Patent Publication Number: US-2009229723-A1

Title: Pneumatic tire, tire molding apparatus and method of molding

Description:
TECHNICAL FIELD 
     The present invention relates to pneumatic tires and apparatuses and methods for building tires. In particular, the present invention relates to pneumatic tires formed by laminating rubber ribbons and exhibiting suppressed cracking and improved durability, and relates to apparatuses and methods for building the tires. 
     BACKGROUND ART 
     A typical pneumatic tire includes a plurality of rubber components such as a tread and sidewalls. Unvulcanized tires (green tires) are built by combining these components, and tire products having predetermined shapes are produced by vulcanizing the green tires. To date, in well-known methods for building green tires, these components are formed so as to have predetermined finished shapes in advance, and are bonded in sequence on building drums. 
     In this method, each component is shaped with, for example, an extruder by being continuously extruded from a nozzle cap attached to an rubber outlet of the extruder so as to have a predetermined finished cross-sectional shape, and by cutting the extruded rubber. Since the cross sectional size of the component is large, the nozzle and the extruder required for shaping the component are increased in size in this method. In addition, various types of nozzles depending on the cross-sectional shapes of different components must be prepared in advance. Moreover, the nozzle of the extruder must be changed and adjusted each time different components are extruded. This causes a considerable reduction in production efficiency during, in particular, small batch production of many different parts. 
     To cope with these problems, a tire building apparatus is well known for building a green tire by helically winding and laminating an unvulcanized rubber ribbon on the outer surface of a cylindrical or toroidal building drum (see Patent Document 1). 
       FIG. 7  is a side view schematically illustrating a known tire building apparatus  80 . 
     As shown in  FIG. 7 , the tire building apparatus  80  includes a cylindrical building drum  81  and an extruder  82  that extrudes a rubber ribbon. The building drum  81  is rotated by driving means (not shown) about a rotating shaft  81 A in the direction of an arrow S. The extruder  82  includes a plurality of (three in  FIG. 3 ) rubber supplying units  83 , a hopper  84  for charging rubber into the extruder  82 , a moving mechanism  85  that moves the extruder  82  in directions parallel to the shaft of the building drum  81 , and a nozzle  86  for extruding a rubber ribbon having a predetermined cross-sectional shape. 
     The tire building apparatus  80  continuously extrudes the rubber ribbon having a predetermined cross-sectional shape from the nozzle  86  (in the direction of an arrow T), and bonds the rubber ribbon at a predetermined position on the outer periphery of the rotating building drum  81  while guiding the rubber ribbon using a guide roller  87 . The extruder  82  is moved parallel to the shaft of the building drum  81  by the moving mechanism  85  during bonding of the rubber ribbon such that the rubber ribbon is helically wound and laminated on the outer periphery of the building drum  81  such that a component having a predetermined cross-sectional shape is formed. In this manner, a green tire is formed. 
       FIGS. 8A to 8D  are cross-sectional views of a rubber ribbon used for such building and laminated states of the rubber ribbon viewed in a direction orthogonal to the longitudinal direction of the rubber ribbon. 
     The tire building apparatus  80  forms each component of a green tire having a predetermined cross-sectional shape by laminating a rubber ribbon  50  having a rectangular cross section as shown in  FIG. 8A  such that one or more layers are formed while partly overlapping the rubber ribbon, as shown in  FIG. 8B . Therefore, only several types of nozzles  86  for shaping rubber ribbons having such shapes need to be prepared for this tire building apparatus  80 , and the number of nozzles  86  can be considerably reduced compared with the case where components of a tire are integrally bonded. Moreover, the extruder  82  does not need to have a large size, and the time required for preparation such as adjustment of the nozzle  86  can be considerably reduced, facilitating small-batch production of many different parts. 
     However, when a green tire is built by laminating a rubber ribbon  50  in this manner, the bonding between the layers of the rubber ribbon  50  and the like may be insufficient, and defects such as cracking may occur in the produced tire. Moreover, steps  51  are formed at the edge of the overlapped rubber ribbon  50  as shown in  FIG. 8B , and can cause lightness and cracks in the produced tire due to insufficient shaping at the steps  51  during vulcanization. This can lead to, for example, a reduction in the durability of the tire product and poor outward appearance of the tire. 
