Abstract:
Excellent durability can be achieved by reducing thermal influences of the excessive load from the engine side for a clutch roller ( 13 ) in a unidirectional rotation clutch system that is helical-splined to a drive shaft that rotates in response to a driving force of a motor and structured such that the motor driving force can be transmitted while an engine driving force cannot be transmitted when an engine stars, wherein a compound film composed of an external mixed phase and an internal diffused phase is formed on the clutch roller ( 13 ) subjected to nitriding by tufftriding with nitrogen being penetrating and diffusing.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the U.S. National Phase of PCT/JP2007/001450, filed Dec. 21, 2007, which claims priority from Japanese Patent Application No. 2006-348819, filed Dec. 26, 2006 and Japanese Patent Application No. 2006-348820, filed Dec. 26, 2006, the entire disclosure of which is incorporated herein by reference hereto. 
     
    
     BACKGROUND 
       [0002]    The present disclosure relates to an engine starter. 
         [0003]    There exists an engine starter that is constructed as shown in  FIGS. 1 ,  2 ,  3 A, and  3 B. A motor (electric motor) M of an engine starter  1  uses a general-purpose brush-type DC motor. A base end of a motor shaft  2  is axially and rotatably supported on an end cover  3   a  that closes a base end side opening of a cylindrical yoke  3 . A commutator  4  is integrally fitted onto a tip end of the motor shaft  2 . To an outer periphery of the commutator  4 , in a tip end side opening of the yoke  3 , a ring-shaped holder stay  5  is mounted. 
         [0004]    Reference numeral  6  denotes a bottomed cylindrical case (see  FIG. 1 ). The case  6  forms a reduction gear D that is disposed to a tip end side of the motor M, that is, adjacent to the holder stay  5 . In the case  6 , a tip end  2   a  of the motor shaft  2  is installed. A base end of a drive shaft  7  is also disposed in the case  6  so as to rotatably fit onto the motor shaft tip end  2   a.  In the case  6 , a plurality of planet gears  8  are also concentrically disposed with respect to the motor shaft tip end  2   a  so as to engage with the motor shaft tip end  2   a  and rotate circumferentially inside the case  6  according to the rotation of the motor shaft  2 . A ring-shaped support plate  9  is also installed in the case  6  so as to be integrated with the planet gears  8  via a support shaft  9   a.  By fitting an inner peripheral surface of the support plate  9  integrally onto the drive shaft  7 , a circumferential rotation of the planet gears  8  is interlocked with and joined to the drive shaft  7 . A driving force of the motor M is thus transmitted to the drive shaft (pinion shaft)  7  in a reduced speed manner. 
         [0005]    A unidirectional rotation clutch system C is disposed on a tip end of the drive shaft  7  (see  FIG. 1 ). A clutch outer  10  of the clutch system C is formed of a stepped cylinder. The clutch outer  10  is fit onto the drive shaft  7  such that a helical spline  10   a  that is formed on an inner peripheral surface of a small-diameter cylinder is engaged with a helical spline  7   a  that is engraved in an outer peripheral surface of the tip end of the drive shaft  7 . When relative rotation occurs between the drive shaft  7  and the clutch outer  10  in a predetermined rotating direction from a side of the drive shaft  7 , the clutch outer  10  rotatively moves along the helical spline  7   a  of the drive shaft  7  and then moves to an active position on a tip end side (a position shown in a lower half of  FIG. 1 ) from an inactive position (a position shown in an upper half of  FIG. 1 ) on the base end side of the drive shaft  7 . To an inside of a large-diameter cylinder on a tip end side of the clutch outer  10 , a clutch inner  12  is joined that includes a pinion gear  11  that is formed on a tip end outer periphery and engages with a ring gear  11  a on an engine side. The clutch inner  12  moves axially and integrally with the clutch outer  10 . 
