Block twisting or cabling is one of the most frequently encountered wire rope problems in the construction industry. When this problem occurs, the wire rope is most often blamed, and other equally important factors in the operation are overlooked.
Personnel experienced with handling wire rope know that conventional wire ropes will twist or unlay slightly when a load is applied. In a reeved hoisting system subjected to loading and unloading, such as a hoist line, it results in block twisting and possibly distortion of the wire rope. Cabling of the block occurs most frequently as the load in the wire rope is released and the falls are in a lowered position. Cabling may be considered as the twisting of the block beyond one-half revolution (180o twisting) of the traveling block. When this condition occurs, the operator shows good judgment in not making additional lifts until the conditions causing the problem are corrected.
The following machine and site conditions should be investigated for possible improvement in block twisting.
- Reduce wire rope length. Longer rope lengths cause more twisting than short rope lengths. This applies particularly to the amount of wire rope in the fall.
- Reduce the amount of load lifted. Heavily loaded ropes have more torque and twist than lightly loaded ropes. This condition also would apply to the speed of loading or shock loading, since this condition also causes higher wire rope loading.
- Eliminate odd-part reeving where the wire rope dead end is on the traveling block. Wire rope torque, from the application of load, is greatest at the rope dead end.
- Relocate the rope dead end at the boom in order to increase the separation between the dead end and the other rope parts. This applies a stabilizing load directly to the traveling block. The original equipment manufacturer should be consulted before making this modification.
- Increase sheave size. This increases the amount of separation between wire rope parts and may improve the situation by applying stabilizing loads and reducing the amount of rope torque transmitted to the traveling block.
- Restrain the twisting block with a tag line. The use of rotation-resistant wire ropes likely will not be required unless the intended length of rope fall exceeds 100 feet, or the length of the hoist line exceeds 600 feet. In the event these latter conditions exist, the end user should anticipate using a combination of the rotation-resistant wire rope and the foregoing field suggestions.