Patent Publication Number: US-2022235828-A1

Title: Wet friction plate and wet multiple disc clutch device provided with wet friction plate

Description:
TECHNICAL FIELD 
     The present invention relates to a wet friction plate used in lubricant oil. Specifically, the present invention relates to a wet friction plate suitable for a wet multiplate clutch device arranged between a motor and a drive target to be rotatably driven by the motor to transmit drive force of the motor to the drive target or block such transmission. Moreover, the present invention relates to a wet multiplate clutch device including the wet friction plates. 
     BACKGROUND ART 
     Typically, on a vehicle such as a four-wheeled vehicle or a two-wheeled vehicle, a wet multiplate clutch device is mounted for transmitting rotary drive force of a motor such as an engine to a drive target such as a wheel or block such transmission. Generally, the wet multiplate clutch device transmits the rotary drive force or blocks such transmission in such a manner that two plates arranged facing each other in lubricant oil are pressed against each other. 
     In this case, one of two plates includes a wet friction plate having a friction member provided along a circumferential direction on a surface of a flat plate annular core metal. For example, Patent Literature 1 below discloses a wet friction member (hereinafter referred to as a “wet friction plate”) having a papermaking groove (hereinafter referred to as a “lubricant oil recessed portion”) depressed in a recessed shape and formed at a surface of a friction member. With this configuration, in the wet friction plate, lubricant oil adhering to a surface of the wet friction plate is easily discharged. Thus, drag torque can be reduced. 
     CITATION LIST 
     Patent Literature 
     
         
         PATENT LITERATURE 1: JP-A-2007-263203 
       
    
     However, in the wet friction plate described in Patent Literature 1, a support layer of the lubricant oil recessed portion into which the lubricant oil enters is compressed and deformed by pressing or cutting for forming the lubricant oil recessed portion. Thus, tendency shows that lubricant oil discharge performance is degraded. For this reason, in the typical wet friction plate, specifically in a case where the lubricant oil is at a low temperature or a case where the pressure of contact between the wet friction plate and a clutch plate is low, a problem that it is difficult to discharge the lubricant oil is caused. 
     The present invention copes with the above-described problem. An object of the present invention is to provide a wet friction plate capable of improving the performance for discharging lubricant oil adhering to a surface of a friction member and a wet multiplate clutch device including the wet friction plates. 
     SUMMARY OF INVENTION 
     In order to achieve the object described above, a feature of the present invention is a wet friction plate which includes: a friction member including a friction slide surface and multiple lubricant oil recessed portions depressed in a recessed shape with respect to the friction slide surface, the friction slide surface and the lubricant oil recessed portions being formed at a surface of a porous layer having multiple hollow portions; and a core metal formed in a flat plate annular shape and provided with the friction member along a circumferential direction. The friction member is configured such that a surface of each lubricant oil recessed portion has no sharp portion in a pointed shape, such as a corner portion, and formed with a smooth continuous surface and a formation rate of the hollow portions in the porous layer forming the lubricant oil recessed portions and a formation rate of the hollow portions in the porous layer forming the friction slide surface are identical to each other. 
     According to the feature of the present invention having such a configuration, a wet friction plate has no sharp portions in a pointed shape, such as corner portions, at the surfaces of the lubricant oil recessed portions. That is, the surface of the lubricant oil recessed portion is formed with the smooth continuous surface. In addition, the porous layer forming the lubricant oil recessed portion and the porous layer forming the friction slide surface are formed to have the same formation rate of the hollow portion. Thus, lubricant oil penetration and discharge performance of the porous layer is the same between the friction slide surface and the lubricant oil recessed portion. Thus, the performance for discharging lubricant oil adhering to a surface of the friction member can be improved. 
     Note that the same formation rate of the hollow portion between the porous layer forming the lubricant oil recessed portion and the porous layer forming the friction slide surface does not mean only perfect match of the formation rate of the hollow portion between these layers. A case where a difference in the formation rate of the hollow portion between the layers falls within such a range (e.g., a difference of equal to or less than ±10%) that a difference in the lubricant oil penetration and discharge performance between the layers falls within a predetermined range (e.g., a difference of equal to or less than ±10%) taken as the substantially same penetration and discharge performance is also included in the same formation rate of the hollow portion. Moreover, the smooth continuous surface forming the surface of the lubricant oil recessed portion may be a lubricant oil recessed portion surface including an inclined surface having a linearly-extending flat surface. Note that the surface of the lubricant oil recessed portion preferably includes the curved surface depressed in the recessed shape. 
     Further, another feature of the present invention is that each lubricant oil recessed portion is configured such that at least one of two sections in directions perpendicular to each other as viewed in plane is formed in an arc shape with one curvature. 
     According to another feature of the present invention having such a configuration, in the wet friction plate, the lubricant oil recessed portion is formed such that the section along at least one of two directions perpendicular to each other as viewed in plane has the arc shape with one curvature. Thus, the lubricant oil recessed portion can be easily formed. 
     Further, still another feature of the present invention is that, in the wet friction plate, each lubricant oil recessed portion is formed in a long hole shape or an oval shape as viewed in plane. 
     According to still another feature of the present invention having such a configuration, in the wet friction plate, the lubricant oil recessed portion is formed in the bottomed long hole shape or oval shape as viewed in plane. Thus, the lubricant oil recessed portion can be formed while a necessary friction contact area can be ensured without locally causing a great missing portion of the friction slide surface at the friction slide surface. 
     Further, still another feature of the present invention is that, in the wet friction plate, each lubricant oil recessed portion is formed to linearly extend, and adjacent ones of the lubricant oil recessed portions are formed in directions perpendicular to each other. 
