Conveyor chain

Section 2
42 I Installation, maintenance & designer guide
Designer Guide
BOX SCRAPER CONVEYOR
Description and chain type
The box scraper type of conveyor can use either
a single strand or two strands of chain. The
general construction is that of an enclosed box
or trunking in which the chain is submerged in
the material. The conveying movement relies
on the ‘en masse’ principle, where the cohesion
of the particles of material is greater than the
frictional resistance of the material against the
internal surface of the box. Because of this
feature, a remarkably large volume of material
can be moved by using flights of quite small
depth.
In general a depth of material approximately
equal to 2/3 of the conveyor width can be
successfully conveyed by the ‘en masse’
principle.
The bottom surface of the box which supports
the material is also used to carry the chain (fig.
33).
Fig. 33
When conveying non-abrasive and free flowing
material, such as grain, a chain speed of up to
0.5-0.6m/sec is practicable. For aeratable
materials however, as for example cement or
pulverised starch, the chain speed must be
reduced to 0.25m/sec maximum.
Excessive speeds reduce efficiency as the chain
and attachments tend to pull through the
material, leaving the top strata either
stationary or moving at reduced speed;
furthermore, turbulent conditions may be set
up. For stringy, flaky and sticky materials, a
speed of 0.2m/sec should not be exceeded.
Abrasive materials increase the amount of
maintenance required, and to keep this within
reasonable bounds the chain speed should not
be more than 0.15m/sec. In ideal handling
situations, e.g. dry grain, it is possible for the
speed to be increased above the 0.5-0.6m/sec
as the conveyor width increases. The effect of
the side friction, which causes a ‘boundary
layer’ of slower moving (and almost stationary)
material becomes less significant as a
percentage of the moving mass of material. On
wide scraper conveyors speeds of up to
0.8m/sec are common.
The operating principle depends, as stated, on
the material having free flowing properties.
Conveying can if desired be carried out contradirectionally
by using both top and bottom
runs of the chain. Operation is confined to
straight sections, but these may incline from
the horizontal. The amount of inclination is
largely governed by the repose angle of the
material and depth of the scraper. Effectiveness
of flow can be prejudiced on inclines, and when
handling grain for example, the maximum
inclination from the horizontal should not
exceed 15°.
Some loss of conveying efficiency will be
caused by the inclines which will become
progressively more significant as the angle
approaches 15°.
The feed-in to the box may take the form of a
manually fed side chute or a hopper with
regulated feed. Alternatively the chain
assembly itself can function as a regulator, as
shown in fig. 34
Fig. 34
DRIVER
FEED OFF OR
DISCHARGE APERTURE
Standard chain of any size or strength may be
adapted to the system. It is usual for the chain
rollers and scraper plate depth to be equal to
chain plate depth thus ensuring that as flat a
surface as possible is moving along the scraper
floor. Bush chain is sometimes used, but the
roller type is recommended wherever possible
to avoid high pressure movement between the
bush and the sprocket tooth as the chain
engages with the sprocket.
The chain pitch is normally governed by the
required proportions of the box allied to the
linear spacing of the scraper. Sprocket sizes are
generally governed by box proportions, 8 or 12
teeth being commonly used. Head and tail
sprockets are normally cast iron with cast teeth
and should have relieved form to reduce
packing of material between the chain and the
teeth. See fig. 55, page 96.
Integral L attachments as illustrated in fig. 35,
may be used in either single or double strand
light duty systems.
HOPPER
Fig. 35
For some series of chain double strength
versions are available, which are dimensionally
identical to the base chain but have twice the
breaking strength. However pin/bush bearing
areas are as the original chain.
Suitable applications for double strength chain
are on systems with long centres but limited
use, where chain wear would not be a problem,
where shock loads are likely on an intermittent
basis and where failure of chains would be
catastrophic to production.
In addition to using L attachments, special
scraper attachments are available, an example
of this is shown opposite.