Abstract:
A disc rotor for a disc brake with a vent hole shape which has an inner peripheral corner with a larger radius to reduce stress generated by braking torque and suppresses an increase in stress generated by pad pressure. The disc rotor includes a first sliding part connected to a bell housing, a second sliding part located parallel to, and spaced in an axle direction from, the first sliding part, a plurality of ribs circumferentially spaced between the sliding parts, and vent holes formed by the ribs and the sliding parts. The inner peripheral shape of each of the vent holes has at least two arc shapes with different curvature radii at an end perpendicular to the disc rotor&#39;s rotation direction. The smallest curvature radius is 2 mm or more. An arc curvature radius on the first sliding part side is larger than that on the second sliding part side.

Description:
CLAIMS OF PRIORITY 
       [0001]    The present application claims priority from Japanese Patent application serial No. 2008-139837, filed on May 28, 2008, the content of which is hereby incorporated by reference into this application. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to disc rotors for disc brakes of vehicles. 
       BACKGROUND OF THE INVENTION 
       [0003]    A disc brake is a kind of braking device as a vehicle component whereby frictional heat is generated by forcing brake pads against both sides of a disc (hereinafter called the disc rotor) rotating together with a wheel so that kinetic energy is converted into thermal energy to produce a braking effect. One example of such a disc rotor is illustrated in  FIG. 20  of JP-A No. 2002-5207. Specifically a disc rotor  71  has fins  83  (hereinafter called the ribs) between a pair of sliding parts  75  and is fixed on a wheel through a main body  73  (hereinafter called the bell housing) having a hub  77 . The sliding parts  75  have vent holes penetrating from the inner periphery to the outer periphery and as the disc rotor  71  rotates, air flows in the vent holes so that heat generated by friction between the brake pads and brake rotor is transferred and dissipated into the air. 
         [0004]    From the viewpoints of fuel efficiency and steering stability, there is a strong demand for weight reduction in this type of brake rotor. Furthermore, with the growing tendency toward sophisticated vehicles, brakes are required to provide a stable braking force at higher vehicle velocities in a higher temperature range. In the past, disc rotors were mainly made of cast iron but in recent years, efforts to develop disc rotors of carbon ceramic (carbon fiber reinforced silicon carbide, hereinafter referred to as C/SiC) have been continued because carbon ceramic is advantageous in terms of weight, heat resistance, corrosion resistance, and abrasion resistance. Disc rotors of C/SiC provide higher abrasion resistance, heat resistance and corrosion resistance than disc rotors of cast iron. According to Walter Krenkel, B. Heidenreich, and R. Renz (“C/C—SiC Composites for Advanced Friction Systems,” Advanced Engineering Materials, Vol. 4, February 2002, pp. 427-436), the density of C/SiC is 2.4 g/cm 3  or about one third of the density of cast iron (7.3 g/cm 3 ). A lighter disc brake leads to reduction in unsprung weight in a vehicle and improvement in driving comfort and safety. 
         [0005]    Although C/SiC disc rotors have many advantages over cast iron ones as described above, C/SiC is lower in strength than cast iron. According to the above article (authored by Walter Krenkel, B. Heidenreich, and R. Renz), the strength of C/SiC is 80 MPa or less than half of the strength of cast iron (200 MPa or more in case of FC200). For this reason, C/SiC disc rotors have a problem that the mechanical stress applied to them during braking may cause cracking. This mechanical stress is a combination of two types of stress: stress generated when a pad compresses a disc rotor (hereinafter called “pad pressure stress”) and stress generated by the torque applied to the disc rotor through a disc rotor surface (surface of contact between the disc rotor and pad) (hereinafter called “torque stress”). Next, these two stresses and vent hole shapes for reducing such stresses will be explained in detail. 
         [0006]    First, pad pressure stress will be explained referring to  FIGS. 1A to 4 .  FIGS. 1A and 1B  show the general structure of an conventional disc rotor  20  with a pad  3  in which  FIG. 1A  is a plan view of the second sliding part of the disc roller and  FIG. 1B  is a sectional view taken along the line A-A′ in  FIG. 1A , and  FIG. 2  shows the disc rotor as seen from direction B (center of rotation of the disc rotor) in  FIGS. 1A and 1B . In the disc rotor  20 , pin holes  4  for fixing it on the bell housing are formed around the rotary shaft and vent holes  5  are formed between ribs  6 . 
