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JUNE 2018
REDECAMGROUP
CONVEYOR MAINTENANCE
Aconcern of many bulk material
handling operators is the damage to
conveyor belts from loading, belt wear
caused by cleaning devices and the
difficulty of cleaning damaged belts.
Belt wear from loading Since the conveyor belt is a major cost
element in bulk material transportation,
much attention is focussed on reducing
wear and damage. Wear from loading
often occurs over a long period of time,
from the discharge of material onto the
belt and from contact with conveyor
components such as idlers and belt
cleaners. This type of wear includes both
impact damage and frictional wear.
Damage to the belt can be caused
by a single event, such as tramp metals
or oversized lumps in the material flow
stream. Such sudden damage can result
in catastrophic failure that requires
immediate attention, demanding a system
shutdown.
The negative effects of long-term wear
are less dramatic and belt replacement
can often be scheduled for planned
outages to avoid affecting conveyor
availability.
Chute design One key to understanding belt wear from
loading is the chute. The development
of discrete element modelling (DEM) as
Figure 1: engineered chutes direct material flow to the receiving belt, centred and as close as possible to the belt's speed
applied to conveyor loading chutes has
provided the industry with a valuable tool
for verifying chute designs and predicting
conveyor belt wear. A survey of the
literature yields evidence indicating belt
life improvements of 40-300 per cent by
using DEM to optimise chute design.'
The primary objective of chute design
is to direct an uninterrupted flow of the
bulk solid from the chute to the receiving
belt, centred in the direction of belt travel
and as close as possible to the speed of the
receiving belt (see Figure 1).
While the interaction between the
belt and the bulk material is complex,
troubleshooting belt wear caused by
100
Figure 2: general wear of rubber based on impact angle
chute design can take
advantage of some
simple relationships.
Q)
Cii a: (;; Q)
~ 50 Q)
. ~ Cii Qi a:
0 o· 3o· eo· eo·
Angle of Impact
The first is the general
relationship between
material impact angles
and the wear rate of
rubber. Figure 2 shows
that as the impact angle
increases, the wear
decreases.
The second
fundamental principle
that can be applied to chute design to
minimise belt wear is the speed of the
bulk material stream, which is affected by
friction and acceleration due to gravity as
the load falls onto the belt. The coefficients
of friction between the bulk material,
chute and belt are important parameters
that are used in DEM programmes to
optimise the shape of the chute, producing
the desired exit velocity and direction of
the discharged bulk material.
Common chute designs include rock
boxes, inclined flat chutes and curved
chutes (see Figure 3). v. is the exit velocity
of the bulk material stream from the chute
and Vb is the belt speed. Other factors to
consider when designing the optimum
chute for a given application include
drop height and preferred liner materials.
However, in most cases, belt wear due
to the choice of chute design is greatest
with rock boxes, which do little to slow the
material 's velocity and introduce a large
amount of disruption as the load cascades
from one shelf to the next, then lands on
the moving belt at a near-perpendicular
angle.
Flat inclined chutes help shift the load
in the general direction of the receiving
JUNE 2018 ICR
II CONVEYOR MAINTENANCE
Figure 3: three different chute design approaches
Rock box chute
vb Rock box chute
Flat chute
vb Flat chute
Curved shute
Figure 4: comparison of loading velocities and vertical component, v.,
l::~ vb
Curved shute
belt's travel but can involve even greater
impacts than a rock box, depending on
the drop height. The violent landing takes
a constant toll on the belt, often creating
significant amounts of fugitive material in
the form of dust and spillage.
Curved chute designs minimise belt
wear from loading impact, as the bulk
material stream velocity can be most
closely matched to that of the belt-Figure
4 shows the relative differences in loading
velocity vectors. v.Y is the bulk material
stream velocity perpendicular to the belt
and is the primary factor in belt wear.
The wear of the belt is proportional to
the magnitude ofV•Y' so minimising this component through chute design is a focus
of a DEM analysis.