     Moreover, when the rubber ribbon  50  is laminated so as to for a plurality of layers, air can be trapped in the steps  51  adjacent to inner layers, and can remain in the tire product. In addition, air tapped between the steps  51  on the outer surface of the tire and the inner periphery of a vulcanizing mold may also be taken in the tire due to flows of rubber during vulcanization. Air trapped in the tire product in this manner leads to ready cracking at the trapped portions due to stress concentration while a vehicle is driven compared with other portions, and the durability of the pneumatic tire can be disadvantageously decreased. Furthermore, extraneous substances such as mold release agent tend to remain at the steps  51  on sidewalls at the sides of the tire. When the green tire is expanded by pressure during vulcanization, the rubber at the steps  51  is deformed and the extraneous substances may be trapped in the rubber. With this, portions into which the extraneous substances bite (adhesion failure) may appear on the surface of the tire product, resulting in cracking. 
     To cope with these problems, a known tire building apparatus squashes projections at such steps  51  formed by laminating a rubber ribbon  50  using a roller (see Patent Document 2). 
       FIG. 9  is a side view schematically illustrating this known tire building apparatus  90 . 
     As does the tire building apparatus  80  shown in  FIG. 7 , the tire building apparatus  90  includes a cylindrical building drum  91 , a supplying device  92  such as an extruder for supplying a rubber ribbon  50 , and two guide rollers  93  and  94  that guide the rubber ribbon  50 . The rubber ribbon  50  supplied from the supplying device  92  is guided by the guide rollers  93  and  94 , and helically wound around the outer periphery of the rotating building drum  91  while being partly overlapped. The tire building apparatus  90  further includes a disc-shaped rotatable roller  95  and a driving mechanism  96  that presses the roller  95  toward the outer periphery of the building drum  91 . The surface of the rubber ribbon  50  is pressed by the outer periphery of the roller  95  while the rubber ribbon  50  is wound around the outer periphery of the building drum  91 . 
     After the rubber ribbon  50  is pressed by such a roller  95 , a brush, or the like, strong adhesion is achieved by pressure between the layers of the rubber ribbon  50  and between the rubber ribbon  50  and other components wound below the rubber ribbon  50 , and protrusions at the steps  51  on the surface of the rubber ribbon  50  are squashed into a smooth surface of the rubber ribbon  50  as shown in  FIG. 8C . The tire building apparatus  90  can therefore control the above-described lightness during vulcanization caused by the adhesion failure between the layers of the rubber ribbon  50  and the steps  51 , or can reduce the cracks on the tire product caused by remaining air or inclusions of extraneous substances. With this, the bonding force between rubber components such as the rubber ribbon  50  can be enhanced, and the durability of the tire product can be improved. 
     However, even when these components are bonded to each other by pressure while the surface of the tire is smoothed, minute adhesion failure or minute projections and recessions may remain on the surface of the tire product, and can cause a crack  52  to occur thereat and to propagate from the surface of the tire product as shown in  FIG. 8D  due to repeated loads, deformations, or the like applied during driving. In particular, the sidewalls that protect the side surfaces of the tire are constantly flexed during driving, and repeatedly undergo deformations such as expansion and contraction. Furthermore, the flexure is the most significant thereat compared with those at other portions. The deformation, the tensile force, and the compressive force during flexure also become the largest accordingly, resulting in the highest possibility that cracks propagate from the surface of the tire. 
     Patent Document 1: Japanese Unexamined Patent Application Publication No. 2000-79643 
     Patent Document 2: Japanese Unexamined Patent Application Publication No. 2004-216603 
     DISCLOSURE OF INVENTION 
     Problems to be Solved by the Invention 
     The present invention is accomplished to solve the above-described problems. It is an object of the present invention to provide a pneumatic tire having a portion formed by laminating a rubber ribbon, and exhibiting improved durability due to reduced propagation of cracks from the surface of the laminated portion of the rubber ribbon at a sidewall, for example. 
