Abstract:
A starter for cranking an internal combustion engine includes a device for absorbing a rotational torque exceeding a predetermined level. In this device, a laminated body composed of fixed disks and the rotatable disks is used. A frictional force in the laminated body is preset by a spring member pressing the laminated body in the laminated direction. To avoid concentration of the pressing force to a position where the spring force is imposed, a certain gap is formed between the spring member and the laminated body. To give a proper surface hardness to an internal gear portion of the rotatable disk and to give an abrasion-resistive property to a portion contacting the fixed disk, the rotatable disk is subjected to soft nitriding treatment. Thus, a stable frictional force is secured in the laminated body, and a long life of the excessive-torque-absorbing device is realized.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based upon and claims benefit of priority of Japanese Patent Applications No. 2007-39849 filed on Feb. 20, 2007 and No. 2007-61937 filed on Mar. 12, 2007, the contents of which are incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a starter for cranking an internal combustion engine, the starter having an excessive-torque-absorbing device. 
         [0004]    2. Description of Related Art 
         [0005]    An example of an excessive-torque-absorbing device used in a starter is disclosed in JP-A-2005-113816. A relevant portion of this device is shown in  FIGS. 15 and 16  attached hereto. Fixed disks  120  and rotatable disks  110  are alternately laminated to form a laminated body. The laminated body is contained in a cylindrical casing  100 , and pushed in the laminated direction by disk springs  130  to thereby control a frictional force between the fixed disks  120  and the rotatable disks  110 . The laminated body contained in the cylindrical casing  100  is pushed in the laminated direction together with the disk springs  130  by a nut  140  disposed at one end of the laminated body. 
         [0006]    Internal gears  150  engaging with planetary gears of a planetary gear speed reduction device used in the starter are formed integrally with the rotatable disks  110 . When a rotational torque exceeding a predetermined level is applied to the rotatable disks  110 , the rotatable disks  110  rotate relative to the fixed disks  120  against friction between the rotatable disks  110  and the fixed disks  120 . Thus, an excessive torque is absorbed by the device. 
         [0007]    In the device described above, the pushing force of the disk springs  130  is concentrated to an outer periphery of the fixed disks  120  and the rotatable disks  110  as shown with an arrow X in  FIG. 15 . Accordingly, the outer peripheral portions of the fixed disks  120  and the rotatable disks  110  closely contact each other as shown in  FIG. 16 . This may cause seizing between the fixed disks  120  and the rotatable disks  110 , making a slipping torque in the laminated body unstable, resulting in a shorter life of the excessive-torque-absorbing device. 
         [0008]    There has been another problem to be solved in the conventional excessive-torque-absorbing device. That is, the internal gear portion  150  of the rotatable disk  110  must have a hardness comparable to that of the planetary gears, while a portion contacting the fixed disk  120  must have a hardness comparable to that of the fixed disks  120  that is made of a material such as phosphor-bronze. When the rotatable disk made of a material such as carbon steel is heat-treated, its surface hardness becomes higher than HV-700 which is, too hard for the contacting portion. 
       SUMMARY OF THE INVENTION 
       [0009]    The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide an improved starter having an excessive-torque-absorbing device, in which a laminated body including fixed disks and rotatable disks is uniformly pushed by a spring member and a slipping torque between the rotatable disks and the fixed disks is stably maintained. 
         [0010]    The starter for cranking an internal combustion engine according to the present invention is composed of an electric motor for generating a rotational torque, an output shaft having a pinion gear that engages with a ring gear of the engine, a planetary gear speed reduction device for transmitting the rotational torque of the motor after reducing its speed, an excessive-torque-absorbing device for absorbing an excessive torque generated when the pinion gear engages with the ring gear and cranking operation is initiated, and other associated components. 
