Pneumatic radial tires

Abstract: A pneumatic tire having improved high-speed running performance is manufactured by conducting vulcanization of a green tire (10) in a mold (12) provided at a position corresponding to a given position between the shoulder portion (33) and the bead portion (16) with at least one protrusion (34) which protrudes inside the mold and continuously extends in the circumferential direction of the mold and consists of a first arc segment (34a) having a center of curvature located outside from the inner surface of the mold and second arc segments (34b) each having a center of curvature located inside from the inner surface of the mold and smoothly connecting to the first arc segment (34a) at upper and lower ends in the radial direction of the mold.

Description: This invention relates to pneumatic radial tires, and more particularly to a method of producing pneumatic radial tires which can advantageously improve high-speed running performances of the tire, and to a bladder used therefor. The pneumatic radial tire comprises a carcass comprised of at least one cord ply of radial arrangement and a belt superimposed about a crown portion of the carcass and comprised of a plurality of cord layers and has many advantages, for example that the motion of tread rubber constituting the tread portion of the tire is made small by a so-called "hoop" effect of the belt as compared with that of bias tires, and the durability, wear resistance and steering stability are excellent and the rolling resistance is low. On the other hand, reinforcement is added to the tread portion and bead portion of the pneumatic radial tire so as to cope with the high-speed running accompanied with recent improvement of vehicle performance, but it is strongly demanded to further improve the high-speed running performances of the tire in accordance with the improvement of vehicle performance. Therefore, there have been proposed various techniques such as change of end count and cord angle in the cords constituting the carcass or the belt, and addition of new reinforcing members and the like for controlling strain or heat generation at portions easily liable to damage or enhancing the rigidity of the tire casing. In these proposed techniques, the rigidity of the tire casing as a whole is increased to create portions having undesirably increased rigidity, which is unfavourable in the running performances of the tire and the ride comfortability, and also there are caused disadvantages such as increased cost, increase of weight and the like accompanied with the increase of the reinforcing members. Attention is also drawn to the disclosure of US-A-3961657. It is an aim of the invention to provide a method of producing pneumatic radial tires which can locally enhance the rigidity of only given portions of the tire casing to effectively improve the running performances of the tire without causing increase of cost and increase of weight in the tire, as well as a bladder used therefor. According to a first aspect of the invention, there is provided a method of producing pneumatic radial tires by vulcanizing in a mold a green tire comprising a carcass comprised of at least one cord ply of radial arrangement toroidally extending between a pair of bead cores and wound at its side portions around the bead core from the inside toward the outside thereof and a belt superimposed around a crown portion of the carcass and comprised of a plurality of cord layers, wherein the vulcanization of the green tire is carried out in a mold provided at its inner surface portion corresponding to a given position between the shoulder portion and the bead portion of a tire to be manufactured with at least one protrusion which protrudes inside the mold and continuously extends in the circumferential direction of the mold and comprises a first arc segment having a center of curvature located outside from the inner surface of the mold and second arc segments each having a center of curvature located inside from the inner surface of the mold and smoothly connecting to the first arc segment at upper and lower ends in the radial direction of the mold, whereby a portion of the carcass is deviated inward from a natural equilibrium profile at the position of the protrusion. In a preferred embodiment of the first aspect, a bladder having a reduced expansion ratio at a position corresponding to the protrusion of the mold is internally contacted with the green tire. According to a second aspect of the invention, there is provided a bladder used for the vulcanization of a green tire together with a mold provided at its inner surface portion corresponding to a given position between the shoulder portion and the bead portion of a tire to be manufactured with at least one protrusion which protrudes inside the mold and continuously extends in the circumferential direction of the mold and comprises a first arc segment having a center of curvature located outside from the inner surface of the mold and second arc segments each having a center of curvature located inside from the inner surface of the mold and smoothly