Patent Publication Number: US-2021189922-A1

Title: Gear, balancer device, and balancer device provided with oil pump

Description:
TECHNICAL FIELD 
     This invention relates to a gear, a balancer device, and a balancer device provided with an oil pump. 
     BACKGROUND ART 
     For example, there has been known devices described in following patent documents 1 and 2, respectively, as a gear and a balancer device. The patent documents 1 and 2 disclose arts in which a plurality of groove portions are provided to the gear. 
     PRIOR ART DOCUMENT 
     Patent Document
     Patent Document 1: U.S. Pat. No. 2,207,290   Patent Document 2: Japanese Patent Application Publication No. 2014-134230   

     SUMMARY OF THE INVENTION 
     Problems which the Invention is Intended to Solve 
     However, in the conventional art, the groove portions are alternately provided on both side surfaces of the gear, so as to suppress the noise generated by the engagement of the gears. Accordingly, the size of the gear may be increased in a radial direction with respect to a rotation axis direction of the gear. 
     Moreover, in a case where the gear including the groove portions alternately provided on the both side surfaces of the gear are employed in the balancer device, the oil pump, and the balancer device provided with the oil pump, the sizes thereof may be increased. 
     It is, therefore, an object of the present invention to provide a gear, a balancer device, and a balancer device provided with an oil pump which are devised to solve the above-described problems, to suppress the noise generated by the engagement of the gears, and to suppress the size increase of the gear. 
     Means for Solving the Problem 
     In one aspect according to the present invention, a gear configured to rotate as a unit with a shaft, the gear includes a plurality of annular grooves formed on both side surfaces of the gear, and partially overlapped when viewed in the direction of the rotation axis and the radial direction. 
     Benefit of The Invention 
     By the present invention, it is possible to provide a gear, a balancer device, and a balancer device provided with an oil pump which are devised to suppress the noise generated by the engagement of the gears, and to suppress the size increase of the gear. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a front view showing a state in which a balancer device according to embodiments of the present invention is mounted on an engine. 
         FIG. 2  is a sectional view taken along a direction II-II of  FIG. 1 . 
         FIG. 3  is a perspective view showing a state where an oil pump according to the embodiments of the present invention is assembled to the balancer device. 
         FIG. 4  is a back view showing the oil pump and the balancer device according to the embodiments of the present invention. 
         FIG. 5  is a view of a housing device when viewed from a bottom portion in a case where a lower housing is detached in the embodiments of the present invention. 
         FIG. 6  is a plan view showing the balancer device according to the embodiments of the present invention. 
         FIG. 7  is a sectional view taken along a line VII-VII of  FIG. 6 . 
         FIG. 8  is a side view showing a plane bearing according to the embodiments of the present invention, 
         FIG. 9  is an exploded perspective view showing components of the oil pump according to the present invention. 
         FIG. 10  is a front view showing the oil pump in a state where a cover member is detached in the embodiments of the present invention. 
         FIG. 11A  is a perspective view showing a main gear according to the embodiments of the present invention when viewed from a pump side. 
         FIG. 11B  is a perspective view showing the main gear according to the embodiments of the present invention when viewed from an opposite pump side. 
         FIG. 11C  is a plan view showing the main gear according to the embodiments of the present invention when viewed from the opposite pump side. 
         FIG. 11D  is a sectional view taken along a direction XID-XID direction of  FIG. 11C . 
         FIG. 12  is a partially enlarged view of a portion XII of  FIG. 2 . 
         FIG. 13A  is a sectional view for explaining an actuation of the main gear and a balancer drive shaft according to a first embodiment of the present invention. 
         FIG. 13B  is a view showing a relationship of a force acted to the main gear according to the first embodiment of the present invention. 
         FIG. 14A  is a sectional view for explaining the actuation of the main gear and the balancer drive shaft according to the first embodiment of the present invention. 
         FIG. 14B  is a view showing the relationship of the force acted to the main gear according to the first embodiment of the present invention. 
         FIG. 15A  is a perspective view showing a crank gear according to the embodiments of the present invention, when viewed from the pump side. 
         FIG. 15B  is a perspective view showing the crank gear according to the embodiments of the present invention, when viewed from the opposite pump side. 
         FIG. 15C  is a plan view showing the crank gear according to the embodiments of the present invention, when viewed from the opposite pump side. 
         FIG. 15D  is a sectional view taken along a direction XVD-XVD in  FIG. 15C . 
         FIG. 16  is a sectional view showing a main gear according to a second embodiment of the present invention. 
         FIG. 17  is a sectional view showing a main gear according to a third embodiment of the present invention. 
         FIG. 18  is a sectional view showing a main gear according to a fourth embodiment of the present invention. 
         FIG. 19  is a sectional view showing a main gear according to a fifth embodiment of the present invention. 
