Method for making aircraft brake disc

Abstract

A method for making an aircraft brake disc includes: a first step of manufacturing a rotary disc preform for manufacturing a rotary disc and a fixing disc preform for manufacturing a fixing disc; and a second step of densifying the rotary disc preform such that density continuously increases from the center to the outside of the rotary disc and of densifying the fixing disc preform such that density continuously decreases from the center to the outside of the fixing disc.

Claims

1. A method for making an aircraft brake disc, comprising: a first step of manufacturing a rotary disc preform and a fixing disc preform, wherein the rotary disc preform is used for manufacturing a rotary disc and the fixing disc preform is used for manufacturing a fixing disc, and wherein each of the rotary disc preform and the fixing disc preform comprises a center portion around a center hole of the respective disc preform and further comprises an outside portion around the center portion of the respective disc preform; and a second step of (i) densifying the rotary disc preform such that the outside portion of the rotary disc preform, which is coupling to a drive key of the rotary disc, has a greater density than the center portion of the rotary disc preform, the densifying step comprising (i-1) densifying the center portion of the rotary disc preform to 1.7 g/cm.sup.3; and (i-2) densifying the outside portion of the rotary disc preform to 1.9 g/cm.sup.3, to thereby make strength higher at the outside portion of the rotary disc preform to prevent the outside portion of the rotary disc from cracking or breaking when a brake system is operated; and (ii) densifying the fixing disc preform such that the center portion of the rotary disc preform, which is coupling to a drive key of the fixing disc, has a greater density than the outside portion of the fixing disc preform, the densifying step comprising (ii-1) densifying the outside portion of the fixing disc preform to 1.7 g/cm.sup.3; and (ii-2) densifying the center portion of the fixing disc preform to 1.9 g/cm.sup.3, to thereby make strength higher at the center portion of the fixing disc preform to prevent the center portion of the fixing disc from cracking or breaking when a brake system is operated.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The above and other aspects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

(2) FIG. 1 is a flowchart illustrating a method for making an aircraft brake disc according to a first embodiment of the present invention;

(3) FIG. 2 is a diagram simply illustrating a thermal gradient chemical vapor deposition apparatus;

(4) FIG. 3A is a graph illustrating a temperature change of an electrode rod with lapse of time;

(5) FIG. 3B is a graph illustrating a flow rate change of a reaction gas with lapse of time;

(6) FIG. 3C is a diagram illustrating a density gradient of the rotary disc preform;

(7) FIG. 3E is a graph illustrating a temperature change of an electrode rod with lapse of time;

(8) FIG. 3F is a graph illustrating a flow rate change of a reaction gas with lapse of time;

(9) FIG. 3G is a diagram illustrating a density gradient of the rotary disc preform;

(10) FIG. 4A is a graph illustrating a temperature change of an electrode rod with lapse of time;

(11) FIG. 4B is a graph illustrating a flow rate change of a reaction gas with lapse of time;

(12) FIG. 4C is a diagram illustrating a density gradient of the fixing disc preform;

(13) FIG. 4E is a graph illustrating a temperature change of an electrode rod with lapse of time;

(14) FIG. 4F is a graph illustrating a flow rate change of a reaction gas with lapse of time;

(15) FIG. 4G is a diagram illustrating a density gradient of the fixing disc preform;

(16) FIG. 5 is a diagram illustrating an aircraft brake disc manufactured by the method for making an aircraft brake disc according to the first embodiment of the present invention;

(17) FIG. 6 is a flowchart illustrating a method for making an aircraft brake disc according to a second embodiment of the present invention;

(18) FIG. 7A is a graph illustrating a temperature change of an electrode rod with lapse of time;

(19) FIG. 7B is a graph illustrating a flow rate change of a reaction gas with lapse of time;

(20) FIG. 7C is a diagram illustrating a density gradient of the rotary disc preform;

(21) FIG. 7E is a graph illustrating a temperature change of an electrode rod with lapse of time;

(22) FIG. 7F is a graph illustrating a flow rate change of a reaction gas with lapse of time;

(23) FIG. 7G is a diagram illustrating a density gradient of the rotary disc preform;

(24) FIG. 8A is a graph illustrating a temperature change of an electrode rod with lapse of time;

(25) FIG. 8B is a graph illustrating a flow rate change of a reaction gas with lapse of time;

(26) FIG. 8C is a diagram illustrating a density gradient of the fixing disc preform;

(27) FIG. 8E is a graph illustrating a temperature change of an electrode rod with lapse of time;

(28) FIG. 8F is a graph illustrating a flow rate change of a reaction gas with lapse of time;

(29) FIG. 8G is a diagram illustrating a density gradient of the fixing disc preform; and

(30) FIG. 9 is a diagram illustrating an aircraft brake disc manufactured by the method for making an aircraft brake disc according to the second embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

(31) Exemplary embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

(32) Hereinafter, a method for making an aircraft brake disc according to a first embodiment of the present invention will be described in detail.

