Rock Products

APR 2018

Rock Products is the aggregates industry's leading source for market analysis and technology solutions, delivering critical content focusing on aggregates-processing equipment; operational efficiencies; management best practices; comprehensive market

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38 • ROCK products • April 2018 Keeping Up the Pressure Tensioning The ability to maintain the proper force required to keep the blade edge against the belt is a key factor in the performance of any cleaning system. Blade-to-belt pressure must be con- trolled to achieve optimal cleaning with a minimal rate of blade wear. There is a popular misconception that the harder the cleaner is pressing against the belt, the better it will clean. But research has shown that there is actually an optimum range of blade pressure, which will most effectively remove carryback material. Increasing tension beyond this range raises blade- to-belt friction, thus shortening blade life, increasing belt wear and increasing power consumption – without improv- ing cleaning performance. Too much pressure can prevent proper contact, forcing the tip away from belt and leaving a small gap that can lead to hydroplaning. Material will wedge between the blade tip and belt, poten- tially leading to premature wear of both the blade and belt. Operating a belt cleaner below the optimum pressure range delivers less effective cleaning and can actually accelerate blade wear. A belt cleaner barely touching the belt may appear to be in working order from a distance, whereas in reality, excessive amounts of carryback are being forced between the blade and the belt at high velocity. This passage of material between the belt and the blade creates channels of uneven wear on the face of the cleaner. As material continues to pass between the blade and the belt, these channels increase in size, rapidly wearing the blade to a jagged edge. Another common source of blade wear that often goes unnoticed – even with a properly installed and adjusted cleaner – is running the belt empty for long periods of time. Even though the cargo may be abrasive, it often has moisture in it that serves as a lubricant and cool- ant. Small particles embedded in the empty belt's surface can be as much as 60 g/m 2 creating a kind of sand paper. In effect, running the belt when empty results in a doubling of the wear rate of both the blade and the belt. There is actually quite a complex geo- metrical relationship between the tensioner's spring force and the blade geometry during its wear life, in order to achieve the most effective cleaning pressure. Most tensioners are designed to be a compromise between optimal pressure at a single point of wear and acceptable pressure over the entire wear profile. In spring tensioners (such as twist or coil types), there is the additional com- plication of the spring force decreasing as the blade wears, bringing one more variable to the equation. So designers are trying to optimize four factors: decreasing spring force, changing blade geometry, blade width and blade type. As urethane cleaner blades wear, the surface area of the blade touching the belt increases. This causes a reduction in blade-to-belt pressure and a corre- sponding decline in cleaner efficiency. Therefore, most mechanically ten- sioned systems require periodic adjustment (re-tensioning) to deliver the consistent pressure needed for effective carryback removal. There are competing theories on belt cleaner tensioning. Linear-tensioned cleaners are pushed up (in a line) against the belt, and radially tensioned cleaners are installed with a mainframe as an axis and rotated into position. In addition, some hybrid systems incor- porate vertical tensioning with a radial relief mechanism. Linear-adjusted cleaners generally require access to both sides to provide balanced pressure. Because of this, the tensioners for these cleaners often have some form of powered adjust- ment that can be remotely controlled. Linear tensioners maintain a constant cleaning angle as the blade wears and can usually be designed to allow easy withdrawal of the cleaner for mainte- nance without removing the tensioner. Radial-adjusted cleaners have sev- eral practical advantages over linear designs. They are easier to install and can more readily rotate away from the belt to absorb the shock inherent in belt motion and splice passage. To overcome the problem of the blade angle changing as the blade wears, a radial-adjusted belt cleaner can be designed with a specially engineered curved blade, known as "CARP" for Constant Angle Radial Pressure. With this innovative design, the changes in contact angle and surface area are min- imized as the blade wears, helping to maintain its effectiveness throughout the cleaner's service life. Air Tensioning Mechanical tensioner designs work well as long as they are properly adjusted, but in most cases this requires periodic attention from maintenance crews as the cleaning blade wears down. In addition, some operators will loosen Figure 3: Linear and radial belt tensioning.

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