i

HAUL TRUCK TIRES AND OPEN PIT MINING APPLICATIONS

by

Patrick McGarry

A thesis submitted to the Department of Mining Engineering

in conformity with the requirements for the

Degree of Bachelor of Science

Queen's University

Kingston, Ontario, Canada

April, 2007

Copyright © Patrick McGarry, 2007

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ABSTRACT

The importance of tires to the vitality and productivity of the mining industry

should not be overlooked. Tires are a critical wear part of material handling in open

pit mines, significant to productivity and operating costs. Currently there is a tire

shortage in the availability of medium and large tires. Due to increased mining

activities at existing mines and the development of new mines, the demand for off the

road tires (OTR tires) has increased significantly. With this in mind, is it particularly

important for mining engineers to be aware of how to best preserve the life of tires.

To do this, a working knowledge of the tires material components and how they work

together is needed.

The most important aspects of tire maintenance are: maintaining recommended tire

pressure, and obeying Tons Kilometer Per Hour and heat restrictions. It is important

to ensure that the pressure in the tire is neither too high or too low, as this will make

the tire more susceptible to damage. As well, obeying the Tons Kilometer Per Hour

ratings is needed to ensure that the tire does not overheat which also causes damage

and reduces the life of the tire. Calculating Tons Kilometer Per Hour is also how

mining engineers select the appropriate tire, so it is very important that mining

engineers understand the heat restrictions of mining tires. Finally, the process of re-

treading - removing the worn tread of a tire and replacing it with a new one - is a

particularly cost effective way to prolong the life of the carcass of the tire.

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ACKNOWLEDMENTS

I would like to thank Garston Blackwell for his guidance and his assistance in

finding research information. I would also like to thank Martin Doyle of Michelin

Canada for providing research information.

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TABLE OF CONTENTS

Chapter Page

1.0 INTRODUCTION……………………………………................ 1

2.0 TIRE COMPONENTS...…………………………….. ………… 2

2.1 The Bead Bundle…………………………………………… 2

2.2 The Inner liner…………………………………………….. 3

2.3 The Body………………………………………..………….. 3

2.4 Belts……………………………………………..…………. 4

2.5 The Sidewall………………………………………………. 4

2.6 The Tread…………………………………………………. 4

3.0 MANUFACTURING TIRES………………………...…………. 6

3.1 Compounding and Mixing………………………………. 6

3.1.1 Compounding……………………...…………. 6

3.1.2 Mixing………………………………..…………. 8

3.2 Component Preparation…………………………………. 10

3.2.1 Calendering…………………………..………….. 10

3.2.1.1 Cord Preparation……………………….. 11

3.2.2 Extrusion……………………………..………….. 12

3.2.3 Bead Building………………………..………….. 14

3.3 Tire Building…………………………………………….. 15

3.4 Curing……………………………………………………. 16

3.5 Final Finishing…………………………………………… 18

3.5.1 Force Variation………………………………….. 18

3.5.2 Conicity………………………………………….. 19

3.5.3 Radial Run Out…………………………………... 19

3.5.4 Lateral Run Out……………………..…………… 19

3.5.5 Sidewall Bulges and Depressions……………….. 20

3.5.6 Tire Balance……………………………………… 20

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TABLE OF CONTENTS

Chapter Page

3.5.7 Tire Testing………………………………………. 20

4.0 RUBBER…………………………………………………………. 22

4.1 Rubber Properties………………………………………… 26

5.0 FUNCTIONS OF A TIRE………………………........................... 27

5.1 Carrying a Load………………………………………….... 27

5.2 Guide or Steer a Vehicle…………………………………... 32

5.3 Transmit Engine or Brake Torque………………………… 32

5.4 Absorb Shock……………………………………………… 32

5.5 Ability to Roll……………………………………………... 32

5.6 Last as Long as Possible…………………………………... 33

5.6.1 Tire Maintenance………………………………….. 33

5.6.2 Tons Kilometer per Hour…….……………….. 38

6.0 WHY TIRES BUILD UP HEAT...………………………………... 41

7.0 RETREADING……………………………………………………. 46

8.0 TIRE ROTATION………………………………………………… 48

9.0 TIRE SHORTAGE………………………………………………… 49

10.0 CONCLUSIONS and RECOMMENDATIONS………………….. 53

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LIST OF TABLES

Table Page

1. Rubber Content Found in Tires in different Types of Tires ………. 7

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LIST OF FIGURES

Figure Page

1. Para Rubber Tree Tapped for Latex Collection …………………… 24

2. Model of Tire as Springs Supporting Load……………………….... 28

3. Force Distribution Around Tire with No Load…………………….. 29

4. Force Distribution Around Tire with Load Applied………………... 30

5. Deflection of Tire vs. Load for Different Pre-Stress Pressures…….. 31

6. Cracks in Bead area of Tire as a Result of Under Inflation………… 34

7. Cracking of the Bead are of Tire as a Result of Under Inflation…… 35

8. Puncturing Damage to Underside of Tread as a Result of Over Inflation 36

9. Chunking Damage to Tread of Tire as a Result of Overinflation..… 37

10. Height of Object Which Poses a Puncturing Threat to Tire vs. Pressure 37

11. Time to Reach Stabilized Temperature…………………………….. 39

12. Force Exerted on Tire vs. Time as Tire Rolls……………………… 41

13. Tire Modeled as Spring with Dampening Device.………………… 42

14. Damage Frequency vs. Tire Temperature………………………….. 44

15. Example of Heat Separation as a Result of Over Heating………….. 45

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CHAPTER 1

1.0 Importance of Tires in Open Pit Mining

Industries such as the mining industry rely on productivity to generate profits. Any

situation where equipment essential to productivity breaks down, production is lost. An

example of such a piece of equipment is haul trucks. When a haul truck stops moving

profit is being lost. Loss of profit does not stop at the mining truck itself, a

maintenance crew must be called out to inspect and correct the problem. Disabled haul

trucks may also be impeding or totally blocking the path of other haul trucks. Thus,

properly maintaining a fleet of haul trucks and loading equipment is crucial to

maintaining high productivity. Wear part monitoring, maintenance, and replacement

are important aspects of maintaining high productivity. Tires are an important wear

part to consider as tires are directly linked to haul truck availability and performance.

Due to a growing shortage of haul truck tires and haul truck tire costs, tire conservation

has become an important issue in today s mining industry.

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CHAPTER 2

2.0 Tire Components

There are several components making up a tire. These components include the Bead

Bundle, the Inner liner, the Body, the Belts, the Sidewall, and the Tread.1

2.1 The Bead Bundle

The bead bundle consists of steel wire, bead filler, and chafer. The bead bundle is

used to strengthen the tire and to hold the tire tightly against the rim. It also acts as an

anchor, holding the plies and belts of the tire in place. The forces applied to the tire are

transferred from the truck through the bead bundle to the rest of the tire, making the

bead bundle a very important component of a tire. The bead bundle also gives the tire

the strength required to handle the forces applied by machinery when mounting the tire

on to the rim. The steel wire loop is the main component of the bead bundle as it acts

as an anchor, holding the tire tightly against the rim and it transfers forces to the rest of

the tire. In the case of haul truck tires, three or four wire loops are included in the bead

bundle. The bead filler consists of a very hard rubber compound and is molded in to a

wedge shape. The primary function of the bead filler is told hold the steel wire in place.

