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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

Receives various type of bulk materials at yard level, following rapid flow discharge to convey them onto a hauling and processing plant with controlled and adjustable flow rate, minimizing the installation costs.

Combines the strenght of a metallic plate extractor with the retaining quality of the rubber belt that seals off any potential excape points for even the finest materials.

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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).