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.
High profile chevron belts
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 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.
Conventionally produced chevron belting – The Achilles Heel
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.
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 1431 testing) to the effects of degradation (surface cracking) created by chemical reactions in the rubber caused by ground-level ozone and ultraviolet light.
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.
Make it once, make it strong
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.
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.
‘Single homogenous structure‘ chevron belting – How do they do it?
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.
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.
How will I know?
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!
Keeping a low profile
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.
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.
Don’t be fooled
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.
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 slippery slope
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.