Patent Publication Number: US-2012043177-A1

Title: Friction-clutch system

Description:
TECHNICAL FIELD 
     This invention relates to a friction-clutch system for transmitting a force from a driving shaft to a driven shaft. Particularly, this invention relates to a friction-clutch system used for motor vehicles, which have an engine, such as four-wheeled vehicles and two-wheeled vehicles (motorcycles). 
     BACKGROUND OF THE INVENTION 
     Clutch systems are used in motor vehicles for connecting and disconnecting the power from a power source, such as engines of motor vehicles, by means of mechanical contact of friction materials. The friction-clutch system may transmit power by means of a friction force generated between two surfaces of the friction materials. The power can be disconnected by separating the two surfaces of the friction materials. The friction-clutch system is frequently used for motor vehicles (four-wheeled and two-wheeled vehicles), because the friction-clutch system can smoothly connect and disconnect the source of power. 
       FIG. 1  shows a sectional view of a conventional friction-clutch system that is used for four-wheeled vehicles (it shows a sectional plane that includes an axis of a central hub). 
     By referring to  FIG. 1 , below the conventional friction-clutch system  101  is explained. 
     Incidentally, to facilitate understanding of the system  101 , a release bearing, a release arm, etc., are omitted in  FIG. 1 . 
     The conventional friction-clutch system  101  comprises a clutch cover  107 , a flywheel  103  connected to an output shaft (a crankshaft) of an engine, a friction disk unit  113 , a pressure plate  115 , and a diaphragm spring  117 . 
     The friction disk unit  113  comprises a central hub  105  (in which a main shaft [not shown] of a transmission is inserted. The central hub  105  includes torsion springs  109 ), a friction disk  111 , and rivets  119  for connecting the friction disk  111  to the central hub  105 . 
     The clutch cover  107  comprises a case, which has a cylindrical shape with a lid and without a bottom plate (namely, its lower surface is opened and its upper surface is closed by a lid that has a central opening  107   h ). The central opening  107   h  has a circular shape, and its center is located on the central axis of the cylindrical case. The diaphragm spring  117 , which has a plurality of reed-shaped elements, is disposed around the central opening  107   h . (Each reed-shaped element of the diaphragm spring  117  is fixed to the lid by means of a screw  118  so that the reed-shaped element can rotate about the axis of the screw  118  within a predetermined angle.) 
     By pushing the protruding portion  115   a  of the pressure plate  115  by means of the outer end  117   a  of the diaphragm spring  117 , the pressure plate  115  is pressed toward the flywheel  103 . Thus, the spring force generated by the diaphragm spring  117  presses the friction disk  111  toward the flywheel  103 . Consequently, the power is transmitted from the flywheel  103  to the friction disk  111 . Namely, this state means that the clutch is “ON,” in which state the power is transmitted from the flywheel  103  to the central hub  105  through the friction disk  111  and the rivets  119 . 
     Clutch facings are attached to both sides of the portion of the friction disk  111  that contacts the flywheel  103  and the pressure plate  115 , to transmit the power from the flywheel  103  to the central hub  105  without slipping. 
     The inner end  117   b  of the diaphragm spring  117  contacts a release bearing (not shown). The inner end  117   b  of the diaphragm spring  117  is moved toward the flywheel  103  by moving the release bearing toward the flywheel  103  by means of the release arm (not shown). Thus, the outer end  117   a  of the diaphragm spring  117  separates from the protruding portion  115   a  of the pressure plate  115 . Consequently, the spring force, which presses the pressure plate  115  toward the friction disk  111  so that the friction disk  111  presses the flywheel  103 , is eliminated. 
     Thus, since the friction disk  111  is not pressed toward the flywheel  103 , no power is transmitted from the flywheel  103  to the friction disk  111 . Namely, this state means that the clutch is “OFF,” in which state no power is transmitted from the flywheel  103  to the central hub  105 . 
     For such a friction-clutch system, it is required that the size of the system be compact, and that great power can be transmitted. Particularly, for the friction-clutch system used for motor vehicles, since it is required to maximize the space in the vehicles and to reduce their weight, it tends to be required that the size of the friction-clutch system be compact, and that great power can be transmitted. The tendency requires that the stable transmission of the power can be achieved within a wide range of the operation of the friction-clutch system, such as the range from a low temperature to a high temperature, the range from low speed to high speed, and the range from a low power to a high power. To achieve the requirements, efforts by trial and error have been carried out. 
     It has been considered to improve the performance of the material of the friction disk  111 , which contacts the flywheel  103  and the pressure plate  115 , so that high power can be transmitted at a high temperature and a high speed. 
     Particularly, since the friction-clutch system used for motor vehicles for racing is used under hard conditions, the system must be able to transmit high power at a high temperature and a high speed. 
     To provide such a friction-clutch system that can transmit high power at a high temperature and a high speed, there is available a friction-clutch system that has its friction disks made from a carbon fiber carbon composite material (see Patent Document 1). 
     The coefficient of friction of the carbon fiber carbon composite material tends to increase as the temperature of its surface increases. 
     Thus, by applying the carbon fiber reinforced carbon composite material (“C/C composite material”) to the friction disks, the friction-clutch system can transmit high power at a high temperature and a high speed. Since the weight and inertia moment is reduced, the friction disks can easily respond to any change of the rate of the rotation. 
     Further, when a carbon/graphite material, like the carbon fiber carbon composite material, is used for the friction disks, since the coefficient of friction of the carbon fiber carbon composite material decreases at a low temperature, it becomes a problem in that the friction-clutch system cannot transmit enough power at a low temperature. To solve this problem, friction disks that are made of different materials, such as a combination of a carbon/graphite material and a metal/ceramics material, are used for the friction-clutch system (see Patent Document 2). 
     Patent Document 1: Japanese Patent Laid-open Publication No. 2002-181072 
     Patent Document 2: Japanese Patent Laid-open Publication No. 2009-97690 
     DISCLOSURES OF INVENTION 
     However, if a carbon/graphite material, such as a C/C composite material, is used for the friction disks, a problem such that the coefficient of friction of the friction disks is reduced at a low temperature, cannot be solved. Further, even if such different materials made from a combination of a carbon/graphite material and a metal/ceramics material are used for the friction disks, a friction-clutch system having an excellent performance cannot necessarily be provided. 
     Thus, it has been required to provide a friction-clutch system that can stably transmit a torque over the wide range of pressure on a friction surface while maintaining a high coefficient of friction, and reducing wear. 
     This invention was conceived in the circumstances explained in the above paragraphs. Namely, the purpose of this invention is to provide a friction-clutch system that uses a carbon/graphite material, such as a C/C composite material, and that can stably transmit power over the wide range of pressure on a friction surface, and a friction disk unit that can be used for the friction-clutch system. 
     To solve the problem explained in the above paragraphs, the first aspect of the invention has the following constitution: 
     a friction-clutch system comprising: 
     a flywheel rotated by an inputted power, 
     a clutch cover attached to the flywheel, and 
     a friction disk unit rotated together with the flywheel by being pressed toward the flywheel, 
     wherein the clutch cover comprises a pressure plate for pressing the friction disk unit toward the flywheel and a spring for forcing the pressure plate against the friction disk unit, 
     wherein the friction disk unit comprises a central hub for outputting the power transmitted from the flywheel and for being connected to the central portion of the friction disk unit, and friction disks for transmitting the rotational power by friction force and for being sandwiched between the flywheel and the pressure plate, 
     wherein the friction disks have a first friction surface facing the flywheel and a second friction surface facing the pressure plate, 
     wherein the flywheel has a third friction surface facing the first friction surface of the friction disks, 
     wherein the pressure plate has a fourth friction surface facing the second friction surface of the friction disks, 
     wherein at least one friction surface of the first to fourth friction surfaces is constituted of different materials, i.e., comprised of a combination of a carbon/graphite material and a metal/ceramics material, 
     wherein the carbon/graphite material forms a friction surface made from a carbon/graphite material, and a metal/ceramics material forms a friction surface made from a metal/ceramics material, 
     wherein the ratio of [the area of the friction surface made from the metal/ceramics material]/[the area of the friction surface made from the metal/ceramics material+the area of the friction surface made from the carbon/graphite material] ranges from 0.05˜0.65, and 