         FIG. 20  is a sectional view showing a main gear according to a sixth embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, balancer devices according to embodiments of the present invention are explained. The present invention is not limited to the below-described embodiments. The present invention includes various variations and applications in a technical concept of the present invention. 
     First Embodiment 
       FIG. 1  is a front view showing a state in which a balancer device according to embodiments of the present invention is mounted on an engine.  FIG. 2  is a sectional view taken along a direction II-II of  FIG. 1 .  FIG. 3  is a perspective view showing a state where an oil pump according to the embodiments of the present invention is assembled to the balancer device.  FIG. 4  is a back view showing the oil pump and the balancer device according to the embodiments of the present invention.  FIG. 5  is a view of a housing device when viewed from a bottom portion in a case where a lower housing according to the embodiments of the present invention are detached.  FIG. 6  is a plan view showing the balancer device according to the embodiments of the present invention.  FIG. 7  is a sectional view taken along a line VII-VII of  FIG. 6 . An arrow P of  FIG. 2  shows a direction of a pump side on which an oil pump  4  is disposed. The direction shown by the arrow P shows an identical direction in  FIG. 11  and  FIG. 15 . 
     As shown in  FIG. 1 , the balancer device  1  is received within an oil pan  30  mounted to a lower portion of a cylinder block SB of an internal combustion engine E. The balancer device  1  is configured to be driven and rotated by a crank gear  3  fixed to a crank shaft  2 . 
     An oil pump  4  is integrally provided to the balancer device  1 . The oil pump  4  is configured to receive the rotation force from the balancer device  1 , and to be driven. Details are described later. 
     As shown in  FIG. 1  to  FIG. 5 , the balancer device  1  includes a main gear (drive gear)  5  which is engaged with a crank gear (input gear)  3 , and to which a rotation force from the crank gear  3  is transmitted; a balancer drive shaft  6  to which the rotation force from the main gear  5  is transmitted; a balancer drive gear  7  fixed to the balancer drive shaft  6 ; a balancer driven gear  8  including teeth engaged with teeth of the balancer drive gear  7 ; a balancer driven shaft  9  to which the rotation force from the balancer driven gear  8  is transmitted. 
     The oil pump  4  is configured to suck the oil stored in the oil pan  30 , and to discharge and supply the oil into the internal combustion engine E. 
     As shown in  FIG. 3  and  FIG. 4 , in the balancer device  1 , a plurality of foot portions  1   a  (four foot portions in this embodiment) are fixed on a lower surface of the cylinder block SB of the internal combustion engine E through four mounting bolts (not shown) which are mounting means. The four foot portions  1   a  are integrally provided on the upper surface of the upper housing  10 . Positioning hollow pins  1   b  protrude, respectively, from upper ends of the four foot portions  1   a  in the upward direction. 
     The balancer device  1  includes the upper housing  10 ; and a lower housing  11  which is tightened to the upper housing  10  on a bottom portion side of the oil pan  30  by tightening bolts  25  which a plurality of fixing means. Each of the upper housing  10  and the lower housing  11  is made from aluminum alloy which is metal material. The balancer drive shaft  6  and the balancer driven shaft  9  are rotatably supported in a receiving portion formed between the upper housing  10  and the lower housing  11 . The balancer drive shaft  6  and the balancer driven shaft  9  are a pair of balancer shafts disposed in parallel with each other. The helical type main gear  5  is provided to one end portion of the balancer drive shaft  6  in the rotation axis direction of the balancer drive shaft  6 . The main gear  5  is engaged with the crank gear  3  configured to be driven and rotated by the crank shaft  2 , so as to receive the rotation force. Besides, as shown in  FIG. 7 , the upper housing  10  and the lower housing  11  are mutually positioned by two pins  25   a  and  25   b.    
     Moreover, as shown in  FIG. 5 , the helical type valance drive gear  7  is fixed on the other end side of the balancer drive shaft  6  in the rotation axis direction of the balancer drive shaft  6  so as to rotate as a unit with the balancer drive shaft  6 . Furthermore, the helical type balancer driven shaft  9  is fixed to the balancer driven shaft  9 . The balancer drive shaft  9  is engaged with the balancer drive gear  7  to receive the rotation force. 
     These upper housing  10  and lower housing  11  constitute a balancer housing which is a housing. 
     The lower housing  11  is formed into a rectangular box shape which is substantially identical to the shape of the upper housing  10 . Moreover, one end surface of the lower housing  11  is a flat mounted surface  28  ( FIG. 6 ) to which the oil pump  4  is mounted. This mounted surface  28  includes a plurality of internal screw holes (not shown) (four internal screw holes in this embodiment) which are formed in a side portion. 
     As shown in  FIG. 5 , the balancer drive shaft  6  includes a pair of journal portions  6   a  and  6   b  which are located on both end sides in the rotation axis direction, and which are rotatably supported by a pair of plane bearings  12  and  13  which are bearing portions (bearing metals) provided between the upper housing  10  and the lower housing  11 . 
