Everything You Need to Know About Loads
In the lifting industry, several different load ratings are used:
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Working Load Limit (WLL)
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Proof Load
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Manufacturing Proof Load
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Minimum Breaking Load (MBL)
In this article, I'll explain each of these terms and what you need to know about them.
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Working Load Limit
The Working Load Limit (WLL) is the maximum load that lifting equipment is designed to safely lift under normal operating conditions. This limit is clearly marked on the equipment, and as a user, you must never exceed it.
The Working Load Limit depends largely on how the load is rigged. To learn more, read our article on safe lifting practices.
Proof Load
The Proof Load varies depending on the type of lifting equipment.
For example, eyebolts and lifting chains are proof tested at 200% of their Working Load Limit (WLL). This means they are subjected to twice their rated capacity during testing. Spreader beams, on the other hand, are typically proof tested at 150% of their WLL.
After proof testing, the lifting equipment must not show any permanent deformation. It must return to its original condition once the test load has been removed.
For more information about inspections and testing, read our article on identifying approved lifting equipment.
Manufacturing Proof Load
The Manufacturing Proof Load test is carried out by the manufacturer. This test goes a step beyond the standard proof load test.
For example, lifting chains are often tested to 250% of their Working Load Limit, placing an additional 50% load on the equipment compared to the standard proof load test.
Minimum Breaking Load and Design Factor
The Minimum Breaking Load (MBL) is closely related to the design factor (also known as the safety factor).
For example, if a lifting chain has a design factor of 4, a chain with a Working Load Limit of 1,000 kg must have a Minimum Breaking Load of at least 4,000 kg.
The chain may deform under loads approaching its Minimum Breaking Load, but it must not fail before reaching this minimum value.
The applicable design factor depends on the type of lifting equipment used, and this automatically determines its Minimum Breaking Load.
Knowledge file Safe Lifting
How do you use lifting equipment safely and correctly? How do you rig loads properly? And how should lifting equipment be checked, inspected, and tested? Learn how to carry out these tasks safely and responsibly.
