Conveyor belts account for 30% to 50% of the entire equipment cost of the conveyor. The correct choice of conveyor belts can not only reduce the equipment cost of the entire conveyor machine, but also affect the design of the conveyor&#;s pulley, driving devices, rollers and other components.
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The selection of the conveyor belt of the belt conveyor is based on the length of the conveyor, the conveying capacity, the tension of the conveyor belt, the characteristics of the conveyed material, the receiving conditions, the working environment and other factors. In addition to the longitudinal tensile stress, the conveyor belt is also subject to the bending stress of the roller and idler during operation. Most of the damage of the conveyor belt is manifested by the wear of the working surface and the edge, and the breakdown, tearing and peeling caused by the impact of large and sharp materials.
1&#;Pre-selection of conveyor belt
The selection of conveyor belt needs to consider many factors, mainly: conveyor system&#;s relevant requirements for conveyor belt; terrain environmental conditions and rated requirements for safety; type, shape, particle size, characteristics of conveying materials, whether there is thermal effect and chemical Function; requires maximum bandwidth, working tension and conveying capacity; minimum roller diameter; troughing ability and lateral stiffness; load support; length of curve section and transition section; tensioning method and stroke; receiving point and receiving conditions, and conveyor belt The operating cycle; requirements for impact resistance and tear resistance; joint conditions; and should meet the following requirements:
1.1&#;For short-distance belt conveyors, polyester fabric core (EP) conveyor belts should be used. Steel cord conveyor belts should be used for belt conveyors with large transport capacity, long transport distance, large lifting height, and high tension.
1.2&#;The material to be conveyed contains large-sized block materials, and when the direct drop of the receiving point is large, the impact-resistant and tear-resistant conveyor should be selected.
1.3 The maximum number of layers of the layered fabric core conveyor belt should not exceed 6 layers; when the conveyed material has special requirements for the thickness of the conveyor belt, it can be appropriately increased.
1.4&#;The downhole belt conveyor must be a flame-retardant conveyor belt.
2&#;Choice of covering layer
The choice of covering layer includes covering material, surface morphology and thickness.
The main components of the covering layer are various rubbers and plastics. In most climates, the rubber can work normally under the condition that the inclination angle is not more than 18°, while the normal operation of the PVC tape is below 12°. PVC is not as good as rubber in resilience, but it has better flame retardancy, discharge properties and cleaning properties. The properties of various rubbers are also very different. For example, natural rubber and styrene-butadiene rubber have good energy absorption and abrasion resistance; ethylene-propylene rubber is particularly heat-resistant; cyanide rubber has good oil resistance; propylene-butyl rubber is both heat-resistant and Resistant to oxidizing chemistry. Therefore, the choice of the type of covering material must be comprehensively considered according to the type of conveying material, operating conditions and working environment.
The increase in the thickness of the covering layer is beneficial to improve the impact resistance and abrasion resistance of the conveyor belt. Especially nowadays cotton canvas belt core has been replaced by synthetic fiber fabric belt core. The performance of the belt core has been greatly improved, while the thickness is relatively thin. In order to improve the overall performance of the conveyor belt, it is necessary to increase the thickness of the covering layer accordingly. Give full play to its advantages and increase the service life of the conveyor belt. However, the lower cover layer is too thick to increase the running resistance of the conveyor belt.
The basis for designing the thickness of the covering layer is the wear speed, material receiving conditions, the heat resistance of the covering layer, impact resistance and the operating cycle of the conveyor belt.
3&#;Conveyor belt joint&#;Conveyor belt joint type should be selected according to the type of conveyor belt and the characteristics of the belt conveyor; steel cord conveyor belts should use vulcanized joints; multi-layer fabric core conveyor belts should use vulcanized joints; fabric whole-core transportation Adhesive bonding head should be used for the belt, and mechanical joints can also be used.
Conveyor belt vulcanization joint form: layered fabric core conveyor belt should adopt step joint; steel cord conveyor belt can adopt one-level or multi-stage vulcanization joint according to the tensile strength level.
