Importance of inflation pressure
We are in the habit of presenting air as one of the components of a tyre. The appropriate inflation pressure is in fact of prime importance for:
- the satisfactory operation of the machine/tyre assembly;
- tyre performance.
Air under pressure, in the correct quantity, enables a tyre to carry the load in approved conditions. The quantity of air necessary for the optimal
functioning of a tyre is given by the correct pressure.
The curve below gives a general indication of the link between inflation pressure and tread life: it should be implied that it applies equally to all types and sizes of earthmover tyres. It clearly shows the consequences of inappropriate pressure on tyre performance.
The correct pressure
Information necessary for establishing the correct pressure
The most rigorous procedure consists in weighing loaded machines, tyre by tyre, failing this, axle by axle, and referring to the technical documentation for the tyre(s) concerned.
In the absence of context-specific, observable data to calculate a pressure (measured weights, operating conditions, etc.), we can provide basic pressures for the majority of machines, from the conditions of use provided for by the manufacturers. Your usual MICHELIN representative would be happy to assist you with weighing your vehicles. Failing this, he will be able to indicate start-up pressures, suitable for standard conditions of use.
Factors that may lead to a modification of the basic pressures
Exceptional conditions of use may entail adapting the basic recommended pressure. Among the factors likely to result in a modification, the following points should receive special mention:
Need for “flotation”
This refers to an enhanced capability to travel on soft or loose soil. In this case, the pressure may be lowered, though it should remain within the limits of the load/pressure scales shown in the technical documentation.
Risks of cuts, chipping or tears
In extreme conditions of use, an excessive inflation pressure increases the chance of shock damage to the tyre casing and damage to the tread and sidewalls. In this case also, the inflation pressure can be reduced to limit the risk whilst at the same time remaining within the load/pressure limits
published in the Technical Data book.
Difference in temperature between the place where are inflated the tyre (workshop for example) and the site of use of these tyres
The inflation pressures indicated in the “tyre load and pressure” tables given in the technical documentation apply in all cases.
However, an exception should be made when there is a difference between the ambient temperature at the time of inflation and the ambient temperature at the time of use. In this case it is advisable to make some corrections.
Difference in ambient temperature
When tyre inflation pressure is checked with a vehicle at rest and the ambient temperature on site is higher than the reference temperature, the pressure measured will be higher than the initial pressure. If the initial pressure was established for an ambient temperature equal to the reference
temperature the difference in the pressure measured at a higher temperature will be:
- from + 25°C to + 29°C (77-84°F)
- from + 30°C to + 34°C (85-93°F)
- from + 35°C to + 39°C (94-102°F)
- from + 40°C to + 45°C (103-113°F)
Low ambient temperatures
Using tyres in cold conditions [temperature < 0°C (32°F)] entails an increase in basic pressures, which is a function of the difference between the workshop where the tyre is inflated and the site. In the event of operating at very low temperatures, some other precautions may be necessary.
For further information, please contact your usual MICHELIN representative.
Under normal conditions of use, our tyres inflated with air pose no particular problems at all, from either an endurance point of view or a safety point of view.
Some manufacturers supply machines inflated with nitrogen tyres.
Inflation with air
This is the most commonly used method. Using the ambient air to inflate tyres is perfectly suitable in the vast majority of applications. Two conditions should be mentioned however: the compressor must have sufficient output (43 m3/h for a minimum pressure of 12 bars) and its tank must be sufficiently large to cater for the size of tyre. (See the chapter on “Organisation of a tyre workshop”).
Inflation with nitrogen
Why inflate with nitrogen?
Nitrogen can be used to inflate tyres with a view to eliminating the risk of internal combustion of the tyre and the associated risk of explosion.
Inflating with nitrogen removes this risk by eliminating the oxygen necessary for combustion and explosion.
Note: when the temperature becomes abnormally high (around 250°C), the rubber enters into internal combustion, a phenomenon known as pyrolysis.
The phenomenon of pyrolysis has two consequences:
- release of inflammable fumes (methane, hydrogen);
- acceleration of the temperature rise inside the tyre.
Under certain conditions, the temperature inside the tyre can reach the point of auto-inflammation of the gas compound produced by the pyrolysis of the rubber. This results in a tyre’s explosion, with much more devastating effects than the near instant deflation commonly known as a “blow-out”.
Note: The exceptional temperatures mentioned above can only be reached with input from an external energy source:
- vehicle struck by lightning;
- vehicle hit by an electric arc (after passing too close to an electricity line);
- welding close to a tyre;
- excessive overheating of a mechanical unit (transmission, electric drive motor, or brakes, for example, or when the tightening nuts on wheels have been heated);
- hot surroundings, as in a steel-works;
- overheating of a tyre caused by underinflation, overloading, exceeding the tyre’s speed capability, or a combination of the three.
Nitrogen is a neutral, non-combustible gas. Inflating tyres with nitrogen improves safety.
Better maintenance of inflation pressure over time- nitrogen, being a neutral gas, diffuses through the rubbers less rapidly than oxygen.
