Manufacture
of Rubber Products
PNEUMATIC TYRES
A
tyre is an annular toroidal-shaped inflatable envelope, made of rubber, which is
reinforced with cord,
enclosing coiled wire bead rings. It is fitted a metallic rim which is secured
to the vehicle (plate 9, Exhibit 17.1).
A pneumatic tyre performs the
following principal functions:
(a)
It supports the weight of the vehicle.
(b)
It transmit the forces on the vehicle to the ground,
e.g. it help to covert the engine torque to movement of the vehicle.
(c)
It gives a more comfortable ride to the passenger or
cargo in the vehicle by.
(i)
Acting as an additional spring in the suspension
system.
(ii)
Elastic deformation over undulations on the road.
(d)
It permits cornering on the road at relatively high
speeds by its capacity to generate higher cornering forces than would have been
possible with a solid tyre.
CLASSIFICATION
Tyres may be classified in
the following ways:
Tyre and Rim Notations See
Fig. 17.1 for basic definitions. The tyre size which is prominently displayed
on the sidewall indicates the approximate dimensions of the tyre. Many systems
of tyre sizing are in vogue and the two most commonly used systems are explained
below:
(i)
Size “9.00-20” indicates that the tyre has a section
width of approximately 9” and is fitted on a rim of 20” nominal bead diameter.
(ii)
For radial ply tyres, size 145/70R12 denotes a tyre
with section width 145 mm, radial play and nominal rim diameter code 12, and an
aspect ratio of 70.
The ply rating of a tyre is
also displayed on the sidewall, and indicates its load carrying capacity. It
should be noted that it does not necessarily represent the number of casing
plies. The actual load carrying capacity may be obtained by reference to
published standards. In India, the Bureau of Indian Standards (BIS) has
published this data. The ratio of tyre section height to width is called
‘aspect ratio’.
Construction
Tyres may be classified by the type of casing construction used and the
principal physical difference between the casing constructions lies in the
angles of the casing and breaker cords. The two basic casing constructions are
as follows:
(i)
Cross ply The casing angle and the breaker angles
are generally equal and in the region of 40°, with cords in alternate plies
rubbing in opposite directions.
(ii)
Radial ply All the cords in the casing run in a
radial direction and breaker cords are at a very low angle, i.e. almost
circumferential.
All aspects remaining the
same, the mileage potential of radials is the best (Fig. 17.2)
Tread Pattern
Design
Tyre used on hard dry roads do not need patterned treads. However, on wet
roads, the water acts as a lubricant between the tread rubber and the road,
thus, drastically reducing the friction and hence grip of the tyre on the road.
The pattern on treads helps to remove much of the water between the tyre and the
road, thus improving the grip.
Tread patterns are of two
basic types which are listed below. Various combinations of these types of
patterns are in use which aim at providing specific properties for specific
service uses and also to improve visual impact.
Figure 17.3 shows in simplest
terms the fundamentals of tread pattern design. The two basic patterns have the
following features:
(2)
Transverse cross-ribbed or lugged patterns
Goods fore-and- aft grip; tendencies to irregular wear and reduced grip; and
noise on running are some of its properties.
From these two basic
patterns, others can be derived by addition and distortion. The simplest of
these is produced by simple addition or combination of the basic two.
(3)
Square studs They have the features of-good grip in
both directions; tendency to irregular wear; noise on running; and instability
on cornering due to flexibility of units.
(4)
Ribs with side studs This is a simple commercial
pattern. It is used for industrial tyres and for front wheel grader tyres. Its
properties are: good resistance to wear; good steering due to longitudinal ribs;
and side studs ensure that rotation and steering are maintained on soft
surfaces.
(5)
Diamonds This was a well-known pattern for earth
mover and other tyres. Properties were very similar to those of square studs,
but slightly improved good grip in all directions; somewhat reduced tendency to
irregular wear, and reduced noise.
(6)
Improved ribs and side studs. The basic properties
are similar to those quoted under ribs with side studs for general on-and-off
the road use.