     Means for Solving the Problems 
     According to the invention as claimed in claim  1 , a pneumatic tire including a laminated portion formed by laminating a rubber ribbon, wherein a rubber sheet for smoothing the outer surface of the tire is disposed on at least a part of the outer surface of the laminated portion. 
     According to the invention as claimed in claim  2 , the pneumatic tire according to claim  1  is characterized in that the laminated portion is a sidewall. 
     According to the invention as claimed in claim  3 , the pneumatic tire according to claim  1  or  2  is characterized in that the thickness of the rubber sheet is from 0.05 to 2 mm. 
     According to the invention as claimed in claim  4 , in a tire building apparatus for building a green tire including a rotatable building drum having a toroidal or cylindrical outer surface; a supplying means for supplying an unvulcanized rubber ribbon to the building drum; and laminating means for winding and laminating the rubber ribbon on the outer surface of the building drum, the laminating means forming a laminated portion having a predetermined cross-sectional shape by laminating the rubber ribbon; the apparatus further includes a supplying means for supplying an unvulcanized rubber sheet to the laminated portion of the rubber ribbon; and bonding means for bonding the rubber sheet on at least a part of the outer surface of the laminated portion of the rubber ribbon, whereby the rubber sheet smoothes the outer surface of the laminated portion. 
     According to the invention as claimed in claim  5 , the tire building apparatus according to claim  4  is characterized in that the rubber sheet supplied by the supplying means for supplying an unvulcanized rubber sheet is at room temperature or heated. 
     According to the invention as claimed in claim  6 , the tire building apparatus according to claim  4  or  5  is characterized in that the supplying means for supplying an unvulcanized rubber sheet includes an extruder that extrudes the rubber sheet, the bonding means bonding the rubber sheet extruded from the extruder on the outer surface of the laminated portion of the rubber ribbon. 
     According to the invention as claimed in claim  7 , the tire building apparatus according to any one of claims  4  to  6  is characterized in that the laminated portion of the rubber ribbon is a sidewall. 
     According to the invention as claimed in claim  8 , a method for building a tire includes forming a laminated portion having a predetermined cross-sectional shape by winding and laminating an unvulcanized rubber ribbon on the outer surface of a building drum having a toroidal or cylindrical outer surface; and bonding an unvulcanized rubber sheet on at least a part of the outer surface of the laminated portion, whereby the rubber sheet smoothes the outer surface of the laminated portion. 
     According to the invention as claimed in claim  9 , the method for building a tire according to claim  8  is characterized in that the rubber sheet bonded during the bonding step is at room temperature or heated. 
     According to the invention as claimed in claim  10 , the method for building a tire according to claim  8  or  9  is characterized in that the laminated portion is a sidewall. 
     ADVANTAGES 
     According to the present invention, a surface layer is formed by bonding a rubber sheet on the outer surface, for example, a sidewall, of a green tire formed by laminating a rubber ribbon so as to smooth the surface of the tire. Thus, starting points of cracks can be removed, and the durability of the pneumatic tire can be improved due to reduced propagation of cracks from the surface of the tire. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a side view schematically illustrating a part of a tire building apparatus according to an embodiment. 
         FIGS. 2A and 2B  are side views schematically illustrating exemplary rubber shaping devices for shaping rubber sheets.  FIG. 2A  illustrates a calender, and  FIG. 2B  illustrates an extruder. 
         FIGS. 3A to 3C  are perspective views schematically illustrating a process of bonding the rubber sheets to both sidewalls of a green tire. 
         FIG. 4  is a side elevation viewed in a radial direction of the tire schematically illustrating a rubber sheet being bonded to a sidewall of a green tire using an extruder in a “hot” method. 
         FIGS. 5A to 5C  are cross-sectional views enlarged in the width direction of a tire schematically illustrating the structure of a sidewall of a rolling pneumatic tire according to an embodiment. 
         FIG. 6  illustrates the results of driving tests. 
         FIG. 7  is a side view schematically illustrating a known tire building apparatus. 
