it s all flat and black, isn t it?

Transcription

1 CONVEYOR BELTING CONSTRUCTIONS FOR MINING AND QUARRYING it s all flat and black, isn t it? The fact is that with the current high demand for materials, any downtime on a conveyor is costly and unplanned downtime can have serious financial consequences. Belt prices have increased with commodity prices, but remain a fraction of the value of a working belt to any operation. As a result the emphasis must move to more mature decision making also considering durability and reliability along with belt price and inevitably the cost per tonne produced. Even with a trained eye it is difficult near impossible non-destructively, to differentiate the best quality belts from much inferior quality belts. To the eye, only obvious defects are apparent, so a working knowledge of belt specifications and constructions is needed to make sound procurement decisions. The cost of conveyor belting is about 75% in the materials used, so in most cases any significant disparities in pricing will reflect compromises in materials used, which in turn will affect performance. The objective of this presentation is to provide some technical basics to help decision making, so that procurement and belt selection decisions can consider more than price and deliver the best value to the business. CONSTRUCTION MATERIALS The elements of a conveyor belt are the fabric carcass, providing the strength and load support, and the two rubber compounds the cover rubber to protect the fabric carcass and the inter-ply skim rubber for ply adhesion and cushioning. BELTING FABRICS Apart from differing strengths or weights, fabrics are further differentiated by the types of yarn used and the weave patterns. Yarn types WARP longitudinal yarn is now almost exclusively polyester because of the technical advantages it offers. Polyester has also become more commercially attractive in recent years. Its features are: Low elongation at working tension - enabling practical take-up designs and deployment on fixed length systems High elongation at break point - helps resist impacts, entrapped materials Very low moisture regain - with negligible effect on strength or stretch High rubber adhesion levels WEFT transverse yarn is now mostly nylon (sometimes polyester) Low moisture regain High stretch Excellent rubber adhesion 1

2 Weave patterns Standard weave patterns for Apex conventional plied belts are Plain Weave and 2x2 Broken Twill. Special 2x2 Broken Twill Crow s-foot weave and 2 ply plain Double weave - are used for high abuse applications. PLAIN WEAVE for fabric strength up to 300 kilonewtons per metre width per ply Modern weaving technology, with better control of twisting and tensioning, and improvements with yarn quality has meant that lighter fabrics are now being used to achieve belt strengths that previously required heavier fabrics. This has had obvious cost advantages but can mean that other attributes of the belting such as load support, impact strength, performance when damaged, can suffer as the yarn content becomes less. For this reason, Fenner Dunlop Australia has retained the premium Apex QuarryMaster and RockMaster series while at the same time introducing the more economical Dunlop Quarryline series. QuarryLine 150kN/m QuarryMaster 160kN/m RockMaster 200kN/m WEAVING RATE -metres per hour Warp strength -kn/m Weft strength -kn/m Warp Folding Decitex Ends/10cm Decitex/10cm Yarn content (relative) 84% 100% 150% 2

3 2 x 2 BROKEN TWILL for STANDARD BELT CONSTRUCTIONS with fabric strength over 300 kn/m/ply and RIPMASTER CROW S-FOOT WEAVE a special 2x2 Broken Twill weave with outstanding rip resistance and excellent fastener holding for Rip Resistant belt constructions above 300 kn/m/ply "RipMaster" 2x2 BT Standard 2x2 BT 'WEAVING RATE -metres per hour 'Warp strength -kn/m 'Weft strength -kn/m Tear strength -kn PLAIN TWILL Warp Folding 3 4 Decitex Ends/10cm Decitex/10cm Yarn content (relative) 100% 98% Weft Folding 5 3 Decitex Picks/10cm Decitex/10cm Yarn content (relative) 100% 51% In this weave each warp (longitudinal) thread passes over two weft (transverse) threads. Each set of four warp threads repeats the pattern where the first threads are in a twill pattern, overlapping one common weft thread and the next two threads are in a plain weave, overlapping opposite wefts. The sketch above illustrates this pattern, which has advantages in heavier fabrics: It opens up the weave, helping weaving and also (in belt manufacture), dip and rubber penetration With the right combination of yarn types, a fabric with outstanding resistance to ripping and tearing can be produced. This is accomplished by using an increased number of lighter warp threads, creating more "space" in the weft for an increased number of heavier weft threads With the construction of the weave (both the right balance of the yarn strengths from warp to weft and the weave pattern), a tear is directed towards the weft. The weave pattern is much less effective in yielding high tear strengths with fabrics lighter than 300kN/m, which is the lowest strength in the RipMaster range. The table above shows a comparison between the two weaves, the RipMaster Crow s foot weave and standard 2x2 Broken Twill. The warp polyester yarn content is the same in each however the RipMaster weave has almost double the amount of Weft nylon with much heavier yarn, and this is reflected in both tear strength and weft belt tensile strength. 3

