38 Powder and Bulk Engineering, July 1998

Do’s and don’ts of belt conveying Andy Marti Martin Engineering

Have belt conveying problems got you down? The material buildup and spillage resulting from these problems can damage your belt and idlers and cre- ate unnecessary friction that wastes conveyor power. Cleaning up the spillage can pose health risks to workers and increase your labor costs. By following the simple do’s and don’ts outlined in this article, you can keep material on your belt and im- prove your conveying efficiency.

belt conveyor is the heart of many bulk materials handling operations. When the conveyor operates A well, material moves efficiently. But when the

conveyor’s operation is less than smooth, it can place a stranglehold on productivity and profits. Here’s some ad- vice on how to keep your belt conveyor running smoothly.

Don ’t be penny-wise and pound-foolish. If experience teaches any lesson in bulk materials han- dling, it’s that conveying demands only increase over time. Your material volume grows, you increase belt speed, and your plant’s expectations increase - often as the con- veyor maintenance schedule is allowed to deteriorate.

In many cases, cutting corners on the initial belt conveyor design proves costly in the long run. Conveying demands increase and adverse conditions - such as changes in your material’s properties -begin to affect the operation.

To avoid this problem, overdesign your belt conveyor at the outset. For instance, use a wider belt, a more powerful

motor, and more belt cleaners. This gives your operation room to grow by ensuring the conveyor can accommodate later volume increases and changing conditions with mini- mal expense and disruption.

Do plan for the worst case. Design your belt conveyor for the worst case rather than just routine operations. This enables the conveyor to oper- ate under the worst possible conditions - when operating in a freezing or wet environment, carrying abrasive or wet material or large chunks, receiving material dropped from great height or at an angle onto a transfer point, or carrying material at very high flowrates.

You need to anticipate which worst-case conditions your conveyor will encounter. For instance, virtually any mate- rial handles differently when wet rather than dry. Because your belt conveyor will probably handle the material in both states, the conveyor - including the loading and discharg- ing hoppers, belt, rolling components, and other accessories -needs to accommodate both dry and wet material.

Planning for the worst case means installing loading chutes and skirtboards that are large enough to handle two of your material’s largest lumps when they’re locked to- gether. You’ll also need to use cleaning components that can remove sticky materials and return carryback to the main material mass. And installing a tracking system will keep the belt away from the conveyor’s steel structure.

A belt conveyor designed for worst-case conditions will be “overkill” for your normal operating conditions. But overdesigning the conveyor will improve its wear life and reduce its service requirements when operating under these normal conditions. When conditions change for the worse, your conveyor will stand up to the challenge.

Powder and Bulk Engineering, July 1998 39

Many belt-support structures can control your belt’s line of travel: closely spaced idlers, impact cradles that absorb the force of material loading, as shown in Figure 3a, and side rail belt-support cradles that stabilize the belt edges to minimize material entrapment and improve sealing along the belt’s edges. Which device will work for you depends on your belt conveyor and material.

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Belt-tracking control methods

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Do control your belt’s tracking. Belt wander can reduce both your belt’s service life and your conveyor’s efficiency. It can be caused by any of these problems: off-center material loading, misaligned stringers (due to a foundation shift or an impact by heavy equipment), material buildup on pulleys and idlers, and belt or belt splice faults.

You can use several techniques to keep the belt in align- ment and control belt tracking. You can install belt-track- ing components such as training idlers, as shown in Figure la, and multiple-pivot belt-steering devices to keep the belt aligned. To correct a wandering belt, you can knock the idlers to introduce deliberate mistracking that compen- sates for the wander, as shown in Figure lb. To determine where the mistracking arises and how you can correct it, you can survey the conveyor’s alignment with laser equip- ment, a conventional surveyor’s transit, or piano wire.

Be careful with belt-tracking components on a belt that runs in two directions. The devices can steer the belt in the wrong direction when belt travel is reversed. And don’t let the forces applied to the belt by different belt-tracking components fight each other.