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The chafer is used to protect the bead bundle from chafing against the rim, to prevent

the tire from being damaged during mounting or dismounting, and to ensure that the tire

does not rotate on the rim during use.2

2.2 The Inner liner

The inner liner normally consists of a double layer of synthetic rubber used to seal

air in the tire. In the past, tires required an inflated inner tube. However, inner liners

now replace the inner tube.3

2.3 The Body

Depending on the type of tire there can be several different layers of plies that make

up the body of the tire. Plies are made of layers of fabric and rubber. The fabric

component is generally made of polyester, nylon or rayon cord. However, large haul

truck tire plies are generally made up of nylon. The plies are orientated in one of two

ways, bias ply or radial ply. When the plies are at an angle to the direction of travel the

tire is called bias ply. Generally speaking the plies of a bias ply haul truck tire are at an

angle of 30 degrees to the centre line of the tire and alternate in direction. Radial ply

tires generally have one heavy ply made up of steel cables. This single ply runs

perpendicular to the direction of travel of the tire from bead to bead. Radial ply tires

were introduced much later than bias ply tires and provide less rolling resistance as well

as better resistance to puncturing and heat build up. Today s haul trucks generally use

radial ply tires to take advantage of the benefits these tires provide. In both radial and

bias ply tires each layer is covered with rubber in order for the plies to stick together

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and to prevent air from leaking out of the tire. The rubber coating also improves the

strength of the tire. The number of plies contained within the tire can be used as a

measure of tire strength. A tire with a strength rating of 30 plies does not necessarily

contain 30 plies. As a comparison, car tires have 2 plies while haul trucks can have 30

to 40 or even more in certain cases.3

2.4 Belts

Steel belts are added to the tire under the tread. These belts improve the tires

resistance to puncturing and help keep the tire s contact area flat which improves the

contact between the tire and the ground. Using steel to construct these belts increases

the rate at which heat can be dissipated.4

2.5 The Sidewall

The sidewall of a tire prevents the tire from bending and folding to the side, adding

lateral stability. Lateral stability is particularly important during cornering and hard

braking. The sidewall facilitates the transmission of force from the ground to the tire

and also gives protection to the body plies of the tire and prevents air from escaping.

Additional belts can be added to the sidewall to increase lateral strength. The air

pressure of the tire holds the sidewall out, allowing it to support some of the load.2

2.6 The Tread

The tread of haul truck tires are made of several different types of rubber, both

synthetic and natural. The tread of a haul truck tire provides protection for the body of

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the tire and traction between the ground and the tire. The tread is a compromise

between soft and hard rubbers. If the rubber compounds used to manufacture the tread

are too soft the tire will wear far too quickly, and if the rubber compounds are too hard

the tire s traction will suffer. Thus, it is important that the tread of the tire has been

properly balanced between good traction and long life.1

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CHAPTER 3

3.0 Manufacturing Tires

There are several processes required for tire manufacturing. These processes require

individual plants. Frequently, these plants are separate factories or separate parts of a

larger factory. In general there are five steps in the tire manufacturing process, these

steps include: Compounding and Mixing, Component Preparation, Tire Building,

Curing, and Final Finishing.4

3.1 Compounding and Mixing

3.1.1 Compounding

Depending on the ultimate usage of a tire, the ingredients and the amount of each

ingredient vary. The ingredients for tire production are processed in batch form and the

time required to process a batch of rubber compound is roughly three to four minutes.

Although there can be many different types of rubber used in tire production, the most

commonly used types of rubber are: natural rubber, styrene-butadiene rubber (SBR),

polybutadiene rubber (BR), butyl rubber, and halogenated butyl rubber. Natural rubber,

SBR, and BR are generally used in the sidewalls, tread, and other parts of the body,

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while butyl rubber and halogenated butyl rubber are common ingredients for the inner

liner of the tire and used to contain air. The amount of each type of rubber used in a

batch depends on the final use of the tire.5 See Table 1.

Table 1. Rubber Content Found in Tires in different Types of Tires.3

Natural rubber is the main elastomer that is used in the tire making process. Styrene-

butadiene is used because it has high resistance to abrasion and good resistance to

aging.6 Polybutadiene rubber is used because it exhibits high resistance to wear and

abrasion, while it also exhibits low heat build up characteristics.7 Butyl rubber and

halogenated butyl rubber are used in the inner liner because of their low permeability to

air. In the case of halogenated butyl rubber, halogen atoms provide a bond between the

layers of rubber and the body of the tire.5

Several types of chemicals are added to the mixture for a variety of reasons.

Reinforcing chemicals and filler chemicals such as black carbon and silica are added to

ensure that the tires have the desired properties for specific applications. Black carbon

is used as an additive due to its high tensile strength characteristics as well as its ability

Type of Tire Synthetic Rubber (%) Natural Rubber (%)

Passenger Tire 55 45

Light Truck Tire 50 50

Haul Truck Tire 20 80

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to conduct heat away from structurally important areas of the tire, such as the tread and

the belts. By weight, black carbon is the most important additive. In some cases, as

much as 50 percent of the weight of the rubber being used will be added in black

carbon.8 Silica is added due to its low heat build up properties. Anti-degradants,

antioxidants, and antiozonants are added to the mixture to prolong the life of the tire

and to help slow down the degradation process which naturally occurs. Oxygen reacts

with the rubber and causes it to lose its adhesive properties. Sunlight is a major source

of degradation as it acts as a catalyst between the rubber and oxygen. Oxidation of the

rubber will cause cracking in the sidewalls of the tire if anti-degradation chemicals are

not used. Curatives such as sulfur and accelerators are added to the rubber mixture to

make the rubber more malleable and to increase the elasticity of the rubber. Curatives

and accelerators are also very important during the vulcanization process.5

3.1.2 Mixing

The ingredients are added in to a mixing unit called a Banbury. Once in the

Banbury, mixing using two counter rotating rotors occurs. The compound is mixed in

several stages, normally three to four, with additives being introduced at the appropriate

stage. The shearing action of the mixing process creates a great deal of heat and thus

the rotors and the housing unit of the Banbury are water-cooled. If the temperature of

the rubber compound is not maintained at suitable levels, vulcanization will occur too

early in the tire manufacturing process and the batch of rubber will be rendered useless.

Temperatures in the Banbury can rise as high as 160 to 170 degrees Celsius. Curatives

are added in the final stage of the mixing process. During this stage of mixing, the

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temperature of the rubber compound is maintained around 110 degrees Celsius. After

mixing the rubber is a black, sticky, hot compound.5

The next phase of the mixing process is milling. The rubber compound is dumped out

of the Banbury and in to a rubber mill. A rubber mill consists of two counter rotating

rotors, normally one of which is serrated. The milling process is similar to the mixing

processes, however no new ingredients are introduced during this process and the

rubber is maintained at a lower temperature. The milling process mechanically works

the rubber, further mixing the different ingredients together. The process is repeated

several times to ensure that proper mixing is achieved. The number of times that the

rubber compound will be milled depends on the type and quality of rubber being

produced. The rubber compound is removed from the milling process as a strip or slab.

The slab of rubber is then cooled so that component preparation can begin.5

Mixing and milling systems are frequently controlled using power integration methods

and the current and voltage supplied to the mixing and milling systems are monitored.

Once a certain amount of energy has been mechanically imposed on the rubber,

compound mixing or milling is stopped.4

After mixing and milling has been completed the rubber compound will ideally be a

very continuous and a uniform mix of the rubber ingredients, however in practice this is

seldom the case. The concentration of ingredients will vary throughout the rubber

compound and as a result each tire produced through the process will not have identical

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properties. Causes of inefficient mixing are frequently the result of; temperature

fluctuations in the Banbury housing unit which cause fluctuations in the rubber

compounds viscosity, excessive rotor clearance which results in inefficient mixing,

worn out rotors, and inefficient flow of the rubber compound through the Banbury

unit.4

3.2 Component Preparation

The component preparation stage of tire production is divided in three categories:

calendering, extrusion, and bead building. Each component will go through one of the

categories based on the process required for each component.5

3.2.1 Calendering

Calendering is the process of bonding rubber to the fabric or steel plies that make up

the body or carcass of the tire by applying pressure. The inner liner of the tire is also

manufactured through calendering. A calendar is a piece of heavy equipment outfitted

with three or more large steel drums. To begin the process, a piece of cord or ply,

which is generally 2 meters wide, is passed between two of the large steel drums. The

rubber compound processed in the preceding steps is then introduced between the two

steel drums both above and below the cord. Due to the pressure that is applied by the

steel drums the rubber will bond with the surface of the cords. To ensure proper

bonding occurs an appropriate amount of tension is kept in the cords as they pass

between the steel drums. Another important factor in the bonding process is

temperature, and to ensure that the proper temperature is maintained, it is controlled

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through the use of steam and water. It is possible to pass more then one cord at a time

through the calendar, provided that rubber is introduced between the cords. To ensure

that bonding has taken place the cords will pass through a series of drums. Poor

bonding will cause the cords and the rubber to separate.5

Quality is critical during calendering because any flaws in the material will result in

faulty tire production, as the components of the tire will unravel after minimal use. To

ensure that newly formed plies are of high quality, the number of cords, space between

cords, rubber bonding, and penetration of rubber through the cords are all monitored.

