Abstract:
A forming method of a tooth trace of a gear according to the present invention includes: a first step of preparing a preformed gear made of metal including a cylindrical main body and a tooth part having a flat tooth flank having a uniform thickness in a tooth trace direction inside of the main body; and a second step of conducting a surface treatment locally to an outside surface of the main body in order to deform the outside surface of the main body in such a manner that the same becomes concave, so that a crowning bulging in the tooth trace direction is formed on the tooth flank of the tooth part.

Description:
FIELD OF THE INVENTION 
     This invention relates to a forming method of a tooth trace of a gear. 
     BACKGROUND ART 
     Conventionally, as a general forming method of a tooth part of an internal gear, a mechanical machining method using a cutting unit having a cutter is known. A forming method of a tooth part using a cutting unit is explained with reference to  FIG. 9 . 
     The cutting unit  101  shown in  FIG. 9  is a cutting unit for conducting a cutting process to a preformed gear, which is cylindrical and which has a small opening at an end and a large opening at the other end. The cutting unit  101  consists of a cutting tool  109  and a holding tool  110 . The cutting tool  109  consists of a cutter  102  having a cutting tooth part  103 , and a holder  104  holding the cutter  102  and movable forward and backward with respect to the preformed gear. The holding tool  110  has a receiving jig  105  arranged opposite to the cutting tool  109 , and a plurality of chuck members  108 . The receiving jig  105  has a first receiving part  106  having an outside diameter that is substantially the same as an inside diameter of the large opening of the preformed gear, and a second receiving part  107  extending outward in the radial direction serially from the first receiving part  106 . 
     The cutting unit  101  is used to form a tooth part of an internal gear. Concretely, at first, the large-opening side of the preformed gear is inserted into the first receiving part  106  of the receiving jig  105 . Then, the end surface of the large opening is brought into contact with the second receiving part  107 , so that the preformed gear is placed on the receiving jig  105 . After that, the chuck members  108  hold the end surface of the small opening of the preformed gear. Then, the cutter  102  integrated with the holder  104  is moved toward the preformed gear, so that the cutter  102  is brought into contact with the inside surface of the preformed gear. Thus, the cutting process by means of the cutter  102  is started, and then the tooth part  153   a  is formed. 
     Herein, in general, for a gear, it is preferable to form a crowning on a tooth flank of a tooth part of the gear, in order to inhibit abnormal sound generation and to improve contact manner of the tooth flank. 
     However, in the cutter  102  of the cutting unit  101  as described above, it is difficult to control minute movement thereof. Thus, it is difficult to form a crowning on the tooth part  153   a . Then, in order to form a crowning on the tooth part  153   a  of the internal gear, after the tooth part  153   a  is formed by the cutting unit  101 , the tooth trace of the tooth part  153   a  is adjusted by another grinding process to form the crowning on the tooth flank. 
     SUMMARY OF THE INVENTION 
     In the method of forming a crowning as described above, there are problems in increase of machining processes and in increase of cost. The object of the present invention is to provide a method of forming a crowning on a tooth flank with less machining processes at less cost. 
     This invention is a forming method of a tooth trace of a gear comprising: a first step of preparing a preformed gear made of metal including a cylindrical main body and a tooth part having a flat tooth flank having a uniform thickness in a tooth trace direction inside the main body; and a second step of conducting a surface treatment locally to an outside surface of the main body in order to deform the outside surface of the main body in such a manner that the same becomes concave, so that a crowning bulging in the tooth trace direction is formed on the tooth flank of the tooth part. 
     According to the invention, the surface treatment (surface modification) is conducted locally to the outside surface of the main body so as to generate distortion locally, so that the outside surface of the main body is deformed to be concave in the tooth trace direction and a crowning bulging in the tooth trace direction may be formed on the tooth flank of the tooth part on the opposite side (inside surface). Thus, the crowning may be easily formed on the tooth flank of the tooth part of the internal gear. 
     In addition, even when gears of different sizes are formed, it is sufficient only to change a receiving jig or the like. That is, flexibility of a gear forming unit is increased. That is, by means of the same surface-treatment unit, it is possible to form gears of different sizes. 
     In addition, if the portion to undergo the surface treatment is changed, the portion to be dented in the tooth trace direction on the outside surface of the main body may be changed accordingly. Thus, a desired shape may be easily formed on the tooth flank of the tooth part. 
     In addition, if condition of the surface treatment such as an extent or a depth is changed, an extent, a shape or the like of a cavity formed on the outside surface of the main body may be changed accordingly. Thus, a crowning of various shapes and/or dimensions may be easily formed on the tooth flank of the tooth part. 