     According to still another feature of the present invention having such a configuration, the wet friction plate is formed such that the lubricant oil recessed portion linearly extends. In addition, adjacent ones of the lubricant oil recessed portions are formed in the directions perpendicular to each other. Thus, weakening of the durability of the friction member in a specific direction can be prevented, and uniform durability can be ensured. Moreover, the lubricant oil recessed portions are formed in directions crossing a rotary drive direction of the wet friction plate. This can improve friction resistance. In addition, the lubricant oil recessed portion is formed to extend outwardly in a radial direction of the wet friction plate. Thus, the lubricant oil discharge performance by the centrifugal force can be ensured. 
     Still another feature of the present invention is that in the wet friction plate, the lubricant oil recessed portions are formed with multiple types of depths. 
     According to still another feature of the present invention having such a configuration, in the wet friction plate, the lubricant oil recessed portions are formed with the multiple types of depths. Thus, in a case where abrasion of the friction member is accelerated and the entire thickness of the friction member decreases, the lubricant oil recessed portions with small depths are brought into a state close to disappearance. Thus, degradation of the durability of the friction member can be reduced. In this case, a similar advantageous effect can be expected in such a manner that the lubricant oil recessed portions of the wet friction plate are formed with multiple types of groove widths instead of or in addition to formation of the lubricant oil recessed portions with the multiple types of depths. 
     The present invention can be implemented not only as the invention relating to the wet friction plate, but also can be implemented as the invention relating to a wet multiplate clutch including the wet friction plates and the method for manufacturing the wet friction plate. 
     Specifically, it is preferred that a wet multiplate clutch device includes: an opposing plate arranged facing a drive plate to be rotatably driven by a motor through a clearance and lubricant oil. It is also preferred that the opposing plate and the drive plate contact each other or separate from each other to transmit rotary drive force between the opposing plate and the drive plate or block transmission of the rotary drive force, and at least one of the drive plate or the opposing plate is the wet friction plate according to any one of claims  1  to  5 . It can be expected that he wet multiplate clutch device having such a configuration provides features and advantageous effects similar to those of the wet friction plate. 
     The method for manufacturing the wet friction plate is the method for manufacturing a wet friction plate including a friction member having a friction slide surface and multiple lubricant oil recessed portions depressed in a recessed shape with respect to the friction slide surface, the friction slide surface and the lubricant oil recessed portions being formed at a surface of a porous layer having multiple hollow portions, and a core metal formed in a flat plate annular shape and provided with the friction member along a circumferential direction. This method may include the original shape formation step of forming, in a sheet shape, the slurry of a raw material including a fibrous material forming the porous layer, the water content adjustment step of reducing the water content of the sheet-shaped raw material to equal to or lower than 90% and equal to or higher than 50%, the lubricant oil recessed portion formation step of pressing a lubricant oil recessed portion molding die with a surface as a smooth continuous surface with no sharp portions in a pointed shape, such as corner portions, against the sheet-shaped raw material having the adjusted water content to form the lubricant oil recessed portions, and the drying step of reducing the water content of the sheet-shaped raw material having the formed lubricant oil recessed portions to equal to or lower than 10%. According to the wet friction plate manufacturing method having such a configuration, the wet friction plate can be manufactured. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a sectional view of an entire configuration of a wet multiplate clutch device including wet friction plates according to one embodiment of the present invention; 
         FIG. 2  is a schematic plan view of an outer appearance of the wet friction plate according to one embodiment of the present invention, the wet friction plate being assembled in the wet multiplate clutch device illustrated in  FIG. 1 ; 
         FIG. 3  is image data obtained in such a manner that an image of the cross section of a friction member forming the wet friction plate illustrated in  FIG. 2  is captured by a scanning electron microscope; 
         FIGS. 4A to 4C  schematically illustrate the lubricant oil recessed portion at the friction member forming the wet friction plate illustrated in  FIG. 2 ,  FIG. 4A  being a plan view of the lubricant oil recessed portion,  FIG. 4B  being a cross-sectional view of the lubricant oil recessed portion, and  FIG. 4C  being a longitudinal sectional view of the lubricant oil recessed portion; 
         FIGS. 5A and 5B  are image data obtained in such a manner that image data trimmed from the image data illustrated in  FIG. 3  within trimming frames TF 1 , TF 2  is binarized,  FIG. 5A  illustrating a porous layer right below a friction slide surface and  FIG. 5B  illustrating a porous layer right below the lubricant oil recessed portion; 
         FIG. 6  is a bar graph showing, for each of the friction slide surface and the lubricant oil recessed portion, a formation rate of a hollow portion in the friction member illustrated in  FIGS. 5A and 5B  and a formation rate of a hollow portion in a friction member according to the prior art; 
         FIG. 7  is image data obtained in such a manner that an image of the cross section of the friction member according to the prior art is captured by the scanning electron microscope; 
         FIGS. 8A and 8B  are image data obtained in such a manner that image data trimmed from the image data illustrated in  FIG. 7  within trimming frames TF 1 , TF 2  is binarized,  FIG. 8A  illustrating a porous layer right below a friction slide surface and  FIG. 8B  illustrating a porous layer right below a lubricant oil recessed portion; 
         FIG. 9  is a schematic view for describing the main steps of manufacturing the wet friction plate and the friction member illustrated in  FIG. 2 ; 
         FIG. 10  is a schematic partially-enlarged view of an external configuration of a lubricant oil recessed portion molding die attached to press rollers illustrated in  FIG. 9 ; 
         FIG. 11  is a partial plan view of an external configuration of a wet friction plate according to a variation of the present invention; 
         FIG. 12  is a partial plan view of an external configuration of a wet friction plate according to another variation of the present invention; 
         FIG. 13  is a partial plan view of an external configuration of a wet friction plate according to still another variation of the present invention; and 
         FIG. 14  is a partial plan view of an external configuration of a wet friction plate according to still another variation of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, one embodiment of a wet friction plate, a wet multiplate clutch device including the wet friction plates, and the method for manufacturing the wet friction plate according to the present invention will be described with reference to the drawings.  FIG. 1  is a schematic sectional view of an entire configuration of a wet multiplate clutch device  100  including wet friction plates  200  according to the present invention. Note that for the sake of easy understanding of the present invention, each figure as a reference in the present specification schematically illustrates the configuration of the invention by, e.g., exaggerating some components. Thus, in some cases, the dimensions of each component, a ratio between components or the like. in the figures might be different from actual dimensions, ratios or the like. The wet multiplate clutch device  100  is a mechanical device configured to transmit drive force of an engine (not shown) as a motor of a two-wheeled vehicle (a motorcycle) to a wheel (not shown) as a drive target or block such transmission. The wet multiplate clutch device  100  is arranged between the engine and a transmission (not shown). 
