A new era in overland conveyor belt design.
1. Capital cost reduction 2. Power consumption reduction 3. Usable strength increase 4. Belt life increase
Longer conveying systems are their own testimonials to the Life Cycle Cost benefits they provide.
Length Company Location Year Flites 20 km Channar Australia 1989 2, curved 16 km ZISCO Zimbabwe 1996 1, curved 24 km Indo Kodeco Indonesia 1998 5 14 km Muja/Collie Power Station Australia 1998 3, curved 14 km Middleburg Mine Services South Africa 1999 3, curved
It is now possible to design overland and high-strength conveyor belts that are substantially more reliable, have greater life expectancy, and reduced life cycle costs. The belt can represent up to 60% of an overland conveyor system's capital cost. A 10-30% reduction in the capital expense can represent big money. The operating life cycle cost, over a period of 10-30 years, can reach many times the capital investment.
New belt technology is good news for those seeking savings, improved reliability, and system efficiency. These benefits are available for both new and retrofit installations. Mines which could not justify an overland system, based on historical design practices, find that these new methods can put them back in the business of long overland conveyor systems.
Overland conveyors can demand a lot of power which is at a premium on-site. A sizable reduction in power can add up to significant cost savings.
Laboratory and field testing show 70% of a conveyor's power maybe needed to overcome the drag due to small indentation of the belt's rubber cover as it passes over each roller. This is typical of belt cover rubber compounds in use today.
Bridgestone, ContiTech, and Goodyear are leaders in developing special rubber compounds that target idler indentation energy reduction. These manufacturers continue to put new compounds into practice that enhance the conveyor's competitive advantage. based on testing, Bridgestone leads in this development.
Energy efficient compounds cost more to produce. They utilize more active ingredients and less bulk agents, such as oils, ash, talc, and clay. This improved efficiency is not recognized by the design standards most engineers use. The belt's true (optimized) cost is lost through the use of antiquated engineering methods, and through poor recognition of advancements in energy efficient rubber compounds. An example illustrates the differences between the design standards and measured rubber efficiencies. Four compounds, spanning three decades are compared with the U.S. belt conveyor standard CEMA (Conveyor Equipment Manufacturers Assn.). The best rubber compound tested requires about 60% of CEMA's power prediction. Stated with respect to the best rubber compound, CEMA would specify a 65% higher power requirement, with the related cost increases for belt strength, structures, pulleys, etc. CEMA is a design standard that was perfected in the 1940s and 1950's by a consortium of companies that contributed to its first edition in 1966. The CEMA method has not been updated in over thirty years and no improvement is planned in the near future. A similar comment applies to the widely used German standard DIN 22101 (Belt Conveyors for Bulk Materials) and its descendent the International Standard ISO 5048 (Continuous Mechanical Handling Equipment).
Power consumption and belt drag are affected by many things that are related to the rubber compound such as temperature, idler roll diameter, belt speed, idler trough shape, idler spacing, belt tension, the combined weight of belt and ore mass upon the idlers, and the overland's undulating terrain. These features must be a part of the engineering exercise.
A chain is only as strong as its weakest link and the weakest "link" in a steel cord conveyor belt is the splice, a connection method of one or more belt ends which make the conveyor's endless loop. Until recently, it was not possible to accurately determine the real strength of the splice and its resulting belt specification. Within the standards (e.g. the German standards DIN 22129 and DIN 22131 - Steel Cord Conveyor Belt Standard), and among manufacturing competitors, different splice patterns are recommended for the same belt strength. It's quite confusing to identify the best splice.
DIN 22101 best describes the connection between the manufacturer's belt breaking strength rating (ST-XXXX N/mm) and the design load strength, using the safety factor (SF) term.
Composition of the DIN 22101 SF is based on four distinct design criteria: running tension, starting tension, elongation and age degradation, and splice dynamic efficiency loss from repeat load cycles based on the tension drop across a drive pulley (10K cycles is the standard for testing).
Equipment has been developed which allows engineers and manufacturers to analyze steel cord belts while they are running under stress. Results of theory and experimental testing clearly show relative strengths of each manufacturer's splice patterns and core rubber fatigue strength. Core rubber is a special polymer which surrounds the belt's steel cord tensile members. It transmits the shear stress in a splice between cords. Core rubber endurance can vary dramatically among manufacturers, so testing must be performed to rate them. DIN 22101 defines a procedure for selecting the steel cord splice dynamic efficiency (core rubber endurance) and strength safety factor. This most respected splice performance procedure needs revision to clarify, and correct technical improvements/superiority.