(33) FIG. 1 is a flowchart illustrating a method for making an aircraft brake disc according to a first embodiment of the present invention.

(34) As illustrated in FIG. 1, a method for making an aircraft brake disc according to the first embodiment of the present invention includes: a first step of manufacturing a rotary disc preform for manufacturing a rotary disc and a fixing disc preform for manufacturing a fixing disc (S11); and a second step of densifying the rotary disc preform such that density continuously increases from the center to the outer side of the rotary disc and of densifying the fixing disc preform such that density continuously decreases from the center to the outside of the fixing disc (S12).

(35) The first step (S11) is described.

(36) A rotary disc preform and a fixing disc preform are formed in the types of a two dimension preform and a three dimension preform.

(37) [Two Dimension Preform]

(38) A heat resistant fiber and resin are put into a mold and then mixed therein. An oxi-pan fiber, a carbon fiber, or silicon carbide fiber is used for the heat resistant fiber. The heat resistant fiber may be mixed and then put into the mold. The resin may be phenol resin, furan resin, coal tar pitch, or petroleum pitch. A preform is obtained by pressing the heat resistant fiber and the resin mixed in the mold and then heating the mixture. The preform is taken out of the mold and then put into a carbonization furnace. The preform is carbonized at a high temperature over 900 C. A carbide with components except carbon removed is obtained. A hole is formed at the center of the carbide to insert an electrode rod of a thermal gradient chemical vapor deposition apparatus. A rotary disc preform and a fixing disc preform are formed in the types of two dimension preforms in this way.

(39) [Three Dimension Preform]

(40) Heat resistant fabrics are formed. The heat resistant fabrics are formed by weaving an oxi-pan fiber, a carbon fiber, or silicon carbide fiber. A staple fiber made of an oxi-pan fiber is applied onto the heat resistant fabrics. The heat resistant fabrics are stacked. Angles such as 30, 45, 60, and 90 may be given, when the heat resistant fabrics are stacked. The stacked heat resistant fabrics are punched with a needle. The needle moves down with staple fiber. The staple fiber combines the stacked heat resistant fabrics, thereby forming a preform. The preform is taken out of the needle punching equipment and then put into a carbonization furnace. The preform is carbonized at a high temperature over 900 C. A carbide with components except carbon removed is obtained. A hole is formed at the center of the carbide to insert an electrode rod of a thermal gradient chemical vapor deposition apparatus. A rotary disc preform and a fixing disc preform are formed in the types of three dimension preforms in this way.

(41) The second step (S12) is described.

(42) FIG. 2 is a diagram simply illustrating a thermal gradient chemical vapor deposition apparatus.

(43) As illustrated in FIG. 2, a thermal gradient chemical vapor deposition apparatus 1 is composed of a chamber 2 and an electrode rod 3 at the center of the chamber 1. The electrode rod 3 is made of graphite. The electrode rod 3 generates heat when receiving electricity. The electrode rod 3 is inserted into a hole H of a preform P. The diameter of the electrode rod 3 is smaller by 0.2 to 0.5 mm than that of the hole H so that the electrode rod 3 can be smoothly inserted into the hole H and heat can transfer well to the preform P. A reaction gas is injected into the chamber through an inlet 2a and then discharged outside the chamber through an outlet 2b. The reaction gas may be a hydrocarbon gas such as methane or propane.

(44) As the electrode rod 3 heats the preform P, the heat propagates from the center to the outer side of the preform P. When the temperature of the preform P reaches 700 C. or more, the reaction gas is thermally decomposed and carbon is deposited by pores in the preform. The preform is densified in this way.

(45) [First Method of Densifying a Rotary Disc According to the First Embodiment (Temperature Changed and Flow Rate Fixed)]

(46) FIG. 3A is a graph illustrating a temperature change of an electrode rod with lapse of time. FIG. 3B is a graph illustrating a flow rate change of a reaction gas with lapse of time. FIG. 3C is a diagram illustrating a density gradient of the rotary disc preform. FIG. 3E is a graph illustrating a temperature change of an electrode rod with lapse of time. FIG. 3F is a graph illustrating a flow rate change of a reaction gas with lapse of time. FIG. 3G is a diagram illustrating a density gradient of the rotary disc preform.

