Scope.
This Technical Bulletin is an overview of BBRG’s process development work to promote control of strand shape and orientation in rope stranding and closing during the manufacturing of Triangular strand wire ropes used for hoisting in mine shaft hoisting applications.Summary.
A two-year interdisciplinary BBRG team has completed a detailed analysis of production methods identifying the main drivers that would improve the final product. To understand where improvements were required, trials were run to reproduce anomalies, then apply potential manufacturing & control improvements which would eliminate such anomalies.
Three manufacturing principles are showing the potential of improvement:
• Rope design - strand and core
• Strand manufacturing
• Rope manufacturing
Reaching maximum rope life potential requires constant diameter stability through optimum shaft rope winding geometry in application together using proven maintenance practices for ropes. The internal investigation project has shown that the BRISTAR core is a proven concept that ensures improved rope diameter stability in the most challenging applications within the mining environment globally.
Background.
In 2016, the Bridon Bekaert Ropes Group was founded from Bekaert with its Mining rope brand WRI in Canada and BRIDON with the former Brand “British Ropes Ltd” and “Thyssen Wire Ropes” in Germany. These companies have over 150 years experience in the manufacturing of steel wire ropes and supporting the mines with their deep knowledge on the applications. As the leading manufacturer of underground mining ropes, the Bridon Bekaert Team is in a constant process to improve our products to ensure reliable quality, we are specialised in deep shaft winding and cater for the most arduous rope duties required.
Triangular Strand Ropes
In general, triangular strand ropes are ropes of choice for many shaft mining installations globally. In China, triangular strand given their application are the most common choice for their Koepe & other multi-rope friction hoists. In South Africa, primarily due to the depth of winds for Drum/ Blair winders, where multi-layer spooling is the only option to reliably perform the duty.
This construction of rope has greater strength per cross sectional area over conventional round strand ropes. These ropes are known for their reliable performance for deep shaft drum winding. The unique shape of the triangular strand provides good crush resistance since the rope evenly distributes the forces concentrically around the rope circumference. This provides diameter stability. Triangular strand ropes are eventually discarded on these hoisting machines mainly due to wear of the outer wires. Best maintenance practices are key to improvement of rope life. Much on this topic, including best practices, is freely available.
Rope life is influenced by many factors and can be split into four categories:
• set up of the hoisting system
• installation
• maintenance
• design and manufacturing
Applying good practices during installation and regular inspection of hoisting ropes executed by qualified experts is crucial for best performance within the underground mining industry. However, these practices vary significantly globally. The South African Mining Regulation SABS 0293 / SABS 0294 can be considered as the most advanced Regulations on Drum / Blair hoists.
Traditionally, a rope manufacturer has only limited influence on the design and installation and maintenance of the hoisting system. This has changed in recent times and more often the rope manufacturers are consulted at feasibility stage of winder installations. BBRG leverages our global historical footprint and the wealth of expertise to promote best practices, thereby optimizing rope design and manufacturing.
Strand and Rope Anomalies
One of the key quality problems of triangular strand ropes are occurrence of unwanted anomalies in diameter concentricity. Any strand distortion will change the concentricity of the rope. This usually will be recognised after installation and during the first inspections as heavy rope wear. These irregularities in the rope can rarely be fixed.
To limit the incidence of rope anomalies, an advanced technology project was started with a detailed analysis of the production of triangular strand ropes. As production plants all have slightly different equipment and methodology, various assessments were conducted on the methods of bettering machinery set up. This led to reducing the probability in occurrence of geometry anomalies.
The detailed analysis of the way of production proved four main drivers for this type of irregularities:
• Torque in the strands during stranding
• Change of lay length in the strands during stranding
• Tension difference of the strands during roping
• Process control
To understand, verify and quantify these detected reasons, trials were set up to reproduce such geometry anomalies. The improvement in the results were obtained from three principles:
• Rope design - strand and core
• Strand manufacturing
• Rope manufacturing
A detailed investigation of these manufacturing principles has highlighted, showing the potential for product improvement.
Strand and Rope Anomalies
The rope choice, as a function of the winding application (shaft type), is crucial. Combining depth and payload, rope speed and number of live rope layers for a given system determines the possible rope options. It has been proven globally that the triangular strand ropes are a preferred solution for multi-layer winders.The rope concepts at BBRG use the experience of various rope application industries. Close customer collaboration in achieving this goal is pivotal.
