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The Working Load Limit, Minimum Breaking Strength, and Safety Factor

By: Dawnerz


Time to read 4 min

Three important terms commonly used in the field of towing and lifting are Working Load Limit (WLL), Minimum Breaking Strength (MBS), and Safety Factor (SF). These terms are often used when discussing the strength and load capacity of various equipment. Understanding these terms is crucial for ensuring safety and efficiency, and also ensure you select the most appropriate equipment for the task at hand. 

Lifting vs Towing - The working load limit

The majority of lifting and rigging equipment typically specifies the permissible load capacities using the Working Load Limit (WLL), whereas towing straps are typically marked by their Minimum Break Strength (MBS). When seeking to acquire fittings and accessories, it becomes imperative to possess the knowledge to accurately ascertain the appropriate ratings.

Minimum Breaking Strength (MBS)

The minimum breaking strength (MBS) or breaking strength refers to the utmost weight or force that the equipment can endure before it fractures. It's also recognized as ultimate strength, tensile strength, or breaking load. This measurement typically surpasses the WLL by a significant margin and is established by subjecting the equipment to destructive testing, revealing its ultimate resilience.


The difference between the Working Load Limit (WLL) and Minimum Breaking Strength (MBS) is important because it determines the safety margin or factor of safety. 

Working Load Limit (WLL)

The Working Load Limit (WLL) stands as the utmost weight or force that lifting equipment is crafted to manage in its regular operation. This parameter is also denoted as the Safe Working Load (SWL), rated capacity, or simply working load.


Manufacturers typically establish the WLL and present it on the equipment, accompanied by key details like model number, serial number, and manufacturing date.


Safety Factor (SF) also referred to as Design Factor (DF)

The safety factor is the ratio of the breaking strength to the WLL, and it represents the degree of safety built into the equipment. The higher the safety factor, the safer the equipment is. 


Determining the Appropriate Rating

The safety factor for lifting equipment is a predefined value clearly stated in the equipment's specifications. While the MBS indicator may not be explicitly mentioned in the specification, it can be easily determined by using a straightforward formula. By multiplying the Working Load Limit (WLL) by the Safety Factor (SF) or Design Factor (DF), you can calculate the Minimum Breaking Strength (MBS) of the equipment.


For instance, if a shackle or sling has a WLL of 5000 lbs and a Safety Factor of 7:1, its Minimum Breaking Strength (MBS) would be 35000 lbs


Equation:         ( WLL x SF = MBS )

Calculation:     ( 5000 x 7 = 35000 )


In the case of towing equipment, the Minimum Breaking Strength (MBS) serves as the installed value. However, the safety factor, also known as the margin of safety, is a variable that you determine based on the specific conditions of use. By determining the appropriate safety or design factor, you can calculate the Working Load Limit (WLL) using a simple formula: ( WLL = MBS / SF ). This calculation enables you to tailor the equipment's capacity to meet your specific requirements.


As a result, the safety factor for the same towing strap may vary depending on the intended application, leading to different Working Load Limits (WLL). For instance, consider a 90,000 lbs tow strap used on a flat road. In this scenario, a double margin of safety may be deemed sufficient, resulting in a safety factor of 2. Consequently, the WLL for this specific situation would be 45,000 lbs


( Calculation: 90000 / 2 = 45000 )


If the 90,000 lbs towing strap is utilized to extract a vehicle stuck in sand or mud, it is advisable to incorporate a triple margin of safety. In this specific scenario, the safety factor would be set at 3, resulting in a corresponding Working Load Limit (WLL) of 30,000 lbs


( Calculation: 90,000 / 3 = 30,000 )


When deciding on towing equipment, it's crucial to establish the appropriate Working Load Limit (WLL) based on the intended usage and the weight or force that will be exerted. The WLL should meet or surpass the weight or force involved in the towing operation. Additionally, factors like load angle, attachment points, and equipment condition play a pivotal role.


By applying this knowledge, individuals can ensure efficient operations and minimize risks, contributing to a safer and more productive working environment.


Understanding the Working Load Limit (WLL), Minimum Breaking Strength (MBS), and Safety Factor (SF) is pivotal for safe lifting, rigging, and towing operations. The distinction between WLL and MBS determines the safety margin, with SF highlighting the built-in safety level.

Calculating appropriate ratings ensures equipment efficiency and minimizes risks, contributing to a safer and more productive working environment.

At Dawnerz, our commitment to excellence extends beyond providing top-quality equipment – we're here to assist you every step of the way. We understand that safety, efficiency, and performance are paramount in your operations, and we're here to ensure you make informed decisions.


Is it acceptable to exceed the WLL?

No, surpassing the WLL should be strictly avoided. Doing so could lead to equipment failure, resulting in potential injury or damage.

Can the breaking strength replace the WLL?

No, utilizing the breaking strength as the WLL is not recommended. This practice lacks the necessary safety margin. Adopting the breaking strength as the WLL might lead to equipment failure and ensuing harm.

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