         FIGS. 8A to 8D  are cross-sectional views of a rubber ribbon used in a known method for building a tire and laminated states of the rubber ribbon viewed in a direction orthogonal to the longitudinal direction of the rubber ribbon. 
         FIG. 9  is a side view schematically illustrating another known tire building apparatus. 
     
    
    
     REFERENCE NUMERALS 
       1 : tire building apparatus,  2 : building drum,  3 : extruder,  4 : roller,  5 : roller,  6 : roller,  10 : calender,  11 : roll,  12 : roll,  20 : extruder,  21 : body of the extruder,  22 : rubber outlet,  23 : cylinder,  24 : hopper,  25 : roller head,  26 : roller,  27 : roller,  30 : green tire,  31 : sidewalls,  40 : laminated portion,  41 : surface layers,  50 : rubber ribbon,  55 : rubber sheets,  56 : rubber compound, and  57 : connecting portions. 
     BEST MODE FOR CARRYING OUT THE INVENTION 
     An embodiment of the present invention will now be described with reference to the drawings. 
       FIG. 1  is an enlarged side view schematically illustrating a part, associated with lamination of a rubber ribbon, of a tire building apparatus  1  according to this embodiment. 
     This tire building apparatus  1  builds a green tire by laminating an unvulcanized rubber ribbon  50  while partly overlapping the ribbon such that sidewalls and the like having predetermined cross-sectional shapes are formed in a manner similar to that performed in the known tire building apparatuses  80  and  90  shown in  FIGS. 7 and 9 . As shown in  FIG. 1 , the tire building apparatus  1  includes a building drum  2 , an extruder  3 , and a set of rollers  4  and  5  disposed between the building drum  2  and the extruder  3 . 
     The building drum  2  has a cylindrical or toroidal shape corresponding to the shape of the green tire to be built, and is rotated about an axis by driving means (not shown). The extruder  3  supplies the unvulcanized rubber ribbon  50 , and includes a nozzle having a predetermined cross-sectional shape. Rubber is continuously extruded via the nozzle so as to form the rubber ribbon  50  (see  FIG. 8A ). The set of rollers  4  and  5  faces the building drum  2 , and the roller  4  (upper roller in  FIG. 1 ) has a diameter larger than that of the roller  5  (lower roller in  FIG. 1 ). These rollers  4  and  5  control the position and the angle of the rubber ribbon  50  while conveying the rubber ribbon  50  to the building drum  2 , and the roller  4  having a larger diameter bonds the rubber ribbon  50  to the outer surface of the building drum  2 . 
     The tire building apparatus  1  having the above-described structure helically winds and laminates the rubber ribbon  50  supplied from the extruder  3  on the outer surface of the rotating building drum  2  while controlling the position and the angle of the rubber ribbon  50  using the rollers  4  and  5  such that the rubber ribbon  50  forms a predetermined cross-sectional shape. In the case of a toroidal building drum  2 , a green tire having a predetermined shape is built by forming, for example, sidewalls and a tread as described above. In the case of a cylindrical building drum  2 , a green tire having a predetermined shape is built by forming, for example, sidewalls and a tread as described above while the central part of a cylindrical rubber member is expanded such that the rubber member forms a toroidal shape. 
     As described above, steps (see  FIG. 8B ) are formed on the surface of the green tire at the edge of the overlapping rubber ribbon  50 . Therefore, when the green tire is vulcanized without any processing or after the steps on the surface are squashed for smoothing, cracking may occur on the surface of the tire product, in particular, on the surfaces of the sidewalls at which flexure during rolling is the most significant as described above. Accordingly, the tire building apparatus  1  according to this embodiment forms surface layers on the outer surface of the green tire at the laminated portion of the rubber ribbon  50  (herein at the sidewalls) after the building by bonding relatively thin and wide rubber sheets so that the steps, projections, and depressions, which can be starting points of cracks, are covered and cracking is prevented from propagating from the surface of the tire. Therefore, the tire building apparatus  1  further includes supplying means that supplies unvulcanized rubber sheets to the green tire and bonding means that bonds the supplied rubber sheets at predetermined positions. 