(7)
Symmetrical transverse. In this case, the simple
tranvansverse bar becomes a chevron, balanced about the centre line of the
pattern. This type of pattern is widely used. The properties are: maximum grip
and drive on rough and loose surfaces; tending to irregular wear if run on
smoother roads; and no sideways reaction on drive or braking.
(8)
Asymmetrical transverse There have been a whole
range of such designs, based on helical gear wheel designs. The properties can
be altered by doing subtle changes to groove shape, tread radius, and shoulder
contour.
The basic types of highway
truck tyre designs are rib, lug, and semi-lug.
Rib Type Treads Tyres with
rib type tread are “all position” tyres. They can be used on all wheel
positions at legal highway speeds. These tyres are always recommended for
steering wheel use on longhaul, high-speed service. The circumferential groove
design provides maximum steering capability and good skid resistance.
Lug Type Treads Lug tyres are
designed for drive wheel service. The design provides greater traction in high
torque application.
Semi-lug Treads Semi-lug type
tyres are designed for drive wheel service and are suitable for many off-the
–road operations. These tread designs provide maximum resistance to wear and
greater traction in high torque service. These designs are also suitable for
steering wheel application.
Some examples of patterns for
specific applications are listed below.
(i)
Tractors used on wet fields have drive axle tyres
which have deep transverse bars which bite into the soil and thus provide the
required traction. The bars are placed at angle, so that the mud can slip off
the tyre and the pattern is always reasonably clean when it makes contact with
the wet surface.
(ii)
Tyres used in deserts where the sandy surface is
very soft and extremely deep have very shallow circumferential patterns. Deep
transverse patterns would dig into the sand and get embedded in it. An
absolutely bald pattern would have been the best in these circumstances so that
the load on the sand is distributed over a larger area thus reducing the chances
of the tyre getting embedded in the sand. However, as these tyres are sometimes
used on hard roads which may be wet, shallow circumferential patterns are
generally used.
Tubed and
Tubeless Tyres
Air at high pressure is required to be enclosed between the tyre and the rim.
In general, this is achieved by using an air-tight elastic tube made of rubber
compound which is placed inside the tyre and rim assembly. These tyres are
referred to as tubed tyres or conventional tyres.
In some cases, it is possible
to achieve air seal between the tyre and the rim without the use of tubes. This
is done by having a thick layer of rubber inside the tyre which acts as an air
envelop. These tyre are known as tubeless tyres. They are used on special rims
(Fig. 17.4)
COMPONENTS
The three main parts of a tyre
are-casing, tread, and bead.
Casing
The casing, which is made of layers of textile cord fabric surrounded by rubber
compounds, provides the strength of the tyre. The following components make up
a typical casing.
(a)
Plies
These are layers of textile cord fabric, coated with rubber compound which are
cut to the required dimensions and bias angle. They are locked around the bead
wire coils. The textile may be cotton, rayon, or nylon. Steel, glass fibre,
and polyester are also used. (Plate 10, Exhibit 17.2).
(b)
Inner liner This is a layer of rubber compound
fitted inside the tyre to protect the plies. In the case of tubeless tyres, a
thick inner liner is used (see item d), and often a special formulation for
lower air permeability is chosen.
(c)
Insulations These are layers of rubber compound
fitted between or over the plies to provide extra reinforcement or cushion to
the casing in critical regions.
(d)
Breakers These are relatively narrow bands of cord
fabric coated with rubber and are fitted where necessary on top of the plies.
They absorb high instantaneous shock load and distribute them evenly over the
casing. They may also be used to provide additional stiffness to the tread.
(e)
Chafers These are layers of textile fabric coated
with rubber which are fitted over the casing in the bead region. They protect
casing from damage due to chafing with the rim or during mounting or dismounting
from the rim.
Tread
It
is a relatively thick layer of rubber which is fitted over the casing/breaker
assembly and protects the casing from external physical and environmental damage
(Plate II, Exhibit 17.3). The top portion of the tread which comes in contact
with the road is patterned as required for the service conditions. The sidewall
region may also have patterns, generally for visual impact. The size, type, ply
rating and manufacturer’s name are generally displayed on the sidewall. In the
case of bicycle tyres, a number of combinations are used, for example, two tone
colours such as white and brown, ribbon or stripes on one or both sides, open
sidewall tyre which has practically no rubber on its sidewell, or fluorescent
line tyre where a fluorescent paint or strip is used.
