Bucket elevators and other lofty ideas...

Optimum performance is the result of good maintenance and common sense.

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By Malia Garner

PlantServices.com

Types of elevators

Centrifugal discharge elevators are the most frequently used type for free-flowing, fine or small lump material, such as grain, coal, sand, clay, sugar, cullet and dry chemicals. Buckets are "A" or "AA" malleable iron or steel and mounted at intervals on a belt or chain.

Figure 2. Buckets types. "A" (left) and "AA" (right)

They usually operate vertically but are sometimes inclined. Buckets are loaded by a combination of material flowing into buckets and by scooping from a boot, the lowest part of the elevator. Speeds are relatively high and governed by head wheel diameter and its relation to centrifugal discharge action.

Figure 1. Four styles of bucket elevator

Perfect discharge elevators are similar to the centrifugal discharge type, except that buckets are snubbed back under head sprockets to gain a more complete discharge at a slower chain speed. The elevators usually handle light aerated, dusty and sticky materials that won't discharge cleanly from a centrifugal discharge elevator. The "A" or "AA" malleable iron or steel buckets are mounted at intervals on two strands of chain. Providing a capacity equal to the centrifugal discharge type requires larger buckets or closer spacing.

Continuous bucket elevators are recommended for large capacity heavy-duty service. The buckets are steel, mounted continuously on single or double strands of chain, or on a belt. The elevators can be either vertical or inclined. As the buckets pass over the head wheel, the flanged front of the preceding bucket acts as a chute to lower the material to the discharge spout. This discharge method, plus the slower speed, minimizes breakage of fragile materials. These elevators are not the self-digging type. As a result, they require a loading leg and a deeper pit than other elevator styles.

Super capacity continuous bucket elevators handle the same materials as the continuous bucket elevators in the larger capacities. They are used for run-of-mine or sized coal, stone, ore and the like. The steel buckets, mounted between strands of roller chain, operate slowly. The construction of the bucket provides a greater capacity than the standard continuous type, but with the same means of loading and discharging. The elevators are built in both vertical and inclined styles. The vertical model has steel guides and the inclined type has tracks. Inclined types can be either open or enclosed.

Operating speed

The elevator's capacity is a function of its speed. The maximum chain speed that still provides proper discharge varies according to elevator type and head sprocket diameter. Table 1 gives the recommended speeds for various combinations of type and head sprocket diameter.

• Recommended bucket speeds for vertical chain elevators are as follows:
• Centrifugal discharge elevators (190 to 275 fpm).
• Perfect discharge elevators (110 to 140 fpm).
• Continuous Bucket elevators (90 to 150 fpm).
• Super capacity continuous bucket elevators (60 to 110 fpm).

For easy pick-up in a digging boot, the foot wheel diameter should be three-fourths that of the centrifugal discharge head wheel. For grain and similar uniform, dry, granular, non-abrasive materials with bulk densities from 50 to 60 lb. per cu. ft., increase centrifugal discharge speeds by 15 or 20 percent. For light powdery material, decrease speeds by 10 to 15 percent. If buckets of centrifugal and continuous elevators are mounted on belts, speeds may be increased 20 percent.

Maintenance and troubleshooting

Efficient bucket elevator performance depends on a good preventive maintenance schedule. Start with a systematic visual inspection of the complete bucket elevator assembly and related components. Repair minor defects observed during operation as soon as current elevator operation has ceased. In the event of a major deficiency that could damage equipment seriously or injure operators, stop the operation immediately and make the repairs.

Proper maintenance includes cleanliness of the equipment and components, regular and periodic inspections, and immediate replacement of defective components and parts. Good housekeeping is critical to maintaining an efficient operation.

Look for signs of structural damage and cracked or broken welds. Pay particular attention to areas subjected to great stress. Repair or replace as necessary.

Inspect the elevator frame and supports for signs of looseness, excessive corrosion, rust and damage. Repair or replace as necessary. Scrape and paint as necessary.

Look for loose bolts, screws and nuts, and tighten as necessary. Replace missing or damaged components. Inspect tapped holes for thread damage. If they are cross-threaded or if spalling is evident, retap the hole to the next larger size. If retapping will weaken the part, or if part cost makes it uneconomical, replace the damaged part. At times, chasing threads with the proper size tap or die is adequate.

Inspect shaft bearings for excessive wear or damage and replace as necessary.

Chains

Before working on the chains or performing any maintenance on the bucket elevator, follow standard lock out/tag out procedures.