Working Load Limits in Trucking
William Thomas / January 2021
What is Working Load Limit as Applied in Heavy Cargo Trucking
The working load limit, as applied to oversize load transport, is the maximum load limit specified by the manufacturer and indicated in the component or system. The maximum load limit of any system is always equal to the slightly less force, which, when applied to a lifting system, or when applied to a system exerting an opposite force, will make the system yield or fail catastrophically.
An excellent example of a working load limit is the ratings of the chains or straps. Any component or system should never be subjected to any force that exceeds the working load limit. Why? Because the system will yield, which may lead to equipment damage. For example, when a driver subjects the straps and chains to larger forces that exceed their working load limit on a flatbed, the straps will get loose and the load will shift position, which may lead to toppling of the trailer. Working load limit ratings apply to all lifting and load securement systems. According to the United States’ department of transportation, the working load limit on any securement system, say a tie-down assembly, used to secure a load against moving either sideways or back and forth should at least be half the weight of the secured shipment.
Another term related to the working load limit (WLL) is the safe working load limit (SWL)
What Is Safe Working Load And How Is It Applied in Heavy Cargo Trucking
Also known as normal working load, a safe working load (SWL) is the maximum force a lifting system, a securement system, or an assembly can safely hold or lift without the fear of the assembly or the system failing. Just like the WLL, SWL is also specified by the manufacturer and the ratings are marked on the equipment. Any force that exceeds the safe working load is considered to pose a risk to the users and other equipment.
Both the safe load limit and the working load limit are both used to determine the safety factor of lifting systems. Since the calculations behind safe working load limits are vague to some extent, the safety regulation bodies stopped using the term and instead adapted WLL as a determining factor. For safe loading, unloading, and towing, tie-downs should never exceed the working load limit specified by the manufacturer. Also, an operator should take into consideration how the downward pressure affects the tie-downs,
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What Determines the Strength of a Tie-Down System in Oversize Load Trucking
With heavy equipment transport, the system is as weak as the weakest component in the system. When other factors are kept constant—such as the efficiency of the straps—the component that is marked with the lowest working load limit ratings represents the maximum strength of the entire system. For this reason, a damaged component affects the integrity of the whole system.
Tie Downs Inspection for Safe Trucking and Delivery of Heavy Equipment
Tie-downs are important equipment that determines how secured a load is during loading, transporting, and unloading. Any lifting machinery that uses tie-down straps is required by law to be inspected after a given period of time. To guarantee safety, a risk assessment should be carried out at shorter intervals, and any damaged strap or strap accessories should be replaced immediately.
Factors Affecting The WLL of A System in Heavy Load Delivery
- Effect of Angle (Effective Downward Pressure)
- The Number of Tie-Down Used
- The Weight of the Load
- The Weight Distribution of the Load
- Presences of A Support System, e.g. A Rigid Sidewall That Prevent Sideway Movements
- The Efficiency of the Tie Downs
- Manufacturer’s Ratings