Beads
Beads consist of one or more coils or rubberized wires fitted generally with
wrappers, fillers, and apices. The bead being relatively inextensible locks the
tyre to the rim. A typical bead is made up of the following components:
(a)
Bead Coil These may be of one wire or more and is
coated with rubber if more than one wire is used. The wires are generally made
from copper-coated steel.
(b)
Wrapper This layer of textile fabric, coated with
compound, which is wrapped around the bead coils, keeps the layers of wires in
position and provides good adhesion between the bead coil and the surrounding
casing or fillers.
(c)
Apex An apex is fitted over the wrapper and is
generally triangular in shape and made of rubber compound. They help in
improving the bead shape and bulk and avoid the formation of air pockets at the
turn-up region.
(d)
Filler This is made of textile fabric coated with
rubber and fitted over the wrapped bead and apex assembly (Fig. 17.5)
Preparation of the
components: Rubberized fabrics from the calender are cut to strips at an angle
in a “bias-cutter”. These strips are jointed together end-wise so that a
continuous strip (cut ply) is formed having the cords at an angle to its
length. The width of the strip is arranged to suit the size of the type to be
made. Two such strips are then plied together so that the cords form a
criss-cross
or trellis pattern. Insulation rubber is applied on the ply as specified.
Beads are produced by passing
a group of wires through a T-head extruder so that they become embedded in a
rubber tape. Layers of this type are wound onto a drum until the required
thickness is built up. The whole is wrapped helically in a strip of
rubber-covered fabric. After applying apex compound strip, a second rubberized
strip, called the “filler” is folded around the bead.
The sidewalls and tread are
frequently made of different compounds, which are extruded either separately, or
as a combination secured by arranging two extruders in tandem.
Tyre Building:
All the components are
assembled at the tyre building machine.
Bicycle tyres are built in two
ways: (I) In the crown overlap method, rubberized tyre cord fabric is cut to 2.5
times the width of the tyre and wound up around a building drum. After placing
the bead wire coils at 1/4th width from either edge, the ends are
folded to have a 1-cm overlap at the centre. Tread rubber is then applied.
(ii) In the band method, tyre
cord fabric is slit in the warp direction and wound up over two bead wires on a
drum building machine at a bias. Tread rubber is then applied.
An automotive tyre building
machine (Plate 12, Exhibit 17.4) consists of a rotatable drum with edges shaped
to receive the bead assemblies. Behind the drum is a set of racks for holding
the strips of fabric and rubber, and lower down are power driven rollers and
wheels for rolling the components into position. To right and left and
supporting rings for beads, so mounted that they place the beads in exactly the
correct position on the drum.
The operator first applies to
the drum the doubled plies of fabric, their edges extending beyond those of the
drum (Plate 13, Exihibit 17.5). Then the beads are set in place. The plies
are turned up around the beads. When the plies and breakers have been fitted
and rolled on, the tread and sidewalls are applied, consolidated, the drum
collapsed, and the ‘green” type removed.
Tyres with bias angle up to 65°
are built of flat-topped drums in a single stage process. Above this angle
(e.g. radial tyres) a two-stage process is necessary.
Radial tyres must be built with
a shape approximating that of the final moulded shape, e.g. a toroidal, rather
than cylindrical shape (plate 13, Exihibit 17.6). The casing is made on a flat
drum machine of the type used for building bias ply tyres. It is transferred to
a second machine which has the ability to expand. Usually, the drum is composed
of metal sections which are covered with a polymer sleeve. After the casing has
been built on the drum, the centre is expanded and the belt plies and tread
built on the expanded casing.
