IE43981B1 - Method of re-treading tyres - Google Patents

Description

The invention relates to a method of renewal of the tread of a worn pneumatic or solid rubber tyre, in which a pre-vulcanized rubber tread ring is applied to a prepared body of the tyre with a layer of bonding rubber applied to the tread ring or prepared body, and with any gaseous inclusions removed from between the applied layer, tread ring and body, the assembly is heated and pressurized in an autoclave for vulcanising the layer of bonding rubber.

In known methods the blank is enclosed in a rubber jacket onto which internal and/or external pressure is then brought into action so that the gaseous inclusions present in the layered structure of the blank escape. Furthermore the layered structure of the blank is fixed by the jacket, especially for the critical state which appears when, under the influence of the heat of vulcanisation, the bonding rubber becomes plastic and finally, shortly before vulcanisation, even liquid. Jackets are costly to produce, they are easily damaged and are then leaky. Their life is limited depending upon the material, vulcanisation temperature and kind of treatment.

If the tread is applied to the prepared body already in the form of a closed ring the danger exists of the plastic layer of bonding rubber becoming non-uniform or of the tread sticking firmly to the layer of bonding rubber before it has been brought into the required position.

Other known methods assume a relatively great mechanical effort, or frequently undesirable imbalances occur through non-uniform distribution of the tread material along the circumference of the body. - 3 5.3981 The object of the invention consists in improving the above-mentioned method in such a way that the blank may, in a simple manner, be prepared ready for the treatment in an autoclave.

In accordance with the invention, a method of renewal of the tread of a worn pneumatic or solid rubber tyre, in which a pre-vulcanized rubber tread ring is applied to a prepared body of the tyre with a layer of bonding rubber applied to the tread ring or prepared body, and, with any gaseous inclusions removed from between the applied layer, tread ring and body, the assembly is heated and pressurized in an autoclave for vulcanising the layer of bonding rubber,is characterised in that a stretch is imparted to the tread ring by mechanical means, the stretch running in the circumferential direction, and, after removal of the tread ring from the mechanical means but maintaining temporarily a part of its stretch such that the inner diameter of the tread ring is greater than the outer diameter of the body, the tread ring-is applied to the body.

In this manner, the tread ring, immediately before applying it to the body, may be stretched very hard in a simple stretching mechanism so that when the tread ring is removed from the stretching mechanism a free stretch of about 3-4% results temporarily, which allows the tread ring to be slipped over the body without further auxiliary means^and the ring, preferably in the horizontal position of the body, to be arranged in the exact correlation with the circumference of the body. The free stretch of, for example, 4% rapidly and continously reduces to about 1% permanent stretch. In any case the duration of the temporary free stretch which is available after the stretching process is adequate for applying the tread ring to the body in a simple manner. - 4 43981 The extent of the necessary temporary stretching of the tread ring depends upon various factors, in particular upon the degree of free stretch desired and upon the composition of the tread rubber. In practical tests with commercial tread material, stretching of, for example, about 100% with subsequent temporary free stretch of about 4% have been applied, but also smaller degrees of stretching has proved adequate.

Since the stretching process and likewise the state of temporary free stretch has included every part of the tread ring, there follows also the gradual reduction of the stretch to a permanent value everywhere uniformly along the circumference of the ring, so that no imbalances can occur.

The internal diameter of the unstretched tread ring may be given just the same dimension as the external diameter of the body. In this case the permanent remainder of the free stretch, which amounts to only a very small percentage, could be easily equalised by rolling on the applied tread ring.

Preferably the diameter of the tread ring before the stretching process is smaller than the diameter of the body, that is, by at least one percent. The consequence of this is that after the reduction of the free stretch no oversize results and the diameter of the tread ring and the body are in the end equal. The selection of a diameter of the tread 43881 ring before the stretching process, which is smaller by more than 1%,leads in the end to the tread ring seating on the body under tension, since the diameter of the tread ring after the reduction of the free stretch is smaller than the outer diameter of the body. In this case the advantage of easy application of the tread ring to which a free stretch has temporarily been imparted, is combined with the further improvement that the tread ring is seated tightly onto the body. Furthermore, in this way automatic removal i.e. escape of gaseous components from beneath the tread ring is achieved as the contact pressure between the tread ring against the usual domed body decreases from the centre towards the sides, so that the gaseous components can be forced out to the sides and not trapped. If still further pressure is needed for this, where necessary a pressure-band or net may be employed in addition around the tread ring.

If in applying the tread ring only a small difference indidmeter between the tread ring and the body is preferred,it is advantageous if the layer of bonding rubber, before application of the tread ring, is covered with a slide-foil of plastics which after pulling on of the tread ring is pulled out again from the applied layers. Pulling out again is advantageously carried out immediately after the application of the tread ring, before its free stretch further diminishes. The slide foil is preferably hung like a tablecloth loosely over the tyre arranged horizontally, so that the edge regions of the slide-foil cover the periphery of the body at least to its centre all round. 439® a. - 6 A central cut-out from the slide-foil may be used for easy centering of the slide-foil with respect to the tyre. Also for the case where the tread ring is coated with the layer of bonding rubber the slidefoil is spread over the body.

The method in accordance with the invention is explained in greater detail below with the aid of the drawings. In the drawings there is shown in: Figure 1 a diagrammatic illustration of the stretching mechanism which may be used in the first phase of the application of a tread ring to a body; Figure 2 a diagrammatic side elevation of the second phase in which a stretched tread ring is being applied to a body; Figure 3 a sectional elevation of the body in the second phase where a slide-foil is being employed; and, Figure 4 a diagrammatic side elevation of a body in the third phase in which the tread ring is resting firmly against the body.