From the day you start up your belt conveyor, keep the belt in the conveyor’s center. This will prevent costly damage to the belt edges and the equipment’s steel structure.

Don’t load material off-center. Loading your material onto the belt at any point other than the belt center can cause mistracking, spillage, and other problems. But loading your material onto the belt center is easier said than done, particularly if you have angular or non-in-line transfer points.

You can use several devices to direct material flow into the belt center, including properly adjusted deflectors, wear liners, baffles, screens, grizzly bars, and curved loading plates, as shown in Figure 2. These can help direct mate- rial onto the belt in the same direction and at the same speed that the belt travels.

Once you correct an off-center loading problem, record de- tails about your solution, such as the design and position of a deflector. These records will come in handy when you need to replace the deflector after wear has altered its shape.

Do stabilize the belt’s line of travel. Stabilizing the belt’s line of travel is the key to reducing spillage, controlling belt tracking, ensuring belt safety by minimizing wear, and maintaining conveying efficiency. A belt with a stable line of travel runs smoothly - as though across a tabletop -particularly through a loading zone.

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40 Powder and Bulk Engineering, July 1998

The belt-support structure should extend from the trans- fer point to the point where the material has settled into the correct conveying profile. As with money, it’s better to have a little extra belt support than to fall short.

Modifying a wing-style tail pulley is another way to maintain your belt’s stability. This type of tail pulley lightly impacts the belt and prevents material lumps from lodging between the pulley and belt. But the pulley’s pad- dle-wheel design can bounce the belt, destabilizing its line of travel and defeating the function of belt-support and sealing components. By wrapping a band of steel around the pulley, as shown in Figure 3b, you can pre- serve its self-cleaning function while minimizing the bounce imparted to the belt.

Don’t allow pinch points. Install all skirtboards (and the wear liners inside them) so they gradually open - in both horizontal and vertical planes -in the direction of belt travel, toward the transfer point’s exit. This prevents pinch points where material lumps can be wedged in by the belt’s motion and abrade the belt cover.

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When placing sections of skirtboard or wear liner adjacent to each other, install them carefully so their edges align precisely. Otherwise, the edges can form a sawtooth pat- tern that creates pinch points, as shown in Figure 4. These pinch points can trap material lumps, which can quickly damage the belt.

42 Powder and Bulk Engineering, July 1998

Don’t expect elastomer sealing strips to contain the full material load.

If you’ve installed elastomer sealing strips along the bot- tom of your skirtboard to keep material on the belt, don’t expect the strips to contain the side forces imparted by the material load. No elastomer strip can contain this weight.

To help control spillage and dust, use a skirtboard with at- tached replaceable wear liners to keep pressure from the material load away from the sealing strips. With the mate- rial load away from the edge, the strips can do the job they’re designed for: keeping stray fines and dust on the belt.

Don’t rely on a one-layer barrier against spillage and dust.

Attach wear liners inside your skirtboard and sealing strips outside the skirtboard to provide an effective multiple-bar- rier defense against spillage. For best results, use sealing strips composed of two layers: a primary seal clamped against the chute wall that keeps material lumps from exit- ing the main material mass and a secondary seal that lays on the belt and contains fines or dust pushed under the pri- mary seal, as shown in Figure 5.

You can also use dust curtains inside the enclosed skirt- board to slow the dust-laden air’s movement above the belt, giving the dust a chance to fall back to the moving belt before it can escape with the air.

Another spillage barrier is a tail-sealing box, which is a tailgate-like structure with a sealing strip attached inside it. You can attach the structure to the chute’s back wall at a transfer point to form a one-way seal that prevents mate- rial from rolling out the back of the transfer point.

Do install multiple belt cleaners. Installing and maintaining an effective belt cleaning system is the best way to control carryback. A system with multiple cleaners provides more than one pass at removing the mate- rial. The system typically includes a precleaner and one or more secondary cleaners, as shown in Figure 6.