     Herein, two methods, i.e., a “cold” method and a “hot” method, can be used for boding the rubber sheets. In the “cold” method, rubber sheets formed by a rubber shaping device are cooled and temporarily wound around rolls such as reels. Subsequently, the rubber sheets are bonded at room temperature while being unwound from the rolls. Therefore, in this method, the tire building apparatus  1  is provided with a device that holds the rolls of the rubber sheets and supplies the rubber sheets to the green tire. This device and the rolls of the rubber sheets constitute supplying means for supplying the rubber sheets. On the other hand, in the “hot” method, rubber sheets formed in a rubber shaping device is directly bonded to the surface of the green tire without being cooled. Therefore, in this method, the tire building apparatus  1  is provided with a shaping device for shaping the rubber sheets, for example, serving as supplying means for supplying the rubber sheets. 
     Although these methods different from each other in the structure of the device and the temperature, for example, during bonding, as described above, each rubber sheet is shaped so as to have a width corresponding to the portion to which the rubber sheet is to be bonded and a predetermined thickness by any rubber shaping device, for example, a calender or an extruder in both methods. 
       FIGS. 2A and 2B  are side views schematically illustrating exemplary rubber shaping devices.  FIG. 2A  illustrates a calender, and  FIG. 2B  illustrates an extruder. 
     As shown in  FIG. 2A , the calender  10  includes a plurality of (two in this embodiment) cylindrical rolls  11  and  12  rotated by driving means (not shown). The rolls  11  and  12  are vertically aligned such that the shafts thereof are parallel to each other and that the outer peripheries of the rolls  11  and  12  have a predetermined spacing corresponding to the thickness of a rubber sheet  55  to be shaped. In this calender  10 , a heated rubber compound  56  is fed between the rolls  11  and  12  (in the direction of an arrow V 1  in  FIG. 2A ), rolled between the rolls  11  and  12  that rotate in opposite directions (in the directions of arrows R in  FIG. 2A ), and continuously shaped into a rubber sheet  55  having a predetermined thickness and width (in the direction of an arrow V 2  in  FIG. 2A ). 
     On the other hand, as shown in  FIG. 2B , the extruder  20  includes a body  21  that heats, kneads, and extrudes a rubber compound  56  and a roller head  25  including a pair of cylindrical rollers  26  and  27  disposed in front of a rubber outlet  22  of the body  21 . The body  21  includes a substantially cylindrical cylinder  23 , a screw (not shown) having a spiral flight rotating inside the cylinder  23 , a hopper  24  disposed at the side surface of the cylinder  23  for charging, for example, the rubber compound  56  into the cylinder  23 , the rubber outlet  22  disposed at an end of the cylinder, and heating means (not shown) that heats the cylinder  23  and other parts so as to heat the rubber inside the extruder to a predetermined temperature. The rollers  26  and  27  in the roller head  25  are vertically aligned such that their shafts are parallel to each other and that the outer peripheries of the rolls  26  and  27  have a predetermined spacing corresponding to the thickness of a rubber sheet  55  to be shaped. 
     The extruder  20  having the above-described structure heats and kneads the rubber compound  56  charged from the hopper  24  into the cylinder  23  (in the direction of an arrow W 1  in  FIG. 2B ) while spirally conveying the rubber compound  56  toward the rubber outlet  22  using a rotating screw (in the direction of an arrow S in  FIG. 2B ), and extrudes the rubber compound  56  from the rubber outlet  22 . The extruded rubber is rolled between the rollers  26  and  27  in the roller head  25  rotating in opposite directions (in the directions of arrows G in  FIG. 2B ), and continuously shaped into the rubber sheet  55  having a predetermined thickness and a predetermined width (in the direction of an arrow W 2  in  FIG. 2B ). Instead of the roller head  25 , a nozzle may be attached to the end of the rubber outlet  22  so that rubber is continuously extruded from the opening of the nozzle and shaped into the rubber sheet  55  having a predetermined cross-sectional shape. 