     (Configuration of Wet Multiplate Clutch Device  100 ) 
     The wet multiplate clutch device  100  includes an aluminum alloy housing  101 . The housing  101  is a member formed in a bottomed cylindrical shape and forming part of a housing of the wet multiplate clutch device  100 . An input gear  102  is, with a rivet  102   b , fixed to a left side surface of the housing  101  as viewed in the figure through a torque damper  102   a . The input gear  102  is rotatably driven by engaging with a not-shown drive gear to be rotatably driven by drive of the engine. On an inner peripheral surface of the housing  101 , each of multiple (eight in the present embodiment) clutch plates  103  is, by spline fitting, held along an axial direction of the housing  101  in a state in which the clutch plates  103  are displaceable and rotatable integrally with the housing  101 . 
     The clutch plate  103  is a flat plate annular component to be pressed against the later-described wet friction plate  200 . The clutch plate  103  is formed in such a manner that a thin plate member made of a SPCC (cold-rolled steel plate) material is punched in an annular shape. A later-described not-shown oil groove having a depth of several μm to several tens of μm for holding lubricant oil is formed at each side surface (front and back surfaces) of the clutch plate  103 . For the purpose of improving abrasion resistance, surface hardening treatment is performed for each side surface (the front and back surfaces) of the clutch plate  103  at which the oil groove is formed. Note that such surface hardening treatment does not directly relate to the present invention, and therefore, description thereof will be omitted. 
     A friction plate holder  104  formed in a substantially cylindrical shape and provided concentrically with the housing  101  is arranged inside the housing  101 . At an inner peripheral surface of the friction plate holder  104 , many spline grooves are formed along an axial direction of the friction plate holder  104 . A shaft  105  is spline-fitted in the spline grooves. The shaft  105  is a shaft body formed in a hollow shape, and one (the right side as viewed in the figure) end portion thereof rotatably supports the input gear  102  and the housing  101  through a needle bearing  105   a . In addition, the shaft  105  supports the spline-fitted friction plate holder  104  in a fixed manner through a nut  105   b . That is, the friction plate holder  104  rotates integrally with the shaft  105 . On the other hand, the other (the left side as viewed in the figure) end portion of the shaft  105  is coupled to the not-shown transmission of the two-wheeled vehicle. 
     A shaft-shaped push rod  106  is arranged in a hollow portion of the shaft  105 . The push rod  106  penetrates the hollow portion of the shaft  105  to protrude from one (the right side as viewed in the figure) end portion of the shaft  105 . The side (the left side as viewed in the figure) of the push rod  106  opposite to the end portion protruding from one (the right side as viewed in the figure) end portion of the shaft  105  is coupled to a not-shown clutch operation lever of the two-wheeled vehicle. By operation of the clutch operation lever, the push rod  106  slides in the hollow portion of the shaft  105  along an axial direction of the shaft  105 . 
     On an outer peripheral surface of the friction plate holder  104 , adjacent ones of the multiple (seven in the present embodiment) wet friction plates  200  sandwich the clutch plate  103 . The wet friction plate  200  is, by spline fitting, held along the axial direction of the friction plate holder  104  in a state in which the wet friction plate  200  is displaceable and rotatable integrally with the friction plate holder  104 . 
     The inside of the friction plate holder  104  is filled with a predetermined amount of lubricant oil (not shown). In addition, each of three tubular support rods  104   a  is formed inside the friction plate holder  104  (the figure illustrates only one). The lubricant oil is supplied to among the wet friction plates  200  and the clutch plates  103 . With this configuration, absorption of friction heat generated among the wet friction plates  200  and the clutch plates  103  and abrasion of friction members  210  are prevented. 
     Each of three tubular support rods  104   a  is formed to protrude outwardly (the right side as viewed in the figure) in the axial direction of the friction plate holder  104 . A pressing cover  107  arranged at a position concentric with the friction plate holder  104  is assembled with the tubular support rods  104   a  through a bolt  108   a , a receiving plate  108   b , and a coil spring  108   c . The pressing cover  107  is formed in a substantially discoid plate having the substantially same outer diameter as the outer diameter of the wet friction plate  200 , and is pressed toward a friction plate holder  104  side by the coil spring  108   c . A release bearing  107   a  is provided at a position facing a right tip end portion of the push rod  106  as viewed in the figure at a center portion inside the pressing cover  107 . 
     (Configuration of Wet Friction Plate  200 ) 
     Specifically, as illustrated in  FIG. 2 , the wet friction plate  200  includes oil grooves  203  and the friction members  210  provided on a flat plate annular core metal  201 . The core metal  201  is a member as a base portion of the wet friction plate  200 . The core metal  201  is formed in such a manner that a thin plate member made of a SPCC (cold-rolled steel plate) material is punched in a substantially annular shape. In this case, an internal-tooth spline  202  is formed at an inner peripheral portion of the core metal  201  such that the core metal  201  is spline-fitted to the friction plate holder  104 . 
     At a side surface of the wet friction plate  200  facing the clutch plate  103 , i.e., a side surface of the core metal  201  facing the clutch plate  103 , the multiple ( 32  in the present embodiment) small piece-shaped friction members  210  are provided along a circumferential direction of the core metal  201  through the oil grooves  203  forming clearances. 
