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conveyors Herringbone Wing—A New Angle on Tail Pulleys BY LEO J. LAUGHLIN Often coal operators use spiral drum pulleys for tail pulleys. These were developed to auger material out and away from the tail pulley and belt. However, they have had some issues with plugging of the spiral, especially with wash down. While some have tried wing pul- leys, these are usually avoided due to concerns of stirring up the dust, or the intermittent contact with the belt causing too much vibration, or the fact that a wing pulley will throw the material back onto the belt. In this case the material will collect with other materi- al falling onto the belt and can eventually plug up the wings. A new device has been developed, the Herringbone Wing pulley, which is a hybrid of the spiral drum and wing pulley. Spiral drums were first developed for use on tail pulleys in coal mines some 40 years ago. The problem was that the wing pulley would move too much air, stirring up too much dust, especially as belt speeds increased. As the spiral bar is only an inch tall it doesn't leave much room for material to be channeled out the side of the pulley. The other problem is that it takes several rotations of the pulley to auger the material out from between the belt and the pul- ley. In some situations, where the tailpiece is frequently overrun, the spiral pulley can plug, (as shown in Figures 1 and 2) rendering the cleaning action ineffective. Wing pulleys can act like a large fan, stirring up the dust. E-wings (wing pulleys with end discs) reduce the amount of air being moved and reduce the dust being stirred. However, the nature of the wing pulley would still cause problems. The first is that the intermittent contact would help clean the belt as it generated a slapping action that shook the material loose from the belt. However this slapping action generated a lot of noise and a lot of belt vibration, which reduced the effectiveness of the skirtboard seals. The conventional wing pulley will throw the material outward. This is not an issue with snubs and bends, as the belt wraps over the top of the pulley and any material thrown out falls to the ground. However, in a tail or take-up pulley the belt wraps under the pulley, and the pulley will throw the material outward, away from the rotational axis and back onto the belt. This happens whenever the centrifugal force exceeds the weight, (force due to gravity). Setting these two forces equal to each other and solving for belt speed yields Equation 1. Eq (1) Belt Speed (FPM) = 240 * square root (pulley diameter in ft) Applying this equation to common pulley diameters used in mining has been tabulated in Table 1. The results show that most conveyors would only have to travel 240 to 340 feet per minute (fpm) to throw the material back onto the belt and not allow the material to fall onto the gusset. Note: Gussets do not have wear problems. 46 www.coalage.com Figure 2—Spiral Drum plugged with material. Problem As the material is thrown back onto the belt, it will be pulled into the pulley again and again. If more material falls onto the return side of the belt, it will add to the material being "recy- cled" by the tail pulley, as seen in Figures 3 and 4. The material will build up until it plugs the pulley, or it may work its way to the end of the pulley where the material can get trapped between the end of the wings and the framework, resulting in bent wings, as shown in Figure 5. As can be seen, the wings are bent in one direction, backward, i.e. opposite the direction of rotation. Analysis Since material doesn't fall and hit the gussets, what good are gussets? They fill in the gap between wings bracing one wing against another. The shape and position of the gusset is based upon the material falling and hitting the gusset, not the bracing of the wings. A better November 2012 Figure 1—Spiral Drum plugged with material.