The precleaner is mounted against the head pulley’s face to remove most of the material. The secondary cleaners are installed further along the belt’s return side to remove residual fines. In some cases follow-up cleaners are posi- tioned further back from the head pulley along the belt’s return side to remove any remaining material.

Consider these criteria when selecting and positioning belt cleaning components:

Choose safe components: Use belt cleaners that mini- mize or eliminate risk of damage to the belt, belt splices,

or belt cleaners themselves. The belt cleaners should apply low pressure to the belt as they clean it and allow belt splices and other obstructions to pass without dam- aging them.

44 Powder and Bulk Engineering, July 1998

Place components farforward: Install the belt clean- ers on the belt’s return side as close as possible to the head pulley’s face. This removes material as close as possible to the trajectory of the material discharged from the conveyor.

Place components out of theflow: Install the belt cleaners below the material’s discharge trajectory so they aren’t barraged by material lumps.

You may also need to install additional components, such as an expanded dribble chute or a scavenger conveyor, to catch material removed by the belt cleaners and return it to the main material flow.

Don’t allow material lumps to damage your tail pulley.

A material lump caught on the belt’s return side will be carried into the tail pulley, where the lump can damage the belt and cause mistracking. To prevent these prob- lems, you can install a tail plow of various shapes on the

belt to remove material. Use a V-shaped plow on a belt that travels in only one direction and a diagonal plow on belt that travels in two directions, as shown in Figures 7a and b.

Don’t select belt conveyor components without considering maintenance requirements.

When choosing a belt conveyor and belt cleaning system, review maintenance requirements. By considering what maintenance work the equipment will need and how much effort that work will require, you can get a better idea of whether you’ll be able to maintain the equip- ment’s performance.

For instance, must a worker lie down on a dust-covered floor to perform a particular maintenance task or can the worker stand? Can each conveyor component be serviced without removing other components? Must the conveyor be shut down for a long period to allow safe service, or is there a method that allows maintenance work during a brief shutdown or, better yet, while the conveyor is run- ning? Does the conveyor manufacturer have a realistic

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understanding of your operating conditions and of the available service timetable in your plant?

Remember: A belt conveyor and conveyor components that are designed to be easy to service will simplify - and, hence, encourage - routine maintenance.

Do properly service your conveyor and components. Without proper maintenance, no belt conveyor - no mat- ter how well it’s designed - can sustain high perfor- mance. Your maintenance workers must be properly trained, equipped, managed, and motivated. More impor- tantly, they need to be aware of what service the conveyor requires and what can happen if the conveyor doesn’t re- ceive that service.

For specialized components such as belt cleaners or belt training systems, you can contract an outside mainte- nance company to provide skilled maintenance service.

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Do protect your investment. -.

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rn Do select suppliers who will be a continuing resource E. for your operation.

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Your belt conveyor represents a large investment - in its steel structure, belt, idlers, and other components. What’s

ciency. If the conveyor goes down, so does the rest of your operation and its profitability. Do everything you can to protect and improve your conveyor investment, from in- stalling belt-speed monitors and belt-alignment switches to providing regular and appropriate maintenance and

more, the conveyor is the key to your operation’s effi-

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3 ra ?. veyor and component suppliers that not only can deliver

equipment and components but can provide these “value- added’ benefits:

Train your workers to operate and maintain the conveyor.

rn Lend problem-solving expertise.

rn Service the conveyor after it’s installed.

Forming this kind of partnership with your suppliers is the best way to achieve continuous conveying improvements over your equipment’s service life. PBE

For further reading Find more information on belt conveyors, their operation, and related components in articles listed under “Mechani- cal conveying” and “Mechanical conveying components,” pages 101-102, in Powder and Bulk Engineering’s com- prehensive “Index to articles,” December 1997.

Andy Marti is marketing communications manager at Martin Engineering, One Martin Place, Neponset, IL 61345; 309/594-2384 cfax 309/594-2432). This material has been adapted from a book he coauthored, Founda- tions*: The Pyramid Approach to Control Dust and Spillage from Belt Conveyors, published by Martin Engineering.