Functions of a tyre
Numerous qualities are required of a tyre, including the following:
- resistance to wear;
- resistance to impacts and cuts;
- low rolling resistance;
- suitable for retreading;
- suitable for repair;
- resistance to heat;
- ability to carry load;
- ability to travel at sustained design speeds.
Moreover, provisioning for tyres always represents a sizeable budget. Applying a few simple rules allows tyres to be used to their full potential, leading to increased productivity and reduced operating costs on a site.
These rules will be examined in this chapter.
The tyre is at rest (position 1); as it rotates (position 2), the sidewalls are compressed, causing heat to be generated in the tyre’s internal components until contact with the ground (position 3) is attained. The heat generated then reduces as the tyre returns to its initial position (position 1).
If the action described above is too rapid, the optimum temperature for the correct functioning of the tyre is exceeded and the tyre is likely to become damaged.
A tyre is usually inflated with air (or nitrogen), and provides contact between the ground and the machine. It is subject to a large number of constraints:
- nature of the ground;
- condition of the haul roads.
The optimum tyre will be the one which is able to satisfy each of these constraints to give the best overall compromise.
Economical load limit of the tyre in use
This is the optimal design load limit beyond which tyre performance is compromised.
If the tyre is operated within this load and pressure limit, optimum performance can be obtained in terms of tread life, ride comfort, resistance to cuts and impacts, etc.
These load and pressure limits are published in our technical documentation and reflect the requirements of the international standards.
It is possible to use our tyres beyond the published load limits for economical use (but remaining within the maximum values indicated in the charts). However, this will reduce the tyre’s tread life and its resistance to damage.
In order to ensure that tyres are being used under optimum conditions, operators are advised to:
- determine tyre loads by weighing the vehicles by axle (preferably by tyre position) under their normal operating loads and set pressures accordingly;
- never exceed the maximum distance that the tyre is designed to travel in any one hour.
Main causes of damage
A large number of earthmover tyres are damaged as a result of:
- Underinflation: if a tyre is run in an underinflated condition for a given load and speed, the increased deflection will lead to i) an increase in its internal operating temperature, and ii) fatigue.
- Overinflation: if a tyre is run in an overinflated condition for a given load and speed, the tread contact area on the ground is reduced leading to i) increased wear and, ii) reduced resistance to cuts and impact damages.
- Overloading: causes increased tread wear, greater deflection of the sidewalls with an increase in tyre temperature, and fatigue.
- Excessive speed: causes increase in the internal temperature of the tyre, fatigue and rapid tread wear.
- Severe impacts.
- or a combination of the above.
Damages can be caused or made worse by:
- lateral forces generated in curves with very small radii;
- impacts on poorly maintained road surfaces.
Factors affecting tyre life
The internal temperature of a tyre
When a tyre rolls, it is heated by:
- its deflection;
- the heat generated in the braking system;
- the heat generated in the reduction gears.
A tyre is likely to become damaged if its critical internal temperature is exceeded.
In the absence of external thermal sources, it is generally accepted that this critical temperature has been reached when the AIR inside the tyre reaches 80°C. This temperature is always lower than the temperature of the internal components of the tyre itself.
It is therefore advisable to check that the temperature has not risen to the point of penalizing the tyre.
How can we check this?
This relation applies according to the from:
- Cold (tyres have not been used)
- Hot (tyres have been used)
As the tyre’s internal volume remains constant, any change in pressure will affect the temperature and vice-versa.
The tyre’s internal temperature can be calculated as follows:
Note: It is preferable to use the same pressure gauge when checking pressures cold and hot (comply with safety precautions when doing so).
Example: Consider a tyre at 6.0 bar (cold) pressure and assume that its internal air temperature is the same as the ambient, say 27°C.
After running for an hour, say the pressure has increased to 7.5 bar.
The temperature of the air in the tyre is:
In this example, the internal temperature is over the limit of 80°C. In such cases, it is important to investigate the factors that may be contributing to the temperature increase and minimise their effects.
Influence of pressure on tyre life
Pressure is one of the prime factors that affects a tyre’s tread life.
As an indication :
- Underinflating a tyre by 10% reduces its tread life by 10%.
- Underinflating a tyre by 20% reduces its tread life by 25%.
- Underinflating a tyre by 30% reduces its tread life by 50%.
- Overinflating a tyre by 10% reduces its tread life by 5%.
- Overinflating a tyre by 20% reduces its tread life by 10%.
- Overinflating a tyre by 30% reduces its tread life by 20%.
Note: a 50% decrease in tread life means that the tyre consumption is doubled and therefore the tyre costs are doubled.
Influence of climatic conditions
Ambient temperature and climatic conditions (wet/dry) will affect tyre life (See the chapter on INFLATION for further details).