     In the balancer drive shaft  6 , the main gear  5  at the one end portion is engaged with the crank gear  3 , so as to receive the rotation force of the crank shaft  2 . Arrows in the drawing represent the rotation directions. In this way, when the balancer drive shaft  6  is rotated, the balancer drive gear  7  fixed at the other end of the balancer drive shaft  6  and the balancer driven gear  8  fixed at the balancer driven shaft  9  are rotated at double the speed of the crank shaft  2  in the opposite directions. That is, the balancer drive shaft  6  and the balancer driven shaft  9  perform two rotations per one rotation of the crank shaft  2 . 
     Moreover, the balancer drive shaft  6  includes a semicircle balancer weight  6   c  integrally provided between the pair of journal portions  6   a  and  6   b  in the axial direction. 
     The balancer driven weight shaft  9  includes a pair of journal portions  9   a  and  9   b  which are formed on the both end sides of the rotation axis direction similarly to the balancer drive shaft  6 , and which are rotatably supported by a pair of plane bearings  14  and  15  which are bearing portions (bearing metals) provided between the upper housing  10  and the lower housing  11 . Moreover, a semi-circular balancer weight  9   c  (second balancer weight) is integrally provided between the pair of journal portions  9   a  and  9   b.    
     As shown in  FIG. 5  and  FIG. 7 , each of the plane bearings  12  to  15  has half circular shapes on the upper housing  10  side and the lower housing  11  side. Each of the plane bearings  12  to  15  has a cylindrical entire shape by contacting confronting end portions. The half portion of each of the plane bearings  12  to  15  is disposed within a semi-circular bearing groove formed between confronting surfaces of a pair of upper and lower partition walls  16   a ,  16   b ,  17   a , and  17   b  provided between the upper housing  10  and the lower housing  11 . 
       FIG. 8  is a side surface view of the lower half portion of the plane bearings  12  to  15  according to the embodiment of the present invention. 
     As shown in  FIG. 8 , each of the plane bearings  12  to  15  has a two layer configuration of the inner circumference portions  12   a  to  15   a  and the outer circumference portions  12   b  to  15   b . The inner circumference portions  12   a  to  15   a  are mainly made from aluminum alloy which is soft metal. On the other hand, the outer circumference portions  12   b  to  15   b  are made from iron series metal. 
     In this way, the inner circumference portions  12   a  to  15   a  are mainly made from the soft aluminum alloy. With this, it is possible to embed the contamination such as the metallic wear debris entering between the inner circumference surfaces of the inner circumference portions  12   a  to  15   a  and the outer circumferences of the journal portions  6   a ,  6   b ,  9   a , and  9   b.    
     Moreover, each of the inner circumference portions  12   a  to  15   a  has a thickness t of substantially 0.2 mm. On the other hand, the outer circumference portions  12   b  to  15   b  has a thickness ti of substantially 1.3 mm. Furthermore, rotation stop protrusions  12   c  to  15   c  are provided on the outer circumference surfaces of the outer circumference portions  12   b  to  15   b . Each of the rotation stop protrusions  12   c  to  15   c  is configured to restrict co-rotation during the rotations of the balancer drive shaft  6  and the balancer driven shaft  9 . 
     Moreover, a passage groove (not shown) is formed in a confronting surface of each of the partition walls  16   a ,  16   b ,  17   a , and  17   b  which confronts the lower housing  11 . Each of the passage grooves is configured to supply the lubricant oil of the plane bearings  12  to  15 . The passage grooves are connected to annular grooves  20   a ,  20   b ,  20   c , and  20   d  shown in  FIG. 5  and  FIG. 7 . Each of the annular grooves  20   a  to  20   d  is formed at a substantially central portion of the inner circumference surface of each of the bearing grooves in the widthwise direction. 
     As shown in  FIG. 7 , the plane bearings  13  and  15  includes connection holes  13   d  and  15   d  which are oil holes formed in the circumferential wall at predetermined positions to penetrate through the circumferential wall, and connected to the annular grooves  20   b  and  20   d . The four connection holes  13   d  are formed on the same circumference at substantially central position of the widthwise direction of the circumferential wall of the plane bearing  13  (two in each bearing). The four connection holes  15   d  are formed on the same circumference at substantially central position of the widthwise direction of the circumferential wall of the plane bearing  15  (two in each bearing). These connection holes  13   d  and  15   d  are configured to introduce the oil into the clearances between the inner circumference surfaces of the inner circumference portions  13   a  and  15   a  and the outer circumference surfaces of the journal portions  6   a  and  9   a . Besides, the oil is introduced into the plane bearings  12  and  14  by the similar configurations, although these are not shown. 
