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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

            48-oz                                                   1100                                        400

 

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

            CN – 175                                                                                175

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

            NN-400                                                                                   400

 

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

            EP-400                                                                                    400

 

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

            Process oil                              6.00                                         6.00

 

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

                        Belt grade                                                       Specification

            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

­­­­­­­                                                                        M-24               M-17                           H.R.

Cover rubber properties                                                    IS:1891 (I)            IS:1891                 IS: 1891 (II)

Tensile strength (MN/m)                                                   min. 24.0               17.0                        11.0

Percentage [email protected] (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                        -

      Ply/ply                                                                             3.00                        5.25                        2.10

 

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. 

For high strength belts, solid-woven fabrics are used.  These lead to improved roughing characteristics, less ply separations, and easier splicing.  In a solid-woven fabric, two or more weft layers are interlaced by warp threads. Solid-woven belting always means using PVC as a matrix material.  Only a low viscosity PVC paste can penetrate into the thick solid woven fabrics.  To get sufficient adhesion, the synthetic filament yarns in warp and weft often are twisted together with cotton. 

Steel Cord Belting 

The longitudinal steel cord reinforcement gives a belt with a high tensile strength, Thus, very long centres and high lifts or falls can be used.  The high strength toweight ratio enables the steel cord belting to carry much more load for a given drive at a given rate of power.

A comparison of the different belting types is given in Table 17.8. 

Comparison of different of conveyor belting

 

Sl. No.    Characteristic                     Cotton                    Nylon                     Polyester/Nylon                   Steel

1.             Longitudinal                         150                         200                                         200                         500

                Strength                                to                             to                                             to                             to

                (kN/m)                                   700                         2000                                       2000                       7000

 

2.             Belt weight                           7.5                          6.5                                          8.0                          12.5

                (kN/m)                                   to                             to                                             to                             to

                                                                22.5                        21.5                                        25.0                        60.0

 

3.                   Extensibility

At working load (%)          3                              3                                              2                              0.5

 

4.                   Maximum haul length

On single endless belt (m) 700                         4000                                       5000                       10,000

 

5.             Typical belt speed (m/s)      1.5                          3                                              3                              9

 

The strength member-steel cord-is a twisted composite of several strands of high-carbon zinc-coated steel wires.  The construction has to be varied for different strength ratings.  In order to ensure long-term corrosion protection during end-use, it is important that rubber penetrates fully into the interstices of the cords, thereby preventing all chances of capillary wicking.  A specially formulated “bonder” compound, in addition to its high adhesion characteristic, which can also flow into the interstices of the cord during the vulcanizing process is used.  Cover rubbers of a variety of grades to suit service requirements are applied. During manufacturing, special care is taken to ensure cord-planarity and tension controls to avoid crooked running.  Special splicing techniques are also used for steel cord belting.

The advantage of steel cord belting.

(a)   Higher tension rating, larger span, and higher throughput compared to textile belting;

(b)   Low elongation and little take-up space;

(c)    Good troughability;

(d)   Good dynamic performance and longer life; and

(e)   Good splicing.

 

HOSES 

Hoses are reinforced polymeric pipes, mainly used for transferring materials (liquids, gases and solids up to a limited particle size) under pressure. Unlike metal pipes, polymeric hoses are flexible, they absorb vibration, handle corrosive fluids, dampen sound, store easily, and are available in a large range of sizes. 

Hoses can be broadly divided into end-use, specific working pressure, and diameter categories.  End-uses may be domestic (e.g. garden hose), automotive, hydraulic and pneumatic, oil and solvent, mining, or general industrial applications. 

Hoses may be divided into low pressure (below 20 bar) medium pressure (20-70 bar) or high pressure (above 70 bar), based on specific working pressure.  A particular hose should be designed taking into account the specific material to be carried (solvent, steam, etc.) application. Specific working pressure and temperature, carrying rate, and life expectancy. 

