Abstract:
A tire has a tread, with the tread having a plurality of ground engaging tread elements creating a tread surface and at least one of the elements has a sipe therein. In planes parallel to the tread surface, the configuration of a radially outer portion of the sipe is defined by a continuous non-linear curvature, the curvature having at least one radius R. The value of the radius R of the sipe curvature increases from the tread surface to a base of the sipe and the base of the sipe has a configuration defined by a radius R having a value of infinity.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention is directed to a pneumatic tire. More specifically, the present invention is directed to the tread of a tire and the sipes in the tire tread. 
       BACKGROUND OF THE INVENTION 
       [0002]    The tread portion of a pneumatic tire generally comprises a plurality of circumferentially and laterally extending grooves defining ground engaging tread elements, the elements being in the form of blocks or ribs or combinations thereof. The particular size and shape of the tread elements contribute significantly to the overall performance of the tire and are for that reason designed to achieve the desired tire characteristics. 
         [0003]    In addition to the grooves provided in the tread configuration, a tire tread is often provided with sipes. A sipe is a groove having a width in the range of about 0.1% to about 1% of the tread width, i.e. the arc length of the tread surface in the axial direction. The sipe tends to close when it is located in the tire footprint at zero speed and under normal load and pressure. Sipes are typically formed by steel blades inserted into a cast or machined mold or tread ring. 
         [0004]    A sipe may extend circumferentially or laterally about the tread and may be as deep as the primary tread grooves or have a depth that is greater than the groove depth. The sipes can pass through the sides of the ribs and tread blocks or be confined to the interior of the tread elements. It is further known to use sipes having a depth that varies along its length. 
         [0005]    The presence of sipes in a tread increases the number of biting edges in the tread. The local high pressure at each biting edge improves the wiping and digging action of the tread surface, conferring to a tire excellent traction on wet roads and on snow and ice. Furthermore, sipes improve the flexibility of the tread elements without destroying their solidity. The easy relative longitudinal sliding between the opposed faces of the sipe weakens the resistance of the tread elements to flexing in the contact area between tread and road and therefor slows down the heat buildup of the tire; however, the sliding of the opposed faces of the sipes creates friction between the opposing sipe faces and can lead to wear of the sipes. 
         [0006]    However, the need for flexibility of the tread element can change during the life of the tire and the present invention is directed to a tire and tire tread with a sipe that addresses such a changing need. 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention is directed to a tire tread having ground engaging elements having sipes therein. The invention also discloses a tire having sipes therein and a blade used to form the sipes. 
         [0008]    Disclosed herein is both a tire and tread, the tread having a plurality of ground engaging tread elements creating a tread surface and at least one of the elements has a sipe therein. The sipe has opposing end points and a sipe length as measured between the opposing end points. In planes parallel to the tread surface, the configuration of a radially outer portion of the sipe is defined by a continuous non-linear curvature, the curvature having at least one radius R. The value of the radius R of the sipe curvature increases from the tread surface to a base of the sipe, the base of the sipe has a configuration defined by a radius R having a value of infinity. 
         [0009]    In one aspect of the invention, the sipe configuration is such that, in a vertical plane, along the sipe length, from a length midpoint of the sipe to the respective sipe end points, the sipe configuration is also defined by an increasing radius Y. 
         [0010]    In another aspect of the invention, the tread element comprising the sipe has opposing edges, and the sipe end points coincide with the tread element opposing edges such that the sipe extends across the full width of the tread element. Alternatively, the sipe may be a blind sipe such that at least one of the sipe end points is located within the tread element and does not intersect with any tread element edge. If both sipe ends are located within the tread element, the sipe is considered a fully embedded sipe. 
         [0011]    In another aspect of the invention, the sipe has a radially innermost portion of at least 10% but not more than the radially innermost 45% of the sipe depth D S  and the radially innermost portion has a configuration defined by a radius R having a value of infinity. 