Figure 3 is a generalisation, but it shows
that the exit velocity of a curved chute is
the lowest of the three design choices. This
is due in part to the force resulting from the
curved chute, which tends to reduce the
impact velocity (V.) relative to a flat chute,
even if the basic discharge angles are
similar. Rock boxes may reduce chute liner
wear but can create significant belt wear
due to the relatively high vertical velocity
CONVEYOR MAINTENANCE
Figure 5: a single scratch can contain significant carryback
VoiScralch= Y2 X b X h x l
VoiScralch = Y2 X 2 X 1 X 1 000 = 1 000 mm3
and the resulting shearing action between
the bulk material and the belt as the load
gets up to belt speed.
Chute liners While belt wear is the main concern, a
significant amount of attention should be
paid to the selection of liners to prolong
chute life. Given the relative cost of the
belt compared to the chute in most
applications, the wear liners should
be considered sacrificial components.
SURFAI:E FEEDER
@spgr. = 1.0 = 1000 g/m or~
Belt Top Cover
Attention would be better spent on
improving chute design, selecting lower
friction liners, and making the liners
easier and quicker to change. Some
manufacturers have engineered new
designs for liners that can be serviced from
outside the chute, for example, eliminating
the need for confined space entry and
drastically reducing replacement time.
Cleaning of damaged belts The US Mine Safety and Health
Administration (M SHA) estimates that 85
per cent of all conveyor issues, including
wear, come from fugitive materials.
Fugitive materials are those that escape
the conveyor other than at the discharge,
including spi llage, dust and carryback.
Since carryback is a significant source
of fugitive materials, wh ich in turn are
a significant contributor to belt and
component wear, it makes sense·to focus
on adequate belt cleaning.
Cleaning efficiency depends on factors
such as material properties, the number of
belt cleaners, the mechanics of a particular
belt cleaner design and the belt surface.
While it is often expected that a conveyor
belt can be cleaned with an efficiency
approaching 100 per cent, even a brand
new belt has macro and micro defects
that make cleaning to this extent nearly
impossible. These imperfections can result
in as much as 60g/m 2 of carryback passing
a belt cleaner station with a new belt.
When the belt surface is damaged, the
amount of carryback that can be sh ielded
from belt cleaning in scratches and gouges
can be even more sign ificant, in the order
of 100-200g/ m2• Figure 5 shows how much
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II CONVEYOR MAINTENANCE
Figure 6: belt cleaner blade angles
Positive rake
carryback can be contained in a single
scratch measuring 2mm wide by lmm
deep in the belt top cover.
When selecting belt cleaning
equipment, much attention is paid to the
price of belt cleaners and replacement
blades. Like chute liners, belt cleaner
blades should be considered sacrificial
components, and the focus - rather than
being on price- should be on installing
an adequate number and appropriate
type of belt cleaners that can be easily
serviced. Cleaning damaged belts is best
accomplished using water in combination
with mechanical scrapers. In severe cases,
brush cleaners are effective in removing
material from damages such as skirtboard
grooves, but brush cleaners require more
frequent adjustment and replacement
than mechanical scrapers.
With a belt in good condition and
professional maintenance, a belt
cleaning station can usually control
carryback to within 10-100g/ m2• The
Conveyor Equipment Manufacturers
Association (CEMA), in its seventh edition
of Belt Conveyors for Bulk Materials,
has established a system for rating the
difficulty of the belt cleaning application
and for desired levels of carryback exiting
a cleaning station to aid users in specifying
belt cleaning performance, rather
than making decisions based on brand
preference or price alone.
Belt wear from cleaning In today 's demanding applications, it is
common to use three or even more belt
cleaners on a conveyor, which produces
concern over belt wear. Several research
studies have shown that there are critical
cleaning pressures and angles for the
various types of mechanical belt cleaners
using elastomer and hard metal blades.2
The pressure ranges are in the order of
15kPa for cleaners with a positive rake
angle and lOOkPa for cleaners with a
ICR JUNE 2018
negative rake
angle. Outside
of these ranges,
either above
or below the
target cleaning
pressures,
the amount
of carryback
passing the
cleaner, the
belt wear and
the blade wear
all increase.