connecting to the first arc segment at upper and lower ends in the radial direction of the mold, wherein an expansion ratio of the bladder at a position corresponding to the protrusion of the mold is smaller than that of the remaining portion except for a bladder bead portion. In a preferred embodiment of the second aspect, the expansion ratio of the bladder at the position corresponding to the protrusion of the mold is reduced by a projection protruding inward from the bladder at this position, or by a thickened portion of the bladder consisting of a projection protruding inward and an outer recess substantially similar to the protrusion of the mold in the bladder at the above position, or by a reinforcing member in the bladder at the position. The invention will be further described, by way of example only, with reference to the accompanying drawings, wherein: Fig. 1 is a partly sectional view of a first embodiment illustrating vulcanization of tire according to the invention; Fig. 2 is a partly sectional view of a second embodiment illustrating vulcanization of tire according to the invention; and Figs. 3a to 3c are schematically sectional views of various embodiments of the bladder according to the invention. In the pneumatic radial tire manufactured according to the method of the invention, a tire portion including an inwardly deviated carcass portion largely expands outward in the radial direction of the tire and hence the carcass bears a large tension over a region ranging from the inwardly deviated carcass portion toward portions adjacent thereto in the radial direction of the tire, e.g. a region ranging from the maximum width portion of the tire toward the shoulder portion under the selection of position in connection with the carcass deviated Portion, whereby the tire rigidity in the vicinity of the maximum width portion of the tire and the tire rigidity in the vicinity of the belt end portion are advantageously increased and also the movement of tread rubber at the belt end portion is effectively controlled. According to the invention, therefore, the high-speed running performances of the tire can largely be improved without special addition and change of known tire constituting parts themselves. The reason why the inwardly deviated carcass portion is formed between the shoulder portion and the bead portion is due to the fact that when the deviated portion is located in the tread portion over the shoulder portion, strain of tread rubber becomes larger due to the expansion of the deviated carcass portion outward from the tire and separation failure of the belt is apt to be caused, while when the deviated carcass portion is located near to the bead core in the bead portion, the deviated portion closely approaches a rim flange or is fixed by contacting therewith, so that even if the tire is inflated under an internal pressure, the tire can not be expanded outward to a desirable extent and hence the function inherent to the deviated carcass portion can not be developed. Moreover, considering the actual effect obtained by the deviated carcass portion, it is preferable that the deviated carcass portion is located between the shoulder portion and the tire maximum width position. According to the production method of the invention, the expansion ratio in such a portion of the bladder that corresponds to the position of the protrusion formed in the mold is made smaller than that of the other portion under the inflation of internal pressure, whereby the force of pushing the green tire to the mold is somewhat weakened at the portion having the reduced expansion ratio to prevent the excessive flowing of outer coating rubber in the tire at a position corresponding to the protrusion of the mold and consequently a danger of air leakage due to outer exposure of carcass cords can sufficiently be removed. In other words, when the tire portion corresponding to the protrusion of the mold is pushed by a bladder portion having an expansion ratio equal to that of the other portion, the pushing force of this tire portion toward the protrusion of the mold tends to be locally increased, so that a greater part of the outer coating rubber flows toward the portions adjacent to the protrusion of the mold and finally the carcass cords are exposed and hence air leakage from the exposed carcass portion is caused at a relatively premature stage. In this case, the reduction of the expansion ratio in the portion of the bladder corresponding to the protrusion of the mold can be easily and reliably attained by arranging a projection protruding inward from the bladder at this position, or by forming a thickened portion consisting of a projection protruding inward and an outer recess substantially similar to the protrusion of the mold in the bladder at the above position, or by embedding a reinforcing member in the bladder at the position. In Fig. 1 is sectionally shown a mold in its widthwise direction when a green tire 10 is vulcanized in a mold 12 according to the method of the invention. Moreover, the tire to be manufactured has substantially a symmetric structure with respect to an