     wherein the pressure between the first and the third friction surface and between the second and the fourth friction surface is 10˜250 N/cm 2  while the power is being transmitted. 
     The second aspect of the invention has the following constitution: 
     The friction-clutch system of the first aspect of the invention, 
     wherein the ratio of [the area of the friction surface made from the metal/ceramics]/[the area of the friction surface made from the metal/ceramics+the area of the friction surface made from the carbon/graphite] ranges from 0.05˜0.40, and 
     wherein the pressure between the first and the third friction surface and between the second and the fourth friction surface is 10˜200 N/cm 2  while the power is being transmitted. 
     The third aspect of the invention has the following constitution: 
     The friction-clutch system of either the first or the second aspect of the invention, 
     wherein the first and the second friction surface is constituted of different materials, i.e., comprised of a combination of a carbon/graphite material and a metal/ceramics material, 
     wherein the carbon/graphite material forms a friction surface made from a carbon/graphite material, and the metal/ceramics material forms a friction surface made from a metal/ceramics material. 
     The fourth aspect of the invention has the following constitution: 
     The friction-clutch system of any one of the first to the third aspect of the invention, 
     wherein the carbon/graphite material is a C/C composite material. 
     The fifth aspect of the invention has the following constitution: 
     The friction-clutch system of any one of the first to the fourth aspect of the invention, 
     wherein the metal/ceramics material is a metal material selected from steel, brass, copper, aluminum, titanium, molybdenum, tungsten, tantalum, and alloys thereof, or a ceramics material selected from alumina, mullite, cordierite, zirconia, titanium oxide, silicon nitride, titanium nitride, aluminum nitride, silicon carbide, boron carbide, tungsten carbide, titanium boride, and sialon. 
     The sixth aspect of the invention has the following constitution: 
     The friction-clutch system of any one of the first to the fifth aspect of the invention, 
     wherein the friction disks comprise a first friction plate, which forms the first friction surface, a second friction plate, which forms the second friction surface, and a central plate for fixing the first and the second friction plate. 
     The seventh aspect of the invention has the following constitution: 
     The friction-clutch system of the sixth aspect of the invention, 
     wherein the first and the second friction plate and the central plate all have an annular shape, 
     wherein the first and the second friction plate are mechanically fixed to the central plate. 
     The eighth aspect of the invention has the following constitution: 
     The friction-clutch system of the seventh aspect of the invention, 
     wherein each of the first and the second friction plate and the central plate comprises a plurality of circumferentially-segmented plates. 
     The ninth aspect of the invention has the following constitution: 
     The friction-clutch system of any one of the sixth, the seventh, and the eighth aspect of the invention, 
     wherein the first friction plate and the second friction plate are made from the carbon/graphite material, which forms a friction surface made from a carbon/graphite material, and 
     wherein fasteners for mechanically fixing the first and the second friction plate to the central plate are made from a metal/ceramics material, which forms a friction surface made from a metal/ceramics material. 
     The tenth aspect of the invention has the following constitution: 
     The friction-clutch system of any one of the first to the ninth aspect of the invention, 
     wherein the friction surface made from a carbon/graphite material and the friction surface made from a metal/ceramics material form one plane. 
     The eleventh aspect of the invention has the following constitution: 
     A friction disk unit comprising the following: 
     a central hub for outputting the power transmitted from a flywheel and for being connected to the central portion of the friction disk unit, and friction disks for transmitting the rotational power by friction force, 
     wherein the friction disks have a first friction surface and a second friction surface, 
     wherein at least one friction surface of the first and the second friction surface is constituted of different materials, i.e., comprised of a combination of a carbon/graphite material and a metal/ceramics material, 
     wherein the carbon/graphite material forms a friction surface made from a carbon/graphite material, and a metal/ceramics material forms a friction surface made from a metal/ceramics material, 
     wherein the ratio of [the area of the friction surface made from the metal/ceramics material]/[the area of the friction surface made from the metal/ceramics material+the area of the friction surface made from the carbon/graphite material] ranges from 0.05˜0.65. 
     The twelfth aspect of the invention has the following constitution: 
     The friction disk unit of the eleventh aspect of the invention, 
     wherein the ratio of [the area of the friction surface made from the metal/ceramics material]/[the area of the friction surface made from the metal/ceramics material+the area of the friction surface made from the carbon/graphite material] ranges from 0.05˜0.40. 
     The thirteenth aspect of the invention has the following constitution: 
     The friction disk unit of either the eleventh or twelfth aspect of the invention, 
     wherein the carbon/graphite material is a C/C composite material. 
     The fourteenth aspect of the invention has the following constitution: 
     The friction disk unit of any one of the eleventh, the twelfth, and the thirteenth aspect of the invention, 
     wherein the metal/ceramics material is a metal material selected from steel, brass, copper, aluminum, titanium, molybdenum, tungsten, tantalum, and alloys thereof, or a ceramics material selected from alumina, mullite, cordierite, zirconia, titanium oxide, silicon nitride, titanium nitride, aluminum nitride, silicon carbide, boron carbide, tungsten carbide, titanium boride, and sialon. 
     The fifteenth aspect of the invention has the following constitution: 
     The friction disk unit of any one of the eleventh to the fourteenth aspect of the invention, 
     wherein the friction disk comprises a first friction plate, which forms the first friction surface, a second friction plate, which forms the second friction surface, and a central plate for fixing the first and the second friction plate. 
     The sixteenth aspect of the invention has the following constitution: 
     The friction disk unit of the fifteenth aspect of the invention, 
     wherein the first and the second friction plate and the central plate all have an annular shape, 
     wherein the first and the second friction plate are mechanically fixed to the central plate. 
     The seventeenth aspect of the invention has the following constitution: 
     The friction disk unit of the sixteenth aspect of the invention, 
     wherein each of the first and the second friction plate and the central plate comprises a plurality of circumferentially-segmented plates. 
     The eighteenth aspect of the invention has the following constitution: 
     The friction disk unit of any one of the fifteenth to the seventeenth aspect of the invention, 
     wherein the first friction plate and the second friction plate are made from the carbon/graphite material, which forms a friction surface made from a carbon/graphite material, and 
     wherein fasteners for mechanically fixing the first and the second friction plate to the central plate are made from the metal/ceramics material, which forms a friction surface made from a metal/ceramics material. 
     The nineteenth aspect of the invention has the following constitution: 
     The friction disk unit of any one of the eleventh to the eighteenth aspect of the invention, 
     wherein the friction surface made from a carbon/graphite material and the friction surface made from a metal/ceramics material form one plane. 
     EFFECTS OF THE INVENTIONS 
     As explained in the above paragraphs, the friction-clutch system and the friction disk unit of the present inventions have the following technical features: 
     At least one of the friction surfaces of the friction-clutch system and the friction disk unit is constituted of different materials, and are comprised of a combination of a carbon/graphite material (for example, a C/C composite material) and a metal/ceramics material, and 
     the ratio of [the area of the friction surface made from the metal/ceramics material]/[the area of the friction surface made from the metal/ceramics material+the area of the friction surface made from the carbon/graphite material] ranges within a predetermined scope. 
     Consequently, it becomes possible to provide a friction-clutch system and a friction disk unit that can achieve a high coefficient of friction over the wide range of pressure on the friction surface of the friction disks while reducing wear, and that can stably transmit a torque. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a sectional view of a conventional friction-clutch system (Prior Art). 
         FIG. 2  shows a sectional view of the friction-clutch system of the present invention. 
         FIG. 3  shows an enlarged sectional view of the friction disk of the friction-clutch system of the present invention (part A of  FIG. 2 ). 
         FIG. 4  shows an elevational view of one embodiment of the friction disk unit of the present invention. 
         FIG. 5  shows an elevational view of another embodiment of the friction disk unit of the present invention. 
         FIG. 5-2  shows an elevational view of yet another embodiment of the friction disk unit of the present invention. 
         FIG. 5-3  shows an elevational view of yet another embodiment of the friction disk unit of the present invention. 
         FIG. 6  shows an enlarged sectional view of the friction disk to explain another embodiment for fixing a friction plate to a central plate. 
     