Inner liner calendering quality must be very high as the gauge control and defect free

inner liner surface are critical to maintaining air pressure within the inner liner of the

haul truck tire.5

3.2.1.1 Cord Preparation

Cords are required in addition to rubber to allow tires to support large amounts of

weight. They act as reinforcement to the tires and provide additional strength

properties. Some of the materials used to fabricate cords are: polyester, steel, rayon,

nylon and fiberglass. There are two types of cords; steel cords and fiber cords.5

Fiber cords are fabricated from fibrous materials that come on a roll, similar to yarn.

Spools of the fibrous materials are twisted together forming a string like material. The

strings of fabric are then twisted together forming a cord. The cords are then treated

with adhesives to ensure that good bonding occurs between the cord and the rubber.

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The tension applied to the yarn while it is being twisted in to cords is important as it

will affect how well the cords and the rubber bond during the calendering process. The

temperature and humidity that the cords are stored at prior to calendering is also critical

to ensure good bonding with the rubber. For this reason, spools of cords are kept in

humidity and temperature controlled rooms. The quality of a fiber cord is measured by

its strength, stretching and shrinking properties, and elasticity.5

Steel cords are fabricated from steel rods having a high carbon fraction. The steel rods

are also coated with brass, giving them a high resistance to corrosion. They are twisted

in to cords to be calendered with rubber to form belts. As with fabric cords, it is

important to control the humidity and the temperature of storage facilities which house

the steel cords. This is to ensure that good bonding will take place during calendering

between the brass and the rubber. Measurement of quality of a steel cord depends on

the type of tire in which it will be used. For tires made up of many plies, fatigue

resistance is especially important. For belted tires, quality is measured by the stiffness

of the cord. Elongation and tensile strength are also important factors for measuring

quality of steel cords.5

3.2.2 Extrusion

Extrusion is a process where objects with a fixed cross sectional size are fabricated

by forcing material through a mold. The material being molded is commonly referred

to as the Barstock while the mold that the barstock is being forced in to is commonly

referred to as the Die . The die has hollow sections and the barstock will be forced to

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flow in to the hollow sections as the pressure on the barstock increases. There are

several different methods for applying pressure to the barstock, some of which are;

screw augers, rams, and hydraulic pressure. In the case of tire production, screw augers

are generally used to apply pressure to the barstock. The extrusion process can be

carried out using both high temperatures, hot extrusion, low temperatures, and cold

extrusion. Hot extrusion takes place in between 50 percent to 75 percent of the melting

temperature of the barstock, while cold extrusion takes place at room temperature or

slightly above. Tire extrusion uses cold extrusion because cold extrusion helps prevent

oxidation during the extrusion process. Cold extrusion also leaves the finished product

with a better surface finish.9 The extrusion process can be continuous, where materials

are extruded to an indefinite length or semi continuous. Continuous extrusion is

generally employed during tire manufacturing.5

Since the sidewalls, tread, and bead filler are manufactured through extrusion it is one

of the most important processes in tire manufacturing. Furthermore, most of the rubber

compound prepared prior to extrusion will be consumed during this process. The tire

extrusion mechanism consists of two main components; the extruder barrel and the

extruder head. To begin the process, the rubber compound is placed in the extruder

barrel. Here the rubber compound will be moderately heated, blended, and pressure

treated in preparation for extrusion. Once the rubber compound is ready for extrusion it

will flow in to the extruder head. The extruder head forces the rubber compound in to

the die for molding of the tire component. The die gives each component the proper

size, weight, and dimensions. If cooling takes place too slowly or too quickly the

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accuracy of the dimensions of the tire components will be compromised. Each

component of the tire requiring extrusion has its respective extruder barrel and die,

however different components may share an extruder head. To ensure quality the twin

screw augers are computer controlled.5

Once a component has finished the extrusion process it is sent down a cooling line

generally 100 to 200 ft long. Cooled components are cut to the appropriate length in

preparation for tire assembly. Although the processes are similar, sidewall extrusion

can be much more complicated then tread extrusion. This is a result of the difference in

shape between of the tread and the sidewall. The tread is long and straight but the

sidewall must be angled as it is circular. The rubber compounds used for these

processes can be quite different as well as the number of extruders required. Sidewall

extrusion can require up to four extruders while tread extrusion may require only one.5

3.2.3 Bead Building

When manufacturing the bead of the tire it is important that quality is assured

throughout every aspect of the process. If the bead has not been made to the proper

specifications the tire will be unuseable. The wire loop of the bead is constructed using

steel wire that has been brass plated. The brass plating helps improve bonding between

the wire and the rubber bead filler. It also helps prevent corrosion of the wire. The

steel wire must have a high tensile strength since all of the forces applied to the tire are

transferred via the bead. Several spools of steel wire are wound together to make a

single high strength wire. The bead filler is made through the extrusion process using a

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very hard strong rubber compound. The steel wire is passed through the extruding

process with the bead filler and this allows the bead filler to completely cover the steel

wire. Once the bead filler has cooled sufficiently, woven nylon is wrapped around the

bead in order to prepare it for the tire building process. After the bead bundle has been

wrapped by nylon it is wound on to spools in preparation for the tire building process.

The wire along with the bead filler is rolled on to a spool several times at the desired

diameter. Once the required number of wires has been achieved, the bead will then be

cut to an appropriate length. After the lengths of the beads are checked to ensure that

they will have the correct circumference they are ready for the tire building process.5

3.3 Tire Building

All of the tire components are assembled and arranged so they will be ready for the

final steps of the tire manufacturing process. The components which will be assembled

during this process are; the inner liner, the bead bundle, the sidewalls, the plies, the

belts, and the tread. All of the components being assembled are required to be spliced.

Splicing the components of the tires together is an important step in the tire building

process. Imperfections in the splicing of the tires results in defects that will alter the

balance of the tire and cause flaws in force variation, and as a result these defects will

hinder the performance of the tire or cause the tire to fail.5

The tire building process consists of two steps. The first process involves assembling

the components of the tire around a tire building drum. The first component to be

wrapped around the drum is the inner liner. The next parts of the tire to be assembled

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are the plies making up the body of the tire. Once the plies are in place the bead bundle

will be added to the drum. Following the bead bundle arrangement on the drum, the

tire will be inflated using a bladder that is a component of the drum. Inflating the tire

will cause the plies to move in to position covering the bead bundle. Additional plies

can then be added to the tire to increase the strength properties. The sidewalls are then

pressed on to the assembly.5

The second phase of the tire building process involves the addition of belts and the

tread of the tire. Once all of the components of the tire have been assembled on the

drum, the drum is collapsed and the tire is removed.5

3.4 Curing

The uncured tire is referred to as a green tire. A green tire has yet to be

chemically treated and still lacks a tread pattern in the tread rubber. Tire curing is also

known as vulcanization. Vulcanization is used to weatherproof the tires and to

strengthen the rubber. Since rubber will begin to oxidize very quickly in air the

vulcanization process is required to prolong the life of the tire. Rubber is made up of

polymer chains linked by double covalent bonds and when exposed to oxygen, the

rubber polymers oxidize and the double bonds are broken. Oxidation causes the

polymer chains to separate by breaking the bonds between the polymer changes, thus

causing the rubber to crumble. UV radiation from natural sunlight will accelerate the

oxidation process. During vulcanization sulfur atom bridges are used to replace the

covalent double bond between chains. The process begins by a sulfur atom replacing

17

the double bond on one of the chains The sulfur atom will then make another bond

either with another sulfur atom or with another polymer chain. Generally sulfur bridges

are between two and ten atoms long. The longer the sulfide bridges are between

polymer chains the better the dynamic qualities of the tire. Good dynamic qualities

provide the tire with better flexing properties because tires with good dynamic

characteristics are less likely to crack or fatigue quickly. The shorter the sulfide bridges

the better the heat resistance of a tire. When components of tires begin to overheat they

may separate from each other, causing the tire to fail. Overheating tires may also catch

fire. Thus a compromise between long and short sulfide bridges is required.

