General stranding structures for cables and conductors, direction, lay, etc





Lay and Lay Selection

Regular (Ordinary) Lay or Lang's Lay. Information about lay and its selection is as under.
Lay

Lay is the direction (hand of lay) and length of the twist (length of lay) of wires in a strand and of the strands forming a wire rope.






"Cross Laid" and "Equal Laid" Principle

Cross laid construction is usually one size wire construction where all the wires, except the King or Centre Wire, are of the same diameter. In the stranding operation each covering layer is provided with a longer lay (or pitch) than the underlaid layer of wires. Therefore, in cross laid ropes, the wires in the covering layer, although spiraling in the same direction as the wires in the underlaid, will repeatedly cross over the inner wires.

The equal (parallel) laid principle is to make each covering layer of wires to the same length of lay as the underlying layer, thereby eliminating any crossing of the wires in these strands. Seale, Filler and Warrington strand forms are examples of equal laid principle.
Lay Selection

A standard wire rope is a right hand regular lay rope composed of six strands laid around a core. All ropes are supplied with this lay unless otherwise specified.

Left hand regular lay ropes are used where drum and anchorages are such that right lay ropes wound under load would tend to roll away from adjacent laps, resulting in uneven winding. They are also used to counter the rotation of a right hand lay rope when two ropes are used as a pair. Usually, a left hand lay rope is used in combination with a right hand lay rope. Left hand lay is also used in ropes for drilling purposes to prevent unscrewing of rods.

Regular / Ordinary lay ropes are suitable for all general work.

Lang's lay ropes are used in construction and mining applications. It is more flexible and has greater abrasion resistance due to longer length of wire exposed to wear than ordinary lay rope. Since Lang's lay ropes have little resistance within themselves (to unlaying) on account of the wires and strands being laid in the same direction, they should not be used where the load is suspended from a free end and allowing the rope to rotate. When a Lang's lay rope is unwound care should be taken since it has got tendency of unlaying.

A six strand wire rope in which three strands are ordinary lay and three strands are Lang's lay is known as alternate lay wire rope and combines some desirable properties of both the regular lay and the Lang's lay type of rope.

The use of equal (parallel) lay strands avoids deformation, internal wear and secondary bending which results from the point of contact between the wires in a cross laid strand. In most fields of application, therefore, equal lay ropes have proved to have a longer life than cross laid ropes.

Wire Rope Manufacturing Process

Various operations carried out for making wire ropes are as under.
Patenting (Heat Treatment)

Steel wire ropes are made from high carbon Steel rods. Patenting is a heat treatment process. In patenting wire rods are heat treated above their transformation temperature. Due to this they attain a homogeneous granular structure.
Pickling (Descaling)

After patenting oxide forms on the surface of rod / wire. This oxide is removed from rod/wire by immersing them in a tank of cold hydrochloric acid, rinsing thoroughly with a jet of water, and finally covering the surface with a layer of protective phosphate coating.
Drawing

It is a cold process involving plastic deformation of steel. In drawing, preheated and pickled rod is cold drawn through a series of successively smaller, accurately shaped and polished dies. After drawing, the material now in wire form acquires its final mechanical properties, shape and size
Galvanizing (Zinc Coating)

Galvanizing is a means of protecting steel against corrosion by coating it with Zinc (spelter). The coating of Zinc is normally applied by the Hot Dip process or by electro-depositing. In Hot Dip process the finished drawn wire is passed through a bath of molten zinc. Galvanized wires made by Hot Dip process are not suitable for high tensile strength application as the bath temperature is sufficiently high to alter the physical and mechanical characteristics. In coating by Hot Dip or electro-depositing, the coating of spelter on wire tends to be porous and also liable to cracking or flaking.

If high tensile strength is required, wires are drawn after galvanizing. They are called "Drawn Galvanized". Due to drawing after coating, the spelter becomes more homogeneous and is bonded to the steel much more firmly. This method produces wires having desirable properties of a bright wire (uncoated wires are called "bright" or "black") with a corrosion resisting Zinc coat.

"Drawn Galvanized" wire has the same strength as bright wire, but wire, "galvanized at finished size" is usually 10% lower in strength.

Standing ropes, such as rigging for ships, guy ropes, bridge ropes, etc., which are exposed to the atmosphere and often a corrosive atmosphere, are normally galvanized, or fully galvanized, i.e. made from wires in the state it comes from the galvanizing process without further treatment. This ensures a heavy coating of spelter and gives maximum protection.