    
    
     PREFERRED EMBODIMENTS OF THE INVENTION 
     Below, the preferred embodiments of the invention are explained by referring to the figures. The preferred embodiments explained below are just for exemplifying the invention. Thus, are not intended to limit the scope of the invention. 
     For this specification, the wording “a carbon/graphite material” means “a material comprising a carbon material or a graphite material,” and the wording “a metal/ceramics material” means “a material comprising a metal material or a ceramics material.” 
       FIG. 2  shows a sectional view of the friction-clutch system of the present invention, which view indicates a plane cut so as to contain the center of the axis of the central hub. To facilitate understanding of the invention, a release bearing, a release arm, etc., are omitted in  FIG. 2 . 
     The friction-clutch system  201  of this invention comprises a clutch cover  207 , a flywheel  203  connected to an output shaft (a crankshaft) of an engine, a friction disk unit  213 , a pressure plate  215 , and a diaphragm spring  217 . 
     The friction disk unit  213  comprises a central hub  205  (in which a main shaft [not shown] of a transmission is inserted, and which includes torsion springs  209 ), a friction disk  211 , and rivets  219  for connecting the friction disk  211  to the central hub  205 . 
     The clutch cover  207  comprises a case, which has a cylindrical shape with a lid and without a bottom plate (namely, its lower surface is opened and its upper surface is closed by a lid that has a central opening  207   h ). The central opening  207   h  has a circular shape, and its center is located on the central axis of the cylindrical case. A diaphragm spring  217 , which has a plurality of reed-shaped elements, is disposed around the central opening  207   h . (Each reed-shaped element of the diaphragm spring  217  is fixed to the lid by means of a screw  218  so that the reed-shaped element can rotate about the axis of the screw  218  within a predetermined angle.) 
     By pushing the protruding portion  215 a of the pressure plate  215  by means of the outer end  217   a  of the diaphragm spring  217 , the pressure plate  215  is pressed toward the flywheel  203 . Thus, the spring force generated by the diaphragm spring  217  presses the friction disk  211  toward the flywheel  203 . Consequently, the power is transmitted from the flywheel  203  to the friction disk  211 . Namely, this state means that the clutch is “ON,” in which state the power is transmitted from the flywheel  203  to the central hub  205  through the friction disk  211  and the rivets  219 . 
     The inner end  217   b  of the diaphragm spring  217  contacts a release bearing (not shown). The inner end  217   b  of the diaphragm spring  217  is moved toward the flywheel  203  by moving the release bearing toward the flywheel  203  by means of the release arm (not shown). Thus, the outer end  217   a  of the diaphragm spring  217  separates from the protruding portion  215   a  of the pressure plate  215 . Consequently, the spring force, which presses the pressure plate  215  toward the friction disk  211  so that the friction disk  211  presses the flywheel  203 , is eliminated. 
     Thus, since the friction disk  211  is not pressed toward the flywheel  203 , the power is not transmitted from the flywheel  203  to the friction disk  211 . Namely, this state means that the clutch is “OFF,” in which state the power is not transmitted from the flywheel  203  to the central hub  205 . 
       FIG. 3  shows an enlarged sectional view of the part (part A) of the friction disk  211  of the friction-clutch system  201  of  FIG. 2 . 
     For the embodiments shown in  FIGS. 2 and 3 , the friction disk  211  comprises a first friction plate  221 , a second friction plate  222 , a central plate  223 , a first friction bush  224 , and a second friction bush  225 , which bushes act as the mechanical fasteners. 
     The first friction plate  221  and the first friction bush  224  form a first friction surface. The second friction plate  222  and the second friction bush  225  form a second friction surface. These friction surfaces define the performance of the friction-clutch system  201 . 
     The first friction plate  221  and the second friction plate  222 , and the first friction bush  224  and the second friction bush  225 , may be made from different materials. Namely, the first friction plate  221  and the second friction plate  222  may be made from a carbon/graphite material, and the first friction bush  224  and the second friction bush  225  may be made from a metal/ceramics material. Further, the first friction plate  221  and the second friction plate  222  may be made from a metal/ceramics material, and the first friction bush  224  and the second friction bush  225  may be made from a carbon/graphite material. 
     The surface of the first friction plate  221  and that of the first friction bush  224 , and the surface of the second friction plate  222  and that of the second friction bush  225 , form one plane, so that the surface of the first friction plate  221  and that of the first friction bush  224 , and the surface of the second friction plate  222  and that of the second friction bush  225 , form one of the friction surfaces. 
       FIG. 3  shows one of the embodiments of the friction disk  211 . It has a friction surface that is constituted of different materials, i.e., comprised of a friction plate and a friction bush that are made from a carbon/graphite material and a metal/ceramics material, respectively. 
     For this embodiment, since the friction plate is made from the carbon/graphite material, and the friction bush is made from the metal/ceramics material, the friction plate forms the friction surface made from a carbon/graphite material, and the friction bush forms the friction surface made from a metal/ceramics material. 
     The friction bush is implanted in the friction plate. The friction bush may be mechanically and directly fastened to the friction plate. Otherwise, for example, the first friction bush  224 , having a female thread, and the second friction bush  225 , having a male thread, may be inserted into and fitted in a through hole disposed in the first friction plate  221 , the second friction plate  222 , and the central plate  223 , and then may be screwed to each other, so as to fix the first friction plate  221  and the second friction plate  222  to the central plate  223 . 
     For the embodiment explained in the above paragraphs, the first and the second friction plate  221 ,  222  are made from a carbon/graphite material, and the first and the second friction bush  224 ,  225  are made from a metal/ceramics material. However, the materials of these parts are not limited to them. The first and the second friction plate  221 ,  222  may be made from a metal/ceramics material, and the first and the second friction bush  224 ,  225  may be made from a carbon/graphite material. 
     The shapes and sizes of the friction surface made from a carbon/graphite material and the friction surface made from a metal/ceramics material, which are comprised of the friction plates and the friction bushes, are not limited to specific values. 
     Any widely-used industrial carbon material, any polycrystalline graphite, and any pyrolytic graphite, may all be used for the carbon/graphite material, which constitutes the friction surface made from a carbon/graphite material. 
     Further, a C/C composite (Carbon Fiber Carbon Composite) material, which is a composite material reinforced by a carbon fiber, may be used for the carbon/graphite material. The C/C composite material is preferable for the carbon/graphite material because of its excellent strength and wear resistance. The C/C composite material is composed of carbon fibers or graphite fibers, which act as reinforcing fibers, and a carbon material or a graphite material, which acts as a matrix. Many kinds of C/C composite materials can be used for a friction-clutch system. For example, the C/C composite material that is made by alternately repeating a processing step for impregnating a pitch into a substrate and a processing step for carbonizing and graphitizing the substrate by heat treating it, can be used for the friction-clutch system. Further, the C/C composite material that is made by densifying a substrate that is formed by carbon fibers or graphite fibers sterically placed by means of a chemical vapor deposition process can be used for the friction-clutch system. 
     For the C/C composite material used for the friction-clutch system, the following materials may be included in the C/C composite material: titanium, silicon, iron, tungsten, molybdenum, copper, aluminum, tin, lead, zinc, magnesium, steel alloys, copper alloys, aluminum alloys, titanium alloys, alumina, mullite, cordierite, zirconia, titanium oxide, silicon nitride, aluminum nitride, silicon carbide, titanium carbide, boron carbide, tungsten carbide, titanium boride, magnesium oxide, iron oxide, zinc oxide, chromium oxide, calcium carbonate, barium sulfate, etc. 
     The following materials may be used for the metal/ceramics material, which forms the friction surface made from a metal/ceramics material: steel, such as chrome molybdenum steel (SCM), brass, copper, aluminum, titanium, molybdenum, tungsten, tantalum, and any alloy that includes these materials, as a metal material, or alumina, mullite, cordierite, zirconia, titanium oxide, silicon nitride, titanium nitride, aluminum nitride, silicon carbide, titanium carbide, boron carbide, tungsten carbide, titanium boride, and sialon, as a ceramics material. 
     As explained in the above paragraphs, when the first and the second friction surface is constituted of different materials that are comprised of a combination of a carbon/graphite material and a metal/ceramics material, the third friction surface (the friction surface of the flywheel) and the fourth friction surface (the friction surface of the pressure plate) may be made from a metal material or a ceramics material as explained in the above paragraph. 
     Generally, for the friction surface formed by a C/C composite material, the coefficient of friction of the friction surface tends to increase as the temperature of its surface increases. Thus, when the temperature of the friction surface formed by the C/C composite material is relatively low, since the coefficient of friction of the friction surface is also low, no high power can be transmitted. 
