A load limit is the amount of weight an item can take before it results in a plastic deformation. There are several different types of load limits and breaking points for any given item. In general, the working load limit is the only one a customer needs, but others are useful as well. Load limits do not take environmental effects and age into account; these factors must be considered by the endpoint customer at the time of loading. A common result of exceeding load limits is deformation, collapse and breaking.
Nearly every structure or metal object is in danger of plastic deformation. This type of deformation is different from common bending or stretching. Once a substance reaches it point of plasticity, it does not return to its normal shape. In order to keep items from reaching this point, systems have maximum load limits. Under optimal conditions, these limits prevent plastic deformation from occurring.
When people refer to a load limit, they typically mean the working load limit. This is the limit where the item is safe from plastic deformation. If kept at this limit, the object should be safe for its lifetime. The exact lifetime of an object is a set amount of time, typically in years, that varies widely based on the object. While many objects continue to see use past their life expectancy, their load capacities are no longer supported past that point.
The working load limit is based on the proof load limit. The proof load is an amount of weight that the item withstood when it was brand new and unused. This limit is then reduced by half for the working limit. The proof load is the extreme point of weight that an item can take, and should never be used as a limit by a consumer. The total proof load rapidly decreases with age and use.
Some items also have a shock load limit. This is the limit of weight that may be placed on the object when the user intends to have sudden or jerking shifts in movement. When a load is suddenly moved, its weight transfers differently to the object it sits on. For a few milliseconds instead of weighing what it normally would, it can weigh significantly more or less. Those few milliseconds can cause plastic deformation and breaking.
There are two main points that any end user needs to take into account when reading load limits. The first is the age of the item. Any item past its expected use life is no longer governed by its weight limit. The other is environment. Load limits are put together in a laboratory environment, extreme heat and cold can weaken a structure. Other environmental factors, such as how wet area is, can have major impacts on certain metals and shouldn’t be discounted even when the item is new.
By M. McGee