Tyres are normally cured by
press cures at high temperatures and pressures. The compound flow into the
mould shape and the cord angle in the ply changes by “pantographing effect” as
the tyre changes to a toroidal shape. In radial tyres with steel belts, where
shaping occurs before the tyre is placed in the mould, it is necessary to use
segmental moulds. With this type, the mould segments approach the crown of the
tyre in radial direction when the press closes.
After curing, the tyre can be
mounted on a rim and permitted to cool while inflated to reduce internal
stresses. This step is called post-cure inflation. Finishing the tyre involves
awlholing, trimming, buffing, balancing, and inspections by quality control
procedures. White sidewall is fitted with a black overlay which is buffed
(Plate 14, Exhibit 17.7). Awlholing removes air pockets that may remain from
tyre building. After buffing and trimming, the tyre is viewed of imperfections,
then balanced. Balancing ensures dynamic equilibrium by testing the uniformity
of mass distribution of a tyre relative to its spin and steer axes.
INNER TUBES
Inner tubes are made of rubber
compound and are provided with a one way valve to enable inflation of the tube.
Tubes were originally made from
natural rubber compounds. However, several other synthetic rubbers are now in
use. The most important of these is butyl which imparts a much higher degree of
impermeability to air and resistance to heat degradation than NR or other
general purpose synthetic rubbers.
Various types of valves are
available to suit specific uses. The function of these valves is to enable
inflation of the tube. Their one way nature also prevents deflation. The type
of valve to be used design of the axle and wheel assembly.
Additional components may be
used in the tube in special cases as dictated by performance requirements. And,
the most important of these are:
(a)
Joint
strips These are thin strips of rubber fitted over the tube joint to improve
joint strength.
(b)
Base
reinforcement strips These are strips of textile fabric coated with rubber
compound which are fitted to the base region of the tube as protection against
heat build up in the rim and/or damage by the rim.
The manufacture of tubes may be
done in many ways depending on the equipment available and the size of the
tube. The following are the most important steps involved:
(a)
Tube extrusion Tubes are extruded as annular
cylinders through dies (fitted with centres) to the required dimensions. If
the tube is extremely big in section, it may not be possible to extrude them in
one piece and may be prepared from two or more pieces joined by hand.
(b)
Valve
fitment. A small hole is made at the appropriate location on the tube and the
valve is fitted over the hole. Valves having rubber bases are fitted to the
tube body by means of rubber-based solutions.
(c)
Joining Raw tubes in their cylindrical forms are trimmed to the correct length
and joined on a machine. If joint strips are required, they are fitted at this
stage over the joint.
(d)
Forming Raw joined tubes are then inflated slightly to a symmetric toroidal
shape and allowed to mature for the required duration of time.
(e)
Moulding Formed raw tubes are vulcanized in moulds at high temperatures.
(f)
Finishing / inspection During this stage, tubes are fitted with all the valve
accessories, checked for defects and leaks. They are then packed in containers
or bags ready for shipment.
Performance Requirements of tyre: Various tyre performance requirements
published by standards organizations include test procedures for tyre strength,
endurance, high speed performance, and tubeless tyre resistance to bead
unseating
In
the tyre strength test, a 19,mm diameter steel plunger with a hemispherical end
is pressed into the tread of the tyre. Different plunger forces are specified
for different tyre constructions. The tyre has passed the test if it does not
break or when the plunger bottoms against the well of the rim.
For
endurance and high-speed testing, a dynamometer or laboratory road wheel of
1.707-m diameter is used under controlled operating and environmental
conditions. These tests are rapid and less expensive than outdoor tests, and
are used for screening purposes, or as minimum customer/compliance tests under
varied loads, speeds, and inflation, pressure – conditions not practical by
other testing means. A tyre will pass if it does not show any evidence of
tread, ply, cord or bead separation, tread chunking, or cord break-up.
In
the bead unseating test for tubeless tyres, a blunt block is pushed against the
sidewall of a properly inflated tyre and the force required to unseat the tyre
is measured.
Rolling resistance coefficient, which is the ratio of rolling resistance force
to the load carried by the tyre, may be determined for the different categories
of tyres.
