Time to expect more from your conveyor belts
Conveying waste is a demanding task even for the very toughest of belts because it contains a multitude of elements that damage and destroy rubber. The oils, fats and greases, found in household waste have a particularly detrimental effect on the performance and life expectancy of conveyor belts.
Throw some acids and chemicals into the mix such as sodium hydroxide or potassium hydroxide (commonly found in soaps for example) and it is little wonder that the recycling industry gets through thousands of conveyor belts every year, the vast majority of which have to be regularly repaired before ultimately being replaced far too prematurely.bThe recycling industry spends millions each year buying, fitting and repairing conveyor belts. They are an essential but very expensive overhead. All too often, however, much of that expenditure is wasted in the pursuit of what are invariably false economies. The fact is that conveyor belt technology has advanced quite significantly, especially in recent years. Today’s belts, even those that carry the most aggressive materials, should be able to run for far longer than they currently do and require far fewer repairs. The technology and the solutions are out there. You just have to know what to look for and where to look. Here, Les Williams of Netherlands-based Dunlop Conveyor Belting explains how taking advantage of improved technology can dramatically reduce the frequency of conveyor belt repair and replacement while boosting productivity and improving safety at the same time.
Oil and chemical resistance
More and more nowadays, conveyor belts used in recycling plants, particularly those dealing in household and industrial waste, need to have the ability to ‘multi-task’ in terms of what they have to deal with. An absolute pre-requisite is the ability to resist the damage caused by oils, fats and greases. Oil is particularly damaging to conveyor belts because it penetrates into the rubber covers causing them to soften, swell and distort. This results in all kinds of problems including a dramatic decrease in the ability to resist abrasion (accelerated wear). As the rubber softens it is also prone to ripping and tearing much more easily. The distortion causes steering and handling problems along with a serious reduction in the elongation at break (the amount of stretch before the belt snaps). Throw in a considerable loss of tensile strength and you have a belt that will need replacing sooner rather than later.
The vast majority of modern-day conveyor belts are made of synthetic rubber rather than natural rubber. Natural rubber is appreciably more expensive and less adaptable compared to the synthetic variety. The oils and fats that have such damaging effects on all types of rubber compound can be divided into two distinct sources – mineral and vegetable/animal.
Mineral oil is usually a liquid by-product of refining crude oil to make gasoline and other petroleum products. It is composed mainly of alkanes and cycloalkanes related to petroleum. High deposits of mineral oil can be found in household and industrial waste. There is a marked difference in the swelling caused by different mineral oils on synthetic rubber compounds. Vegetable oil is the most predominate source in household waste and is defined as being all forms of oil (and resin) that is derived from flora and fauna.
Testing for oil resistance
Unfortunately for the end-user, belt manufacturers and traders can confidently claim that the belt they are supplying has good oil resistance while safe and secure in the knowledge that there are no ISO or DIN international standards for oil resistance that the belt has to meet. My best advice is to always ask for precise details of the oil resistance test methods they have used to support their claim. At Dunlop for example, we use the American ASTM ‘D’ 1460 test method, which is widely regarded as being the most demanding test of its kind.
Different oils have different effects
Although different oils have different effects on rubber, most conveyor belt manufacturers still only produce one oil resistant rubber cover quality compound. This is often referred to as ‘MOR’ (medium oil resistance). In our experience, to provide the best possible protection against the differing effects of the two primary oil types requires two specific compounds.
The first and generally the most commonly used is a synthetic rubber compound (Dunlop ROM), which is based on a combination of SBR (Styrene Butadiene Rubber) and NBR (Nitrile butadiene rubber). This compound is specifically designed to resist the penetration of vegetable and animal oils, fats and wood oils and resins.
Good quality SBR provides excellent resistance to abrasion combined with very good tensile strength while the NBR provides the necessary resistance to avoid the swelling, distortion and softening caused by the oil. These two characteristics will ensure that the rubber covers are durable and long lasting.
This combination of SBR and NBR is also better than natural rubber when it comes to ageing and has excellent mechanical properties including good elongation at break.
Mineral oils tend to be much more aggressive than vegetable oil. For this reason our engineers developed the extremely successful Dunlop ROS Nitrile butadiene rubber compound. The higher the concentration of nitrile that there is within the polymer, the higher the resistance it will provide. Although nitrile butadiene is more expensive than Styrene Butadiene, it does provide the essential levels of resistance needed to cope with mineral oils, fuel plus a wide variety of chemicals found in waste. In situations that involve products that have particularly high concentrations of animal and vegetable oils we always recommend the use of the Nitrile butadiene (ROS) cover grade. For the benefit of the scientifically minded amongst you, nitrile butadiene rubber is a family of unsaturated copolymers of 2-propenenitrile and various butadiene monomers (1,2-butadiene and 1,3-butadiene). Although its physical and chemical properties vary depending on the polymer’s composition of nitrile, this form of synthetic rubber is unusual in that it is generally resistant to oil, fuel, and other chemicals.