         [0006]    Reference numeral  13  denotes a clutch roller to be interposed between the clutch outer  10  and the clutch inner  12  (see  FIGS. 1 ,  2 ,  3 A, and  3 B). Reference numeral  14  denotes a spring that urges the clutch roller  13  toward a side of a clutch outer wall. The clutch roller  13  is housed in a roller chamber  10   g  that is recessed in an inner peripheral surface of the clutch outer  10 . In the roller chamber  10   g,  as shown in  FIG. 2  and  FIGS. 3A and 3B , an opposing distance is larger between the clutch inner  12  and the clutch outer  10  at a rotation side end  10   b  of a clockwise side so as to allow the clutch roller  13  to freely rotate. The opposing distance is narrower toward an engagement side end  10   c  of a counterclockwise side. When the motor M is halted, as shown in  FIG. 2  and  FIG. 3A , the clutch roller  13  is positioned at an intermediate position between ends  10   b  and  10   c  because of an urging force of the spring  14 . In this state, because the clutch roller  13  does not engage with the clutch outer  10  and the clutch inner  12 , a driving force is not transmitted. When the clutch outer  10  rotates clockwise, as shown by an arrow in  FIG. 2 , in response to driving of the motor M, then the clutch roller  13  moves to an engagement side end  10   c,  which is shown in  FIG. 3B . An engaged state is thus set; and a rotating force of the clutch outer  10  is transmitted to the clutch inner  12  via the clutch roller  13 . As a result, an engine starts. 
         [0007]    When the engine starts, an overrun occurs such that rotation of the clutch inner  12  is faster than that of the clutch outer  10 . As shown in  FIG. 3A , the clutch inner  12  then rotates counterclockwise (an arrow direction) relatively to the clutch outer  10 . The clutch roller  13  then moves toward the rotation side end  10   b  and freely rotates. This clutch unit thus functions as a one-way clutch unit that prevents the engine driving force from being transmitted to a side of the clutch outer  10  from the clutch inner  12  (see Japanese Published Examined Utility Model Application No. S59-26107 and Japanese Published Unexamined Utility Model Application No. H05-42675, for example). 
       SUMMARY 
       [0008]    However, the ring gear  11   a  on a side of the engine may abnormally rotate while the engine starts because of some defective actuation, such as a defective ignition in the engine. A torque also may impact the drive shaft  7  from the pinion gear  11  because of the abnormal rotation of the ring gear  11   a.  In such a case, a load from the engine side that exceeds a normal torque load affects the clutch system C. The clutch roller  13  then reaches the engagement side end  10   c  as shown in  FIG. 3B . If the torque impact is so large that the abnormal load exceeds a maximum normal force of the clutch roller  13 , then the clutch roller  13  slips while engaging with the clutch inner  12 . A high temperature state then occurs because of a frictional heat caused by the slipping. As a result, a softened portion  13   a  of the clutch roller  13  is generated, as shown in  FIGS. 4A and 4B . Being subjected to shearing stress, such softened composition plastically flows. A drawn portion  13   b  is then formed (see  FIG. 4A ). If the drawn portion  13   b  exceeds a ductility limit of the material, then a breakage (exfoliation)  13   c  occurs. The clutch roller  13  then deforms (see  FIG. 4B ). 
         [0009]    If the clutch roller  13  deforms, an engagement allowance (overlapping allowance) of the clutch roller  13  cannot be secured at the time when the engine starts. The clutch system C then idles, and as a result, the engine cannot smoothly start. The present disclosure solves the problems and is able to achieve various advantages. 
         [0010]    The disclosure according to a first exemplary aspect provides an engine starter that includes a motor that includes a drive shaft that rotates in response to a motor driving force; and a unidirectional rotation clutch system that is helical-splined to the drive shaft, the clutch system being structured such that the motor driving force can be transmitted while an engine driving force cannot be transmitted when an engine starts. The clutch system includes a clutch inner; a clutch outer; a clutch roller that is interposed between the clutch outer and the clutch inner; and a spring that urges the clutch roller. A soft nitriding treatment is applied to a surface of the clutch roller or surfaces of the clutch outer and the clutch inner, the soft nitriding treatment being gas soft nitriding. 