On operative ropes, however, such as hoisting ropes, crane ropes, etc., Drawn Galvanized wires are used. The wire used in the manufacture of the rope in this instant is drawn after the wire is galvanized. This reduces to a degree the amount of spelter per sq. mm. of wire but bonds the spelter to the wire more firmly than in the case of fully galvanized wire. This is essential when the rope is working over pulleys or drums.
Winding

Finished wires after drawing / galvanizing are wound onto bobbins to be fed into stranding machines.
Stranding

In stranding, predetermined number of wires are helically laid around a center to make a strand.
Closing (Roping)

In closing predetermined number of strands (generally six) are laid helically around a core (fibre or steel) to make steel wire rope.
Performing

Performing is the process in which each individual strand and each individual wire is permanently formed (shaped or set) during fabrication process in the helical shape it will assume in the finished wire rope. This process causes the strands to lie in place and removes the tendency of wires and strands to fly apart when cut. As performed rope is more inert than non performed rope, it is easier to handle during installation and is less susceptible to the formation of kinks. It can be put to work at full capacity after a shorter "running in" period.

Outer wires of preformed ropes after breaking do not protrude from the rope to injure workmen's hands, do not distort adjacent wires or cause excessive wear to sheaves and drums.

Usually, a general engineering rope is preformed, whereas an elevator rope is non-preformed.
Postforming

When required, ropes are subjected to postforming in addition to the aforesaid performing process, where the residual stress resulting out of the stranding and closing operations are minimized and the rope becomes "dead".
Pre-stretching

Wire ropes for certain uses as in suspension bridge should not elongate while in use. So, after manufacture, the whole length of the wire rope is per-stretched to prevent further elongation. This is done by fixing the ends of the wire rope at two terminal and applying safe working load for a certain period of time as per specification. As a result of this, the structural and elastic stretch that may arise during the valuable life of the rope is done away with.

For more information on pre-stretching, please refer IS 9282: Wire ropes and strands for suspension bridges.
Rope Dressing (Lubrication)

Lubrication of ropes serves a double purpose. Firstly the lubrication resists corrosion and secondly, it enables the wires to move in smoothly resulting in less wear.

During the process of manufacture the strands of the core are well lubricated. In case of ropes for general use the fibre core is thoroughly impregnated with suitable lubricants which prevent the fibre core from wroughting and also act as a preservative of lubricant providing internal lubrication.

Finally, the rope itself is passed through a bath of a special wire rope dressing compound which thoroughly lubricates the rope and also leaves a thin film of lubricant on the surface of the wire rope. The rope thus lubricated, permits strands and wires to move smoothly and is preserved against corrosion during the time of transport and storage.
Testing

For quality control, testing is carried out at various stages of manufacturing like raw material testing, testing for thickness of Zinc coating after galvanizing, testing of finished wires for shape and size (diameter, roundness in case of round wires, and smoothness) before stranding and testing of wire ropes after closing operation to destructive test or proof load test as per requirement.

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Regular lay:

In a regular lay cable, the wires and strands are laid opposite to each other. In other words, all of the wires are laid in one direction as they are made into strands, and those strands are then laid in the opposite direction of the wires as they are combined into cable. When looking along a length of regular lay cable, the wires will appear to run parallel and straight the entire way. You can tell a cable is right regular lay because the strands will all flow to the right, or clockwise direction, compared to a regular left lay cable which flows the strands in a leftward, counterclockwise direction.   

Lang lay:

In a lang lay configuration, the wires and strands are laid in the same direction. If all the wires are laid to the right as they are made into strands, then those strands will also be laid to the right as they are combined into cable. The same applies to a left lang lay configuration, where both the wires and strands would lay to the left. When looking along a length of lang lay cable, the wires will appear to angle across the rope, following the general flow of the strands. You can tell a cable is a right lang lay cable because both the wires and strands will flow rightward, in a clockwise direction. A left lang lay cable flows both wires and strands in a leftward, counterclockwise direction.


Functionality:

Cable is generally manufactured with a standard right regular lay because it is useful for a variety of different applications and complies with most equipment. In general, regular lay cable is more resistant to crushing forces than lang lay cable of an identical material and size, though lang lay cable is typically more flexible. Lang lay cables are usually more susceptible to pinching and kinking than regular lay, which means they are better suited to hoisting applications where the cable only moves along one axis.

Right and Left Hand Regular Lay:

Right hand regular lay is the most common type of lay. With regular lay, the wire rope denotes the direction of wire twists in the strand. The wires in each strand lie in the opposite direction from the strand. By design, most machines that wire rope is used on are meant for right hand regular lay rope as it is the most common lay type. Right hand regular lay is the most common as it can be helpful for a variety of different applications and complies with most equipment. In general,regular lay cable is more resistant to crushing forces than lang lay cable of identical material and size. Left hand regular lay has the same characteristics as regular right hand lay type, but the strands in left hand lay rotate counterclockwise around the wire rope rather than clockwise. The top left image is an example of right hand regular lay and the image below is left hand regular lay.