Design of Hoses:

A rubber hose consists of three main components:

(a)   a lining or rubber tubing;

(b)   reinforcement (which may be braided, spiralled, circular woven, knitted, or wrapped with a range of reinforcing textiles (cotton, rayon, nylon, polyester, polypropylene, steel wire, glass fibre, or aramid); and

(c)    a cover for outer protection (Fig. 17.9). 

Lining carries the material being handled, transmits the pressure to the reinforcement, and is made of suitable compounded rubbers which are impervious to and unaffected by the material being handled.  For braided hoses, the lining compound should have good extrudability and flow readily around the braid.  The compound should have sufficient stiffness to withstand the tension during braiding and vulcanize to a smooth and seamless finish so that it offers minimum resistance to the flow of material in contact with it. 

Reinforcement: The reinforcement of hoses may be done in five different ways braiding, spiralling, weaving, knitting, and wrapping.  Braiding is a simple process which gives a flexible hose and is more resistant to misuse (better impulse and flex performance) than spiralling.  Spiralling can be done at higher speeds (10 times) than braiding, giving high bursting pressures and good impulse resistance. 

Circular woven hoses are foldable and are useful in fire hoses.  Vertical knitting can be done at four4 times the production rate of horizontal braiding machines and the hoses can have good flexibility and range of diameters, e.g. automotive coolant hoses.

Wrapping is suitable for large diameter hoses, e.g. marine hose.

The last three processes are not suited for high burst pressure applications. When more than one layer of reinforcement is used, an additional insulation rubber is generally applied between the layers. 

Cover: To prevent damage to the reinforcement member in service, a suitable rubber compound is used as cover.  The cover finish may be smooth or fluted.  The burst pressure is generally determined to be four times the operating pressure.  Burst pressure of a hose is calculated by the formulae below:

(a)   Braided or spiralled hose:

2 N R Sinθ

                        __________

            P =        DL

 

            P = burst pressure, 

Where N = no. of yarns or cords, R = breaking strength of single yarn/cord,  D = diameter, L = pitch length, and θ = braid angle.

(b)   Wrapped hose:

 

2 V F                           2 (2S R Sin θ) N

            P =       DL                  =                      DL

Where P = burst pressure, V = hoop force, D = mean ply diameter, L = length of fabric unit, S = fabric strength, N = no. of plies, F = efficiency factor, and θ = braid angle.

The angle of application of the reinforcing material around a hose lining is known as braid angle.  Braiding is normally done at 54° 44’ which is mathematically derived as the natural angle.  At this angle, the length and diameter change under pressure will be the minimum. 

Testing procedures and requirements for hoses are specified in various standards such as BIS, BS, ISO, DIN, and SAE. 

Hose Manufacture 

Although there are many different processes and machinery used for  making hoses these can be divided into the following broad categories by diameter, pressure, and production methods (plate 16, Exhibit 17.9). a classification of different hose types is presented in Fig. 17.10. 

Hand-made hoses: These cover a range of sizes from fractional bore vacuum hoses to 40” diameter sleeves. This process is most suited for large diameter hoses for oil suction and discharge (up to 600-mm diameters), dreading, etc.  These hoses are generally hand-built on mandrels using wrapping and are cured in vulcanizers.  Calendered inner liners is applied on mandrels, then rubberize woven fabric, and lastly, Calendered cover rubber, all to the requisite gauge and width.  Many layers of wrappers are applied and the mass cured in vulcanizers.  A common feature of many hand-built hose constructions is inclusion of several types of wire, braided wire, or woven fabric to improve the strength or collapse resistance.  For example, for oil suction and discharge hoses, a helix of heavy metal strip or a thick wire is built onto the carcass to provide rigidity.  Special end fittings r couplings may be built in to provide for special

Manufacture

 

 

 

 

High

Spiralled or

Braided,

Lead press/

Lead extruder

vulcanizer

Hand built on mandrels, reinforced with

Several types of wire, braided wire,

Woven fabric, wrapped and cured in

Vulcanizer,

Not suitable for automated or continuous manufacture.