     The balancer driven shaft  9  includes one end portion in the rotation axis direction (the opposite side of the balancer driven gear  8 ). The oil pump drive gear  21  is fixed at the one end portion of the balancer driven shaft  9 . The oil pump drive gear  21  is an outer gear having a diameter smaller than that of the main gear  5 . This oil pump drive gear  21  is configured to drive the oil pump  4 . 
       FIG. 9  is an exploded perspective view of the oil pump according to the embodiment of the present invention.  FIG. 10  is a front view of the oil pump in a state where the cover member is detached. 
     The oil pump  4  is a normal variable displacement vane pump. Accordingly, the oil pump  4  is briefly explained. The pump housing  1  is mounted to the mounted surface  28  sides of the both housing  10  and  11  of the balancer device  1  by a plurality of bolts  26  (four bolts in this embodiment) which are fixing means. 
     This pump housing includes a housing main body  31  made from the resin and the metal such as the aluminum alloy; and the cover member  32  made from the aluminum alloy. 
     The housing main body  31  includes an opened one end side; and a pump receiving chamber. With this, the housing main body  31  has a U-shaped section. Moreover, the cover member  32  is mounted to close the opening of the housing main body  31 . The cover member  32  has a thickness smaller than that of the housing main body  31 . 
     Moreover, the oil pump  4  includes a pump shaft  33 : a rotor  34 ; and vanes  35 . The pump shaft  33  is disposed at a substantially central portion of the pump receiving chamber. The pump shaft  33  includes both end portions in the rotation axis directions. The both end portions of the pump shaft  33  penetrate through the housing main body  31  and the cover member  32 . The both end portions of the pump shaft  33  are rotatably supported. The rotor  34  are rotatably received within the pump receiving chamber. A central portion of the rotor  34  is spline-connected to the pump shaft  33 . The vanes  35  are received within a plurality of slots (seven slots in this embodiment) cut and formed in the outer circumference portion of the rotor  34  in the radial directions to be projectable and retractable from and into the slots. 
     Moreover, the oil pump  4  includes a cam ring  37 ; a coil spring  38  which is an urging member; and a pair of vane rings  39  and  39 . The cam ring  37  has a ring shape having a circular hole formed in the inner circumference. Furthermore, the hole of the cam ring  37  is contacted on the outer circumference sides of the vanes  35 . 
     Moreover, the cam ring  37  is configured to be swung. The cam ring  37  is configured to be swung to vary an eccentric amount of the hole of the cam ring  37  with respect to the center of the rotation of the rotor  34 . A plurality of pump chambers  36  are formed by the inner circumference of the cam ring  37 , the outer circumference surface of the rotor  34 , and the adjacent vanes  35  and  35 . 
     The coil spring  38  is received within the housing main body  31 . The coil spring  38  is configured to constantly urge the cam ring  37  in a direction in which the eccentric amount of the center of the hole of the cam ring  37  with respect to the center of the rotation of the rotor  34  is increased. 
     The vane rings  39  and  39  are abutted on the inner circumference sides of the vanes  35  disposed within the slots of the rotor  34 . 
     The cam ring  37 , the pump shaft  33 , the rotor  34 , and the vanes  35  constitute a pump element. 
     The pump main body  31  includes a bearing hole  31   a  which is formed at a substantially central position of a bottom surface of the pump receiving chamber of the pump main body  31  to penetrate through the pump main body  31 , and which rotatably supports the one end portion of the pump shaft  33 . Furthermore, the pump main body  31  includes a pivot pin hole (not shown) which is formed in the bottom surface of the pump receiving chamber of the housing main body  31 , and into which a pivot pin  40  is inserted. Moreover, a pin groove is formed in the inner circumference wall of the pump receiving chamber. The pin groove extends in the axial direction of the pivot pin  40 . 
     Furthermore, a seal sliding surface  31   c  is formed on the inner circumference wall of the pump receiving chamber. A seal member  27  (described later) of the cam ring  37  is slidably abutted on the seal sliding surface  31   c.    
     The housing main body  31  includes a plurality of bolt insertion holes  31 F (three bolt insertion holes in this embodiment) formed in boss portions formed on the outer circumference side. A plurality of second bolts  29  (three second bolts in this embodiment) which are mounting means are inserted into these bolt insertion holes  31 F, so as to connect the housing main body  31  and the cover member  32 . 
     The housing main body  31  includes three bolt insertion holes  31   g  which penetrate through the housing main body  31 , and into which three bolts  26  of four bolts  26  are inserted. Moreover, the housing main body  31  includes a positioning hole  31   h  which is similarly formed on a lower portion side to penetrate through the housing main body  31 , and into which a positioning pin  63  to position the cover member  32  to the balancer device  1  is inserted. 