equatorial plane S, so that only one half of the tire is shown for convenience. In the green tire 10, a sidewall portion 20 and a tread portion 22 are reinforced with a carcass 18 comprised of at least one carcass cord ply of radial arrangement toroidally extending between a pair of bead portions 16 separated from each other in the rotating axial direction of the tire. In the carcass 18, at least one carcass cord ply is wound around a bead core 24 in each of the bead portions 16 from the inside toward the outside to form a turnup portion. In the green tire 10 of this embodiment, the carcass 18 is comprised of one carcass cord ply. Moreover, the ply number may be increased in accordance with the specification of the tire to be manufactured. In the latter case, at least one carcass cord ply is wound around the bead core 18 in the same manner as described above. A bead filler 26 and a reinforcing layer 28 are disposed between the carcass 18 and its turnup portion to reinforce a region ranging from the bead portion 16 to the sidewall portion. In this case, the number and arranging position of the reinforcing layer or layers are not limited to the illustrated embodiment and may be changed, if necessary. Further, a belt 30 comprised of a plurality of cord layers is superimposed about an outer periphery of a crown portion of the carcass 18 in the tread portion 22 constituting a ground contacting surface of the tire. Each side end portion of the belt 30 in the widthwise direction is reinforced with an auxiliary layer 32 in the circumferential direction as in the known high-speed running radial tires. On the other hand, the mold 12 is provided at its inner surface portion corresponding to a given position between the shoulder portion 33 and the bead portion 16 of the tire to be manufactured with a protrusion 34 continuously extending in the circumferential direction of the mold and protruding inward from the inner surface of the mold. As shown in Fig. 1, the profile of the protrusion 34 is a convex shape inward of the mold 12 in a section of the mold 12 in its widthwise direction, and is comprised by a first arc segment 34a having a center of curvature located outside from the inner surface of the mold 12 and two second arc segments 34b each having a center of curvature located inside from the inner surface of the mold 12 and smoothly connecting to the first arc segment 34a at upper and lower ends in the radial direction of the mold. Moreover, the shape of the protrusion 34 is so selected that when the tire vulcanized with the use of the mold 12 is mounted onto a normal rim and inflated under a given internal pressure, an outer profile length P of a recess portion formed in the side surface of the tire by the protrusion 34 is 4%-25%, preferably 10%-20%, of an outer tire profile length L. The term "outer tire profile length L" used herein means a curved length measured from the equator S of the tire to a bead base along the outer profile of the tire in the radial section of the tire. Moreover, when grooves are formed in the tread portion, the above tire profile passes a line segment connecting to surfaces of adjoining island portions separated by the groove. The term "outer profile length P of recess portion" used herein means a curved length measured along the outer profile of the tire from a contact point to the other contact point of a straight line circumscribedly drawn between portions adjacent to both sides of the recess portion in the radial direction of the tire, i.e. between peaks of the second two arc segments 34b sandwiching the first arc segment 34a. In the vulcanization of the green tire 10 with the mold 12, the green tire is first placed in the mold provided at its inner surface with the protrusion 34 and then the mold is closed, while a bladder 14 is expanded by supplying a pressurized fluid into the inside of the bladder 14 disposed inside the green tire, whereby the green tire is forced toward the inner surface of the mold, during which the heating is carried out. Thus, there is obtained a vulcanized radial tire in which a portion of the green tire corresponding to the protrusion 34 and hence a portion of the carcass corresponding to the protrusion 34 is deviated inward in the tire. Moreover, numeral 36 designates a protrusion for forming a groove in the tread portion of the tire to be manufactured, and broken lines show an outer profile of a tire and a carcass line thereof when the green tire is vulcanized by using a conventionally known mold. When the thus vulcanized tire is inflated under a given internal pressure, the carcass portion deviated inward in the tire is expanded toward the outside of the tire to increase tension bearing of the carcass portions sandwiching the deviated carcass portion, i.e. carcass portions located at the tire shoulder portion and the tire maximum width portion as previously mentioned, so that the rigidity of these carcass portions and hence the rigidities of the belt end portion and the sidewall portion in the tire can sufficiently be enhanced, and consequently the running performances of the tire inclusive of the steering