20,000 psi

to

11,000 psi

 

Pressure

Psi

 

 

 

Medium

3000 psi

 

 

 

 

 

 

 

 

 

Low

20 psi

 

 

 

 

Hydraulic

 

 

 

Rotary oil drilling

 

 

Sand blast

 

 

Oil suction and

Discharge

Fire hoses

 

Off shore

Oil transportation,

Aircraft refuelling

marine

 

Food

Welding

Fuel & emission

Automotive

-hydraulic brake

-radiator

-air brake

-power steering

water

gas/air

LPG

 

 

 

 

 

 

 

 

 

 

Oil dock

Water

 

 

 

 

 

 

 

 

 

 

Chemical

Hose

(XLPE cover)

0

38                                                                      150

       Hose bore (mm)→

 

The process is not suited for continuous vulcanization methods, but development efforts are going on to make large bore hoses with semi-automatic or automatic devices. 

Circular woven hoses: Fire hoses are mostly produced by this method.  Yarns are applied with a circular knitting machine.  The pattern may be plain knit, lock stitch, or warp knit, depending on the design of the knitter.  The process is usually limited to low burst pressure use such as garden hose and radiator hose.

Reinforcement for fire hoses consists of long lengths of fabrics woven on circular looms.  The warp is run in the longitudinal direction and weft threads are interlaced to produce a seamless circular weave.  The lining is either extruded or hand-built from Calendered material, either unvulcanized or partly vulcanized; this is inserted into the woven jacket. 

The ends are clamped and steam is admitted through the lining.  This blows out and presses the lining against and into the jacket during vulcanization.  The woven fabric itself acts as a cover and is usually treated to prevent mildew formation.  The main advantage of this hose is that it can be coiled flat and, therefore, a long length can be accumulated in a small coil-size range is 37-62 mm is diameter. 

Wrapped ply hoses: In  this process, low pressure hoses up to 40 m continuous length can be made on simple, inexpensive equipment with accurate bore diameter.

The process involves wrapping several plies of rubber impregnated square woven fabric cut at 45° around an extruded tube on a mandrel in three roll building machine.

The mandrels are usually made of steel for smaller diameter hoses and of light alloy nominal bore diameter of the hose, and its maximum length can be 40m.  The cut fabric is applied lengthwise in strip form through rollers.  Extruded cover is also applied lengthways and consolidated. A wrapping cloth (usually of nylon) is applied over the built hose and the assembly is vulcanized.  After vulcanization, the cloth is removed and the cured hose extracted from the mandrel.  These hoses have poor appearance and low flexibility.  For suction applications, hoses can be built with metal helix or springs under or above the inner lining. 

Braided/spiralled hoses

Braided reinforcement: Braiding is the most popular method of reinforcing industrial hoses.  Before braiding, a mandrel which is equal in diameter to the inner.  Tube is inserted into the tube.  This is done in order to avoid the tube from collapsing under pressure of the braiding threads.  For convenience, the maximum lengths produced at a time are from 10-15 m. 

A braid is formed by the interweaving of yarns while they are helically spiralled over the tube.  Half of the reinforcing yarns spiral in a clockwise direction while the rest move counter-clockwise giving usually a 2 up-2 down braid pattern. 

Size range: 3-100 mm bore diameter.

A braiding machine may have one or two decks.  On double deck machines, two layers of braiding can be applied simultaneously. There are two types of braiding machine: vertical braider, also called planetary or “maypole” braiders and non-planetary or horizontal braiders (Table 17.9) 

Spiral reinforcement: Spiral reinforcement is applied in separate plies which are always in opposite bias and remain separated by a Calendered rubber sheet, dough, or cement. Since it requires two spiral layers to replace one braid, it is important that there is “strike-through” from tube to cover to bond the two layers, otherwise an extra layer of rubber must be added. 

Vertical and horizontal braiders-a Comparison

 

            Vertical braider                                              Horizontal braider

1.                   Economical for long length moulded hose                      Not economical for manufacture of long

Long continuous length eliminates use of                     length moulded hose. Length is governed

Couplings and hence the possibility of                            by length of mandrels and ancillary

Leakage from them                                                             equipment such as covering and

                                                                                                Wrapping machines and pans

 

2.             Variation in bore and other components                         May use both solid and flexible mandrels,

Cannot be closely controlled unless a flexible               Therefore, dimensions can be accurately

Mandrel is used.                                                                  Controlled.