     In the tire building apparatus  1  according to this embodiment, rubber sheets  55  shaped as described above are bonded at predetermined positions of both sidewalls on the sides of a green tire by the bonding means such that surface layers, each composed of one rubber sheet  55 , are formed on the outer surface of the laminated portion formed by laminating the rubber ribbon  50 . Herein, the rubber sheets  55  can be bonded to the sidewalls by tackiness of the rubber itself by pressing the rubber, or can be bonded to the sidewalls by pressing the rubber toward the sidewalls after an adhesive is applied to the sidewalls or bonding surfaces of the rubber sheets  55 . In particular, in the “hot” method where the rubber sheets  55  are bonded while being shaped, the temperature of the rubber is higher than that in the “cold” method, and the tackiness is also higher. Therefore, the rubber sheets  55  can be sufficiently firmly bonded by only the tackiness of the rubber sheets  55 . 
       FIGS. 3A to 3C  are perspective views schematically illustrating a process of bonding the rubber sheets  55  to both sidewalls  31  of a green tire  30 . 
     In this tire building apparatus  1 , the rubber sheets  55  are bonded to both sides of the green tire  30  at the same time. Since the process is the same, a procedure for bonding the rubber to one of the sidewalls  31  at the sides of the green tire  30  will be described. 
     First, as shown in  FIG. 3A , the tire building apparatus  1  supplies a rubber sheets  55  from means (not shown) for supplying the rubber sheets  55  to the sidewall  31  (in the directions of arrows K in  FIG. 3 ) while the green tire  30  is not rotated. Next, when a sensor (not shown), for example, detects the arrival of the leading end of the rubber sheet  55  at a predetermined position on the sidewall  31 , the leading end of the rubber sheet  55  is pushed from the outside of the tire in the width direction of the tire by a pusher (not shown) such that the rubber sheet  55  is bonded at the predetermined position of the sidewall  31  by pressure. Subsequently, the pusher is detached from the rubber sheet  55 , and a rotatable cylindrical roller  6  is pressed to substantially the same position. At this moment, the shaft of the roller  6  is substantially parallel to the outer surface of the sidewall  31 , and is substantially orthogonal to a rotating direction of the green tire  30  (the direction of an arrow F in  FIG. 3B ). 
     Next, as shown in  FIG. 3B , the rubber sheet  55  is bonded along the sidewall  31  of the green tire  30  rotating in the same direction (the direction of the arrow F in  FIG. 3B ) in synchronization with the speed of supplying the rubber sheet  55 . At this moment, the rubber sheet  55  is pressed by the roller  6  and bonded by pressure while air between the rubber sheet  55  and the lower component is removed and the rubber sheet  55  is deformed so as to fit projections and depressions on the surface of the laminated portion of the rubber ribbon. Moreover, the outer portion of the rubber sheet  55  in radial directions of the tire is expanded and bonded such that the rubber sheet  55  is deformed into a substantially ring shape corresponding to the shape of the sidewall  31 . 
     In this state, the supply of the rubber sheet  55  and the rotation of the green tire  30  are continued. When a sensor (not shown), for example, detects that the leading end of the rubber sheet  55  reaches the bonding start position as shown in  FIG. 3C  after one rotation of the green tire  30 , the rotation of the green tire  30  is stopped and the rubber sheet  55  is cut by cutting means (not shown). Subsequently, the roller  6  is separated from the surface, and the boding operation is completed. At this moment, the trailing end of the cut rubber sheet  55  is bonded to the leading end by the pressure of the roller  6 . 
     Herein, with the rubber sheet  55 , a thickness of less than 0.05 mm may lead to tearing of the rubber sheet  55  by deformation during bonding due to insufficient tensile strength, and cracking caused by projections and depressions on the surface of the laminated portion of the rubber ribbon. In contrast, a thickness exceeding 2 mm precludes the bonding operation, for example, due to the high stiffness of the rubber sheet  55 , and causes air trapping between the rubber sheet and the lower component due to insufficient deformation of the rubber sheet with reduced flexibility along the projections and the depressions on the surface of the laminated portion. Thus, the thickness of the rubber sheet  55  preferably ranges from 0.05 to 2 mm. 