The
safe performance of a tyre is influenced by tyre design, tread pattern, rubber
composition, and inflation pressure. Other safety requirements require
acceptable performance in the following tests: drive and breaking traction, low
pattern noise, side forces, high speed capability, directional control, low heat
generation, damage resistance, low hydroplaning potential, and high-skid
resistance on wet and dry road surfaces. Users must guard against the
indiscriminate exchange of tyres and tyre sizes.
Tyre
Retreading: Considering that only about one-fifth of a tyre is worn out in
service, it makes economic sense to retread the tyres for multiple use.
Although both cross ply and radial tyres can be retreaded, 4 to 8 ply bias tyres
can be the most easily retreaded. However, the structural performance may fall
short of the new tyres and service conditions may have to be more closely
controlled. Two main methods, currently in use, are (a) conventional “hot”
capping and (b) precured-tread rubber process.
The
hot capping process uses “Camelback”, a thick section of unvulcanized tread
rubber, backed with cushion gum, which is applied to the (buffed) tyre carcass
during retreading. Generally, the techniques used for retreading and remoulding
compounds follows the practice of compounding used for new types, except that
the practical and economic aspects are given greater emphasis in order to get a
combination of good physical properties and simplified inventories. Casings are
repaired prior to remoulding, with unvulcanized cross ply patches. Different
types of retreading can be done, covering portions of the tyre-top cap (for
tread wearing area retreads only), re-cap (a shoulder to shoulder retread,
including the buttress), and remoulds (complete bead to bead retread).
In
the precured tread rubber (cold process) retreading, better mileage is obtained
than with the hot capping process. The tyre is inspected for cuts, ply
separations, etc. and after inspection, it is repaired with repair compound or
patches, and then buffed under inflated conditions to facilitate bonding.
Thereafter, the tyre is again inflated on an expandable hub and coated with
vulcanizing cement on its buffed surface. A layer of bonding/cushion gum is
applied around the surface of the tyre. The trapped air between the layers is
removed and the vulcanized tread strip is applied, its ends spliced and
stitched. After this operation, the expandable hubs are collapsed and the tyre
is deflated and removed for the vulcanization. The tyre is fitted on suitable
rims and inflated with the use of a tube. The inflated tyre is then slipped
into an envelope and vulcanized in a press at lower temperatures than is
normally used for tyre vulcanization. The low temperature curing condition must
be good for each retread, i.e. number of retreads. However, the casing
condition must be good for each retread, i.e. substantially free from major cuts
and punctures. Tyres can be retreaded with different patterns and various sizes
can be moulded simultaneously, unlike in conventional remoulding.
CONVEYOR BELT
A
conveyor belt is used to carry materials from its loading point to discharge
end. A conveyor system, in its simplest form, consists of built-up structure
having a driving pulley, a trail pulley, and a number of three pulley idler,
placed at a regular intervals between the driving and trail pulleys. A conveyor
belt moves round the pulleys, driven by the driving pulley and the material
discharged on it is taken away and transferred to the other end. A conveyor
system may be a few km in length. It is used in steel, cement, thermal power,
mines, port, fertilizer, and other major industries.
A
conveyor belts is normally of 300-1600 mm in width and of length up to 400m. It
is actually used as a mechanical device for conveying materials in horizontal,
inclined, and declined line.
Belt
Design: The structural design of the conveyor belt is based on the conditions of
the individual drive which carries varied amount of materials of different
grades. Therefore, each conveyor belt is designed for specific service
conditions and components are designed to ensure maximum belt life.
In
order to decide the belt grade and cover thicknesses, one has to consider the
conveyor systems in totality, the temperature of the material to be carried,
abrasive nature, size, chemical effect of the materials on belt, surrounding
conditions, etc.
When
a conveyor installation is planned, the driving power (drive pulley) and the
tensile strength of the conveyor belt can be calculated on the basis of industry
and manufacturers’ standards. In general, belts have a factor of safety of 8to
10. To obtain sufficient friction between the belt and drive pulley, the belt is
given a pretension of about 2% of its nominal load. A load of 10% of the
nominal strength corresponds to the average operating conditions. Peak loads
may reach 25-30% of the nominal strength.