         [0007]      FIGS. 3A ,  3 B and  4  illustrate the structure for one period in the periodic structure  24  including vent holes and ribs as shown in  FIG. 2 . When the disc rotor  20  is compressed by the pads  3  as shown in  FIG. 2 , the vent hole  5  of the disc rotor  20  is deformed as illustrated in  FIG. 3A . At this time, distribution of principal stress σ 1  of the disc rotor  20  is as shown in  FIG. 3B  and portions C and D in  FIG. 3B  are stretched toward the circumferential direction, causing an increase in stress. This stress distribution may be considered to be equivalent to that for beams with both ends fixed which are under uniformly distributed load. According to the beam theory, the maximum stress generated on a beam surface is proportional to the square of the beam length and inversely proportional to the square of the beam thickness. Therefore, in order to decrease the stress on the portions C and D ( FIG. 3B ), it is necessary to decrease width W of the beam part  8  ( FIG. 4 ) and increase thicknesses H 1  and H 2  of the first sliding part  1  and second sliding part  2  (in other words, vent hole height H 3  should be decreased). 
         [0008]    Next, torque stress will be explained referring to  FIGS. 5A to 7B . Like  FIGS. 1A and 1B ,  FIG. 5A  shows the planar structure of the disc rotor and  FIG. 5B  is a sectional view taken along the line A-A′ in  FIG. 1A .  FIG. 6A  is a perspective view of the disc rotor and  FIG. 6B  is a partially enlarged perspective view thereof.  FIG. 7A  shows the shape of an conventional vent hole and  FIG. 7B  is a sectional view taken along the line G-G′ in  FIG. 7A . 
         [0009]    In braking the vehicle, the pads  3  are pressed against the disc rotor  20  rotating together with the wheel and bell housing (not shown) and the disc rotor  20  receives a frictional force from the contact surface  7  of each pad  3  in the opposite direction to the rotor rotation direction. When the relative motion of the disc rotor  20  and pad  3  is seen from the rotating bell housing, it can be said that the disc rotor  20  remains still because it is fixed on the bell housing with pins (not shown) and relatively speaking, the pad  3  is rotating. In this condition, it may be considered that the disc rotor  20 , displacement of which is restricted by the pins, receives a frictional force generated by friction with the pad  3  (the direction of the frictional force is opposite to the rotor rotation direction) from its surface. In order to find stress distribution of the disc rotor  20 , a shear stress (circumferential-axial shear stress) along the circumferential direction was applied to the surface  7  of contact between the disc rotor  20  and pad  3  ( FIGS. 5A and 5B ) and stress analysis was conducted using the finite element method on the condition that displacement around the pin holes  4  was restricted. As a consequence, a large stress was found at point E ( FIGS. 6A and 6B ) in the inner peripheral corner of each vent hole (the corner is on the rib  6  and the first sliding part  1  connected to the bell housing, and also on the opposite side to the rotation direction  9  of the disc rotor  20  with respect to the left-right center of the vent hole). 
         [0010]    Generally, reduction of stress in a corner of a structure can be achieved by increasing the radius of the corner, so the stress at point E can be reduced by decreasing R 1  ( FIG. 7A ) and R 2  ( FIG. 7B ). However, this approach has the following problem.  FIG. 7A  shows the disc rotor as seen from direction F in  FIG. 5A  and  FIG. 7B  is a sectional view taken along the line G-G′ in  FIG. 7A . For the conventional vent hole shape shown in  FIG. 7A , R 1  is limited to half of vent hole height H 3  or less. Hence, vent hole height H 3  must be increased in order to reduce the stress at point E by increasing R 1 . This means that thicknesses H 1  and H 2  of the first sliding part  1  and second sliding part  2  must be decreased. 
         [0011]    As described above, for reduction in pad pressure stress, the vent hole height should be decreased and for reduction in torque stress, the vent hole height should be increased in order to increase vent hole corner radius R 1 . In other words, a problem with the conventional vent hole shape as shown in  FIGS. 7A and 7B  is that when the vent hole height is increased to reduce torque stress (disc thickness is decreased), the pad pressure stress largely increases in inverse proportion to the square of the disc thickness. 