 

3.             Maximum strength of reinforcing yarn cannot             Maximum strength of yarn can be utilized

                be obtained since raw hose is not made to                       as raw hose is made to finished design

                finished design

 

4.             Calendered rubber sheet cannot be used for Calendered rubber sheet may be used for

                lining                                                                                     lining

 

5.             Rigid mandrel cannot be used                                           Rigid mandrel can be used

 

6.             Takes less floor space                                                        Takes more floor space

 

A Comparison of braiders and spiral machines is gives in Table 17.10.

 

Comparison of braiders and spiral machines

 

                                                            Vertical           Horizontal                   Spiral

                                                            braider            braider                        machine

                                                            (24 carrier)     (24 carrier)

            Deck speed (rpm)                  33                    100                              240-600

            Bobbin size (cm)                     640                  190                              476-2300

            Carrier tension (kg)               0.9-2.7             0.2-0.7                         0.05-0.45

 

The speed of rotation determines the output.  Bobbin size and number of bobbins also influence the output.  Carrier tensions are linked with the type of reinforcement yarns used.  Higher tensions are required to give maximum burst pressure and the minimize dilation, so for some hoses, vertical braiders are still a necessity.

            Spiralling machine operate at a very high speed.

            Size range: 3-100 mm bore diameter.

 

Flexible and rigid mandrels: In the flexible mandrel method, lining is extruded over flexible mandrel made of nylon or polypropylene through a cross head extruder and then reinforced by braiding or spiralling through a horizontal braiding or spiralling machine.  After reinforcement, the hose is rubber covered through a cross-head extruder and then lead covered by lead extruder of lead press. 

In the rigid mandrel method, lining is blown to a rigid mandrel made of steel (for low diameters) of light alloy (for larger diameters), treated to prevent sticking.  Since the length is restricted to 40 m only, flexible mandrels are preferred for critical uses such as high pressure hydraulic hoses, even though the production cost is very high.  Hydraulic hoses are generally reinforced with rayon, polyester, polyvinyl alcohol, aramid, or high-tensile brass coated steel wire, depending on working pressure requirement. 

Society of Automotive Engineers (SAE) has formulated standards for hydraulic hose-grades R1 to R11. For example, SAE 100 R1 is a rubber hose with one layer of wire braid reinforcement, sizes ranging from 10 mm to 50 mm and for working pressure ranging from 50 bar to 200 bar.  It is used for medium pressure applications.  Hose SAE R11 has six layers of wire spiraled in opposite directions around the tube.  Hydraulic hoses may be manufactured for very high pressure u9p to 11000 psi or 750 kg/cm².  NBR based lining compound and CR cover compounds are usually applied to meet the service conditions. 

The lining is stiffened by freezing with liquid nitrogen before steel cord reinforcement under high tension is applied over it. 

Moulding: Raw hose is now inflated up to the desired outer diameter by light air pressure in lining and then lead covered by passing it through a lead extruder or lead press. Lead extruders, similar to that used in lead cable industry, are used.

Lead (99.9% purity) is melted in an iron kettle and is poured into a water-jacketted mould to cast billet.  The billet is allowed to cool and is placed inside the lead press (a ram type extruder cavity) by mechanical aid.  The ram then moves into position and presses the lead billet through the die assembly to extrude a lead pipe.  The hose is inflated with air and is fed into the press for lead covering.  The lead sheath may heave a smooth or a fluted inside surface which is imprinted on rubber cover of the hose.  The hose in the lead sheath is then coiled on iron reels. 

Curing: Ends of the lead covered hoses are cut, the hoses in lead sheath are filled with water, and the reels are transferred to a horizontal curing pan.  The hose is given as open-steam cure, maintaining a constant water pressure inside the lead-covered hose through out the cure. 

