Steel Wire Ropes
Steel Wire Ropes
Steel wire rope, is also known as steel cable. It is a type of rope which consists of several strands of steel wire laid (twisted) into a helix. Modern wire rope was invented by the German mining engineer Wilhelm Albert in the years between 1831 and 1834 for use in mining. It was quickly accepted because it proved superior to metal chains and ropes made of hemp which was used before. Wilhelm Albert’s first ropes consisted of three strands consisting of four wires each. Wire rope industry in India, which was founded very early, started with coconut fibre ropes, switched to jute fibre, on to ‘Manila’ ropes and then to steel ropes. With the change in the needs, the designing of the wire ropes also undergone major changes with respect to the core, overlay and the weight requirement etc.
Wire rope is a machine which consists of a number of precise moving parts, designed and manufactured to bear a definite relation to one another. In fact, some wire ropes contain more moving parts than many complicated mechanisms. For example, a six strand wire rope, laid around and independent wire rope core each strand and core with 49 wires, contains a total of 343 individual wires. All these wires are to work together and move with respect to one another if the rope is to have the flexibility necessary for successful operation.
Wire rope has three components (Fig 1) consisting of wires, strands and core. The basic unit of a wire rope is wire which is carefully processed and drawn from selected grades of steel to predetermined physical properties and sizes. A predetermined number of finished wires are helically laid together in a uniform geometric pattern to form a strand. The process is carried out with precision and exactness to form a strand of correct size and characteristics. The required numbers of suitably fabricated strands are laid symmetrical with a definite length of lay around a core to form the finished wire rope.
Fig 1 Wire rope components (left) and wire rope lay (right)
Wire ropes are identified by a nomenclature that is referenced to (i) the number of strands in the rope, (ii) the number (nominal or exact) and arrangement of wires in each strand and (iii) a descriptive word or letter indicating the type of construction i.e. the geometric arrangement of wires (Fig 2).
Fig 2 Geometric arrangement of wires in a wire rope
The term ‘bright’ refers to a wire rope manufactured with no protective coating or finish other than lubricant can provide. These wire ropes are usually manufactured from high carbon steel. The chemistry of the steel used and the practice employed in drawing the wire are varied to supply the ultimate combination of tensile strength, fatigue resistance and wear resistance in the finished wire rope. Galvanized finished wire ropes have improved corrosion resistance. These wire ropes are produced from the drawn wires which have been galvanized. Wire ropes are usually supplied in three grades namely
- Improved Plow Steel (IPS) -This steel is strong, tough, durable steel that combines good strength with high resistance to fatigue. Its minimum tensile strength varies from 154 to 178 Kg/Sq mm depending upon wire diameter.
- Extra Improved Plow Steel (EIP) – In this steel minimum tensile strength varies from 169 to 196 Kg/Sq mm depending on the wire diameter.
- Extra Extra Improved Plow Steel (EEIP) – It is a grade where a high breaking strength is required. This grade typically provides a breaking strength a minimum of 10 % higher than the EIP grade.
Wire rope lay
The helix or spiral of the wires and strands in a wire rope is called the lay (Fig 1). The word ‘lay’ has got three meanings in the rope design. The first two meanings are descriptive of the wire and strand position in the rope. The third meaning is a length measurement used in manufacturing and inspection.
- The first meaning describes the direction in which strands rotate around in the wire rope i.e. right lay or left lay. If the strands rotate around the wire rope in a clock wise direction, the rope is said to be right lay. When the strands rotate in the counterclockwise direction, the wire rope is left lay.
- The second meaning shows the relationship between the direction strands lay in the wire rope and the direction wire lay in the strands. Regular lay denotes rope in which the wires are twisted in one direction, and the strand in opposite direction to form the rope. Due to the difference in direction between the wires and strand, regular lay ropes are less likely to untwist or kink. Regular lay roes are also less subject to failure from crushing and distortion because of shorter length of exposed outer wires. Lang lay is the opposite; the wires and strands spiral in the same direction and appear to run at a diagonal to the centre line of the rope. Due to the longer length of t6he exposed outer wires, Lang lay ropes have greater flexibility and abrasion resistance. These ropes are more likely to twist, kink and crush.
- In the third meaning it is the linear length along the rope that a strand makes one complete spiral around the rope core. Lay length is measured in straight line parallel to the centre line of the rope, not by following the path of the strand. The appropriate time to replace a wire rope in service is frequently determined by counting the number of broken wires in the length of one rope lay.