In
general, a conveyor belt has to meet the following basic requirement: high
strength, low growth, high impact resistance good throughability, low bending
resistance, effective mechanical splicing, resistance to outdoor and special
conditions like fire, etc.
In
operation, the conveyor belt has to transfer the tension through the
longitudinal member of the carcass and to carry the load via the cover by both
longitudinal and transverse members of the carcass. Besides, the top and bottom
cover rubber have to protect the carcass and to bear impact and pulley
rotational forces, respectively.
The
conveyor belt is exposed to changes of tension and elongation at the start-up,
when in use, and at rest. The elastic properties determine the design of a
conveyor belt, especially its correct tension rating. A special problem is
growth of carcass under service conditions. The growth is determined by the type
reinforcement, its construction, and treatment. A special test method has been
established to draw the tension-elongation curve (belt characteristic curve) of
any conveyor belt.
High
impact resistance is required to absorb the impact forces on the belt in the
loading area. This impact resistance is proportional to the breaking energy of
the belt. Apart from the reinforcement, the construction of the top cover also
plays an important role.
Good
troughability of the conveyor belt in transverse direction is essential for
running on high idler angles (therefore, larger load capacities). Mechanical
fasteners are used for fabric reinforced light belts and where the length of the
fabric conveyor belt has to be changed frequently, for example, in a coal mine.
The efficiency of joints made with mechanical fasteners relative to the nominal
belt strength is up to 90% There are seven major systems of belt jointing to
form an endless belt.
There are numerous designs of fabric belting, for example:
(a)
One-ply belts-solid woven or single ply.
(b)
Two-ply belts-duo-ply or two plies
(c)
Multi-ply belts.
The
main ranges for the use of conveyor belts with different reinforcement materials
is shown in Fig. 17.7. The most widely used reinforcement is woven belting duck
(with ends and picks at right angles to each other). The warp crimp resulting
from the fabric construction absorbs the compression of the inner fabric plies
when the belt goes around pulleys.
The
breaking strength of conveyor belts in expressed in kN/m, (length-wise
strength). According to the International standards Organization
classification, the breaking strength classes may be expressed in the schedule
100, 125, 160, 200, 250, 315, 400, 500, 630, 800, and 1000 kN/m. The number of
plies by which a belt is reinforced is indicated by the figure following the
determination of its breaking strength. Usually, the belts have two to six
plies. The corss-wise belt strength is calculated as 25/32/40/50/65/80/100/120
kN/m multiplied by the number of plies. Similar classification is used for
belting ducks also, but the actual strengths are specified at 20-25% higher due
to the conversion efficiency losses. The ratio between length-wise and
cross-wise strength is usually 4:1 or 3:1, and occasionally, 2:1 where an
extremely good fasteners resistance is desired. A desired type of belt of can
be made up from different fabrics and a wide range of strength classes can be
built up with only a few varieties.
PARTS OF CONVEYOR BELT
A
conveyor belt consists of (Plate 15, Exhibit 17.8):
(1)
Strength/tension members,
(2)
Inter-ply rubber compound
(3)
Cover
rubber, and
(4)
Breaker if required
Strength member Strength members are normally woven fabric made from cotton,
cotton/nylon, rayon/nylon, nylon/nylon, on polyester/nylon. Number of plies
varies from two to six. Cotton fabrics only were used earlier and typical
constructions are given in Table 17.1.
Cotton belting ducks
Fabric
type Average breaking load
(min.)
Cotton/Cotton __________________________________________
Warp
way Weft Way
(N/cm
width) (N/cm width)
28-oz
630 294
31-oz
630 343
32-oz
687 321
34-oz
687 441
36-oz
736 343
42-oz
883 441
Cotton
belts tend to be bulky, and, at the same time, have strength limitation of
500kN/m only. But with increasing demand of carrying more material, higher
strength of belt was urgently needed. Hence, cotton/nylon fabric came into use.
Fabrics used are given in Table 17.2.
