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Second time around

19th July 1980, Page 46
19th July 1980
Page 46
Page 47
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Page 46, 19th July 1980 — Second time around
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Which of the following most accurately describes the problem?

If you're thinking of buying retread Brian Chalmers-Hunt assures yo( the're as good as nen

THE British Rubber Manufacturers Association definition of retreading is "a process of reconditioning a worn tyre as a means of extending its useful life."' With today's spiralling inflation and analysts' predictions of limited oil supplies, cv retreads, as a concept, become more feasible both from the economical and the ecological point of view. Not only are they initially less expensive to buy than new tyres and on a cost per mile basis but their production consumes a lot less energy than does a new tyre.

Statistics can be meaningless or can be distorted, cost analyses tend to ignore the calorific value of the tyre casings or the cost of disposal, the affluent (are there any left?) may regard retreading as something which does not concern them, the rest of us may be forced to look upon retreading as an economic necessity.

Whatever the view, the facts are these. (a) A properly retreaded tyre on a sound casing will give it a second life at a fraction of the relative cost per mile — this can be less than half of that for the new tyre: (b) less relative cost means more profit.

As in so many other cases in the cv world, it is not possible to generalise as to whether retreading should be used or not. A lot will depend on the nature of the operation.

Many operators who use retreads do not in fact realise that they themselves are growing their own seed corn. It is a truism in the industry that retreading starts and finishes with a casing, which must be in sound condition, if it is to be retreaded.

There are a number of factors which cause casings to be rejected. The most common is casings which have been run underinflated, or extensive overloading or speeding.

A new tyre is essentially an air container which is designed to operate within three parameters — load, speed and pressure.

If the tyre is run overor under-inflated or at excessive speed, then the heat build-up will be such that there is small chance that the casing will be accepted for retreading.

Another major factor which affects casing acceptance is poor regrooving. Regrooving really came into its own following changes in legislation in 1968. It is an inexpensive and effective way of gaining perhaps an extra 10,000 miles for a cost of about £5.

But if it is not carried out skilfully and when the tyre has at least two millimetres of tread remaining right across its surface, it is likely that the regrooving operation will lead to damage to the breaker, and subsequent rejection of the casing.

However, it is true that with Dunlop and certain other manufacturers, a slave or protective breaker has been incorporated into the new tyre and this breaker can be removed at the retreading stage without harm to the casing.

Recognising the operators' prime needs, Dunlop in fact employs the concept of building a multi-life tyre (which can withstand around 250,000 miles of running) from the start with the hope that provided it is properly maintained it will be suitable for regrooving and then retreading.

There are three types of retreading. Top capping, in which the worn tread only is removed; recapping, in which new rubber is applied to the tread area and over the shoulders; and remoulding, in which the new rubber extends from bead to bead.

The two most popular types of retreading in Britain are the top cap — such as the Dunlop Duraband — and the bead to bead remould. The essential

difference between the two prc cesses is that in the case of th Duraband top cap or precure, retread, a high density tread i applied to the casing after bu• fing in a manner which involve less heat and pressure than i the case with a bead to bee, remould.

The top cap mileage potentiz is equivalent to, or sometime even better than, that of a ne% tyre. Because of its length of lif most manufacturers involve with the precured process wi insist that only a first-life casin in first-class condition i accepted for this treatment.

Dunlop offers a wide range c sizes and patterns, both i Duraband and in bead to beac While retread manufacturer take great care in the selectio of suitable casings and use series of sophisticated techn ques, these will vary throughout the retreading industry dependent on individual needs and financial resources.

There have been great developments in tyre quality in the last decade — and of course this has affected the quality of retreads. Tread patterns have greatly improved but even so manufacturers are obliged to build a safety factor into the original casing which is equivalent to much more than the expected life of the tread.

The tyre casing is the crux of the remoulding process. Before cases are considered suitable for

reconditioning they are minutely examined. Members of the RMA subscribe to a -Code of Minimum Standards.

This code sets out the minimum acceptable casing standards, specifies the maximum amount of damage that can be repaired, and lists methods to be used in carrying out these repairs.

