MATERIALS HANDLING
of fugitive materials in the load zone. Sag (Ys) is proportional to the
weight (force) of the belt and bulk material (Wb + Wm) [newtons (lbf)] and the idler spacing (SI ) [mm], and it is inversely proportional to the minimum belt tension in the load zone (Tm) [newtons (lbf)]. [Fig. 1] To control fugitive materials, designers should consider managing the belt tension and idler spacing in the load zone to keep belt sag at no more than 3mm and preferably 0.0. Even with very little sag, if belt support is not continuous, fugitive materials can escape and cause wear. The example in Figure 1 shows that,
FIGURE 1: Belt Sag Calculation
allowing the rubber skirting to form a tight seal along the entire length of the chute. Not only does the cradle/ skirting combination eliminate gaps, it allows for greater airflow control through the loading, settling and stilling zone for superior dust and spillage suppression. However, a tight seal can induce
additional friction. Although field tests have shown minimal erosion of the belting or splice, it calls for slightly more power, the ongoing cost of which is no small concern to operators. This article provides the calculations required to determine the distance between idlers to reduce belt sag and will discuss how engineers can calculate the power requirements of a cradle system. The data will help operators decide
if installing preventive measures – such as modern conveyor transfer point sealing equipment like cradles and skirting – is more cost effective over the long run than reactive measures like cleanup and ongoing equipment replacement.
DETERMINING SAG AND IDLER DISTANCE In the sixth edition of Belt Conveyors for Bulk Materials, the Conveyor Equipment Manufacturer’s Association (CEMA) recommends that conveyor belt sag between idlers be limited to 2% for 35-degree idlers and 3% for
20-degree idlers. The CEMA method refers to limiting sag outside the load zone to prevent spillage. To fully prevent spillage, dust,
premature belt wear, wearliner depreciation, and skirt seal wear in the load zone, the sag must be significantly less than that recommended by CEMA. For example, using the CEMA method results in a recommended maximum sag between idlers of 12.5mm for 35° idlers and 19mm for 20° idlers. [Fig. 1] For loading zones with many gaps and tons of material escaping from the chute, field tests have shown that this is clearly unacceptable sag for control
with idler spacing of 600mm, there is 3.37mm of sag. If the idler spacing in the example is reduced to 178mm, the belt sag drops to 1.0mm. On the other hand, if a belt-support system such as an impact cradle or air-supported conveyor section is used, idler spacing (SI) can be assumed to be 0.0. The calculation then yields belt sag of 0.0, because there should be no sag when the belt is a continuous, flat surface.
CRADLES AND POWER REQUIREMENTS Belt-support systems have a significant effect on the power requirements of a conveyor. Changes in belt support will have a particularly noticeable effect on short or under- powered systems. Conveyor designers should ensure there is adequate conveyor drive power available to compensate for the additional friction placed on the conveyor when
FIGURE 2: Tension added to the belt due to sealing support
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