         [0012]    In another aspect of the invention, a theoretical line can be drawn between opposing sipe end points and the theoretical line and the sipe base are in the same vertical plane. Alternatively, the theoretical line and the sipe base may be in axially offset vertical planes. 
         [0013]    In another aspect of the invention, at the sipe end points, the sipe has a straight line vertical configuration. 
         [0014]    Also disclosed is a blade that forms the disclosed sipe. The blade has a portion that is the positive to the negative space that the sipe forms in the tread element. The blade thus has a shape creating portion that has opposing end points, a length as measured between the opposing end points, and a depth. In planes perpendicular to the blade depth, the configuration of a radially outer portion of the blade is defined by a continuous non-linear curvature, the curvature having at least one radius R, the value of the radius R increasing from a top of the blade to a base of the blade, the blade base having a configuration defined by a radius R of infinity. 
       Definitions 
       [0015]    The following definitions are controlling for the disclosed invention. 
         [0016]    “Axial” and “axially” are used herein to refer to lines or directions that are parallel to the axis of rotation of a tire. 
         [0017]    “Blade” is typically a small width element used to create a protrusion in a tire curing mold that forms part of the tread design. The protrusion forms a corresponding depression in the finished tire tread. Conventionally, a blade is distinct from a rib in a tire curing mold. A rib in a mold is also a protrusion; however, a rib forms a groove in the finished tread, while blades are used to form sipes. 
         [0018]    “Radial” and “radially” are used to mean directions radially toward or away from the axis of rotation of the tire. 
         [0019]    “Sipes” refer to small grooves molded into tread elements of a tire that subdivide the tread elements and improve traction characteristics. Sipes have a width in the range of about 0.1% to about 1% of the tread width and tend to close completely in a tire footprint. The depth of a sipe may vary around the circumference of the tread, or the depth of one sipe may be constant but vary from the depth of another sipe in the tire. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    The invention will be described by way of example and with reference to the accompanying drawings in which: 
           [0021]      FIG. 1  illustrates a tread element with a sipe therein; 
           [0022]      FIGS. 2A-2D  are sectional cuts along the length of the tread element sipe; 
           [0023]      FIG. 2E  is an alternative sipe configuration; 
           [0024]      FIG. 3  is a top view of a tread element with the sipe therein; 
           [0025]      FIG. 4  is a top view of the tread element at approximately 25% wear; 
           [0026]      FIG. 5  is a top view of the tread element at approximately 50% wear; and 
           [0027]      FIGS. 6-9  illustrate alternative sipe configurations. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0028]    The following language is of the best presently contemplated mode or modes of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims. 
         [0029]      FIG. 1  illustrates a tread element  10  having a sipe  12  in accordance with the present invention. The tread element  10  is a tread feature on a tire, preferably on a pneumatic tire. In a manner known to those skilled in the art, the tread element  10  is formed from any combination of lateral and/or circumferential grooves. While the tread element  10  is illustrated as an individual tread block, those skilled in the art will appreciate that this tread element  10  may have any overall configuration desired by the tire designer. The element  10  may be defined as having both a length or width along any horizontal plane and a depth along the vertical, or radial, plane of the tread element and associated tire. Additionally, features of the tread element  10 , such as the sipe  12 , may be similarly defined as having a length or width along horizontal planes and a depth along vertical or radial planes. 
         [0030]    The illustrated sipe  12  extends from one tread element edge  14  to the opposing tread element edge  16 , wherein the opposing sipe end points  18  are located on the same hypothetical line  20  drawn the length of the sipe  12 . Between the sipe end points  18 , in a plane parallel to the tread surface  26 , the centerline of the sipe  12 , which is located halfway between the opposing surface walls  22 ,  24  of the sipe  12 , has a configuration defined by an arc of a circle having a defined radius R, this being the radius R S  at the initial tread surface  26 , see also  FIG. 3 . The initial tread surface  26 , defining the original tread element height and sipe depth D S , is determined when the tire is new and has not been subjected to any significant tire wear. 
