The first element of tyre retreading involves the casing preparation and the first element of this involves an inspection of the tyres.
Initial inspection of tyres is carried out visually with the aid of high intensity lights magnifying any shadows that are caused by irregularities.
All significant cuts are assessed for severity, and where necessary non-destructive testing (NDT) is carried out. Any major repairs required, separations, run flat injuries or exposed areas of cord will most likely cause the tyre to be rejected at this point.
Any remaining tread is then removed in the buffing stage and the tyre is profiled to standard crown radius.
In a three stage process the tyre is then brought back to being a clean, smooth buffed surface.
The side-rasping or skiving process basically is hand buffing all the areas that the larger machine is not capable of reaching without removing large sections of the tyre. It is at this stage that all stone penetrations, cuts and minor stell belt abrasions are cut out. It is often where the real damage to a tyre is found and where the decision to retread or reject a tyre is finalised. Once past this point the tyre will not touch the ground until it is finished.
Any exposed steel is then cleaned and treated to prevent oxidisation and allow a bond between the steel and the fresh rubber that will be applied.
All cracks cut out and voids in the shoulder area are filled with rubber in preparation for building. Once this is completed the tyre is wrapped in cushion gum, which is basically a highly adhesive natural rubber compound. This creates a very strong bond between the casing and the new rubber on the retread.
Once the tyre has gone through the casing preparation it is ready for retreading. There are many different types of retread processes available, each explained overleaf.
Once built and grooved the tyre is ready for curing. All systems of retreading with the exception of remoulding are cured in open steam chambers (autoclaves).
The temperature, pressure and time are precisely calculated for the size, type and thickness of the carcass and tread.
The rubber also shrinks slightly during this process helping increase the durability of the bond.
All Australian OTR retreaders currently use the ‘slab build slick and groove’ process for its versatility and hard-wearing tendencies.
The rough side of a loader tyre can be ‘scuffed’ during the process, giving a fresh sidewall with impact protection restored.
This process is generally not done to truck tyres.
Tread patterns and compounds can be varied easily.
There are tread designs to suit applications from heavy-duty underground loaders to high speed long haul trucks.
With a retread you can confidently predict to get at least 70-110 per cent of your average new tyre life again (dependant on process used and application).
Retreads are recommended for use on the rear of machines only.
This is primarily because of the age of the tyre and the likelihood that at some stage in its life the tyre has sustained some damage.
Retreads have been trialled successfully in a wide range of operations, from high speed, long haul to severe hard rock. A retread can be produced to suit almost any application.
Retreading is not isolated to truck tyres. Retreaded loader tyres have been successfully used for many years in a wide variety of applications.
When starting to use retreads talk to your retreader before the tyres are sent. In the same way there is more than one type of tyre from your new tyre supplier retreaders are able to offer a variety of tread patterns and compounds for most applications.
Retreading has been proven both locally and overseas to give the lowest operating cost per hour, dependant on process used and application.
During normal usage the only part of the tyre to wear is the tread. Retreading simply replaces this worn rubber leaving the structure of the casing intact. Most tyres are manufactured for the casing life to exceed the tread life allowing for this process to be viable. Because the structure of the tyre is not altered in the process tyre kilometre per hour (TKPH) is not generally affected by retreading a tyre.
When considering retreading tyres it is important to consider the worn out tyre (casing) as an asset. Stock availability from retreaders is very low, and like new tyres supply cannot be guaranteed. Generating casings from your own sites is the best option for machinery operators, giving the benefits of predicting supply and knowing the history of the tyre. While all care is taken during the retreading process it is not able to tell what sort of damage lies behind a seemingly solid patch, or how many hours a tyre has done at what speeds or loads.
The most common cause of premature failure in a retreaded tyre is due to casing damage. A retread is dependant on the casing provided to last the distance. Apart from minimising sidewall damage, monitoring tyre pressures and using tyre management systems the other important consideration is removing the tyre with some tread remaining to protect the underlying belts from stone penetration.
Consider a casing as you would the motor in a large machine. It is an asset, which at a scheduled point in its life you will rebuild to get extended life.
The depth that needs to remain will vary from site to site, with some being able to wear tyres to 5mm, others needing to remove at around 20mm to ensure casing viability.
This may seem excessive but when considering that a retread can out perform a new tyre in terms of tread wear it still makes economic sense.
If a tyre is suffering significant belt damage below 20mm you are sacrificing maybe 200-300 hours of useable tread before the tyre starts to expose steel and fail. To get the best results from retreading operators need to establish their critical wear point, the point at which maximum utilisation of initial tread is counterbalanced by the likelihood of belt damage if worn further. This can be done either as a blanket rule for the tyres on site based on tread depth and machine hours or can be done on an individual tyre basis.
What is retreadable?
The new tread of a retreaded tyre should confidently be expected to last a considerable number of hours. To ensure that the full benefit of this is realised the casing needs to be in good condition before retreading. A tyre that has been run flat, had multiple major repairs or has suffered considerable belt damage is not going to give the best possible result.
Retreaders need to be able to bond to rubber. The bond between rubber and steel is one of the most complex and difficult to achieve. In new tyre manufacture this is overcome by brass coating the steel, a coating that is removed very quickly once the belts are exposed, 80 per cent of tyres that are rejected for retreading are rejected due to belt damage.
Consider a retread as ideally performing as a new tyre. A maximum of three major repairs is acceptable in a casing depending on size and location. A major repair to the shoulder section of a tyre, where the stress points are highest will most likely cause the tyre to be rejected for retreading, or, depending on condition be done without warranty. More than one major repair in any quarter of the tyre will also cause concerns about the long-term viability of the casing, as will more than two sidewall repairs.
The age of the tyre needs to be taken into consideration as well. The curing process of rubber is continuous throughout the life of the tyre, and is accelerated by heat and in the case of older tyres sunlight. Modern tyres have UV protection built into the compound to reduce the effects of this. Tyres that are starting to perish or become brittle will have difficulty in achieving maximum wear before failure.
With the outlook of the world tyre shortage predicted to continuing for several more years, retreading is a proven and cost effective solution to ensure your equipment stays operational.
Tim Prest is a Director of Flextred International.