Heavy duty pneumatic tires

Abstract: A heavy duty pneumatic tire comprising a tread having a treading surface (1) provided with three or more zigzag grooves (3,2,3) extending in a tire-circumferential direction, wherein opposed groove walls (4,5) of each of at least the zigzag grooves (3) which are located nearest to axial edges of said tread are each bent in a radially outer area spaced from the bottom (3a) of the zigzag groove as viewed in a tire-widthwise sectional plane, and the following inequalities are satisfied. 0.25 ≦ h/H ≦ 0.5, γ₁ ≦ α₁, β₁ ≦ α₁, γ₁ ≦ δ₁, β₁ ≦ δ₁, and γ₂ ≦ α₂, β₂ ≦ α₂, γ₂ ≦ δ₂, β₂ ≦ δ₂, in which: (i) H is the radial depth of the zigzag groove (3), and h is the depth of a location where the groove wall is bent, as measured from an outer contour surface of the tread, (ii) α₁, β₁, γ₁ and δ₁ define angles of a portion of the zigzag groove (3) which is bent to axially outwardly project toward a shoulder portion of the tire, (iii) α₂, β₂, γ₂ and δ₂ define angles of a portion of the zigzag groove (3) which is bent to axially inwardly project toward a central portion of the tire.

Description: The present invention relates to heavy duty pneumatic tires provided with three or more zigzag grooves extending in the tire-circumferential direction thereof. More particularly, the invention is aimed at the effective prevention of so-called river wear along edge portions of the zigzag grooves as well as prevention of entrapment of stones or gravel into the grooves. For example, Japanese patent publication No. 50-22,282 discloses a technique for preventing river wear in a tire of this kind in which a plurality of zigzag grooves are provided in a treading surface of a tread, and extend in the circumferential direction of the tire. As show in Figs. 2(a) to 2(c) of the accompanying drawings, according to Japanese patent publication No. 50-22,282, each zigzag groove 31 has side walls 32 and 33, which form intersecting angles ε and η with normal lines to land portions of the tread. These angles ε and η are made smaller at an axially projecting corner portion 34 of a land portion defined by the zigzag groove 31, and made greater at an axially retracting corner portion 35 of the land portion. This publication states that river wear is reduced by such a construction. However, according to this technique, since the rigidity of the land portion at the axially retracting corner portion 35 becomes too high on the tread center side of the zigzag groove 31, uneven wearing unfavorably occurs there. In addition, since the intersecting angle ε of the groove wall 32 becomes too small at the axially projecting corner portion 34 of the land portion on the tread shoulder portion of the zigzag groove 31, a stone may be unfavourably entrapped in the zigzag groove 31, particularly during an initial wearing stage as illustrated by an imaginary line. Attention is also drawn to the disclosure of US-A-4 449 601. The present invention aims to provide a heavy duty pneumatic tire which can effectively prevent uneven wear of land portions defined by zigzag grooves and can also prevent entrapment of stones or gravel. The present invention provides a heavy duty pneumatic tire comprising a tread having a treading surface provided with three or more zigzag grooves extending in a tire-circumferential direction, wherein opposed groove walls of each of at least the zigzag grooves which are located nearest to axial edges of the tread are each bent in a radially outer area spaced from the bottom of the zigzag groove as viewed in a tire-widthwise sectional plane, characterized in that the following inequalities are satisfied: 0.25 ≦ h/H ≦ 0.5, γ 1 ≦ α 1 , β 1 ≦ α 1 , γ 1 ≦ δ 1 , β 1 ≦ δ 1 , and γ 2 ≦ α 2 , β 2 ≦ α 2 , γ 2 ≦ δ 2 , β 2 ≦ δ 2 , in which: (i) H is the radial depth of the zigzag groove, and h is the depth of a location where the groove wall is bent, as measured from an outer contour surface of the tread, (ii) α₁, β₁, γ₁ and δ₁, define angles of a portion of the zigzag groove which is bent to axially outwardly project toward a shoulder portion of the tire, α₁ and β₁ forming intersecting angles between radially outer and inner areas of the groove wall on a shoulder side of the tread and normal lines to a land portion or an extension line