4 DOUBLE WEAVE (two ply plain weave) 450kN/m/ply for severe impact and applications requiring excellent fastener holding. Warp Folding 4 Decitex 1100 Ends/10cm 188 Decitex/10cm Weft Folding 3 Decitex 940 Picks/10cm 82 Decitex/10cm Weaving rate - metres/hour 25 This weave pattern is also known as 2 ply plain weave. In this weave each warp (longitudinal) thread passes over three, then under three weft (transverse) threads, and then over and under successive single weft threads, with the pattern then repeated. This pattern is replicated adjacent warp threads, offset or staggered by two weft threads and so repeats with every four warp threads. The end result is that the fabric has a weft yarn in two planes, with a warp layer separating the two weft planes. The sketch below illustrates this pattern, which has advantages in heavier fabrics: There are more weft threads, because there are weft threads in two planes, so lighter weft yarns can be used while still achieving high weft strength Each ply is effectively two plies interwoven, so less plies are required to achieve the required belt strength and load support Where a single warp thread passes over a single weft thread, it passes between the weft thread planes, away from the outer faces of the fabric. Warp threads passing over weft threads in the outer face, will span three separate weft threads. Impact strength is enhanced by two features of the belt/fabric construction: Heavy cushioning skims because the double weave fabric gives additional load support and fabric strength with each double ply, less plies are needed, enabling heavy 1.5mm skims to be incorporated without overly increasing belt carcass thickness. Three BulkMaster double weave plies is effectively six plies. Avoiding the anvil effect with warp yarns being crushed-cut by heavy weft yarns. The weft yarns are finer and outer warp threads span three weft threads so that the impact load is shared. With correct fasteners, the weave provides fastener pullout strength of two thirds of the belt strength, which is the same efficiency as a stepped vulcanised splice for a three ply belt, so the three ply BulkMaster has the same rated working tension whether hot spliced or mechanically fastened. Other weave patterns used in plied conveyor belting are: STRAIGHT WARP WEAVE for high abuse applications, more difficult to splice and often fastened. (Most common in USA) 4

5 OXFORD or SEMI-OXFORD (Basket) WEAVE used for conventional constructions in heavier fabrics BELT COVER GRADES There are a number of international standards that define belt covers with the available grades generally falling into groups with similar physical properties and application areas. This is both because of market needs and also because of practical compounding. International Standard Cover Grades SERVICE COVER TYPE TENSILE STRENGTH BREAK ABRASION LOSS Harsh service - cut & gouge DIN22102 AS1332 ISO10247 X 25MPa 450% 120mm³ M 24MPa 450% 125mm³ H 24MPa 450% 120mm³ Harsh service - abrasive fines AS1332 DIN22102 ISO10247 A 17MPa 400% 70mm³ W 18MPa 400% 90mm³ D 18MPa 400% 100mm³ General service - medium to heavy duty General service - light to medium duty DIN22102 RMA AS1332 ISO10247 DIN22102 RMA Y 20MPa 400% 150mm³ I 17MPa 450% - N 17MPa 400% 200mm³ L 15MPa 350% 200mm³ Z 15MPa 350% 250mm³ ll 14MPa 400% - Consideration of the physical properties listed above only provides an indication of the service which can be expected and is no substitute for first-hand experience with an application or material. Other factors such as tear strength, resilience, ageing and the combination of properties will ultimately determine the end performance. The listing above only shows cover types which are specifically covered by the Standards (excluding fire/oil & heat resistant types) and covers exceeding and/or combining the above properties are available from many manufacturers. 5

