Excessive sag between conveyor idlers can cause load shifts that abrade belts, spill materials, and in the case of steeply sloped belts, create an avalanche. Belt sag can be kept to a maximum of 2% with conveyor idler spacing at four-foot intervals along the carrying side. By increasing the tension, the idler spacing can be increased and still maintain the 2% maximum sag.
Sag can be controlled by spacing idlers closer together at the tail pulley and farther apart towards the head. This method saves on belt wear, power and the total number of idlers required.
The principle behind this concept is simple. Belt tension is lower at the tail end, so that sag is more of a problem. In addition to possible load shifts, material jamming and spillage can occur between loads and skirtboards. To counteract sagging in this area, idlers are spaced at, perhaps, five-foot intervals for half the flight, and at six-foot intervals for the remainder. In some instances, idlers must be placed 12 to 18 inches apart to prevent excessive spillage and belt vibration.
Although six feet is generally considered the maximum spacing between carrying idlers, a spacing of eight to ten feet can be used on very high tension or heavy-duty troughing applications. On undulating or declining conveyors, tension should be calculated for all loading conditions as the conveyor accelerates or decelerates. Idlers should be spaced to minimize belt sag along these systems.
To avoid conveyor belt edge overstress or idler junction fatigue, the curve radius and idler spacing must be designed properly. In cases of space limitations a lower trough angle may have to be installed. The drawback to that solution is reduced conveyor capacity.
Troughing idlers through the convex curve should be positioned on the curve arc, not on structural chords, and may require shimming to fit the arc profile. Idler spacing should be no more than half the spacing distance on the remaining idlers on the conveyor.
On concave curves, radii should be large enough to prevent bending of belt edges. Idlers should be spaced to keep sag to a minimum when the belt is fully loaded. As with a convex curve, idlers should be placed on the arc of the curve - not on the structural chords. Shim idlers, if necessary.
As a belt moves from a flat to a troughed configuration, the stress of the change can be eased through the use of idlers with adjustable concentrating angles. These idlers minimize junction strain through a gradual curving of the belt to a troughed contour.