     A rotatable cylindrical brush to be pressed toward the surface of the sidewall  31  may be disposed downstream of the roller  6  in the tire-rotating direction so that both ends of the bonded rubber sheet  55  in the width direction are reliably bonded to the sidewall  31  with the brushing. Moreover, the surface of the sidewall  31  may be, for example, pressed by a roller or brushed by a brush in advance for smoothing the surface of the laminated portion of the rubber ribbon before bonding the rubber sheet  55 . 
     Furthermore, the rubber sheet  55  to be supplied may be cut into a predetermined length required for bonding, and may be bonded to one of the sidewalls  31  at a time. That is, after one rubber sheet  55  is bonded to one of the sidewalls, the green tire  30  may be rotated about an axis of a radial direction of the tire by 180°, and another rubber sheet  55  is bonded to the other sidewall. In this single-side bonding case, the tire building apparatus  1  needs an additional rotating device for turning the green tire  30  over after the bonding at one of the sidewalls and for directing the other sidewall toward the bonding means. In this case, the means for supplying and bonding the rubber sheets  55  needs to be disposed only at one side of the green tire  30 . 
     In the bonding by the “hot” method, the rubber sheet  55  can be bonded with the rollers in the devices for shaping the rubber sheets  55  (see  FIGS. 2A and 2B ). 
       FIG. 4  is a side elevation viewed in a radial direction of the tire schematically illustrating a rubber sheet  55  being bonded to one of the sidewalls  31  using the extruder  20  in the “hot” method. 
     In this case, as shown in  FIG. 4 , the diameter of the roller  26  (upper roller in  FIG. 4 ) in the roller head  25  at the tip of the extruder  20  is larger than that of the roller  27  (lower roller in  FIG. 4 ), and the outer periphery of the roller  26  is pressed toward a predetermined position of the sidewall  31  during bonding of the rubber sheet  55 . In this state, rubber is extruded from the rubber outlet  22 , and rolled between the rollers  26  and  27  in the roller head  25 . While the rubber is shaped into a sheet having a predetermined cross section, the rubber sheet  55  is bonded at a predetermined position of the sidewall  31  by the roller  26 . That is, the rubber sheet  55  is supplied (in the direction of an arrow K in  FIG. 4 ) by rotating the rollers  26  and  27  in opposite directions (in the directions of arrows G in  FIG. 4 ), and is bonded to the sidewall  31  of the green tire  30  (rotating in the direction of an arrow F in  FIG. 4 ) as in the same manner shown in  FIGS. 3A to 3C . 
     After the rubber sheet  55  is bonded as described above, a substantially ring-shaped surface layer connected at a connecting portion  57  is formed on the outer surface of the sidewall  31  at each side of the green tire  30  as shown in  FIG. 3C . The green tire  30  is then removed from the tire building apparatus  1 , transferred to a vulcanizing apparatus by a conveying apparatus (not shown), and vulcanized in a mold into a pneumatic tire (tire product) having a predetermined shape. 
       FIGS. 5A to 5C  are cross-sectional views enlarged in the width direction of the tire schematically illustrating the structure of one sidewall  31  of the rolling pneumatic tire after vulcanization. 
     As shown in  FIG. 5A , this pneumatic tire includes a laminated portion  40  serving as inner layers formed by helically winding and laminating a rubber ribbon  50  and a surface layer  41  outside the laminated portion formed by bonding a rubber sheet  55  at each sidewall  31 . That is, the outer surface of the sidewall  31  at each side of the tire is composed of a single rubber layer. 
     Herein, the flexure of the sidewall  31  is the most significant in the rolling tire, and the sidewall  31  is repeatedly deformed from an unloaded state ( FIG. 5A ) to an expanded state or a contracted state ( FIG. 5B  or  5 C). That is, during flexure, the sidewall is curved outward in the width direction of the tire as shown in  FIG. 5B , and tensile forces (arrows P in  FIG. 5B ) act so as to expand, for example, the surface layer  41  in the direction of the arrows P. Alternatively, the sidewall is oppositely curved inward in the width direction of the tire as shown in  FIG. 5C , and compressive forces (arrows Q in  FIG. 5C ) act so as to compress, for example, the surface layer  41  in the direction of the arrows Q. 