         [0011]    The excessive-torque-absorbing device includes fixed disks and rotatable disks laminated alternately with the fixed disks, forming a laminated body. The laminated body is contained in a cylindrical casing having a circular rear end portion. The fixed disks and the rotatable disks are ring-shaped. The fixed disks are fixedly held in the cylindrical casing while the rotatable disks are disposed in the casing so that the rotatable disks rotate relative to the fixed disks when a torque exceeding a predetermined frictional force is imposed on the laminated body. 
         [0012]    The frictional force in the laminated body is given by disk springs pushing the laminated body in the laminated direction, and an amount of the frictional force is preset by fastening a screw at a front end of the cylindrical casing. The pushing force of the disk springs is imposed on a pushing plate, which is disposed at the front end of the laminated body, at a specific position, such as at the outer periphery of the fixed disk or at a potion a certain distance inside of the outer periphery. If the pushing force concentrates to the specific position, the laminated body is not uniformly pressed. To avoid the concentration of the pushing force, a certain gap is formed between the pushing plate and a fixed disk disposed at an end of the laminated body. The gap is formed at a position where the pushing force is imposed. Thus, the pushing force is uniformly imposed on the laminated body, realizing a stable frictional force in the laminated body. 
         [0013]    The rotational disk is formed integrally with an internal gear of the planetary gear speed reduction device. The internal gear engaging with the planetary gears has to have a hardness comparable to a hardness of the planetary gears, while a portion contacting the fixed disk must have a good abrasion-resistant property. To give the rotatable disk these properties, it is subjected to a treatment of soft nitriding. In addition, a confined grease space is formed in the cylindrical casing at an outer peripheral portion of the rotatable disks to keep the grease longer. 
         [0014]    According to the present invention, the frictional force set in the excessive-torque-absorbing device is stabilized and a life of the device is prolonged. Other objects and features of the present invention will become more readily apparent from a better understanding of the preferred embodiments described below with reference to the following drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is a cross-sectional view showing an excessive-torque-absorbing device as a first embodiment of the present invention; 
           [0016]      FIG. 2  is a cross-sectional view showing the excessive-torque-absorbing device and other structures in its vicinity; 
           [0017]      FIG. 3  is a cross-sectional view showing a starter having the excessive-torque-absorbing device; 
           [0018]      FIG. 4A  is a plan view showing a fixed disk used in the excessive-torque-absorbing device; 
           [0019]      FIG. 4B  is a plan view showing a rotatable disk used in the excessive-torque-absorbing device; 
           [0020]      FIG. 5  is a plan view showing a cylindrical casing in which a laminated body of rotatable disks and fixed disks is contained, viewed from a rear side of the starter; 
           [0021]      FIG. 6  is a plan view showing a cylindrical casing in which a laminated body of rotatable disks and fixed disks is retained by a nut, view from a front side of the starter; 
           [0022]      FIG. 7  is a schematic view showing a size of a gap formed in a pushing plate relative to its diameter; 
           [0023]      FIG. 8  is a plan view showing a portion of a fixed disk and an area contacting a rotatable disk; 
           [0024]      FIG. 9  is a cross-sectional view showing a half of an excessive-torque-absorbing device as a second embodiment of the present invention; 
           [0025]      FIG. 10  is a partial cross-sectional view showing a pushing plate formed by stamping; 
           [0026]      FIG. 11  is a cross-sectional view showing an excessive-torque-absorbing device as a third embodiment of the present invention; 
           [0027]      FIG. 12  is a cross-sectional view showing an excessive-torque-absorbing device as a fourth embodiment of the present invention; 
           [0028]      FIG. 13  is a cross-sectional view showing an excessive-torque-absorbing device as a fifth embodiment of the present invention; 
           [0029]      FIG. 14  is a partial cross-sectional view showing an iron-nitride compound layer and a nitrogen-diffused layer formed on a surface of a rotatable disk by a soft nitriding process; 
           [0030]      FIG. 15  is a cross-sectional view showing a half of a conventional excessive-torque-absorbing device used in a starter; and 
           [0031]      FIG. 16  is a partial plan view showing a fixed disk and an area contacting a rotatable disk in the conventional excessive-torque-absorbing device. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0032]    A first embodiment of the present invention will be described with reference to  FIGS. 1-8 . First, referring to  FIG. 3 , an entire structure of a starter  1  in which an excessive-torque-absorbing device  4  is installed. The starter  1  includes: an electric motor  2  generating a rotational torque; a planetary gear speed reduction device  3  for reducing a rotational speed of the electric motor  2 ; an excessive-torque-absorbing device  4  for absorbing an excessive torque in a starting operation; an output shaft  6  connected to the planetary gear speed reduction device  3  via a clutch  5 ; a pinion gear  7  supported on the output shaft  6 ; and an electromagnetic switch  9  forming a circuit for turning on the electric motor  2 . 
