The difference between WLL and SWL

The terms Working Load Limit (WLL) and Safe Working Load (SWL) are used for the maximum load capacity of a construction, tool, or equipment between the crane and the load. But what are the differences between both terms and when and where are they applied? And what is Eager.one’s philosophy on this subject?

The Working Load Limit (WLL) is the maximum load an item could lift under ideal conditions. The Safe Working Load (SWL) is a reduction of the WLL under particular operating conditions. This follows an engineering assessment, by a competent person, of the maximum load the item can sustain under the conditions in which the item is being used.

To explain the differences between WLL and SWL we will also take a closer look at the Dynamic Amplification Factor (DAF) and the psi (ᵠ).

Working Load Limit (WLL)

Working Load Limit (WLL) is the maximum load for which a piece of lifting equipment is designed to raise, lower, or suspend. The WLL is the total load capacity at the optimal position without any kind of resistance. You can look at it like the ‘best case scenario’. In reality, the WLL is not applied because of the conditions in the field. To determine how much these conditions affect the final rating of the equipment (i.e. SWL) we must consider the Dynamic Amplification Factor (DAF).

Dynamic Amplification Factor (DAF)

The Dynamic Amplification Factor (DAF) is a variable factor that represents the dynamic effects that the working load is exposed to during lifting off or lowering/grounding of the load. In the best-case scenario you have a DAF that is close to 1 but always bigger than 1. The DAF is used for the below-the-hook calculations of rigging and for the lift capacity of the crane.

Safe Working Load (SWL)

The Safe Working Load (SWL) is the maximum load (as certified by a competent person) that a piece of lifting equipment may raise, lower or suspend under particular operating conditions. The SWL is calculated by dividing the WLL by the DAF.

This means that the SWL is always lower than the WLL, because a DAF of 1 and lower is impossible.

Relation to load factor (psi) in cranes

Cranes are designed and calculated to suspend loads. During pick-up and grounding of the load, dynamics occur which are covered by a factor called psi (ᵠ). Stiff cranes like gantry cranes will have a higher psi than very flexible cranes as, for instance, mobile cranes. Since the psi and DAF cover the same dynamics, these factors may negotiate. This means that in case a high DAF occurs due to, for instance, lifting through the splash zone, the capacity of the crane needs to be reduced with the surplus of the DAF.

The crane capacity is normally denoted as SWL XXX ton at a given psi (load factor). The DAF, and from that the SWL and WLL are used in the same way.

And what about the Center of Gravity (CoG)?

In this article we explained the difference between the SWL and WLL. This showed that these two terms are not isolated. The DAF and psi also play an important role. In addition, to complete the calculation, we must also look at the Center of Gravity (CoG). We will cover that subject in more detail in a future article.

Structural engineers and designers of Eager.one design the proper construction, tool or equipment, to fulfill each specific customer requirement. Please contact us to discuss the requirements for your specific lifting or handling project.