(47) Referring to FIGS. 2, 3A, 3B, 3C, 3E, 3F and 3G, the electrode rod 3 is put into the hole H of the rotary disc preform P.

(48) As illustrated in FIG. 3A, the temperature of the electrode rod 3 is continuously decreased, as time passes. As illustrated in FIG. 3B, the flow rate of the reaction gas is kept constant, even though time passes. Accordingly, as illustrated in FIG. 3C, the carbon of the reaction gas is much deposited, as it goes from the center to the outside of the rotary disc preform P.

(49) The reason is as follows.

(50) The higher the temperature of the electrode rod 3, the more the heat rapidly transfers and the more the area where the carbon of the reaction gas can be deposited rapidly increases, whereas the lower the temperature of the electrode rod 3, the more the heat slowly transfers and the more the area where the carbon of the reaction gas can be deposited slowly increases.

(51) Since the flow rate of the reaction gas is constant, as the area rapidly increases, the amount of carbon of the reaction gas that can be deposited per unit area decreases and the density decreases, but as the area slowly increases, the amount of carbon of the reaction gas that can be deposited per unit area increases and the density increases. Accordingly, the density of the rotary disc preform P continuously increases from the center to the outside. The density is indicated by the depth of a color. That is, lower density is illustrated lighter and higher density is illustrated darker. Those are the same in the following description.

(52) For reference, to avoid complication in description, the increase amount of the reaction gas for the unit area, which increases from the center to the outside because the rotary disc preform P has a circular shape, is not considered. This is the same in the following description.

(53) Accordingly, the density of the rotary disc made of the rotary disc preform P also continuously increases from the center to the outside.

(54) [First Method of Densifying a Fixing Disc According to the First Embodiment (Temperature Changed and Flow Rate Fixed)]

(55) FIG. 4A is a graph illustrating a temperature change of an electrode rod with lapse of time. FIG. 4B is a graph illustrating a flow rate change of a reaction gas with lapse of time. FIG. 4C is a diagram illustrating a density gradient of the fixing disc preform. FIG. 4E is a graph illustrating a temperature change of an electrode rod with lapse of time. FIG. 4F is a graph illustrating a flow rate change of a reaction gas with lapse of time. FIG. 4G is a diagram illustrating a density gradient of the fixing disc preform.

(56) Referring to FIGS. 2, 4A, 4B, 4C, 4E, 4F and 4G, the electrode rod 3 is put into the hole H of the fixing disc preform P.

(57) As illustrated in FIG. 4A, the temperature of the electrode rod 3 is continuously increased, as time passes. As illustrated in FIG. 4B, the flow rate of the reaction gas is kept constant, even though time passes. Accordingly, as illustrated in FIG. 4C, the carbon of the reaction gas is less deposited, as it goes from the center to the outside of the fixing disc preform P.

(58) The reason is as follows.

(59) The higher the temperature of the electrode rod 3, the more the heat rapidly transfers and the more the area where the carbon of the reaction gas can be deposited rapidly increases, whereas the lower the temperature of the electrode rod 3, the more the heat slowly transfers and the more the area where the carbon of the reaction gas can be deposited slowly increases.

(60) Since the flow rate of the reaction gas is constant, as the area rapidly increases, the amount of carbon of the reaction gas that can be deposited per unit area decreases and the density decreases, but as the area slowly increases, the amount of carbon of the reaction gas that can be deposited per unit area increases and the density increases. Accordingly, the density of the fixing disc preform P continuously decreases from the center to the outside.

(61) For reference, to avoid complication in description, the increase amount of the reaction gas for the unit area, which increases from the center to the outside because the fixing disc preform P has a circular shape, is not considered. This is the same in the following description.

(62) Accordingly, the density of the fixing disc made of the fixing disc preform. P also continuously decreases from the center to the outside.

(63) [Second Method of Densifying a Rotary Disc According to the First Embodiment (Temperature Fixed and Flow Rate Changed)]

(64) As illustrated in FIG. 3E, the temperature of the electrode rod 3 is kept constant, even though time passes. As illustrated in FIG. 3F, the flow rate of the reaction gas increases, as time passes. Accordingly, as illustrated in FIG. 3G, the carbon of the reaction gas is much deposited, as it goes from the center to the outside of the rotary disc preform P.