         [0031]    As the tread surface is worn and the sipe depth decreases, the radius R of the arc of the sipe  12 , as measured in a plane parallel to the tread surface, gradually increases while the end points  18  of the sipe  12  remain constant, see  FIGS. 4 and 5 .  FIG. 4  illustrates the tread element after approximately 25% wear of the tread element  10 , the level of wear based on the original sipe depth D S . The sipe centerline has a radius R 25 ; R 25  being greater than R S .  FIG. 5  illustrates the tread element after approximately 50% wear of the tread element  10 , the level of wear based on the original sipe depth D S . The sipe centerline has a radius R 50 ; R 50  being greater than R 25 . For the curved portion of the sipe, as measured in a horizontal plane parallel to the tread surface, at each depth of the sipe, the radius is constant along the arc configuration of the sipe. 
         [0032]    The radially innermost portion  30  of the sipe  12  has a length of D I  corresponding to at least 10% but not more than the innermost 45%, more preferably not more than the innermost 30%, most preferably, not more than the innermost 25%, of the sipe depth D S , see  FIGS. 2B-2D . The radially innermost portion  30  of the sipe  12  has a radius R of infinity so the base  28  of the sipe  12  has a straight configuration. Preferably, the straight line configuration of the sipe base  28  is in the same vertical plane as the theoretical line  20  drawn between the sipe end points  18 . When the sipe base  28  is aligned with line  20 , at the sipe edges  18 , when viewed in a radial plane perpendicular to the tread element surface  26 , the sipe  12  has a straight vertical presentation, see  FIG. 2A ; the sipe edge  18  may be slightly tapered due to a inclination of the tread element sidewall. Thus, along the depth of the sipe  12 , when viewed from radially above the tread surface, the sipe  12  gradually progresses from a highly curved configuration to a constant configuration. 
         [0033]      FIGS. 2A-2D  illustrate the cross-sectional cuts of the tread element  10  along the various line of A-A, B-B, C-C, D-D. Along section line D-D, the sipe  12  has mirror symmetry. As already mentioned, at the sipe end points  18 , as seen in  FIGS. 1 and 2A , in a radial plane of the tire, the sipe has a straight vertical configuration. Along the length of the sipe, as determined parallel to line  20  between the sipe ends  18 , from the sipe edge  18  to the mid-point  32  of the sipe length (at the section line D-D for the illustrated sipe), the vertical configuration of the sipe has a varying curved configuration. As seen in  FIGS. 2B-2D , the 20-30% of the sipe depth radially outward and adjacent to the radially innermost portion  30  of the sipe  12  has a curvature of Y that gradually decreases as it approaches the sipe length mid-point  32 . Thus, irrespective of the increasing radius R of the sipe in the horizontal plane of the sipe  12  and tread element  10 , in the vertical plane (or radial plane of the tire) of the sipe  12 , along the sipe length, the sipe  12  has an decreasing and then increasing curvature from one sipe end to the opposing sipe end. 
         [0034]    The sipe  12  of  FIG. 2E  illustrates a cross-sectional view of a variation of sipe  12  at the mid-point  32  of the sipe length; it is at the same cross-sectional location as the view of  FIG. 2D . In this sipe, the radially innermost portion  30  has a length of DI of approximately 25%. 
         [0035]    Additionally, variations in the curvature of the radius, as seen in the plane parallel to the tread surface, are also within the scope of the present invention, so long as the curvature radius is decreasing with wear and flows into the straight sipe base as described above. A few embodiments of variations in the curvature are shown in  FIGS. 6 and 7 . For the illustrated sipe of  FIG. 6 , the sipe  12 ′ has a parabolic curvature, while the sipe  12 ″ has a double curvature defined by two radii, R S1 , R S2 . The two radii R S1 , R S2  defining the curvature in the plane parallel to the tread surface preferably are equal to one another, but one radius may be greater than the other. The sipe curvature may include more than two curves, such as three curves, wherein the sipe centerline begins to resemble a zig-zag pattern. 