     The oil groove  203  is a flow path configured to guide the lubricant oil between an inner peripheral edge and an outer peripheral edge of the core metal  201  of the wet friction plate  200 . In addition, the oil groove  203  is also an oil holding portion configured to hold the lubricant oil between the wet friction plate  200  and the clutch plate  103 . Each oil groove  203  is formed to linearly extend between adjacent ones of the multiple small piece-shaped friction members  210 . 
     The friction member  210  improves friction force for the clutch plate  103 . The friction member  210  is made of a small piece-shaped paper material bonded along the circumferential direction of the core metal  201 . As illustrated in  FIG. 3 , the friction member  210  includes a hard porous layer  211 . The porous layer  211  is obtained in such a manner that thermosetting resin with which a paper base material is impregnated is hardened. 
     In this case, the paper base material includes a filler added to at least one of organic fibers or inorganic fibers. The organic fibers may be made of one or more types of fibers such as wood pulp, synthetic pulp, polyester-based fibers, polyamide-based fibers, polyimide-based fibers, modified polyvinyl alcohol fibers, polyvinyl chloride fibers, polypropylene fibers, polybenzimidazole fibers, acrylic fibers, carbon fibers, phenol fibers, nylon fibers, and cellulose fibers. Moreover, the inorganic fibers may be made of one or more types of fibers such as glass fibers, rock wool, potassium titanate fibers, ceramic fibers, silica fibers, silica-alumina fibers, kaolin fibers, bauxite fibers, kayanoid fibers, boron fibers, magnesia fibers, and metal fibers. 
     Moreover, the filler fulfills a function as a friction modifier and/or a solid lubricant. Such a filler may be made of one or more types of materials such as barium sulfate, calcium carbonate, magnesium carbonate, silicon carbide, boron carbide, titanium carbide, silicon nitride, boron nitride, alumina, silica, zirconia, cashew dust, rubber dust, diatom earth, graphite, talc, kaolin, magnesium oxide, molybdenum disulfide, nitrile rubber, acrylonitrile-butadiene rubber, styrene-butadiene rubber, silicon rubber, and fluorine-containing rubber. Further, examples of the thermosetting resin include phenol-based resin, melamine resin, epoxy resin, urea resin, and silicone resin. 
     A number of hollow portions  212  are formed at the porous layer  211 . The hollow portions  212  are pore portions formed in clearances among the materials including the paper base material and the thermosetting resin. Neither the paper base material nor the thermosetting resin is present in the pore portions. The lubricant oil adhering to surfaces of the friction members  210  penetrates and flows in the pore portions, or is held in the pore portions. Many hollow portions  212  with various sizes are randomly formed inside the porous layer  211 . In this case, the hollow portions  212  include hollow portions  212  opening at a surface of the porous layer  211  and hollow portions  212  formed inside the porous layer  211  without opening at the surface of the porous layer  211 . Moreover, the hollow portions  212  include adjacent hollow portions  212  communicated with each other and adjacent hollow portions  212  not communicated with each other. 
     As illustrated in each of  FIGS. 4A to 4C , a friction slide surface  213  and lubricant oil recessed portions  214  are formed at the surface of the porous layer  211 . The friction slide surface  213  is a portion configured to friction-slide in contact with the clutch plate  103 . The friction slide surface  213  includes a flat surface. The lubricant oil recessed portion  214  is a portion configured to increase the friction resistance of the friction member  210  and hold the lubricant oil. The multiple lubricant oil recessed portions  214  are depressed in a recessed shape with respect to the friction slide surface  213 . In this case, the recessed surface of each lubricant oil recessed portion  214  has no sharp portions in a pointed shape, such as corner portions. That is, the recessed surface of the lubricant oil recessed portion  214  is formed with a smooth continuous surface. 
     In the present embodiment, each lubricant oil recessed portion  214  is formed in a long hole shape as viewed in plane. More specifically, the lubricant oil recessed portion  214  is formed in such a shape that in a longitudinal sectional shape along a longitudinal direction of the lubricant oil recessed portion  214 , a deepest portion of the recessed portion linearly extends in the horizontal direction and both end portions of the deepest portion gently curve toward the friction slide surface  213  and incline in a curved shape. Moreover, the lubricant oil recessed portion  214  is formed with an arc cross-sectional shape along a width direction perpendicular to the longitudinal direction. In this case, the lubricant oil recessed portions  214  include lubricant oil recessed portions  214  formed with multiple types of sizes including multiple types of lengths in the longitudinal direction, lengths in the width direction, and depths of the deepest portion. In the present embodiment, the lubricant oil recessed portions  214  are formed as lubricant oil recessed portions with multiple types of sizes of about 1.0 to 2.0 mm in the longitudinal direction, about 0.6 to 0.7 mm in the width direction, and 0.25 to 0.4 mm in depth. 
     Of these lubricant oil recessed portions  214 , adjacent lubricant oil recessed portions  214  are formed to extend in directions perpendicular to each other through the friction slide surface  213 . Of the lubricant oil recessed portions  214  adjacent to each other in the longitudinal direction and the width direction, one lubricant oil recessed portion  214  is formed to extend in a direction perpendicular to an adjacent lubricant oil recessed portion  214  in the present embodiment. That is, in the present embodiment, the lubricant oil recessed portions  214  are formed in a mesh shape across the entire surface of the friction slide surface  213  as viewed in plane. 