     Since the sidewall  31  is repeatedly expanded and contracted by large forces acting during driving in this manner, cracking may occur at minute projections and depressions remaining on the surface and may propagate from the surface of the sidewall  31  formed by laminating only the rubber ribbon  50 , as described above. Moreover, when adhesion failure of the rubber ribbon  50  occurs, portions of the adhesion failure may be split by the tensile forces generated during expansion, or portions of the adhesion failure may be shifted in a transverse direction by the compressive forces and cracks can propagate, for example, inward. 
     Since this pneumatic tire has the surface layer  41  formed by bonding the rubber sheet  55  to the outer surface of the laminated portion  40  of the rubber ribbon  50 , projections and depressions serving as starting points of cracks are covered, and the surface is smoothed. Thus, cracking can be prevented from propagating therefrom. At the same time, since the surface layer  41  prevents split and shift of portions of the adhesion failure, cracking can be prevented from propagating from the surface of the sidewall  31 , resulting in an improved durability of the pneumatic tire. 
     In this embodiment, a wide rubber sheet  55  is bonded to the entire surface of the sidewall  31 . However, a narrower rubber sheet  55  can be bonded to the sidewall  31  by, for example, shifting the position of the rubber sheet such that the rubber sheet partly overlaps with itself in radial directions of the tire. In addition, the narrow rubber sheet  55  can be bonded to only a portion that significantly flexes in particular for forming a surface layer  41  on only the portion required on the sidewall  31 . Moreover, in order to enhance the strength of the surface layer  41 , two or more plies of the rubber sheet  55  can be bonded to the same portion for forming the surface layer  41  composed of two or more plies of the rubber sheet  55 . 
     Furthermore, additional surface layers can be formed by bonding rubber sheets  55  to other portions, for example, the tread of the green tire  30  formed by laminating the rubber ribbon  50 . Also in this case, cracking can be prevented from propagating from the surface, and the durability of the pneumatic tire can be improved as in the case of the sidewall  31 . 
     (Driving Test) 
     In order to confirm the effect of the present invention, tires according to the above-described embodiment (hereinafter referred to as implementation products) formed by bonding rubber sheets  55  to sidewalls  31  as described above, tires according to a comparative example (hereinafter referred to as comparative products  1 ) formed by laminating a rubber ribbon and by pressing the surface thereof for bonding the components to each other by pressure and for squashing projections, and tires according to another comparative example (hereinafter referred to as comparative products  2 ) formed by only laminating a rubber ribbon were produced for driving tests. Conditions for production and the shapes of the tires, for example, other than those described above were identical. The number of the tires produced was identical for each example, and the incidence of cracking after driving a predetermined distance was compared. 
       FIG. 6  illustrates the results of the driving tests. The abscissa represents the travel distance (10,000 km) while the ordinate represents the incidence of cracking (%). 
     The incidence of cracking of each tire, which is expressed in percentage, is determined by dividing the total number of tires cracked before reaching a predetermined travel distance by the total number of tires used for the driving tests. In  FIG. 6 , circles indicate the incidence of cracking of the implementation products, triangles indicate that of the comparative products  1 , and crosses indicate that of the comparative products  2 . 
     As shown in  FIG. 6 , the incidences of cracking after about thirty-thousand kilometer driving were 0% for the comparative products  1  and the implementation products as against 30% for the comparative products  2 , and cracking did not occur in the comparative products  1  and the implementation products. The incidences of cracking after about sixty-thousand kilometer driving were 0.1% for the comparative products  1  and 0% for the implementation products as against 55% for the comparative products  2 ; and cracking did not occur in the implementation products whereas the comparative products  1  had a few cracks. The incidences of cracking after about one hundred-thousand kilometer driving were 100% for the comparative products  2 , 0.2% for the comparative products  1 , and 0% for the implementation products; and cracking did not occur in the implementation products whereas all the comparative products  2  had cracks and a larger number of comparative products  1  had cracks. These results proved that the present invention could prevent cracking from propagating from the surface, and could improve the durability of the pneumatic tire.