OPERATIONS MANAGEMENT


standard cross-sectional, As, area by 15% and provides recommended ranges of belt speeds for different materials and applications. The best approach is to base the belt width and speed on the life cycle cost and not on the price of installation. When increasing the cross-sectional area or belt speed, confirm that the existing belt, idlers and structure can handle the increased load. The design options of a new system dwindle as the price outweighs the cost. By applying the traditional 2/3 belt


Figure 3: Free edge calculation


COMPARISON OF TROUGH ANGLE FREE EDGE & CAPACITY 1,200 MM BELT (MM) Trough Angle (deg) Free Edge (mm)


0 CEMA Std. Belt Edge (mm)


Recommended CEMA Guide 01 – Critical Mistracking (mm)


Available for Sealing System (mm) CEMA Standard Cross-sectional Area, As (m2


CEMA Dms Material Rubbing on Skirtboard Sides (mm)


Figure 4: Table of results In Figure 4, the trough angle affects


the free edge and therefore the space for the sealing system. In addition, mistracking becomes problematic for 35- and 45-degree trough angles. A common low-cost OEM sealing system consists of an internal wear liner, a vertical external seal and angle iron clamps. The seal clamping arrangement needs to clamp close to the bottom of the skirtboards to reduce seal deflection. As the trough angle increases, the space available for the clamp decreases, raising the danger of the clamp rubbing on the belt and damaging it. The external seal is typically a


13mm thick rubber slab. A common problem with OEM sealing systems is the skirtboard and liner are often spaced well above the belt leaving the rubber seal to contain the material pressure. This means the cargo is


26 www.engineerlive.com


continuously rubbing on the liners or exerting pressure on the seal. This prematurely erodes the rubber seal and wearliner resulting in leakage and belt damage. Material trapped in the space between the liner and the seal can cause belt grooving requiring more frequent cleaning and maintenance attention. The capacity (cross-sectional area


x belt speed x bulk density) rises by increasing the trough angle but often this is the only consideration when upgrading. Spreading the distance between the skirtboard walls reduces the available free edge and widening the belt increases the initial price. The solution is often to increase the belt speed while holding the tons-per- hour constant. This lowers the cross- sectional area needed to convey the same volume at the slower speed. CEMA recommends reducing the


200 88.9


± 60 51.1


20


188.6 88.9


± 60 39.7


) 0.062 0.128 40 73 35


160.9 88.9


± 60 12.0


0.157 65 45


126.6 88.9


± 60 -22.3 0.175


41


width rule for the skirtboard widths and loading the conveyor at the CEMA 100% full (As) cross-sectional area, designs can stay competitive and efficient. Not applying these principles results in material continuously rubbing on the internal structure walls, prematurely eroding them and causing more spillage. The height of material rubbing on the skirtboards or wearliners is called Dms in CEMA terminology. The theoretical Dms is 40 mm and for the 20-degree troughed belt and Dms is 73 mm for the flat belts used in this article. While a little more spillage may


seem to be a minor cost compared with the next wider belt, consider the lifetime of additional cleanup and maintenance costs. Although there are other causes, spillage and wear are directly proportional to the square of the belt speed. There are also labor and replacement component costs, as well as indirect costs associated with more frequent cleaning and maintenance. Recent projects applying the


principle of long-term cost over initial installation price resulted in an increase of 25% to belt life, 40% to idler life, 60% reduction in cleanup. Improving the access to transfer points made maintenance easier and faster which is widely acknowledged to drastically increase workplace safety.


INCREASING PRODUCTION Almost invariably the capacity is expected to increase to meet production demands over the life of a conveyor system. If operators anticipate that production capacity may need to be increased, they should consider future production needs early and factor those into the original design. Rather than derating


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