Accelerating and curing agents are added to the rubber during the mixing process to

facilitate this process.10

The vulcanization process is a batch procedure which involves applying pressure to the

tire to force the tread and sidewalls in to their respective molds and applying heat to set

in motion the chemical reactions required during vulcanization. Molds are used to give

the tread the pattern required for the tire s application and to apply writing to the

sidewalls. The green tire is placed in a mold and an inflatable bladder is inserted in to

the tire. The mold is closed around the tire and the bladder inflated, forcing the tire in

to the mold of the tread pattern and the lettering to appear on the sidewall. The

temperature of the tire during this process is 350o F and the pressure exerted on the tire

can be 350 PSI and higher. Heat is applied to the tire through the inflated bladder,

which is heated through circulating hot water, steam, or hot inert gas. Once the tire has

cured for an appropriate amount of the time, the bladder is deflated and the tire is

18

removed from the mold. Haul truck tires require curing times of 24 hours while

passenger car tires require as little as 15 minutes.4

3.5 Final Finishing

Final finishing involves inspecting the tires to ensure that there are no defects which

is a very important step to ensure safety, performance and quality. There are many

parameters that affect tire performance that must be taken in to account through tire

inspection, they are; force variation, conicity, radial runout, lateral runout, sidewall

bulge and depression, and tire balance.11

3.5.1 Force Variation

There are three forces acting on a tire at any given moment, these forces are the

radial force, the lateral force, and the tangential force. The radial force acting on a tire

is the force acting from the centre of the tire up and this force supports the load of the

truck. The lateral force is the force acting parallel to the axle, this force acts from the

contact point of the tire. The tangential force is the force applied parallel to the

direction of travel. Imperfections in the tire will change the elasticity of the tire in

certain areas. As the tire rotates the surface of the tire supporting the weight will be

constantly changing, thus the amount of support provided will vary as a result of

imperfections depending on the section of tire in contact with the ground. Small

fluctuations are acceptable, however large fluctuations cause the truck to pull to one

side, causing excessive wear on the tires, poor fuel economy, and excessive heat build

up. All types of force variation are measured during final finish inspecting except

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tangential force variation. Tire force variation is observed using harmonic analysis.

Harmonic analysis consists of observing the frequency and timing of the force variation

as the tire rotates. This creates a complex waveform that can be analyzed; certain

patterns seen in the harmonic analysis are known to be linked to manufacturing defects

and flaws.11

3.5.2 Conicity

The conicity of a tire describes the tires tendency to roll like a cone. This property is

related to the lateral forces acting on the tire and will affect the steering of a truck, the

tire wear, heat build up, and the fuel economy. This property is inspected during the

final finishing stages of tire production.11

3.5.3 Radial Run Out

Radial run out is a measure of how the shape of the tire differs from that of a perfect

circle. Normally it is measured around the centre of the tire, however some tire

manufactures measure radial run out around the left and right side of the tire as well as

around the centre. Radial run out can be expressed as the difference between the

highest and lowest values or as a harmonic waveform.11

3.5.4 Lateral Run Out

Lateral run out measures how the sidewall differs from a continuous plane. Similarly

to radial run out it can be expressed as the difference between the highest and lowest

values or as a harmonic waveform.11

20

3.5.5 Sidewall Bulges and Depressions

Sidewall bulges are imperfections that can be detected by visual inspection of a tire.

Bulges in a tire represent areas of weakness where the tire is bulging as a result of the

air pressure being applied. Depressions represent areas which are significantly stronger

then other areas of the tire and as a result, these areas will not bulge as much as the rest

of the tire. Detecting bulges in tires is particularly important as they represent areas

lacking belts or cords and these weaknesses could become safety hazards if the tire is

brought in to service. The safety hazard lies in the potential of a violent failure of the

tire while machinery or workers are nearby.11

3.5.6 Tire Balance

Tire balance is a measure of the weight distribution of a tire. Tires with an uneven

weight distribution will create centrifugal forces propagating through the vehicle and

these forces will alter the ride of the truck. The centrifugal forces created by an

unbalanced tire depend on the speed of the truck, more specifically the speed at which

the tire is rotating and will increase as the truck s speed increases.11

3.5.7 Tire Testing

A tire uniformity machine is used to inspect the tire for the stated defects. A tire is

placed on a rim and inflated to the recommended pressure. Once inflated, a load is

applied to the tire and the tire is accelerated. A load cell is then used to measure the

forces transmitted through the tire to a load wheel. A load wheel is a wheel spinning in

contact with a tire and imperfections in a tire will cause variations in the forces applied

21

to the load wheel by the tire. This is the most common method of detecting flaws in a

tire, although new methods have recently been introduced. Two of these new methods

are the Contact Stylus machine and the Sheet of Light Laser system.11

The Contact Stylus machine consists of a probe in contact with the tread of a tire. The

tire is put in to motion and the probe measures changes in its position as the tire spins.

Changes in position represent defects in the shape of the tire. The Sheet of Light Laser

system consists of a line of laser light moving over the tread and sidewall of the tire.

The laser detects defects such as bulges by sensing changes in the shape of the tire.11

X-rays are also used to inspect the interior of the tire for defects in the structure. Visual

inspections are also made of the tire to detect defects such as exposed cords and

incomplete filling of the tire molds.5

22

CHAPTER 4

4.0 Rubber

Natural Rubber is the main ingredient in commercial tire production and haul truck

tires contain up to 80 percent natural rubber. Natural rubber is produced from latex

secreting plants, although many plants secrete different types of latex such as Para

Rubber Trees, which produce a latex secretion that is used for the commercial

production of tires. The Para Rubber Tree, also known as the Rubber Tree, uses the

natural latex to protect wounds from exposure to the environment. The latex secretions

are also used by the Para Rubber Tree as a deterrent for herbivores, as storage for

products required for photosynthesis, and for disposal of metabolic wastes. Natural

rubber is collected by tapping at a rubber tree and collecting the latex secreted from the

wound. The latex secretion appears as a white milky sap and is produced by special

cells called laticifers. 12

The Para Rubber Tree is indigenous to Brazil but during the 19th

century cultivation

began in Southeast Asia, Africa, and other South American countries. Today Asia is

the largest producer of natural rubber, accounting for 94 percent of global production.

23

The largest rubber producing countries are Malaysia, Thailand, and Indonesia. These

three countries produce around 76 percent of global production. In 2005 global

production of rubber, both natural and synthetic, was around 21 million tones and 42

percent was natural rubber.13

Latex is collected from the rubber tree by first slicing off a thin section of bark. If the

section is cut too thin the rubber will not flow from the tree and if the cut is too deep

the tree will be damaged and may not be able to continue growing or producing latex.

Once the bark has been stripped off a section of the tree, thin lines are cut in to the tree

and this will facilitate the flow of latex. The lines are cut in spirals travelling up and to

the left at an angle of 30 degrees. This is because latex flows through vessels in the tree

which spiral up the tree in a right-handed spiral at an angle of roughly 30 degrees.