     As shown in  FIG. 9 , the cover member  32  includes a bearing hole  32   a  which is formed at a position confronting the bearing hole  31   a  so as to penetrate through the cover member  32 , and which rotatably supports the other end side of the pump shaft  33  in the axial direction of the pump shaft  33 . This cover member  32  includes a housing mounting surface  32   b  which is on the inner end side, and to which the housing main body  31  is mounted; and a balancer mounting surface  32   c  which is on the outer end side, and which is abutted and mounted on the mounted surface  28  of the balancer device  1 . 
     The cover member  32  includes three internal screw holes  32   d  which are formed on the outer circumference portion side, and in which three second bolts  29  are fixed. Moreover, the cover member  32  includes four bolt insertion holes  32   e  which penetrate through the cover member  32 , and into which the four bolts  26  are inserted. 
     The cover member  32  includes two positioning holes  32 F which penetrate through the cover member  32 , and into which the positioning pins  25   c  and  63  are inserted. 
     Each of the housing main body  31  and the cover member  32  include a suction port  41  and a discharge port  42  which are formed on the outer circumference sides of the mounting surfaces  31   e  and  32   e  confronting each other. The suction port  41  is a suction portion. The discharge port  42  is a discharge port. The suction port  41  is formed and opened in a region (suction region) in which the internal volumes of the pump chambers  36  are increased in accordance with the pump operation of the pump element. The suction port  41  has an arc recessed shape. On the other hand, the discharge port  42  is formed and opened in a region (discharge region) in which the internal volumes of the pump chambers  36  are decreased in accordance with the pump operation of the pump element. The suction port  41  has an arc recessed shape. The suction port  41  and the discharge port  42  substantially confront each other to sandwich the bearing holes  31   a  and  32   a.    
     As shown in  FIG. 10 , the suction port  41  includes a suction hole  41   a  which is disposed on a spring receiving chamber  44  (described later) side, and which is formed and opened to the outside to penetrate through the bottom wall of the cover member  32 . With this, the lubricant oil within the oil pan  30  is sucked through a strainer  46 , a suction passage  47 , the suction hole  41   a , and the suction port  41  to the pump chambers  36  in the suction region. 
     The discharge port  42  is connected to a discharge passage  48  formed in the bottom wall of the housing main body  31  to penetrate through the housing main body  31 . This discharge passage  48  is connected through a discharge hole (not shown) on the downstream side of the discharge port  42  to the main oil gallery  18 . Besides, the discharge passage  48  includes a part of the lower side of the discharge port  42 , that is, a part between the discharge port  42  and the discharge hole. 
     The main oil gallery  18  is configured to supply the oil to an oil jet configured to inject the cooling oil to the piston, a valve timing control device, and bearings of the crank shaft  2 . 
     An oil filter  49  is provided in the main oil gallery  18 . The oil filter  49  is configured to catch the foreign object within the pressurized oil supplied from the discharge passage  48 . 
     Moreover, a relief valve  24  is provided in the discharge passage  48 . The relief valve  24  is configured to suppress the damage of the oil filter  49  when the discharge pressure is excessive. As shown in  FIG. 8 , the relief valve  24  includes a ball valve element  24   a  configured to open and close an opening end of a bifurcated passage bifurcated from the discharge passage  48 ; and a coil spring  24   b  configured to urge the ball valve element  24   a  in the closing direction; and an annular spring retainer  24   c.    
     The main oil gallery  18  includes a supply passage  18   a  formed and bifurcated to supply the oil through the electromagnetic switching valve  22  to a control hydraulic chamber  45  described later. 
     The electromagnetic switching valve  22  is connected to a supply and discharge passage  23 . The supply and discharge passage  23  is configured to introduce the hydraulic pressure of the main oil gallery  18  through the supply passage  18   a  to the control hydraulic chamber  45 , and to discharge the oil within the control hydraulic chamber  45  to the oil pan  30 . Moreover, the electromagnetic switching valve  22  includes a pilot port connected to a pilot passage (not shown) bifurcated from the supply passage  18   a ; a supply and discharge port connected to the supply and discharge port  23 ; a drain port connecting the supply and discharge passage  23  and the discharge passage; and a supply port connected to the supply passage  18   a . The discharge passage is connected to the oil pan  30 . 
     An oil pump driven gear  43  (driven side helical gear) is fixed by the press fit to the one end portion of the pump shaft  43  in the rotation axis direction, which protrudes from the bearing hole  32   a . The oil pump driven gear  43  is engaged with the oil pump drive gear  21  (drive side helical gear). With this, the rotation force of the balancer driven shaft  9  is transmitted through the oil pump drive gear  21  and the oil pump driven gear  43  to the pump shaft  33 . 
     Moreover, the pump shaft  33  is set to have the substantially identical rotation speed of the crank shaft by the speed reduction ratio between the oil pump drive gear  21  and the oil pump driven gear  43 . 
     The rotor  34  includes an insertion hole which is formed at a center to penetrate through the rotor  34 , and into which the pump shaft  33  is inserted. This insertion hole includes an inner circumference surface having a spline groove formed in the axial direction. 