Right and Left Hand Lang Lay:

Similar to regular lay, the right hand vs left hand is merely the way the wire rope closes with the strands in left hand lay rotating counterclockwise and right hand lay rotating clockwise. With lang lay rope, the wires in each strand lie in the same direction as the strands. When looking along a length of lang lay cable, the wires will appear to angle across the rope, following the general flow of the strands. Lang lay cables are more susceptible to pinching and kinking than regular lay, which best suits hoisting applications where the cable only moves along one axis. Lang lay is typically more flexible than regular lay. The third image down on the left is an example of right hand lang lay and the image below is left hand lang lay.

Lay

An important consideration in wire rope construction is the way the wires have been laid to form strands and the way the strands have been laid around the core.

Lay is classified by both direction and type. The lay direction of the wires within a strand and of the strands within a rope is either left or right. Rope lay is further classified as either regular or lang. In a regular lay rope, the wires in the strands are laid in the opposite direction as the strands in the rope. In a lang lay rope, the wires in the strands are laid in the same direction as the strands in the rope.

Regular and lang lay ropes are easily identified by the appareance of the outer wires with respect to the rope axis as shown by the examples to the right.

Right regular and right lang are the most common types of lay in use. Each possesses unique characteristics important to proper selection. Wire rope can be manufactured with five types of lay.

Regular lay ropes are generally more stable and more resistant to crushing. Lang lay ropes are significantly superior in fatigue and abrasion resistance. However, lang lay ropes are more susceptible to crushing and require good winding conditions. They are also extremely prone to rotate under load; they must never be used unless both ends are restrained.

Alternate lay rope combines the best features of regular and lang lay ropes. It offers the advantages of both constructions while minimizing the disadvantages. This construction is ideal where high bending stresses (fatigue) are combined with high rope-to-sheave pressure (crushing); for example, as applied to boom hoist rope.

Compared to other wire rope lay types, the superiority of lang lay rope in certain applications derives from the fact that when bent over sheaves, its life span is longer than the others. Stated in another way, the advantage of lang lay rope is its greater fatigue resistance. Yet another claim is made for the lang wire rope lay types: they are more resistant to abrasion. Broadly speaking, this is true, but there are some reservations that should be taken into account.

It is important to understand the reasons for the advantages of lang lay rope. To begin with, consider its fatigue and bending properties. Figure 4A shows,  in part, how the lang lay construction characteristics result in greater fatigue resistance than is found in regular lay rope. Note how the axis of the wire relates to the axis of the rope in both cases. When the regular lay rope is bent, the same degree of bend is imparted to the crowns of the outer wires.

Superior fatigue life in lang lay rope is also attributable to the longer exposed length of its outer wires. In the upper photograph of a regular lay rope (Fig. 4A), the valley-to-valley length of individual wires is about 7/8″; the length of the lang lay wires in the lower photograph is about 1-1/8″ or 30% longer. Bending the lang lay rope results in less axial bending of the outer wires and greater torsional  flexure. These combined stresses notwithstanding, the lang lay rope displays
15 to 20% superiority over regular lay when bending is the principal factor affecting service life.

It is said that lang lay is more flexible, but flexibility should not be confused with fatigue resistance. These two attributes may, under certain circumstances, bear some relationship, but they are distinctly separate characteristics. Flexibility defines the relative ease with which a rope "flexes" or bends. Fatigue resistance defines the rope's ability to endure bending.




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The illustration shows the individual wires within the outer strands twisting in a Z (right) direction and the outer strands twisting around the core in a Z (right) direction. The lay of this rope is therefore referred to as zZ (right-hand Lang's lay - RHLL) the smaller letter first being the direction of the wires in the strands and the second larger letter being the direction of the strands in the rope.

Alternative descriptions for the direction and type of rope lay

 RHOL (Right Hand Ordinary Lay)    RHRL (Right Hand Regular Lay)     sZ
 LHOL (Left Hand Ordinary Lay)            LHRL (Left Hand Regular Lay)     sZ
          
 RHLL (Right Hand Lang's Lay)          zZ
 LHLL (Left Hand Lang's Lay)          sS

Wire rope is manufactured in "Regular Lay" and "Lang Lay".

Regular lay indicates ropes in which the direction of lay of the wires in the strands is in  the opposite direction to the lay of the strands in the  r ope. Due to this difference in direction, regular lay ropes are less likely to untwist or kink. They are alsa less subject to failure from crushing and distortion because of the shorter length of exposed outer wires.

Lang lay denotes wire rope in which the direction of lay of the wires in the strand is the same as that of the strands in the rope. Because of the longer length of exposed outer wires, lang lay ropes have greater flexibility and abrasion resistance than do regular lay ropes. These ropes are more likely to twist , crush and kink than regular lay ropes.

Strands in wire rope  can be twisted  in  right  or le direction even if the wire rope is regular or  lang lay. lf the strands are laid around the wire rope in a clockwise direction, the  rope  is right lay. When the strands are laid in a counterclockwise direction, the rope is said to be le lay