Bucket elevators are common industrial devices used
throughout bulk materials handling industries. They offer a
compact footprint for the vertical elevation of a wide variety
of bulk materials. There is little restriction on the elevation
height, and throughputs are broadly scalable. The mechanical
construction of a bucket elevator is relatively simple with
most elevators using a fabric reinforced ‘conveyor belt’ material
for power transmission and bucket attachment, large elevators
may utilise steel core belts, hybrid belts or chain. For
the purpose of this paper, a flexible belt is assumed in the
construction.
Typically, buckets are bolted to the belt using a purpose
designed bucket elevator bolt with a large diameter head
that embeds into the belt cover to provide a flush finish
on the underside of the belt. The buckets generally have a
flat inner/back wall through which the fasteners pass and
these determine the effective pivot of the bucket on the belt.
Figure 1 illustrate the shape of a Starco Jumbo bucket and the
mounting of these buckets onto a typical belt (hidden behind
the buckets).
Operationally, the buckets within a bucket elevator pass
through the lowermost section of the elevator called ‘the
boot’. The material to be elevated is supplied by chutes
on either the downward, or upwards strands of the elevator
located close to the boot end. A percentage of material elevated
will return to the boot through spillage at the head
pulley end. Irrespective of the mode of feeding, at some point
soon after the boot area, the buckets are filled to a level commensurate
with the nominal tonnage rate. Figure 2 illustrates
where these sections are on a laboratory bucket elevator.
During the traverse between the head and tail pulleys, the
bucket, its contents, and the beltmust travel at the same velocity.
However, as the bucket passes around either pulley, the
tip speed of the bucket must increase by the ratio of bucket tip
radius divided by pulley radius.As it is clearly impossible for
this increase in speed to occur instantaneously, it is evident
that the belt deforms over a finite time period to facilitate the
acceleration phase. This has been observed with high speed
video footage revealing the bucket undergoing a damped
oscillatory response to the step change in speed (Table 1).
During these oscillations, the material carried in the bucket
will have a greater propensity to discharge due to a reduction
in the contact force between material and bucket in the local
deceleration phase.