Inspection of Crane Hooks

In a recent incident, a tower crane was hoisting a load weighing approximately 4000 kilograms (8800 pounds). The hook assembly on the load block failed, causing the load to fall. It narrowly missed hitting two workers below.

After the incident, the threads on the hook and nut were found to be corroded and excessively worn. The damage was not visible when the load block was assembled. This type of wear and corrosion could be found on any crane hook with a threaded shank.

The worn hook had surrounded threads and showed signs of corrosion. A similar hook had threads with sharp edges and no signs of excessive wear or damage.

Inspecting the load hook assembly

1. Remove the hook from the load hook assembly and clean the threads on the hook and the nut.
2. Examine the threads for cracks using magnetic particle or dye penetrant test methods. The first three threads from the bottom and the thread relief area are the most critical areas to inspect. If a crack is found in one of these areas, remove the hook from service.
3. Examine the threads for wear and corrosion. The threaded area may appear tapered from top to bottom if corrosion has rounded the thread profile over time. If the threads are badly deformed and/or corroded for a total length that is more than 20% of the thread length, remove the hook from service.
4. Examine the hook. If a hook has any of the following signs of damage or excessive wear, remove the hook from service:
• The throat opening, measured at the narrowest point, has increased by more than 15% of the original opening.
• The hook has twisted more than 10 degrees from the original plane of the hook.
• The hook has lost 10% or more of its cross-sectional area.
• The hook is cracked or otherwise defective.
• Wear or damage exceeds any criteria specified by the manufacturer.



This is one example of a load hook assembly. The components may vary with the make and model of crane.



5. Check that the nut retaining pin, set screw, or key prevents relative movement between the hook and the nut.
6. Examine the hook pivot and the axial and cross pin bearings. Look for wear, and ensure the pivot and bearings provide a freely rotating support for the hook.
7. Examine the block bearing plate for cracks, wear, or other damage. Damage affecting the safe use must be repaired before returning the block to service.
8. Before reassembling the load hook, grease the bearings as required and coat the clean threads on the hook and nut with a commercial thread sealant to provide corrosion resistance.
9. When reassembling, check that the length of threads engaged between the hook and the nut is at least equal to the hook thread diameter. Check that the nut retaining pin, set screw, or key that prevents the nut from backing off the hook is properly in place and secure.