4&#;Safety factor of conveyor belt
4.1 The importance of reducing the safety factor of conveyor belts and according to the fact that conveyor belts, especially high-strength conveyor belts, account for up to 30% to 50% of the investment in belt conveyors. To choose conveyor belts reasonably and reduce the investment of conveyor belts, an important way is to improve the conveyor belt joint technology to reduce the safety factor. For example, a 20km long conveyor in Australia reduces the safety factor of the steel cord conveyor belt from the usual 6.7 to 5.0, the weight of the conveyor belt is reduced by 14%, the life of the bearing roller is increased by 7%, and the life of the return roller is increased by 40%. The reduction of the conveyor belt tension reduces the corresponding structural investment, and the power consumption is reduced by 4%. The investment and operating costs can be reduced by about 10% within the 20-year service life.
4.2 Definition of international safety factor
The German Industrial Standard DIN- defines the safety factor of the steel cord conveyor belt as the ratio of the tensile strength of the steel cord conveyor belt to the tension of the conveyor belt when the conveyor is running stably, with a minimum of 6.7. In , Germany revised the original standard, and the new version adopted a brand-new concept and calculation method. The conclusion is that the safety factor of steel cord conveyor belt can be less than 6.7, and the lowest can be 4.5. In the past 10 years, some countries have already broken through the regulations. The belt conveyors in operation in Germany, Britain, New England, Australia and other countries use 4.5-5.5. According to the research results of foreign institutions such as CDI in the United States, the safety factor of steel cord conveyor belts can be reduced to below 5.5, ranging from 4.5 to 5.0.
After the above and the provisions in the standard: the safety factor of the conveyor belt should be selected according to different conditions: that is, the safety factor of the general belt conveyor and the steel cord conveyor belt can be 7-9; when the conveyor is controlled by soft start, For braking measures, 5&#;7 can be taken.
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Tags: CONVEYOR BELT SELECTION
Despite the critical function they fulfil, technically they remain something of a mystery. Consequently, even less is understood as to why some chevron belts are so much more efficient and less troublesome than others. Here, conveyor belt specialist Leslie David explains the different production methods and how they ultimately affect reliability and how to use this knowledge to avoid being ripped off.
Belts with chevron-patterned profiles ranging from 15mm up to 32mm in height above the belt surface that guide and control the flow of materials such as sand or small size aggregates are the most commonly used profiled belts. One of the most frequent problems experienced by many operators is that the chevron profiles split and ultimately become detached entirely.
A common problem &#; profiles that split and eventually detach from the base belt
A second and sadly very common problem that affects all types of profiled belting is that the profiles wear down within an unacceptably short period of time. The origins of these problems (and the solutions) are to be found within the manufacturing processes and the rubber compounds that are used.
Because of its adaptability, most of the rubber used to make modern-day conveyor belting is synthetic. Because of the technical difficulties (and higher cost) involved in creating a synthetic rubber compound that will flow uniformly, the vast majority of chevron profile belts are effectively created using a two-stage vulcanisation process. Firstly, a belt carcass consisting of layers of fabric reinforcing ply and covered by a layer of uncured rubber compound on the top and bottom surfaces are placed in a vulcanisation press. At the same time, a mould plate is filled with uncured rubber, and the base structure is then placed on top of the filled mould. Alternatively, the mould plate is extracted, filled with uncured rubber and then replaced back under the base structure. In both cases, the complete structure is then vulcanised to create the finished belt.
Constant flexing can cause dynamic stress fractures in conventionally produced chevron belts
The key problem with this production process is that the uncured rubber compound used to construct the base belt structure cannot be the same as the rubber used on the top and bottom cover surfaces. This is because the rubber used to fill the chevron profile mould has to be more malleable so that it can flow and completely fill the mould cavities. However, in almost all chevron belts that are made in this way, the contact point where the two different rubber compounds join instantly becomes a point of weakness because chevron profiles constantly stretch and flex under significant tension each time they run around a pulley or drum. This means that unless the bond between the base belt carcass and the chevron profile is absolutely flawless, then sooner or later dynamic stress fractures in the profile will begin to occur, eventually causing the profile to split and all too often completely part company with the rest of the belt.