         [0012]    So far the problem with conventional vent hole shapes exemplified in the vent hole shape shown in  FIG. 7A  has been discussed. Another example is disclosed in JP-A No. 11 (1999)-257386 where the vent hole is square or has curved corners on the second sliding part side. In this vent hole shape, since the corner corresponding to point E in  FIG. 6B  has no radius (no curve), torque stress is larger than in the shape shown in  FIG. 7A . 
         [0013]      FIG. 2  in JP-A No. 59 (1984)-194139 shows that in the vent hole shape, the first sliding part side corner has a smaller radius than the second sliding part side corner. In this shape, the corner corresponding to point E in  FIG. 6B  has a smaller radius than in the vent hole shape shown in  FIG. 7A , so torque stress is larger than in the vent hole shape shown in  FIG. 7A . 
         [0014]    Although the problem discussed so far is related to disc rotors for ceramic disc brakes, a solution to the problem can also reduce stress which is applied to a cast iron disc rotor during braking. 
         [0015]    The present invention has been made in view of the above related art and has an object to provide a vent hole shape which reduces stress generated by braking torque (torque stress) as mentioned above and also prevents an increase in stress generated by compression with pads (pad pressure stress). For this purpose, the invention provides such a vent hole shape that the radius of the inner peripheral corner of the vent hole is larger while the vent hole height is constant. 
       SUMMARY OF THE INVENTION 
       [0016]    In order to achieve the above object, according to one aspect of the present invention, there is provided a disc rotor for a disc brake which includes a first sliding part connected to a bell housing, a second sliding part located parallel to, and spaced in an axle direction from, the first sliding part, a plurality of ribs circumferentially spaced between the sliding parts as a pair, and vent holes formed by the ribs and the paired sliding parts. In the disc rotor, an inner peripheral shape of each of the vent holes has at least two arc shapes with different curvature radii at an end perpendicular to the rotation direction of the disc rotor, the smallest curvature radius Rs is 2 mm or more, and an arc curvature radius on the first sliding part side is larger than an arc curvature radius on the second sliding part side. 
         [0017]    According to the invention, since the inner peripheral corner of the vent hole, where torque stress in braking is relatively large, can have a larger radius while the vent hole height is constant, torque stress can be reduced. 
         [0018]    Similarly, torque stress in braking can be reduced while the vent hole height is constant. In this case, even when the vent hole shape is changed, the width W of the beam part of the sliding part is the same as in the conventional structure, so an increase in pad pressure stress due to the change in the vent hole shape can be suppressed more effectively than with the above structure. In addition, since the radius of the inner peripheral corner of the disc rotor is larger than in the conventional shape, torque stress can be further reduced. 
         [0019]    Alternatively, in the disc rotor, at an end perpendicular to the rotation direction of the disc rotor on an opposite side to the disc rotation direction, the arc curvature radius on the first sliding part side may be smaller than the arc curvature radius on the second sliding part side. 
         [0020]    According to a second aspect of the invention, in the disc rotor, an inner peripheral shape of each of the vent holes includes at least two arc shapes with different curvature radii at an end perpendicular to the rotation direction of the disc rotor, an arc curvature radius on the first sliding part side is smaller than an arc curvature radius on the second sliding part side, the smallest curvature radius is 2 mm or more, and a curvature radius larger than the smallest curvature radius Rs is as large as 1.5 times or more of Rs. 
         [0021]    According to a third aspect of the invention, in the disc rotor, an inner peripheral shape of each of the vent holes includes at least two arc shapes with different curvature radii at an end perpendicular to the rotation direction of the disc rotor. At the end on the same side as the rotation direction of the disc rotor with respect to a left-right center in a circumferential direction of each vent hole, an arc curvature radius on the first sliding part side is smaller than an arc curvature radius on the second sliding part side, and at the end on the opposite side to the rotation direction the disc rotor with respect to the left-right center in the circumferential direction of each vent hole, an arc curvature radius on the first sliding part side is larger than an arc curvature radius on the second sliding part side, and the smallest curvature radius Rs is 2 mm or more. 
         [0022]    Preferably an inner peripheral corner of a connection between the first sliding part connected to the bell housing and each of the ribs has a curvature radius R larger than the height of the vent hole. 
