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The
manufacturer determines maximum recommended operating tension per inch of
width by a consideration of:
1. Stretch
characteristics of the belt.
2. Fastener/bolt
holding capability
3. Load
characteristics.
4. Stiffness.
5. Impact
resistance of the belt construction.
6. Customer
perception.
There is
relationship between the maximum recommended operating tension per inch of
width of the belt and the ultimate tensile strength (breaking strength) of the
belt. However, this relationship is relatively unimportant since modern day
conveyor belts seldom “break”.
Ⅱ.
Carcass Design
1. Multi-Piles
+ Rubber = BELTOP
The most
common carcass design is made up of layers or “piles” of woven fabrics bonded
together. This “conventional piled” belt construction, like HS BELTOP,
generally employs a plain weave carcass which is built up into as many layers
as is required to provide the necessary belt strength …bound together with
rubber, Usually.
The rubber
between the two piles is called a “skim”. Skims are important contributors to
internal belt adhesion, impact resistance, and play a significant role in
determining belt “load support” and “troughability.”
Improper
or marginal “skims” can adversely affect belt performance in general and can
lead to ply separation and/or idler junction failure.
In the
“plain weave”, the “wrap yarns” (lengthwise yarns) and the “fill yarns”
(crosswise yarns) pass over and under each other. This means that both
members are “crimped” (Essentially, each assumes a sine-wave-like
configuration.) This fact, plus the basic characteristics of the fiber used
give the belt its stretch characteristics.
Conventional
plied carcass belts have been used for decades. Consequently, they are the
most common belt design used today. Most Conveyor Engineers and Millwrights
are familiar with constructions and their characteristics. Virtually, all
belting mechanics know how to “splice” conventional plied belts. This
familiarity with the belt’s characteristics and the “ease of endlessing”
gives the conventional plied belting design its broad customer acceptance.
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Conventional
plied belting constructions, employing all synthetic carcasses and rubber
covers appropriate to the end use, like BELTOP, are particularly recommended
for:
1. Hard
Rock Mining:
(A) Aggregate,
Sand,
Ore,
(B) Soft
Minerals,
(C) General
Purpose Applications,
(D) Vulcanized
Splices, and
(E) Forest
Products.
In the
days when cotton and similar materials were widely used as carcass components
in plied belts, a breaker strip was added into the top cover for heavy abuse
construction to help absorb the loading impact. The switch to modern
synthetic carcass materials (like polyester and nylon) has essentially
eliminated the need for the breaker strip. Today, breaker strips are seldom found
in current plied belt constructions.
2.
Single-Ply + Rubber = Monocon
The straight
warp carcass design, yields a carcass construction wherein the basic
lengthwise (wrap) yarns are essentially
uncrimped. These are the main load-carrying tension yarns. Fill yarns
are then laid transversely and alternately, above and below the main tension
yarns. This construction gives greater dimensional stability to the belt, and
does employ a “beam” effect for better load support.
The yarns
used are much thicker than yarns in conventional fabrics. Further, they are
locked together by means of another series of lengthwise yarns, known as the binder
warp system. The binder warp system locks the tension and fill cords
tightly together, creating a belt which is unusually tough and which has
exceptional tear and impact resistance, as well as unusual fastener and bolt
holding ability.
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The Monocon
belting series lends itself particularly to applications requiring
exceptional impact and abuse resistance, high load carrying capacities, and
extended life, coupled with very low stretch. HS Monocon is ideal for
mechanical fasteners of vulcanized splice applications.
Monocon is recommended for:
1.
Hard Rock
Mining,
(A)
Aggregate,
Sand,
Ore,
(B)
High
Impact Applications,
(C)
Soft
Minerals, and
(D) General Purpose Applications.
3.
Single-Ply + PVC = VINYL TOP
The
single-ply, straight warp carcass concept can also be coupled with PVC as the
elastomer, rather than rubber. In the case of HS VINYL TOP, a single-ply,
straight warp, all synthetic, carcass construction is essentially encased
within a solid block of a high performance PVC elastomer..
The VINYLTOP
belting series also introduces a solid woven carcass design into the higher
tension HS VINYL TOP belting products.
The solid woven design can be considered an extension of the straight warp
concept. Polyester filament yarns, as well as spun polyester staple yarns,
are coupled in a highly complex fabric construction, which is somewhat
similar to the straight warp. However, because of the high performance
requirements of these constructions, more than one layer of basic warp yarns
are used. The whole is interlocked and tied into one single mass by means of
a uniquely designed binder warp system. Spun polyester staple yarns protect
the two faces of the carcass construction and combined with the high
performance PVC, form the working surface of the belt itself.