     At the porous layer  211 , the hollow portions  212  are formed with the same formation rate between the portion forming the friction slide surface  213  and the portion forming the lubricant oil recessed portion  214 . The formation rate of the hollow portion  212  as described herein is the rate of the hollow portion  212  in a certain space in the porous layer  211 . The formation rate of the hollow portion  212  can be calculated from the amount of fluid injected into the porous layer  211  by pressure injection, such as oil, mercury, or helium. Alternatively, the formation rate of the hollow portion  212  can be approximately calculated by image processing using a digital image obtained by imaging of the porous layer  211 . Specifically, the formation rate of the hollow portion  212  can be calculated by the image processing of binarizing images acquired by imaging of each section of the porous layer  211  right below the friction slide surface  213  and the porous layer  211  right below the lubricant oil recessed portion  214 . 
     First, a worker cuts the lubricant oil recessed portion  214  of the friction member  210  in a cross-sectional direction. Thereafter, digital image data (hereinafter merely referred to as “image data”) is acquired in such a manner that the state of the cross section is imaged with a predetermined magnification (e.g., ×120) by means of a magnification imaging device such as a scanning electron microscope (see  FIG. 3 ). In this case, the worker captures a state image including the section of the friction slide surface  213  formed around the lubricant oil recessed portion  214  targeted for imaging. 
     Next, the worker performs, using a computer device capable of processing the image data, such as a personal computer, trimming for image data of a portion right below the lubricant oil recessed portion  214  and image data of a portion right below the friction slide surface  213  in the acquired image data within trimming frames TF 1 , TF 2  with predetermined sizes. In this case, the trimming frame TF 1  defining the trimming area of the image data of the portion right below the lubricant oil recessed portion  214  can be formed in a quadrangular shape as a whole. This shape has a length of at least equal to or greater than ⅓ of the width of the lubricant oil recessed portion  214  with respect to the center position of the portion right below the lubricant oil recessed portion  214  in the width direction thereof and a length of at least equal to or greater than 70% of the thickness of the porous layer  211  right below the lubricant oil recessed portion  214 . 
     The trimming frame TF 2  defining the trimming area of the image data of the portion right below the friction slide surface  213  is formed to have the same size as that of the trimming frame TF 1 . In addition, the trimming frame TF 2  is arranged sufficiently apart from the lubricant oil recessed portion  214  at the portion right below the friction slide surface  213 . In this case, the trimming frames TF 1 , TF 2  are arranged at positions close to each surface of the lubricant oil recessed portion  214  and the friction slide surface  213 . Note that in the present embodiment, the trimming frame TF 1 , TF 2  is formed with 330 pixels×660 pixels. 
     Next, as illustrated in each of  FIGS. 5A and 5B , the worker binarizes two pieces of image data trimmed within the trimming frames TF 1 , TF 2  by means of image processing software (a well-known computer program) capable of binarizing the image data. In this case, a threshold for accurately converting image data of the hollow portion  212  in the image data into the black or the white is set to the image processing software. With this configuration, the worker can acquire the binarized image data of the portion right below the lubricant oil recessed portion  214  and the binarized image data of the portion right below the friction slide surface  213 . In the present embodiment, the hollow portion  212  is converted into the black. In addition, the porous layer  211  other than the hollow portions  212  is converted into the white. 
     Next, the worker calculates, using image processing software (a well-known computer program) capable of calculating each area of the black portion and the white portion in the binarized image data, the cumulative total of the area of the portion (the black portion) equivalent to the hollow portions  212  in the binarized image data. Such total calculation is performed for each trimming frame TF 1 , TF 2 . 
     In this manner, when a difference between the total area of the hollow portions  212  within the trimming frame TF 1  on the image data and the total area of the hollow portions  212  within the trimming frame TF 2  on the image data falls within a predetermined range, the worker determines that the formation rate of the hollow portion  212  is the same between these portions. The difference falling within the predetermined range as described herein preferably means that a difference between both totals is equal to or less than a range of ±10%. Note that the worker can also check the identicalness of the formation rate by comparison of the rate of the total area of the hollow portions  212  to the total area of the trimming frame TF 1  or the rate of the total area of the hollow portions  212  to the total area of the porous layer  211  (the white portion) excluding the hollow portions  212  within the trimming frame TF 1  with a similar total area rate in the trimming frame TF 2 . 
     Verification results obtained by the inventor(s) of the present invention will be described herein.  FIG. 6  is, for the friction member  210  according to the invention of the present application and a friction member  90  according to the prior art, a bar graph of the cumulative value of the area of the hollow portions  212  obtained by the image processing by binarization. The horizontal axis represents the total area of the hollow portions  212  for each of the friction slide surface  213  and the lubricant oil recessed portion  214  in the friction member  210  and the total area of hollow portions  92  for each of a friction slide surface  93  and a lubricant oil recessed portion  94  in the friction member  90 . Moreover, the vertical axis represents the rate of the total area of the hollow portions  212  to the entire area of each of the trimming frames TF 1 , TF 2 . 
     As illustrated in  FIG. 7 , the lubricant oil recessed portion  94  in a recessed shape is formed at the friction member  90  in such a manner that a die is pressed against a surface of a porous layer  91  having the same quality as that of the porous layer  211 . In this case, at the lubricant oil recessed portion  94 , a corner portion with an angle of about 120° is formed at each end portion of a bottom portion in a cross-sectional direction. Note that  FIG. 7  shows an image of only one (the left side as viewed in the figure) of two corner portions. The non-imaged other corner portion is formed at a right end portion of this image as viewed in the figure. 
     For the friction member  90 , the inventor(s) of the present invention has acquired image data by imaging of the porous layer  91  in a manner similar to that for the friction member  210 . Thereafter, as illustrated in  FIGS. 8A and 8B , the acquired image data is trimmed within trimming frames TF 1 , TF 2  and is binarized. Then, the inventor(s) of the present invention calculates, in a manner similar to that described above, the cumulative total of the area of a portion (a black portion) equivalent to the hollow portions  92  in the binarized image data. Note that from the image data illustrated in  FIGS. 7, 8A and 8B , it can be also confirmed that the porous layer  91  as a support layer forming the lubricant oil recessed portion  94  is more compressed and crushed in the upper-lower direction as viewed in the figure than the porous layer  91  as a support layer forming the friction slide surface  93  in the friction member  90 . 