     The movements of the vanes  35  to the inner circumference side of the rotor  34  are restricted by the vane rings  39  and  39 . Accordingly, the rotor  34  can be moved relative to the cam ring  37  and the vane rings  39  and  39  in a state where the vanes  35  are abutted on the inner circumference surface of the cam ring  37  and the outer circumference surfaces of the vane rings  39  and  39 . 
     The cam ring  37  is integrally formed into the cylindrical shape by molding the iron series metal by the sintering method. This cam ring  37  is configured to be swung about the pivot groove  37   a  formed in the outer circumference portion, and the pivot pin  40  which is a swinging fulcrum, and which is supported by the pin groove. Moreover, the cam ring  37  includes an arm portion  37   b  which is formed at a position substantially opposite side of the pivot groove  37   a  with respect to the center of the cam ring  37 , which protrudes in the radial direction, and which is linked with the coil spring  38 . 
     In this case, the coil spring  38  which is the urging member is received within the spring receiving chamber  44  of the housing main body  31  which is connected through the suction hole  41   a  to the pump receiving chamber. 
     This coil spring  38  is configured to constantly urge the cam ring  37  through the arm portion  37   b  in a direction in which the eccentric amount of the cam ring  37  with respect to the center of the rotation of the rotor  34  is increased (in the counter-clockwise direction in  FIG. 10 ), by the elastic force based on a set load W. With this, an outer surface of the arm portion  37   b  of the cam ring  37  is pressed on a stopper surface  44   a  formed on the wall surface of the spring receiving chamber  44 . In this state, the cam ring  37  is held at a position at which the eccentric amount of the cam ring  37  with respect to the center of the rotation of the rotor  34  is maximum. 
     Moreover, as shown in  FIG. 10 , a seal member  27  is received and held in a seal holding groove provided in the outer circumference portion of the cam ring  37  to confront the seal sliding surface  31   c.    
     The control hydraulic chamber  45  is provided in the outer circumference region between the pivot groove  37   a  of the cam ring  37 , and the seal member  27 . This control hydraulic chamber  45  is separated by the pivot pin  40  and the seal member  27  between the inner circumference surface of the housing main body  31  and the outer circumference surface of the cam ring  37 . 
     The control hydraulic chamber  45  is connected through the supply and discharge passage  23  and the electromagnetic switching valve  22  to the supply passage  18   a . Accordingly, the hydraulic presser from the main oil gallery  18  is supplied through the supply passage  18   a , the electromagnetic switching valve  22 , and the supply and discharge passage  23  to the control hydraulic chamber  45 . Moreover, the internal hydraulic pressure of the control hydraulic chamber  45  is discharged through the supply and discharge passage  23  and the electromagnetic switching valve  22 . 
     The cam ring  37  includes a pressure receiving surface  37   e  which is an outer circumference surface confronting the control hydraulic chamber  45 . The cam ring  37  is configured to provide the swing force (the movement force) in a direction in which the eccentric amount with respect to the center of the rotation of the rotor  34  is decreased (in the clockwise direction in  FIG. 10 ) against the urging force of the coil spring  38 , by the hydraulic pressure received by the pressure receiving surface  37   e  from the supply passage  18   a.    
     That is, the internal hydraulic pressure of the control hydraulic pressure  45  is acted to the cam ring  37  in the direction in which the eccentric amount with respect to the center of the rotation of the rotor  34  is decreased, so as to control the movement amount of the cam ring  37  in the concentric direction. 
     In this case, the swing position of the cam ring  37  is balanced by a predetermined force relationship between the urging force in the eccentric direction of the cam ring  37  by the urging force of the coil spring  38 , and the urging force based on the internal pressure of the control hydraulic chamber  45 . 
     The electromagnetic switching valve  22  is configured to produce the solenoid thrust to be proportional to the duty ratio by the pulse current from the control unit, and to act the thrust to a three-way valve in a direction identical to the pilot pressure. 
     That is, when the pulse current from the control unit to the electromagnetic switching valve  22  is stopped, and the electromagnetic switching valve  22  is in the deenergized state (the duty ratio=0), there is no solenoid thrust. The electromagnetic switching valve  22  has a set pressure determined by the spring force. 
     With this, by the three-way valve, the supply and discharge passage  23  is disconnected to the supply and discharge port, and the supply and discharge passage  23  is connected to the drain port. With these, the internal hydraulic pressure of the control hydraulic chamber  45  is discharged to be the low pressure state. 
     When the control unit outputs a signal to energize the coil of the electromagnetic switching valve  22 , and the energization amount (the duty ratio) is increased, the solenoid thrust is increased to assist the pilot pressure. Accordingly, in the electromagnetic switching valve  22 , the three way valve is acted against the spring force so that the supply and discharge port is connected to the supply port, and so that the supply and discharge port is disconnected to the drain port. With this, the electromagnetic switching valve  22  can be actuated by the hydraulic pressure equal to or smaller than the set pressure of the spring force, and controlled to the constant low pressure. 