This problem is significantly magnified on conveyors with relatively small pulley diameters, especially where mobile equipment is concerned. The smaller the pulley, the higher the dynamic stress. Failure will happen even sooner if one (or both) of the rubber compounds used are not fully resistant (as per ISO testing) to the effects of degradation (surface cracking) created by chemical reactions in the rubber caused by ground-level ozone and ultraviolet light.
Chevrons as high as 25mm or 32mm can wear almost completely flat in a remarkably short time.
Having to use a rubber that is sufficiently malleable (pliable) so that it will fill the mould cavities and accept the dynamical strains of belt operation often creates a second major &#;belt life-threatening&#; weakness. Research and experience has shown that the rubber used to make the chevrons in the conventional two-step production process almost invariably has much lower resistance to abrasive wear than would normally be acceptable. It is not unusual, especially among so-called &#;economy&#; belts imported from Asia, that even chevrons as high as 25mm or 32mm can wear almost completely flat within a matter of a few months and in some cases, weeks. And while this is happening the efficiency of the belt steadily diminishes.
There are two essential requirements needed to avoid the inherent weaknesses I have described. The first is to use a single rubber compound that has been specially engineered for both the base belt structure and the chevrons and which can be vulcanised virtually simultaneously within the mould and the base belt structure. While being sufficiently malleable to allow it to flow smoothly and evenly into the moulds, the rubber also needs to have good wear resistance, tensile strength and durability. Also, the compound needs to be fully resistant to the effects of ozone and ultraviolet light (for the longevity of working life) and conform to European REACH regulations so that the end product is also safe to handle.
The second essential requirement is to manufacture the belt as a single, homogenous structure using a one-step production process rather than a two-step process. This is because a belt with a completely homogenous structure, even if damaged or split, is significantly stronger and more resilient against spreading damage or having profiles shear off entirely compared to belts where two non-identical rubber compounds have been vulcanised (bonded) together. In other words, making the belt once makes it considerably stronger.
A &#;single structure&#; chevron belt needs a highly specialised rubber
That may sound a relatively straightforward proposition, but it is a surprisingly tall order. Firstly, it is extremely difficult (and more costly) to produce such a versatile rubber compound. This is largely due to the huge number of different chemicals, polymers and additives that are used to create the synthetic rubber. All of the various components have to be very precisely balanced and mixed so that the final compound &#;cocktail&#; possesses all of the necessary physical properties.
To create a single homogenous structure the base belt (at this stage comprising of only uncured rubber) is placed in the vulcanising press between the base plate of the press and a chevron mould plate positioned immediately below it. The base belt will already also have a specific quantity of uncured rubber on the top cover surface in addition to the volume of rubber needed to achieve the minimum thickness of the top cover of the base belt once it has been vulcanised. The actual amount of &#;extra&#; rubber needed depends on the design and depth of the chevron pattern. The compression of the upper and lower plates then forces the additional rubber to flow into and fill the mould cavities. Vulcanisation of both the base belt structure and the rubber-filled moulds then takes place simultaneously to form a single homogenous unit.
The mould plate used to make a &#;Single homogenous structure&#; chevron belt
Bearing in mind the technical complexity and the higher costs involved, perhaps it is not surprising that apart from Dunlop Conveyor Belting, hardly any other manufacturer of note produces profiled chevron belt in this way. However, I would argue that the significantly superior strength, reliability and much longer working life that is created by chevron belting being made in this way certainly makes it worth the investment, both in terms of reliability and efficiency and also &#;whole life&#; cost.
