Production increases such as a boost in cargo volume, material size or conveyor speed can result in changes in the conveyor tracking, spillage and safety. One consideration is how loaded (or overloaded) the belt is compared to the trough angle. Proper belt width and cradles with adjustable supports should be factors to consider.
By R. Todd Swinderman, President Emeritus/Martin Engineering.
The trough angle is initially selected based on experience or the existing idlers for standardisation. Belt width is selected by calculating the cross-sectional area of the bulk material by assuming a troughing angle, an idler with three equal roll lengths and the surcharge angle, lump size and flowability of the bulk solid being handled. There are two important cross-sectional areas to consider, CEMA 100% full and full edge-to-edge.
CEMA 100% full
The 100% full area is based on a standard belt edge required to prevent spillover between idlers as the belt sags on the carrying run. The full edge-to-edge loading is used to calculate the maximum potential load on the structure. The best practice is to select the belt width based on 85% of the CEMA 100% cross-sectional area to allow for surge loads, off-center loading or normal mistracking (see Fig 1).
If operators are planning on upgrading the transfer chute to prevent spillage from mistracking, it may be possible to use a non-standard belt width, because the wing lengths of most troughing idlers allow more room than what is considered acceptable for mistracking belts. It may also be possible to change the standard trough angle or use a custom designed idler to allow for more cross-sectional area. Two common techniques can be incorporated into a new or complete conveyor design to make future upgrades less costly.
The first technique is changing the trough angle of the idlers to raise the capacity by increasing the cross-sectional area. In new designs, consider using 20-degree idlers. Upgrading to 35-degree idlers is a 27% increase in cross sectional area and going from 20- to a 45-degree trough angle is a 37% increase. Although 35-degree idlers are fairly standard, it is important to note that for retrofit upgrades, going from 35 to 45-degree idlers is only an 8% cross-sectional area increase (see Figure 2).
Full edge-to-edge
The second common technique for new construction is to design the structure for the next wider belt width and use CEMA wide-base idlers or Martin® Slider Cradles. The mounting dimensions of wide-base idlers or bumpers allow for a future replacement with a wider belt.
For example, if the structure for the 1 200 mm wide belt and 20-degree surcharge angle using 35-degree trough idlers was designed for wide base idlers, the belt width could be increased to 1 400 mm, resulting in a 33% capacity increase with the same trough angle and belt speed. Changing from a 35 to 45-degree trough angle and the wider belt and idlers would result in a 90% increase in cross-sectional area. This method is not often used, because there is resistance to increasing capital cost for a wider and higher load-bearing structure, higher material mass and larger drive. However, it is an excellent approach if there is an expectation of increasing capacity in the future.
