Sunday, January 27, 2008


Standard wire rope has six strands. Classifications group wire ropes according to weight, flexibility, and strength. The 6 x 19 classification has 6 strands and 19 wires per strand. The 6 x 37 classification has 6 strands and 37 wires in each strand. Rope of numerous small wires is more flexible, but less resistant to external abrasion. Wire rope of a smaller number of larger wires is less flexible but more resistant to abrasion. Two ropes of the same size have the same strength even though, for example, one is 6 x 19 and the other is 6 x 37.
In most wire rope the wires and strands are preformed. Preforming means presetting wires in the strands into a permanent corkscrew form which they will have in the completed rope. Preformed wire rope does not have the internal stresses found in nonpreformed wire rope, does not untwist as nonpreformed wire rope, and is more flexible.
Lay means the direction of winding of the wires in the strands and the strands in the rope. Both may be wound in the same direction or in opposite directions.
In regular lay, the strands and wires are wound in opposite directions. Most common is the right regular lay which the strands are wound right and the wires wound left. This lay is used in marine operations.
In lang lay, the strands and wires are wound in the same direction. This type of wire rope is used on the blades of bulldozers and scrapers.

MEASUREMENT. Wire rope is usually measured by its diameter. To measure wire rope correctly, you should place it in the caliper so that the outermost points of the strands will be touching the jaws of the caliper. Here are some common causes of wire rope failures.
Using rope of incorrect size, construction, or grade.
Allowing rope to drag over obstacles.
Operating over sheaves and drums of inadequate size.
Overwinding or crosswinding on drums.
Operating over sheaves and drums that are out of alignment.
Permitting rope to jump sheaves.
Subjecting rope to moisture or acid fumes.
Permitting rope to untwist.
Using kinked rope.
You should inspect weak points and points of stress. Worn or weak spots show up as shiny, flat spots on the wires. If the outer wires have been reduced in diameter by one-half, the wire rope is unsafe.
Inspect broken wires, since they show where the greatest stress occurs. If individual wires are broken next to each other, unequal load distribution at this point will make the rope unsafe. Broken wires are called fishhooks. To determine the extent of damage to the wire rope, you can slide a finger along one strand of wire for one complete turn, equal to the length of one wire rope lay. Next, count the number of fishhooks. If six or more fishhooks are found, the wire rope is unsafe and should be replaced.