     When the first and the second friction plate are made from the C/C composite material, and the first and the second friction bush are made from the metal/ceramics material, and when the temperature of the friction surface is low, a stable torque can be transmitted based on the friction between the surface of the metal/ceramics material and the surface of the metal/ceramics material. When the temperature of the friction surface becomes high, high power can be transmitted because of the high coefficient of the friction of the C/C composite material (or the carbon/graphite material). 
     Namely, the friction-clutch system that is constituted of the friction surfaces made by the materials explained in the above paragraphs can compensate for the defect (this means that when the temperature of the friction surface is low, high power cannot be transmitted) of the C/C composite material (or the carbon/graphite material). 
     However, based on recent research on friction-clutch systems, it becomes clear that the friction-clutch system that is constituted of the friction surfaces made by the materials explained in the above paragraphs does not always have a high performance. Below this point is explained in detail. 
     In this embodiment, the following is explained. Namely, the first and the second friction surface are comprised of a combination of the C/C composite material (the carbon/graphite material) and the metal/ceramics material. However, the configuration of the first and the second friction surface is not limited to such a one. For at least one friction surface of the first and the second friction disk, which are the friction surfaces of the friction disks, the third friction surface, which is the friction surface of the flywheel and faces the friction disk, and the fourth friction surface, which is the friction surface of the pressure plate and faces the friction disk, the friction surfaces made from the combination of the C/C composite material (the carbon/graphite material) and the metal/ceramics material may be used. 
       FIG. 4  shows an elevational view of the friction disk unit  213  of  FIGS. 2 and 3 . In this embodiment, a friction disk  211  has a circular ring shape. It is mechanically fixed to a central hub  205  by means of a central plate  223  of the friction disk  211 . A plurality of rivets  219  are used for mechanically fixing the central plate  223  to the central hub  205 . However, a plurality of screws, or welding, also may be used for mechanically fixing them. 
     In this embodiment, the friction disk  211  and the central hub  205  of the friction disk unit  213  are made separately and then are mechanically assembled. However, the friction disk  211  of the friction disk unit  213  and the central hub  205  may be made integrally. 
     For the embodiment shown in  FIG. 4 , a plurality of friction bushes are disposed on a circle at even intervals. However, it is not necessary that they be disposed on a circle at even intervals. 
       FIG. 5  shows an elevational view of the friction disk unit  213  of another embodiment. In this embodiment, the friction disks  211  have a sector-form shape that is made by cutting a friction disk  211  having a circular ring shape in the circumferential direction. The friction disks  211  are made by mechanically fixing a plurality of friction disks  211  to the central hub  205  by means of the central plates  223 , which all have a sector-form shape. For the embodiment of the friction disk unit  213  shown in  FIG. 5 , the central plate  223  is mechanically fixed to the central hub  205  by a plurality of rivets  219  in the same fashion as that shown in  FIG. 4 . However, the central plate  223  may be mechanically fixed to the central hub  205  by screws or welding. 
     For this embodiment, the friction disk  211  and the central hub  205  of the friction disk unit  213  are also made separately and then are mechanically assembled. However, the central hub  205  and the central plate  223  of the friction disk  211 , which has a sector-form shape, may be made integrally. 
     When the first friction plate  221  and the second friction plate  222  have a circular ring or a sector-form shape, since the amount of the C/C composite material, which is very expensive, can be reduced, it is possible to reduce the cost for producing the friction disk unit  213 . When the first friction plate  221  and the second friction plate  222  have a sector-form shape, since these friction plates can be efficiently cut out from a large plate, it is possible to further reduce the cost for producing the friction disk unit  213 . 
     In the above paragraphs, the method for fixing the first and the second friction plates  221 ,  222  to the central plate  223  is explained as follows. Namely, first, a male and a female threaded portion are formed on the first and the second friction bush  224 ,  225 , respectively. Then, the first and the second friction bush  224 ,  225  are inserted into through holes, which holes are disposed at the first and the second friction plates  221 ,  222  and the central plate  223 . Next, the second friction plates  221 ,  222  are fixed to the central plate  223  by engaging the first and the second friction bush  224 ,  225 , and by screwing them together. 
     However, the method for fixing the first and the second friction plates  221 ,  222  to the central plate  223  is not limited to such a way. 
     Below, another embodiment of the method for forming the first and the second friction surfaces of the friction disk unit  213 , which surfaces are constituted of different materials, i.e., comprised of a combination of a C/C composite material (or a carbon/graphite material) and a metal/ceramics material, is explained. 
     Namely, the first friction plate  221  and the first friction bush  224 , and the second friction plate  222  and the second friction bush  225 , may be located as shown, and may have the shape as shown, by the denotations  221 - 2 ,  224 - 2 ,  222 - 2 , and  225 - 2  of  FIG. 5-2 . Further, they may be located as shown, and may have the shape as shown, by the denotations  221 - 3 ,  224 - 3 ,  222 - 3 , and  225 - 3  of  FIG. 5-3 . 
     Further, as shown in  FIG. 6 , in addition to the first and the second friction bush  224 ,  225 , shearing pins  226  can be used for firmly fixing the first and the second friction plates  221 ,  222  to the central plate  223 . For this case, the force caused by screwing the first and the second friction bush  224 ,  225  acts to press the first and the second friction plates  221 ,  222  against the central plate  223  at a predetermined pressure. The shearing pins  226  are tightly inserted into the through holes disposed at the central plate  223 . The hollows, which fit the shearing pins  226 , are machined on the first and the second friction plates  221 ,  222 . Then, the first and the second friction plates  221 ,  222  and the central plate  223  are assembled by inserting the shearing pins  226  into the hollows. This structure allows the shearing pins  226  to transfer the friction force generated between the first and the second friction plates  221 ,  222  and the central plate  223  when the friction disk unit  213  transmits power. 
     As explained in the above paragraphs, by applying the method for fixing the first and the second friction plates  221 ,  222  to the central plate  223 , the friction disk unit  213  can definitely transmit the power. 
     As previously mentioned, even if the method for fixing the first and the second friction plates  221 ,  222  to the central plate  223 , which method is explained in the above paragraphs, is applied to the friction disk unit  213 , the friction-clutch system  201  does not always perform excellently. 
     To evaluate the performance of the friction-clutch system  201 , it is necessary to consider the coefficient of friction of the friction surfaces, the amount of wear of the friction surfaces, and whether the torque fluctuations are within a predetermined range. Particularly, the torque fluctuations causes judders (vibrations) on the friction-clutch system  201 . The judders (vibrations) interrupt smoothly transmitting the power. 
     Thus, these properties should comply with the requirements in a wide range of the power to be transmitted. 
     From recent research on the friction-clutch system  201 , it was found that there is a close correlation among the magnitude of the coefficient of friction of the friction surfaces, the amount of the wear of the friction surfaces, the magnitude of the torque fluctuations, the magnitude of the power to be transferred, and the ratio of [the area of the friction surface made from the metal/ceramics material]/[the area of the friction surface made from the metal/ceramics material+the area of the friction surface made from the carbon/graphite material]. 
     To clarify these facts, the following tests were conducted. 
     The ratio of the friction surface (a) and the magnitude of the power to be transmitted (it corresponds to the pressure of the friction surfaces) were selected as parameters. The tests for transmitting the power by the friction-clutch system  201  were conducted to evaluate the magnitude of the coefficient of friction of the friction surfaces, the amount of the wear of the friction surfaces, and the magnitude of the torque fluctuations. In the test, the ratio of the friction surface (a) is defined as the ratio of [the area of the friction surface made from the metal/ceramics material]/[the area of the friction surface made from the metal/ceramics material+the area of the friction surface made from the carbon/graphite material]. 
     Below, the method of the tests and the results of the tests are explained in detail. 
     Examples 101-110 
     A C/C composite material as the carbon/graphite material and steel as the metal/ceramics material are used. After changing the ratio of the friction surface (a) from 3% to 75%, ten samples of the friction disk unit  213 , which corresponds to examples  101 - 110 , were manufactured. Table 1 shows the constitution of the materials of examples 101-110 of the friction disk unit, which examples were used for the tests. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Constitution of the materials of the friction 
               