Inflation with nitrogen reduces the risks of oxidation of the tyre components (rubbers, cords, etc.) and the wheel/rim equipment.
When is it advisable to use inflation with nitrogen?
For obvious safety reasons, this method of inflation is systematically recommended for the following applications:
- operating in an atmosphere with risks of explosion;
- operating on or near incandescent materials (smelting works, steelworks, glassworks, etc.);
- operating with risks of electric arcs (near high-voltage lines or cables, lightning, etc.);
- applications susceptible to entail significant overheating of tyres as a result of:
- intensive tyre use (speed, distance, intensity of the cycles)
- substantial transmission of heat from the engine, hubs, brakes, etc.
When the tyre’s operating pressure is below 5.0 bars1, it necessitates a system to drain the tyre or put it under vacuum in order to significantly lower the proportion of oxygen. These systems are complicated to use, and only justified by exceptional conditions (explosive atmosphere, for example).
The use of deoxygenized air can represent an alternative to using pure nitrogen (of the medical type).
In the latter case, its implementation may be delicate, as cylinders of pure nitrogen are filled at very high pressures.
Lowering the proportion of oxygen in the inflation gas also reduces the possibility of oxidation of the metallic surfaces.
CAUTION: inflation with nitrogen from cylinders of compressed gas must imperatively be performed by authorized, fully-trained operators. Never use a cylinder of nitrogen that is not equipped with a regulator, and always comply with the safety rules issued by the supplier.
The inflation of a tyre with nitrogen averts the risk of explosion by eliminating the oxygen or reducing the proportion of oxygen inside the tyre (O2 is an essential gas for the combustion). Inflation with nitrogen reduces the effects of the pyrolysis of a tyre, but does not eliminate its causes.
(In strict terms, we should talk about deoxygenating the inflation mixture.)
Inflation with nitrogen should be particularly encouraged for extreme operating conditions.
For other cases, it depends on the operating and maintenance conditions; a site-specific study will enable us to establish the most appropriate inflation method.
Filling of tyres with polyurethane (PU fill)
This operation consists in filling the tyre with a semi-solid material like polyurethane.
The advantages of filling with polyurethane
- no machine downtime as a result of punctures;
- no risk of instant flats due to lack of air;
- improved stability due to reduced tyre deflection;
- no more monitoring of tyre pressure. The disadvantages of filling with polyurethane
- faster wearing of mechanical parts (axles, transmission and brakes) because of the increased weight in movement (we advise checking with the machine’s manufacturer before any PU filling any tyres);
- lower operating speeds to limit heat generation and the resulting dilation of the polyurethane that would involve an excessive pressure on the casing;
- elimination of the machine’s “suspension” function;
- reduction in tyre wear life and traction capability (caused by the reduction in it’s deflection and contact patch);
- the cost of filling is comparable to the cost of the tyre itself;
- increase in fuel consumption caused by greater rolling resistance.
The filling operation
After normally mounting the tyre (see the chapter on Mounting/Demounting), the tyre is inflated so that the beads take up the correct position on the rim, then it is deflated. The next operation consists in piercing a hole in the centre of the tread to reach the interior of the tyre. The polyurethane fill comes in the form of 2 different liquids. The 2 components are mixed in equal proportions before being injected through the valve. Filling continues until the liquid starts to escape through the hole at the top of the tyre made previously.
The hole is then blocked, and injection resumes until the desired pressure is reached. Taking into account the inextensible nature of the radial casing, the pressure increases rapidly and therefore requires a particular vigilance. The solidification of the 2 liquids starts immediately and continues for 72 hours.
MICHELIN’s is not responsible for either the filling material or its use, which involve procedures established by the supplier of polyurethane.
It is generally accepted that a filling pressure of approximately 60% of the pressure recommended by MICHELIN (with the same load conditions) gives
satisfactory results. (The importance of the correct inflation pressure is explained in section 1.2 at the beginning of this chapter.)
However, if the polyurethane filling has been chosen to solve a problem of frequent punctures of L2, L3 or even diagonal tyres, the use of type L5 (X
MineD2 or XSMD2+) tyres inflated with air would solve the problem quite satisfactorily.
Precautions for protecting operators
The operators must be equipped with helmet, safety shoes or boots and gloves. We recommend that they also wear protective glasses because of the risk that dust will be propelled towards them when the air hose is disconnected from the valve.
During the entire operation, the operator should stand facing the tread of the tyre being inflated, at a minimum distance of 3 meters.
Precautions to avoid accidents with tyres
The tyre must be firmly secured during inflation to avoid any risks of falling. Components may be propelled into the air by the pressure. During inflation, the side of the tyre with demountable components should be placed facing a wall.
On receipt of a new vehicle, check the inflation pressures before commissioning; vehicles are very often delivered with overinflated tyres to ensure stability during transport. Adjust the pressures if necessary.
MICHELIN has developed specific training modules on tyre use. Your usual MICHELIN representative would be happy to present them to you.