There are many different belt carcass constructions and a wider range of rubber compounds have been designed to protect the carcass. With different requirements, dazzling test methods and quality standards have been produced.
Dimensions and tolerances
In the terms of standards and acceptable tolerances, such as length/width/thickness, etc. All textile fabric construction conveyor belts should be subject to ‘ISO’ 14890-2013. These specify the dimension requirements about rubber (and plastic) -covered conveyor belts for general surface use on flat or trough idlers.
Tests for different demands
There are different types of belts cover, the primary rubber cover grades are:
- abrasion resistant;
- heat resistant;
- oil-resistant;
- fire-resistant;
Rubber covers often need to cope with combination factors, such as fire & oil. However, the ability to resist abrasion is common to all. The most commonly used belt, also to be abrasion-resistant, and so that should be the best place to start.
Abrasion resistance
The wear resistance is normally the most important factor which determines its lifetime of operation and therefore its cost-effectiveness. There is two internationally recognized standard for abrasion, EN-ISO 14890 and DIN-22102. In Europe, it is the longest established DIN standards that are commonly used. Generally, DIN Y (ISO-14890 L) relates to normal service conditions; and DIN W (ISO-14890 D) is for a particularly high level of abrasive wear. However, the DIN X (ISO 14890 H) is regarded as the most versatile, because in addition to resisting the abrasive wear, it also has good resistance to cutting/ impact (high drop heights) / gouging, which is usually caused by heavy and sharp materials.
Abrasion testing
The test for abrasion (ISO 4649, DIN 53516) is very simple. Abrasion resistance is normally measured by moving a test piece of rubber across the abrasive sheet surface mounted on a revolving drum. Expressed as volume reduction in cubic millimeters, like 150mm³.
The most important when checking the abrasion test result is the higher figures mean greater loss of rubber on the surface, which also means lower resistance abrasion. A lower figure means better wear resistance.
Heat resistance
For all the industrial conveyor belts, heat always is regarded as most damaging and unforgiving. High-temperature material and working environment cause acceleration of the aging process, which results in hardening and cracking of rubber covers.
Heat usually has very destructive effects on the belt carcass, because it can damage the adhesion between covers on top and bottom of carcass, and between inner plies within the carcass. The belt will fall apart when core temperature of the carcass becomes too high. This is usually referred to as the “de-lamination”.
ISO 4195 testing
To provide the most accurate measurement of heat resistance, accelerated aging tests are conducted by placing rubber samples in high-temperature ovens for seven days. The reduction in mechanical properties is then measured. The three ‘classes’ of aging within ISO 4195 are Class 1 (100°C), Class 2 (125°C), and Class 3 (150°C).
There are three key factors to consider when choosing a heat-resistant belt. The most critical considerations are the actual temperature range of the materials being carried; the level of ambient temperatures of enclosed running environments and the length of the conveyor. All of these factors can have a major influence on the speed of the aging process.
Oil resistance
These two distinct sources of oil — mineral and vegetable/animal. When any form of oil penetrates rubber, it swells and distorts. This causes major tracking and steering issues, as well as premature wear and eventual replacement. For oil resistance, there are two widely accepted test techniques, both of which use nearly identical test methodologies. ISO 1817 (2015) and ASTM ‘D’ 1460.
Test methods
Oil’s effect on vulcanized rubber is measured using the ISO 1817 and ASTM “D” 1460 test procedures. Rubber samples are immersed in the applicable test liquid for a set amount of time. The time of immersion and the temperature at which the liquid and sample are held can be changed, but three or seven days at ambient or 70°C are the most usual.
The ambient temperature of the surroundings is kept under strict control. When the samples are removed, the changes in the geometry and dimensions of the specimen induced by absorption are measured.
Despite the fact that there are no formal performance standards, it is nonetheless vital to seek for references to the manufacturer’s or supplier’s test methodologies.
Cold-resistance
When the temperature outside drops below –0°C, the rubber loses its flexibility. Rubber loses elasticity and its ability to resist abrasion, impact, and cutting as the temperature drops. The belt eventually loses its ability to trough and pass over pulleys and belt covers, causing the rubber in the carcass to break. The belt will eventually snap because frozen rubber is as fragile as glass.
Testing for cold resistance
There are currently no internationally approved test techniques for determining the ability of a conveyor belt to perform in extremely cold temperatures. A liquid nitrogen freezing cabinet is used in the laboratory to examine samples at extremely low temperatures.
The elastic modulus of rubber belt samples is initially determined at a temperature of 20°C. After that, the samples are placed inside the cabinet. After then, the temperature in the cabinet is gradually dropped in 5°C increments. At each stage, the elastic modulus is measured to determine when the rubber’s flexibility begins to deteriorate too much, resulting in the identification of the rubber’s lowest allowed ambient temperature.
Always ask for confirmation of the minimum operating temperature when there is a potential of extremely low temperatures. Abrasion-resistant belts can endure temperatures of –30°C to –40°C. Other cover qualities (such as oil or fire) can often only survive a temperature of –20°C. Conveyors with belts specially engineered to endure extreme cold must be used in temperatures below this.
Fire-resistant
Because fire safety is such a critical concern, there are various safety categories and international standards for measuring the self-extinguishing properties of conveyor belts, as well as a variety of tests. Depending on whether a belt is for use above or below ground, test techniques and performance standards differ dramatically.
Basic testing
EN/ISO 340 is the foundation for most belting testing in common industrial applications. This standard differentiates between fire resistance with covers (K) and fire resistance with or without coverings (S). Although grades ‘K’ for testing with covers and ‘S’ for testing with and without covers are no longer used in the current EN ISO 340, they are still extensively utilized in the market.
Six individual samples of the belt are exposed to a naked flame in EN/ISO 340 tests, causing them to burn. The flame’s source is then extinguished. After the flame has been removed, a stream of air is applied to the test component for a set amount of time. After the flame has been extinguished, the time it takes for the belt sample to self-extinguish is measured. For each sample, the length of continuous burning (visible flame) should be less than 15 seconds, with a total duration of 45 seconds for each group of six test samples. This regulates how fire can be transported along a belt that is moving.
Ozone & ultraviolet resistance
Along with abrasion resistance, all rubber belting has the capacity to withstand the destructive effects of ozone and ultraviolet light. Despite the fact that it is not a cover grade in and of itself, all rubber belts must be fully resistant to ozone and UV light. This is because ozone becomes a contaminant at low altitudes. Carbon black surfaces become more acidic as a result of exposure, and reactions occur inside the rubber’s molecular structure.
This has a number of effects, including surface cracking and a significant reduction in the rubber’s tensile strength. UV radiation from the sun and fluorescent lights, on the other hand, accelerates deterioration by promoting photochemical processes that promote the oxidation of the rubber’s surface, resulting in a loss of mechanical strength.
Samples are placed under stress (e.g. 20% elongation) within the ozone testing cabinet and exposed to extremely concentrated quantities of ozone for a period of time to scientifically measure resistance to ozone (eg. up to 96 hours).
At two-hour intervals, samples are checked for signs of cracking, and the results are meticulously quantified and recorded. The rubber sample must not show any evidence of cracking after 96 hours (at 40°C, 50pphm, and 20 percent strain) within the ozone chamber, according to experience.