Wire rope core
The core is the foundation of a wire rope. Its primary function is to support the wire strands in the rope, maintaining them in their correct relative positions during the operating life of the wire rope. Fibre cores are used for wire ropes made from fibre yarns while steel wire ropes are always with a steel core. Steel cores provide more support than the fibre cores. Steel cores resist crushing, are more resistant to heat, reduce the amount of stretch, and increase the strength of the wire rope. Steel cores are of two types.
- The first type is wire strand core (WSC). This type of the core is used in case of small diameter ropes and in some rotation resistant wire ropes.
- The second type of steel core is independent wire rope core (IWRC). IWRC usually provide increased strength to the rope, greater resistance to crushing and resistance to excessive heat. IWRC increases the strength of the wire rope by 7 %, increases its weight by 10 %, and decreases the flexibility slightly. These ropes are recommended for use on installations where severe loads are placed on ropes running over sheaves or wound on drums.
Wire rope lubrication
Lubrication of wire ropes is a difficult proposition, regardless of the construction and composition. Wire rope lubricants have two principal functions:
- To reduce friction as the individual wires move over each other.
- To provide corrosion protection and lubrication in the core and inside wires and on the exterior surfaces.
There are two types of wire rope lubricants namely (i) penetrating and (ii) coating. Penetrating lubricants contain a petroleum solvent that carries the lubricant into the core of the wire rope then evaporates, leaving behind a heavy lubricating film to protect and lubricate each strand. Coating lubricants penetrate slightly, sealing the outside of the cable from moisture and reducing wear and fretting corrosion from contact with external bodies.
Both types of wire rope lubricants are used. But because most wire ropes fail from the inside, it is important to make sure that the center core receives sufficient lubricant. A combination approach in which a penetrating lubricant is used to saturate the core, followed with a coating to seal and protect the outer surface, is usually recommended. Wire rope lubricants can be petrolatum, asphaltic, grease, petroleum oils or vegetable oil-based.
Petrolatum compounds, with the proper additives, provide excellent corrosion and water resistance. In addition, petrolatum compounds are translucent, allowing the technician to perform visible inspection. Petrolatum lubricants can drip off at higher temperatures but maintain their consistency well under cold temperature conditions.
Asphaltic compounds generally dry to a very dark hardened surface, which makes inspection difficult. They adhere well for extended long-term storage but will crack and become brittle in cold climates. Asphaltic compounds are of coating type.
Various types of greases are used for wire rope lubrication. These are the coating types that penetrate partially but usually do not saturate the rope core. Common grease thickeners include sodium, lithium, lithium complex, and aluminum complex soaps. Greases used for this application generally have a soft semi fluid consistency. They coat and achieve partial penetration if applied with pressure lubricators.
Petroleum and vegetable oils penetrate best and are the easiest to apply because proper additive design of these penetrating types gives them excellent wear and corrosion resistance. The fluid property of oil type lubricants helps to wash the rope to remove abrasive external contaminants.
Wire ropes are lubricated during the manufacturing process. In case of wire rope with a steel core, the lubricant (both oil and grease type) is pumped in a stream just ahead of the die that twists the wires into a strand. This allows complete coverage of all wires.
After the cable is put into service, re-lubrication is required due to loss of the original lubricant from loading, bending and stretching of the cable. Field re-lubrication is necessary to minimize corrosion, protect and preserve the rope core and wires, and thus extend the service life of the wire rope.
Usage of wire ropes
Wire ropes have to fulfill different requirements depending on where they are used. The main uses of the wire ropes are as follows:
- Running ropes – They are also called stranded ropes. These wire ropes are bent over sheaves and drums. They are therefore stressed mainly by bending and secondly by tension.
- Stationary ropes – These ropes are stay ropes (spiral ropes, mostly full-locked) and have to carry tensile forces. These ropes are therefore mainly loaded by static and fluctuating tensile stresses. Ropes used for suspension are often called cable.
- Track ropes – Track ropes (full locked ropes) have to act as rails for the rollers of cabins or other loads in aerial ropeways and cable cranes. In contrast to running ropes, track ropes do not take on the curvature of the rollers. Under the roller force, a so called free bending radius of the rope occurs. This radius increases (and the bending stresses decrease) with the tensile force and decreases with the roller force.
- Wire rope slings – Wire (stranded ropes) are used to harness various kinds of goods. These slings are stressed by the tensile forces but first of all by bending stresses when bent over the more or less sharp edges of the goods.