         [0033]    The electric motor  2  is a known direct current motor composed of a yoke  10  forming a magnetic circuit, field coils  11  disposed in the yoke  10 , an armature  13  having a commutator  12 , brushes  14  slidably contacting the commutator  12  and other components. It is possible to use permanent magnets in place of the field coils  11 . The armature  13  includes an armature core  16  connected to an armature shaft  15  and armature coils  17  wound around the armature core  16 , and the armature coils  17  are connected to segments forming the commutator  12 . The armature shaft  15  is rotatably supported by a bearing  18  fixed in an end frame  19  at the rear side and by a bearing  20  fixed in a center plate  21  at a front side. The front side and the rear side of the starter are indicated by an arrow in  FIG. 3  and other drawings. 
         [0034]    The center plate  21  is disposed between the armature  13  and the planetary gear speed reduction device  3 , so that foreign particles including brush powders are prevented from entering into the planetary gear speed reduction device  3 . An outer periphery of the center case  21  is sandwiched between a center case  22  and the yoke  10 . The center case  22  is disposed between a front housing  23  covering a front side of the starter  1  and the yoke  10 , and covers the outside of the clutch  5  and the planetary gear speed reduction device  3 . The front housing  23 , the center case  22 , the yoke  10  and the end frame  19  are connected together with plural through-bolts  24 . 
         [0035]    The planetary gear speed reduction device  3  is disposed coaxially with the armature shaft  15 . As shown in  FIG. 2 , the device  3  is composed of a sun gear  25  formed on the armature shaft  15  at a position extending through the center plate  21 , an internal gear  26  formed as part of the excessive-torque-absorbing device  4  and planetary gears  27  engaging with both of the sun gear  25  and the internal gear  26 . The internal gear  26  is usually restrained, and the planetary gears  27  orbit around the sun gear  25 . The orbital movement of the planetary gears  27  around the sun gear  25  is transmitted to the output shaft  6  via the clutch  5 . The planetary gears  27  are rotatably supported by pins  27   a  fixed to a clutch outer  29  of the one-way clutch  5  via respective bearings  28  such as needle bearings. 
         [0036]    The one-way clutch  5  transmits a rotational torque of the electric motor  2  to the output shaft  6  while preventing torque transmission from the output shaft  6  to the electric motor  2 . The one-way clutch  5  is composed of: a clutch outer  29  that is rotated according to the orbital rotation of the planetary gears  27 ; an inner tube  31  rotatably supported in the center case  22  via a bearing  30 ; and rollers  32  disposed between the inner tube  31  and the clutch outer  29  to connect or interrupt torque transmission between the clutch outer  29  and the inner tube  31 . 
         [0037]    As shown in  FIG. 3 , a front end portion of the output shaft  6  is rotatably and slidably supported by the front housing  23  via a bearing  33 , and its rear end portion is spline-coupled to an inner bore of the inner tube  31  of the one-way clutch  5 . A pinion gear  7  is coupled to a front end of the output shaft  6  to be movable in the axial direction, and is biased in a frontward direction by a pinion spring  35  to abut a stopper  36 . The pinion gear  7  engages with the ring gear  34  of the engine and transmits the rotational torque of the electric motor  2  to the engine when the output shaft  6  is shifted frontward in the manner described below. In  FIG. 3 , portions above a centerline of the output shaft  6  and a centerline of the electromagnetic switch  9  show a non-engaging state where the pinion gear  7  is not engaged with the ring gear  34 , while portions below those centerlines show an engaging state where the pinion gear  7  engages with the ring gear  34 . 
