Specific Flaws To Look For During An Inspection Of Wire Rope Used In Overhead Cranes
No matter the rated capacity of an overhead crane, the lifting strength of any type of hoist is only as good as the integrity of its lifting cable. A weakened or poorly maintained cable, which is composed of wire rope, could lead to disastrous failure that can be costly in terms of dollars and lives lost. That's why it is critical that wire rope be properly inspected. A close inspection will reveal early warning signs that failure may be imminent. Below is more information on what to look for when inspecting wire rope:
The diameter of wire rope should be measured with an outside micrometer across its widest point, which is the outside edge of the outer strands. To measure, place the anvil on one strand edge and adjust the spindle until its face just touches the opposite strand. Record the reading and compare it to the nominal diameter of the wire rope.
If the measured diameter is more than five percent less than the nominal diameter, then the rope should be immediately removed from service. For example, if you are using a nominally-demarcated 1-inch wire rope, but it is measured as a 0.93-inch diameter during inspection, then the rope has decreased in diameter by 7% and is unsuitable for lifting.
Broken outer strands
One "random" broken wire in a rope's outer strands is generally not reason to disqualify a rope for service, though if there is any doubt, the matter should be reviewed by an engineer. However, if any broken wires appear at locations where the wire rope terminates, then the rope is unfit for service and should be withdrawn. These types of breaks are a warning sign that excessive force placed upon the rope has caused specific types of damage that could lead to failure.
Kinking or permanent bends
Wire rope is a fluid mechanical device, and its proper functioning depends on the ability of wire strands to flow past one another during use. Kinks or bends that do not disappear when the rope is straightened is cause for removal from service. This type of damage prevents the wire rope from operating as a fluid, dynamic unit and causes internal stresses that can ultimately lead to collapse.
Inner core avulsion
Wire rope contains a central wire core that serves to hold together the outside strands. This inner core remains hidden as the outside strands serve as a visual shield. However, if a wire rope is stressed too much, then the inner core can be pushed through the outside strands and become kinked. Any appearance of the inner wire core to the naked eye should result in the wire being removed from lifting duty.
Corrosion and rust
Light red and orange discoloration of the surface of wire rope is an indicator that mild corrosion is occurring. As long as this discoloration does not involve pitting or is easily removed with a swipe of sandpaper, then lubricating the rope will help eliminate and prevent problems.
However, significant corrosion, including pitting and full-scale rusting, is a definite sign of the need to replace the wire rope. Since moisture can be trapped inside the wire rope more readily than on the outside, external corrosion is almost certainly going to be preceded by internal corrosion and that renders the wire rope ineffective and weakened.
Wire rope deformities
Other types of problems with wire rope that should cause users to pull it from service immediately include various deformities of the rope. These deformities include open kinking and bird-caging, two problems that result from sudden load releases. Both of these problems are apparent when inspection shows separation of the strands from each other.
If you do need to replace some of your equipment, don't hesitate to reach out to a local overhead crane hoist supplier.