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Clearance Above the Conveyor Belt

Even under the most ideal conditions, steel skirtboards can be hazardous to the belt. Fluctuations in the belt's line of travel may allow the belt to move up against the steel, where it can be gouged or cut. In addition, material can wedge under the skirtboard to abrade the belt's surface.

It is critical to raise the bottom edge(s) of the skirtboard far enough above the conveyor so they never come in contact with the belt cover. As the distance above the belt increases, so does the difficulty in providing an effective seal. Skirtboard is sometimes installed with a clearance of several inches above the belt to facilitate belt replacement. When the steel is placed this far above the belt surface, it is virtually impossible to provide an effective seal on the outside of the skirtboards when there is side pressure.

Clearance Above The Belt
To reduce the risk entrapped material might gouge the belt, the skirtboard should open (or self-relieve) both horizontally and vertically in the direction of belt travel.

An ineffective seal perpetuates itself. Material leaks out, accumulating on idlers, leading to mistracking and other problems that result in an unstable belt line. The belt flexes up and down and wanders from side to side. Plant engineers and maintenance staffs, mindful of the need to prevent the belt from coming into contact with the chutework, increase the belt-to-skirting clearance. This dramatically increases the difficulty in sealing the transfer point, resulting in increased spillage. This increased spillage results in a continuing vicious cycle of belt wander, rolling component failures, and increased operating costs.

The closer the steel and belt are placed together, the easier it is to maintain a seal between them. It is critical to provide relief in the direction of belt travel. The gap under the steel should form a wedge-shaped opening that allows conveyed material to ride along the steel skirting and sealing rubber, rather than become wedged into an opening by the ceaseless force of belt motion. The skirtboard should open gradually, both horizontally and vertically, from the loading point in the direction of belt travel to permit entrapped material to free itself.

It is recommended that the lower edges of the skirt plates be positioned 6 millimeters (1/4 or 0.25 in.) above the belt at the belt’s entry into the loading zone. This dimension should be uniformly increased in the direction of belt travel to 9 to 12 millimeters (3/8 or 0.38 to 0.5 in.) as the belt exits the skirtboard. This close clearance cannot be accomplished unless the belt travel is stabilized within a plus-or-minus tolerance of 1.5 millimeters (1/16 or 0.063 in.) at the entry (tail pulley) end of the chute.

It is critical that the centerline of the skirtboard construction be in line with the centerline of the belt to prevent belt mistracking. If the two are not in line, the unequal forces from the cargo’s center of gravity and the friction against the skirtboard will cause a chronic mistracking of the belt and accelerated wear on the wear liners and skirt seal. With the steel positioned close to the belt line, it is critical to the safety of the belt that the belt be prevented from rising up off the idlers during is critical to the safety of the belt that the belt be prevented from rising up off the idlers during conveyor startup. This is one reason why the elevation of the tail pulley, commonly known as the half-trough arrangement, is not a good idea, as this practice encourages the belt to rise. The employment of using a half-trough arrangement is generally done in the interest of shortening the transition distance. It is important that the belting specifications and tension be calculated correctly to minimize the risk of the belting lifting off the idlers. Hold-down rollers can be installed to keep the belt on the idlers.

Rough bottom edges or warped steel can create difficult conditions, capturing material to increase the drag on the conveyor drive and/or abrade the belt surface. Ceramic blocks or wear plates must be carefully installed to avoid jagged or saw-toothed edges that can trap material or damage the belt. The rule is to maintain a smooth flow surface on the bottom edge of the skirtboard and eliminate all entrapment points. Skirtboard steel and chute liners must be installed very carefully, with all seams well matched.

The gap left between the skirt and the belt surface should be sealed by a flexible, replaceable elastomer sealing system applied to the outside of the skirtboard.

Topics: Conveyor Belt Component Damage, Material Spillage

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