Once the thin lines are cut into the tree a small cup is placed at the bottom of the

incision to collect the latex. At the end of the work-day the small cups are emptied in

to a larger container to begin processing. At the beginning of every day a new area of

the tree must be stripped and prepared for rubber collection and this is done early in the

morning as this is when the latex flows the most freely. Generally latex will not be

collected from the same tree every day as this will damage the tree and jeopardize

rubber production.14

24

Figure 1. Para Rubber Tree Tapped for Latex Collection15

The latex secreted by the tree will begin to coagulate in the collection cup and this is

acceptable for the production of rubber, however this is unacceptable for the production

of latex. To prevent the coagulation of the latex, additives such as ammonia are added

to the cups to keep the rubber in a liquid state, a 3 percent ammonia solution is

sufficient to prevent coagulation.14

25

The rubber tree is capable of secreting around 6 ounces of rubber per tapping, of which

generally 30 percent will become rubber. Aside from the quality of rubber produced

from the Para rubber tree the quantity produced also makes this tree the most important

economically. As the tree gets older and is tapped more and more it will increase latex

production until it reaches a maximum yield. Once an area of the tree has been stripped

of bark and tapped it will generally not be tapped again for 3 to 4 years. If a sufficient

period of time has not passed between tappings the yield and quality of the latex

collected with suffer. Young trees are expected to produce 1 to 2 lbs of rubber per year

while larger older trees can produce 12 to 15 lbs or rubber per year. Young trees

cannot be tapped for latex production until they are 5 to 7 years old and 20 to 25 inches

in diameter. Once tapped Para rubber trees can continue to produce rubber for years,

and some trees have productive lives extending beyond 20 years.16

Once the latex has been collected it must be coagulated and purified to a certain extent

before it can be shipped to manufacturers. Methods of centrifuging the latex have been

attempted, however they were found to be ineffective at separating the different

elements of the natural latex of the Para rubber tree. However, centrifuging has been

successful in the processing of natural latex from other types of latex secreting plants.

The latex is coagulated by adding an acidic solution such as acetic acid or lime juice.

After standing long enough to allow the rubber to separate from the fluids in the natural

latex the rubber is removed and the raw rubber is known as biscuit . The rubber is cut

in to small pieces and washed to remove impurities then it is dried. Washing is

accomplished by passing the rubber through rollers and water forcing the rubber to take

26

on the form of thin sheets. The rollers are ridged to perforate the rubber, thus allowing

the water to wash a greater surface area of the rubber. Large solid impurities are

crushed and washed away during the washing process. The washing process causes a

loss of 10 to 15 percent of the rubber. Drying is conducted by hot air or by vacuum

chamber and once dried a curing process is initiated to prevent any proteins in the

rubber from decomposing. Curing is generally performed in two different ways; either

by exposing the coagulated rubber to the smoke of burning wood or by adding

antiseptic chemicals to the freshly collected latex prior to coagulation. Once received

by a tire manufacturer, the rubber is washed repeatedly to remove any impurities still

present. Recently rubber producers have begun to recognize and importance of the

purity of their rubber exports and have improved rubber purifying methods.16

4.1 Rubber Properties

Natural rubber is a hydrocarbon with the chemical formula C5H8, and10 to 20 of

these molecules bunch together giving rubber unique properties. When heated

excessively the rubber molecules will no longer bunch together, causing the rubber to

break down and this is an important aspect to consider for tire applications as tires can

accumulate heat. When heated to 150 to 200 degrees Celsius, the rubber will become a

liquid and not coagulate upon cooling. Oxygen penetrates rubber and causes chemical

changes making the rubber hard, brittle and stiff. This process is accelerated in the

presence of sunlight, this is an important aspect to consider for tire applications as tires

are constantly exposed to both sunlight and oxygen.16

27

CHAPTER 5

5.0 Functions of A Tire

Tires perform 6 essential functions for vehicles17

;

1) Carry a load

2) Guide or Steer the Vehicle

3) Transmit engine or brake torque

4) Absorb Shock

5) Roll

6) Last as long as possible

5.1 Carrying a Load

The capability of a tire to carry a large load is crucial in the design of haul truck and

loader tires. The way that a tire carries a load is more complex then the simple model

of the air pressure alone supporting the load. When no load is applied to a tire the air

pressure is exerting an equal amount of pressure and force along the inside of the tire,

thus the resultant force is zero. When a load is applied to the tire it must be transmitted

through the rim, beads, sidewalls, and belts before it is balanced by the surface contact

28

area with the ground. As a result of the transmission of forces through the different

components of the tire a portion of the load is supported structurally by the tire. In the

case of haul truck tires up to 20 percent of the load is supported structurally by the tire

and as much as 80 percent supported pneumatically through the air pressure in the

tire.17

A useful model when describing how a tire carries a load is by looking at the tire as a

set springs (see Figure 2).

Figure 2. Model of Tire as Springs Supporting Load17

Where: L represents the load

Ks represents the spring constant of the tire Structure

Kp represents the spring constant of the Air within the Tire

Ktread represents the spring constant of the tread

29

Since the structure of the tire is flexible the spring constant of the pneumatic pressure is

much stiffer and as a result supports more of the load.

The load supported by the structure of the tire is not distributed evenly through the tire.

The lower hemisphere of the bead is transmitting more load to the rest of the tire than

the upper hemisphere. Furthermore, the top of the casing of the tire is experiencing

much more tension then the bottom section of the tire. The difference between the

tension through the top of the casing and the bottom of the casing is the load being

supported. Figure 3 shows the pressure distribution of a tire before a load is applied to

it. Figure 4 shows the force distribution on the bead and the tension distribution around

the casing of a tire when a load is applied.17

Figure 3. Force Distribution Around Tire with No Load, All Force are

Distributed Evenly around the Tire17

30

Figure 4. Force Distribution Around Tire with Load Applied. Lower

Hemisphere of Bead Transmits Forces to the rest of the Tire, while the Load is

Supported through Tension in the Upper Hemisphere of the Body17

Figures 3 and 4 demonstrate that the load is actually carried by the upper part of the tire

through its shear strength. In order to carry a load the structure of the tire must be pre-

stressed by inner liner air pressure. By pre-stressing the tire it must deflect less to carry

a load and this is important because too much deflection can result in damage to the

sidewall of the tire. Figure 5 shows how pre-stressing a tire reduces the amount of

deflection necessary to support a load. Since the air pressure in the tire is greater the

contact area required to support the load is reduced.17

Tension in the

casing is much

greater at the

top of the tire

then at the

bottom, the

difference

between the

tensions is

equal to the

load

The force that the

rim is exerting on

the bead is higher

in the lower

regions of the bead

then in the higher

regions

31

Figure 5. Deflection of Tire vs. Load for Different Pre-Stress Pressures, Tires with

Higher Pressure Require less Deflection to Support a Load17

The amount of load balanced pneumatically is equal to the pressure in the tires

multiplied by the size of the contact area of the tire. An example calculation of a load

supported pneumatically is:

Contact Area: 10 000 cm2 = 1 m

2

Tire Pressure: 775 kPa = 775 kN/m2

22 m/kN775m1Load

Load = 775 kN =174 220 lbs

Y axis

represents load

applied to tire

Curves display the

load deformation

relationship for tires

with different pre-

stress pressures. Tires

with higher pre-stress

pressure require less

deflection to support a

load

X axis represents

deflection of the tire

32

5.2 Guide or Steer the Vehicle

Another important function of a tire is to allow the operator to guide and steer the

vehicle. The tread of the tire provides traction with the ground providing friction. The

friction prevents the tire from sliding side to side thus forcing the tire to roll in the

direction being steered in. The strength of the sidewall and the air pressure prevents the

tire from collapsing when lateral forces are applied to the tire through steering.

5.3 Transmit Engine or Brake Torque

Transmitting engine and brake torque is required for accelerating and braking.