     According to the verification results obtained by the inventor(s) of the present invention, the formation rate of the hollow portion  212  right below the friction slide surface  213  in the friction member  210  is 27.4%. The formation rate of the hollow portion  212  right below the lubricant oil recessed portion  214  is 26.1%. Moreover, the formation rate of the hollow portion  92  right below the friction slide surface  93  in the friction member  90  is 28.8%. The formation rate of the hollow portion  92  right below the lubricant oil recessed portion  94  is 21.2%. Note that each of these formation rates is the average of multiple samples. 
     (Manufacturing of Wet Friction Plate  200 ) 
     Next, the method for manufacturing the wet friction plate  200  configured as described above will be described with reference to  FIGS. 9 and 10 . First, the worker performs the raw material mixing step of mixing a raw material. Specifically, the worker injects the raw material of the paper base material, i.e., the organic fibers and/or the inorganic fibers, the filler, and a flocculant, into water in a mixing tank  300 , and thereafter, mixes such a material to generate the raw material in the form of slurry. Subsequently, the worker transfers the slurry of the raw material in the mixing tank  300  to a papermaking tank  301  by means of a pump  300   a.    
     Next, the worker performs a papermaking step. The papermaking step mainly includes an original shape formation step, a water content adjustment step, a lubricant oil recessed portion formation step, and a drying step. Specifically, the worker rotatably drives a delivery device  303  including an endless belt-shaped papermaking net  302  arranged facing the inside of the papermaking tank  301 . Then, the worker delivers the raw material filtered in the form of a sheet from the inside of the papermaking tank  301  to a pair of press rollers  305  (the original shape formation step). The delivery device  303  includes, among the papermaking tank  301  and the press rollers  305 , a water absorbing roller  304   a  and a suction box  304   b . With this configuration, moisture is removed from the sheet-shaped raw material on the papermaking net  302  (the water content adjustment step). 
     The press rollers  305  are components configured to form the lubricant oil recessed portions  214  at the sheet-shaped raw material on the papermaking net  302 . The press rollers  305  include a pair of rollers arranged facing each other. In this case, a lubricant oil recessed portion molding die  305   a  formed of metal or resin threads woven in a grid shape as illustrated in  FIG. 10  is wound around a surface of one of two rollers forming the press rollers  305 . The press rollers  305  are provided such that the water content (% by weight) of the sheet-shaped raw material on the path of the papermaking net  302  falls within a range of 90% to 50%. In the present embodiment, the press rollers  305  are provided such that the water content (% by weight) of the sheet-shaped raw material on the path of the papermaking net  302  falls within a range of 60% to 50%. 
     Thus, after having drawn up from the papermaking tank  301 , the sheet-shaped raw material adjusted to have a predetermined water content passes through the press rollers  305 . In this manner, the friction slide surface  213  and the lubricant oil recessed portions  214  are formed at the surface facing the lubricant oil recessed portion molding die  305   a  (the lubricant oil recessed portion formation step). In this case, the sheet-shaped raw material is adjusted to have a relatively-high water content. In addition, the lubricant oil recessed portion molding die  305   a  is formed with a smooth curved surface. This reduces the absence of the hollow portions  212  due to compression deformation of a portion of the friction member  210  pressed against the lubricant oil recessed portion molding die  305   a . Thus, a difference in the formation rate of the hollow portion  212  in the porous layer  211  between the friction slide surface  213  and the lubricant oil recessed portion  214  can be reduced. Note that according to experiment by the inventor(s) of the present invention, the lubricant oil recessed portion molding die  305   a  includes the woven resin threads, and therefore, flexibility can be more easily ensured as compared to the case of including the metal threads. Moreover, compression deformation of the porous layer  211  right below the lubricant oil recessed portion  214  can be reduced. 
     Subsequently, the sheet-shaped raw material at which the friction slide surface  213  and the lubricant oil recessed portions  214  are formed passed through a drying device  306  including a drying roller arranged on a downstream side of the press rollers  305 . In this manner, the water content is further decreased to 10% or lower (the drying step). In this case, the water content of the sheet-shaped raw material at which the friction slide surface  213  and the lubricant oil recessed portions  214  are formed is preferably equal to or higher than 3%. Thereafter, the sheet-shaped raw material is rolled up by a recovery roller  307 , and the papermaking step ends. 
     Next, the worker performs a hardening step. Specifically, the worker impregnates, with the thermosetting resin, the sheet-shaped raw material dried until the water content reaches 10% or lower. Thereafter, the worker hardens the sheet-shaped raw material while heating the sheet-shaped raw material and pressing and shaping the sheet-shaped raw material. In this manner, the worker can manufacture the friction member  210  including the porous layer  211  hardened in a state in which the friction slide surface  213  and the lubricant oil recessed portions  214  are formed. 
     Next, the worker performs the step of bonding the friction members  210 . Specifically, the worker bonds, with an adhesive, small pieces of the friction members  210  to the surface of the core metal  201  produced by machining such as pressing at another step along the circumferential direction. In this case, the worker may bond the friction members  210  cut in a small piece shape in advance to the core metal  201 . Alternatively, the worker can cut the friction member  210  in a small piece shape when bonding the friction member  210  to the core metal  201 . In this manner, the worker can manufacture the wet friction plate  200  configured such that the small piece-shaped friction members  210  are bonded to both surfaces of the core metal  201  along the circumferential direction through the oil grooves  203 . Note that the steps of manufacturing the wet friction plate  200  may include, for example, machining steps other than above and the steps of adjusting and examining friction characteristics. Note that these steps do not directly relate to the present invention, and therefore, description thereof will be omitted. 