6 Rubber compound cost and performance Savings in rubber compound cost inevitably involves some penalty in cover performance. The standard benchmark criteria for evaluation of rubber compounds for belt covers are Tensile Strength (MPa), Elongation at break (%) and Relative abrasion loss (mm 3 ). There are other measures such as mentioned above, but these are generally accepted as providing the most practical guide to performance. Rubber compounds are susceptible to degradation due to environmental factors such as ozone attack, UV, heat ageing and will generally harden (depending on environmental factors, natural rubber may revert ), while losing tensile strength and elongation. As well as measuring the properties of belt covers when new, the properties are also measured after simulated ageing of the covers by holding the samples at elevated temperature for a specified time after which the tensile strength and elongation are again tested and the comparative loss is recorded. Rubber compounds can be cheapened with the substitution of fillers, the use of lower grades of carbon black, cheaper cure systems, omission of property enhancements. Conversely, premium compound formulations with properties well exceeding the requirements of the Standards can be offered. These may target specific features such as cut and gouge resistance, abrasion resistance, ozone resistance etc, or combinations of these. The table below shows how the cost of typical cover compounds is made up, how the cost of the compound is affected and what the consequences can be to the physical properties of the cover: COVER TYPE Polymer Carbon black Process aids Property enhancers Fillers Cure system Premium high tensile / abrasion resistant High quality M grade standard Medium quality M grade standard Good quality grade N or RMA I RMA II or JIS general purpose $2.30 $2.07 $2.02 $1.50 $1.27 $0.60 $0.59 $0.58 $0.58 $0.58 $0.00 $0.07 $0.07 $0.15 $0.23 $0.16 $0.14 $0.07 $0.07 $0.05 $0.00 $0.00 $0.03 $0.15 $0.12 $0.25 $0.22 $0.15 $0.20 $0.11 COST PER LITRE $/L $3.30 $3.08 $2.92 $2.65 $2.35 $3.08 7% $2.92 5% $2.65 9% $ % SG Tensile strength MPa Elongation at break % Abrasion loss mm³ <90 <115 <125 <150 <200 Ageing -? TS % Ageing -? EB % Ageing -? Shore A % *Typical properties and indicative costs shown 6

7 BELT CONSTRUCTION The most common construction for mining and quarrying is the conventional multi-ply belt, from 3 to 5 plies with an inter-ply skim of 0.35 to 0.7mm clear of the fabric knuckles, depending on the thickness of the fabric. The skim needs to be thick enough to adequately separate the fabric knuckles and provide cushioning while too thick a skim places too much stress on the outer ply and compression of the inside ply around pulleys which can cause premature splice failure. Reduced ply constructions with 2 or 3 very heavy Double Weave or Straight Warp plies and much heavier skims are not uncommon and popular in some markets. There are many aspects of belt manufacture that will affect belt performance, such as tension control and alignment during manufacture, the critical quality of calendaring, gauge control, adhesion levels, control of curing and many more areas where quality control is essential. There are too many to comment on in individual detail. As an example, the control of fabric and uncured belt tension during all parts of manufacture is critical. The fabric must have adequate crimp to cope with the compression and extension of pulleys and transitions and to absorb impact. The crimp in the Warp yarn can be seen in the picture that follows: With inadequate crimp, the extension of the outside ply passing around a pulley, and the compression of the same ply on reverse bends can cause yarn fatigue and fractures. The belt below has almost no longitudinal crimp and failed in service with fractures and splice failures. The nominal belt strength is 630 kn/m and the new belt will have met this on test. The belt was subsequently installed on a stacker with forward and reverse high tension bend pulleys and as a consequence; the belt was weakened to less than half of its original tensile strength by flexing, compression and extension initiated fractures. The unusual stress/strain results above show that the yarn threads are stretching and breaking at different rates due to the damage incurred. CONCLUSION In a competitive market, the price of conveyor belting is likely to be a reasonable reflection of the cost of the inputs, being materials used and the quality control through the manufacturing processes. Procurement decisions need to consider the value of the belt to the business and the cost of potentially compromised performance. In some applications, the belt can be treated as a consumable and purchased on price alone. In most applications, investing in quality or specifications that directly address the needs of the application is likely to provide the best cost-effective solution and warrants performance monitoring and a technical input into decision making. 7

8 WEAVING TERMS Decitex - Linear density of the yarn Ply folding Number of strands twisted together to make each thread of yarn. (A single strand of 1100 decitex polyester has 192 individual filaments) Ends or Picks per 10cm - Number of warp threads (ends) or weft threads (picks) per 10cm of fabric width RUBBER COMPOUND COMPONENTS Polymer - the most common polymers for belt cover compounds for mining and quarrying are: NR - natural rubber, SBR - styrene butadiene BR - polybutadiene. They may be blended to provide different properties or sometimes substituted for cost reasons. Carbon black fine particles of carbon which provide reinforcement in rubber compounds increasing hardness and abrasion resistance, available in a range of grades and qualities. Process aids improve the processing compound and are cheaper than the rubber and enable more carbon black and filler to be added. Property enhancers - provide protection from weathering, UV, ozone and reduce property loss through ageing. Fillers - used to reduce compound cost as substitute for polymer and carbon black. May vary greatly in quality (and cost). Cure system - Ingredients required for the efficient insertion of a cross-link between chains of rubber molecules. 8