There are
several research studies showing the
proper adjustment of belt cleaners and
documenting the use of water to keep
the belt cleaner free of debris, improving
cleaning efficiency by 5-15 per cent.3
A common belief is that more cleaning
pressure is the answer to ineffective belt
cleaning. In reality, increasing the cleaning
pressure beyond the target range will
result in additional belt wear and there
is an increased power consumption
penalty that often goes undocumented.
When dealing with rising energy costs
and carryback amounts that can easily
reach several tonnes per hour, the
selection, installation and maintenance
of belt cleaners should be performed by
professionals trained in their application
and service. New cleaner designs feature
constant pressure to maintain cleaning
performance through all stages of blade
life, particularly on high-speed belts and
those with multiple splices.
Figure 6 illustrates a variety of rake
angles for mechanical blade cleaners.
Designs using the different rake angles all
have positive and negative characteristics,
but the basic question of how much belt
wear is caused by the cleaner is similar for
similar rake angle designs.
Field data is scarce and it is difficult to
separate the dozens of variables that affect
belt cleaning from the factors primarily
responsible for the wear of cleaner blades
and belts. The best evidence of the trade
off between belt cleaning and no belt
cleaning would be obtained from long
term belt life records, which are rarely
kept. However, one such data review of
detailed records over a 40-year period
at an Indian power plant showed that
engineered belt cleaning compared to the
facility's homemade cleaners was actually
related to a doubling of belt life. Most belts
fail from reasons other than simple old age
and wear, but this study at least suggests
that belt cleaning is likely not a significant
cause of reduced belt life.4•5
To try and answer qualitatively the
question of how much belt wear is caused
by cleaning, the author has conducted
several laboratory studies. The results
show that elastomeric blades wear the belt
faster than hard metal blades by a factor of
roughly 2:1. It is assumed that this can be
attributed to differences in blade contact
area and the differences between the
coefficients of friction of elastomers and
hard metals against the belt. However, belt
cleaners with hard metal blades are more
likely to cause catastrophic damage if not
properly installed and maintained .
Using a major belt supplier's method of
estimating top cover wear due to loading
and experimental results of top cover wear
from belt cleaners, it is estimated that
elastomer precleaners represent about
five per cent of the top cover wear and
secondary hard metal-tipped cleaners
represent about 2.5 per cent of top cover
wear. However, loading under optimum
conditions is responsible for an estimated
25 per cent of top cover wear.6
Conclusions To minimise belt wear from loading,
curved chutes are particularly effective. In
addition, while belt cleaners do result in
belt wear, this occurs at a much lower rate
than the wear due to loading.
Furthermore, rather than focussing
on extending the life of sacrificial wear
materials, design and maintenance
engineers should aim to make servicing of
wear materials easier and faster.
These measures can be expected to
help prolong conveyor belt life. •
REFERENCES 1 ALDRICH, J AND ZHANG, Y (2014) Minimizing Belt Wear and Damage from Optimized Chute Design. Bellingham (WA), USA: Conveyor Dynamics Inc, p3-7. 2 RHOADES, CA, HEBBLE, TL AND GRANNES, SG (1989) Basic Parameters of Conveyor Belt Cleaning. Pittsburgh (PA), USA: US Department of the Interior - Bureau of Mines, p3. 3 PLANNER, JH (1990) 'Water Washing Belt Scrapers for Conveyor Cleaning' in: Proceedings International Coal Engineering Conference, Institution of Engineers, Australia, Sydney, 19-21 June.
' SWINDERMAN, RT (2012) Belt Wear from Belt Cleaning Review. Martin Eng internal study (unpublished). 5 KASTURI, TS (1995) Conveyor Belting Wear, A Critical Study. Madras, India: Jay Kay Engineers and Consultants, p86.
' SWINDERMAN, RT A Study of Urethane Belt Cleaner Blades and Belting Wear. Martin Eng internal study (unpublished).
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