Table 17.2
Cotton/nylon belting ducks
Fabric type
Strength/ply from
Cotton/nylon
finished belting (kN/m)
CN –
70
70
CN –
80
80
CN –
90
90
CN –
105
105
CN –
130
130
Belt strength achieved up to
500 kN/m.
All nylon ducks undergo dipping
and heat setting to improve their adhesion to rubber and to reduce elongation.
The latter is essential as high elongation will cause excessive belt growth
which needs adjustment for the take up system.
Different fabrics used are
given in Table 17.3
Table 17.3
Nylon belting ducks
Fabric
type Ply
strength (kN/m)
Nylon/nylon
from finished belting (min.)
NN-100
100
NN-125
125
NN-160
160
NN-200
200
NN-250
250
NN-315
315
NN-350
350
Belt strength achieved is
maximum 1800 kN/m only. In longhaulage conveyor, even nylon fabrics are not
suitable due to high elongation (i.e. minimum achievable by heat setting).
Therefore, polyester/nylon fabrics are used as shown in Table 17.4.
Table
17.4
Polyester/nylon belting ducks
Fabric
type Ply
strength (kN/m)
Polyester/nylon from
finished belting (min.)
EP-100
100
EP-125
125
EP-160
160
EP-200
200
EP-250
250
EP-315
315
EP-350
350
Maximum belt strength achieved
is 1800 kN/m. At present, this strength is not enough to cope with the
increasing demand of higher belt strength with maximum elongation. Hence, steel
cord becomes the answer. It has as low as 0.2% elongation at working load
compared to 4% for nylon and 3% for polyester fabrics. Using steel cord as
tension member, belt strengths that can be obtained are ST500 to ST7000. Steel
has many advantages but its high relative density makes the belt very heavy.
Inter ply rubber Its main
function is to give adequate bondage between plies so that the plies are not
separate from each other during the service period. It also gives cushioning
effect between plies and protects them to some extent from shock load due to the
fall of the materials. Inter ply rubber thickness depends upon type of ply and
severity of service conditions.
COVER RUBBER
Face cover: It protects the
carcass, i.e. the assembled plies, from impact damage abrasion loss, cutting,
gouging, etc. of the carcase materials. Cover rubber which faces the carrying
materials is termed as face cover or top cover. NR, SBR, NBR. etc. are used for
making the cover rubber.
Breaker: If additional
protection of the carcass from impact is required, a loosely woven fabric is
used in between the face cover and the carcass. This diversifies the impact
force in all directions leaving only a little at the place of impact.
Back cover: It protects the
carcass from abrasion of the pulley and disperses the shock to some extent.
17.3.2 Manufacturing Process
Fabric preparation: The fabric
plies of required width are dried by passing through a number of bowls heated by
steam. The dried fabrics are then rubberized by frictioning in a three-bowl
calender. These are then topped in three-bowl calender with topping compound.
In case of
nylon/nylon and polyester/nylon fabrics, the fabrics are stabilized by heat
setting and dipped in RFL dip to get adequate bonding in finished product. Due
to dipping, the fabric interstices are almost filled up and hence no frictioning
is required. These fabrics are, therefore, topped only either in three-bowl
calender or preferably in a four-bowl calender. In a three-bowl calender, two
pass is required for both side topping. Table 17.5 gives typical formulations
for conveyor belt frictioning and topping compounds.
Table 17.5
Typical frictioning and
topping compound formulations
Frictioning compound Topping
compound
for
cotton/cotton for cotton and
and cotton/nylon phr cotton/nylon x 2 phr.
NR
100.00 100.00
Zinc oxide 4.00
4.00
Stearic acid
1.50 1.50
Sulphur
3.75 3.75
Carbon black
20.00 20.00
Accelerator
0.75 0.75
Tackifying resin
3.00 3.00
Preparation of cover rubber: The compound is calendered into specified gauge on
a three-bowl calender. The width of the calendered rubber is kept approximately
75mm more than nominal belt width.
Slitting of plies: The rubber fabric is cut to width on a slitting machine,
appropriate to the size and construction of the belt to be made.