         [0011]    The disclosure according to a second exemplary aspect provides the engine starter according to the first exemplary aspect, wherein the gas soft nitriding is formed on the surface of the clutch roller. 
         [0012]    The disclosure according to a third exemplary aspect provides the engine starter according to the first or second exemplary aspect, wherein a clutch roller contacting surface of the clutch inner extends long in an axial direction. 
         [0013]    The disclosure according to a fourth exemplary aspect provides an engine starter that includes a motor that includes a drive shaft that rotates in response to a motor driving force; and a unidirectional rotation clutch system that is helical-splined to the drive shaft, the clutch system being structured such that the motor driving force can be transmitted while an engine driving force cannot be transmitted when an engine starts. The clutch system includes a clutch inner; a clutch outer; a clutch roller that is interposed between the clutch outer and the clutch inner; and a spring that urges the clutch roller. A soft nitriding treatment is applied to a surface of the clutch roller or surfaces of the clutch outer and the clutch inner, the soft nitriding treatment being tufftriding. 
         [0014]    The disclosure according to a fifth exemplary aspect provides the engine starter according to the fourth exemplary aspect, wherein the tufftriding is formed on the surface of the clutch roller. 
         [0015]    The disclosure according to a sixth exemplary aspect provides the engine starter according to the fourth or fifth exemplary aspect, wherein a clutch roller contacting surface of the clutch inner extends long in an axial direction. 
         [0016]    According to the first exemplary aspect, one of the sliding contact surfaces between the clutch roller and the clutch outer and the clutch inner that come into sliding contact with the clutch roller includes a mixed phase of Fe 2 +3Nε (epsilon) and Fe 3 C; and a diffused phase of Fe 4 Nγ′ (gamma prime) inside the mixed phase. As a result, the sliding contacting surface can be stabilized as being a heterogeneous contact. Abrasion and fatigue resistance is thus excellent. Durability can also be improved against the excessive load from the engine side. 
         [0017]    According to the second exemplary aspect, because the gas soft nitriding can be formed on the clutch roller, an easy compound film formation can be achieved. 
         [0018]    According to the third exemplary aspect, because the contacting surface of the clutch roller with the clutch inner is axially long, even if an end face of the clutch inner is chamfered or a bottom surface of the clutch outer is drafted, the clutch roller can come into contact with circumferential surfaces that are not chamfered or drafted. Local contacting with the clutch roller can thus be prevented. As a result, a uniform force can be applied, and the clutch life can also last longer. 
         [0019]    According to the fourth exemplary aspect, one of the sliding contact surfaces between the clutch roller and the clutch outer and the clutch inner that come into sliding contact with the clutch roller includes a mixed phase of Fe 2 +3Nε (epsilon) and Fe 3 C; and a diffused phase of Fe 4 Nγ′ (gamma prime) inside the mixed phase. As a result, the sliding contacting surface can be stabilized as being a heterogeneous contact. Abrasion and fatigue resistance is thus excellent. Durability can also be improved against the excessive load from the engine side. 
         [0020]    According to the fifth exemplary aspect, because the tufftriding can be formed on the clutch roller, an easy compound film formation can be achieved. 
         [0021]    According to the sixth exemplary aspect, because the contacting surface of the clutch roller with the clutch inner is axially long, even if an end face of the clutch inner is chamfered or a bottom surface of the clutch outer is drafted, the clutch roller can come into contact with circumferential surfaces that are not chamfered or drafted. Local contacting with the clutch roller can thus be prevented. As a result, a uniform force can be applied, and the clutch life can also last longer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    Various embodiments of the disclosure will be described with reference to the drawings, wherein: 
           [0023]      FIG. 1  is a partial sectional front view of an engine starter; 
           [0024]      FIG. 2  is a sectional view of a clutch system; 
           [0025]      FIG. 3A  is a main portion enlarged sectional view of the clutch system at a time of cranking, and  FIG. 3B  is a main portion enlarged sectional view of the clutch system at a time of torque limiter actuation; 
           [0026]      FIGS. 4A and 4B  are explanatory views showing a mechanism of deformation of a clutch roller; and 
           [0027]      FIG. 5A  is an explanatory view showing a mechanism of friction between irons, and  FIG. 5B  is an explanatory view showing a mechanism of friction between irons and nitrided-irons. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0028]    Focusing on the fact that the deformation of the clutch roller  13  has been caused by heating, a measure is formulated in order to prevent the deformation of the clutch roller by (1) reducing heating values; and (2) increasing strength against thermal softening. 
