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New LTe for Old Tyres

4th May 1962, Page 125
4th May 1962
Page 125
Page 126
Page 125, 4th May 1962 — New LTe for Old Tyres
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Retreading and re-capping is a world-wide activity By A. J. P. Wilding

FYRES account for between 8 and 18 per cent, of the total maintenance costs of cars and commercial vehicles. It is not surprising, therefore, that tyre conditioning of one type or another is employed as much it is today to get these costs down, or at least to stop em rising. The degree of use of reconditioned tyres is parent from figures for 1960 which show that of 500,000 tyre replacements, over 4,000,000 were retreads. The increased confidence of vehicle users in retreaded res. is shown by the fact that in 1948 only about 20 per nt. of the 6,750,000 tyre replacements were retreads. is confidence is more or less equally shared between car d commercial vehicle users, for in the three years up to 60 just over 50 per cent, of the car tyre replacements re retreads,_ whilst the figure for commercials was 1 per cent.

It is thus obvious that retreading is big business. And there many concerns engaged in it. In addition to the new-tyre mufacturers there are about 90 independent retreaders, 58 them members of the Retread Manufacturers' Association 2w-tyre makers cannot join theR.1VI.A.). These 58 members oduce some two-thirds of the total number of retreads and the makers of new tyres have an appreciable share of the tread market it means that the production of the 30 or so dependent retreaders who are non-members is relatively iall.

All retreading is carried out following the same basic ocedure. On receipt of a worn tyre it is inspected, the crown buffed to provide a level, clean and rough surface, new tread bber is applied and the new rubber is vulcanized onto the own by the application of heat and pressure. The main fferences between the systems in existence are in the method vulcanizing, as the first three operations are generally the me whatever system is used.

There are four basic types of tyre reconditioning: a. top cap, ien the new tread is a rectangular strip of rubber: and does It extend down the wall of the tyre at all; a re-cap, when the w rubber extends a small way over the shoulder; a retread, aich extends a little farther down the wall.; and a remould, ien new rubber is applied from bead to bead. Technically, I four types are equally satisfactory. That is, there is no ason why a full remould should give a better result than top cap, because the rubber applied to the walls is a very in veneer and cannot add to the strength of the bond at the .a.ci. The main advantage of the bead-to-bead remould is in r pear a n ce , but a coat of paint would do almost as much and ere is a possibility that the second curing of the existing side ill could be detrimental.

Initial inspection is, of course, of the utmost importance and e aim should be to allow through only those casings which li give at least one more tread life. All stages of tyre reconHoning are covered by a "Code of Minimum Standards," to aich all members of the R.M.A. subscribe. This defines the inimum acceptable casing standards and specifies the maxi mum amount of damage that Can be repaired, together with the methods to be used in effecting repairs.

Most damage can be discovered on visual examination, but it is recommended that an air-injection needle is used to identify any points of separation of the casing plies. Air at a minimum pressure of 80 p.s.i. is injected into 'several places around each sidewall, and any separation is indicated by inflation of the casing. In addition to this method, infra-red ray equipment can be used to test tubeless tyres for separation.

Tubeless tyres can be processed quite satisfactorily, but if there is slight damage to the bead or more than a certain amount of damage from cuts, the retreads are identified as tubed tyres and not passed out as tubeless. Defects which make a casing unsuitable for 'retreading include ply separation and excessive weathering cracks which cannot be prevented, but most of the casings rejected have been damaged because of bad maintenance or driving.

Under the latter categories. are excessive tread cuts caused by allowing stones to remain embedded in the rubber; excessive shoulder or side-wall damage as a result Of " kerbing "; bead breakage or damage to the rubber and plies close to the bead • beyond the limits of repair due to misuse of tyre levers or chafing by the split flange on two-piece wheels (periodical repositioning of the flange is recommended); and oil saturation through to the cords. Casings of steel construction with wire showing .are unsuitable if the area exposed has rusted.

It goes without saying that excessive cord damage will be caused by allowing the tyre to run until the main casing cords are worn. These must not be confused with the "breaker" plies, which on conventional textile tyres are laid between the tread ;rubber and the " cushion" rubber -above the structural plies. These breaker plies actually give an indication of the limit of wear and when they can be seen show that further wear can damage-the main plies.

Tyres such as the Michelin "X,' Dunlop Duraband (car sizes only) and Goodyear Uni-Steel, which have steel or textile casing plies and circumferential steel plies in the crown of the tyre below the tread, are very often referred to as " rigid breaker tyres." The crown plies are quite different from the breaker plies, however, and perform a totally different function—they stabilize the tread. Michelin, who originated the design, say the correct term is stabilizing plies and in fact true rigidbreaker tyres are those such as the U.S. Royal Safety Steel Shield tyre, which are of conventional textile-ply construction but have steel breaker strips below the tread.

Many names are used for tyres based on Michelin "X "steel cord, radial, radial steel cord and braced tread. It must be remembered that these tyres are of a different construction from Michelin Metallic tyres, which are generally similar to conventional tyres but have up to four steel casing plies instead of up to 14 textile plies, and steel instead of textile breaker strips.

