Anyone in the vicinity of cranes, hoists, and other rigging equipment has likely heard the term “safe working load” (SWL) being referenced. SWL seems easy to grasp, yet each year many workplace incidents are attributed to a failure to properly understand or perform calculations pertinent to SWL. Knowing the correct SWL is critical to the success of a given lift, and is certainly a better scenario when compared to the disastrous alternative.
What Does Safe Working Load Mean?
The safe working load (SWL) for any piece of rigging or lifting equipment is its maximum load, under the most reasonable and standard operating conditions, that is safe for the equipment to handle. SWL is always set below the equipment’s break strength and incorporates a margin to cover wear, shock loading, and other unforeseen factors when operating potentially dangerous heavy equipment. It is standard practice for equipment manufacturers to place the SWL value on a piece of equipment or on an accompanying certificate. As a first and most important practice, always check the equipment label.
Laying the Foundation
Before the calculations begin, the load to be lifted, the number of lifting slings to be used, and the lift sling angle must all be known. Any of these missing pieces of information can change your calculations, and in certain situations, can have dangerous consequences.
Weight figures can typically be found on delivery notes, data plates, or engineering drawings. In the absence of these, it is preferable to estimate weight conservatively, as opposed to optimistically. If conservative estimates are not successful, this will undermine everything that follows.
The Angle Factor
This is where a lot of people tend to make mistakes. As the angle between the slings and vertical increases, tension on the legs also increases. For example, lifting slings suspended at right angles to each other will bear a considerably greater load per leg than if they were suspended fairly vertically. The capacity of slings when the angle is greater than 45 degrees, from vertical, starts to diminish quite rapidly, and rigging tables are developed to assist in determining this reduction accurately, rather than estimating.
An approximate working method can be described as follows:
– Reduce the total load by the number of slings in use.
– Referring to a rigging chart, apply the angle factor.
– Compare with the capacity indicated on the sling.
If the calculated load per leg of a sling exceeds the capacity, then the configuration is definitely unsafe, and must be remedied by changing the angle, by bringing in more lifting slings, or sourcing slings of a greater capacity.
Remember the Mode of Use
The rated capacity of slings will vary based on the method of rigging. Whether using a choke hitch, a straight vertical lift, or a basket hitch, the capacity will be different. Riggers should check certificates, as they usually specify all three configurations, to avoid verging on unsafe practice by assuming a universal capacity.
Damage, Wear, and Common Sense
The condition of the equipment, and not the calculations, is the most important factor. Deformed links, missing identification tags, and corrosion should all warrant a regulated stop to further investigate the condition of the sling, regardless of the calculations.
Summary
Once you learn the method for calculating the SWL, it is very easy, but it does take discipline. Ensuring you know the angles, the load, and the equipment, and doing a check to mitigate the risk of habitual practice. Lifting slings are designed to specific tolerances, and the practice of lifting slings within these tolerances is what protects the people who are beneath them.