Most of the damage is usually visible, but an air injection needle is used to check possible points of separation of the casing plies. Retreaders .reject as many as 30 per cent of the casings tested.

On the initial visual inspection common failings are discoloration of the sidewall interior because of heat build up.

usually as a result of underinflation, injuries through

penetrations; and damage to bead caused by excess friction between tyre and wheel often as a result of under-inflation or overloading.

Obviously overloading and under-inflation will impose severe stress on tyres, but other factors contributing to stress include fitting the wrong size tyres and incorrect ply rating of tyres in relation to the load carried.

The British Standards Institution publish specifications for "Retreaded Car and Commercial Tyres (BSAU 144b: 1977) and these latest recommendations were requested by the Department of Environment (now the Department of Transport) to reflect developments and, in particular, to allow for the raising of the speed capability of commercial vehicle tyres to a minimum of 96km /h (60mph) in accordance with the permitted speeds of such vehicles on motorways in the UK.

The specifications detail the minimum requirements for the retreading of commercial vehicle pneumatic tyres. They specify certain constructional features, the examination, limits of damage preparation and permissible repairs of the tyre carcass before processing, marking and inspection of the tyre after curing, and minimum performance levels (not applicable to -off the roador -passenger transport'. (PT) tyres) are also given. The speeds for PT tyres should be agreed between the purchaser and the retread manufacturer.

There are strict guidelines on the limits of carcass injury and the extent of permissible repair.

These include. crown, shoulder or sidewall penetrations or cuts

which, after preparation, would be outside recommended limits: the break-up of the carcass; chemical attack or oil contamination; and bead damage, other than rubber or chafer damage.

Limits applying to textile diagonal ply tyres are damage to more than three local areas or any area more than 25mm long, or any two areas less than 300mm apart or any damage penetrating more than 25 per cent of the carcass plies.

For radial ply repair is not allowed if there is damage to more than five local areas, or any area more than 60mm long or 30mm wide, or any two areas less than 100mm apart, or more than four carcass cords damaged for a length of more than 5mm in the area from the bead heel to the rim fitting line. (This latter is limited to three such areas per bead.)

Repair is also prohibited if carcass cords are exposed and damaged due to sidewall scuffing, carcass cords are exposed due to tread wear; there is belt ply separation or tread separation from the carcass.

Restrictions on repair, which apply to radial ply only, are that not more than four major repairs should be made in any one tyre (this includes successful previous repairs).

There is no limit to the number of minor repairs permitted on radial ply tyres, but if the area of a sidewall or shoulder repair may bulge slightly when the tyre is fitted and inflated to recommended operational pressure, reinforced patches to restrict the height of the bulge to not more than 4mm should normally be used.

If the belt plies are exposed due to wear or damage.extending beyond the specified injury limits they should be replaced before retreading, and when a protective breaker is fitted and only this is damaged it may be removed and need not be renewed.

Repairs to commercial vehicle tyres prior to retreading fall into two categories — minor and major repairs.

Minor repairs comprise repairs involving the preparation, solutioning and application of rubber where cord reinforcement is not necessary..

The BSI specifications regarding both diagonal ply and radial ply permit repair to any rubber damage; repair to any acceptable bead damage: repair to any penetration up to 6mm diameter in the crown or shoulder areas: repair to any penetration up to 3mm in the sidewall area: repair to carcass ply damage which is limited to 25 per cent of the actual carcass plies.

Permitted repairs to radial ply only cover any penetration up to 3mm diameter in the shoulder area: damage up to 20rnm diameter in any belt ply except that which is adjacent to the carcass plies: and rubber tears

between cords of the radial carcass.

Major repairs involve the exceeding of the dimensions referred to above as "minor repairs" in the case of diagonal or radial ply tyres or when cord reinforcement is necessary. Where dimensions are in excess, the maximum recommended values must not be exceeded.

We move on now to the actual process of retreading.

Before processing begins the casing is rechecked and prepared, To help with inspection There is now a range of sophisticated equipment either cur rently available or under investigation.

