VERSUS / CONVEYING
Bucket elevators: centrifugal vs continuous discharge
How centrifugal and continuous discharge bucket elevators differ — speed, bucket spacing, feeding, materials and wear — plus belt versus chain, and what goes wrong in the head and the boot.
Part of the guide: Belt, pipe, screw, airslide or bucket elevator? Conveyor types compared
A bucket elevator is the plainest way to lift bulk material vertically, and the head is where most of its design lives. How the bucket lets go of its load decides the speed of the machine, how it is fed at the bottom, how much dust and breakage it makes and which materials it can carry. That choice comes before belt or chain, before capacity and before price.
How does each type discharge?
Centrifugal discharge
The buckets are spaced apart on a belt or chain and run fast — commonly in the region of one to two metres per second. At the boot they scoop, digging their fill out of the material fed into the casing. At the head, centrifugal force lifts the load out of the bucket as it passes over the pulley and throws it over the lip into the spout: a compact, high-capacity machine for fine, free-flowing material that does not mind being thrown, such as cement, sand and dry granules.
Continuous discharge
The buckets are mounted close together and run slowly — commonly well under a metre per second. They are fed directly from a loading leg above the boot, so they do not dig. At the head each bucket tips its load over the pulley, and the extended back of the bucket in front acts as a chute into the spout. Nothing is thrown, so lumpy, fragile, abrasive, sluggish or hot materials — clinker, glass batch and cullet, lime — arrive unbroken and without grinding the boot away.
Which one suits which material?
| Centrifugal discharge | Continuous discharge | |
|---|---|---|
| Speed | High; the throw depends on it | Low; gravity does the work |
| Bucket spacing | Spaced, so the load clears the bucket ahead | Close or continuous; the bucket ahead is the chute |
| Feeding | Scooped from the boot; fed on the up-leg side | Fed directly into the buckets above the boot |
| Materials | Fine, free-flowing, non-fragile, moderately abrasive at most | Lumpy, fragile, abrasive, sluggish, hot |
| Degradation | Higher: impact at the head and in the boot | Low: the load slides rather than flies |
| Dust | More, from the throw and the digging | Less, at lower speed |
| Wear | Boot and bucket lips from digging; head casing from the throw | Slow; bucket backs and the chain or belt carry it |
| Capacity per casing width | High, from speed | Lower per bucket, made up with larger, closer buckets |
Belt or chain?
- Belt
- Quieter, faster and cheaper per metre of lift; the usual carrier for centrifugal machines on fine material. Limited by temperature unless heat-resistant, by slip on the head pulley when the boot floods, and by the bucket-bolt holes through the carcass.
- Central chain
- A single chain with the buckets bolted to its back. Takes hot, heavy and abrasive material at low speed and tolerates dust in the boot; the usual choice for continuous discharge. Chain and sprocket wear are the maintenance items.
- Twin chain
- Two strands with the buckets slung between them, for the largest buckets and highest lifts. Heavier, costlier and noisier; both strands must be kept in matched tension so the buckets stay square.
- Temperature
- Standard rubber belts tolerate only moderate temperatures; heat-resistant grades extend the range and chains take what is left. Bearings, seals and casing expansion must be designed for the same temperature.
What goes wrong in the head and the boot?
- Back-legging — material that fails to leave the bucket at the head falls down the return leg and recirculates until the boot fills. Pulley diameter, speed and bucket shape must match so the throw lands in the spout.
- Boot flooding and choked feed — feeding faster than the buckets can take, or on the wrong side so they dig on the way down, packs the boot, overloads the drive and causes belt slip. Feed rate and position are design decisions.
- Belt slip — a flooded boot, worn pulley lagging or a take-up at the end of its travel lets the belt slip, and the elevator stalls with its buckets full. A speed switch catches it before the motor does.
- Bucket wear and loose buckets — lips worn thin by digging, and bolts working loose, until one bucket catches the casing and takes the next ten with it. Inspect buckets on a fixed round, through proper doors.
- Bearings and speed monitoring — bearing temperature, belt or chain speed, misalignment in the casing and a plugged discharge chute are the signals worth wiring in; the note on safety switches and flow aids covers each. Combustible dusts also call for explosion protection on the casing, a design item, not an accessory.
The discharge choice is one line in the wider comparison of conveyor types; the clinker handling and glass batch and cullet notes show why hot and abrasive duties land on continuous discharge. Both types are part of our conveying systems work; see the elevating duties in the Flow Finder.
QUESTIONS WE HEAR
Can a centrifugal bucket elevator handle lumps?
Small lumps in a mostly fine material, yes; large or frequent lumps, no. The buckets have to dig the material out of the boot, which breaks lumps and wears the lips, and the throw at the head scatters them. Lumpy materials belong on a continuous discharge machine, fed directly into the buckets at low speed.
Which bucket elevator is best for hot material?
A continuous discharge machine on a central chain is the usual answer for hot clinker, lime or similar. The low speed limits wear and dust, and the chain is not limited by temperature the way a rubber belt is. Bearings, seals, casing expansion and the boot design all have to be rated for the same temperature.
What causes a bucket elevator to back-leg?
The material leaves the bucket too late or too early to reach the spout, so it falls down the return leg. The causes are a belt speed that no longer matches the head pulley diameter, worn or wrongly shaped buckets, a spout set too high or too low, or sticky material hanging in the bucket. Check the speed first.