         [0023]    According to a fourth aspect of the invention, in the disc rotor, the vent hole&#39;s shape is oval as slanted toward the rotation direction of the disc rotor. Regarding the slanting direction, a pointed part of the oval on the rotation direction side of the disc rotor is at a lower position than the rest of the oval (closer to the first sliding part). 
         [0024]    Preferably the material of the disc rotor is ceramic. Alternatively the material of the disc rotor may be cast iron. Preferably at least the first sliding part, the second sliding part, and the ribs are united. Specifically these members are united by casting or molding. 
         [0025]    Preferably the inner peripheral shape of the vent hole is rotationally symmetric in its plane. As experimentally demonstrated, in order to minimize torque stress, it is preferable that the vent hole shape is rotationally symmetric (for example, the shapes shown in  FIGS. 13 and 17 ). 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]      FIGS. 1A and 1B  show the general structure of a conventional disc rotor with a pad, in which  FIG. 1A  is a plan view and  FIG. 1B  is a sectional view; 
           [0027]      FIG. 2  is a side view of the disc rotor shown in  FIG. 1A , as seen from direction B; 
           [0028]      FIG. 3A  shows that a structure for one period in the periodic structure including vent holes and ribs as shown in  FIG. 2  is deformed as a result of compression from above and below and  FIG. 3B  shows main stress distribution of the deformed structure; 
           [0029]      FIG. 4  illustrates the structure for one period in the periodic structure including vent holes and ribs as shown in  FIG. 2 ; 
           [0030]      FIG. 5A  is a plan view of the whole disc rotor excluding the pad shown in  FIG. 1A  and  FIG. 5B  is a sectional view thereof; 
           [0031]      FIG. 6A  shows the general structure of the conventional disc rotor and  FIG. 6B  is a partially enlarged view thereof; 
           [0032]      FIG. 7A  is a sectional view showing the inner peripheral shape of the vent holes shown in  FIG. 6A  and  FIG. 7B  is a sectional view taken along the line G-G′ in  FIG. 7A ; 
           [0033]      FIG. 8  shows the general structure of a disc rotor according to a first embodiment of the present invention; 
           [0034]      FIG. 9  shows a cross section of the disc rotor shown in  FIG. 8  in combination with an enlarged view of its vent hole shape; 
           [0035]      FIG. 10  shows the general structure of the disc rotor according to the first embodiment of the present invention; 
           [0036]      FIG. 11  is a graph of comparison in torque stress between the conventional vent hole shape and the vent hole shape in the first embodiment; 
           [0037]      FIG. 12  shows the general structure of a disc rotor shape according to a second embodiment of the present invention; 
           [0038]      FIG. 13  shows a cross section of the disc rotor shown in  FIG. 12  in combination with an enlarged view of its vent hole shape; 
           [0039]      FIG. 14A  shows the conventional vent hole shape,  FIG. 14B  shows the vent hole shape in the second embodiment, and  FIG. 14C  shows the vent hole shape in the first embodiment; 
           [0040]      FIG. 15  shows the boundary condition for two-dimensional analysis of pad pressure stress; 
           [0041]      FIG. 16  is a graph showing the result of two-dimensional analysis of pad pressure stress; 
           [0042]      FIG. 17  shows a vent hole shape in the second embodiment; and 
           [0043]      FIG. 18  shows another example of a radial-axial cross section of the vent hole shapes in the first and second embodiments. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0044]    Next, the preferred embodiments of the present invention will be described in detail referring to the accompanying drawings. In all the drawings that illustrate the preferred embodiments, elements with like functions are designated by like reference numerals and repeated descriptions of such elements are omitted. 
       First Embodiment 
       [0045]    The first embodiment of the present invention is described below referring to  FIGS. 8 and 9 .  FIG. 8  is a perspective view of the disc rotor according to the first embodiment where the whole disc rotor assembly including a bell housing  21  is shown.  FIG. 9  shows a cross section of a disc rotor  20  and its vent hole shape in enlarged form. As the disc rotor material, an aluminum alloy with dispersed cast iron or ceramic particles or carbon fiber reinforced silicon carbide (C/SiC) is chosen. As the bell housing material, iron, aluminum alloy or titanium is chosen. Although  FIG. 8  shows that the disc rotor  20  and bell housing  21  are separate from each other, it is also possible that the disc rotor  20  and bell housing  21  are integrally molded. Alternatively, the bell housing  21  may lie over the disc rotor  20  shown in  FIG. 8 . 
