Abstract:
An engine starter includes a motor that comprises a drive shaft; and a unidirectional rotation clutch system that is helical-splined to the drive shaft. 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, wherein a compound film is formed on a surface of the clutch roller or surfaces of the clutch outer and the clutch inner.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is the U.S. National Phase of PCT/JP2006/325844, filed Dec. 26, 2006, which claims priority from Japanese Patent Application No. 2005-377908, filed Dec. 28, 2005, the entire disclosure of which is incorporated herein by reference hereto. 
     BACKGROUND 
     The present disclosure relates to an engine starter. 
     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 the 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. 
     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. 
     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 . 
     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   d  that is recessed in an inner peripheral surface of the clutch outer  10 . In the roller chamber  10   d , 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  due to 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. 
     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 
     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 due to 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 ). 
     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 also able to achieve various advantages. 
     The disclosure addresses an exemplary aspect of an engine starter that includes a motor that comprises a drive shaft; and a unidirectional rotation clutch system that is helical-splined to the drive shaft. 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, wherein a compound film is formed on a surface of the clutch roller or surfaces of the clutch outer and the clutch inner. 
     In another exemplary aspect, the compound film is formed on the clutch roller. 
     In another exemplary aspect, the clutch roller is made of bearing steel, the compound film is formed by nitriding, and the nitriding is applied to the bearing steel that is quenched and tempered. 
     In another exemplary aspect, the nitriding is gas nitriding. 
     In another exemplary aspect, the nitriding is carbonitriding. 
     In another exemplary aspect, a clutch roller contacting surface of the clutch inner extends long in an axial direction of the clutch roller. 
     According to various exemplary aspects of the disclosure, because the compound film is formed on 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, 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. 
     According to various exemplary aspects of the disclosure, because the compound film can be formed on the clutch roller, easy compound film formation can be achieved. 
     According to various exemplary aspects of the disclosure, because the compound film is formed by nitriding, durability can be improved. 
     According to various exemplary aspects of the disclosure, because gas nitriding or carbonitriding is applied with friction coefficient being lowered, the durability can be further improved. 
     According to various exemplary aspects of the disclosure, 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 
       Various embodiments of the disclosure will be described with reference to the drawings, wherein: 
         FIG. 1  is a partial sectional front view of an engine starter; 
         FIG. 2  is a sectional view of a clutch system; 
         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; 
         FIGS. 4A and 4B  are explanatory views showing a mechanism of deformation of a clutch roller; 
         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; and 
         FIG. 6A  is an enlarged photo after a clutch impact test is carried out for a blank,  FIG. 6B  is an enlarged, photo of a surface portion of the clutch roller before the clutch impact test is carried out for sample 2, and  FIG. 6C  is an enlarged photo of a surface portion of the clutch roller after the clutch impact test is carried out for sample 2. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     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. 
     First, a heating value is examined. The heating value Q is as follows:
 
Q=μPV
 
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.
 
     Frictions between the clutch inner  12  and the clutch roller  13  are frictions 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 portions adhere to each other between the clutch inner  12  and the clutch roller  13 . A force that separates the portions is thus a frictional force. In other words, a measure for reducing such friction coefficient is to prevent the electron migration. 
     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 . 
     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. 
     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 nitriding or carbonitriding is preferable. 
     Reference numeral  13  denotes the clutch roller to be interposed between the clutch outer  10  and the clutch inner  12 . Reference numeral  14  denotes the spring that urges the clutch roller  13  toward a side of the clutch outer wall. The clutch roller  13  is made of a bearing steel (for example, SUJ2), and its surface is nitrided by gas nitriding. The 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 35 hours in an atmosphere with a temperature of 500 to 550 degrees Celsius, and penetrating and diffusing nitrogen. 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, two surface layers are formed: an A layer (Fe 2 N) and a B layer (Fe 4 N) with a thickness of approximately 14 micrometers. Inside the surface layers, a (Fe+Fe 4 N) phase (hereinafter, referred to as “internal phase”) is formed as a diffusion phase with a thickness of approximately 4 micrometers. Hardness of the surface is slightly lowered to Hv 670 from Hv 760 before being nitrided. 
     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 after being quenched and tempered (sample 2). In addition, as a blank, a clutch roller that is just quenched is also used for the tests. Test results are shown in the photos of  FIGS. 6A-6C . 
     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 after being quenched and tempered is thus more excellent in durability. 
     Next, a clutch roller  13  whose surface is carbonitrided will be described. This method hardens a steel surface with nitrogen as well as carbon being penetrating into the steel surface. Carburizing is carried out for 50 to 150 minutes by adding 0.5 to 1.0 percent of ammonia (NH 3 ) in an atmosphere of carbonaceous gas, such as natural gas or LPG, and heating to 500 degrees Celsius or higher. A quenching temperature thereof can be lower than simple carbonizing because of the penetrating nitrogen. Quenching transformation is then reduced. The clutch roller  13  being examined further, friction coefficient of the steel that has been carbonitrided is lowered to 0.16, whereas friction coefficient of the steel that has not been carbonitrided is 0.21. In addition, surface hardness of the steel is increased to Hv 1000 from HV 760 before being carbonitrided. 
     Actual machine tests are repeated 20 times with the clutch roller  13  thus carbonitrided. For the actual machine tests, two different samples of the clutch roller  13  are being used: a clutch roller that is carbonitrided after being quenched (sample 3) and a clutch roller that is carbonitrided after being quenched and tempered (sample 4). In addition, as a blank, a clutch roller that is just quenched is also used for the tests. 
     According to test results, tissue changes under thermal influences can be seen 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 seen. As for the samples 3 and 4, on the other hand, although surfaces of the treated layers completely decay because of abrasions, internal layers mostly last without being exfoliated. Thermal influences can also hardly be observed on those clutch rollers, which can continuously be used. In addition, the samples 3 and 4 being observed further, remaining carbonitrided layers of sample 4 are thicker than those of sample 3. The clutch roller  13  that is carbonitrided after being quenched and tempered is thus more excellent in durability. 
     The disclosure is not limited to the embodiment described above. The compound film can also be formed by using surface-treatment materials, such as chrome carbide (CrC) or titanium nitride (TiN). In addition, the compound film can be formed on the clutch inner and the clutch outer instead of the clutch roller. 
     The present disclosure relates to an engine starter that starts an engine (internal combustion) in a vehicle and especially can improve durability of a clutch roller. With the engine starter, further reliability can thus be achieved.