stability can effectively be improved without changing the known tire constituting parts. Moreover, when the protrusion 34 formed in the inner surface of the mold 12 is located near to the bead portion of the tire to be manufactured as shown in Fig. 2, the tension bearing of the carcass portion near to the bead portion 16 can be increased to effectively enhance the rigidity of the sidewall portion. As previously mentioned, the reason why the protrusion 34 is formed in the mold at a position between the shoulder portion and the bead portion in the tire to be manufactured is due to the fact that when the deviated portion of the carcass 18 is located in the tread portion over the shoulder portion, the tension bearing of the deviated portion and the carcass portions adjacent thereto is increased as compared with those of the other carcass portions to increase strain of tread rubber at the belt end portion and hence separation failure is apt to be caused to lower the belt durability, while when the protrusion 34 is located close to the bead core, the function inherent to the deviated carcass portion can not be developed as mentioned above. In the preferred embodiment of the invention, when the vulcanized tire is mounted onto a normal rim and inflated under a given internal pressure, the outer profile length P of the recess portion formed in the side surface of the tire is selected to satisfy a range of 4%-25% of the outer tire profile length L. That is, when the outer profile length P is less than 4% of the outer tire profile length L, the ratio of P to L is too small and hence it is difficult to expect the increase of tension bearing of the carcass portion corresponding to a given position between the shoulder portion and the bead portion in the tire when being inflated under the internal pressure, while when the ratio exceeds 25%, the tire tends to expand outward as a whole, so that tension acting to the carcass is dispersed over the whole of the tire and it is difficult to expect the local increase of tension bearing in the given carcass portion. As seen from the above, the recess portion formed in the side surface of the tire according to the invention is essentially different from a so-called decoration slightly recessed in the outer surface of the sidewall portion near to the tread end side. A tire having a tire size of 255/40ZR17 was manufactured according to the method of the invention and its high-speed running performances were compared with those of a comparative tire mentioned later. In this test tire, the carcass was comprised of two carcass cord plies of radial arrangement containing nylon-6,6 cords of 1260d/2 arranged at an angle of approximately 80° with respect to the equatorial plane of the tire, the cords of which plies being crossed with each other. One of the two plies located inside the tire was wound around a bead core from the inside toward the outside, while the other ply located outside the tire was wound around the bead core from the outside toward the inside. A reinforcing layer 28 was arranged between the turn-up portion of the inner carcass ply and a bead filler 26 disposed about the bead core 24 and between the above turn-up portion and the outer carcass ply, respectively. Each of these reinforcing layers contained aromatic polyamide fiber cords of 1500d/2 arranged at a cord angle of approximately 30° with respect to a line segment in the circumferential direction of the tire and had a maximum height of 43 mm as measured from a bead base. A belt 30 was comprised of two cord layers containing aromatic polyamide fiber cords of 1500d/2 arranged at a cord angle of approximately 35° with respect to the equatorial plane of the tire, the cords of which layers being crossed with each other. An auxiliary layer 32 was constructed by spirally winding nylon-6,6 cord of 1260d/2 in the circumferential direction of the tire so as to cover each side end portion of the belt 30 over a width corresponding to 30% of the belt width. Invention tire I This tire is a tire shown in Fig. 1, which is manufactured by using a mold provided with a protrusion 34 continuously extending in the circumferential direction of the tire at a position corresponding to a side surface of the tire to be manufactured. A recess portion formed by the protrusion 34 is located at such a position that a height of the bottom of the recess portion measured from the bead base is approximately 77% of a tire section height (=102 mm) when the vulcanized tire is mounted onto a normal rim (9x17) and inflated under a given internal pressure (2.5 kg/cm²), and has such a size that the outer profile length P is approximately 10.3% of the outer tire profile length L (=210 mm). Although the carcass 18 is comprised of single cord ply of radial arrangement in Fig. 1, the actual test tire comprised a carcass 18 of two carcass cord plies as mentioned above. Invention tire II This tire is a tire shown in Fig. 2, which has the same structure as in the invention tire I except that the bottom of the recess portion formed in the side surface of the tire by the protrusion 34 is located at a height