
1. Breaking Strength
Breaking Strength refers to the maximum force or load that a piece of equipment (e.g., a rope, chain, or shackle) can withstand before it fails or breaks. It is determined through rigorous testing under controlled conditions. Breaking strength is a theoretical value that represents the upper limit of the equipment’s capacity, but it does not account for real-world variables such as dynamic forces, wear, or improper use.
Key Points:
• Measured in units such as pounds (lbs) or kilograms (kg).
• Represents the absolute maximum load an item can handle before failure.
• Is not a safe value for operational use.
• Testing involves applying force to the equipment until it physically breaks.
Example:
A steel cable with a breaking strength of 10,000 kg will fail if a load exceeding that value is applied. However, this does not mean it is safe to use the cable at or near this limit.
2. Working Load Limit (WLL)
Working Load Limit (WLL) is the maximum load that a piece of equipment can safely handle during regular use. It is derived by applying a factor of safety (FOS) to the breaking strength, accounting for real-world variables like dynamic forces, wear, and environmental factors.
Formula:
WLL = Breaking Strength divided by the factor of safety (FOS)
The safety factor typically ranges from 4:1 to 10:1, depending on the application and regulatory requirements. Under the UK’s LOLER regulations for textile ropes and slings the factor of safety is normally 7:1, wire rope 5:1. For pulleys it is normally 6:1 and for steel shackles and chains it is 4:1. When lifting people the factor of safety should be doubled, so for a textile rope it becomes 14:1.
Key Points:
• Represents the maximum safe load under normal conditions.
• Always includes a safety margin to prevent equipment failure.
• Must not be exceeded during use.
Example:
If a rope has a breaking strength of 10,000 kg and a safety factor of 7:1, the WLL would be 1,428 kg. This means the rope should only be used to lift or support loads up to 1,428 kg.
3. Safe Working Load (SWL)
Safe Working Load (SWL) is an older term that was widely used before WLL became the standard in industry terminology. It is essentially synonymous with WLL, representing the maximum load that equipment can handle safely under normal operating conditions. In modern practice, WLL has replaced SWL as the preferred term to avoid confusion and standardize safety practices.
Key Points:
• SWL and WLL are often used interchangeably but refer to the same concept.
• SWL is now less commonly used in professional and regulatory contexts.
• Represents the safe operational limit for equipment, calculated using safety factors.
Example:
A shackle with a SWL or WLL of 1,000 kg is rated to safely support or lift up to that amount, assuming proper use and maintenance.
Real-World Application of the Terms
Let’s consider a scenario where a lifting sling is being used to hoist equipment:
1. The breaking strength of the sling is 5,000 kg. This is the absolute maximum load the sling can handle before failure.
2. With a safety factor of 7:1, the WLL (and effectively the SWL) is calculated as 714 kg. This is the safe limit for lifting loads (the safe limit for lifting a person would be 7:1 x 2 = 14:1, so 357Kg.
3. If a load of 2,000 kg is applied, the sling would likely not break immediately but would operate well beyond its WLL, risking sudden failure and endangering personnel.
4. We must also consider that in many entertainment performance scenarios the load may not be static. In addition you may need to allow for additional forces generated through static or shock loading, reducing the safe WLL even further.
Conclusion
Understanding the differences between Breaking Strength, Working Load Limit (WLL), and Safe Working Load (SWL) is essential for maintaining safety in lifting and rigging operations. Breaking strength is a theoretical maximum capacity, while WLL and SWL represent safe, practical limits for real-world applications. By adhering to WLL and SWL guidelines, professionals can ensure the longevity of equipment and, more importantly, the safety of everyone involved in the operation.
See Also: Safety Factors