Proof Load vs Breaking Load: What Is the Difference for Lifting Chains?

Understand proof load, breaking load and WLL to correctly interpret lifting chain strength and test data.

When reviewing lifting chain specifications, you may see three different load values: Working Load Limit (WLL), proof load, and breaking load. Although they are related, they serve very different purposes.

The main difference in proof load vs breaking load is straightforward:

Proof load verifies the chain without intentionally destroying it, while breaking load is associated with destructive testing of the chain's ultimate strength.

The WLL is different from both. It represents the rated capacity used for normal lifting operations.

A simple way to understand the relationship is:

WLL → Normal rated lifting capacity
Proof Load → Manufacturing and strength verification
Breaking Load → Ultimate strength verification

Proof testing applies a specified force above normal working conditions to confirm that the lifting chain can withstand the required test load without unacceptable failure or permanent deformation. A conforming chain is not supposed to break during this test.

Breaking testing serves another purpose. A sample is subjected to increasing tensile force to verify its minimum breaking-force performance and, in an ultimate test, is loaded to failure. Because this process is destructive, the tested sample does not return to lifting service.

For many alloy steel chain sling systems, a minimum design factor of 4:1 is used. In simplified terms:

Minimum Breaking Force ≥ 4 × WLL

However, the exact relationship between WLL, proof force and minimum breaking force depends on the applicable chain standard, grade and product specification. A single proof-load multiplier should not be assumed for every lifting chain.

Most importantly, neither proof load nor breaking load represents additional usable lifting capacity.

If a chain has a 2-ton WLL, it remains a 2-ton rated lifting chain even when its proof and minimum breaking forces are substantially higher.

When selecting lifting chains, use WLL to determine working capacity. Proof-load and breaking-load information should instead help verify manufacturing quality, testing and compliance with the applicable technical requirements.

In short:

Proof load verifies. Breaking load measures ultimate strength. WLL defines normal rated use.