     (Actuation of Wet Friction Plate  200 ) 
     Next, actuation of the wet friction plate  200  configured as described above will be described. As described above, the wet friction plates  200  are, upon use, assembled in the wet multiplate clutch device  100 . The wet multiplate clutch device  100  is arranged between the engine and the transmission of the vehicle as described above. By operation of the clutch operation lever by a driver of the vehicle, the wet multiplate clutch device  100  transmits the drive force of the engine to the transmission, or blocks such transmission. 
     That is, in a case where the driver (not shown) of the vehicle operates the clutch operation lever (not shown) to retreat (displace to the left side as viewed in the figure) the push rod  106 , the tip end portion of the push rod  106  does not press the release bearing  107   a . Thus, the pressing cover  107  presses the clutch plates  103  by elastic force of the coil spring  108   c . Accordingly, the clutch plates  103  and the wet friction plates  200  are in a friction-coupled state. In this state, all plates are pressed against the outer peripheral surface of the friction plate holder  104  while displacing toward a receiving portion  104   b  formed in a flange shape. As a result, the drive force of the engine transmitted to the input gear  102  is transmitted to the transmission through the clutch plates  103 , the wet friction plates  200 , the friction plate holder  104 , and the shaft  105 . 
     On the other hand, in a case where the driver of the vehicle operates the clutch operation lever (not shown) to advance (displace to the right side as viewed in the figure) the push rod  106 , the tip end portion of the push rod  106  presses the release bearing  107   a . Thus, the pressing cover  107  displaces to the right side as viewed in the figure against the elastic force of the coil spring  108   c . Then, the pressing cover  107  and the clutch plates  103  are separated from each other. Accordingly, the clutch plates  103  and the wet friction plates  200  displace to a pressing cover  107  side, and the state in which all plates are pressed against each other and coupled to each other is cancelled. Thus, all plates are separated from each other. As a result, the drive force is no longer transmitted from the clutch plates  103  to the wet friction plates  200 . As a result, transmission of the drive force, which is transmitted to the input gear  102 , of the engine to the transmission is blocked. 
     In this state in which the clutch plates  103  and the wet friction plates  200  friction-contact each other, the lubricant oil present on the surfaces of the friction members  210  of the wet friction plates  200  is pushed by the clutch plates  103 . Then, part of the lubricant oil is discharged to the outside of the friction members  210  through outer edge portions thereof. In addition, another part of the lubricant oil penetrates the friction members  210 . In this case, the lubricant oil penetrating the friction members  210  includes lubricant oil held in the lubricant oil recessed portions  214  and lubricant oil penetrating the porous layers  211 . Part of the lubricant oil penetrating the porous layers  211  is discharged from end surfaces of the porous layers  211  (the friction members  210 ) through the hollow portions  212 . In addition, another part of the lubricant oil remains in the hollow portions  212 . 
     In this case, there are no sharp portions in the pointed shape, such as the corner portions, at the surfaces of the lubricant oil recessed portions  214  holding the lubricant oil. That is, these surfaces are formed with the smooth continuous surfaces. In addition, the above-described surfaces are formed such that the formation rate of the hollow portion  212  in the porous layer  211  forming the lubricant oil recessed portion  214  and the formation rate of the hollow portion  212  in the porous layer  211  forming the friction slide surface  213  are the same as each other. With this configuration, a difference in lubricant oil penetration and discharge performance of the porous layer  211  is small between the friction slide surface  213  and the lubricant oil recessed portion  214  in the wet multiplate clutch device  100 . Thus, the performance for discharging the lubricant oil adhering to the surface of the friction member  210  is improved. Moreover, temperature characteristics (cooling characteristics) and surface pressure characteristics can be stabilized. As a result, the durability of the friction member  210  can be improved. 
     In the wet multiplate clutch device  100 , when the clutch plates  103  and the wet friction plates  200  are separated from each other, the amount of lubricant oil remaining among the clutch plates  103  and the wet friction plates  200  is less than that in the prior art. Thus, the wet multiplate clutch device  100  can reduce a state in which the clutch plates  103  and the wet friction plates  200  are indirectly connected to each other due to the lubricant oil present among these plates, i.e., drag torque, upon clutch-OFF. 
     As can be understood from description of actuation above, according to the above-described embodiment, there are no sharp portions in the pointed shape, such as the corner portions, at the surface of the lubricant oil recessed portion  214  of the wet friction plate  200 . That is, such a surface is formed with the smooth continuous surface. In addition, the above-described surface is formed such that the formation rate of the hollow portion  212  in the porous layer  211  forming the lubricant oil recessed portion  214  and the formation rate of the hollow portion  212  in the porous layer  211  forming the friction slide surface  213  are the same as each other. Thus, the lubricant oil penetration and discharge performance of the porous layer  211  is the same between the friction slide surface  213  and the lubricant oil recessed portion  214 . Thus, the performance for discharging the lubricant oil adhering to the surface of the friction member  210  can be improved. 
     Further, implementation of the present invention is not limited to the above-described embodiment, and various changes can be made without departing from the object of the present invention. Note that in each variation described below, reference numerals corresponding to those assigned to the wet friction plate  200  are used to represent components similar to those of the wet friction plate  200  in the above-described embodiment. Moreover, description thereof will be omitted. 
     For example, in the above-described embodiment, the wet friction plates  200  are held by the friction plate holder  104  to be rotatably driven integrally with the shaft  105 . That is, the wet friction plate  200  is applied as an opposing plate arranged facing the clutch plate  103  to be rotatably driven by the rotary drive force of the engine and rotatably driven integrally with the shaft  105  as an output shaft in the wet multiplate clutch device  100 . However, the wet friction plate  200  can be also applied to the clutch plate  103  as a drive plate to be rotatably driven by the rotary drive force of the engine. 
