Tyre Manufacturing Process
Tyre Manufacture
The pneumatic Tyre was invented by a Scot, R. W. Thomson, and first patented by him in 1845. A set of Tyres made according to Thomson’s design was fitted to a horse-drawn carriage and covered more than 1000 miles (1600km) before the needed replacing. It was not until nearly 50 years later, however, that the Tyre industry was founded by J. B. Dunlop, an Irishman from Belfast.
A vehicle Tyre consists of an inner layer of fabric plies which are wrapped around bead wires at their inner edges. The bead wires hold the Tyre in position on the wheel rim. The fabric plies are coated with rubber, which is moulded to form the sidewalls and the thread of the Tyre. There are two main types of Tyres. In the cross-ply or bias Tyre the plies run diagonally from bead to bead. Making an angle of around 40° to the beads. Successive plies run in opposite directions to give a criss-cross pattern. With the radial-ply Tyre, the plies run directly from bead to bead at 90° o the Tyre circumference. Tread-bracing layers, or belts, are fitted around the circumference of the Tyre between the radial plies and the tread. The radial-ply design offers significantly better cornering and wear characteristics than the bias Tyre and is rapidly replacing it for automotive use. Some o the advantages offered by the bias-belted Tyre which has cross plies with a tread belt.
The process of making a Tyre involves three separate stages: the preparation of the components, the assembly into the shape of a Tyre, and heating with sulphur in a suitable mould to vulcanize the rubber. During vulcanization the rubber in the Tyre structure combines with the sulphur, which sets it in its final pattern and gives the rubber components the required physical properties.
The main materials in Tyre construction are steel wire for the inextensible beads. Textile fabric or steel for the casing and reinforcing breakers, and rubber mixed with various additives to give the required strength and resistance to wear and fatigue. For Tyre casings, rayon, nylon, Kevlar and polyester are the most commonly used materials, although thin steel cable is often found in truck Tyres. The tread belt is usually made of steel in truck Tyres and care Tyres, but rayon and other materials such as glass fibre are also used in car Tyres. The rubber mixes are made from synthetic rubbers for care Tyres: heavier truck Tyres are made of natural rubber, which runs cooler than synthetic rubbers.
Rubber mixes
The first operation is to prepare the rubber mixes, working the rubber into a plastic state, in an internal mixer, and milling into it sulphur for vulcanization and other ingredients for the different types of rubber mix needed in the various parts of the Tyre. In the tread, a high loading of finely powdered carbon black is used to give resistance to tread wear, with the tread mix being chosen to give the required combination of wear characteristics, and wet and dry grip. These conflicting requirements result in a compromise. An alternative solution tried by some manufacturers uses a combination of rubbers for tread area. With tubeless Tyres, the inner surface of the Tyre and the bead are covered with a soft rubber mix to give good sealing between the bead and the wheel, and to provide sealing around any object that punctures the Tyre – this reduces the risk of sudden deflation and loss of control.
The casing rubbers are mixed to have strength in the thin layers which bond the casing cords together, and to have resistance to fatigue under repeated flexing and continuous tension. For use on the bead wires, a hard mix with a high sulphur content is prepared, which will set into a solid mass on moulding under heat and pressure.
The bead wires are prepared from high-tensile steel wire, assembled as a ribbon of five or six strands, side by side, and enclosed in the hard rubber mix to form a tape. A number of runs of this tape are wounded up on a former of the correct size and, on vulcanization, the bead is set in a solid and virtually un-stretchable form.
As well as holding the Tyre on the rim, the bead wires provide structural strength, so their breaking force must never be exceeded.
Fabric preparation
Cotton, which was originally used, has been replaced in Tyre casings by synthetic textiles, which are made of long, continuous filaments and have much greater strength for a given thickness than the old cotton thread spun from a collection of short hair like fibres.
The material for the plies and for the breakers consists of a practically weft less fabric, with the strength all in the wrap, and with the sheet held together only by a system of fine, widely spaced weft threads. This conjunction is used to eliminate the knuckles which occur in a normal cross-woven fabric (where wrap and weft threads cross) and to reduce the sawing and chafing which takes place when such a fabric is flexed under load. The sheet of weft less fabric is made into a sandwich, between two films of rubber, in a calendering operation (pressing between rollers). The rubber-coated fabric is then cut into strips, with the threads running in one direction, for use in building up the casings of the different types of Tyres.
Tyre building
Both radial and cross-ply Tyres are built up on collapsible steel formers. The layers of fabric plies are secured by turning the edges around the coil of bead wire. the structure is then enclosed in suitable reinforcing and packing strips.
The process of adding the smooth strip of extruded rubber compound which will form the thread differs in the two types of Tyres. The cross-ply receives its thread while still on the almost flat on the building drum. Afterward the complete cylindrical Tyre is shaped up to the usual doughnut form as it is introduced into the mould. The radial-ply Tyre requires different treatment. Here the casing must be taken from the nearly flat cylindrical building drum and shaped up to the required toroidal form before the un-stretchable rigid bracing bands are added. The tread is then fitted on top of these bands.
The final stage in the Tyre manufacture consists of moulding the built-up raw Tyre in a suitably designed steel shell mould. The mould is either engraved with the tread and wall pattern or has die-castings riveted inside it, to make up the complicated pattern of ribs, blocks, grooves and fine slots that give the final Tyre its all-important road grip and even wear characteristics.
The mould is in a press containing a cylindrical rubber diaphragm which is inflated under high pressure inside the Tyre as the mould closes. This operation forces the raw plastic Tyre into the pattern on the inside of the mould. Heat is then applied, in the form of steam, both from cavities in the press through the outer mould, and from the inside of the diaphragm within the Tyre. This heating causes the chemical combination of the rubber with the sulphur which has been included in the various rubber mixes in the components of the Tyre. The result, when the moulding is finished, is a Tyre of permanent shape and specific physical properties for its intended use.
Provided that the main body, or carcass, of the Tyre is not damaged, a worn Tyre can be tread for further use. In this process the tread and wall rubber is stripped from the Tyre body and a new tread and wall layer bonded on. With heavy-duty truck Tyres, the tread rubber is often made thick enough to allow the tread pattern to be recut when the top layer has become worn.
Safety Tyres
The outstanding weakness of the pneumatic Tyre has always been the risk of puncture. Cars carry spare wheels and tools for changing the wheel. Such roadside wheel changes are hazardous, however, and the sudden deflation of the Tyres, as when a blowout occurs, can cause loss of control.
Various safety Tyres have been developed. Most rely on the use of locking devices to keep the Tyre on the wheel rim when it was deflated, giving a measure of stability and steering control. Some deigns allow the Tyre to be run at low speeds in a flat state for some distance to reach a service station, where a replacement can quickly be fitted.