January 2012 Recycling & Waste World

Moving items are the most vulnerable parts of any machinery and none more so than conveyor belts, especially when dealing with materials such as construction and demolition waste. Sytze Brouwers looks at a new product which claims to offer increased impact and tear resistant characteristics.

In many recycling plants, even the strongest of conventional belts such as 5-ply fabric belting can be ripped or torn by large lumps of heavy, sharp objects, either falling from a height or becoming trapped. In extreme cases, even the heaviest of belts can be destroyed within a matter of weeks or months. To find an answer to this problem, engineers at Fenner Dunlop USA and Netherlands-based Dunlop Conveyor Belting set out to create a new concept of conveyor belt construction specifically designed to increase belt life and reduce operational costs in applications where hard, sharp or irregular shaped product are being handled by the conveyor system. The result of this development work was their UsFlex range of belting. Dunlop claims that it has a rip and cutting resistance that is more than four times greater than conventional multi-ply belts and far superior to steel reinforced, solid woven and other conventional heavy-duty belt constructions of similar tensile strength. However, is it the ultimate solution?

Belt characteristics

The toughness of UsFlex lies within its unique carcass, which differs fundamentally from that of standard warp and weft construction conveyor belts. UsFlex owes its outstanding impact and tear resistant characteristics to an innovative woven straight-warp carcass. This consists of heavy strands of polyester running lengthwise and nylon running crosswise. The strands are completely straight in both directions and not interlocked as in a conventional belt carcass.

This allows the weft (the crosswire threads under which other threads, the warp, are passed) to float free from the warp, thereby minimising the peak point of impact because the energy is absorbed over a larger area, providing maximum protection of the carcass. When compared to conventional multi-ply and solid woven belting, the superior impact resistance of UsFlex is quite amazing.

For example, a single ply UsFlex type 630/1 has the impact absorbing qualities of a 4-ply EP belt type 1600/4 or an EPP 1250/2. The chart on the right shows the results of impact tests on 630 rated UsFlex, solid woven and multi-ply belting. Rip resistance is over five times that of conventional multi-ply belts with similar tensile strength. The rip resistance is also far superior to solid woven and EpP constructions. To illustrate this fact, the lightest duty UsFlex belt has greater rip resistance than the heaviest type of EP2000/4.

Tear resistance is also greatly increased. The tear resistance of UsFlex, measured according to the international EN ISO 505 standard, also exceeds that of conventional multi-ply belts with a comparable tensile strength. As with any product, there is no shortage of imitators and rival products such as belts fitted with steel breakers. Having spent years refining UsFlex, we continually test not only our own belts but also belts manufactured by our competitors so that we can prove its strengths and advantages.

Tested to destruction

It is important to point out that tests for rip and tear resistance are only made on the actual belt carcass so the top and bottom covers are always removed. This ensures that the thickness and quality of the cover do not influence the validity and consistency of the tests. Although some potential alternatives to UsFlex may have a fairly good rip or tear resistance, we have yet to find a belt made by a competitor that has the combined qualities of both outstanding rip and tear strength.

Life at the sharp end

There is, of course, no truer test of a belt’s durability, resilience and length of operational life than seeing it perform in the toughest possible conditions. Over the years, we have been fortunate to be given the opportunity to put UsFlex to the test in a wide range of industries carrying an equally wide range of aggressive materials. One of the earliest and toughest tests took place in the UK. The site was handling in excess of one million tonnes per annum of Dolerite, a hard rock used in a variety of applications from road surfacing to ready-mixed concrete. The primary jaw crusher was producing up to 650 tons per hour of sub-250mm material that fell some two meters on to the conveyor, which was widely recognised as being one of the toughest applications in the country.

New belts were lasting as little as two months before signs of deterioration due to impact damage started to appear. Desperate for a solution, the site management agreed to undertake trials to assess the long-term benefits of UsFlex in terms of durability, performance and especially the opportunity for planned preventive maintenance. The performance of three previous ‘conventional’ multi-ply conveyor belts was measured. The first belt operated for 26 weeks before replacement. The second belt only lasted for 9 weeks and the third ran for some 15 weeks before failing.

A Dunlop UsFlex 1000/2 8 + 3 conveyor belt was then fitted. After carrying more than one million tonnes and reducing the cost per ton by almost 50% the trial was considered a complete success. Examination of the belt did reveal cuts in the top cover, but importantly with no evidence of carcass damage. The introduction of UsFlex improved continuous production and reduced belt maintenance and downtime. After three years, the UsFlex belt was still operating successfully, ultimately resulting in a reduction in cost per ton of more than 80%. In Sweden, a wood and paper plant recycling wood including tree branches, roots and wooden pallets containing nails had tried several different kinds of conventional belting over the years. The belts were being destroyed within 12 to 18 months due to tears and ripping. Following a change to an UsFlex 1000/2 8+3 belt, the operational lifetime increased to more than three years.

Added protection

UsFlex is available as a one or two ply construction, from a 400/1 through to a 1600/2. To protect the carcass, Dunlop RS covers are fitted as standard because of its excellent resistance to cutting combined with a resistance to abrasion that exceeds the highest abrasion standard (DIN W) by nearly 30% and the equivalent ISO ‘D’ standard by more than 40%. Other qualities, such as oil, fire and heat resistant covers are available. UsFlex is also available with homogenous chevron profiles.

Vulcanised splice

It is widely recognised that the weakest part of any conveyor belt is the splice joint. For maximum splice strength, we recommend finger splicing. Some people in the industry seem to have something of a phobia when it comes to finger splicing but it is quite straightforward. In destructive testing, it achieves well in excess of 90% of the belt strength at failure. This is far stronger than conventional multiply belting where splice strength is typically 60-70% of the breaking strain. On two-ply UsFlex belts such as the 1000/2, conventional step splicing techniques can be used.

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