         [0038]    The electromagnetic switch  9  includes an electromagnetic coil  37  that is excited by supplying current from an on-boar battery and a plunger  38  that is slidably movable in the axial direction within an inner bore of the electromagnetic coil  37 . When the plunger  37  moves in the rear side by excitation of the electromagnetic coil  37 , a main switch for supplying electric current to the electric motor  2  is closed. When the electromagnetic coil  37  is de-energized, the plunger  38  returns to its original position by a biasing force of a return spring  39 , and the main switch is opened. 
         [0039]    The main switch is composed of a pair of fixed contacts  42  connected to respective external terminals  40 ,  41  and a movable contact  43  connected to the plunger  38 . When the movable contact  43  contacts the pair of fixed contacts  42 , the main switch is closed. When the movable contact  43  is separated from the pair of contacts  42 , the main switch is opened. The external terminals  40 ,  41  are fixed to a resin cover  9   a  of the electromagnetic switch  9 . The external terminal  40  is a B-terminal connected to a plus terminal of the on-board battery through a battery cable, and the external terminal  41  is an M-terminal connected to the electric motor  2  through a motor terminal  44 . The motor terminal  44  is held by a grommet  45  sandwiched between the yoke  10  and the end frame  19 , and one end of the motor terminal  44  is connected to the field coil  11  of the electric motor  2 . 
         [0040]    A shift lever  8  is pivotally supported by a fulcrum  8   a . One end of the shift lever  8  is connected to a shift rod  46  of the electromagnetic switch  9 , and the other end thereof is coupled to the output shaft  6 . The shift rod  46  is assembled to the plunger  38  together with a driving spring  47 , and the movement of the plunger  38  is transmitted to the shift rod  46  via the driving spring  47 . The output shaft  6  is shifted frontward according to the movement of the shift rod  46  in the rearward direction. 
         [0041]    Now, the excessive-torque-absorbing device  4  will be described with reference to  FIG. 1 . The excessive-torque-absorbing device  4  is composed of a cylindrical casing  48 , fixed disks  49 , rotatable disks  50 , a pushing plate  51 , a washer  54  and disk springs  52 . The cylindrical casing  48  having a circular rear end  48   a  bent from a cylindrical portion is inserted into an inner bore of the center case  22  (refer to  FIG. 2 ) and fixed to it not to rotate. An inner diameter of the circular rear end  48   a  is made not to interfere with the planetary gears  27  of the planetary gear speed reduction device  3 . Depressed portions  48   b  that prevent rotation of the fixed disks  49  are formed on an inner periphery of the cylindrical casing  48  (refer to  FIG. 5 ). A female screw  48   c  is formed at a front end of the cylindrical casing  48 . 
         [0042]    As shown in  FIG. 1 , the fixed disks  49  and the rotatable disks  50  are alternately laminated, and the fixed disks  49  are disposed at both axial ends, forming a laminated body. The laminated body is contained in the cylindrical casing  48 . The fixed disk  49  is made of a material such as phosphor-bronze, and is formed in a ring-shape by stamping as shown in  FIG. 4A . Dimples  49   a  are formed on both surfaces of the fixed disk  49 . Projected portions  49   a  are formed on the outer periphery of the fixed disk  49 , so that the projected portions  49   a  engage with the depressed portions  48   b  of the cylindrical casing  48  to thereby prevent rotation of the fixed disk  49  in the cylindrical casing  48  (refer to  FIG. 5 ). The inner diameter of the fixed disks  49  is made not to interfere with the planetary gears  27  of the planetary gear speed reduction device  3 . 