The potential danger of having high levels of nitrile within the polymer is that it can reduce the flexibility of the rubber. This means that the recipe formula for a nitrile-based rubber compound has to be extremely precise. Every new batch produced during the mixing process has to be absolutely consistent. That means creating fresh, new compound for every belt rather than trying to artificially keep the cost down by using high quantities of reject tyre compound or recycled rubber.
Oil and wear resistant
Another common downside of having rubber that has good resistance to oil is that the ingredients used to create that resistance tend to have an adverse effect on the wear resistant properties of the rubber. In other words, oil resistant belts usually wear out faster. However, this need not be the case.
At Dunlop the oil resistant rubber compounds developed by our technicians have an abrasion resistance that is even superior to most ‘abrasion only’ rubber covers. Buyers should always insist on documented evidence that shows the level of abrasion (wear). Never accept an abrasion resistance that is more than 150mm³.
It’s important to remember that when comparing resistance to abrasion, lower figures represent higher resistance to wear. In basic terms, the lower the figure then the longer the wear life will be.
Fire and oil resistant belting
Conveyor belts that are resistant to both oil and fire are increasingly in demand. There are more than 300 reported fires every year at waste and recycling plants in the UK and the number and seriousness of the incidents is steadily growing.
This worrying situation is hardly surprising given the combustibility of the materials – paper, plastic, wood, cardboard and so on. It is impossible for waste businesses to take too many precautions but sadly the reality is that having fire resistant conveyor belts is still a rarity because of the cost. Conveyors can carry a fire very quickly. Thanks again to advances in technology, it is now possible to buy good quality fire resistant (self-extinguishing) conveyor belt that also has first class resistance to wear and oil. It requires a highly advanced rubber compound that possesses all of these properties so naturally it does not come cheap. Having said that, given its much-extended wear life coupled with added fire safety protection (very much appreciated by the insurers in the form of lower premiums) makes such belting very cost-effective.
Impact and rip resistant belting
Some waste, such as building materials for example, can create a big problem for recyclers because the belt surface and the actual carcass itself can easily ripped and torn. Belt that has been clipped (stitched) back together is a common sight. I have seen belts in use that remind me of a Frankenstein monster. Apart from fitting increasingly thicker belts (which is not actually the best answer) many operators accept what they believe is inevitable and resort to fitting the cheapest belt they can lay their hands on. These are commonly referred to as ‘sacrificial’ belts.
This may seem to be a cost ‘saving’ in the short term but the repeated stoppages, running repairs and replacements are definitely not cost-effective. In fact quite the opposite.
Nowadays, belts are available that have been specifically designed to cope with heavy impact and sharp objects. At Dunlop we have a number of different belt constructions for heavy-duty use. The most successful of these is UsFlex, which has a totally unique fabric ply that provides a longitudinal rip resistance that is more than five times stronger than multi-ply belts of equivalent rating.
Although the carcass is only a single ply or dual ply construction (usually 630/1 or 1000/2) it has up to three times greater impact resistance compared to conventional plied belting. The carcass is protected by very high grade abrasion and cut resistant rubber covers. The result is a belt that consistently outlasts conventional belting many times over.
For some, trying to imagine how a conveyor belt with only one or two plies could be so much stronger than a belt with multiple (4 or 5) plies may seem impossible to comprehend but the fact that it is far stronger is due (yet again) to advances in technology and engineering.
The analogy I would use is armour plating on military tanks. The modern composite armour used on modern-day military vehicles is a much thinner and lighter construction than the extremely thick and heavy steel plating used in the past yet it provides far greater protection.
Longer belt life = lower belting costs
In my experience, the vast majority of conveyor belts in use within the recycling and waste industry should be lasting at least 50% longer and need far fewer repairs.
A significant cause of unduly short belt life is simply a consequence of poor maintenance. Seized or missing rollers, damaged drums and build-up of waste around pulleys are very common sights. But the single biggest reason of all for the high frequency of repairs and replacements lies in the quality of the conveyor belts themselves.
It may sound cynical but the reality is that it is not really in the best interests of belt manufacturers, traders and service companies to supply conveyor belts that run and run and run, particularly if they are trying to compete on price, which is pretty much the usual approach. This is especially so in the UK where the vast majority of belting is directly or indirectly imported from Asia. The fact is that there is no such thing as a ‘cheap’ conveyor belt. True, there are low priced ones but that’s a very different thing because they are low priced for a reason.
As often as not, the quality of a belt (including its ability to resist oil or fire or wear) is reflected in its price. Virtually 80% of the cost of making a conveyor belt lies in the raw materials so the most effective way of minimising costs (prices) is to use as lower grade materials as possible and to minimise the use of the more costly components such as nitrile.
Belts that have inadequate resistance to the substances they carry will cut, rip and wear far too easily. Consequently, they prove to be considerably more expensive in the end. In my view, the true cost of a conveyor belt can only be measured over its operating lifetime. It is always worth the effort to check the original manufacturer’s specifications very carefully. Always insist on a fully detailed manufacturer’s technical datasheet and a warranty certificate as well as documented evidence of tested performance before placing your order. Taking these simple precautions could save you from making a very expensive mistake.
Read the full article here
Recycling & Waste World 2018