Position of tyres on a vehicle
It is generally acknowledged that, due to torque transmission, the tread life of tyres mounted on drive wheels is 25% lower than those mounted on steer wheels.
Exception in industrial applications (forklift trucks, reach stackers…). In these applications the opposite occurs due to the small turning radius ; tyres on the steer axle wear more quickly than those on the drive except where the machine is used mainly in straight lines.
Difference in diameter of tyres mounted on a vehicle
See also the chapter on TWINNING.
A difference in diameter (due to tyres worn to different tread depths, different types or brands) in the two tyres of a twinned fitment, or the front and rear axle tyres (loaders), generate to rapid and uneven wear on all the tyres fitted to a machine.
Certain tolerances are accepted.
- For rigid dumpers, a 1% difference in diameter is tolerated between the left and right-hand sets of tyres on the same axle. A difference of 0.5% is tolerated between two twinned tyres.
- For loaders, a 6% difference in diameter is tolerated between the front and rear axles, and a 3% difference between the two tyres on the same axle (tolerances specified by the SAE J2204 standard).
- For articulated dumpers, a 2% difference in diameter is tolerated between the front and rear axles. A 1.5% difference is tolerated between tyres on the same axle, and a 1.5% difference between the two rear axles.
Overloading of tyres is often encountered and is generally due to the nature and condition of the material being worked, and the way it is loaded.
As an indication:
- Overloading a tyre by 10% reduces its tread life by 15%.
- Overloading a tyre by 20% reduces its tread life by 30%.
- Overloading a tyre by 30% reduces its tread life by 50%.
Tyre life is affected by the way a machine is operated and the following factors are known to have a notable effect:
- heavy and repeated braking;
- rapid acceleration;
- cornering at high speed (will also cause heat generation);
- spinning the drive wheels (ex: scrapers during loading);
- improper operation of a loader (wheel spin) during face loading.
Cycle length and duration
Long cycles, especially on established haul roads, encourage high speeds and therefore significant temperature increase inside the tyres.
This may also occur when the machine’s running time is much longer than its standing time, and as seen in “The internal temperature of a tyre”,
the internal temperature of a tyre affects its life.
Poor mechanical condition of a machine can affect the life of its tyres.
- Faulty brakes can cause excessive heat generation in the wheels and tyres.
- Poor alignment of a transport machine’s steer wheels.
- Play in the suspension or hub components.
In the last 2 cases, tyres are likely to wear abnormally and rapidly.
Design and maintenance of haul roads
The profile of the haul roads (length, width, camber and elevation of curves, gradients, etc) have a considerable effect on dynamic overloading and on tyre scrub.
Travelling downhill laden increases the load on the front axle, generally by the value of the gradient. A cambered haul road or banked curves will increase the load carried by the tyres on one side of the machine.
Regular maintenance of the haul roads and removal of debris (spillage from machines), and cleaning of the loading areas helps to protect tyres from accidental damage.
Choosing a suitable tyre
As good as our products may be, they can only perform satisfactorily if their choice is compatible with the conditions in which they will be used, and if they are used in accordance with our recommendations.
Operating the tyre at the recommended inflation pressure is paramount to optimising its tread life potential.
Changes in the operating conditions of a site, the nature of the ground, cycle lengths or the haul roads may render unsuitable a tyre that previously had performed well. It then becomes necessary to conduct a site evaluation and recommend an alternative tyre. In such cases, you can call upon MICHELIN earthmover tyre specialists for assistance.
This document provides you with the information that would enable you to select the most appropriate tyre for your needs.
Ensuring an appropriate pressure:
- check the pressure regularly. (Chapter on vehicle inspection);
- adjust the pressure, if necessary. Allow for increase of pressure in a hot tyre!;
- check the load and its centering (this point is covered later);
- check the actual operating speed.
- check the cycle length;
- check the machine’s braking equipment;
- clean the haul roads to eliminate obstacles as far as possible.
Loads on the vehicle must be reduced. The material being worked may have a higher density than usual. Is the loading machine suitable?
Very often linked to bad loading, which results in most of the load being carried by one axle, or one side of the machine, or one tyre, etc.
Give the operators adequate training.
The operator should:
- operate the machine correctly by using its hydraulics and minimise wheel spin;
- keep the working area free of debris to minimise tyre damage on the loader and trucks;
- optimize the positioning of the different vehicles during loading operations at the fac.
Construction of haul roads
When the haul roads are being constructed, bank the curves correctly. Travel at speeds compatible with the radius and banking of the curve.
Damage incurred by a tyre may also lead to damage to a machine and/or personal physical injury.
Undertaking to reduce the damage done to tyres protects not only the machine but also the operators and other personnel who service it.
MICHELIN has developed training modules to assist its customers in managing their tyres. Please consult your MICHELIN representative for further information.