Engine and brake torque are transmitted through the tire by friction acting between the

tread of the tire and the ground.

5.4 Absorb Shock

In order to assist in the absorption of shock, tires are required to deform in order to

adjust the contact area. When a haul truck or other vehicle bounces as a result of

uneven terrain the load applied to the tire changes. As a result the contact area between

the tire and the ground must change to compensate. A certain amount of flexibility is

required for the tire to allow this action to take place readily.

5.5 Ability to Roll

The ability to roll is the most basic requirement of a tire. The rolling resistance of

the tire influences fuel consumption and tire wear. To roll, tires are round and as

symmetric as possible. Friction between the ground and the tread forces the tire to roll

33

instead of slide along the ground. Tires displaying a larger amount of deflection to

support a load have a larger rolling resistance which results in higher fuel

consumption.17

5.6 Last as Long as Possible

In order to minimize operating costs it is necessary for tires to last as long as

possible. Proper tire care and effective retreading programs can greatly increase the

useful life of tires. In order to maximize tire life it is necessary for operators to

maintain appropriate speeds for the load being carried and for the ground conditions.

Maintaining proper ramp care and road care in a mine will also increase the life of tires.

Examples of road maintenance include minimizing the amount of water which haul

trucks must travel through and grading roads as frequently as needed to remove rocks

which may puncture tires. The presence of water greatly increases the chances of a tire

being punctured because water lubricates the rubber allowing rocks to penetrate the

rubber far more easily. Proper tire maintenance is critical to maximizing the life of

tires. Tire maintenance includes maintaining the recommended tire pressure, respecting

TKPH (Ton Kilometer per Hour) or TMPH (Ton Mile per Hour) (see section 5.6.2),

being considerate of tire temperature, and removing tires in time to carry out retreading.

5.6.1 Tire Maintenance

It is important to properly maintain tires to ensure they have a long useful life.

Maintaining a tire at its recommended pressure is the most important factor relating to

tire life. When a tire is under inflated the contact area with the ground must be larger to

34

support the load, thus the sidewall and tread must deflect more to accommodate this.

Excessive deflection results in cracking where the sidewall meets the bead and it also

results in a faster propagation of existing cracks. Excessive deflecting also results in

increased heat building up in the tire.17

Figure 6. Cracks in Bead area of Tire as a Result of Under Inflation, Tire Inflation is

the Most Important Aspect of Tire Maintenance17

35

Figure 7. Cracking of the Bead are of Tire as a Result of Under Inflation, Under

Inflation adds Additional Stress to Bead and Sidewall Area Causing Cracks17

When a tire is over inflated the tread begins to wear much more quickly and this is

because the tire becomes less flexible, resulting in an increased susceptibility to

puncturing. Chunking is another problem associated with over inflated tires. Chunking

is when large pieces of rubber are gouged out of the tire as a result of puncturing. This

is particularly damaging to tires because it can leave the ply layers of the tire exposed

to the ground.

36

Figure 8. Puncturing Damage to Underside of Tread as a Result of Over Inflation, Over

Inflated Tires are Far More Susceptible to Puncturing Damage17

Properly

inflated tire (73

Psi) has some

puncturing

damage to

underside of

tread

Over Inflated

Tire (102 Psi

instead of 72 Psi)

has over 3 times

the amount of

puncturing

damage

compared to

properly inflated

tire

37

Figure 9. Chunking Damage to Tread of Tire as a Result of Over Inflation17

Figure 10. Height of Object Which Poses a Puncturing Threat to Tire vs. Pressure, as

Pressure Increases More Objects poses a Puncturing Threat17

Influence of Pressure at Constant Load

18.00R25 - 13T, 30km/h

190

200

210

220

230

5 5.25 5.5 5.75 6 6.25 6.5 6.75 7 7.25 7.5

Pressure (bars)

Ob

ject

Heig

ht

(mm

)

As pressure increases

smaller objects pose a threat

to the tire

38

5.6.2 Tons Kilometer per Hour

As a tire is used to carry a load, heat is generated and retained in the tire. With this

heat build up, heat can have serious consequences for tire performance and tire life.

Through out a working shift a tire will continue to heat up until it has reached a

stabilized temperature. The stabilized temperature depends on the truck speed and the

load which is being exerted on the tire. Tons kilometer per hour (TKPH) or Tons Mile

per Hour (TMPH) are a measurement of a tires carrying capacity. The TKPH rating of

a tire is a function of the maximum temperature that a tire can reach before the tire

begins to suffer from heat separation. Once a tire has over-heated, irreversible damage

will occur. TKPH is the product of the load that a tire is carrying, including the weight

of the vehicle multiplied by the average speed that the truck travels at. To find the

average speed the number of cycles per shift is multiplied by the distance of a round

trip and divided by the time worked in a shift. Using this calculation the TKPH rating

of a site can be found and an appropriate tire will be a tire with at least the TKPH rating

of the site. However, it is recommended that a tire with a greater TKPH than that of the

site is used.17

Aside from load and distance there are other factors that need to be considered and

accounted for. Other factors include, but are not limited to, external sources of heat,

vehicle configuration limiting heat dispersion, and road conditions. Experience relating

to the site and to tire performance must be used to choose an acceptable TKPH site

rating. The wheel position also has an effect on the TKPH, and that generally the tires

suffering the most strain are on the front axel.17

39

Figure . Time to Reach Stabilized Temperature throughout a shift a Tire s

Temperature will Increase until it Reaches a Stabilized Temperature where it will

Remain17

The stabilized temperature of a tire also depends on the distance that is being traveled.

As a result a correction factor k1 is used to make adjustments for the length of a haul.

When the haul length is quite long, k1 will increase, thus increasing the TKPH of the

site. For haul distance of 5km and greater, k1 must be used to account for the increase

in tire stabilization temperature. Another correction factor is used to account for

ambient temperature. When the atmospheric temperature is quite high, the stabilized

temperature of the tire will also be higher. The correction factor k2 is used to account

for the temperature of the surrounding environment as this will affect the TKPH of a

site. The TKPH of a tire is rated for a temperature of 100oF and any deviation from this

temperature will require the use of k2.17

Stabilized

Temperature

40

Currently there are several software packages that keep track of tire TKPH for each

truck and each tire, as each tire location will have a different TKPH. These software

packages, in conjunction with truck dispatch and monitoring, are capable of monitoring

TKPH of each tire in real time allowing engineers to plan production and equipment

usage appropriately.18

41

CHAPTER 6

6.0 Why Tires Build Up Heat

As a tire rolls along the ground it must deflect to form the contact area with the

ground in order to support the load. As the tire deforms it stores energy in a manner

similar to a spring, and as it continues to roll the tire returns to its original shape, thus

releasing the stored energy. Due to internal friction between rubber elements and other

inefficiencies, some of the energy is retained in the tire in the form of heat. If the rate

of energy build up in the tire is greater than the rate at which it can be dissipated to the

surrounding environment, the temperature of the tire will increase.17

Figure 12. Force Exerted on Tire vs. Time as Tire Rolls, Force of Load is not

Distributed Evenly, as the Tire Rolls Section of Tire Supporting the Load Changes17

42

Figure 12 demonstrates how a tire must deflect as it rolls to and from the contact area

with the ground. This is the source of heat build up.

Figure 13. Tire Modeled as a Spring with Dampening Device, Energy is Stored in the

Spring then Released However, some Energy is Storage in the form of Heat17

Figure 13 demonstrates how as the tire rolls, energy is stored but as the tire returns to

its original shape, not all of the energy is released and some is stored as heat.

The majority of heat build up occurs in the tread of the tire as this is the section of the

tire deflecting the most. Heat build up in the tread represents approximately 60 percent

of the heat build up in a radial tire. The remaining heat build up is split between the

bead and the sidewall.17

Difference between

the areas of the two

curves represents

stored energy

A tire is modeled as a spring

with a dampening device.

