Bucket elevators are often used to raise free-flowing bulk solid materials to a higher elevation in a range of industries, including chemicals, minerals, foods, wood products and wastewater treatment. As the name implies, bulk solid materials are conveyed via buckets, which are firmly attached to a belt or chain. As shown in Figure 1, each bucket gets filled as it pushes through a pile of material at the elevator’s bottom inlet (or boot) section. The buckets then travel upwards and empty at the elevator’s top (or head) section.
Bucket elevators excel at conveying material upwards vertically, which most other mechanical conveyors struggle with, especially if the vertical distance exceeds 10 or 20 feet. Bucket elevators can be small or large and can convey over short or long vertical distances. One of the most common applications is filling silos — the material is moved to the top of the bucket elevator, where it falls out of the bucket into spouting, through which it flows by gravity into the silo.
Industrial bucket elevators are available in a few configurations. Centrifugal bucket elevators rely on centrifugal force to fling material out of the bucket as it passes over the head pully at high speed. Belt speeds of 225 to 300 feet per minute are typical, and capacities can be very high, up to 5,000 cubic feet or 150 tons of material per hour.
Continuous bucket elevators move at about half the speed of centrifugal bucket elevators, so their capacities are correspondingly lower. However, the lower speed makes them more suitable for abrasive or fragile materials. Instead of flinging material out of the bucket at the top, the slower speed allows material to fall out of the bucket by gravity right after the bucket turns over at the top. The loose material then flows past the back of the previous bucket and into the outlet chute.
Most bucket elevators are unidirectional, moving material only in the vertical direction. However, a few designs use buckets that are not rigidly attached to the belt — they swivel to remain horizontal even if the belt changes direction. The buckets are then mechanically tipped at the discharge point to empty the buckets. These elevators typically form an S- or L-shaped layout in the plant.
Bucket elevator maintenance
Bucket elevators are not maintenance-free. Wear is a factor, especially since the buckets fill by pushing through a pile of material in the bottom boot. Regular maintenance and parts replacement are needed, so it is important to ensure that critical areas at the top and bottom are easily and safely accessible.
Belts and chains stretch, so an adjustment or take-up is required to maintain the correct tension. Various take-up designs are available. Frequent checks of slack or proper tension are important for reliable operation.
Moreover, economical instrumentation is available to monitor factors such as belt alignment, speed, bearing temperature, and material plugging. These can provide early warnings to greatly decrease downtime and increase safety.
Bucket elevator sizing
The capacity of a bucket elevator is a function of the bucket volume and how many buckets go past in a given time. In other words, volumetric throughput equals the volume of each bucket times the linear speed of the belt divided by the bucket spacing. Gravimetric throughput equals the volumetric throughput times the bulk density of the material.
Bucket volume is determined by the geometry of the bucket and the fill efficiency, or percent fill, during operation. Various bucket sizes are available from each manufacturer, but lengths of 6, 8, 10, 12, 14, 16, and 18 inches are common. As described earlier, belt speed is typically 225 to 500 feet per minute for a centrifugal discharge and about half that for other bucket elevators.
Typical spacing between buckets is 2 times the bucket’s projection, which is a measurement of the distance from the bucket’s outer lip to the belt, as shown in Figure 3.
Bucket elevator advantages
Bucket elevators are simple and reliable and are one of the most efficient methods for conveying materials vertically. They can handle a wide variety of free- to medium-flowing materials and are available in a wide range of standard sizes and capacities. While conveying rates can be small, high rates are more common, up to hundreds of tons per hour.
The initial expense is reasonable since bucket elevators are built in modular designs and then assembled for the particular application. Furthermore, components are economical since designs are standardized and mass-produced.
Bucket elevator disadvantages
Standard bucket elevators only convey in a straight direction, vertically upwards, and direction changes are not possible.
Bucket elevators have one inlet and one outlet. Accordingly, if a system has multiple destinations, then the bucket elevator’s outlet will need to feed a gravity diverter valve with spouting to each destination, as shown in Figure 4. Alternatively, another conveyor, such as a horizontal screw conveyor, may be used to move material to the alternate destinations.
Bucket elevators are unsuitable for powdery materials, especially those that might fluidize and flow out of the bucket. Furthermore, they do not work well with sticky, cohesive materials that could adhere to the buckets.
Since the buckets plow through a pile of material at the bottom boot, care must be taken when handling abrasive or heavy materials. Gravel, for instance, would tend to quickly wear the buckets or even tear them off of the belt. If material is not free-flowing, or if it tends to set up or consolidate, then a few of the buckets can be replaced with “digger” buckets that are oversized and more robust. The digger buckets loosen the pile so other buckets fill more easily.
The pile of material in the boot will not clean out at the end of a run. Therefore, if cross-contamination or hygienic cleaning is important, then a different type of conveyor may be better.
Like most mechanical conveyors, bucket elevators will generate some dust. If the dust is potentially harmful, then dust collection will be required. Dust exhaust ports and explosion protection devices can be incorporated into the bucket elevator housing.
As with all mechanical conveyors, regular maintenance is required.
Conclusion
Bucket elevators are useful and efficient for moving bulk solid materials vertically upwards and shine when high conveying rates are needed for free-flowing materials. Dust collection, wear, and maintenance must be considered, but economical instrumentation can help reduce equipment downtime.
Todd Smith has helped manage Kansas State University's Bulk Solids Technology Center for five years. KSU-BSTC is the only university-centered facility and staff in North America dedicated to helping industry with bulk solid challenges and education. Previously, Todd spent his nearly forty-year career in industry working with bulk solids — first at four DuPont plants, then at Mac Equipment (now Coperion) and Coperion K-Tron. Todd is also a member of Processing’s editorial advisory board.
KSU Bulk Solids Technology Center