thereof, respectively, and γ₁ and δ₁ being intersecting angles between radially outer and inner areas of the groove wall on a center side of the tread and normal lines to a land portion of the tread or an extension line thereof, respectively, and (iii) α₂, β₂, γ₂ and δ₂ define angles of a portion of the zigzag groove which is bent to axially inwardly project toward a central portion of the tire, α₂ and β₂ forming intersecting angles between radially outer and inner areas of the groove wall on a shoulder side of the tread and normal lines to a land portion of the tread or an extension line thereof, respectively and γ₂ and δ₂ being intersecting angles between radially outer and inner areas of the groove wall on a center side of the tread and normal lines to a land portion of the tread or an extension line thereof, respectively. According to the present invention, the following are preferred. (a) β₁ ≧ β₂ and δ₁ ≦ δ₂. (b) 17.5° ≦ α₁, α₂ ≦ 22.5° and 0° ≦ γ₁, γ₂ ≦ 5°. (c) 17.5° ≦ α₁, α₂ ≦ 22.5°, 0° ≦ γ₁, γ₂ ≦ 5°, 7.5° ≦ β₁ ≦ 12.5°, 22.5° ≦ δ₁ ≦ 27.5°, 0° ≦ β₂ ≦ 5°, 27.5° ≦ δ₂ ≦ 32.5°. (d) The number of the zigzag grooves provided in the treading surface of the tread in the circumferential direction of the tire is 3. (e) The cross sectional shape of the zigzag groove extending in a central portion of the tread is laterally symmetrical as viewed in a cross sectional plane. (f) All of the zigzag grooves satisfy the inequalities: γ₁ ≦ α₁, β₁ ≦ α₁, γ₁ ≦ δ₁, β₁ ≦ δ₁, γ₂ ≦ α₂, β₂ ≦ α₂, γ₂ ≦ δ₂, and β₂ ≦ δ₂ (g) β ₁ ≧ β₂ and δ₁ ≦ δ₂. For a better understanding of the invention, reference is made to the accompanying drawings, wherein: Figs. 1(a) through 1(c) illustrate an embodiment of the present invention; and Figs. 2(a) through 2(c) illustrate the conventional pneumatic tire. The present invention will be explained in more detail with reference to the specific embodiment of Fig. 1. Fig. 1(a) is a plan view illustrating a tread pattern as an embodiment of the present invention. In this embodiment, three zigzag grooves 2, 3, 3 are formed at the same phase in a tread surface of a tread, extending in the circumferential direction of the tire. Among these zigzag grooves, each of two zigzag grooves 3, 3 located near opposite edges of the tread has opposed groove walls 4 and 5, which are bent in a radially outer area spaced from the bottom of the zigzag groove 3 as viewed in a tire-widthwise sectional plane as shown in Figs. 1(b) and 1(c). The groove walls 4, 5 are inclinedly bent, preferably smoothly. According to the present invention, the depth h of the location where the groove is bent and the depth H of the groove satisfy the following inequalities: 0.25 ≦ h/H ≦ 0.5, when measured from the surface of the land portion of the tread. Further, when various intersecting angles between portions of groove walls 4, 5 of the zigzag groove 3 being bent to axially outwardly project toward a shoulder or center portion of the tread and normal lines to a land portion of the tread or its extension (hereinafter referred to as "tread normal line") are defined as shown in Figs. 1(b) and 1(c) mentioned below, these intersecting angles satisfy the following inequalities: γ 1 ≦ α 1 , β 1 ≦ α 1 , γ 1 ≦ δ 1 , β 1 ≦ δ 1 , and γ 2 ≦ α 2 , β 2 ≦ α 2 , γ 2 ≦ δ 2 , β 2 ≦ δ 2 , In these inequalities, α₁, and β₁, denote intersecting angles between tread normal lines and areas 4a and 4b located radially outwardly and inwardly relative to the bent position of a portion of the groove wall 4 on the shoulder side of the the zigzag groove 3, respectively, whereas γ₁, and δ₁ denote intersecting angles between tread normal lines and areas 5a and 5b located radially outwardly and inwardly relative to the bent position of the groove wall 5 on a center side of the tread, respectively. On the other hand, α₂ , β₂, γ₂ and δ₂ define intersecting angles between tread normal lines and the groove walls 4, 5 of a portion of the zigzag groove which is bent to axially inwardly project toward a central portion of the tread, α₂ and β₂ denoting intersecting angles between radially outer and inner areas 4a, 4b of the groove wall 4 on a shoulder side of the tread and tread normal lines, respectively, and γ₂ and δ₂ being intersecting angles between radially outer and inner areas 5a, 5b