The spring stores energy

and releases it while the

dampener absorbs some of

the energy

43

As a tire is used, the temperature will rise, and after a certain temperature has been

reached the tire becomes more susceptible to damage. The temperature at which the

tire s susceptibility to damage begins to increase is the optimum operating temperature

of the tire. At the optimum temperature, the tire is hauling as much as it can as quickly

as it can without damaging itself. Once the tire s temperature heats up past the

optimum operating temperature the tire s susceptibility to heat separation as well as

puncturing and chunking increases. Full heat separation of a tire rarely occurs,

however once heat separation begins the tire will quickly deteriorate resulting in a very

short operating life.17

44

Figure 14. Damage Frequency vs. Tire Temperature, Once the Temperature has risen

past the Optimum Temperature as Temperature Increases the Tires Susceptibility to

Damage Increases. 17

45

Figure 15. Example of Heat Separation as a Result of Over Heating, Once a Tire

Surpasses a certain Temperature Rubber Components begin to Separate17

46

CHAPTER 7

7.0 Retreading

Retreading of a tire is an important method of extending a tires life. The cost of

retreading a tire can be as low as 30 percent of the cost of a new tire. Furthermore, the

carcass of a tire is designed to last much longer then the tread of the tire. Thus re-

treading is necessary to ensure that a tire has the longest life possible.19

The retreading process starts with a thorough inspection the tire to ensure that the tire is

not damaged from previous use and this can be done several ways including x-ray

methods. After a tire has been deemed suitable for retreading the tread is worn down

through buffing to make a smooth defect free surface. A clean, smooth, defect free

surface is required to ensure good bonding between the new tread which is being

applied to the tire and the carcass of the tire.19

There are two methods of applying a new tread to the carcass of a tire; Mold Curing

and Pre-Curing. The Mold Curing method is performed by applying a soft uncured

rubber suitable for use as a tread to the tire carcass. The tire is then placed in a mold

47

having the desired tire tread design and is subjected to heat and pressure. The soft un-

cured rubber is forced in to the mold and is cured by the heat and pressure. Each tread

pattern requires is own mold for this process.19

The Pre-Curing method begins with the application of a thin layer of rubber around the

carcass of the tire. This layer of rubber is used to bond a tread to the tire carcass. Once

the bonding rubber has been applied to the surface of the tire, a previously molded tread

is wrapped around the tire carcass and cut to the appropriate length. Some tire

manufacturers such as Goodyear have developed tire treads that are manufactured in a

loop rather then a strip. These treads fit more tightly against the carcass of the tire

providing better adhesion during the curing process. Once the tread has been fitted on

the tire it is subjected to heat and pressure to cure the bonding rubber.19

The Pre-Curing method is considered to be superior and represents 80 percent of tire

retreading. This is because a specific mold is not required for each type of tire and

because it is subjected to less heat then the Mold Curing method. The temperatures

required to cure the tread in Mold Curing method range between 295 to 310 degrees

Celsius, while the temperatures required for the Pre-Curing method range from 210 to

250 degrees Celsius. Subjecting the tire to less heat decreases the chance of damaging

the tire or deteriorating the rubber of the tire.19

Inspection of the tire is repeated when retreading is complete to make sure that the

retread has been carried out properly and that the tire was not damaged during the

48

process. If a tire does not pass the post retreading inspecting the tread must be removed

and the process repeated.19

49

CHAPTER 8

8.0 Tire Rotation

Rotating tires appropriately is an important method of maximizing safety and tire

life. The position of the tire on the truck has an effect on the amount of load and wear

that is exerted on tire as well as consequences should the tire fail during operation.

Generally the tires are arranged so that on the front axle there are two tires, one per side,

and on the rear axle there are four tires, two per side. Should a tire on the front axle fail

while the truck is traveling down a hill there is a high risk of the truck rolling over on to

its side. This is very dangerous to the operator of the haul truck and other people and

equipment in the area and it is also very damaging to the haul truck its self. In order to

prevent this from occurring, new tires are generally put on the front axle until they have

reached a certain amount of wear. Once this predetermined amount of wear has been

reached the tires are removed and used as spare tires for the rear axle. Since there are

fewer tires on the front of the vehicle and since the load is not evenly distributed, the

tires do not wear evenly and this must be taken in to account and accommodated for

through tire rotation.20

50

There are two methods commonly used to determine when a tire should be removed

from the front axle; wear based and performance based rotation. Wear based rotation is

centered around the extent of wear of the tread. Once the tread depth has been worn

down to a predetermined percent (generally around 66%) of the original tread depth,

the tire is taken off the front axle and used as a spare for the rear axle. Performance

based rotation rotates tires based on a predetermined percentage of useful life. Tire life

is based on operating hours so that once a certain number of hours of operation has

been reached, generally around 33% of the useful life, the tire is removed from the front

axle and used as a spare for the rear axle.20

Rotating tires based on performance is believed to be the most effective method.

Performance based rotation is the most responsive method to changing site conditions

as well as providing an adequate supply of spare tires for the rear axle. Currently there

are several commercial products that keep track of tire location as well as tire

inventories, however a spread-sheet can be used to keep track of tire location and

rotation.20

In the past, the worst tire was placed on the outside rear as it was most likely to be

punctured or wear out. Changing the outside rear tire is a simpler procedure than

changing the inside rear. Today, the worst tire is placed on the inside rear, as if it fails

in that position, personnel and equipment would be less likely to be injured.20

51

CHAPTER 9

9.0 Tire Shortage

Currently there is a serious shortage in the availability of medium and large tires.

Due to increased mining activities at existing mines and the development of new mines,

the demand for off the road tires (OTR tires) has increased significantly. It has been

estimated that tire manufacturers have a surplus of 25 to 30 percent of tire orders that

they are unable to fill. As a result customers are unable to obtain tires when they are

needed. Another factor impacting the shortage is demand for OTR tires for

construction purposes both domestically and abroad. Countries seeing recent enormous

economic growth such as China and India have a high demand for large tires used for

construction purposes. North American construction activities have also been strong

the last few years, adding addition pressure to the tire shortage. A third factor leading

to the current situation is tire demand for military purposes. America s military

presence in Iraq and other unstable areas of the world has increased the demand for

tires as well. It is expected that the tire shortage will continue in to the next few years

as tire manufacturers take the necessary steps towards increasing production. 21

52

Although this is not the first tire shortage, experts in OTR tire manufacturing have not

seen tire demand this high in 40 years leaving them unprepared.21

Before the mining

boom there was a five-year period of poor OTR tire sales which left manufacturers

unprepared for one of the largest booms in tire demand in recent history. Currently all

OTR tires produced are allocated to large companies which have been in business with

the tire producers for many years, forcing them to turn away new customers coming in-

to the tire market. Even though tire producers are allocating tires to fewer companies

then in previous years, many of them are still filling back orders dating as much as two

years ago.21

Allocating tires to specific companies has forced new tire consumers to look abroad for

tire supplies, which has brought foreign tire producers from countries such as Taiwan,

Russia, and China in-to the North American market. In the past, concerns regarding

product quality have kept producers from these countries out of the North American

OTR tire market. Prior to the tire shortage, American and Japanese producers

dominated the market. These dominant companies have maintained their market share

by providing quality dealers with whom the tire consumers can rely on for advice,

maintenance, and retreading programs helping give tires an extended useful life.21

Factors working against tire producers meeting current supply demands are the cost of

raw materials and the time required to increase production and production facilities.

Rubber production must also be increased to meet tire production demands, however

Para rubber trees must reach a certain age, between 5 to 7 years, before latex collection

53

can begin. Tire manufacturers have begun taking steps to increase production, a few

examples are:

Michelin has begun construction of a new OTR tire plant in Brazil that is planned to

come online this year23

, and in 2005 Michelin announced an 85 million dollar

investment over 5 years in its OTR plant in Lexington, South Carolina.21

Yokohama Tire Corp has built a new OTR tire plant in Japan in 2005 and is expected to

reach full production capacity this year.22

Purcell Tire Company have announced the development of a new tire retreading facility

that will use segmented molds for re-treading for the first time at its facility in Phoenix,

Arizona.24

In order for tire consumers to maintain desired production it is necessary to take tires in

to account. Many mining companies have begun training haul truck and loader

operators on how to extend the life of tires by instructing them how to avoid parts of the

road which are at high risk of puncturing the tires, maintaining proper pressure of the

tires, and maintaining appropriate speeds when turning.