Raw
belt making: The cut plies are transferred to the belt- making table
(hand-building table or automatic belt – making machine), where the required
number of plies are assembled centrally and passed through pressure rollers for
consolidation. The back cover rubber is then assembled over the build-up
carcass and passed through pressure rollers, is then assembled over the build-up
carcass and passed through edges, rolled down, and trimmed flush. The face
cover is then assembled and turned at edges similarly. The raw belt, thus made,
is passed through a chalk box and pricker rollers and batched on itself.
Vulcanization: Vulcanization is carried out on hydraulically operated presses. A
frame is prepared with the help of moulding irons, metals frames, and packing
bars on the lower platen and raw belt is fitted to that frame. The frame
controls the thickness and width o8f the belt. With the help of movable clamps,
raw belts are stretched to requirement prior to closing the press for
vulcanization. The stretched belt is then compressed between steam heated
platens and vulcanized. Temperature, time, and hydraulic pressure being
automatically controlled and recorded. After vulcanization, the press is opened,
the vulcanized portion is withdrawn and raw portion which then enters inside is
vulcanized similarly and this cycle of operations continues until the whole
length of belt is cured.
Dimensions of moulding presses are different. A common press is 9.75 m long x
1875 mm wide having a stretching device at both ends. It has cool end at the
incoming end of the belt.
Raw
belt is stretched as per requirement. The curing time varies between 16' to 30'
and temperature between 140°C and 150°C, depending upon the thickness of the
belt and cure characteristics of the compounds. One should remember that a good
sequence of curing characteristic between all the compounds should be made.
Moulding pressure varies between 1.2 Mpa and 2.8 Mpa depending upon the type of
belt. Spew is trimmed off from belt edges and the belt is packed after
inspection and repair.
Inspection and repair: Defects market during Moulding are then repaired and the
belt is inspected. Sample from each belt is taken and tested for different
parameters laid down in specification for respective type of beltings.
Finished belt testing
(1) Measurement-Belt width thickness, and face and back cover
thicknesses, etc.
(2)
Full belt strength and elongation in warp and weft way.
(3)
Tensile strength and elongation of cover rubbers, initial and after
ageing as
Specified in the specification.
(4) Peel adhesions of face cover ply and ply/ply and back cover/ply.
(5) Oil swell test (for oil resistant belt).
(6) Abrasion test.
(7)
Flame
and electrical resistant test (for PVC belt).
(8)
Tear
test (for PVC belt.)
Belt
grade: Depending upon the end-use, different grades of beltings are given in
Tables 17.6 and 17.7.
Table 17.6
Conveyor belt grades
M-24
IS: 1891 (Part I) 1978
M-17
IS: 1891 (Part II) 1978
HR
IS: 1891 (Part III) 1978
Hygienic IS:
1891 (Part IV) 1978
Flame resistant (surface)
Flame resistant (coal
mines) IS: 3181 – 1978
Table 17.7
Specification for belting grade
Cover rubber
properties IS:1891
(I) IS:1891 IS: 1891 (II)
Tensile strength
(MN/m) min. 24.0
17.0 11.0
Percentage
elongation@break (min.)
250 400 350
Hardness (Shore
A) 65° ±
5° 65° ± 5° -
Abrasion loss
(mm³)
150 200 -
After Ageing
for 70°C
for 70°C for 100°C for
72 hours 72 hours 72 hours
Percentage drop
tensile strength +10
+10 +10
-20 -20 -45
Percentage
change elongation at
+10 +10 +10
Break
-25 -25 -45
Adhesion (kN/m)
For cotton For snthetic
Cover/Ply
(a) Cover up to
1 mm No
test No test No test
(b) Cover up to
1.5mm
2.20 3.15 1.95
(c) Cover over
1mm
2.60 4.50 -
PVC
Belting
These
are used in flat running belts and in underground mining belts. They are of the
following types: (a) monoply or solid woven; (b) duo-ply.
The ply
belts have doubled cotton/nylon or doubled viscose/nylon as a-reinforcing
fabric. The reinforcing component should meet the flame test requirements,
create sufficient mechanical fastening, and increase the belt modulus.