     Accordingly, the internal pressure of the control hydraulic chamber  45  is increased. The cam ring  37  is continuously swung in the concentric direction against the spring force of the coil spring  38  to decrease the pump discharge pressure. 
     The control unit is configured to control the actuation of the electromagnetic switching valve  22  based on the oil temperature and the water temperature of the engine, the driving state of the internal combustion engine such as the engine speed and the load, the hydraulic pressure information signal from the hydraulic pressure sensor (not shown) provided on the downstream side of the oil filter  49  of the main oil gallery  18 , and so on. That is, the electromagnetic switching valve  22  is configured to continuously control the hydraulic pressure within the control hydraulic chamber  45  based on the hydraulic pressure information signal from the hydraulic pressure sensor. With this, the fuel economy is improved. 
     In the engagement portion of the gears, the teeth hitting noise is generated by the engagement of the gears. In particular, the semi-circular balancer weight  6   c  is attached to the balancer drive shaft  6 . Accordingly, the balancer drive shaft  6  is rotated while being deformed in the arcuate shape. Consequently, the main gear  5  mounted to the balancer drive shaft  6  is rotated in a state where the main gear  5  is inclined. The teeth hitting noise is generated between the main gear  5  and the crank gear  3  engaged with the main gear  5  in accordance with the inclination of the main gear  5 . Hereinafter, means to decrease the noise generated by the engagement of the gears is explained with reference to the drawings. 
       FIG. 11A  is a perspective view showing a main gear according to the embodiments of the present invention when viewed from a pump side.  FIG. 11B  is a perspective view showing the main gear according to the embodiments of the present invention when viewed from an opposite pump side.  FIG. 11C  is a plan view showing the main gear according to the embodiments of the present invention when viewed from an opposite pump side.  FIG. 11D  is a sectional view taken along a direction XID-XID direction of  FIG. 11C .  FIG. 12  is a partially enlarged view of a portion XII of  FIG. 2 . 
     The main gear  5  includes an opening portion  5   d  which is formed at a central portion of the main gear  5 , and through which the balancer drive shaft  6  passes. The teeth portion  5   a  of the main gear  5  has a predetermined torsion angle with respect to the rotation axis. 
     The teeth portion  5   a  of the main gear  5  is engaged with the teeth portion  3   a  of the crank gear  3 . The teeth portion  3   a  of the crank gear  3  has a predetermined torsion angle with respect to the rotation axis. The balancer drive shaft  6  is inserted into the opening portion  5   d  of the main gear  5 . The base portion  5   b  of the main gear  5  is fixed to the balancer drive shaft  6 . 
     The main gear  5  includes a first annular groove  51  and a second annular groove  52  which are formed on both side surfaces of the main gear  5  in the rotation axis direction of the balancer drive shaft  6 , and which are a plurality of annular grooves. The first annular groove  51  is formed on a first side surface which is in a direction of the thrust received by the teeth portion  5   a . The second annular groove  52  is formed on a second side surface opposite to the first side surface on which the first annular groove  51  is formed. Moreover, the second annular groove  52  is overlapped with the first annular groove  51  when viewed from the direction of the rotation axis. In addition, the second annular groove  52  is partially overlapped with the first annular groove  51  in the radial direction with respect to the rotation axis. The overlapping portion in which the first annular groove  51  and the second annular groove  52  are overlapped is positioned on a side opposite to the side receiving the thrust. 
     A small thickness portion  5   c  is formed between the first annular groove  51  and the second annular groove  52 . The small thickness portion Sc is disposed and inclined from the base portion  5   b  to the teeth portion  5   a  of the main gear  5  in the extension direction (the outside) of the rotation axis of the balancer drive shaft  6 . That is, the small thickness portion  5   c  is inclined so that the side opposite to the balancer weight  6   c  side in the rotation axis direction directs to the outside in the radial direction. 
     The first annular groove  51  is recessed in the axially inward direction from the end surface portion  5   b   1  of the base portion  5   b  in the axial direction. The first annular groove  51  includes a curved surface portion  51   a  which is formed on the recessed bottom portion, and which has a predetermined radius of curvature R1. The end surface portion  5   b   1  and the curved surface portion  51   a  are connected by a first inner circumference surface  5   b   3  of the base portion  5   b . The first inner circumference surface  5   b   3  is provided on the rotation axis side in the radial direction with respect to the rotation axis. A linear line portion  51   b  is formed on the surface of the small thickness portion  5   c  linearly extending from the curved surface portion  51   a  in the radially outward direction. The linear line portion  51   b  is connected to the first outer circumference surface  5   a   3  of the teeth portion  5   a  of the main gear  5 . The first outer circumference surface  5   a   3  is connected to the end surface portion  5   a   1 . The first inner circumference surface  5   b   3  confronts the first outer circumference surface  5   a   3 . A depth of the first outer circumference surface  5   a   3  in the rotation axis direction is smaller than that of the first inner circumference surface  5   b   3 . 