HS
VINYLTOP, a single-ply conveyor and elevator belting construction which
incorporates an all polyester carcass with a high performance PVC elastomer,
has found wide acceptance in:
1. A
broad range of Industrial Applications,
2. Agricultural
equipment,
3. Food
processing,
4. Grain
handling (conveyor and elevator),
5. Underground
mining, such as coal, potash, and other soft minerals, and
6. Forest
products.
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Monocon belt constructions employ the
straight warp carcass principal, coupled with appropriate rubber covers.
Reduced weight and gauge with improved strength are offered.
The
Monocon I (WL I) belting series yields a range up to 400 # PIW.
The HS
Monocon Ⅱ belting series extends the
desirability of the straight warp minimum stretch concept to the higher
tension ranges by using two plies of straight warp carcass. Monocon Ⅱ represents the state of the art in minimum-ply
conveyor belting, with a range of 400 # PIW to 1500 # PIW.
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4. Steel Cord +
Rubber
Steel
cord-type constructions utilize a single layer of steel cords as tension
members; and either use plies of woven fabric to reinforce the cords
crosswise, or merely encase the cords in rubber. Steel cord belts are
generally found in high tension application ranging from 600 PIW to 5,000 PIW
and/or where extremely low stretch is a necessity. Steel cord belts must be
manufactured to width, are difficult to splice, and are subject to
deterioration because of rusting of the steel components.
5.Single Ply Kevlar
+ Rubber
A single
ply carcass construction made of Kelvar (A new “space age” fiber material
stronger than steel.) can be coupled with rubber to provide a belt
construction which is superior to the steel cord approach. Higher strength is
coupled with lower weight and freedom from rust deterioration. ARATOP is the
high tension belt of the future.
6. Strength
Designations
In the
past, when cotton was the primary fabric for carcass construction, all fabric
were designated by the weight of a piece of fabric 42” ´ 36”. As
new carcass materials were developed that varied in strengths and weights,
new methods of designation were required. As a general rule, current fabrics
in use are designated by the working tension or strength of the fabric, shown
in pounds per inch of width (PIW) ie., 80, 110, 150, 200 and 250 pound
fabrics.
When
dealing with carcass fabrics, we work with two separate strength
measurements. The first is the Maximum Working Tension or Strength of
the belt. This is the highest tension occurring in any portion of the belt on
the conveyor system, under normal operating conditions. This is the strength
measurement used to determine the proper belt for the system. The second
measurement is the Ultimate Tensile Strength of the belt. The Ultimate
tensile strength of a belt is the point at which that belt will rupture
and fail due to excessive tension.
The
difference between the maximum working tension and the ultimate tensile
stre1ngth of the belt is often referred to as the “service factor”. On top
quality domestic polyester belting, this service factor is 8-10 to 1. Most HWASEUNG
R&A belting has a 10 to 1 service factor. This means that if the maximum
working tension is 220 pounds PIW the ultimate tensile strength would be 2200
pounds PIW. Belting utilizing nylon constructions generally has a service
factor of 15 to 1 and more. This higher service factor is necessary to
overcome some of the instabilities inherent in nylon; particularly, its
excessive stretch.
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Cotton
Rayon
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Natural
Cellulose
Regenerated
Cellulose
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Only
natural fiber used to any great extent in belting Manufacture. Increases in
strength when wet. High Moisture absorption – consequently, poor dimensional Stability.
Susceptible to mildew attack. At one time Represented 80% of the raw fiber
input into belt man-ufacture. Currently, something less than 5%.
Slightly
stronger than cotton, but tensile strength is lowered by water. Chemical
resistance similar to cotton. High moisture absorptionㅡconsequently,
poor dimen-sional stability. Susceptible to mildew attack. Used very little
in belt manufacture currently.
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Glass
Nylon
Polyester
Steel
Kevlar
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Glass
Polyamide
Polyester
Steel
Aramid
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Very
high strength compared to rayon. Low elongation. Mainly used in high
temperature applications. Poor flex life. Limited use in belt manufacture
currently.
High
strength, high elongation, good resistance to abra-sion, fatigue and impact.
While moisture absorption not as high as cotton, it will absorb up to 10% of
its own weight in moisture Consequently, poor dimensional stability. High
resistance to mildew. At one time, nylon represented 40% of the raw material
input into belt man-ufacture. Today, it is something less than 20%…and shrinking.
High
strength, exceptionally good abrasion and fatigue Resistance. Extremely low
moisture absorption…cones-quently good dimensional stability. Unaffected by
mildew. Polyester usage in the manufacture of belting has grown from 0% in
1960 to something in the range of 70-75% today.
Used
where high strength and extremely low stretch are a necessity. A small amount
of woven steel carcass is found in today’s market. However, more steel is
used in steel cable-like belting constructions. Use limited because of tendency
to rust and the “manufacture to width requirement.