(65) The reason is as follows.

(66) Since the temperature of the electrode rod 3 is constant, heat uniformly transfers and the area, where the reaction gas can be thermally decomposed, uniformly increases. As the flow rate of the reaction gas increases, the amount of carbon that is deposited in the same unit area gradually increases. Accordingly, the amount of carbon that is deposited increases from the center to the outside of the rotary disc preform P.

(67) Accordingly, the density of the rotary disc preform P continuously increases from the center to the outside.

(68) Therefore, the density of the rotary disc made of the rotary disc preform P also continuously increases from the center to the outside.

(69) [Second Method of Densifying a Fixing Disc According to the First Embodiment (Temperature Fixed and Flow Rate Changed)]

(70) As illustrated in FIG. 4E, the temperature of the electrode rod 3 is kept constant, even though time passes. As illustrated in FIG. 4F, the flow rate of the reaction gas decreases, as time passes. Accordingly, as illustrated in FIG. 4G, the carbon of the reaction gas is less deposited, as it goes from the center to the outside of the fixing disc preform P.

(71) The reason is as follows.

(72) Since the temperature of the electrode rod 3 is constant, heat uniformly transfers and the area, where the reaction gas can be thermally decomposed, uniformly increases. As the flow rate of the reaction gas decreases, the amount of carbon that is deposited in the same unit area gradually decreases. Accordingly, the amount of carbon that is deposited decreases from the center to the outside of the fixing disc preform P.

(73) Accordingly, the density of the fixing disc preform P continuously decreases from the center to the outside.

(74) Accordingly, the density of the fixing disc made of the fixing disc preform. P also continuously decreases from the center to the outside.

(75) FIG. 5 is a diagram illustrating an aircraft brake disc manufactured by the method for making an aircraft brake disc according to the first embodiment of the present invention.

(76) As illustrated in FIG. 5, an aircraft brake disc 10 manufactured by the method for making an aircraft brake disc according to the first embodiment of the present invention includes a pressure disc 11, a rear disc 12, and rotary discs 13 and fixing discs 14 that are alternately disposed between the pressure disc 11 and the rear disc 12. A drive groove 13a where a drive key is inserted is formed on the outside of the rotary disc 13. A spline groove 14a where splines are inserted is formed at the center of the fixing disc 14.

(77) The density of the rotary disc 13 continuously increases from the center to the outside. The density of the fixing disc 14 continuously increases from the outside to the center.

(78) In the first embodiment, the density of the rotary disc 13 continuously increases from 1.7 g/cm.sup.3 at the center to 1.9 g/cm.sup.3 at the outside. Since the larger the density, the higher the strength, strength is high around the drive groove 13a.

(79) The density of the fixing disc 13 continuously increases from 1.7 g/cm.sup.3 at the outside to 1.9 g/cm.sup.3 at the center. Since the larger the density, the higher the strength, strength is high around the spline groove 14a.

(80) Hereinafter, a method for making an aircraft brake disc according to a second embodiment of the present invention will be described in detail.

(81) FIG. 6 is a flowchart illustrating a method for making an aircraft brake disc according to a second embodiment of the present invention.

(82) As illustrated in FIG. 6, a method for making an aircraft brake disc according to the second embodiment of the present invention includes: a first step of manufacturing a rotary disc preform for manufacturing a rotary disc and a fixing disc preform for manufacturing a fixing disc (S21); and a second step of densifying the rotary disc preform such that density is uniform from the center to the portion close to the outside of the rotary disc and is higher only at the portion around the outside, and of densifying the fixing disc preform such that density is uniform from the portion close to the center to the outside of the fixing disc and is higher only around the center of the fixing disc (S22).

(83) The first step (S21) is described.

(84) The rotary disc preform and the fixing disc preform may be formed in the types of a two dimension preform and a three dimension preform. This was described in the first embodiment of the present invention, so it is not described here.

(85) The second step (S22) is described.

(86) [First Method of Densifying a Rotary Disc According to the Second Embodiment (Temperature Changed and Flow Rate Fixed)]

(87) FIG. 7A is a graph illustrating a temperature change of an electrode rod with lapse of time. FIG. 7B is a graph illustrating a flow rate change of a reaction gas with lapse of time. FIG. 7C is a diagram illustrating a density gradient of the rotary disc preform. FIG. 7E is a graph illustrating a temperature change of an electrode rod with lapse of time. FIG. 7F is a graph illustrating a flow rate change of a reaction gas with lapse of time. FIG. 7G is a diagram illustrating a density gradient of the rotary disc preform.