Lead Sheath Stripping: Cured hose is passed through a stripping machine where the lead sheath is taken off the hose, cut into small pieces, and the pieces are fed back to lead kettle for reuse.  The lead sheath imparts a smooth or corrugated finish which is characteristic of long length hose. 

TESTING OF HOSE: 

Common tests which are used for hose are:

(a)               visual check on appearance and finish;

(b)               dimensional check;

(c)                braid angle and adhesion between the components;

(d)               physical properties of rubber components (initial and aged); and

(e)               proof test and burst pressure test.

 

Hoses cannot be used at high operating pressures in hydraulic systems without end fittings.  Couplings or end fittings must also be made to close dimensional tolerances to resist high fluid pressures.  The proof pressure test is conducted at twice the working pressure.  In the case of rotary oil drilling hose, for example, the proof test pressure is 10,000 psi and 5000 psi in actual service, while swirling and twisting in an oil rig.  With couplings applied, a length of rotary petroleum drilling hose can weigh 500 kg or more. 

Couplings may be permanent (one piece) or re-usable (with connecting ends).  Toi basic components make up a hose connection-the hose attachement which holds on to the hose end and the fitting connection which holds the hose assembly to the point of transfer.  Hose can be purchased in relatively long lengths or in complete assemblies. 

Hoses and assembly life is influenced by the following factors:

(a)             the design of the hose;

(b)             the design of the fitting/coupling;

(c)              the technique of fitting;

(d)             the technique of mounting the hose assembly onto a machine;

(e)             the working conditions; and

(f)               environmental conditions.

 

The tests usually specified in hydraulic hose specifications are the leakage test, burst test, and impulse test.

 

Leakage test: This is a destructive test which is used for sampling only.  Here, a hose with end fitting is subjected to 70% of the minimum burst pressure for 5 min., then the pressure is reduced to zero after which the initial pressure is reapplied for 5min.  These should be absolutely no sign of leakage or any evidence of failure in hydraulic hoses.

 

Brust test : This is a destructive test done on hose assemblies which are gradually pressurized up to the burst pressure to check if any leaks or indication of failure occurs below the burst pressure.

 

Impulse Test: In this test, hose assemblies are impulse on suitable test rigs with the hose bent to its minimum recommended bend radius.  Hoses less than 25mm nominal size are bent within 90° or 180° and hoses of 25mm and above are bent 90°.  The test fluid is circulated at the specified temperature at the impulse rate of 30-100 cpm.  the pressure wave applied may be in square or secant or half-omega form.  Specifications indicate the minimum number of impulse cycles to failure (Fig. 17.11)

 

Other type tests include swelling, steam ageing, ozone and low temperature resistance, concentricity, whip, pull-out, and salt-spray tests.

 

FAN AND V-BELTS

 

Fan belts are used in automobiles for driving the fan pulley and the dynamo pulley, and are designed to suit the layout of the automobile.  V –belts are used for transmitting power between V-Shaped pulley (sheaves) of short-centre distances.  The V-belt is chosen in preference to the flat belt since the wedging action between V-belt and pulley grooves enables higher power transmittance.  Transmission systems may be standard, synchronous, variable speed, or free-clutch systems.

 

The main types of V-belts used for power transmission may be classified into wrapped, raw edge (cogged or uncogged), V-ribbed (serpentine), or self-tensioning (maintenancefree) categories (Fig. 17.12).  The traditional V-belt is jacketted with woven fabric, raw V-ribbed belt replaces sets of V-belts by a single belt tracked in a serpentine fashion through the accessory sheaves, the tension being automatically maintained by one single take up. The maintenance free V-belt is self-tensioning during operation by means of frictional heat which causes the reinforcing polyester cords to shrink.

 

Construction:

 

Generally fan belts and V-belt have the following components:

(a)             base rubber

(b)             cord;

(c)              top rubber; and

(d)             rubberized fabric jacket.

 

The cord is held in place or supported by rubber in the finished belt, which also transfers stress from one cord to another, of from the pulley to the cord.  In some belt section, it is usually to place the cord layer in neutral axis so that the cord is less fatigued during run on drive. 