     Preferably, in the second step, the surface treatment is adapted to be conducted to a portion of the outside surface of the main body corresponding to a central portion in the tooth trace direction of the tooth part. In the case, a crowning bulging at a central portion in the tooth trace direction may be easily formed on the tooth flank of the tooth part. 
     For example, the surface treatment may be conducted by using a laser beam. Alternatively, the surface treatment may be conducted by using an electron beam. Alternatively, the surface treatment may be conducted by an induction hardening process. Alternatively, the surface treatment may be conducted by a shot peening process. 
     In addition, the concept of the present invention can be applied to an external gear as well as the internal gear. That is, the present invention is a forming method of a tooth trace of a gear comprising: a first step of preparing a preformed gear made of metal including a cylindrical main body and a tooth part having a flat tooth flank having a uniform thickness in a tooth trace direction outside the main body; and a second step of conducting a surface treatment locally to an inside surface of the main body in order to deform the inside surface of the main body in such a manner that the same becomes concave, so that a crowning bulging in the tooth trace direction is formed on the tooth flank of the tooth part. 
     According to the invention, the surface treatment (surface modification) is conducted locally to the inside surface of the main body so as to generate distortion locally, so that the inside surface of the main body is deformed to be concave in the tooth trace direction and a crowning bulging in the tooth trace direction may be formed on the tooth flank of the tooth part on the opposite side (outside surface). Thus, the crowning may be easily formed on the tooth flank of the tooth part of the external gear. 
     Even when external gears of different sizes are formed, it is sufficient only to change a receiving jig or the like. That is, by means of the same surface-treatment unit, it is possible to form external gears of different sizes. 
     In addition, if the portion to undergo the surface treatment is changed, the portion to be dented in the tooth trace direction on the inside surface of the main body may be changed accordingly. Thus, a desired shape may be easily formed on the tooth flank of the tooth part. 
     In addition, if condition of the surface treatment such as an extent or a depth is changed, an extent, a shape or the like of a cavity formed on the inside surface of the main body may be changed accordingly. Thus, a crowning of various shapes and/or dimensions may be easily formed on the tooth flank of the tooth part. 
     Preferably, in the second step, the surface treatment is adapted to be conducted to a portion of the inside surface of the main body corresponding to a central portion in the tooth trace direction of the tooth part. In the case, a crowning bulging at the central portion in the tooth trace direction may be easily formed on the tooth flank of the tooth part. 
     In a case of an external gear as well, for example, the surface treatment may be conducted by using a laser beam. Alternatively, the surface treatment may be conducted by using an electron beam. Alternatively, the surface treatment may be conducted by an induction hardening process. Alternatively, the surface treatment may be conducted by a shot peening process. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plan view of an internal gear; 
         FIG. 2A  is a sectional front view of a preformed gear (internal gear) before a tooth trace thereof is adjusted in accordance with an embodiment of the present invention; 
         FIG. 2B  is a sectional front view of an internal gear after a tooth trace thereof has been adjusted in accordance with the embodiment of the present invention; 
         FIG. 3A  is a sectional view of a tooth part of an internal gear formed by the embodiment of the present invention; 
         FIG. 3B  is an explanatory view of the deformation state of the tooth part of the internal gear formed by the embodiment of the present invention; 
         FIG. 4A  is a perspective view of a preformed gear before a tooth trace thereof is adjusted in accordance with the embodiment of the present invention; 
         FIG. 4B  is a perspective view of an internal gear after a tooth trace thereof has been adjusted in accordance with the embodiment of the present invention; 
         FIG. 5  is a partial sectional view showing a surface-treatment unit by means of an electron beam, used in the embodiment of the present invention; 
         FIG. 6  is a partial sectional view showing a surface-treatment unit by means of an induction hardening process, used in a second embodiment of the present invention; 
         FIG. 7  is a partial sectional view showing a surface-treatment unit by means of a shot peening process, used in a third embodiment of the present invention; 
         FIG. 8A  is a sectional front view of a preformed gear (external gear) before a tooth trace thereof is adjusted in accordance with an embodiment of the present invention; 
         FIG. 8B  is a sectional front view of an external gear after a tooth trace thereof has been adjusted in accordance with the embodiment of the present invention; and 
         FIG. 9  is a partial sectional front view showing a conventional cutting unit. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A forming method of an internal gear according to an embodiment of the present invention is explained based on  FIGS. 1 to 5 . 