Opinions vary as to when a tyre should be removed for retreading. The limit of wear would appear to be when the breaker strip starts to show, but apart from the danger of running with bald tyres on wet roads, on car-size tyres once the tread has gone there is more danger of damage to the crown plies. It is therefore best that a tyre be removed for retreading when the tread pattern at the centre has just disappeared. On giant tyres with a heavy undertread, regrooving (within limits) is possible. Many users, however, prefer to play it safe and change the tyre as soon as the tread pattern starts to go, to ensure that the casing will not be rejected.

The first stage in any retreading process after inspection and removal of flints, is buffing of the casing to a definite contour to provide a true and roughened surface to take the new tread rubber. The Michelin Rebuildex process which they use on their" X" tyres differs from that employed for textile and metallic casings. Michelin remove the three stabilizing plies completely and replace them with new before rebuilding the tyre in the same way as a new one. This makes the process comparatively costly and uneconomical for car-size tyres, and it is available only for giant " X " tyres.

After buffing, any permissible repairs to the casing are carried out and then the new tread rubber—called camelback at this stage—is applied to the casing. Except in the case of top caps, the camelback is wider than the tread and tapers at the edges. These tapered edges are brought down to the side walls. With full remoulds, thin veneers of rubber are laid on both side walls.

The most important part of retreading is, of course, the vulcanizing of the camelback on to the casing. There are three or four basic methods of doing this. New-tyre manufacturers (who only do bead-to-bead remoulding) use moulds similar to those used for new-tyre production. Matrices havir the tread and side-wall patterns are split into halves down ti centre-line of the tread, one fitted into the base of a curir machine, the other into a hinged upper part. The built-t casing with a curing bag inside it is laid in the lower half the matrix and the top half clamped on to it. The curing br is then inflated and heat applied to the outside of the moul for a set time.

This method is used by some independent retreaders als One of the drawbacks is that there is no adjustment for possib growth in the old casing and some firms employ spacer rim between the upper and lower moulds to make allowance f( this.

Another vulcanizing system is the open-steam process. 1 this the camelback is laid on in the usual way and the rebui casing put in a high-pressure steam chamber. The pressure c the inside and outside of the tyre has the same effect as tl pressure of a curink bag on a casing in a fixed mould and tl heat of the steam provides the curing temperature. Aft curing, a tread pattern is obtained by machining.

Tyresoles, Ltd., with their associated companies, are possib the largest retreading organization in the world and the Tyr soles process is operated in the concern's 17 factories in tt U.K. and by nearly 1,000 factories throughout the world. T1 main feature of vulcanizing as carried out in this process that the size of the mould in which the new tread is cured determined by measuring the actual overall diameter of ti tyre with its camelback applied to the casing. The moulds al one-piece rings with small flanges at the sides to cover tt shoulder region of the new tread. Heat for vulcanizing provided by pipes on the outside diameter of the ring throug which steam is passed.

Fitting the casing into the mould is done by spreading tt beads apart to reduce the outside diameter. When the beat are released, the casing assumes its normal shape and the trea area enters the mould. A curing bag is put in the casing an the tyre fitted on what is in effect a dummy road wheel. T1 whole assembly is then laid on a circular steel table an clamped down with an upper steel plate. Heat is then appliei the curing bag inflated and the tread cured for a set time.

As the top and bottom plates are not heated, second curin cannot harm the old side-wall rubber. On bead-to-bea remoulds of car-size tyres, however, which can be processe with similar equipment, the top and bottom plates incorporai moulds for the side walls and are heated for a shorter tim just to cure the veneer of new rubber applied to them. An interesting development by Tyresoles is a modification t their curing equipment to enable braced-tread tyres to b processed. The difficulty with these is that they cannot b expanded into a mould. This process is basically the same a the standard one, the casing being assembled on a dumm wheel disc with a curing bag inside it, but the mould ring i in three segments. These are heated and clamped to the trea rubber with the top plate holding the assembly down and th curing bag inflated to keep the casing rigid.

The price charged for retreading by the many firms engage in this work varies quite considerably. For a good qualit job, however, a retread of a giant tyre will cost about hal the new price of the tyre. As the size gets smaller the per centage increases, so for car sizes the charge is about two-third of the new price. More extensive work is involved in th Michelin Rebuildex process—this is for giants only—and th charge for these is about two-thirds the cost of a new tyre.

What life can be expected from a retread? Some retreader _claim 75 to 80 per cent. of the new tyre life. Tyresoles clair 100 per cent. Michelin Rebuildex should give the same life a new. The actual life attained is naturally wholly dependen on adequate inspection by the retreader and adequate main tenance by the user. The number of times a casing can b processed will depend on how well the tyre is maintained an the amount of damage the side walls receive. Retreading twic is quite usual with textiles and, because steel-cord tyres are les prone to side-wall damage, third and fourth retreads can b expected.

On th c question of using retreads, one thing is mos important. to remember, that buying the cheapest, regardles of other considerations, is not always good policy. It i sometimes dearest, in the long run!

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Organisations: R.M.A.

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