Conventional wire rope has a tendency to rotate or twist when tensioned. This comes from the fact that the individual strands of wire that form the rope are twisted during assembly. Rotational resistant wire rope is manufactured in a fashion that prevents rotation under load. To fully appreciation this process you must have a basic understanding of wire rope make-up.
What Is Wire Rope?
Used in mining operations, wire rope came into existence in the early 19th century in Germany. It was originally fabricated from wrought iron but steel is used exclusively today. The average populace refers to wire rope as cable. Wire rope consists of several strands of wire twisted into a helix. This twisting process is commonly referred to as laying. Wire rope is composed of wires, strands, and a core. The basic unit is the wire, which is formed into strands. The strands are wound around the core.
What is Lay?
The way in which the wires and strands are wound about each other is the lay. Conventional wire rope has either left-hand or right-hand lay. In rotational resistant wire rope two or more layers of strands having right and left directions of lay (shown below) are used. When tensioned, as under load, one layer’s directional rotation is counteracted by the tendency of the other one or more layers to rotate in the opposite direction. In order to create improved resistance to rotation, these ropes are fabricated with a larger number of smaller diameter strands. This design introduces a need for greater attention with regard to proper use in handling and operating with rotational resistant wire rope.
It's All About Balance
Wire rope balance has to do with the in place interaction of the individual wires and strands. Balanced rope limits the movement of strands by a ratcheting effect thus holding the rope's intended shape during operations. This evenly distributes working stresses to all the wires and strands and greatly reduces wear and therefore reduces fatigue breaks and increased service life. The degree of a given rope's ability to become unbalanced is referred to as rope sensitivity. The combination of smaller diameter wires and differing rope lays in rotational resistant wire rope creates a delicate balance associated with greater sensitivity.
Handle It, Handle It, Handle It
Heightened sensitivity demands that careful handling of rotational resistant wire rope occur to ensure acceptable service life. If sharp turns are imparted into the rope, the outer strands may become shorter in length, and the inner strands in turn slip and project from the rope assembly. This is called core slippage. In this situation, under load the outermost strands become overstressed, and the innermost strands no longer share an equal amount of the load.
Incorrect winding methods are the origin for many rotational resistant wire rope problems. Hoisting difficulties and even failures leading to rope replacement can stem from improper winding procedures. Maintaining constant tension when winding prevents core protrusion. The first or beginning layer, as well as all of the following layers, must be tightly placed without gaps or cross winding. This prevents crushing, the damage caused by distorting the cross-sectional shape of the rope, its strands, its core, or all three.
When spooling, winding from the top to bottom, or vice versa, introduces unwanted reverse bending in rotational resistant wire rope. Always wind from top to top or bottom to bottom of the payout reel to the hoist drum.
So, while rotational resistant wire rope has its advantages, it must be given special consideration to work safely over extended periods of useful life. This is evidenced by the fact that OSHA requires a design factor of 10 for rotational resistant wire rope whereas conventional rope is afforded a design factor of five (5). [See the previous article entitled Understanding Rigging Design Factors]