How is the loading station affecting the belt conveyors?

Two of the more common reasons of belt failure are a direct result of the loading stations. These are: Holes being punched in the belt due to impact and carry cover deterioration due to abrasion.

IMPACT

Large product lumps such as minus 8" limestone, granite or heavier products with irregular shapes will punch holes in a conveyor belt if not properly introduced to the belt's surface.

The following are the more common methods of presenting large product lumps to the belt and minimizing the damage that these lumps will cause.

ROCK BOXES

Rock boxes can absorb the impact from large lumps as the box fills up first, then other lumps bounce off of the pile onto the belt. The purpose of the rock box is to transfer the impact energy to the box rather than the belt. The box also will allow the height of the free fall to the belt to be greatly reduced.

FEEDERS

Various types of feeders, such as belt feeders, pan feeders, reciprocating feeders and vibratory feeders can be utilized. The particular style of feeder that is used will be determined by product characteristics such as: lump size, product weight, moisture content and abrasion.

GRIZZLY BARS

Grizzly bars, if installed properly, can be used rather than a rock box. The bars need to be installed to absorb the impact energy from the lump and redirect the lump to the belt in the line of travel. Grizzly bars must be installed so that the bars do not fill up with product and clog the loading station. Grizzly bars also allow a bed of fines to be introduced to the belt before the larger objects fall to the belt. This cushion effect helps to reduce the damage from larger lumps.

CHUTES/SPOONS

Chutes, spoons and deflector plates are quite common methods for diverting material onto the belt and reducing the vertical impact onto the belt. All of these various methods serve the same purpose; The introduction of material to the center of the belt, loading in the direction of travel, loading product with velocity approaching belt speed, and reducing product freefall and impact to the belt.

CARRY COVER ABRASION DUE TO LOADING

Rubber cover abrasion due to poor loading is an ongoing problem for the conveyor operator. The first step in correcting this problem is the installation of a rock box, feeder, grizzly bar, chutes, spoons or deflector plates into the loading area. As previously discussed, the product needs to be introduced to the belt with a minimum of impact, loaded in the center of the belt, and product velocity approaching belt speed. As the product free fall decreases, the time required for the product to settle down on the belt also decreases. When products bounce around on the belt before settling down, the rubber cover gets cut, gouged and abraded.

Proper loading is best accomplished when products are loaded on the horizontal.

Loading on a decline belt conveyor increases product movement, increases abrasion and increases the opportunity for spillage. Loading on an incline increases cover cutting, gouging and cover wear.

Introducing large lumps at the feed point can result in impact energy that a given conveyor belt will be unable to absorb without damage to the cover and carcass. Impact energy is calculated in terms of foot-pounds of energy. The weight of the product times the free fall in feet to the belt will yield the impact energy imparted.

A lump of limestone that measures 12"x10"x18" equals 2160 cubic inches. 2160 cubic inches divided by 1728 (cubic inches in a cubic foot) equals 1.25 cubic feet. If limestone weighs 100 pounds per cubic foot, this lump would weigh 125 pounds. With a free fall drop of 7 feet, this lump would impart 875 foot pounds of impact energy to the belt. This example does not take into account sharp edges or abrasion. It is suggested that impact energy be calculated by the above method, then refer to the manufacturer’s catalog for guidance in belt selection.

In addition to the product side of the belt, you must also consider the support area of the loading station. The belt needs to deflect under impact.

Rubber impact idler should be placed so that the load is introduced to the belt between the impact idlers as much as possible. This will allow the belt to deflect under impact. If the deflection of the belt is excessive, then product will be trapped between the skirt boards and the belt. This causes additional cutting, gouging and abrasion.

When impact rollers are moved closer together, it is impossible to load product between idlers. Loading over the idlers is quite common. With closely spaced impact idlers, it is imperative that these idlers be soft enough to deflect under impact.

Another common method of impact absorption is through the utilization of impact beds. impact beds have a low durometer soft rubber that allows the impact energy to be transmitted through the belt, into the bed, and then absorbed by the bed. Impact beds are more suitable for applications where dust suppression or sealing is required. Impact beds are not recommended for heavy impact applications. Special care is needed to make sure the impact beds are kept clean and free of build-up to avoid causing conveyor pulley cover abrasion.

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