         [0036]    The technical benefits of the invention sipe configuration are as follows. When the tread depth is relatively high, as with a new tread, the tread element has a high degree of flexibility. To stiffen the tread using tread features, a curved, three dimensional sipe will create a locking effect. The use of a three dimensional sipe also permits a stiffening of the blade used to manufacture the sipe; especially in instances wherein the sipe is fully embedded in the tread element, i.e. a blind sipe. As the tread wears, the need for stiffening of the tread via tread features gradually reduces as the shorter tread element in and of itself because stiffer due to the reduced element height. Thus, the need for a stiffening effect created by a three dimensional sipe is also reduced. In the present invention, at the tread surface, the sipe  12  has a greater three dimensional aspect which gradually reduces as the need for tread stiffening features is also gradually reduced. When the need is significantly less, the sipe  12  has only a two-dimensional configuration, with no stiffening effect, thus no longer providing increased tread element stiffness, but still providing desired traction features by the presence of the sipe itself. 
         [0037]    The blade used in manufacturing the sipe of the present invention has a significant portion of the blade having a shape corresponding to the sipe, and is in essence, the positive of the blank, or negative, space that is the formed sipe such that the same language that describes the sipe also describes the blade that forms the sipe. The blade is defined as opposing end points that defines the length of the blade and a depth. In planes perpendicular to the depth of the blade, the radially outer portion of the blade has a non-linear curvature, the curvature having at least one radius R, the value of the radius R gradually increasing along the depth of the blade to a blade base having a radius R having a value of infinity. Preferably, not more than the 45% of the bottom most portion of the blade has a radius R of infinity. 
         [0038]    To create the sipes  12 ,  12 ′ of  FIGS. 1 and 6 , in the vertical plane, the blade has a mirror symmetry about the mid-length of the blade of a decreasing radius from the blade edges. 
         [0039]    The blade is made of metal, preferably steel, and for molding the sipe  12 , is mounted into a tread mold such that the straight bottom portion of the blade projects into the open space of the mold. Those skilled in the art will readily understand the appropriate method of mounting and using a sipe blade in a tire mold. 
         [0040]      FIGS. 8 and 9  illustrate variations of the inventive sipe and tire tread. 
         [0041]    In  FIG. 8 , the radially innermost portion  30  of the sipe  12  has an increased width in comparison to the radially outermost portion of the sipe  12 . The increased width portion provides for multiple benefits. By providing an increased width portion, it reduces potential friction contact in the lower portion of the sipe as the tread element passes through the contact patch during tire rotation. 
         [0042]    As previously discussed, the straight configuration of the radially inner portion  30  of the sipe  12  is preferably in the same vertical plane as the theoretical line  20  connecting the sipe end points  18  of the radially outermost portion of the sipe. Conversely, the sipe base  28  may be displaced from the vertical plane coincident with the theoretical line  20 . In such an embodiment, the sipe  12  has a configuration as shown in  FIG. 9 . The sipe base  28  is axially displaced from the radially outer portion of the sipe  12 . The amount of displacement, x, measured along the sipe centerline, is not more than five times the width w of the sipe  12 . 
         [0043]    Additionally, while the sipes are illustrated as extending between the tread element edges  14 ,  16 , to achieve tread characteristics, the sipe may have at least one end  18  located within the tread element and not corresponding to either tread element edge  14  or  16 . Such a sipe is often referred to as a “blind” sipe. Alternatively, both sipe ends  18  may be completely embedded with the tread element  10  such that the sipe edges  18  do not intersect the tread element edges  14 ,  16 ; i.e. an embedded sipe. In regards to placement of the sipe  12  in a tread element  10  in regards to the circumferential direction of the tire, the sipe  12  may be oriented in any direction desired by the tire designer.