         [0043]    As shown in  FIG. 4B , the rotatable disk  50  is made of a metallic plate such as a steel plate by stamping into a ring shape. Dimples  50   a  are formed on surfaces of the rotatable disk  50 . The outer diameter of the rotatable disk  50  is made a little smaller than the inner diameter of the cylindrical casing  48 . The rotatable disks  50  are disposed in the cylindrical casing  48  to be able to rotate relative to the fixed disks  49 . The internal gear  26  is formed on the inner periphery of each rotatable disk  50 , i.e., the internal gear  26  is formed integrally with the rotatable disk  49 . The internal gear engaging with the planetary gears  27  of the planetary gear speed reduction device  3  is formed by laminated plural internal gears  26 . Surfaces of the fixed disks  49  and the rotatable disks  50  are coated with lubricating grease. The laminated body of the fixed disks  49  and the rotatable disks  50  is disposed in the cylindrical casing  48  as shown in  FIGS. 5 and 6 . 
         [0044]    As shown in  FIG. 1 , the pushing plate  51  is formed in a ring shape similar to the shape of the fixed disk  49  and disposed at a front end of the laminated body. Two disk springs  52  in a ring shape are disposed in the cylindrical casing  48  to push the pushing plate  51  in the axial direction of the laminated body. A frictional force between the rotatable disks  50  and the fixed disks  49  is properly adjusted by fastening a nut  53  to a female screw  48   c  formed at the front end of the cylindrical casing  48 . Two disk springs  52 , with a washer  54  disposed therebetween, are used in this particular embodiment. It is possible, however, to use a single disk spring  52 . 
         [0045]    Now, a pushing force of the disk springs  52  generating the frictional force in the laminated body will be described in detail. As shown in  FIG. 1 , the pushing force of the disk spring  52  is imposed on the outer peripheral portion of the pushing plate  51 . A gap  51   a  is formed on the pushing plate  51  so that the pushing force of the disk spring  52  does not concentrate to the outer periphery of the front fixed disk  49 A (a fixed disk  49  disposed at the front end of the laminated body is referred to as a front fixed disk  49 A). The gap  51   a  is formed by making a step on a surface of the pushing plate  51  as shown in  FIG. 1 . In this manner, the pushing force is imposed on the front fixed disk  49 A at a portion inside the gap  51   a , and the concentration of the pushing force to the outer periphery of the front fixed disk  49 A is avoided. 
         [0046]    On an inner surface of the circular rear end portion  48   a  of the cylindrical casing  48 , a step forming a gap  48   d  is made as shown in  FIG. 1 . The gap  48   d  serves to avoid concentration of the pushing force on the outer periphery of the rear fixed disk  49 B (a fixed disk  49  disposed at the rear end of the laminated body is referred to as a rear fixed disk  49 B). In other words, the pushing force is imposed on the rear fixed disk  49 B at a position inside the gap  48   d . The pushing force generated by the disk springs  52  is imposed on the front fixed disk  49 A as shown with an arrow “a” in  FIG. 1  and on the rear fixed disk  49 B as shown with an arrow “b”. 
         [0047]    A size of the gap  51   a  relative to a friction area between the fixed disk  49  and the rotatable disk  50  is shown in  FIG. 7 . That is, length I of the gap  51   a  in the radial direction is made in a range from ⅓ to ⅔ of the radial length L of the friction area. Namely, I=(⅓ to ⅔)L, where L=½(D−d), D is an outer diameter of the fixed disk  49 , and d is an inner diameter of the fixed disk  49 . Most preferably, I is made a half of L. 
         [0048]    Operation of the starter  1  will be briefly explained. Upon turning on a starter switch, the electromagnetic coil  37  in the electromagnetic switch  9  is energized, and the plunger  38  is attracted to the electromagnetic coil  37 . The movement of the plunger  38  is transmitted to the output shaft  6  via the shift lever  8 . The output shaft  6 , helical-coupled to the inner tube  31 , is shifted frontward while rotating. The pinion gear  7  coupled to the output shaft  6  abuts an axial surface of the ring gear  34  and stops there, while the pinion spring  35  being compressed. Then, the plunger  38  further moves rearward, while compressing the driving spring  47 , and the main switch is closed to supply electric power to the electric motor  2 . 