       FIG. 1  is a plan view of an internal gear  51  having a tooth trace formed by the embodiment of the invention on a tooth flank of a tooth part  53  thereof. The internal gear  51  has a cylindrical main body  52 , which has a small opening  55  at one end and a large opening  56  at the other end. A plurality of projections  54  are formed at predetermined intervals on the outside periphery of the large opening  56  of the main body  52 . The tooth part  53  is a straight tooth part, and formed inside the main body  52 . In the tooth part  53 , as shown in  FIG. 2B , a central portion of the tooth part  53  in the tooth trace direction budges inwardly, that is, a circular arc crowning is formed. 
     The crowning on the tooth flank of the tooth part  53  of the internal gear  51  is formed by adjusting a flat tooth flank of a preformed gear, which is to be formed into the internal gear  51 , in the tooth trace direction. 
     In  FIG. 5 , a surface-treatment unit  1  used in the present embodiment is shown. The surface-treatment unit  1  has an electron-beam gun  2  and a holding tool  3  that can hold a work. The holding tool  3  has a plurality of chuck members  4 , a chuck body  5  on which the chuck members  4  are mounted, and a positioning tool  6  arranged inside the chuck members  4 . The positioning tool  6  has a contact end  7  opposite to the chuck body  5 , for positioning the work. 
     A method of forming (adjusting) the tooth flank(s) of the tooth part(s)  53  of the internal gear  51  by using the surface treatment unit  1  is explained with reference to  FIG. 5 . 
     At first, as shown in  FIG. 2A , the preformed gear  51   a  is prepared, in which a preformed tooth part  53   a  having a flat tooth flank of a uniform thickness in the tooth trace direction has been formed. Then, an end portion of the preformed gear  51   a  on the side of the large opening  56  is brought into contact with the contact end  7  of the positioning tool  6  so as to be positioned. Under that situation, an outside surface of a preformed main body  52   a  of the preformed gear  51   a  on the side of the large opening  56  is held by the chuck members  4 . 
     Then, as shown in  FIG. 5 , the electron-beam gun  2  is positioned at a position corresponding to a substantially center in the tooth trace direction of the preformed tooth part  53   a  of the preformed gear  51   a . Under that situation, the holding tool  3  starts to rotate. Thus, the preformed gear  51   a  held by the chuck members  4  also rotates together with the holding tool  3 . At the same time, the electron-beam gun  2  starts to irradiate an electron beam to the outside surface of the preformed gear  51   a . Thus, the outside surface of the preformed gear  51   a  receives the electron beam circumferentially, and hence the portion receiving the electron beam is distorted because of surface modification thereof. Thus, the outside surface of the preformed main body  52   a  is deformed to be dented in a circular arc in the tooth trace direction. Thus, the central portion(s) of the preformed tooth part(s)  53   a  in the tooth trace direction is deformed to bulge inwardly. 
     Herein, as shown in  FIG. 3A , the preformed tooth part  53   a  of the preformed gear  51   a  is formed in a substantially saddleback shape tapering from a tooth bottom portion B to a tooth tip portion A. In the preformed gear  51   a , as described above, when the outside surface of the preformed main body  52   a  is deformed to be dented inwardly, the tooth tip portion A at the substantially central portion of the preformed tooth part  53   a  in the tooth trace direction protrudes inwardly, as shown in  FIG. 3B . Thereby, from both ends of the preformed tooth part  53   a  to the substantially central portion in the tooth trace direction, the position of the tooth bottom portion B having a large thickness is also moved inwardly. In addition, as shown in  FIG. 4A , the tooth flank of the preformed tooth part  53   a  of the preformed gear  51   a  is flat and has a uniform thickness in the tooth trace direction before the electron beam is irradiated. However, since the portion receiving the electron beam is distorted because of surface modification thereof, as shown in  FIG. 4B , a substantially central portion of a tooth-trace center line C extending in the tooth trace direction through the median between the tooth tip portion A and the tooth bottom portion B is deformed to bulge outwardly maximally. That is, a crowning is formed on the tooth flank of the preformed tooth part  53   a , so that the desired tooth part  53  is formed. 
     Next, a second embodiment of the present invention is explained. Then, a preformed gear prepared for the second embodiment is substantially the same as the preformed gear  51   a  in the previous embodiment. 
       FIG. 6  shows an induction hardening unit  11  as a surface-treatment unit used in the second embodiment. The induction hardening unit  11  has a circular induction heating coil  12  and a supporting tool  13 . The supporting tool  13  has a cylindrical part  14  whose diameter is substantially the same as the inside surface of the large opening  56  of the preformed main part  52   a  of the preformed gear  51   a . A plate-like supporting part  15  extends serially from the cylindrical part  14 . A shaft  16  protrudes from a surface of the supporting part  15  on a side opposite to the cylindrical part  14 . The shaft  16  is connected to a controlling unit not shown. 