               
                 disk unit used for the tests (No. 1) 
               
            
           
           
               
               
               
               
            
               
                   
                 Type of 
                 Type of 
                 The ratio of the 
               
               
                   
                 carbon/graphite 
                 metal/ceramics 
                 friction surface 
               
               
                   
                 material 
                 material 
                 (a) (%) 
               
               
                   
                   
               
            
           
           
               
               
               
               
            
               
                 Example 101 
                 C/C composite 
                 Steel 
                 3 
               
               
                   
                 material 
               
               
                 Example 102 
                 C/C composite 
                 ″ 
                 5 
               
               
                   
                 material 
               
               
                 Example 103 
                 C/C composite 
                 ″ 
                 10 
               
               
                   
                 material 
               
               
                 Example 104 
                 C/C composite 
                 ″ 
                 20 
               
               
                   
                 material 
               
               
                 Example 105 
                 C/C composite 
                 ″ 
                 40 
               
               
                   
                 material 
               
               
                 Example 106 
                 C/C composite 
                 ″ 
                 50 
               
               
                   
                 material 
               
               
                 Example 107 
                 C/C composite 
                 ″ 
                 60 
               
               
                   
                 material 
               
               
                 Example 108 
                 C/C composite 
                 ″ 
                 65 
               
               
                   
                 material 
               
               
                 Example 109 
                 C/C composite 
                 ″ 
                 70 
               
               
                   
                 material 
               
               
                 Example 110 
                 C/C composite 
                 ″ 
                 75 
               
               
                   
                 material 
               
               
                   
               
            
           
         
       
     
     Examples 201-210 
     A C/C composite material as the carbon/graphite material and a brass as the metal/ceramics material were used. By changing the ratio of the friction surface (a) from 3% to 75%, ten samples of the friction disk unit  213 , which corresponds to examples 201-210, were manufactured. Table 2 shows the constitution of the materials of examples 201-210 of the friction disk unit, which examples were used for the tests. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Constitution of the materials of the friction 
               
               
                 disk unit used for the tests (No. 2) 
               
            
           
           
               
               
               
               
            
               
                   
                 Type of 
                 Type of 
                 The ratio of the 
               
               
                   
                 carbon/graphite 
                 metal/ceramics 
                 friction surface 
               
               
                   
                 material 
                 material 
                 (a) (%) 
               
               
                   
                   
               
            
           
           
               
               
               
               
            
               
                 Example 201 
                 C/C composite 
                 Brass 
                 3 
               
               
                   
                 material 
               
               
                 Example 202 
                 C/C composite 
                 ″ 
                 5 
               
               
                   
                 material 
               
               
                 Example 203 
                 C/C composite 
                 ″ 
                 10 
               