of the groove wall 5 on a center side of the tread and tread normal lines. According to the present invention, the following are preferred. (a) β₁ ≧ β₂ and δ₁ ≦ δ₂. (b) 17.5° ≦ α₁, α₂ ≦ 22.5° and 0° ≦ γ₁, γ₂ ≦ 5°. (c) 17.5° ≦ α₁, α₂ ≦ 22.5°, 0° ≦ γ₁, γ₂ ≦ 5°, 7.5° ≦ β₁ ≦ 12.5°, 22.5° ≦ δ₁ ≦ 27.5°, 0° ≦ β₂ ≦ 5°, 27.5° ≦ δ₂ ≦ 32.5°. (d) The number of the zigzag grooves provided in the treading surface of the tread in the circumferential direction of the tire is 3. (e) The cross sectional shape of the zigzag groove extending in a central portion of the tread is laterally symmetrical as viewed in a cross sectional plane. (f) All of the zigzag grooves satisfy the inequalities: γ₁ ≦ α₁, β₁ ≦ α₁, γ₁ ≦ δ₁, β₁ ≦ δ₁, γ₂ ≦ α₂, β₂ ≦ α₂, γ₂ ≦ δ₂, and β₂ ≦ δ₂ (g) β₁ ≧ β₂ and δ₁ ≦ δ₂. According to the heavy duty pneumatic tire of the present invention, γ₁ ≦ α₁ and γ₂ ≦ α₂. As a consequence, entrapment of stones or gravel into the grooves can be effectively prevented. Further, β₁ ≦ α₁, γ₁ ≦ δ₁, β₁ ≦ δ₁, β₂ ≦ α₂, γ₂ ≦ δ₂, and β₂ ≦ δ₂ are combined with γ₁ ≦ α₁ and γ₂ ≦ α₂. As a consequence, although large ground-contacting pressure is applied to the groove wall 4 when the tire receives side forces, the ground-contacting pressure can be reduced through the bent portion being swelled out toward the interior of the groove. In addition, although large side forces are applied to the top portion of the groove wall 5, these side forces can be mitigated because rigidity is low at this top portion and this location is also likely to be swelled inside the groove. Therefore, railway wear can be sufficiently suppressed. Further, when β₁ ≧ β₂ and δ₁ ≦ δ₂, rigidity of the groove wall in the axially projecting corner portion which is more likely to receive side forces is made smaller than at the groove wall in the axially retracting corner portion positioned on the same groove wall side and adjacent to the axially projecting corner portion. Consequently, rigidity of the entire tread can be made well balanced and the depth of the groove can be made small. Thus, railway wear can be prevented in an intermediate wearing stage and thereafter during which uneven wearing-suppressing effect due to movement (swelling) of the groove walls in the initial wearing stage as mentioned above becomes smaller. In addition, when 0.25 ≦ h/H ≦ 0.5, nuclei of uneven wearing can be prevented in the initial wearing stage, and entrapment of stones or gravel can be prevented. If h/H is less than 0.25, a stone is likely to enter the groove deeper than the bent location, so that the stone entrapment-preventing effect is deteriorated. If h/H is more than 0.5, rigidity on the side of the groove wall 4 becomes too low, and the swelling amount of the bent portion on the side of the groove wall 5 becomes smaller, so that good railway wearing-preventing effect cannot be obtained. According to the present invention, the intersecting angles α₁, α₂, γ₁, and γ₂ preferably satisfy the following inequalities irrespective of the bending directions of the zigzag groove 3. 17.5° ≦ α 1 , α 2 ≦ 22.5° and 0° ≦ γ 1 , γ 2 ≦ 5° In this case, stone entrapment can be more effectively prevented. In addition, when β₁, β₂, δ₁, and δ₂ further satisfy the following inequalities, the groove wall structure most suitable for the prevention of the railway wear can be realized. 7.5° ≦ β 1 ≦ 12.5°, 22.5° ≦ δ 1 ≦ 27.5°, and 0° ≦ β 2 ≦ 5°, 27.5° ≦ δ 2 ≦ 32.5° As mentioned above, in the pneumatic tire of the present invention, occurrence of uneven wearing on the land portion on the tread-center side of the zigzag groove 3 can be effectively prevented, and stone-entrapment in the portions of the zigzag groove 3 being bent to project toward the shoulder portion of the tread can be effectively prevented. In the following, the present invention will be explained in further detail with reference to the accompanying drawings. Fig. 1 illustrates one embodiment of the present invention. The internal reinforcing structure of this tire is the same as that of an ordinary radial tire, and illustration thereof is omitted. As mentioned above, three circumferentially continuous zigzag grooves 2, 3, 3 are provided in a treading surface 1 of a tread. The width of the treading surface 1 is 204 mm, and that of each of the zigzag