X-ray machines have been used by the manufacturers of new tyres for some time, but because of their expense, similar models are rarely found among the smaller retreaders.

However, small portable units are available which are invalu able in identifying suspect cord fractures and previous casing repairs in steel lorry casings.

Although ultrasonic detection is effective in detecting casing separations, it has yet to be proved as a reliable production tool. The electronic signals are transmitted through the casing and any irregularities are identified on a visual display unit. Prototype models, which operated in either air or in water, were developed in 1974/75 by the Transportation System Centre, NHTSA. A visual display unit analyses the result, Holographic interferometry, at present largely employed for aero tyres, utilises laser light to photograph casings while both in a relaxed state and under slight tension in a vacuum chamber.

The holograms so produced reveals areas of separations, cord damage and porosity. Interest among retreaders was shown in the early 1970s, but once again the high capital outlay restricts its use.

There are also British standards which apply to the retreading process itself, but it must be remembered that before a casing is submitted for retreading it may well have done over 100,000 miles.

During the buffing stage the entire surface to be re-rubbered must be prepared, with care being taken to avoid gouging or scorching. The contour of the prepared area should suit the tread profile and the matrix to be used. All loose cord ends should be trimmed.

Local buffing damage to an overlap joint is permissible but no damage to the carcass should be present in the sidewall region.

In the case of diagonal ply tyres, buffing damage must not extend beyond the top carcass ply in the crown. It should be assumed that the first play encountered is a carcass ply unless a breaker can be identified.

With radial ply tyres, localised buffing damage to the belt should be allowed.

When repairs are being carried out all injuries to the carcass must be prepared so that all loose and noticeably oxidised material is removed.

Any carcass damage exceeding 6mm in diameter and exceeding 25 per cent ply penetration after preparation should be cord reinforced. Where a reinforcing patch is fitted, the retreader is responsible for ensuring the patch is suitable in all respects for the tyre being processed.

Radial ply carcass repairs in the sidewall or shoulder regions must be reinforced with a radial ply patch. Crown regions must be reinforced with a radial ply patch. Crown repairs can be backed by either radial or diagonal ply patches, but in the latter case they should not extend beyond the tread shoulder width and should be fitted so that one ply at least is radial.

The spacing of reinforcing patches should be such as to avoid overlapping. To decrease balance and uniformity problems, concentrated areas of reinforced repairs should be avoided.

Final preparation should include reinforced repair cavities which should be filled with the correct rubber compounds; the internal sealing of inner lining damage and punctures: the covering of exposed cords with the correct rubber compound; and the building up of low spots and minor repairs with the correct rubber compounds — except when using a direct extrusion building technique.

A further inspection should be carried out within an hour of curing because most defects are best observed when the retread is still warm. Thereafter, the final inspection should be carried out when cold One of the major factors influencing an operator's decision to use remoulds will be the likelihood of any increased downtime as a result of using retreads.

Those who decide against retreads should still not neglect their tyres because if the tyre is fit for retreading it may still fetch a reasonable price.

For instance, at present rates a sound 10.00R20 casing could bring in up to £30.

What then is the retread market? Since 1975 the total level for lorry retreads has been consistently around a million tyres a year, of which some 80 per cent are accounted for by heavy tyres. This reflects the severe damage which often oc curs on the smaller tyres operating in an urban environment.

The retread market is split about evenly between com panies who also manufacture new tyres, that is members of the British Rubber Manufac turers Association and those whose business is largely or exclusively retreading — members of the Retread Manufacturers Association.

While the market for retreads will mainly continue to be de pendent on the number of casings available, because of the general improvement in casing standards by manufac turers and allied to the increase in the use of premium tyres, it is likely that over the next four or five years the percentage of retreads of the total market could climb to 53 or 54 per cent.

Retreads properly maintained are an economic proposition and on the basis of, for instance, a bead to bead retread costing approximately half that of a new tyre and giving around 90 per cent of new tyre mileage, it will pay operators to look closely at the retread option when deciding what their tyre policy should be.


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