         [0046]    The disc rotor  20  ( FIG. 8 ) is connected to the bell housing  21  through pins (not shown) and the bell housing  21  is connected to the wheel (not shown). In braking the vehicle, pads are pressed against the disc rotor  20  to apply a braking torque to the disc rotor  20  in the opposite direction to the rotor rotation direction  9  and this braking torque is transmitted through the bell housing  21  to the wheel so that the wheel rotation speed decreases. In this process, the kinetic energy of the vehicle and wheel is converted into frictional heat between the pads and disc rotor  20 , resulting in a rise in the temperature of the disc rotor  20 . For this reason, vent holes  5  are provided in the disc rotor  20  to allow cooling air to flow therein. The upper illustration in  FIG. 9  shows the shape of the vent holes ( FIG. 8 ) as seen from the inner periphery of the disc rotor  20 . 
         [0047]    Next, the vent hole shape ( FIG. 9 ) characteristic of the present invention and its effect will be explained in detail. 
         [0048]    First, the shape of the vent hole  5  is described below. The vent hole inlet shape has two types of arcs with different curvature radii where the radius (R 3  in  FIG. 9 ) of the vent hole inlet corner on the first sliding part  1  side is larger than the radius (R 4  in  FIG. 9 ) of the vent hole inlet corner on the second sliding part  2  side. With this shape, R 3  can be larger than half of the vent hole height H 3  and torque stress can be reduced while the vent hole height is kept constant. As a concrete example, if the vent hole height H 3  is 8 mm, R 3  and R 4  may be 6 mm and 2 mm respectively. 
         [0049]    The effect of this embodiment is as follows. In order to investigate how much this embodiment reduces torque stress, a shear stress τzθ (the direction of stress is opposite to the rotor rotation direction  9 ) was applied to the contact surfaces  7  of the disc rotor  20  with the upper and lower pads  3  along the circumferential direction and stress analysis was conducted using the finite element method in the condition that displacement around the pin holes  4  was restricted. The material, which was used for the disc rotor  20  in this test is C/SiC which has a Young&#39;s modulus of 35 GPa and a Poisson&#39;s ratio of 0.14. The stress distribution of the conventional vent hole shape ( FIGS. 6A and 6B ) and that of the vent hole shape in this embodiment ( FIG. 9 ) were calculated. Comparison in maximum main stress among ribs a through e ( FIG. 10 ) is shown in  FIG. 11 . The maximum stress with the conventional vent hole shape is used as standard in  FIG. 11 . The graph indicates that the vent hole shape in this embodiment reduces stress by 20% in comparison with the conventional vent hole shape. Thus it has been demonstrated that this embodiment reduces torque stress effectively even when the vent hole height is the same as that of the conventional vent hole shape. Although the disc rotor structure shown in  FIGS. 8 and 9  has a vent hole shape which is uniform in the radial direction from the inner periphery to the outer periphery, even a disc rotor structure with radially varying vent hole widths and heights will produce an effect similar to the above. 
         [0050]    The disc rotor  20  shown in  FIG. 8  has a radial-axial cross section as illustrated in  FIG. 7B . Here, the radius R (R 2  in  FIG. 7B ) of the inner peripheral corner of the connection between the first sliding part  1  and rib  6  is smaller than the vent hole height H 3  but it may be larger than H 3  like the shape shown in  FIG. 18  (the corner here means a corner in a r-z plane in the cylindrical coordinate system representing the disc rotor where r denotes the radial direction and z denotes the axial direction). In the shape shown in  FIG. 18 , the radius R of the corner at point E in  FIG. 6B , where torque stress is relatively large, can be larger than in the shape shown in  FIG. 9 , so torque stress can be smaller than in the disc rotor structure shown in  FIG. 9 . 