grooves 2, 3, 3 is 13.2 mm. As illustrated in Figs. 1(a) and 1(b), opposed groove walls 4, 5 of each of the zigzag grooves 3 located near a shoulder of the tread are both axially bent toward the shoulder of the tread at radially outer areas 4a, 5a, respectively, spaced from a groove bottom 3a of the zigzag groove 3 as viewed in a tire widthwise sectional plane, and h/H is set at 0.254 in which h and H are depths of the bent location and the zigzag groove 3 as measured from the surface of the tread. In this embodiment, with respect to a portion of the zigzag groove 3 being axially bent to project toward the shoulder of the tread, α₁ = 20°, β₁ = 10°, γ₁ = 0°, and δ₁ = 25°, in which α₁ is an intersecting angle between a tread normal line and a radially outer area 4a of the groove wall 4 on the tread-shoulder side positioned on the radially outer side of the bent portion; β₁ is an intersecting angle between a tread normal line and a radially inner area 4b of the groove wall 4 located on the radially inner side of the bent portion; γ₁ is an intersecting angle between a tread normal line and a radially outer area 5a of the groove wall 5 on the tread-center side positioned on the radially outer side of the bent portion; and δ₁ is an intersecting angle between a tread normal line and a radially inner area 5b of the groove wall 5 located on the radially inner side of the bent portion. With respect to a portion of the zigzag groove 3 being axially bent to project toward the central portion of the tread, α₂ = 20° identical with α₁, β₂ = 0°, γ₂ = 0° identical with γ₁, and δ₂ = 30°, in which α₂ is an intersecting angle between a tread normal line and a radially outer area 4a of the groove wall 4 on the tread-shoulder side positioned on the radially outer side of the bent portion; β₂ is an intersecting angle between a tread normal line and a radially inner area 4b of the groove wall 4 ; γ₂ is an intersecting angle between a tread normal line and a radially outer area 5a of the groove wall 5 on the tread-center side; and δ₂ is an intersecting angle between a tread normal line and a radially inner area 5b of the groove wall 5. The thus constructed tire having a tire size of 10.00R20 was fitted to a steering wheel of a vehicle having a normal load of 10 tons, which was run over 20,000 km. Thereafter, a worn amount and a stone-entrapped state were checked. The results obtained are shown in Table 1. For comparison purpose, a conventional tire having the same construction as the test tire according to the present invention except that the groove wall structure was the same as in Fig. 2 was prepared, and similarly tested. The results obtained are also shown in Table 1. In Table 1, A, B, C and D are locations shown in Figs. 1 and 2. With respect to the worn amount, the results are shown by index by taking those of the conventional tire as 100 as control tire. The greater the figure, the better is the result. Conventional tire Invention tire Worn volume location A 100 105 location B 100 110 location C 100 105 location D 100 125 Stone-entrapment (number of stones per groove) 10 3 As is clear from Table 1, wearing state could be effectively improved at each of the locations A through D. Among them, at the location D, uneven wearing can be extremely effectively prevented. Further, the number of stones entrapped per one zigzag groove can be reduced to less than a half of that in the conventional tire. Although the present invention has been explained with reference to the illustrated embodiment, zigzag grooves may be provided in a desired number not smaller than three. In this case, the technical idea of the present invention may be applied to all of these zigzag grooves. Alternatively, the zigzag groove extending in the central portion of the tread may be designed laterally symmetrical as viewed cross-sectionally, or may be designed in an appropriate shape depending upon necessity. As is clear from the above explanation, according to the present invention, the formation of uneven wear nuclei can be sufficiently prevented in the initial wearing stage, so that progressing of the uneven wearing can be effectively suppressed after the initial wearing stage, and in addition stone-entrapment in the initial wearing stage can be sufficiently prevented.