         [0029]    First, a heating value is examined. The heating value Q is as follows: 
         [0000]      Q=μPV 
         [0000]    wherein μ represents a friction coefficient; P represents load; and V represents velocity. Reduction in contact surface pressure and friction coefficient are thus to be achieved. As being about the mechanical, however, the former is provisionally excluded from the present development theme. The reduction in friction resistance will thus be examined herein. 
         [0030]    A friction between the clutch inner  12  and the clutch roller  13  is a friction of metals (irons). As a normal condition, grease (lubricant or clutch grease) is interposed at a space  15  between the clutch inner  12  and the clutch roller  13 . However, when a load is imposed, the clutch inner  12  and the clutch roller  13  directly contact with each other and slide as shown in  FIG. 5A . At sliding portions, electron migration then occurs. As a result, the sliding portions adhere to each other between the clutch inner  12  and the clutch roller  13 . A force that separates the sliding portions is thus a frictional force. In other words, a measure for reducing such friction coefficient is to prevent the electron migration. 
         [0031]    An effective proposal includes a chemically stable substance being on the sliding and contacting surfaces. A stable compound film can thus be formed on one of the sliding surfaces of the clutch system C, in which the clutch roller  13  comes into sliding contact with both the clutch outer  10  and the clutch inner  12 . By simply forming a compound film on the surface of the clutch roller  13 , the sliding portions of both the clutch outer  10  and the clutch inner  12  can be reduced in friction resistance as shown in  FIG. 5B . The compound film may instead be formed on both surfaces of the clutch outer  10  and the clutch inner  12 , which ultimately reduces the friction resistance as coming into sliding contact with the clutch roller  13 . 
         [0032]    A circumferential surface  12   a  of the clutch inner  12  (see  FIG. 1 ), with which the clutch roller  13  comes into contact, extends toward a side of the motor M in an axial direction. An extending end of the circumferential surface  12   a  is fit and incorporated in an attaching portion  10   f  that is recessed in a bottom surface  10   e  of the clutch outer  10 . The clutch roller  13  comes into contact with the circumferential surface  12   a  of the clutch inner  12 . Even if a clutch inner end face is chamfered or the clutch outer bottom surface  10   e  is drafted, the clutch roller  13  comes into contact with a circumferential surface that includes no chamfered or drafted portions. Local contacts with the clutch roller  13  do not occur, and a uniform force is also applied. As a result, a longer lifetime of the clutch can be achieved. 
         [0033]    When such a compound film is formed, nitrogen can be used because of its firm bonding with iron. The compound film can thus be easily formed by nitriding. As such nitriding, gas soft nitriding is exemplified. 
         [0034]    According to a first embodiment, the clutch roller  13  is made of a bearing steel (for example, SUJ2), and its surface is nitrided by gas nitriding. Carbon nitride is formed on the surface by a method via steps of adding 30 to 50 percent of ammonia (NH 3 ) gas into a carbonaceous gas or nitrogen gas atmosphere, heating and holding for 30 minutes to 5 hours in an atmosphere with a temperature of 550 to 600 degrees Celsius, and penetrating and diffusing nitrogen and carbon together. By this method, a nitride phase with 0.7 to 0.8 mm is formed on the surface of the clutch roller  13 . As the nitride phase, formed on the surface is a mixed phase (hereinafter, referred to as “external phase”) of ε (Fe 2 3N) and Fe 3 C with a thickness of approximately 14 μm (micrometers) that contains iron as a main component. As a diffused phase, formed inside the external phase is a ν′ (Fe 4 N) phase (hereinafter, referred to as “internal phase”) with a thickness of approximately 4 μm. Hardness of the surface is slightly changed to Hv 700-800 from Hv 760 before being nitrided. 