In the method of applying a tread in the form of a ring to a body 2 the tread ring !, preferably with a preformed rough face, is subjected to stretching of the order of magnitude of about 100% (depending upon the rubber mixture, form of profile, thickness of tread rubber, desired free stretch), in which the tread ring 1 is laid over mechanical means constituted by two rollers 3 and a roller 5, which can be displaced mechanically,electrically or hydraulically in the direction of the arrow 4, and is stretched by appropriate displacement of the roller 5. 43881 In the case of this stretching of only brief duration the tread ring 1 directly after its unloading exhibits a free stretch in the circumferential direction of, for example about 4%, which after the unloading of the tread ring 1 gradually reduces again down to a residual value of about 1%, which is reached in a short while.

If the inside diameter of the unstretched tread ring 1 is just as large as the diameter of the periphery of the body 2 when coated with a layer 6 of bonding rubber, or is smaller, that is, for example, by 1-2% than this body G ffGCt diameter, the stretched section ring 1 (Figure 2) can be easily and without/ 10 be slipped over the body coated with bonding rubber 6, in which case, because of the temporary stretch,there is a very small clearance between the body 2 and the tread rinq 1. For this reason it is essential that the tread ring 1 be applied to the body 2 directly after the stretching process. With originally equal diameters and a temporary free stretch of 12 cm the clearance amounts to about 2 cm all round, so that un-impeded slipping of the tread ring 1 over the body 2 is possible.

Because of the decrease in the stretch, in a short while, depending upon the choice of diameters before the stretching process, the phase illustrated in Figure 4 is reached , in which the tread ring 1 is resting firmly against the body coated with bonding rubber 6. 43SS3The application of the tread ring 1 can be performed without special auxiliary means and the ratio between the diameters existing directly after the stretching process affords the possibility of bringing the tread ring 1 by appropriate shifting with respect to the body 2, into an exactly centered position on the body 2. But advantageously, in order to be able to slip the tread ring 1 without any Special precautions over the body 2, a slide-foil 7, preferably of superfine S clinging plastics fabric having a central cut-out/for centering (Figure 3), is hung over the body 2 arranged horizontally. After the application of the tread ring 1 the slide-foil 7 is pulled effortlessly out of the blank.

A further considerable advantage consists' in the fact that the same size of tread ring may be applied for the whole field of tolerance of the bodies of one tyre size, that is, by appropriate stretching the necessary equalisation may be carried out.

After the third phase in accordance with Figure 4 is reached, the blank thus processed is, for vulcanising the layer 6 of bonding rubber, inserted in an autoclave and pressurized and heated. In case of necessity another reduced pressure treatment of any kind may also be applied beforehand in order to lead away possible gaseous inclusions between the applied layers. It is also possible,before the treatment in the autoclave^to connect the tread ring 1 in the region of the lateral points of outlet from the layer 5 of bonding rubber by mechanical or chemical action firmly and tightly to the body 2 in order to prevent possible slipping of the tread ring 1 when the layer 6 of bonding rubber becomes liquid directly before vulcanising. - 9 43881 If the inside diameter of the tread ring 1 is chosen smaller than the greatest outside diameter of the domed body 2, as already stated above there follows automatic escape of gaseous media from the gaps because the tread ring 1 makes contact with the body 2 from the centre progress5 ively to the sides. The escape is promoted by a network of passages present in the preformed rough surface.

For slipping the tread ring 1 over, the body 2 is advantageously laid on a centering table having a funnel-shaped receiving or table face, so that bodies of different sizes may be covered on the same table.

The tread ring 1 may slide down to the oblique receiving-face and then always adopts a centred position on the body 2.

Claims (10)

1. A method of renewal of the tread of a worn pneumati c or solid rubber tyre, in which a pre-vulcanized rubber tread ring is applied to a prepared body of the tyre with a layer of bonding rubber applied to the tread ring or prepared body, and, with any gaseous inclusions removed from between the applied layer, tread ring and body, the assembly is heated and pressurized in an autoclave for vulcanising the layer of bonding rubber, characterised in that a stretch is imparted to the tread ring by mechanical means, the stretch running in the circumferential direction, and, after removal of the tread ring from the mechanical means but maintaining temporarily a part of its stretch such that the inner diameter of the tread ring is greater than the outer diameter of the body, the tread ring is applied to the body,

2. A method according to claim 1, in which the temporary free stretch resulting from the stretching of the tread ring amounts directly after removal from the mechanical means to 3-45S.

3. A method according to claim 1 or claim 2, in which the diameter of the tread ring before the stretching process is smaller than the diameter of the body,

4. A method according to claim 3, in which diameter of the tread ring is smaller than the diameter of the body by at least 1%.

5. A method according to any one of claims 1 to 4, in which the body coated with the layer of bonding rubber is provided before application of the tread ring with a slip-foil of plastics material,which after pulling on of the tread ring is pulled out 5 again from the applied layers.

6. A method according to claim 5, in which the slip-foil is spread over the body, which is horizontally arranged, so that the edge regions of the slip-foil cover the periphery of the body at least to its centre, and has a central cut-out for centering. 10

7. A method according to any one of claims 1 to 6, in which the tread ring is slipped over the body lying on a centering table and guided down until the bottom edge of the tread ring rests all round on an oblique receiving and centering surface.

8. A method according to any one of the preceding claims, 15 wherein the stretch is imparted to the tread ring by laying the tread ring over rollers which are moved apart.

9. A method according to claim 1, substantially as described with reference to the accompanying drawings.

10. , A tyre which has been retreaded by a method according to 20 any one of the preceding claims.