     A method of forming (adjusting) the tooth flank(s) of the tooth part(s)  53  of the internal gear  51  by using the induction hardening unit  11  is explained with reference to  FIG. 6 . 
     At first, as shown in  FIG. 2A , the preformed gear  51   a  is prepared, in which the preformed tooth part  53   a  having the flat tooth flank of the uniform thickness in the tooth trace direction has been formed. Then, the cylindrical part  14  is inserted into the large opening  56  of the preformed gear  51   a , and the end portion of the preformed gear  51   a  on the side of the large opening  56  is brought into contact with the supporting part  15 . Thus, the preformed gear  51   a  is supported by the supporting tool  13 . 
     Then, as shown in  FIG. 6 , the induction heating coil  12  is positioned concentrically with the preformed gear  51   a , at a position corresponding to a substantially center in the tooth trace direction of the preformed tooth part  53   a  of the preformed gear  51   a . Under that situation, the supporting tool  13  starts to rotate by means of the controlling unit not shown. Thus, the preformed gear  51   a  also rotates together with the supporting tool  13 . At the same time, a high-frequency electric current is supplied to the induction heating coil  12 , so that a circular portion corresponding to the substantially center(s) of the preformed tooth part(s)  53   a  of the preformed gear  51   a  in the tooth trace direction is heated. Just after that, a quenching liquid is ejected to the heated portion. After the heating process is completed, a cooling liquid W is ejected from a cooling unit  17  to the heated portion of the preformed gear  51   a . Thus, the preformed gear  51   a  is cooled, and then the forming (adjusting) method of the tooth trace is completed. That is, the heated portion is distorted because of surface modification thereof, so that the outside surface of the preformed main body  52   a  is deformed to be dented in a circular arc in the tooth trace direction. Thus, the central portion of the preformed tooth part  53   a  in the tooth trace direction is deformed to bulge inwardly. 
     Thus, according to the second embodiment, similarly to the above embodiment, the outside surface of the preformed main body  52   a  is deformed to be dented inwardly, and the tooth tip portion A at the substantially central portion of the preformed tooth part  53   a  in the tooth trace direction protrudes inwardly, as shown in  FIG. 3B . Thereby, from the both ends of the preformed tooth part  53   a  to the substantially central portion in the tooth trace direction, the position of the tooth bottom portion B having a large thickness is also moved inwardly. In addition, as shown in  FIG. 4B , the substantially central portion of the tooth-trace center line C extending in the tooth trace direction through the median between the tooth tip portion A and the tooth bottom portion B is deformed to bulge outwardly maximally. That is, a crowning is formed on the tooth flank of the preformed tooth part  53   a , so that the desired tooth part  53  is formed. 
     Next, a third embodiment of the present invention is explained. Then, in a preformed gear  51   a ′ prepared for the third embodiment, no protrusion is formed on the outside periphery of the preformed main body  52   a . However, the other structures of the preformed gear  51   a ′ are substantially the same as the preformed gear  51   a  of the previous embodiments. 
       FIG. 7  shows a shot peening unit  21  as a surface-treatment unit used in the third embodiment. The shot peening unit  21  has a nozzle head  22  and a supporting tool  23  for supporting the preformed gear  51   a ′. The supporting tool  23  has an upper cover  26  and a lower cover  29 . The upper cover  26  has an upper cup portion  24  of a cup shape having an inside surface of substantially the same diameter as the outside surface of the preformed gear  51   a ′ on the side of the large opening  56 , and an upper shaft  25  extending upward from the upper cup portion  24 . The lower cover  29  has a lower cup portion  27  of a cup shape having an inside surface of substantially the same diameter as the outside surface of the preformed gear  51   a ′ on the side of the small opening  55 , and a lower shaft  28  extending downward from the lower cup portion  27 . The upper shaft  25  of the upper cover  26  and the lower shaft  28  of the lower cover  29  are connected to a controlling unit not shown. A predetermined gap is formed between the upper cover  26  and the lower cover  29 , correspondingly to the substantially center of the preformed gear  51   a ′ in the tooth trace direction. In addition, the shot nozzle  22  is connected to a shot conveying hose not shown. 
     A method of forming (adjusting) the tooth flank(s) of the tooth part(s)  53  of the internal gear  51  by using the shot peening unit  21  is explained with reference to  FIG. 7 . 