The cord is usually polyester or rayon, with some usage of steel or glass fibre for special applications.  Polyester and rayon have good dimensional stability, fatigue resistance, and flexibility.  A single layer of cord is used in smaller cross-sections and multi-layers in larger sizes.  The jacket helps in achieving positive traction, flexibility wear resistance, and is resistant to oils. 

V-Belt Manufacture 

Base and cushion (top) compounds are calendered into thin sheets of specified thickness and batched on liners.  These are not extruded, but built up from sheet, in order to avoid a transverse joint. 

For jacket, cotton or blended cotton canvas is frictioned or spread with a suitable rubber compound, and then cut into specified width and bias angle, and batched on liner.  Carcass building is done on single-drum or two-drum building machines. 

The single-drum building machine essentially consists of an expandable metal drum, over which a rubber sleeve is fitted.  The drum is rotated, and a cylinder composed of the components of V-belt is built over it.  The calendered base rubber is first built up to the required thickness.  One layer of calendered cushion rubber is applied over it and then the treated cord is spirally wound on the drum at the specified pitch under tension.  Thereafter, further layers of cushion and filler rubber are built over it.  Then the built up cylinder is slit into rectangular sections of desired width with circular rotating knives and taken out from the drum after collapsing the drum.  The cut sections are skived to trapezoidal shape with the help of two circular angularly placed rotating knives.  The raw belts are jacketted with rubberized fabric in a jacketing machine.  

Long belts are usually built on a two-drum machine, with the distance between the drum axes being adjustable, both for setting the required length and also for removal of the belt.  Equal cord tensions are an important factor in achieving minimum spread of belt lengths in the finished product.  This is necessary in industrial drives where several belts are fitted in parallel on multi-groove pulleys.  The cord elongation must be minimal. Predictable, and uniform. 

Vulcanization: The processes usually employed for vulcanizing V-belts use ring moulding, press moulding, or rotary vulcanizing method (Plate 16, Exhibit 17.10; Fig. 17.13). 

Ring Moulding: Jacketed raw belts are placed between two rings of a multi-cavity circular mould. The moulds with the belts are then bolted up axially and wrapped with wet wrapping (impression fabric) cloth under tension so that it can exert pressure on the top of the belt during cure.  The wrapped mould is then placed in an autoclave for vulcanization in open steam.  After cure, the belts are taken out from mould cavities.  A separate set of rings has to be available for each size-and cross-section of belt required. 

Double-deck daylight: Presses may be used with V-grooved moulds for vulcanizing long V-belts.  The load, consisting of a number of belts, will be supported by drums or pulley sets mounted so that tension can be maintained in the belts during vulcanization.  The mould ends are water cooled so that pressure can be maintained in the hot rubber as it passes through a low viscosity stage before vulcanization commences.  The belts are rotated through the machine between vulcanization steps so that sections are treated successively. 

Rotary Vulcanizers: Consist of a rotatable grooved drum maintained at a high temperature.  The load of belt carcasses in hung over the drum, one belt to each groove, and an apron covering about half the circumference of the drum is applied at a high tension.  The drum and the back of the apron are heated.  The drum speed is controlled so that the belt is vulcanized in half a revolution.   An auxiliary cold grooved roller maintains the tension or length of the belts during the process with the rotary vulcanizer, one machine can be used for a wide range of belt lengths (Fig 17.13).  In the case of raw-edge belts, the individual belts are cut from vulcanize sleeves. 

MAIN TYPES OF POWER TRANSMMISION BELTS 

Conventional V-belts are of standard sections (namely A, B, C, D, E, and Y, Z) for specified top width and depth as given in Table 17.11 

V-belt standard section                                                                      Table 17.11

            Section                        Top width (mm)                      Depth (mm)

            A                                             13.0                                         8

            B                                             17.0                                         11

            C                                             22.0                                         14

            D                                             32.0                                         19

            E                                              38.0                                         23

            Y                                              6.5                                           4

            Z                                              10.0                                         6

 

Wedge section belts SPZ, SPA, and SPC have similar top widths to the sections Z, A, B, C but with a lower top width to height ratio (1.2).  With a narrower cross-section, belts are smaller and weigh less; the sheaves are also smaller and weigh less.  The belts transmit more power at the same pulley diameter or it is possible to achieve a required horsepower rating with fewer wedgebelts, or to downsize drive motors, or to increase drive efficiency. 