(88) Referring to FIGS. 2, 7A, 7B, 7C, 7E, 7F and 7G, the electrode rod 3 is put into the hole H of the rotary disc preform P.

(89) As illustrated in FIG. 7A, the temperature of the electrode rod 3 is kept high for a predetermined time, and when the carbon of the reaction gas is deposited at the end of the rotary disc preform P, the temperature is rapidly decreased and then maintained at the level. For this purpose, a cooler (not illustrated) is disposed inside the electrode rod 3. As illustrated in FIG. 7B, the flow rate of the reaction gas is kept constant, even though time passes.

(90) Accordingly, as illustrated in FIG. 7C, the carbon of the reaction gas is uniformly deposited from the center to the portion close to the outside of the rotary disc preform P, but is suddenly much deposited at the outside of the rotary disc preform P.

(91) The reason is as follows.

(92) The higher the temperature of the electrode rod 3, the more the heat rapidly transfers and the more the area where the carbon in the reaction gas can be deposited rapidly increases, whereas the lower the temperature of the electrode rod 3, the more the heat slowly transfers and the more the area where the carbon in the reaction gas can be deposited slowly increases.

(93) Since the flow rate of the reaction gas is constant, as an area increases at the same speed, the amount of the carbon of the reaction gas that can be deposited per unit area is also constant. When the temperature of the electrode rod 3 rapidly decreases, the increase speed of the area also rapidly decreases, so the carbon of the reaction gas is deposited in the decreased area.

(94) Accordingly, the density of the rotary disc preform P is uniform from the center to the portion close to the outside and increases only around the outside.

(95) Therefore, the density of the rotary disc made of the rotary disc preform P is also uniform from the center to the portion close to the outside and increases only around the outside.

(96) [First Method of Densifying a Fixing Disc According to the Second Embodiment (Temperature Changed and Flow Rate Fixed)]

(97) FIG. 8A is a graph illustrating a temperature change of an electrode rod with lapse of time. FIG. 8B is a graph illustrating a flow rate change of a reaction gas with lapse of time. FIG. 8C is a diagram illustrating a density gradient of the fixing disc preform. FIG. 8E is a graph illustrating a temperature change of an electrode rod with lapse of time. FIG. 8F is a graph illustrating a flow rate change of a reaction gas with lapse of time. FIG. 8G is a diagram illustrating a density gradient of the fixing disc preform.

(98) Referring to FIGS. 2, 8A, 8B, 8C, 8E, 8F and 8G, the electrode rod 3 is put into the hole H of the fixing disc preform P.

(99) As illustrated in FIG. 8A, the temperature of the electrode rod 3 is maintained at a low level for a predetermined time until the carbon of the reaction gas is deposited at the center of the fixing disc preform P, but when the deposition of the carbon of the reaction gas at the center is finished, the temperature of the electrode rod 3 is rapidly increased and maintained at the level. As illustrated in FIG. 8B, the flow rate of the reaction gas is kept constant, even though time passes.

(100) Accordingly, as illustrated in FIG. 8C, the carbon of the reaction gas is much deposited only at the center of the fixing disc preform P, and is uniformly deposited from the center to the outside of the fixing disc preform P.

(101) The reason is as follows.

(102) The higher the temperature of the electrode rod 3, the more the heat rapidly transfers and the more the area where the carbon in the reaction gas can be deposited rapidly increases, whereas the lower the temperature of the electrode rod 3, the more the heat slowly transfers and the more the area where the carbon in the reaction gas can be deposited slowly increases.

(103) Since the flow rate of the reaction gas is constant, as an area increases at the same speed, the amount of the carbon of the reaction gas that can be deposited per unit area is also constant.

(104) When the temperature of the electrode rod 3 rapidly increases, the increase speed of the area also rapidly increases, so the carbon of the reaction gas is deposited in the increased area.

(105) Accordingly, the density of the fixing disc preform P is high only around the center and is uniform from the portion around the center to the outside.

(106) Therefore, the density of the fixing disc made of the fixing disc preform P is also high only around the center and is uniform from the portion around the center to the outside.