54

CHAPTER 10

10.0 Conclusions and Recommendations

As a result of increasing prices of tires and decreases in the availability of tires it is

important for mines to ensure that tire life is being optimized. Proper tire care, road

maintenance, appropriate tire selection, and retreading programs have become

increasingly important. Tires are crucial wear parts that can have a significant effect on

both productivity and costs of a mine. Mining engineers must have an understanding of

how tires work, how tires fail, and how to optimize the life of tires in order for mining

projects to be as profitable as possible.

Understanding the different components of a tire and how they work together to carry a

load is important in ensuring tire use is optimized for a given application. A knowledge

of the tire manufacturing process as well as the limiting factors such as rubber

production and production rates, will help mining engineers understand the tire crisis

and adopt new strategies to prolong the life of their tires as well as acquiring new tires.

55

Ensuring that tires are properly inflated decreases the rolling resistance, helping to

optimize fuel consumption, further minimizing production costs. In addition, tire

inflation pressure has a direct impact on tire wear depending upon whether the tire is

over or under inflated. Over inflated tires are less flexible and as a result are more

prone to puncture, while under inflated tires must deflect more to form an adequate

contact area with the ground, thus causing excessive movement of the sidewall that will

cause cracking. When the tire must deflect excessively, extra heat is built up in the tire

causing heat separation as well as increased wear. Therefore, rolling resistance, tire

wear, puncture susceptibility, and heat generation are all functions of tire pressure

which greatly impact the life and performance of a tire.

Understanding how to calculate Tons Kilometer per Hour (TKPH) and how to make

site specific adjustments is very important in preserving tires and in maximizing their

utility. Calculating the TKPH is important because it determines what tires are selected.

If the wrong tire is chosen, tire life will be greatly reduced and subsequently tire

consumption and costs will rise. Making site-specific adjustments is critical to

calculating an accurate TKPH as road conditions, haul length, and tire arrangement all

effect tire temperature stabilization. Tire performance varies from site to site and

therefore it is important to choose an appropriate tire for that particular site.

Since heat generation is the limiting factor of tire capacity mining engineers must

understand how it is generated in order to minimize heat build up. As a tire deflects to

form the contact area with the ground it stores energy, most of this energy is then

56

released as the tire continues to roll. The remainder of this energy is retained in the tire

in the form of heat. Excessive heat generation can cause the rubber holding the

components of a tire together to deteriorate causing the tire to break down. Excessive

heat build up also increases the rate at which the tire wears.

Retreading is critical to tire life optimization as it greatly increases the life of a tire

carcass for substantially less than the price of a new tire. In order for a tire to be

successfully retreaded it must be removed from service before it is too damaged to be

retreaded. The carcass of a tire must be in fairly good condition to be retread. Tire

rotation is important for safety reasons as well as ensuring that tires wear evenly. Tire

failure can be catastrophic under the right circumstances, for example, the failure of a

front axle tire while a haul truck is traveling down hill can easily cause the truck to roll,

endangering the life of the operator and causing severe damage to the haul truck. To

minimize the chances of this happening, new tires are placed on the front of the truck.

Tires are then moved to the rear of the truck where there are additional tires available to

support the load should one fail.

Due to the current tire shortage and mining boom tires have never been more important.

Since tires can be exceedingly difficult to acquire and have become quite expensive,

tire care is critical to maintaining productivity and to minimizing costs. It is vital that a

multimillion or even a multibillion dollar mining project does not under produce due to

a lack of available tires, which by comparison are extraordinary inexpensive. Mining

engineers must plan production and tire consumption appropriately to ensure that this

57

does not occur. Through the understanding of tire use and tire consumption this

situation can be avoided. It is the mining engineer s responsibility to minimize

production costs and so it is very important that they have a working knowledge of tire

limitations and longevity. Tires represent a significant portion of material handling

costs and thus must be minimized. This can only be done through a sound knowledge

of tire use and tire limitations.

58

REFERENCES

1. Nice, K. How Tires Are Made. How Stuff Works.

<http://auto.howstuffworks.com/tire1.htm>

2. Laferre, S. Tire Talk, Taking it to the Streets. The Tire Review.

<http://www.tomorrowstechnician.com/tt/tt50632.htm>

3. Radial Tire Production. Goodyear Tire.

<http://www.goodyeartires.com/about/testing/radial.html>

4. Tire Manufacturing. Wikipedia. March 21. 2007. <

http://en.wikipedia.org/wiki/Tire_manufacturing>

5. Tire School, How a Tire is Made. Maxxis. 2005

<http://www.maxxis.com/Services/how_a_tire_is_made.asp>

6. Styrene-Butadiene. Wikipedia. January 15, 2007.

<http://en.wikipedia.org/wiki/Styrene-butadiene>

7. Polybutadiene. Wikipedia. February 26, 2007.

<http://en.wikipedia.org/wiki/Polybutadiene>

8. Black Carbon. Wikipedia. March 22, 2007.

<http://en.wikipedia.org/wiki/Carbon_Black>

9. Extrusion. Efunda, Engineering Fundamentals.

<http://www.efunda.com/processes/metal_processing/extrusion.cfm>

10. Vulcanization. Wikipedia. <http://en.wikipedia.org/wiki/Vulcanization>

11. Tyre Uniformity. Wikipedia. <http://en.wikipedia.org/wiki/Tire_Uniformity>

59

REFERENCES

12. Armstrong J. Rubber Production: Tapping Rubber Trees, Latex Collection, and

Processing of Raw Rubber.

<http://www.bio.ilstu.edu/Armstrong/syllabi/rubber/rubber.htm>

13. Rubber. Wikipedia. <http://en.wikipedia.org/wiki/Rubber>

14. Kinnamen, B. A Brief Natural History of Latex Rubber Allergy. March 8, 2007.

<http://www.immune.com/rubber/nr1.html#d>

15. Para Rubber Tree. Biology Daily.

<www.biologydaily..com/biology/Rubber_Tree>

16. Rubber. Love to Know <http://www.1911encyclopedia.org/Rubber>

17. Doyle, M. Functions of a Tire. Michelin

18. Off The Road Tire Engineering Data. Goodyear. 2005. pp 9

19. Baxter, J. Born Again. 2003. <http://www.retread.org/PDF/BornAgain.pdf>

20. Performance-Based Haul Truck Tire Rotation. Michelin. 2004

21. OTR Tire Shortage Could Hinder Mining Boom. Modern Tire Dealer. January

4, 2007.

<http://www.moderntiredealer.com/t_inside.cfm?action=news_det&storyID=55

48>

22. Unit Drain, the Impact BRIC and Offshore comes Ashore. Modern Tire Dealer.

February 2007.

<http://www.moderntiredealer.com/t_inside.cfm?action=art_det&storyid=1367

&pgNum=2>

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REFERENCES

23. Michelin Announces the Construction of a New Earthmoving Tire Plant In

Brazil. Michelin. April 11,

2005.<http://earthmover.webmichelin.com/na_eng/News/132.html>

24. Purcell Tire Company Launches Revolutionary Retread Service for Large

Haulage Tires. Purcell Tire. March 2, 2007.

<http://prweb.com/releases/2007/3/prweb508535.htm>

61

VITA

Department of Mining Engineering

ueen s University

Kingston, Ontario

3pm15@qlink.queensu.ca

B.Sc ueen s University ingston Ontario

Engineering Intern

The Hard Rock Group

Port Colborne, Ontario