corresponding to approximately 68.6% of the tire section height (=102 mm) as measured from the bead base and the outer profile length P of the recess portion is approximately 9.5% of the outer tire profile length L (=210 mm). Comparative tire This tire has the same structure as in the above invention tire except that a portion corresponding to the recess portion of the invention tire is not formed in the side surface of the vulcanized tire. Test method A vehicle provided with the test tires was run on a circuit course of 6 km, during which lap time was measured and straight running performance and cornering performance were evaluated by feeling test. The results are shown in Table 1 by index value based on the comparative tire as a control. Moreover, the larger the index value, the better the result. Invention tire I Invention tire II Comparative tire Lap time 105 103 100 Straight running performance 102 105 100 Cornering performance 107 105 100 As seen from Table 1, when the tires manufactured by the method of the invention are compared with the comparative tire, the lap time, straight running performance and cornering performance during the high-speed running are effectively improved to obtain excellent high-speed running performances. Thus, according to the method of the invention, pneumatic tires having advantageously improved high-speed running performances can be provided by sufficiently enhancing the rigidity only at given positions of the tire without enhancing the rigidity of the tire as a whole. Although the method of the invention has been described with reference to Figs. 1 and 2, when the expansion ratio of the bladder 14 internally touching with the green tire 10 and pushing it toward the inner surface of the mold 12 at a position corresponding to the protrusion 34 of the mold in the inflation of internal pressure is made smaller than that of the other portions of the bladder except for the bladder bead portion in the vulcanization of the green tire according to the invention, the force of pushing the tire 10 can properly be lowered at the portion having a reduced expansion ratio, whereby the excessive flowing of outer coating rubber due to large force of pushing the green tire 10 toward the protrusion 34 of the mold can effectively be prevented to substantially completely prevent the exposure of the carcass 18 to the outside of the tire. Figs. 3a to 3c are schematically sectional views of various embodiments of the bladder capable of use together with the mold 12 having the protrusion 34 in the vulcanization of the green tire. The bladder 14 shown in Fig. 3a is provided at a position corresponding to the protrusion 34 of the mold with a projection 38 protruding inward from the bladder. The expansion ratio in the inflation of internal pressure is reduced by the projection 38. In the bladder 14 shown in Fig. 3b, the expansion ratio in the inflation of internal pressure is reduced by a thickened portion 40 having a thickness thicker than the other portions and consisting of a projection 40a protruding inward and an outer recess 40b substantially similar to the protrusion 34 of the mold. In the bladder 14 shown in Fig. 3c, the expansion ratio is reduced by embedding a reinforcing member 42 such as fiber cords or the like in the bladder at the position corresponding to the protrusion 34 of the mold. In order to compare the performances of these bladders, tires having the same structure as in the invention tire I except that the height of the bottom of the recess portion in the tire to be manufactured as measured from the bead base was approximately 68.6% of the tire section height (=102 mm) were manufactured by using the above bladders, and then the thickness of the outer coating rubber at the bottom of the recess portion in the tire (thickness of rubber located outside the carcass) was measured to obtain results as shown in Table 2. Moreover, the target thickness was 2.0 mm. Bladder of Fig. 3a Bladder of Fig. 3b Bladder of Fig. 3c Conventional bladder having uniform expanding ratio over a whole Thickness of outer coating rubber 1.5 (mm) 2.0 (mm) 1.8 (mm) 0.2 (mm) Appearance OK (Thickness is somewhat thin, but there is caused no problem) OK (no problem) OK (no problem) Carcass cords were visually observed. As seen from Table 2, all of the bladders according to the invention can give sufficient thickness in the outer coating rubber as compared with the conventional bladder, so that the leakage of internal pressure can completely be prevented. As mentioned above, according to the method of the invention, only the rigidity of a given portion in the tire is locally enhanced without unnecessarily enhancing the rigidity of the tire casing as a whole, whereby the high-speed running performances of the tire can largely be improved without causing increase of cost and increase of weight in the tire. Furthermore, when the vulcanization of the tire is carried out by using the bladder according to the invention, excessive flowing of the outer coating rubber can sufficiently be prevented to effectively remove a danger of causing the leakage of internal pressure.