       Second Embodiment 
       [0051]    The second embodiment of the present invention will be described referring to  FIG. 12  to  FIG. 14C .  FIG. 12  is a perspective view of a disc rotor shape according to the second embodiment where the whole disc rotor assembly including a bell housing  21  is shown.  FIG. 13  shows a cross section of the disc rotor  20  and its vent hole shape in enlarged form.  FIG. 14A  shows the conventional vent hole shape,  FIG. 14B  shows the vent hole shape in this (second) embodiment, and  FIG. 14C  shows the vent hole shape in the first embodiment. Next, the vent hole shape ( FIG. 14B ) characteristic of this embodiment and its effect will be explained in detail. 
         [0052]    First, the shape of the vent hole  5  is described below. As with the vent hole shape in the first embodiment ( FIG. 14C ), with the vent hole shape in the second embodiment ( FIG. 14B ), the radius R of the corner with relatively large torque stress (point G in  FIG. 14B ) is large and torque stress is thus smaller than with the conventional shape. The vent hole shape shown in  FIG. 14B  includes two different curvature radii R 5  and R 6  (R 5 &gt;R 6 ) and width W of the beam part  8  of the disc (described earlier) can be equal to that in  FIG. 14A , so the pad pressure stress with the vent hole shape shown in  FIG. 14B  is almost equal to that in  FIG. 14A . In other words, with the vent hole shape shown in  FIG. 14B , the increase in pad pressure stress is smaller than in the first embodiment ( FIG. 14C ) even though the vent hole shape is changed to reduce torque stress. As a concrete example, if the vent hole height H 3  is 8 mm, R 5  and R 6  may be 6 mm and 2 mm respectively. 
         [0053]    The effect of this embodiment is as follows. In order to investigate the effect of this embodiment, as illustrated in  FIG. 15  which shows the two-dimensional structure  24  (simulated circumferential axial cross section of the disc rotor  20 ) for one period in the periodic structure  24  including vent holes  5  and ribs  6 , stress analysis was conducted using the finite element method in the condition that circumferential displacement on lines corresponding to periodic boundaries was restricted and a given pressure was applied from above and below the disc rotor. The material which was used for the disc rotor  20  in this test is C/SiC which has a Young&#39;s modulus of 35 GPa and a Poisson&#39;s ratio of 0.14. The vent hole shapes shown in  FIGS. 14A to 14C  were tested and comparison in maximum main stress among the three shapes is shown in  FIG. 16 . The maximum main stress with the conventional shape is used as standard in  FIG. 16 .  FIG. 16  indicates that the maximum main stress with the shape in the first embodiment ( FIG. 14C ) is 20% larger than with the conventional shape. The reason for this is that although the vent hole height in the first embodiment is equal to the vent hole height in the conventional shape, the width W of the disc beam part  8  in the shape in the first embodiment is larger than in the conventional shape. On the other hand, it is apparent that the difference in main stress between the conventional shape ( FIG. 14A ) and the shape in the second embodiment ( FIG. 14B ) is very small (2% or less). The reason for this is that the vent hole height and beam part  8  width W in the second embodiment are the same as those in the conventional shape. This means that the second embodiment can suppress an increase in pad pressure stress even though its vent hole shape is changed for the purpose of torque stress reduction. 
         [0054]    Although the vent hole shape shown in  FIGS. 12 ,  13 , and  14 B includes a linear portion  23 , it does not always have to include a linear portion. A curve may be used in place of the linear portion  23 , forming a slanted oval as shown in  FIG. 17 . 
         [0055]    The disc rotor shown in  FIG. 13  has a radial-axial cross section as illustrated in  FIG. 7B . Here, the radius R (R 2  in  FIG. 7B  of the inner peripheral corner of the connection between the first sliding part  1  and rib  6  is smaller than the vent hole height H 3  but it may be larger than H 3  like the shape shown in  FIG. 18  (the corner here means a corner in a r-z plane in the cylindrical coordinate system representing the disc rotor where r denotes the radial direction and z denotes the axial direction). In the shape shown in  FIG. 18 , the radius R of the corner at point E in  FIG. 6B , where torque stress is relatively large, can be larger than in the shape shown in  FIG. 13 , so torque stress can be smaller than in the disc rotor structure shown in  FIG. 13 . 
         [0056]    The invention made by the present inventors has been so far explained in reference to the preferred embodiments thereof. However, the invention is not limited thereto and it is obvious that these details may be modified in various ways without departing from the spirit and scope of the invention.