(107) [Second Method of Densifying a Rotary Disc According to the Second Embodiment (Temperature Fixed and Flow Rate Changed)]

(108) As illustrated in FIG. 7E, the temperature of the electrode rod 3 is kept constant, even though time passes. As illustrated in FIG. 7F, the flow rate of the reaction gas is kept low, and when the carbon of the reaction gas is deposited at the end of the rotary disc preform P, the flow rate is rapidly increased and then maintained at the level. Accordingly, as illustrated in FIG. 7G, the carbon of the reaction gas is uniformly deposited from the center to the portion close to the outside of the rotary disc preform P, but is suddenly much deposited at the outside of the rotary disc preform P.

(109) The reason is as follows.

(110) Since the temperature of the electrode rod 3 is constant, heat uniformly transfers and the area, where the reaction gas can be thermally decomposed, uniformly increases. Since the flow rate of the reaction gas is constant, as an area increases at the same speed, the amount of the carbon of the reaction gas that can be deposited per unit area is also constant. When the amount of the reaction gas is rapidly increased, more carbon of the reaction gas is deposited per unit area.

(111) Accordingly, the density of the rotary disc preform P is uniform from the center to the portion close to the outside and increases only around the outside.

(112) Therefore, the density of the rotary disc made of the rotary disc preform P is also uniform from the center to the portion close to the outside and increases only around the outside.

(113) [Second Method of Densifying a Fixing Disc According to the Second Embodiment (Temperature Fixed and Flow Rate Changed)]

(114) As illustrated in FIG. 8E, the temperature of the electrode rod 3 is kept constant, even though time passes. As illustrated in FIG. 8F, the flow rate of the reaction gas is kept much only when the carbon of the reaction gas is deposited at the center of the fixing disc preform P, and then it is rapidly decreases around the center of the fixing disc preform P and then maintained at the level.

(115) Accordingly, as illustrated in FIG. 8G, the carbon of the reaction gas is much deposited only at the center of the fixing disc preform P, and is uniformly less deposited from the portion around the center to the outside of the fixing disc preform P.

(116) The reason is as follows.

(117) Since the temperature of the electrode rod 3 is constant, heat uniformly transfers and the area, where the reaction gas can be thermally decomposed, uniformly increases. Since the flow rate of the reaction gas is constant, as an area increases at the same speed, the amount of the carbon of the reaction gas that can be deposited per unit area is also constant. When the amount of the reaction gas is rapidly decreased, less carbon of the reaction gas is deposited per unit area.

(118) Accordingly, the density of the fixing disc preform P is high only around the center and is uniform from the portion around the center to the outside.

(119) Therefore, the density of the fixing disc made of the fixing disc preform P is also high only around the center and is uniform from the portion around the center to the outside.

(120) FIG. 9 is a diagram illustrating an aircraft brake disc manufactured by the method for making an aircraft brake disc according to the second embodiment of the present invention.

(121) As illustrated in FIG. 9, an aircraft brake disc 20 manufactured by the method for making an aircraft brake disc according to the second embodiment of the present invention includes a pressure disc 21, a rear disc 22, and rotary discs 23 and fixing discs 24 that are alternately disposed between the pressure disc 21 and the rear disc 22. A drive groove 23a where a drive key is inserted is formed on the outside of the rotary disc 23. A spline groove 24a where splines are inserted is formed at the center of the fixing disc 24.

(122) The density of the rotary disc 23 is uniform from the center to the portion around the outside and is high only around the outside. The density of the fixing disc 24 is high only around the center and is uniform from the portion around the center to the outside.

(123) In the second embodiment, the density of the rotary disc 23 is uniform at 1.7 g/cm.sup.3 from the center to the portion around the outside, and is 1.9 g/cm.sup.3 only around the outside. Since the larger the density, the higher the strength, strength is highest around the drive groove 23a.

(124) The density of the fixing disc 24 is 1.9 g/cm.sup.3 only around the center and is uniform at 1.7 g/cm.sup.3 from the portion around the center to the outside. Since the larger the density, the higher the strength, strength is highest around the spline groove 24a.

(125) As set forth above, according to exemplary embodiments of the invention, it is possible to increase the density around the outside of a rotary disc where a drive key groove is formed. Accordingly, it is possible to sufficiently protect the rotary disc from torque and shock transmitted from a drive key.

(126) Further, according to the present invention, it is possible to increase the density around the center of a fixing disc where a spline groove is formed. Accordingly, it is possible to sufficiently protect the fixing disc from torque and shock transmitted from splines.

(127) While the present invention has been illustrated and described in connection with the exemplary embodiments, it will be apparent to those skilled in the art that modifications and variations can be made without departing from the spirit and scope of the invention as defined by the appended claims.