Even when armed with this understanding of the huge difference in strength and overall durability between chevron belts made using the conventional &#;two-step&#; process and the &#;single homogenous structure&#;, the dilemma then is how to establish which kind is being offered by the manufacturer/supplier who is providing the quotation. Unfortunately, the only way to find out is to ask the would-be supplier and then hope that the salesperson you are speaking to actually does know the difference!
Chevron belts with a single homogenous structure are stronger and longer-lasting by far
Several belts on the market that have low profile patterns, usually no more than 5mm high and sometimes even less. Belting of this kind is mostly used for the transportation of packaged goods such as boxes, bags and baggage as well as bulk materials including agricultural products, oily materials, woodchips and wet sand and can successfully be used on inclines as steep as 30° in some cases.
Still going strong &#; a five-year old Dunlop Multiprof belt in action.
Unlike their high-chevron cousins, making single, homogenous structures is relatively easy to achieve with low profiles because the rubber only has to flow a small amount. Yet again, the key influencer as far as performance and value is concerned is the quality of the rubber. In this case it is ability of the rubber to resist wear (abrasion) and to resist the effects of ozone & ultra violet that are the most crucial. As far as European-based, quality led manufacturers like Dunlop are concerned, achieving working lifetimes as long as five years is more than possible.
When it comes to profiled belting, appearances can be very deceptive. One belt may look virtually identical to another belt, and the basic specifications such as tensile strength and number of plies may also appear to be identical. Therefore, expecting that the actual performance and working lifetime will be roughly the same would seem to be a reasonable assumption. However, the alarm bells should start to ring if there is a significant difference in the asking price. Actually, the reason why one belt can have a dramatically lower price compared to one of a seemingly identical specification is very easy to explain. Ultimately, there is a direct correlation between the price and the quality of the rubber. And there is no doubt that the quality of the rubber that will have the biggest bearing on performance and the cost-effectiveness of the end product.
Splitting the difference in price and quality
The rubber used for conveyor belts usually constitutes at least 70% of the material mass and therefore it is the single biggest element of cost when manufacturing a conveyor belt. Consequently, in the highly price-competitive conveyor belt market, for those who want to compete for orders based on price rather than performance and operational longevity, rubber is the single biggest opportunity for manufacturers to minimise costs.
The two most common methods used to keep the cost of the rubber to an absolute minimum are the use of recycled rubber (usually of highly questionable origin) and the use of cheap &#;bulking&#; fillers such as chalk to replace part of the rubber polymers in the rubber compound. Another practice is the burning of used rubber car tyres to create a cheap form of carbon black. Some 20% of rubber compound is made up of carbon black, so it has a notable impact on the overall cost of making a conveyor belt. Good quality carbon black is quite costly because it is created by the process of burning oil in a strictly controlled, low oxygen environment so that combustion is incomplete. But burning used car tyres not only pollutes the atmosphere it also means that any oils and greases contained within &#;regenerated&#; materials compared to good quality carbon black will have a detrimental effect on the physical properties of the rubber.
The harsh reality is that conveyor belts used in quarries are viewed by a great many as readily disposable &#;sacrificial&#; components where the aim is to pay as little per meter as possible even though the need to regular repair and replace worn and damaged belts is expensive both in terms of lost productivity and day-to-day running costs. A cynic might also say that for some manufacturers, belt suppliers and service companies, regularly replacing belts is a highly lucrative business that would not be nearly so profitable if the belts were a lot more durable and lasted a lot longer. Fortunately, for the higher-quality belt manufacturers, not everyone thinks that way.
Making profiled belts that perform reliably as well as giving the best return on investment by providing the longest possible working life, is something very few belt manufacturers can achieve. Fortunately, although they are a very rare breed indeed, there are at least one or two still out there that continue to prove that chevron belts that are genuinely durable and last much longer than the &#;cheap&#; imitations cost appreciably less and are much less hassle in the long run.
About the author
After spending 23 years in logistics management, Leslie David has specialised in conveyor belting for over 14 years. During that time, he has become one of the most published authors on conveyor belt technology in Europe.
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