Steel wire ropes used in mining and general engineering industries consist of many individual wires laid into a number of strands, which are in turn laid around a centre core.
They may be broadly grouped under the two heads: round ropes and flat ropes. In mine shafts, they are used as main or head ropes and balance or tail ropes.
Main Hoist Ropes
Whereas round ropes are used with drum and friction hoists in mine shafts, flat ropes are used exclusively with reel hoists.
Round ropes may be further divided into: stranded ropes and non-stranded locked-coil ropes.
Round-stranded hoisting ropes are manufactured in size from 19 to 80 mm using wire sizes ranging from 1.4 to 3.0 mm. Ropes larger than 80 mm can be manufactured without any serious difficulties, but problem arise in their transport, installation, examination, rope changing and shortening, and making rope attachment. Furthermore, they are subjected to greater wear due to large twist and larger diameter of driving drum/friction sheave and pulleys will be required, resulting in larger size of the hoist. When the calculated diameter of rope of a single-rope hoist exceeds 50 mm, multi-rope friction hoists may be considered.
Full-locked coil ropes are manufactured up to a maximum diameter of 65 mm.
Flat ropes for reel winders are usually made of six parallel alternating right- and left- lay ropes sewn together by ‘single stitching’ utilizing a number of wires, or ‘ordinary stitching’ utilizing a strand instead of single wires.
Stranded Round Ropes
There are five basic elements of rope design, each of which influences the characteristics of the stranded ropes. The design objective is to provide a rope with characteristics balanced to withstand the demands of a particular service.
These are:
- Grade of wire and wire finish;
- Rope construction (number of strands per rope strand constructions);
- Type of lay;
- Performing; and
- Type of core.
Stranded wire ropes are characterized by their strand shape (see figure) as follows:
- Round-strand ropes;
- Triangular (Flattened)-strand ropes; and
- Oval (Flat)-strand ropes.
To obtain ropes possessing varying degrees of resistance to bending fatigue and abrasion, the ordinary strand construction is modified by using wires of varying sizes and number and arranging them in certain geometric patterns called constructions.
There are three basic types of parallel-laid construction; Seale, Warrington, and Filler Wire. Combination of rope classification and strand constructions are used to build large-diameter ropes with varying degrees of resistance to bending fatigue and abrasion and flexibility characteristics.
Type of Lay
The term ‘lay’ signifies the direction of rotation of wires in the strands and of the strands in the rope about their respective cores. Rotation may be either to the right, clockwise (Right Lay) or to the left, counter-clockwise (Left Lay). Right lay is the standard. The lay of the rope affects its flexibility and resistance to wear.
Strands may be laid in a hoist rope in two ways: Regular Lay and Lang Lay. In the Regular Lay rope, the wires in the strands have a lay opposite to that of the strands in the rope.
The outer wires run approximately parallel to the longitudinal axis of the rope and are bent on a relatively short are at the wearing surface.
In the Lang Lay rope , the wires in the strands and the strands in the rope have the same direction of lay. The outer wires run diagonally to the longitudinal axis of the rope and are on a longer arc so that greater lengths of individual wires are exposed to wear. The Lang Lay ropes must be secured at both ends to prevent untwisting.
Regular Lay ropes offer greater resistance to distortion and crushing on drums than Lang Lay ropes, which, however, possess greater resistance to abrasion and bending fatigue and are more flexible.
Types of Core
The wire rope core forms the heart of the rope. It provides a support to the main rope strands laid around it in their relative positions throughout the rope life.
There are two main types of cores: fibre core and wire cores.
A natural fibre core is the standard core for most hoist rope constructions. It gives adequate support to the strands and makes the rope resilient. The core must be free from water-soluble acids and be treated with water- and rust-repelling solutions before it is impregnated with a compatible lubricant. They may also be manufactured from man-made fibres, such as polypropylene or nylon. Fibre-core ropes should not be used when multi-layer coiling is used, since they are susceptible to crushing.
The following figure shows the different types of wire rope cores.
The symbolic cores such as nylon and polypropylene thermoplastics have uniform quality, repel water, are resistant to many acids and rotting, and are less susceptible to compacting.
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