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Earthmover tyre production at Michelin’s Vitoria plant

Radial tyre technology was invented by Michelin in 1946, but it was not until 1959 that the company produced its first radial earthmover tyre. Today, over 40 years later, Michelin are the market leaders in radial earthmover tyres and offer the largest product range in the market, with 45 different tread patterns and some 420 different sizes, ranging from the smallest 5.3kg fork-lift tyre to the largest dumptruck tyre weighing 5,450kg.

Over the years Michelin have achieved a number of notable milestones. By 1980 the company was offering a complete radial earthmover range, with a tyre suitable for every machine and every application. In 1997 the company launched its first low-profile radial tyre for medium-sized loaders, and a year later low-pressure technology (which reduces the risk of punctures and can increase tyre life by up to 20%) was applied to giant dumptrucks with the introduction of the 55/80 R63 XKD1.

In 1999 the XDR tyre was launched with its revolutionary tread pattern designed for large dumptrucks working in demanding mine and quarry environments, and a year later saw the introduction of the company’s MEMS (Michelin Earthmover Management System) preventive maintenance system.

Michelin’s current earthmover product segment, which includes tyres for surface mine, underground mine, quarry, construction, industrial handling, and port and intermodal applications, was established in 1996 and today accounts for some 6-7% of the company’s turnover, making it the third largest of Michelin’s nine business segments.

The company has a commercial presence in over 170 countries around the world and the nine business areas (car tyres, truck tyres, aircraft tyres, agricultural tyres etc) operate 82 manufacturing facilities on five continents, which between them produce a total of 844,000 tyres a day. Earthmover tyre production, however, is centred at six locations: Lexington in the UK; Waterville in Canada; Montceay-les-Mines, Le Puy, Clermont-Ferrand and Poitiers in France; and Vitoria in northern Spain.

Vitoria plant

Located some 50km south-east of Bilbao in the heart of the Basque region, the Vitoria plant is one of four Michelin production facilities in Spain. Opened in 1966 and recently accredited to ISO 14001, the 43ha plant has both car tyre and earthmover tyre production lines as well as its own in-house wire works. It employs just over 3,000 staff, of which around a third are dedicated to earthmover tyre production.

Vitoria produced is first radial earthmover tyre (25in) in 1966, but throughout the 1970s and early 1980s the facility underwent significant expansion and modernization of its workshops to cater for the increasing trend towards larger sizes. The production of 51in tyres commenced at the plant in 1971, followed two years later by 57in versions.

In 1976 the plant produced what was at the time the world’s largest radial tyre – a 60/65 R57 SRD1 weighing 4,280kg. Sixteen years later Vitoria achieved a Guinness world record for a 55/80 R57 X Mine D2 Cat 994 loading shovel tyre which weighed in at 5,782kg. This upward trend in tyre sizes culminated in 1997 with the start of 63in production.

Tyre production

The Vitoria plant imports both natural and synthetic rubbers from countries such as Brazil, Nigeria, Indonesia and Malaysia and produces over 100 different rubber compounds to cater for the wide range of earthmover tyre applications, but all compounds consist of a combination of five key groups of ingredients.

Not surprisingly, the main component in all compounds is the raw rubber itself, which comprises some 56-58% of the total by volume. By varying the proportions of the natural and synthetic rubber content in a compound, earthmover tyres can be given particular characteristics depending on their specific application – synthetic material giving the finished product greater resistance to cuts and tears while natural rubber helps to reduce heat generation.

The second key ingredient by volume is carbon black, which increases the resistance of the rubber compound during vulcanization in the latter stages of tyre production. Various chemical additives, including zinc oxide, antioxidants and anti-ultraviolets, constitute the third ingredient group, and special oils which aid the mixing process make up the fourth.

The accelerators necessary for effective vulcanization form the fifth and final group. These are added after the first four have been amalgamated and cooled, as the temperature reached during the mixing process is sufficient to initiate premature curing of the rubber.

Notwithstanding these five broad component groups, each tyre compound actually contains up to 200 individual constituents, and each finished tyre comprises several different rubber compounds, as the various parts of the tyre (tread, sidewall, inner etc) require different levels of resilience, strength and flexibility.

Tyre production is a time-sensitive operation because unvulcanized rubber has a shelf-life ranging from 8h to 15 days, so storage and usage is carefully controlled using a computerized barcode system. Various blocks of raw rubber weighing up to 35kg are loaded on to semi-automatic mixing lines in a precise sequence depending on individual compound requirements. At the outset these blocks are chopped into smaller, more workable pieces to ensure homogeneity before being check-weighed in batches and topped up with additional rubber if necessary.

Mixing of the rubber compounds takes place in 200kg batches, with the weighed quantities of rubber being delivered to the mixing area via an overhead bucket system, while precisely weighed amounts of carbon black and the various other key ingredients are fed directly into the mixers.