Cogged construction provides high flexibility for small sheaves or for drives with relatively short centre distances.  The cogs also provide a larger surface area to dissipate heat and prolong belt life. 

Variable speed belts have to fit between the flanges of the variable speed pulleys, consequently they are much wider than the conventional V-belt.  They can be used on small pulley drives where the belt stresses are the greatest. 

Hex belts (also know as double V-belts) are designed for use on drives with one or more reverse bends.  They usually transmit power from both sides of the belt. 

Sometimes V-belts in a drive can resonate into pronounced vibration, turn inside out, and rapidly break up.  To avoid this, a set of belts can be built with multiple belts joined by a backing fabric that regulates belt travel so that all ribs pull together as a single, perfectly matched group. 

Mismatching of individual belts in a drive, misalignment, shock loads, pulsation, and vibration can reduce drive efficiency.  Open and V-belting is used in applications where endless V-belts are difficult or impossible to install.  They are fastened and are usually employed as an emergency replacement when the exact length to endless belt is not readily available. 

Poly V-belts are used on automobiles to drive the accessory equipment.  One belt drives all of the accessories and a spring loaded tensioning device eliminates need for belt adjustment.  The belt design incorporates the power transmission capabilities of V-belt with flexibility of flat belts. 

LATEX PRODUCTS 

There are several advantages in using a latex for producing rubber articles. 

(1)   It can be shaped and worked easily and thus find certain application where dry rubber or a rubber solution cannot be used.  (2). It does not require any heavy or expensive machinery.  (3) Power requirement is less. (4) Certain latex processes are amendable to continuous operation.  (5) The physical properties of latex articles are usually good.  (6) Latex is safer to handle than rubber solutions. 

The disadvantages are: (a) Latex processing requires strict controls and batch-to-batch adjustments due to variability in latex properties.  (b) Latex Processes are not suitable for making rubber goods of heavy or bulky nature. (c) Fillers do not reinforce latex, they act as diluents reducing the product properties.  (d) Latex is usually more expensive then dry rubber of comparable rubber contents. 

PREPARATION OF INGREDIENTS 

Like dry rubber compounding, the latex should be mixed with vulcanizing ingredients in order to cure the dried film.  Other additives like anti-oxidant, softeners, and fillers are also added to latex to impart the desired properties in the finished articles.  In compounding latex, the fillers and vulcanizing ingredients must be in the form of small particles completely wetted.  Otherwise, coagulation or quick settling out of the powders from latex will occur.  Both grinding and wetting are carried out by means of pebbles and is mechanically rotated.  Grinding of the powders takes place between the pletely wet.  The efficiency of ball milling depends on the size of the pebbles, amount of pebbles and water used, and speed of rotation.  At least 24 hours is usually necessary to prepare a satisfactory dispersion by ball milling. 

To ensure uniform mixing of compounding ingredients and to avoid subsequent settling out, water-soluble ingredients are added to latex as aqueous solutions and water insoluble ingredients (except for fillers) as aqueous dispersions or emulsions. 

Sulphur, zinc oxide, accelerator and solid anti-oxidant dispersions are prepared in a ball mill, vibro-energy mill, or attritor. A suitable general purpose formulation for variety of compounding ingredients is given below: 

Parts by weight

Additive

Dispersant

(sulphonate type)

Soft water

50.0

 

2.0

48.0

100.0

 

Ball mill for 24-48 hours.  The dispersion is checked once or twice during milling to deal with any problems that may arise, such as, excessive foaming, which will reduce dispersion efficiency.  Certain anti-oxidants, such as substituted phenols may require a different formulation such as 

Parts by weight

Anti-oxidant

Dispersant

Bentonite clay

Polyvinyl pyrollidone

Sodium dodecyl sulphate

Water

50.0

2.0

0.5

1.0

0.1

46.4

100.0

 

Anti-oxidant dispersions frequently became viscous on milling due to foaming or wetting effects.  Dilution to 40% of active ingredient or use small amounts of antifoaming or wetting agents will overcome the problem. 