         [0035]    Actual machine tests are repeated 20 times with the clutch roller  13  thus gas-nitrided. For the actual machine tests, two different samples of the clutch roller  13  are used: a clutch roller that is nitrided after being quenched (sample 1) and a clutch roller that is nitrided without being quenched (sample 2). In addition, as a blank, a clutch roller that is just quenched is also used for the tests. 
         [0036]    According to the test results, tissue changes under thermal influences can be observed in surface layers of the blank clutch roller. The surface layers are then plastically deformed and exfoliated. Several tens of micrometers of the exfoliation can also be observed. As for the samples 1 and 2, on the other hand, although external phases completely decay because of abrasions, the internal phases mostly last without being exfoliated. Thermal influences can also hardly be observed on those clutch rollers in themselves, which can continuously be used. In addition, the samples 1 and 2 being observed further, the internal phases remain thicker in the sample 2 than in the sample 1. The clutch roller  13  that is nitrided without being quenched is thus excellent in durability. 
         [0037]    According to a second embodiment, a clutch roller  13  is made of a bearing steel (for example, SUJ2) and nitrided by tufftriding (salt bath soft nitriding). Carbon nitride is formed on a surface of the clutch roller  13  by a method via steps of adding potassium cyanide or potassium carbonate into a titanium crucible so as to be molten, blowing air into the crucible, heating for 30 to 240 minutes with a temperature of approximately 570 degrees Celsius, and cooling by oil or water after the heating. By this method, a nitride phase with 0.7 to 0.8 mm is formed on the surface of the clutch roller  13 . By this method, a nitride phase with 0.7 to 0.8 mm is formed on the surface of the clutch roller  13 . As the nitride phase, formed on the surface side is a mixed phase (hereinafter, referred to as “external phase”) of ε (Fe 2 3N) and Fe 3 C, containing iron as a main component, with a thickness of approximately 14 μm (micrometers). As a diffused phase, formed inside the external phase is a ν′ (Fe 4 N) phase (hereinafter, referred to as “internal phase”) with a thickness of approximately 4 μm. Hardness of the surface is slightly changed to Hv 670 from Hv 760 before being nitrided. 
         [0038]    Actual machine tests are repeated 20 times with the clutch roller  13  thus gas-nitrided. For the actual machine tests, two different samples of the clutch roller  13  are used: a clutch roller that is nitrided after being quenched (sample 3) and a clutch roller that is nitrided without being quenched (sample 4). In addition, as a blank, a clutch roller that is just quenched is also used for the tests. 
         [0039]    According to the test results, tissue changes under thermal influences can be observed in surface layers of the blank clutch roller. The surface layers are then plastically deformed and exfoliated. Several tens of micrometers of the exfoliation can also be observed. As for the samples 3 and 4, on the other hand, although external phases completely decay because of abrasions, the internal phases mostly last without being exfoliated. Thermal influences can also hardly be observed on those clutch rollers in themselves, which can continuously be used. In addition, the samples 3 and 4 being observed further, the internal phases remain thicker in the sample 4 than in the sample 3. The clutch roller  13  that is nitrided without being quenched is thus excellent in durability. 
         [0040]    The present invention is useful for an engine starter for starting an engine (internal combustion) installed in a vehicle. One of the sliding contact surfaces between the clutch roller, and the clutch outer and the clutch inner that come into sliding contact with the clutch roller can be stabilized as being a heterogeneous contact. Abrasion and fatigue resistance is thus excellent. Durability can also be improved against the excessive load from the engine side.