         [0049]    Upon closing the main switch, the electric motor  2  begins to rotate. The rotational torque of the electric motor  2  is transmitted to the output shaft  6  via the one-way clutch  5  while the rotational speed is reduced by the planetary gear speed reduction device  3 . The pinion gear  7  is forcibly rotated up to a position where engagement with the ring gear  34  is possible, and the pinion gear  7  engages with the ring gear  34 . The ring gear  34  is rotated by the rotational torque of the pinion gear  7 , thereby cranking up the engine. 
         [0050]    At the moment when the pinion gear  7  engages with the ring gear  34  and starts cranking operation of the engine, an excessive torque (an impact torque) is imposed on the internal gear  26  through the pinion  7 , the output shaft  6 , the inner tube  31 , the rollers  32 , the clutch outer  29  and the planetary gears  27 . If the impact torque exceeds a predetermined frictional torque between the fixed disks  49  and the rotatable disks  50 , slippage occurs between the fixed disks  49  and the rotatable disks  50 . In other words, the rotatable disks  50  rotates against the preset frictional force in the excessive-torque-absorbing device  4 , and thus the excessive torque is absorbed. 
         [0051]    After the engine is cranked up, the electromagnetic coil  37  is de-energized by turning off the starter switch. The plunger  38  returns to its initial position by the spring-back force of the return spring  39 . Power supply to the electric motor  2  is terminated, and the output shaft  6  returns to its initial position by the shift lever  8  returning to its initial position. 
         [0052]    Advantages attained in the first embodiment will be summarized below. Since the gap  51   a  is formed on the pushing plate  51 , the pushing force of the disk springs  52  are imposed on the front fixed disk  49 A at the position shown with the arrow “a” in  FIG. 1 . Similarly, since the gap  48   d  is formed on the circular rear end portion  48   a  of the cylindrical casing  48 , the counter-force is imposed on the rear fixed disk  49 B at the position shown with the arrow “b”. This means that the laminated body is not pressed at its outer periphery, but it is pressed at a middle portion between the outer diameter and the inner diameter of the fixed disk  49 . 
         [0053]    Thus, concentration of the pushing force to the outer periphery of laminated body is avoided, and substantially uniform pushing force is imposed on the contacting area between the fixed disks  49  and the rotatable disks, as shown in  FIG. 8 . Thus, a stable frictional force in the laminated body can be obtained. Seizing between the fixed disks  49  and the rotatable disks  50  is prevented, and a life of the excessive-torque-absorbing device is prolonged. In addition, the excessive-torque-absorbing device  4  of the present invention may be applied to a starter for a diesel engine that requires a high torque. 
         [0054]    A second embodiment of the present invention is shown in  FIG. 9 . In this embodiment, the gap  48   d  formed on the circular rear end portion of the cylindrical casing  48  is modified to a tapered form. Other structures and functions are the same as those of the first embodiment. The pushing plate  51  may be modified to a form shown in  FIG. 10 . In the pushing plate shown in  FIG. 10 , a step for forming the gap  51   a  is formed by stamping. 
         [0055]    A third embodiment of the present invention is shown in  FIG. 11 . In this embodiment, another pushing plate  55  is disposed between the rear fixed disk  49 B and the circular rear end portion  48   a  of the cylindrical casing  48 . The outer diameter of the pushing plate  55  is made smaller than the outer diameter of the fixed disk  49 , thereby forming a gap  55   a  that corresponds to the gap  48   d  of the first embodiment. On the outer periphery of the pushing plate  55 , projected portions  55   b  engaging with the depressions  48   b  of the cylindrical casing  48  are formed. The pushing plate  55  is prevented from rotating and moving in the radial direction by the projected portions  55   b . Other structures and functions of the third embodiment is the same as those of the first embodiment. 