     Moreover, in the above-described embodiment, the lubricant oil recessed portion  214  is formed such that the cross-sectional shape thereof is a single arc shape. However, it may only be required that the surface of the lubricant oil recessed portion  214  has no sharp portions in the pointed shape, such as the corner portions, and is formed with the smooth continuous surface. Thus, the lubricant oil recessed portion  214  may include a linear portion as included in the longitudinal section of the lubricant oil recessed portion  214 . Alternatively, the lubricant oil recessed portion  214  may be formed to have a curved surface with two or more curves. 
     Further, in the above-described embodiment, the lubricant oil recessed portion  214  is formed in the long hole shape as viewed in plane. However, it may only be required that the surface of the lubricant oil recessed portion  214  has no sharp portions in the pointed shape, such as the corner portions, and is formed with the smooth continuous surface. Thus, as illustrated in  FIG. 11 , the lubricant oil recessed portion  214  can be formed in a dimple shape recessed in a hemispherical shape. Alternatively, the lubricant oil recessed portion  214  can be formed in an oval shape. As illustrated in  FIG. 12 , the lubricant oil recessed portions  214  can be formed in shapes including the long hole shape and the oval shape. Alternatively, as illustrated in  FIG. 13 , the lubricant oil recessed portion  214  can be formed to linearly extend in a radial direction of the core metal  201  and penetrate the friction member  210 . Alternatively, the lubricant oil recessed portion  214  can be formed to extend in the circumferential direction and a tangential direction instead of or in addition to the radial direction of the core metal  201 . Alternatively, as illustrated in  FIG. 14 , the lubricant oil recessed portions  214  can be formed to extend in a grid shape or a mesh shape in two directions crossing each other. Alternatively, the lubricant oil recessed portion  214  can be formed in a curved shape instead of or in addition to the linear shape as viewed in the plane. 
     In addition, in the above-described embodiment, the lubricant oil recessed portions  214  are formed to have the multiple types of sizes including the multiple types of lengths in the longitudinal direction, lengths in the width direction, and depths. However, all of the lubricant oil recessed portions  214  can be formed in the same shapes. 
     Moreover, at the original shape formation step of the above-described embodiment, the endless belt-shaped papermaking net  302  is used to form the raw material in the sheet shape extending in a band shape. However, it may only be required that the papermaking net  302  forms the raw material in the sheet shape. Thus, the papermaking net  302  may be formed in a quadrangular shape or a circular shape. In this case, the papermaking net  302  formed in the quadrangular shape or the circular shape can be housed in a bottomed tubular molding die, and can form the raw material in a quadrangular or circular sheet shape. 
     Further, in the above-described embodiment, the water content of the sheet-shaped raw material is adjusted to equal to or lower than 60% and equal to or higher than 50% at the water content adjustment step. However, it may only be required that at the water content adjustment step, the water content of the sheet-shaped raw material is adjusted to equal to or lower than 90% and equal to or higher than 50%. Preferably, such a water content is adjusted to equal to or lower than 70% and equal to or higher than 50%. With this configuration, the lubricant oil recessed portion  214  can be easily formed with high accuracy. 
     In addition, in the above-described embodiment, at the lubricant oil recessed portion formation step, the lubricant oil recessed portions  214  are formed by the lubricant oil recessed portion molding die  305   a  formed in a roll shape. However, at the lubricant oil recessed portion formation step, the planar lubricant oil recessed portion molding die  305   a  can be pressed against the raw material to form the lubricant oil recessed portions  214 . 
     Moreover, in the above-described embodiment, the lubricant oil recessed portion molding die  305   a  includes the resin threads woven in the grid shape. However, it may only be required that the lubricant oil recessed portion molding die  305   a  forms the lubricant oil recessed portions  214  on the raw material. Thus, the lubricant oil recessed portion molding die  305   a  may be made of a resin material or a metal material formed in a grid shape by processing such as injection molding. Alternatively, the lubricant oil recessed portion molding die  305   a  can be formed of a resin or metal plate-shaped body having multiple recessed-raised portions or through-holes formed for formation of the lubricant oil recessed portions  214 . 
     Further, in the above-described embodiment, the example where the wet friction plate according to the present invention is applied as the wet friction plate  200  used for the wet multiplate clutch device  100  has been described. However, it may be only required that the wet friction plate according to the present invention is a wet friction plate used in oil. In addition to the wet multiplate clutch device  100 , the wet friction plate according to the present invention can be applied as a wet friction plate used for a brake device configured to put a brake on rotary motion by the motor. 
     LIST OF REFERENCE SIGNS 
     
         
         TF 1  Trimming frame for acquiring image data of portion right below lubricant oil recessed portion 
         TF 2  Trimming frame for acquiring image data of portion right below friction slide surface 
           90  Typical friction member 
           91  Porous layer 
           92  Hollow portion 
           93  Friction slide surface 
           94  Lubricant oil recessed portion 
           100  Wet multiplate clutch device 
           101  Housing 
           102  Input gear 
           102   a  Torque damper 
           102   b  Rivet 
           103  Clutch plate 
           104  Friction plate holder 
           104   a  Tubular support rod 
           105  Shaft 
           105   a  Needle bearing 
           105   b  Nut 
           106  Push rod 
           107  Pressing cover 
           107   a  Release bearing 
           108   a  Bolt 
           108   b  Receiving plate 
           108   c  Coil spring 
           200  Wet friction plate 
           201  Core metal 
           202  Spline 
           203  Oil groove 
           210  Friction member 
           211  Porous layer 
           212  Hollow portion 
           213  Friction slide surface 
           214  Lubricant oil recessed portion 
           300  Mixing tank 
           300   a  Pump 
           301  Papermaking tank 
           302  Papermaking net 
           303  Delivery device 
           304   a  Water absorbing roller 
           304   b  Suction box 
           305  Press roller 
           305   a  Lubricant oil recessed portion molding die 
           306  Drying device 
           307  Recovery roller