               
                   
                 material 
               
               
                 Example 204 
                 C/C composite 
                 ″ 
                 20 
               
               
                   
                 material 
               
               
                 Example 205 
                 C/C composite 
                 ″ 
                 40 
               
               
                   
                 material 
               
               
                 Example 206 
                 C/C composite 
                 ″ 
                 50 
               
               
                   
                 material 
               
               
                 Example 207 
                 C/C composite 
                 ″ 
                 60 
               
               
                   
                 material 
               
               
                 Example 208 
                 C/C composite 
                 ″ 
                 65 
               
               
                   
                 material 
               
               
                 Example 209 
                 C/C composite 
                 ″ 
                 70 
               
               
                   
                 material 
               
               
                 Example 210 
                 C/C composite 
                 ″ 
                 75 
               
               
                   
                 material 
               
               
                   
               
            
           
         
       
     
     Examples 301-310 
     A C/C composite material as the carbon/graphite material and a copper as the metal/ceramics material are used. After changing the ratio of the friction surface (a) from 3% to 75%, ten samples of the friction disk unit  213 , which corresponds to examples 301-310, were manufactured. Table 3 shows the constitution of the materials of examples 301-310 of the friction disk unit, which examples were used for the tests. 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Constitution of the materials of the friction 
               
               
                 disk unit used for the tests (No. 3) 
               
            
           
           
               
               
               
               
            
               
                   
                 Type of 
                 Type of 
                 The ratio of the 
               
               
                   
                 carbon/graphite 
                 metal/ceramics 
                 friction surface 
               
               
                   
                 material 
                 material 
                 (a) (%) 
               
               
                   
                   
               
            
           
           
               
               
               
               
            
               
                 Example 301 
                 C/C composite 
                 Copper 
                 3 
               
               
                   
                 material 
               
               
                 Example 302 
                 C/C composite 
                 ″ 
                 5 
               
               
                   
                 material 
               
               
                 Example 303 
                 C/C composite 
                 ″ 
                 10 
               
               
                   
                 material 
               
               
                 Example 304 
                 C/C composite 
                 ″ 
                 20 
               
               
                   
                 material 
               
               
                 Example 305 
                 C/C composite 
                 ″ 
                 40 
               
               
                   
                 material 
               
               
                 Example 306 
                 C/C composite 
                 ″ 
                 50 
               
               
                   
                 material 
               
               
                 Example 307 
                 C/C composite 
                 ″ 
                 60 
               
               
                   
                 material 
               
               
                 Example 308 
                 C/C composite 
                 ″ 
                 65 
               
               
                   
                 material 
               
               
                 Example 309 
                 C/C composite 
                 ″ 
                 70 
               
               
                   
                 material 
               
               
                 Example 310 
                 C/C composite 
                 ″ 
                 75 
               
               
                   
                 material 
               
               
                   
               
            
           
         
       
     
     Below, the data on the friction disk unit used for the tests is explained. 
     Configuration of the friction disk unit: See  FIGS. 2 ,  3 , and  5 . 
     Shape of the friction plates: A sector-form shape (which is made by dividing the friction disk having a circular ring shape into three parts) 
     Diameter of the outer side of the friction plates: 240 mm 
     Diameter of the inner side of the friction plates: 160 mm 
     Thickness of the friction disk: 7.8 mm 
     Test 
     The test for transmitting the power by using the Examples of the friction disk unit explained in the above paragraphs was conducted. 
     Below, the conditions of the tests are explained. 
     Temperature of the friction plates when the tests start:
         Room temperature       

     Mechanical work performed during each operation of the friction-clutch system: 8.8 Kcal 
     Pressure of the friction surfaces: 5˜300 N/cm 2    
     Under the conditions explained above, the tests were conducted, and the magnitude of the coefficient of friction of the friction surfaces, the amount of the wear of the friction surfaces, and the magnitude of the torque fluctuations, were measured. 
     The measurements are defined as follows: 
     The magnitude of the coefficient (μ) of friction of the friction surfaces is determined by the following equation: 
       λ= T /(2× P×R )
 
     where T is thethetorque to be transferred, P is a load to press the friction surfaces, and R is the effective radius of the friction plates. 
     The amount of the wear of the friction surfaces, which is defined as a rate of wear, is determined as an amount of change in the thicknesses of the friction plates when the friction-clutch system is operated one time (the mechanical work: 8.8 Kcal). Thus, the unit of the rate of wear is 10 −3  mm/one operation. 
     The magnitude of the torque fluctuations is determined based on the characteristic curve of the torque obtained by the results of the tests for transmitting the power. It is defined as a vibrational component of the torque. It is determined by eliminating the transitional component of the vibration of the torque caused in the tests for transmitting the power. The unit of the torque fluctuations is given in kgm. 
     Table 4 shows the coefficient of friction, the rate of the wear, and the torque fluctuations that were obtained by the tests using Examples 101-110 in relation to the pressure on the friction surfaces. 
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 Results of the tests for evaluating the performance of the friction-clutch system 
               
            
           
           
               
               
            
               
                   
                 Pressure of the friction surfaces 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 (N/cm 2 ) 
                 5 
                 10 
                 25 
                 100 
                 150 
                 200 
                 250 
                 300 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 Example 101 
                 Coefficient of friction 
                 0.15 
                   
                   
                 0.15 
                   
                 0.11 
                   
                 0.1 
               
               
                   
                 Rate of wear 
                 0.8 
                   
                   
                 0.99 
                   
                 1.23 
                   
                 1.69 
               
               
                   
                 Torque fluctuations 
                 15 
                   
                   
                 39 
                   
                 34 
                   
                 51 
               
               
                 Example102 
                 Coefficient of friction 
                   
                 0.26 
                   
                 0.31 
                   
                 0.32 
                 0.3 
               
               
                   
                 Rate of wear 
                   
                 0.84 
                   
                 0.89 
                   
                 0.88 
                 1.23 
               
               
                   
                 Torque fluctuations 
                   
                 4 
                   
                 7 
                   
                 8 
                 19 
               
               
                 Example 103 
                 Coefficient of friction 
                 0.16 
                   
                 0.32 
                   
                 0.34 
                   
                   
                 0.18 
               
               
                   
                 Rate of wear 
                 0.61 
                   
                 0.69 
                   
                 0.79 
                   
                   
                 1.55 
               
               
                   
                 Torque fluctuations 
                 5 
                   
                 4 
                   
                 6 
                   
                   
                 48 
               
               
                 Example 104 
                 Coefficient of friction 
                 0.19 
                 0.27 
                   
                 0.31 
                 0.41 
                   
                 0.29 
               
               
                   
                 Rate of wear 
                 0.4 
                 0.49 
                   
                 0.63 
                 0.8 
                   
                 1.21 
               
               
                   
                 Torque fluctuations 
                 4 
                 5 
                   
                 4 
                 8 
                   
                 13 
               
               
                 Example 105 
                 Coefficient of friction 
                   
                 0.29 
                   
                 0.36 
                   
                 0.37 
                 0.3 
                 0.24 
               
               
                   
                 Rate of wear 
                   
                 0.63 
                   
                 0.77 
                   
                 0.79 
                 0.98 
                 1.69 
               
               
                   