Kevlar
(the material used in flak jackets and buller-proof vests) has twice the
strength of steel, with stretch char-acteristics roughly halfway between
steel and polyester. It is significantly lower in weight than steel and will
not rust. Kevlar appears to be the high tension material of the future. It is
expensive and is currently considered experimental.
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7. Covers
Covers
are used in conveyor belt constructions in order to protect the base conveyor
belt carcass and, if possible, to extend its service life. In addition,
covers do provide the finished belt with a wide variety of desirable
properties, including the following:
A. Textures,
To increase traction
To increase inclinability
To control product
B.
Ease of cleanability,
C.
A specific coefficient of friction,
D.
A specific color,
E.
Cut resistance, and
F.
Enhanced impact resistance, etc.
Cover
type, quality and thickness are matched to the service life of the belt
involved. A specific
cover
formulation used in an individual belt construction, is determined by the
material to be carried and the environment in which the belt will operate.
Historic
belt constructions were highly susceptible to moisture and chemical attack
because of their cotton (and nylon) carcass components. Accordingly, it was
common to extend the belt covers over the edges of the belt in what is known
as a “molded edge” construction. This type of manufacture is expensive
because of the additional labor and machine time involved.
Modern day
belt constructions, with their high adhesion levels and synthetic carcass,
are considerably less susceptible to moisture and chemical attack, and do not
require edge protection. They make possible the “slitedge belt distribution”
program currently used in the Belting industry. Costs are minimized since an
84” slitedge belt can be manufactured about as quickly (if not more so) as a
24” molded edge construction. Further, the labor involved is somewhat less.
Georgia
Duck uses an extremely wide variety of polymers for our cover needs,
including: Polyvinylchloride, Natural Rubber, various Synthetic Rubbers, and
urethaneㅡto
meet individual customer needs. Quality competitors offer covers made of
similar polymers although their individual “recipe” may be somewhat
different.
Individual
cover formulations are usually blends consisting of one principal polymer and
assorted modifiers, such as other polymers, anti-oxidants, accelerators,
curatives, pigments, extending and reinforcing fillers, plasticizers, etc.
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be a
compromise, which seeks to meet the customer’s criteria and still remain cost
effective. For many applications, the blending of polymers adds properties
that could not be obtained in a single polymer compound.
In
addition to selecting proper compounds for cover material, it is also
necessary to determine the proper cover thickness. The thickness of a cover
is influenced by the amount of abuse and wear the belt will receive.
The severity of the wear depends on the
nature of the material and on the size, weight, and shape of the lumps
conveyed. Sharp edges, particularly on large pieces, can quickly cut a cover
badly. On the other hand, if loading conditions are ideal, with the material
being loaded in the direction of travel of the belt, and with only a slight
impact onto the belt, even very sharp material may not seriously cut or wear
the belt surface.
The
following table provides a good “rule of thumb” for selecting minimum covers
on PVC and Grade Ⅱ
rubber belt constructions.
Wearability
of rubber-like compounds can be characterized by a “PICO” abrasion test. This
test assigns “wearability levels” or “abrasion numbers” to various
elastomers. The higher the number,
the more
durable the elastomer. For example, Grade Ⅰ rubber normally will test out at a PICO rating of 135,
while a Grade Ⅱ
rubber will yield a PICO of 100, and PVC a PICO of 50.
Fillers
and additives added to a given recipe can affect the PICO adversely. It is
not uncommon, for example, for and oil resistant, MSHA, rubber elastomer to
yield a PICO in the 50’s or 60’s.
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Recommended
cover thickness
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Class of
material Examples
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Minimum top cover
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Light or
non-abrasive Soft minerals,
salt,
Bituminous coal, potash ore
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A.
favorable B. adverse
conditions conditions
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Standard HS
VINYILTOP HS VINYILTOP plus*
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Rubber
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1/16˝ 1/18˝
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Fine and
abrasive Sharp sand,
clinker
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1/8˝ 3/16˝
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Heavy, crushed, to 3” Sand, gravel, crushed stone
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1/8˝ 3/16˝
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Heavy, crushed, to 8” ROM coal, rock, ores
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3/16˝ 1/4˝
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Heavy, large lumps Hard ores, slag
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1/4˝ 3/8˝+
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*Consider
“Thicker” HS VINYILTOP construction to get benefit of additional “Binder
Warp” cover yarns.
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Minimum bottom cover
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1/16˝ 1/8˝
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Specific conveyor belt
applications seldom require the belt cover to satisfy one or two conditions. More
usually, a broad variety of required and desired properties are encountered.
The specific cover formulation is quite likely to | | |