         [0056]    A fourth embodiment of the present invention is shown in  FIG. 12 . In this embodiment, a pushing force of the disk springs  52  is imposed on the pushing plate  51  at an inner position, not at the outer peripheral position. The gap  51   a  of the pushing plate  51  is formed at the position where the pushing force is imposed. In this manner, concentration of the pushing force is avoided and the laminated body is pressed substantially uniformly. The gap  48   d  at the rear end of the laminated body is similarly formed as in the first embodiment. 
         [0057]    A fifth embodiment of the present invention is shown in  FIG. 13 . In this embodiment, the pushing force of the disk springs  52  is imposed on the inner position of the pushing plate  51  in the same manner as in the fourth embodiment. The gap  51   a  is formed at the position where the pushing force is imposed in the same manner as in the fourth embodiment. A second pushing plate  55  having a gap  55   a  formed at an inside position is additionally used in this embodiment. Concentration of pushing force to the inside position where the pushing force is imposed is avoided, and the laminated body is uniformly pressed, generating a stable frictional force therein. Projected portions  55   b  engaging with the depressed portion  48   b  of the cylindrical casing  48  are formed on the outer periphery of the second pushing plate  55  to prevent rotation and radial movement of the second pushing plate  55 . 
         [0058]    Grease for the excessive-torque-absorbing device  4  is contained in a grease space  55  shown in  FIG. 2 . The grease space  55  is a space confined by an outer periphery of the rotatable disks  50 , an inner periphery of the depressed portions  48   b  and fixed disks  49  disposed both end of the laminated body. For example, the grease may be lithium-type grease containing lithium soap added to base lubricant as a thickener, and further containing an extreme-pressure additive and a solid additive such as molybdenum disulfide. Since the grease space  55  is a confined space, the grease can be kept for a long time without easily flowing out. 
         [0059]    The same grease may be used for both the planetary gear speed reduction device  3  and the excessive-torque-absorbing device  4 . If a different type of grease from the grease used for the excessive-torque absorbing device  4  is used for the planetary gear speed reduction device  3 , it is most preferable to make an amount of grease used in the planetary gear speed reduction device  3  less than one half of an amount of the grease used in the excessive-torque-absorbing device  4 . This is because a friction coefficient of the lithium type grease containing additives mentioned above changes when another type of grease is mixed in an amount in excess of 50% of own grease. The preset frictional torque in the excessive-torque-absorbing device  4  is changed according to changes in the friction coefficient of the grease. 
         [0060]    The rotatable disk  50  is made of low carbon steel or medium carbon steel, and soft nitriding treatment is performed to form an iron-nitride compound layer (A) and a nitrogen-diffused layer (B) thereon, as shown in  FIG. 14 . The iron-nitride compound layer (A) is 10-30 μm thick and has a hardness of HV 500-650. The thickness and the hardness of the layer (A) may be adjusted by changing a period of time for performing the soft nitriding treatment. Underneath the layer (A) the nitrogen-diffused layer (B) is formed. An entire surface of the rotatable disk  50  including a portion forming the internal gear  26  is subjected to the soft nitriding treatment. It is also possible to perform the soft nitriding only to the surface contacting the fixed disk  49  masking the surface forming the internal gear  26 . 
         [0061]    Since the iron-nitride compound layer (A) has a hardness of HV 500-650 that is comparable to a hardness of the planetary gear  27  engaging with the internal gear  26  and a thickness of 10-30 μm, abrasion wear of the internal gear  26  is suppressed. Since the iron-nitride compound layer (A) has an excellent property in lubrication, abrasion wear of the fixed disk  49  contacting the rotatable disk  50  is suppressed. This means that the rotatable disk  50  subjected to the soft nitriding treatment satisfies both properties required by the rotatable disk  50  and the internal gear  26 . Further, a friction coefficient of the iron-nitride compound layer (A) is low and stable, a stable frictional torque can be obtained in the laminated body. 
         [0062]    While the present invention has been shown and described with reference to the foregoing preferred embodiments, it will be apparent to those skilled in the art that changes in form and detail may be made therein without departing from the scope of the invention as defined in the appended claims.