Claims: A heavy duty pneumatic tire comprising a tread having a treading surface (1) provided with three or more zigzag grooves (3,2,3) extending in a tire-circumferential direction, wherein opposed groove walls (4,5) of each of at least the zigzag grooves (3) which are located nearest to axial edges of said tread are each bent in a radially outer area spaced from the bottom (3a) of the zigzag groove as viewed in a tire-widthwise sectional plane, characterized in that the following inequalities are satisfied: 0.25 ≦ h/H ≦ 0.5, γ 1 ≦ α 1 , β 1 ≦ α 1 , γ 1 ≦ δ 1 , β 1 ≦ δ 1 , and γ 2 ≦ α 2 , β 2 ≦ α 2 , γ 2 ≦ δ 2 , β 2 ≦ δ 2 , in which: (i) H is the radial depth of the zigzag groove (3), and h is the depth of a location where the groove wall is bent, as measured from an outer contour surface of the tread, (ii) α₁, β₁, γ₁ and δ₁ define angles of a portion of the zigzag groove (3) which is bent to axially outwardly project toward a shoulder portion of the tire, α₁ and β₁ forming intersecting angles between radially outer and inner areas (4a,4b) of the groove wall (4) on a shoulder side of the tread and normal lines to a land portion or extension line thereof, respectively, and γ₁ and δ₁ being intersecting angles between radially outer and inner areas (5a,5b) of the groove wall (5) on a center side of the tread and normal lines to a land portion of the tread or an extension line thereof, respectively, and (iii) α₂, β₂, γ₂ and δ₂ define angles of a portion of the Zigzag groove (3) which is bent to axially inwardly project toward a central portion of the tire, α₂ and β₂ forming intersecting angles between radially outer and inner areas (4a,4b) of the groove wall (4) on a shoulder side of the tread and normal lines to a land portion or an extension line thereof, respectively, and γ₂ and δ₂ being intersecting angles between radially outer and inner areas (5a,5b) of the groove wall (5) on a center side of the tread and normal lines to a land portion of the tread or an extension line thereof, respectively. A pneumatic tire as claimed in claim 1, characterized in that β₁ ≧ β₂ and δ₁ ≦ δ₂. A pneumatic tire as claimed in claim 1 or 2, characterized in that 17.5° ≦ α₁, α₂ ≦ 22.5° and 0° ≦ γ₁, γ₂ ≦ 5°. A pneumatic tire as claimed in any of claims 1 to 3, characterized in that 17.5° ≦ α₁, α₂ ≦ 22.5°, 0° ≦ γ₁, γ₂ ≦ 5°, 7.5° ≦ β₁ ≦ 12.5°, 22.5° ≦ δ₁ ≦ 27.5°, 0° ≦ β₂ ≦ 5°, 27.5° ≦ δ₂ ≦ 32.5°. A pneumatic tire as claimed in any of claims 1 to 4, characterized in that the number of zigzag grooves (3,2,3) provided in the treading surface (1) of the tread in the circumferential direction of the tire is three. A pneumatic tire as claimed in any of claims 1 to 5, characterized in that the cross-sectional shape of the zigzag groove (2) extending in a central portion of the tread is laterally symmetrical as viewed in a cross-sectional plane. A pneumatic tire as claimed in any of claims 1 to 6, characterized in that all of the zigzag grooves (3,2,3) satisfy the inequalities: γ₁ ≦ α₁, β₁ ≦ α₁, γ₁ ≦ δ₁, β₁ ≦ δ₁, γ₂ ≦ α₂, β₂ ≦ α₂, γ₂ ≦ δ₂, and β₂ ≦ δ₂. A pneumatic tire as claimed in claim 7, characterized in that β₁ ≧ β₂ and δ₁ ≦ δ₂.

Inventors: IKEDA HIROMICHI, IKEDA, HIROMICHI
Applicants: BRIDGESTONE CORP, BRIDGESTONE CORPORATION
Application year: 1992
Main classification: B60C11
IPC codes: B60C11