Working Load Limit
- Working Load Limit Defined and Replaces Safe Working Load Terminology
Faded Term “Safe Working Load”
We are examining the reasons for the industry changes in load limit terminology when evaluating the maximum force allowable during lifting or holding. Because of numerous environmental factors, manufacturers no longer find the term “Safe Working Load” accurately descriptive for maximum rated loads in equipment standards. Manufacturers all over the globe are concerned with effectively assigning load limits to avoid “plastic deformation” or stress effects during continued use of rigging equipment. These limits do not infer safety because so many factors can affect equipment with use in various environmental heat, cold and wet extremes over time.
Working Load Limit
What is the working load limit (WLL)? As a most frequently used Rigging Terminology, manufacturers tag all load carrying equipment, rigging and its components, with a Working Load Limit, or WLL. A simple Definition of Working Load Limit is the maximum load that includes mass or force, that should ever be applied to load carrying equipment in a specified configuration or application. Any rigging device or “configuration” is only as strong as its weakest, or lowest WLL rated, parts.
Working Load Limit Calculation
How to calculate the working load limit (WLL)? The load carrying equipment manufacturer calculates load bearing limits for all products. When setting up a rigging configuration, the total Working Load Limit is actually lower than the component with the lowest rated WLL. That is why it is important to make sure that all components match the load carrying standard needed to do the job. When calculating the working load limit (WLL) needed, all components including shackle, turnbuckle, hook and links attached to chains, cords and ropes must be made of same strength materials with equal working load limit (WLL) ratings.
Advantages of Work Load Limit
U.S. Engineering Standards, as well as European and ISO Standards no longer use the term Safe Working Load or SWL, officially. This term, at best, reflects the maximum load a product can carry or lift at optimum or brand new conditions. There is no way to predict the down rating or “derate” of load limits due to use of the equipment over time in various environmental conditions.
Never exceed the Working Load Limit of any equipment, match components properly and avoid shock loads. Shock Loads are identified as impacting, jerking and swinging loads which actually causes the working load to drastically increase. The high stress of these actions on a load can exceed the breaking strength or working load limit. This can cause the rigging to fail. Never allow workers or other people to stand under a raised load or in the line of force of any load. Workers should never put additional force or weight by riding on the load. Breaking strength is the force that a brand new rigging device has been tested to find its breaking point. This factor, in combination of other factors used by the manufacturer are included when calculating the Working Load Limit.
The only way to guardedly determine the safe use of a lifting device is by the consistency and thoroughness of inspection of rigging components for indications of cracks, rust, elongation, visible stress changes and damage. Reduce rigging hazards with regular inspections and teach crews that rigging equipment does not operate at rated capacity indefinitely. Always protect components from corrosion. Frequency of use, application, storage conditions and environment all impact rating of WLL.
Matters Need Attention
Working load limits are calculated on straight line pulls only. Never side load. Other conditions such as extreme temperatures, chemicals solutions or spills, vapors, or immersion in salt water can reduce the Working Load Limit. Welds to any steel products can also void out a Working Load Limit rating.