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Reducing Cost Using an Advanced Roll Form Stranding Process

Reaching for the STARs
January 18, 2018
An Intern’s Perspective
March 19, 2018

The roll form stranding process combines the advantages of two highly productive processes, namely roll forming and double­ twist stranding These two systems are forged together to create one high-speed continuous manufacturing cell. This system not only produces compact strand at high speeds but also allows significant savings to be made throughout the stranding process, from wire drawing all the way through to the extrusion process.

Introduction

Recent years have presented a great challenge to the cable industry, especially through the most current world recession when demand for cable was reduced, in some cases, by 50 percent. At the same time, raw material prices and energy costs have continued to rise. This made the competition between manufacturers increasingly aggressive in product range, product quality, and costs.

This article presents a cost-saving technological solution for the wire drawing process through the use of the roll form strander, which allows the use of a single input diameter wire to be utilized throughout the stranding process giving significant process savings and advantages, including:

  1. Higher stranding productivity (typically 40 tonnes /day compact Al)
  2. Higher productivity in wire drawing Ill. Lower drawn wire scrap
  3. Lower insulating costs
  4. Lower capital investment
  5. Improved return on investment
    • Quicker set-up
    • Reduced work in process
    • Shorter cycle times
    • Smaller input wire storage area
    • 75% more energy efficient than a typical rigid strander

The roll form stranding process combines the advantages of two highly productive processes; roll forming of shaped wire and double twist stranding of conductors. These two systems are forged together to create one high-speed continuous manufacturing cell.

This system not only produces compact strand at high speeds but also allows significant savings to be made throughout the stranding process, from wire drawing through to the extrusion process. Round wire is paid off from stem-type payoff packages allowing up to 1,000kg (2,000lb) of aluminum or 3,000kg (6,000lb) of copper conductor material to be staged behind the roll form strander. The stem package allows continuous operation and is the preferred solution for this process, maximizing productivity when the single input wire method is an integral part of the manufacturing process. The round wire passes into the external roll forming section, an integral part of the roll form strander.

The object is to present profiled material to the strander. The round wire is re-shaped into the optimum format for the desired strand construction and finish ed cable size. This roll form system can be as simple as a two-layer (1+ 6) construction or as intricate as a five -layer (1+6+12+18+24) construction. The scope of the strand design determines the application configuration; there are many different designs available for each layer.

  • Roll forming of the input wires.  This is achieved with a driven roll stand shaping each wire in a layer to a precisely designed shape.
  • Round wire layers where a number of round wires are closed without any change in section.

Depending on the strand design, up to four layers of roll forming strand may be produced. Each roll strand is driven individually, allowing slight changes in speed to be made at each position to compensate for the variances in the spiral length.

It further allows more precise distribution of wire tension in each layer, optimizing the straightness of the resulting conductor.

The shaped conductors are formed into a compact unilay strand using a high-speed double twist strander. The machine is a side-loading unit equipped with a standard floor loader, designed to load and unload the take-up reel with minimum operator effort. The scope of products that can be manufactured includes:

  • Copper and aluminum stranded conductors between 8AWG and 600kcmil or 10mm and 300mm
  • Bunched conductors to compact strand with fill factors between 76% and 97%
  • Aluminium conductors steel reinforced (ACSR) using a single steel wire core
  • All aluminum and aluminum alloy conductors (AAC & AAAC)
  • Cabling of insulated conductors

SIW represents a strand and design mentality, with a manufacturing methodology that effectively reduces the conversion cost from rod to strand without compromising conductor performance.

This concept replaces the traditional stranding of wires using different wire diameters with the stranding of wires using the same wire diameter for a wide range of cross section s. SIW diameter meets major conductor standards such as IEC 60228, HD 383 and the ASTM standards. By incorporating a single input wire diameter program into the strand design, significant savings can be achieved in wire drawing, stranding, and the insulation process.

Traditionally a finished stranded conductor requires its own drawn wire diameter. Each wire diameter typically requires a new string-up in the wire drawing machine. Some conductor designs require more than one drawn wire size.

The setup time associated with drawing, combined with inventory levels that are necessary to manage the number of wire diameters, represent unnecessary activities that add to the cost of conversion from rod to strand. The SIW approach, using the same input wire diameter to finish a range of stranded conductors, eliminates much of the unnecessary activity associated with the traditional setup.

This leads to increased efficiency in the wire drawing process. Instead of having to produce a large number of different wire sizes only one or two are required using the SIW system. The improvements can be seen in the following areas:

  • Higher productivity in wire drawing Lower drawn wire scrap
  • Quicker set-up
  • Reduced work in process
    • Shorter cycle times
    • Smaller input wire storage area Reduced drawing die inventories

The single input wire method can save between 15% and 20% on wire drawing costs, including the elimination of re-strings for size changes, lower die inventory and reduced in-process wire. Double twist stranding has always been among the most productive methods of producing strand. Its incorporation into the roll form strander, with the application of the individual shaping of the wire, has further extended its performance range. In the following table, its performance can clearly be seen. Each machine type works the wire differently, and this impacts on the strand design that can be used for that process. The figure on the right highlights some of the advantages and disadvantages of each machine type as they relate to product capability and relative cost. It is important to recognize that if the roll formed or die shaped wire is used in the strand construction a ‘rigid’ machine or a machine that puts a twist in the wire for each lay length, is a prerequisite for manufacture.

Capital cost per twist

Determining the range of equipment to cover the strand designs is an important consideration in achieving the lowest conversion cost. For example, the double twist machine offers the lowest cost per twist but is the most limiting in terms of the construction possibilities. By its incorporation into the roll form strander this range of construction possibilities has been greatly expanded. The planetary machine, at the other end of the spectrum, has the highest cost per twist but the greatest construction possibilities, which is why it is used for special purpose products.

