In lifting operations such as the use of jib cranes, hoists or slings, plus applications such as rigging, load restraint, material handling, fall protection, construction and demolition, it is essential that safe lifting limits are respected.
The use of terminology is how manufacturers of lifting equipment can guide users on how to use any equipment or lifting equipment accessories safely.
Two commonly used terms are working load limit (WLL) and safe working load (SWL).
However, while both SWL and WLL may be used interchangeably, there are some key differences to note.
Here is an overview to explain the differences between a safe working load and a working load limit from the lifting experts here at Metreel.
WLL – Working Load Limit
Working load limit definition: The maximum load that lifting equipment such as chains, hooks, ropes or slings are designed to raise, lower or suspend.
How Is the Working Load Limit Calculated?
The working load limit (WLL) is the maximum load a piece of lifting equipment is designed to lift in a straight, vertical pull under normal conditions. It is set by the manufacturer and should be marked on the equipment or its identification tag.
To calculate the working load limit, the manufacturer first establishes the minimum breaking load (MBL) of the equipment. This is the point at which the item would fail or break under testing. The MBL is then divided by a factor of safety, also known as a design factor, to give the WLL.
For example, a polyester webbing sling with a minimum breaking load of 7 tonnes and a factor of safety of 7:1 would have a working load limit of 1 tonne.
Typical factors of safety
Factors of safety vary depending on the type of equipment and the standard it is manufactured to. Common examples include:
Grade 8 chain slings: 4:1
Wire rope slings: 5:1
Synthetic webbing and roundslings: 7:1
The higher the factor of safety, the greater the margin between the working load limit and the point of failure.
Why the factor of safety is not a spare capacity
The factor of safety only applies to equipment in new condition. It is there to account for variables such as dynamic loading, minor wear and manufacturing tolerances, not to provide extra lifting capacity. Once equipment is in service, its strength can be reduced by:
- Abrasion and general wear
- Age
- Chemical exposure
- Cuts and nicks
- Knots
- Heat damage
- UV (solar) degradation, particularly in synthetic slings
Any of these can bring the true breaking load much closer to the working load limit, which is why equipment must never be loaded beyond its WLL. Exceeding it significantly increases the risk of sudden failure, dropped loads and serious injury.
When the working load limit needs adjusting
The WLL assumes a straight, vertical lift. When equipment is used differently, such as in a choker hitch, a basket hitch or with multi-leg slings at an angle, the safe capacity changes. A choker hitch, for instance, typically reduces the capacity of a sling to around 80% of its vertical WLL, while increasing the angle between sling legs reduces the load each leg can safely carry.
In these situations, a competent person must assess the configuration and determine the safe working load (SWL) for that specific lift. Under the Lifting Operations and Lifting Equipment Regulations 1998 (LOLER), every lifting operation must be properly planned by a competent person, and lifting equipment must be thoroughly examined at regular intervals: at least every six months for lifting accessories and every 12 months for other lifting equipment.
SWL – Safe Working Load
Safe working load definition: Safe working load is the maximum load that the equipment can safely lift, lower, move or suspend safely under normal operation conditions.
The key aspect to know about SWL is that it is a term which has since been replaced by WLL. Specifically, SWL was replaced for everything below the hook. Rated capacity became the new rating system for cranes, hoists and winches.
While it’s still common to hear SWL being referenced, it doesn’t hold any standing from a safety standpoint in modern times. The reason is that legally, it is very difficult for manufacturers to state that a load may be safe to lift.
Furthermore, there was often a lot of ambiguity over SWL which created risk management challenges. WLL was brought in to create clarity so that everyone from the engineers of the lifting equipment to the end users can best understand what constitutes a safe amount to lift, including based on the different types of conditions which may occur.
Metreel – Discover Effective Lifting Solutions For Your Business
Now you know the differences between SWL and WLL, are you keen to discover our vast range of material handling solutions?
Based in Derbyshire, Metreel supplies industrial lifting solutions to businesses across the UK and beyond.
Check out our products on our website to learn more, or give us a call on 0115 932 7010 to speak to our sales team.