Jan 2015 Recycling & Wasteworld - part 1
This is the first part of a two part series. In this part the key factors to consider when choosing a conveyor belt are explained.
Getting the most out of conveyor belts
The conveyor belt market is a highly competitive market that is seeing a steady increase in what can best be termed as “unscrupulous practices”. Conveyor belts are often treated as an everyday commodity rather like photocopier paper, whereas in reality they are highly sophisticated and costly components. Their durability and reliability are critical factors in productivity, safety and cost management.
What do my conveyor belts have to cope with?
The first thing to consider when choosing conveyor belts is the chemical and physical demands they have to cope with. To those outside of the industry, general household waste may sound relatively harmless, but in reality it is a real ‘belt killer’ because it contains countless things that damage rubber. Chemically; oils, fats and greases, chemicals, acids, bleaches, caustic soda, you name it and it will be in there somewhere. Physically; there is also an almost endless range of causes of damage to the belt such as glass, sharp metal objects and masonry. Is it any wonder that conveyor belts are cut and ripped to shreds so easily? Perhaps it is therefore understandable that so many recycling plants seem to believe that it does not make much difference what belt is fitted to a conveyor because the end result will always be the same. The tendency often seems to be ‘fit the cheapest belt and hope that it lasts as long as possible.’
Conveyor belts: the basic structure
Conveyor belts most commonly used within the recycling industry are composed of two basic elements. The first is the carcass, which consists of multiple layers of synthetic fabric plies. Each layer of fabric is bonded together by a thin layer of rubber known as the ‘skim’. It is the carcass that provides the inherent characteristics of a conveyor belt such as its tensile strength and elongation (elasticity or ‘stretch’ under tension). For rubber multi-ply belts, the use of good quality fabrics (most commonly polyester/ nylon) is extremely important. Poor quality fabric plies invariably have inadequate tensile strength; are prone to excessive stretching (elongation) and tear and rip far too easily. These weaknesses also lead to premature failure of the splice joint, tracking and steering problems, de-lamination (where the belt literally starts to fall apart) and, most commonly of all, longitudinal tearing.
It’s all about the rubber
The second basic element is the layer of rubber on the top and the bottom of the carcass. These outer layers are known as ‘covers’. They not only act as the carrying surface but also, and most importantly of all, protect the carcass. Although the actual construction of the carcass is important, it is the physical qualities of the rubber outer covers that will ultimately determine the lifetime of the conveyor belt and which is also the truest test of its value for money.
Many different types of rubber compound are used for rubber multi-ply covers; each designed to withstand damaging effects such as wear caused by abrasion (the most common type), greases and fats, chemicals, tearing and cutting, heat, fire, extreme cold, ozone and oil penetration. These different types of rubber compound are generally referred to as “cover grade qualities’.
Wear (abrasion) resistance
The wear resistance quality of a conveyor belt is usually the single most important factor. As a general rule, 80% of conveyor belt surface wear occurs on the top cover of the belt and 20% on the bottom cover. Wear on the top cover is primarily caused by the abrasive action of the materials being carried, especially at the loading point or ‘station’ and at the discharge point where the material is effectively ‘accelerated’ along the surface of the belt.
Belts on short conveyors (less than 50 metres) wear more rapidly compared to those fitted on longer conveyors because the belt passes the loading and discharge points more frequently. For this reason, choosing belts that have a good level of resistance to abrasion is even more important than usual. Wear on the bottom cover of the belt is mainly caused by contact with the drum surface and idlers. The rate and uniformity of this type of wear can be adversely affected by factors, but in my experience of the recycling industry, it is seized or missing idlers and waste build-up near the drums and pulleys that are the most common causes of wear and damage to the bottom cover.
Cutting back on what should be routine maintenance such as immediately replacing damaged or seized rollers is a false economy.
There are two internationally recognised sets of standards for abrasion, ISO 10247 (H, D and L) and DIN 22102 (Y, W and X). The longer established DIN standards are most generally recognised and accepted. DIN Y relates to ‘normal’ service conditions; DIN W for more abrasive materials and DIN X for resistance to cutting, impact (caused by large lump sizes falling on the belt from height), abrasion and gouging caused by hard, sharp materials. The ISO and DIN test methods for measuring abrasion (wear) resistance are identical. The level of resistance (wear) is established by measuring the amount of rubber lost (in cubic millimetres) during the test.
When comparing abrasion resistance between one manufacturer and another it is important to remember that lower figures represent superior resistance and therefore longer wear life. For example, a belt with an abrasion resistance of 100 mm³ compared to a belt with an abrasion resistance of 150 mm³ can reasonably be expected to provide 50% longer wear life.
DIN and ISO abrasion resistance standards are only the minimum benchmark of acceptability so even wear resistant covers that conform to international standards can still need to be replaced after unacceptably short periods. Despite the claims of the manufacturers and traders, laboratory tests reveal that more than 50% of belts tested are found to be significantly below the specified standards.
Oils, fats and chemicals
Oil and fats have a particularly detrimental effect on the performance and life expectancy of a conveyor belt because they penetrate the rubber causing it to swell and distort, often resulting in serious operational problems. Oil and fat resistance can be divided into two sources - vegetable and animal oils and fats and mineral oils. When recycling household or automotive waste and indeed any waste that contains chemicals, it is absolutely essential to use conveyor belts made using a good standard of oil resistant rubber.
At Dunlop we use a special rubber compound formula (Dunlop RO M) cover grade, which is specifically designed for these types of oils and fats.
Mineral oils are the most aggressive and therefore demand a particularly high level of protection. For these oils, as well as aggressive chemicals and acids and high concentrations of animal and vegetable oils, we have developed our particularly resilient Dunlop RO S compound. Despite the untold damage caused by oils and greases there are, as yet, no recognised EN /ISO test standards for oil and fat resistance. This in itself can pose a big question mark against the oil resistance claims made by some manufacturers. In the Dunlop laboratories we apply the stringent ASTM D 1460 test standard, which originated in the USA and which is generally accepted as being the toughest standard of its type in the world.
When choosing conveyor belts that are resistant to oils and chemicals it is still essential that you look for good wear (abrasion) resistance. As is also the case with conveyor belts primarily designed to resist heat or fire, oil resistant belts often have a very poor resistance to wear. It is almost as if the manufacturer has given with one hand and taken away with another. Do not accept this. Always demand that the abrasion resistance of the oil resistant belt you are considering must have an average abrasion resistance that is less than 150 mm³, which is the DIN Y standard for abrasion resistant belting. When requesting quotations, my advice is to always ask for a technical datasheet and a certificate of origin showing the country of actual manufacture. The technical datasheet should contain all key data relating to the belt carcass, the covers and other information such as operating temperatures and minimum pulley diameters. The easiest and most important data to use for making comparisons is the average level of abrasion resistance.
Article was published in Recycling Waste World (Feb 2015)