The rubber compound (still without its vulcanizing accelerators) exits the mixing process at a temperature of around 170?C, whereupon it is cooled and rolled into flat sheets. When the temperature has been lowered sufficiently the vulcanizing agents are added and the material is formed into rolls for check-weighing. Further cooling takes place in a large water bath, after which the material is gently dried by warm air before being cut into pieces (plates) or folded into rolls suitable for storage prior to further processing.

Once the vulcanizing agents have been added to the rubber sampling of the compound is carried out on a continual basis. Small discs of rubber are punched out of the sheets and sent to the laboratory for analysis. This involves vulcanizing the sample and checking that the resulting composition, properties and tolerances of the compound are within specification before allowing the source batch to proceed to the next stage of production.

The rubber plates and rolls from the mixing section are delivered to the extrusion workshop where reheating and remixing further improves homogeneity and returns the compound to a workable state. When suitably warm and plastic the material is rolled and sheared before being forced through precision extruders which produce the various profiled sections necessary for tyre construction – a total of 2,400 different profiles are used in the production of the full range of Michelin radial earthmover tyres.

The extruded profiles are cut to length, passed through a water bath and then dried before being fed on to thin aluminium sheets and rolled up for storage until required for the first stage of tyre assembly. These thin metal sheets are used to feed the rubber profiles on to rotating metal reels which are used to assemble the tyre casing, reels of different sizes being used to create the various tyre rim diameters.

Working in teams of two, skilled technicians use these reels to build up layers upon layer of flat and profiled sections in a precise order and pattern to create what eventually forms the inner, sidewall and tread area of the tyre. Bead wires, which are produced in house to suit the various rim diameters, are also incorporated within the casing during this stage of assembly.

At this point in production the casing effectively consists of a composite rubber tube with a profiled surface which bears little resemblance to the tyre into which it is about to be transformed. However, with this first stage of assembly completed, the casing undergoes a process by which it is ‘conformed’ into a more familiar shape to produce a ‘green’ tyre.

In simple terms, the outermost ends of the rubber tube (which form the rims of the tyre) are forced towards each other under pressure while the entire casing is slowly inflated to a low pressure to create the basic, but as yet formless, tyre shape.

Once the casing has been conformed in this way, second-stage assembly begins with the addition of final profiles and bracing plies (ie working plies for structural strength and protective plies which offer more elastic properties). The last component to be added is a thick layer of tread rubber which can constitute 50% or more of the finished tyre. This is located over the green tyre casing and then rolled on to the crown area by means of large, mechanical, vacuum-assisted fitment rings.

The green casing now closely resembles the finished product, although as yet the tyre is still without its distinctive tread pattern and markings. In total, the 50 or so individual components that make up a typical earthmover tyre, most of which are used in pairs due to the symmetrical nature of the product, require over 100 individual stages to assemble.

With assembly complete, vulcanization of the product marks the final stage of production. This takes place in a vast curing area which houses an array of different-sized curing presses, each of which can be fitted with interchangeable moulds that give the tyres their particular tread patterns and descriptive markings. Curing (vulcanization) takes place under high pressure at a temperature of around 120-130?C, the smallest earthmover tyres spending around one-and-a-half hours in the press, while the largest sizes can take up to 10h to cure.

Post production the tyres are moved on to the verification section where visual inspection and non-destructive quality assurance tests are conducted. As an initial check on build quality, every tyre produced is accurately weighed to ensure that all component parts have been included during assembly. In addition, a sample representing around 50% of production is subjected to X-ray and ultrasonic analyses. These provide quality control specialists with an ‘internal’ view of the tyres, allowing them to check that all component parts have been fitted in the correct position and all plies have been correctly aligned.

Finally, highly accurate laser-guided measuring systems are used to check the dimensions and thickness of a representative sample of the tyres produced. The resulting measurements are digitized and stored on computer for statistical and product-development purposes.

Future developments

By adopting a continuous improvement approach which both ensures and accelerates progress, supported by stringent quality assurance measures to consolidate this progress and inspire customer confidence, Michelin are constantly seeking technical solutions to help improve earthmover tyre performance and hence boost customers’ productivity and profitability.

At their 4,500ha test facility in Almeria, Spain, the company logs over 3 million earthmover tyre kilometers a year on 100km of test tracks using a wide range of vehicles up to 600 tonnes in weight in an effort to develop earthmover tyres capable of operating safely and comfortably at higher speeds and lower pressures with greater loads, while maintaining optimum service life.

In addition, Michelin’s long-term permanent partnerships with the world’s leading construction plant manufacturers allow them to design and develop tyres adapted to each vehicle and application, and to introduce innovative new tyre concepts which give the manufacturers the freedom to design the earthmoving vehicles of the future.

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