     At first, as shown in  FIG. 2A , the preformed gear  51   a ′ is prepared, in which the preformed tooth part  53   a  having the flat tooth flank of the uniform thickness in the tooth trace direction has been formed. Then, the preformed gear  51   a ′ is inserted into the lower cup portion  27  of the lower cover  29 . Thus, substantially a half of the preformed gear  51   a ′ on the side of the small opening  55  is covered by the lower cup portion  27 . Then, the upper cover  26  is placed on the preformed gear  51   a ′, so that substantially a half of the preformed gear  51   a ′ on the side of the large opening  56  is covered by the upper cup portion  24 . Thus, the preformed gear  51   a ′ is supported by the upper cover  26  and the lower cover  29 . Under that situation, rotation of the upper cover  26  and the lower cover  29  is started by the controlling unit. Thus, the preformed gear  51   a ′ also rotates. At the same time, a shot S is supplied to the shot nozzle  22  through the conveying hose, and then the shot S is ejected from the shot nozzle  22  to the preformed gear  51   a ′. The shot S passes through the gap between the upper cover  26  and the lower cover  29  and hits the preformed gear  51   a ′. The portion hit by the shot S is distorted because of a local compression stress. Concretely, the outside surface of the preformed main body  52   a  of the preformed gear  51   a ′ is deformed to be dented in a circular arc in the tooth trace direction. Thus, the central portion of the preformed tooth part  53   a  in the tooth trace direction is deformed to bulge inwardly. 
     Thus, according to the third embodiment, similarly to the above two embodiments, the outside surface of the preformed main body  52   a  is deformed to be dented inwardly, and the tooth tip portion A at the substantially central portion of the preformed tooth part  53   a  in the tooth trace direction protrudes inwardly, as shown in  FIG. 3B . Thereby, from the both ends of the preformed tooth part  53   a  to the substantially central portion in the tooth trace direction, the position of the tooth bottom portion B having a large thickness is also moved inwardly. In addition, as shown in  FIG. 4B , the substantially central portion of the tooth-trace center line C extending in the tooth trace direction through the median between the tooth tip portion A and the tooth bottom portion B is deformed to bulge outwardly maximally. That is, a crowning is formed on the tooth flank of the preformed tooth part  53   a , so that the desired tooth part  53  is formed. 
     As a surface-treatment unit, a unit using a laser-beam may be used to form (adjust) the tooth trace(s) of the tooth part(s) of the internal gear, as well as the case of the electron beam. 
     As a surface-treatment unit, each unit using an electron beam, a laser beam, an induction hardening process or a shot peening process has been explained. However, the surface-treatment unit is not limited thereto. The surface-treatment unit may be any other unit for generating distortion by means of various surface modifications, compression stresses, and so on, as far as it can deform the outside surface of the internal gear in such a manner that the same is dented inwardly in a circular arc. 
     In addition, in the above embodiments, the central portion of the tooth part of the internal gear in the tooth trace direction bulges to form the crowning. However, the bulging portion is not limited to the central portion in the tooth trace direction. The bulging portion is suitably changeable depending on an object and/or an application. 
     Furthermore, if a size and/or a depth of the distortion caused by the surface-treatment unit is changed, a size and/or a shape of the circular arc dented inwardly on the outside surface of the internal gear may be changed accordingly. Thus, a size and/or a shape of the crowning may be changed. 
     In addition, the concept of the present invention can be applied to an external gear as well as the internal gear. For example, it can be also applied to an external gear wherein a tooth part is formed on an outside surface of a cylindrical main body. 
       FIG. 8A  shows a preformed gear  71   a  in which a preformed tooth part  73   a  having a flat tooth flank of a uniform thickness in the tooth trace direction has been formed. Then, correspondingly to substantially a center in the tooth trace direction of the preformed tooth part  73   a  of the preformed gear  71   a , from an inside of the preformed main body  72   a  of the preformed gear  71   a , a surface treatment is conducted using an electron beam, a laser beam, an induction hardening process or a shot peening process. Thus, the inside surface of the preformed gear  71   a  is deformed to be dented outwardly, and a tooth tip portion at the substantially central portion of the preformed tooth part  73   a  in the tooth trace direction protrudes outwardly. Thereby, from both ends of the preformed tooth part  73   a  to the substantially central portion in the tooth trace direction, the position of a tooth bottom portion having a large thickness is also moved outwardly. In addition, a substantially central portion of a tooth-trace center line extending in the tooth trace direction through the median between the tooth tip portion and the tooth bottom portion is deformed to bulge outwardly maximally. That is, a crowning is formed on the tooth flank of the preformed tooth part  73   a , so that the desired tooth part  73  is formed (see  FIG. 8B ).