Proof Load Test: What is it and Why is it Important?
A proof load test involves applying a load to a structure or item of equipment to beyond its working load limit (WLL).
A proof load test demonstrates the fitness of that structure / equipment. Such tests may be required upon manufacture (before use), as part of a thorough examination, and / or required by regional legislation.
A proof test ensures that a lifting item works correctly and safely through its full cycle of operation. It verifies that there are no material or welding flaws, or other fundamental mechanical defects.
A proof test is usually expressed as a percentage of the WLL that equipment is designed to withstand. It does not compromise equipment because even when you test beyond rated capacity (to, say, 200%) you test well within manufacturing safety factors.
Det Norsk Veritas (or DNV), for example, mandates proof testing of loose gear such as chains, shackles, hooks and swivels to be performed thus:
WLL ⋜25t – proof test to two times WLL
WLL > 25t – proof test to (1.22 x WLL) +20t
This is the calculation used by Straightpoint when proof testing their load cells.
Proof testing actually serves as a way of “settling down” the strain gauges (electrical sensors used to measure force—or strain connected in a wheatstone bridge arrangement) at manufacture and tests the strain gauge bond to the substrate. If a newly strain-gauged load cell is not proof tested, return to zero after loading in normal use will be problematic.
Numerous load-testing applications worldwide require a load cell or dynamometer to verify the load applied, from crane testing using water bags and a tension load link, to cylinder testing using a hydraulic test rig and a compression load cell.
The exact requirements of load testing and the extent of each test can vary depending on standards, equipment or industries involved, as well as requirements from customers.
Here are the standards bodies to look out for in North America:
- OSHA: Occupational Safety and Health Administration
- ASME: The American Society of Mechanical Engineers
- ANSI: American National Standards Institute
And here are some specific standards that may require a load test to be performed on lifting and ancillary equipment:
- ASME B30.9 (lifting slings)
- ASME B30.10-1.7 (hooks)
- ASME B30.16 (overhead hoists, underhung)
- ASME B30.17 (overhead and gantry cranes, and trolleys)
- ASME B30.20: (below-the hook lifting devices)
- ASME B30.20: (manual lever hoists)
- ASME B30.26: (lifting accessories: shackles, eyebolts, turnbuckles, links, rings, swivels and compression hardware)
OSHA Section 1926.251(a)(4), meanwhile, covers special custom design lifting accessories that shall be proof tested prior to use to 125% of WLL, while ANSI N14.6 is application specific. ANSI singles out lifting devices, shipping containers, or nuclear materials weighing more than 9,921 lbs., for instance.
In mission-critical industries, additional tests may be required. NASA (The National Aeronautics and Space Administration) specifies the occasions and frequency of proof load tests and periodic load tests to be performed on lifting devices and equipment in its NASA-STD-8719.9 technical standard on lifting.
What equipment must be load tested?
As stated above, a load test might need to be performed on any load-bearing structure or item of equipment. Here are some examples of tests that Straightpoint equipment would be used for on a regular basis:
- Bollard pull tests
- Tug tests
- Crane tests (using water bags, block weights)
- Pad eye or fly point tests
- Crash barrier tests
- Lifting equipment tests (slings, chains, wire rope, hooks)
- Construction equipment tests (shoring columns, lintels)
- Lifting and spreader beam tests
- Hydraulic cylinder load tests
It is important to note that some of these items will be proof tested in tension (slings, chains, wire rope and cranes) and others will be tested in compression (spreader beams and hydraulic cylinders).