OPERATIONS MANAGEMENT B
elt conveyor trough angles are not an arbitrary matter of ‘preference,’ they are based on precise decisions made
according to several operational factors. Material size, the way it settles when loaded (angle of surcharge), and how it centres when the full trough angle is achieved are all factors that need to be considered both in the initial design and during retrofits. The selection of the conveyor belt
trough angle is also a compromise between the belt width, the skirtboard width, liner wear, throughput and the desired reduction in spillage. The balancing of these variables is important for reliable and safe operation over the long term. Failure to examine this issue may result in lower capacity, increased liner wear, excessive spillage and belt damage.
THE CEMA GEOMETRY In Chapter 4 of Belt Conveyors for Bulk Materials, 7th edition published by The Conveyor Equipment Manufacturers Association (CEMA), there are dimensionless ratios developed so that any 3-roll idler configuration can be evaluated.[1] The CEMA terminology is being used in this idealised analysis of the loading chute cross-section. For simplification, the thickness of
the belt and skirtboards are assumed to be zero, which is customary in bulk
Figure 1: CEMA flat belt capacity Figure 2: CEMA troughed belt standard capacity
material conveyor belt engineering. The tolerances this introduces are minimal compared with the normal variations in the material’s bulk density and other factors such as loading and weather which affect throughput. Only one configuration of belt width geometry is studied but the method applies to any belt troughed by three roll idler sets. Similar ratios can be developed for non-standard idlers with more or less than three rolls. The detail of interest is the belt’s Free Edge (FE) that is
available for the sealing system and belt mistracking. [Fig 1 & 2] The theoretical CEMA cross-
sectional area that a particular arrangement can convey is called the “CEMA Standard Area” (As). The area is developed using the standard edge distance and the bulk material surcharge angle. The top surface of the Standard Area is a circular arc tangent to the surcharge angle beginning at the Standard Edge Distance, Bwe. The standard edge distance is
calculated through a formula and is intended to prevent material spillage from the belt as it sags between the widely spaced carrying idlers outside the load zone but not to determine the width of the skirtboards. The FE width of the skirtboards should be based first on reducing spillage by having adequate room for a side sealing system and secondly on how much bulk material is rubbing on the inside of the skirtboards or wearliners. [Figure 3] While holding the skirtboard width
ratio constant, the change in trough angle results in decreasing free belt edge as the trough angle increases, while the standard cross-sectional area, As, increases with a larger trough angle. The results for the four common trough angles are shown in Figure 4. The CEMA mistracking allowance is based on the dimensions of the standard CEMA idlers and pulleys.
www.engineerlive.com 25
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52
