Results are cast not in stone, but magnetic rubber
Crane owners pay for thousands of OSHA-mandated inspections every year. Components of each crane are examined with a critical eye, reports are generated for management, and repairs are made when needed. But inspections can vary in depth, complexity and thoroughness. One key component that sometimes falls victim to a less-than-thorough inspection is the hook itself.
By Dave Hermanowski, Technical Trainer
Unfortunately, the part of the hook that is most susceptible to stress cracks and damage is the part nobody can see. The hook shank is the portion that is encased inside the bottom block, and defects here can cause all kinds of catastrophic damage–including a dropped load and loss of life. But because it takes more time to disassemble and remove the hook from the bottom block, test and then reassemble it, production-conscious managers may resist taking the crane out of service to examine the shank.
New frontiers in non-destructive testing
Before Konecranes began doing magnetic rubber inspections in 2010, inspecting the hook shank and showing the result to the customer seemed a little like a magic act. A traditional magnetic particle test employed airborne iron particles coated in a dye material that contrasted with the color of the hook. After inducing a magnetic field into the quenched and tempered steel of the hook, iron particles would congregate on surface breaks, discontinuities or defects. But creating a permanent record of the result that management could review later was tricky. Technicians could attempt to lift up the magnetic particles with clear tape, like fingerprints at a crime scene. Or, they could take a photo and hope the ambient lighting would adequately record the result.
Magnetic rubber has changed all that. Magnetic rubber creates a record of the test that encases and holds the image of the defect. The technician can take it down to show it to the customer, and it can be kept on file forever and a day.
How does it work?
The magnetic rubber technique works like a traditional magnetic particle test, except that the suspended magnetic particles are encased in a latex-based material.This approach was developed for detecting very fine cracks and is capable of revealing finer cracks than other magnetic techniques. In addition, it can be used to examine difficult-to-reach areas, such as the threads on the inside diameter of holes, where the molded plugs can be removed and examined under ideal conditions and magnification if desired.
Once the hook shank is removed from the bottom block and accessible, the technician encases it in a clear plastic collar, secured at the base with a dam of modeling clay or plumber’s putty. Then the liquid rubber is catalyzed and poured around the shank. The hook is magnetized using the same magnetizing systems that are used for dry method magnetic particle tests: yokes, prods, clamps or central conductors. The magnetic current draws the particles to the leakage fields. The direct current yoke is the most common magnetization source for magnetic rubber inspection.
The catalyzed rubber is allowed to completely set, which takes from 10 to 30 minutes. After this rubber cast is removed, it is examined for evidence of discontinuities, which appear as dark lines on the surface of the molding. It can be retained as a permanent record of the inspection.
Is it worth it?
Crane owners can simply inspect the visible portion of the hook without violating OSHA crane inspection requirements. And because it takes more time to inspect the shank of the hook, they may question whether it is worth the extra time. Looking at sample cost-benefit scenarios, in a paper mill, a single dropped roll could dent the bottom line to the tune of $20,000. In a steel mill, a dropped coil could mean a loss of $50,000 or more.
So let’s review. If you have a 20-ton hook, you could spend a hundred and fifty dollars on a quick dye-penetrant test of the visible portions of the hook. Or, you could spend three to four hundred dollars on a thorough magnetic rubber inspection of the hook and shank, and potentially save your company thousands of dollars in lost productivity or a potential loss of life. Which would you choose?