                 Torque fluctuations 
                   
                 5 
                   
                 6 
                   
                 8 
                 15 
                 22 
               
               
                 Example 106 
                 Coefficient of friction 
                 0.14 
                   
                 0.24 
                 0.31 
                   
                   
                 0.29 
               
               
                   
                 Rate of wear 
                 0.67 
                   
                 0.93 
                 1.06 
                   
                   
                 1.48 
               
               
                   
                 Torque fluctuations 
                 12 
                   
                 12 
                 8 
                   
                   
                 14 
               
               
                 Example 107 
                 Coefficient of friction 
                   
                 0.21 
                   
                   
                 0.29 
                 0.22 
                   
                 0.21 
               
               
                   
                 Rate of wear 
                   
                 0.94 
                   
                   
                 0.99 
                 1.07 
                   
                 1.88 
               
               
                   
                 Torque fluctuations 
                   
                 11 
                   
                   
                 8 
                 18 
                   
                 28 
               
               
                 Example 108 
                 Coefficient of friction 
                   
                 0.23 
                   
                 0.29 
                 0.36 
                   
                 0.29 
               
               
                   
                 Rate of wear 
                   
                 1 
                   
                 1.43 
                 1.22 
                   
                 1.43 
               
               
                   
                 Torque fluctuations 
                   
                 28 
                   
                 31 
                 38 
                   
                 36 
               
               
                 Example 109 
                 Coefficient of friction 
                 0.15 
                 0.24 
                   
                 0.33 
                   
                 0.34 
                   
                 0.21 
               
               
                   
                 Rate of wear 
                 1.19 
                 1.33 
                   
                 1.48 
                   
                 1.89 
                   
                 2.09 
               
               
                   
                 Torque fluctuations 
                 46 
                 45 
                   
                 47 
                   
                 55 
                   
                 61 
               
               
                 Example 110 
                 Coefficient of friction 
                   
                   
                 0.22 
                 0.32 
                 0.34 
                   
                 0.22 
               
               
                   
                 Rate of wear 
                   
                   
                 1.6 
                 1.58 
                 1.81 
                   
                 2.16 
               
               
                   
                 Torque fluctuations 
                   
                   
                 59 
                 66 
                 51 
                   
                 78 
               
               
                   
               
            
           
         
       
     
     Table 5 shows the coefficient of friction, the rate of the wear, and the torque fluctuations that were obtained by the tests using Examples 201-210 in relation to the pressure on the friction surfaces. 
     
       
         
           
               
             
               
                 TABLE 5 
               
             
            
               
                   
               
               
                 Results of the tests for evaluating the performance of the friction-clutch system 
               
            
           
           
               
               
            
               
                   
                 Pressure of the friction surfaces (N/cm 2 ) 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                 5 
                 10 
                 25 
                 100 
                 150 
                 200 
                 250 
                 300 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 Example 201 
                 Coefficient of friction 
                   
                 0.12 
                   
                 0.16 
                   
                   
                 0.1 
                 0.18 
               
               
                   
                 Rate of wear 
                   
                 0.91 
                   
                 1.12 
                   
                   
                 1.36 
                 1.73 
               
               
                   
                 Torque fluctuations 
                   
                 28 
                   
                 41 
                   
                   
                 43 
                 45 
               
               
                 Example 202 
                 Coefficient of friction 
                   
                 0.28 
                   
                   
                 0.25 
                   
                 0.29 
               
               
                   
                 Rate of wear 
                   
                 0.74 
                   
                   
                 0.92 
                   
                 1.15 
               
               
                   
                 Torque fluctuations 
                   
                 5 
                   
                   
                 5 
                   
                 20 
               
               
                 Example 203 
                 Coefficient of friction 
                 0.18 
                 0.26 
                   
                 0.38 
                   
                 0.31 
               
               
                   
                 Rate of wear 
                 0.66 
                 0.6 
                   
                 0.77 
                   
                 0.96 
               
               
                   
                 Torque fluctuations 
                 5 
                 3 
                   
                 4 
                   
                 7 
               
               
                 Example 204 
                 Coefficient of friction 
                   
                 0.28 
                 0.36 
                   
                   
                 0.38 
                 0.31 
                 0.29 
               
               
                   
                 Rate of wear 
                   
                 0.55 
                 0.48 
                   
                   
                 0.81 
                 1.15 
                 1.48 
               
               
                   
                 Torque fluctuations 
                   
                 8 
                 6 
                   
                   
                 9 
                 17 
                 48 
               
               
                 Example 205 
                 Coefficient of friction 
                 0.17 
                 0.27 
                   
                 0.33 
                   
                 0.36 
                   
                 0.31 
               
               
                   
                 Rate of wear 
                 0.69 
                 0.59 
                   
                 0.69 
                   
                 0.77 
                   
                 1.91 
               
               
                   
                 Torque fluctuations 
                 8 
                 7 
                   
                 9 
                   
                 9 
                   
                 29 
               
               
                 Example 206 
                 Coefficient of friction 
                 0.14 
                   
                   
                 0.29 
                 0.38 
                   
                 0.28 
               
               
                   
                 Rate of wear 
                 0.67 
                   
                   
                 1.12 
                 0.99 
                   
                 1.36 
               
               
                   
                 Torque fluctuations 
                 12 
                   
                   
                 6 
                 10 
                   
                 16 
               
               
                 Example 207 
                 Coefficient of friction 
                   
                 0.23 
                   
                 0.31 
                   
                 0.28 
                 0.27 
               
               
                   
                 Rate of wear 
                   
                 1.22 
                   
                 0.79 
                   
                 1.21 
                 1.39 
               
               
                   
                 Torque fluctuations 
                   
                 13 
                   
                 18 
                   
                 17 
                 13 
               
               
                 Example 208 
                 Coefficient of friction 
                 0.16 
                 0.22 
                 0.21 
                   
                   
                 0.24 
                 0.28 
                 0.24 
               
               
                   
                 Rate of wear 
                 0.93 
                 1.18 
                 1.26 
                   
                   
                 1.44 
                 1.41 
                 2.06 
               
               
                   
                 Torque fluctuations 
                 23 
                 22 
                 25 
                   
                   
                 29 
                 39 
                 51 
               
               
                 Example 209 
                 Coefficient of friction 
                   
                 0.21 
                   
                 0.31 
                   
                 0.33 
                 0.26 
               
               
                   
                 Rate of wear 
                   
                 1.39 
                   
                 1.44 
                   
                 1.99 
                 1.78 
               
               
                   
                 Torque fluctuations 
                   
                 48 
                   
                 52 
                   
                 45 
                 58 
               
               
                 Example 210 
                 Coefficient of friction 
                 0.16 
                   
                 0.24 
                   
                 0.29 
                   
                 0.26 
               
               
                   
                 Rate of wear 
                 1.4 
                   
                 1.73 
                   
                 1.94 
                   
                 2.23 
               
               
                   
                 Torque fluctuations 
                 53 
                   
                 52 
                   
                 66 
                   
                 69 
               
               
                   
               
            
           
         
       
     
     Table 6 shows the coefficient of friction, the rate of the wear, and the torque fluctuations that were obtained by the tests using Examples 301-310 in relation to the pressure on the friction surfaces. 
     