Material limitations

Each machine type works the wire differently. For this reason alone it is necessary to identify the differences to be able to use the same drawn wire size for the multitude of stranding possibilities. This applies not only to the principle of the machine but also to the area reduction that can be expected from different machine types. Keep in mind that in most cases the area reduction through the machine varies at different speeds and, to some extent, all machines used to manufacture strand require that the stress in the wire during the stranding point exceed the yield point of that material. For example, the double twist, single twist, and rigid strander put a twist into each input wire along the axis of the wire for each lay length; the tubular and planetary machines are more forgiving and put almost no twist into each wire, which is important when stranding steel wire.

Lay and layer limitations

Both the double twist and single twist machines currently can manufacture up to four layers (typically a 37-wire construction) in one pass. The lay length and the lay direction are identical, which is a limitation for some specifications. A tubular strander is typically a one-layer machine manufacturing a reverse concentric strand. Rigid and planetary machines, in the correct configuration, effectively have no limitations for the majority of conductor materials. With the roll form strander it is possible to use this as a highly productive feeder into the rigid strander for larger products, while still optimizing the SIW concept.

The optimum mix of machines in a manufacturing plant will not be discussed at length in this paper. Suffice it to say that this analysis represents perhaps the most significant economic risk in the installation of any stranding capacity. The process of defining the scope of what constructions need to be made, both present and future, is an important prerequisite to determine the optimum manufacturing cell for conductor strand. Using the roll form strander not only to produce finished compact conductor, ut as a feeder into a rigid strander for larger products such as 400mm2 and 500mm’, allows for a flexible stranding manufacturing cell.

When a comparison inline speed is made between compact conductor production and other high-speed stranding processes, the impact on performance using the roll form strander and the SIW process is dramatic, with the roll form strander achieving double the productivity. The benefits of the roll form strander are more apparent when compared to conventional stranding processes, such as rigid stranding.

Important points to remember:

  1. The speed of the roll form strander is 1,200tpm, product dependent. In comparison, the rigid strander will operate at 300tpm maximum.
  2. While the loading time of 19 DIN 630 reels can be minimized,  the rigid strander must still be stopped in order to replenish the reels and allow welding of the wires. Even with modern automatic loading, it is estimated that two operators will take 45 minutes to complete a loading sequence. In comparison with the use of the automatic changeover facility at the payoff system, the operator is able to change the 19 coils of wire and weld them together while the roll form strander is running. Therefore the only time the machine stops is to change the take-up drum, which should not take longer than 10 minutes.
  3. The whole roll form stranding process requires only one operator.

After the strand has been formed it is often insulated; the ease and cost of this process are greatly dependent on the stability, tightness, and surface of the strand. If the geometry of the strand is unstable, the strand elements will shift and, ultimately, birdcaging will result.

This not only makes the process much more difficult, but the incurred losses due to scrap and downtime can be significant. A tightly wound conductor is less likely to be subject to birdcaging; again, the tightness of the strand is greatly dependent on the geometry of the elements.

Figure 6 shows two strand designs. Both designs consist of the same number of wires, of identical

input diameter,  and both have the same cross-sectional area. The difference is that the construction on the left is unilay or unidirectional lay, while the construction on the right is of a reverse concentric lay design. The elements of the unilay/unidirectional strand are nested; all of the elements touch and each element of a layer rests on an element of the layer below. The result is a more stable and a more compact geometry.

Comparing unilay and reverse concentric strands of the same round element input diameter, the unilay strand will inherently have a smaller conductor diameter (4.86d versus Sd) and thus a higher fill factor (80.3% versus 76%).

Note: the fill factor represents the ratio of conductor area to the total circular area enclosing the elements.

The amount of extrusion material necessary is defined by the strand design; the smaller the outer diameter of the bare conductor, the less extrusion material is necessary. Figure 6 shows how a unilay/ unidirectional lay conductor is inherently smaller in diameter than a reverse concentric lay conductor. The more compact the conductor, the smaller the outer diameter.

The surface of the outer diameter is critical. A smooth outer layer, such as one found on a solid conductor or a roll formed layer, has fewer interstices and, therefore, fewer gaps that need to be filled with insulation. This can be clearly seen when comparing a compressed conductor with a compacted conductor, as seen in Figure 7. As the conductor is compacted the diameter of the conductor and the interstices are reduced in size, leading to a reduction of used extrusion material. The extrusion process is most economical and productive when using a stable, tight conductor with the minimum outer diameter and smoothest possible surface.

Conventional stranders can only achieve a maximum fill factor of 92%, whereas the roll form strander can achieve fill factors of 96% and above. The effective saving in insulation costs between the two processes is around 2%.

Case studies have been carried out from wire drawing to final insulation of the conductor, taking all downtime parameters into consideration. The comparison was between a conventional 19-wire rigid strander and a roll form strander, each producing 3,000km of 150mm ‘ compact aluminum per year. The predicted annual savings were demonstrated to be in the region of $525,000.

It should be remembered that savings in production costs depend on many factors such as existing manufacturing facilities, whether the strand is currently manufactured in-house or purchased, the care and control exercised over input copper and aluminum wire, general housekeeping and the control of high-speed roll form stranding machines. Under the most advantageous conditions savings can provide extremely short payback periods, but should, of course, be calculated for each individual application.

The high performance of roll form stranders coupled with the Bartell patented roll forming process will allow the cable manufacturer to reduce costs without compromising finished conductor performance. An awareness of this and other new technologies, combined with enlightened specifications, will further enhance the development of strand design and the potential to optimize further the manufacture of stranded conductors.

 

Sean Harrington

Wire and Cable Products,

Bartell Machinery Systems LLC

Email:   sales@bartellmachinery.com

Website: www.bartellmachinery.com