Liquid compounding ingredients which are not soluble in water, such as certain accelerators, anti-oxidants, and plasticizers, are normally added to latex as oil-in-water emulsions. Emulsions are prepared using a high-speed stirrer or homogenizer to a general formulation of the type given as follows. 

Parts by weight

Liquid compounding ingredient

30% Potassium oleate solution

Soft Water

50.0

7.5

42.5

100.0

 

The liquid and surfactant may be added to water in a slow stream until a satisfactory particle size has been obtained.  Alternatively, in the case of viscous liquid ingredients, the water may be added to the oil phase gradually to give first a water-in-oil emulsion and then an oil-in-water emulsion. 

Fillers are usually added to the latex compound as aqueous slurries.  They essentially act as diluents and have the effect of reducing the strength properties of the vulcanizate, unlike dry rubber compounding where “reinforcing” fillers are used. 

STABILITY OF LATEX COMPOUNDS 

Prior to further concentration, the addition of fillers and other compounding ingredients,  the raw latex has to be stabilized agaist conagulation.  This is done with 0.25% caustic soda made up as a 5% solution in water.  The addition of zinc oxide to the latex is generally left until the last, as the oxide a thickening and destabilizing effect.   

When added a latex in the presence of ammonia, a small amount of zinc oxide dissolves in accordance with the equations. 

            ZnO + 2NH3 + 2NH4 ÕZn (NH3)4++ + H2O

                        Zn(NH3)4++ Õ(NH3)x++ + (4-x) NH3,

Where x = 0 to 3. 

The lower zinc ammine ions, such as Zn (NH3)2++ from uncharged compounds (zinc ammine soaps) of low solubility by reaction with the fatty acid soaps at the surface of the rubber particles.  Typically, this leads to a slow increase in the viscosity of the latex (zinc oxide thickening) accompanied by a loss in mechanical stability.  Therefore, further addition of a stabilizer such as anionic or non-ionic surfactant is usually necessary. 

In many important latex manufacturing processes, and e.g. production of dipped gloves, moulded foam, and latex manufacturing processes, and e.g. production of dipped product.  In the course of gelation, the rubber particles aggregate into a three-dimensional network without separation of the aqueous phase, which is retained in the interstices of the network.  There are three basic methods of effecting gelation involving (a) use of acids or acid-producing substances; (b) use of calcium salts; and (c) heat latex thread, and in the Dunlop process for moulded foam rubber.  Calcium salts are polyed in dipping compounds and foam manufacture.  However, certain processes, such as adhesives, do not require a gelation step.

 Manufacture of Latex Products 

The preparation of rubber articles from latex is briefly described in Table 17012 

Latex Processes                                                                                 Table 17.12

            Process                                                           Typical product

            Dipping                                   Balloons, gives, nipples, fountain pen sacks, etc.

            Casting

            (i) Heat sensitization              Foamed latex sponge,

            (ii) Plaster casting                  Moulded toys, etc.

            Spreading, doubling,              Toys

            Impregration                          Coated fabrics, paper, etc.

            Bonding                                  Adhesives

 

Manufacture of latex foam The manufacturing process for latex foam essentially consists of: (I) Compounding of latex; (ii) expanding (foaming) to a desired volume; (iii) setting (gelling) the rubber particles; and (iv) vulcanizing. 

There are mainly two methods of making latex foam, namely, (a) Dunlop process and (b) Talalay process, which differ mainly in foaming and gelling method.

Dunlop process Table 17.13 gives the typical formulations for the Dunlop Process for making foam. 

The Dunlop process for making latex foam                                     Table 17.13

A typical formulation             Compounding             Parts by weight

 

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