Claims: A method of producing a pneumatic radial tire by vulcanizing in a mold (12) a green tire (10) comprising a carcass (18) comprised of at least one cord ply of radial arrangement toroidally extending between a pair of bead cores (24) and wound at its side portions around the bead core from the inside toward the outside thereof and a belt (30) superimposed around a crown portion of the carcass and comprised of a plurality of cord layers, wherein the vulcanization of the green tire is carried out in a mold (12) provided at its inner surface portion corresponding to a given position between the shoulder portion (33) and the bead portion (16) of a tire to be manufactured with at least one protrusion (34) which protrudes inside the mold and continuously extends in the circumferential direction of the mold, characterized in that the said protrusion (34) comprises a first arc segment (34a) having a center of curvature located outside from the inner surface of the mold and second arc segments (34b) each having a center of curvature located inside from the inner surface of the mold and smoothly connecting to the first arc segment (34a) at upper and lower ends in the radial direction of the mold, whereby a portion of the carcass (18) is deviated inward from a natural equilibrium profile at the position of the protrusion (34). A method as claimed in claim 1, characterized in that a bladder (14) having a reduced expansion ratio at a position corresponding to the protrusion (34) of the mold (12) is internally contacted with the green tire. A bladder (14) used for the vulcanization of a green tire (10) together with a mold (12) provided at its inner surface portion corresponding to a given position between the shoulder portion (33) and the bead portion (16) of a tire to be manufactured with at least one protrusion (34) which protrudes inside the mold and continuously extends in the circumferential direction of the mold and comprises a first arc segment (34a) having a center of curvature located outside from the inner surface of the mold and second arc segments (34b) each having a center of curvature located inside from the inner surface of the mold and smoothly connecting to the first arc segment (34a) at upper and lower ends in the radial direction of the mold, wherein an expansion ratio of the bladder (14) at a position corresponding to the protrusion (34) of the mold (12) is smaller than that of the remaining portion except for a bladder bead portion. A bladder as claimed in claim 3, characterized in that said expansion ratio of the bladder (14) at the position corresponding to the protrusion (34) of the mold (12) is reduced by a projection (38) protruding inward from the bladder at said position. A bladder as claimed in claim 3, characterized in that said expansion ratio of the bladder (14) at the position corresponding to the protrusion (34) of the mold (12) is reduced by a thickened portion (40) of the bladder consisting of a projection (40a) protruding inward and an outer recess (40b) substantially similar to the protrusion (34) of the mold in the bladder at said position. A bladder as claimed in claim 3, characterized in that said expansion ratio of the bladder (14) at the position corresponding to the protrusion (34) of the mold (12) is reduced by a reinforcing member (42) embedded in the bladder at said position.

Inventors: USHIKUBO HISAO, YAMAZAKI TADASHI, USHIKUBO, HISAO, YAMAZAKI, TADASHI
Applicants: BRIDGESTONE CORP, BRIDGESTONE CORPORATION
Application year: 1992
Main classification: B29D30
IPC codes: B29C35, B29K21, B29C33, B60C9, B29L30, B29D30, B29K105