       
         
           
               
             
               
                 TABLE 6 
               
             
            
               
                   
               
               
                 Results of the tests for evaluating the performance of the friction-clutch system 
               
            
           
           
               
               
            
               
                   
                 Pressure of the friction surfaces (N/cm 2 ) 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                 5 
                 10 
                 25 
                 100 
                 150 
                 200 
                 250 
                 300 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 Example 301 
                 Coefficient of friction 
                   
                   
                 0.13 
                   
                   
                 0.12 
                   
                 0.11 
               
               
                   
                 Rate of wear 
                   
                   
                 0.83 
                   
                   
                 1.02 
                   
                 1.21 
               
               
                   
                 Torque fluctuations 
                   
                   
                 28 
                   
                   
                 22 
                   
                 33 
               
               
                 Example 302 
                 Coefficient of friction 
                 0.17 
                 0.25 
                   
                 0.29 
                   
                 0.26 
                 0.33 
                 0.29 
               
               
                   
                 Rate of wear 
                 0.75 
                 0.98 
                   
                 0.68 
                   
                 0.82 
                 1.19 
                 1.6 
               
               
                   
                 Torque fluctuations 
                 6 
                 8 
                   
                 9 
                   
                 7 
                 22 
                 54 
               
               
                 Example 303 
                 Coefficient of friction 
                 0.15 
                   
                 0.27 
                   
                   
                 0.33 
                 0.34 
               
               
                   
                 Rate of wear 
                 0.77 
                   
                 0.89 
                   
                   
                 0.91 
                 1.3 
               
               
                   
                 Torque fluctuations 
                 8 
                   
                 8 
                   
                   
                 7 
                 32 
               
               
                 Example 304 
                 Coefficient of friction 
                   
                 0.26 
                   
                 0.29 
                   
                 0.33 
               
               
                   
                 Rate of wear 
                   
                 0.66 
                   
                 0.88 
                   
                 0.95 
               
               
                   
                 Torque fluctuations 
                   
                 6 
                   
                 6 
                   
                 9 
               
               
                 Example 305 
                 Coefficient of friction 
                 0.22 
                   
                   
                 0.29 
                   
                   
                 0.28 
                 0.27 
               
               
                   
                 Rate of wear 
                 0.59 
                   
                   
                 0.63 
                   
                   
                 1.22 
                 1.68 
               
               
                   
                 Torque fluctuations 
                 5 
                   
                   
                 8 
                   
                   
                 15 
                 25 
               
               
                 Example 306 
                 Coefficient of friction 
                   
                 0.27 
                 0.23 
                   
                 0.29 
                 0.33 
                   
                 0.22 
               
               
                   
                 Rate of wear 
                   
                 0.69 
                 0.88 
                   
                 1.14 
                 1.16 
                   
                 1.71 
               
               
                   
                 Torque fluctuations 
                   
                 15 
                 25 
                   
                 29 
                 19 
                   
                 32 
               
               
                 Example 307 
                 Coefficient of friction 
                   
                 0.2 
                 0.24 
                   
                 0.22 
                   
                 0.29 
               
               
                   
                 Rate of wear 
                   
                 1.25 
                 0.92 
                   
                 1.35 
                   
                 1.42 
               
               
                   
                 Torque fluctuations 
                   
                 26 
                 32 
                   
                 19 
                   
                 23 
               
               
                 Example 308 
                 Coefficient of friction 
                 0.22 
                 0.21 
                   
                 0.29 
                   
                   
                 0.26 
                 0.22 
               
               
                   
                 Rate of wear 
                 0.88 
                 1.12 
                   
                 1.43 
                   
                   
                 1.99 
                 2.14 
               
               
                   
                 Torque fluctuations 
                 25 
                 33 
                   
                 31 
                   
                   
                 39 
                 62 
               
               
                 Example 309 
                 Coefficient of friction 
                   
                 0.22 
                   
                 0.31 
                   
                 0.32 
                 0.29 
               
               
                   
                 Rate of wear 
                   
                 1.43 
                   
                 1.39 
                   
                 1.76 
                 1.91 
               
               
                   
                 Torque fluctuations 
                   
                 43 
                   
                 41 
                   
                 48 
                 45 
               
               
                 Example 310 
                 Coefficient of friction 
                   
                 0.2 
                   
                 0.31 
                   
                 0.29 
                 0.28 
               
               
                   
                 Rate of wear 
                   
                 1.49 
                   
                 1.68 
                   
                 2.22 
                 2.34 
               
               
                   
                 Torque fluctuations 
                   
                 52 
                   
                 49 
                   
                 51 
                 68 
               
               
                   
               
            
           
         
       
     
     Generally, for the friction-clutch system and for the friction disk unit, it is preferable, for example, that the coefficient of friction be more than 0.2, that the rate of the wear be less than 1.5×10 −3  mm/one operation, and that the torque fluctuations be less than 40 Kgm. 
     Thus, from Tables 4, 5, and 6, the following conclusions were obtained. 
     It was found that a friction-clutch system having a preferable performance can be provided by setting the properties of the friction disk unit as follows: 
     (1) The ratio of the friction surface (a), namely, the ratio of [the area of the friction surface made from the metal/ceramics material]/[the area of the friction surface made from the metal/ceramics material+the area of the friction surface made from the carbon/graphite material] is as follows: 0.05˜0.65 
     (2) While the power is being transmitted, the pressure between the first and the third friction surface and between the second and the fourth friction surface is as follows: 10˜250 N/cm 2    
     Further, it was found that a friction-clutch system having a more preferable performance can be provided by setting the properties of the friction disk unit as follows: 
     ( 1 ) The ratio of the friction surface (a), namely, the ratio of [the area of the friction surface made from the metal/ceramics material]/[the area of the friction surface made from the metal/ceramics material+the area of the friction surface made from the carbon/graphite material] is as follows: 0.05˜0.40 
     (2) While the power is being transmitted, the pressure between the first and the third friction surface and between the second and the fourth friction surface is as follows: 10-200 N/cm 2    
     Further, it was found that the friction disk unit having a preferable performance can be provided by setting the ratio of the friction disk unit as follows: 
     The ratio of the friction surface (a), namely, the ratio of [the area of the friction surface made from the metal/ceramics material]/[the area of the friction surface made from the metal/ceramics material+the area of the friction surface made from the carbon/graphite material] is as follows:  0 . 05 ˜ 0 . 65   
     Further, it was found that the friction disk unit having more preferable performance can be provided by setting the ratio of the friction disk unit as follows: 
     The ratio of the friction surface (a), namely, the ratio of [the area of the friction surface made from the metal/ceramics material]/[the area of the friction surface made from the metal/ceramics material+the area of the friction surface made from the carbon/graphite material] is as follows: 0.05˜0.40 
     EXPLANATIOMN OF DENOTATIONS 
       201  a friction-clutch system 
       203  a flywheel 
       205  a central hub 
       207  a clutch cover 
       211  a friction disk 
       213  a friction disk unit 
       215  a pressure plate 
       217  a diaphragm spring 
       219  rivets 
       221  a first friction plate 
       222  a second friction plate 
       223  a central plate 
       224  a first friction bush 
       225  a second friction bush 
       226  shearing pins