The optimisation of the rope concept for triangular strand ropes focuses on:
• Geometric stability of the strands
• Geometric stability of the rope - core design
• Controlled bedding of the strands
Geometric Stability of the Strand.
The triangular shape of the strands designed to fit together when closed into a rope provide perfect product cross-sectional concentricity. In operation, the strands will face significant compression due to crushing of the other wraps on top and in contact with the drum and sheaves. The contact surface of the strands is optimised by striving to match the circumference of the rope as this will result in better load distribution over all wires in contact. The “contactwires” transmit their share of the load into the supporting wires. The force geometry is crucial to assure distribution thereof and limit high Hertzian stresses, considering contact pressures
and Elastic Moduli at play.
By using larger outer wires in the outer surface, the robustness of the strands is assured. The “inside” of the strands consist of the strand core, the brangle and the layer of wires around it. The brangle shape and lay consistency are crucial to give the strand the triangular shape. Several options are possible to meet the requirement and balance between maximum flexibility, breaking load, and high radial stiffness. Today, the best options are the 3-wide core, the plaited core or the compacted 3+9 as shown in Figure 1.
Geometric Stability of the Rope - Core Design.
The standard core of a triangular rope is typically made of a natural fibre, Polyamide or Polypropylene. Figure 1.
The well-known patent from BBRG to extrude a core in the shape of a star, called Bristar, reassures the positioning of the outer-strands and improves the diameter stability even further.
Figure 2 shows a cross-section of a triangular stranded rope with a Bristar core and the closing point of a Bristar rope on a rope making machine.
The use of a Bristar core has several advantages. Additionally to high radial stiffness, the Bristar core has ability to assist the equal force distribution of the strands during manufacturing and in the application. The enhanced distribution is ideal to transmit the loads to all strands. During operation, depending on the depth and load, the rope is opening and closing. Use of the Bristar core assists better positioning of the different strands in the rope.
In the field, the Bristar concept has shown excellent performance in terms of lifetime, diameter stability and crush resistance.
Manufacturing
The concept of the high performing rope needs to translate in a design for manufacture. During manufacturing, care for detail is essential to realise the desired quality standards.The manufacturing stability during stranding has direct impact on the robustness of the closing process. By optimising the process in stranding and closing, the occurrence of strand irregularity will be significantly reduced.
Stranding
During the stranding operation, extra care is taken to realize consistency of the key quality parameters. To realise geometric stability from a cross section and longitudinal point of view, over all stress levels in the strand and individual wires, synchronisation of the different parts and overall torque level, need to be under control.Overall torque level is linked with the stress state of all the components of the strand. When torque levels or residual torsions on the final strand are too high, handling the strand can be difficult and even unsafe for the operator.
Additionally, the synchronisation of the different bobbins with the capstan is essential.
The stress level induced during stranding is driven by pay off tension, friction between the wires and the different contact points and the type of strander.
By advanced control of the flat lay, the stability of our processing can be guaranteed.
Figure 3 is showing the variation of the helix. The boundary conditions can be set to any required parameter.
Closing.
Based on the excellent geometric stability of the strands used by BBRG it is possible to rely on a predictable behaviour of controlled manufactured strands. This allows use of the common practices employed for Langs lay round strand ropes.
Matching the flat lay with the closing lay is crucial. The stable strand geometry allows the lay factors to be matched on the closer. By doing so, robust manufacturing is realised.
The Bristar assists the closing process. The Bristar helps to bring the strands at the closing point in position (see Figure 2).
To achieve the desired orientation, the machine is equipped to track orientation of the strands which flags the system to automatically correct, notifying the operator that remedial action has taken place. Figure 4 shows a screenshot of the system controls.
Conclusion
To reach maximum rope life from a set of ropes, constant diameter stability is needed. It is self-evident that a suitable shaft setup, rope installation and rope maintenance need to be in place too to ensure and improve a consistent quality of triangular stranded ropes.
This interdisciplinary project was undertaken over a two-year period. Advanced processing techniques now enable the manufacturing team and equipment to react automatically to the occasional irregularity experienced during rope manufacturing.
The Bristar core is a proven concept that assures a stable rope diameter for the most challenging applications in multi-layer spooling on drum and Blair hoists.