DIN 22101 running tension is usually set at the peak running force. Such is rarely achieved. First, the nominal running force, which fatigues the splice, is about 80% of the peak design force. Second, the 0.4 torque multiple for starting running load specified in DIN 22101 is an infrequent cyclic condition and shouldn't be computed as a continuous fatigue factor. In-plant conveyors stop and start more often than overlands and should be factored into the equation. Third, belt elongation stress (pulleys, alignment error, etc.) and degradation (age, splice, construction, etc.) should be added to the running value as a continuous load. Fourth, the number of cycles a splice is subjected to in a ten-year period exceeds 10K. A 1,000 m long conveyor, running at 5 m/s for 5,000 hr/yr, will accumulate about 450K load cycles in 10 yr. This more realistic number should be used in the specific fatigue criterion or SF.
In 1996, Conveyor Dynamics, Inc. (CDI) built a machine, now owned by Goodyear, to carry out dynamic testing on continuous belt loops up to ST-10K N/mm (8,500 PIW) belt ratings. This machine, combined with advanced analytical technologies such as Finite Element Method (FEM), allows for the determination of the mode of failure in the splice as it occurs on the loop tester.
CDI has designed a two-step ST-5,100 N/mm belt splice pattern that achieved over 60% splice dynamic efficiency verses the standard 36%. Four-step and five-step splices have been tested to more than 50% efficiency, which lowers the necessary belt strength by 40% per DIN 22101. The strongest successful belt tested to date is a Bridgestone ST-8,800 N/mm. That splice has a dynamic strength rating of ST-4,400 N/mm, and is the world's strongest steel cord splice per DIN 22110. Improvements in splice dynamic efficiency is reflected in the belt SF plotted verses a 40 yr history.
None of the Bridgestone high strength belt splices tested have failed in the core rubber. In every case, the cables failed outside the splice zone. Much of the credit is due to the rubber endurance and metal-bonding efficiency of the Bridgestone compound. Goodyear (U.S.) and Phoenix (Germany) have also performed successful high strength tests on the machine with constructions above ST-6,600 N/mm.
Applying new technologies, the benchmark is elevated above published works and standards. Belt splice dynamic strength has achieved new levels of performance. Cable endurance is now the limiting factor to higher splice efficiency. Advanced rubber properties can yield reliable high performance splice efficiencies, above 50%, for all belt strengths. Increased belt strength allows lighter belts to be used with fewer splices. Lighter belts can greatly reduce power consumption. Designers must take care to validate the rubber, steel cord, splice pattern, splicing machine, and construction efficacy if they are to apply these gains.
Advancements with ore flow control in the transfer station can also add years to the belt's life. Predicting belt wear due to ore turbulence at transfer stations has, until recently, eluded engineers. Research engineers have developed a mathematical tool that provides information on the forces which damage belts and the product at transfer stations. The tool is based on the Discrete Element Method (DEM).
DEM can simulate ore flow by modeling physical motion and forces of a representative number of particles and determine abrasion and gouging damage that ore flow will cause on belt and chute liners, and within the product.
The DEM model was first applied at the Palabora copper mine in South Africa. That belt is 1,800 mm wide with a rating of ST-6,600 N/mm and has 18 x 9 mm covers. The belt conveys primary crushed copper-ore up a 16 [degrees] slope. The original transfer station was a conventional rockbox. The belt wore down to the steel cords in three years.
Palabora issued a contract to engineer and replace the rockbox with a curve chute. The impact of the ore falling from the rockbox onto the belt and then sliding along it, simply ground the belt away. The new transfer chute is designed to make the ore flow in the same direction and at the same speed as the belt. As a result, impact and sliding damage is greatly reduced.
Palabora built and installed the curved chute in early April 1994. After more than four years of operation, there is little discernible belt wear. Frequent inspection projects belt life to exceed 20 years. Other chute designs using the DEM model approach have also been implemented, such as the Los Pelambres chute.
Engineering tools can now rank the performance of rubber compounds resulting in the selection of belts that offer lower power consumption and capital and operating cost savings which can exceed 20%
Splice fatigue-strength and efficiency now reach far beyond industry standards. Belts have been tested up to ST-8,800 N/mm and achieved a 50% splice dynamic efficiency as set forth in DIN 22110. New materials and methods of analysis make it possible to reduce the overland safety factor for steel cord belts from 6.7-4.0 and beyond.
Belt wear is greatly reduced by utilizing new rubber compounds and curved chute technologies. Curved chute design, guided by the DEM ore-flow modeling technique, has resulted in improved belt life, increased puncture protection, and reduced dust pollution.
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|Author:||Nordell, Lawrence K.|
|Publication:||E&MJ - Engineering & Mining Journal|
|Date:||Apr 1, 1999|
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