     On the other hand, the second annular groove  52  is recessed in the axially outward direction from the end surface portion  5   a   2  of the teeth portion  5   a  in the axial direction. The second annular groove  52  includes a curved surface portion  52   a  which is formed on the recessed bottom portion, and which has a predetermined radius of curvature R2. The end surface portion  5   a   2  and the curved surface portion  52   a  are connected by a second outer circumference surface  5   a   4  formed on the radially outer side (the teeth portion  5   a  side) with respect to the rotation axis. A curved surface portion  52   b  is formed on the surface of the small thickness portion  5   c  extending from the curved surface portion  52   a  in the radially inward direction. The curved surface portion  52   b  is bulged (raised) toward the opposite extension direction (the inner side) of the rotation axis to have a predetermined radius of curvature R3. The curved surface portion  52   b  is connected to the base portion  5   b  of the main gear  5  through a curved surface portion  52   c  having a predetermined radius of curvature R4. The curved surface portion  52   c  and the end surface portion  5   b   2  of the base portion  5   b  are connected by the second inner circumference surface  5   b   4  formed on the radially inner side (the rotation axis side) with respect to the rotation axis. The second inner circumference surface  5   b   4  confronts the second outer circumference surface  5   a   4 . A depth of the second outer circumference surface  5   a   4  in the rotation axis direction is greater than that of the second inner circumference surface  5   b   4 . Moreover, the first annular groove  51  is deviated from the second annular groove  52  in the radial direction with respect to the rotation axis. 
     The axial end surface portion  5   b   2  of the base portion  5   b  is positioned inside the axial end surface portion  5   a   2  of the teeth portion  5   a  in the rotation axis direction. 
     The first annular groove  51  is largely recessed in the axially inward direction at a position near the base portion  5   b  located radially inside the main gear  5 . The first annular groove  51  has a recessed amount (a depth of the groove) which is smaller toward the radially outward side on which the teeth portion  5   a  is located. Conversely, the second annular groove  52  has a recessed amount (a depth of the groove) which is greater from the position near the base portion  5   b  located radially inside the main gear  5 , toward the radially outward side on which the teeth portion  5   a  is located. That is, in the first annular groove  51 , the recessed amount (the depth of the groove) on the radially inner side is greater than that of the radially outer side. In the second annular groove  52 , the recessed amount (the depth of the groove) on the radially outer side is greater than that of the radially inner side. A section of the main gear  5  taken along the rotation axis has a Z shape by the teeth portion  5   a , the base portion  5   b , and the small thickness portion  5   c . Moreover, the first annular groove  51  and the second annular groove  52  are at least partially overlapped with each other when viewed from the rotation axis direction. In the first embodiment, the first annular groove  51  and the second annular groove  52  are entirely overlapped with each other when viewed from the rotation axis direction. 
     In a case where the radius of curvature R1 of the curved surface portion  51   a  is compared with the radius of curvature R2 of the curved surface portion  52   a , the radius of curvature R1 is greater than the radius of curvature R2 (R1&gt;R2). Moreover, a radial width D1 of the first annular groove  51  is identical to a radial width D2 of the second annular groove  52  (D1=D2), Furthermore, an angle formed by the extension line of the curved surface portion  52   b  in the second annular groove  52 , and the line on the second inner circumference surface  5   b   4  is represented by θd2. An angle formed by the extension line of the linear line portion  51   b  in the first annular groove  51 , and the line on the first outer circumference surface  5   a   3  is represented by θd1. The angle θd2 is greater than the angle θd1 (θd&gt;θd1). Moreover, θd2 and θd1 are acute. Besides, in this embodiment, the line on the first outer circumference surface  5   a   3  and the line on the second inner circumference surface  5   b   4  are parallel to the balancer. However, the line on the first outer circumference surface  5   a   3  and the line on the second inner circumference surface  5   b   4  may not be parallel to the balancer. 
     The first annular groove  51  formed on the first side surface includes the first inner circumference surface  5   b   3  provided on the rotation axis side; and the first outer circumference surface  5   a   3  which is provided on the teeth portion Sa side in the radial direction with respect to the rotation axis, and which has the axial depth (the depth in the rotation axis direction) smaller than the depth of the first inner circumference surface  5   b   3 . A first bottom portion is positioned in the curved surface portion  51   a  connecting the first inner circumference surface  5   b   3  and the first outer circumference surface  5   a   3 . An angle formed by the first bottom portion and the first inner circumference surface  5   b   3  is acute. 
     The second annular groove  52  formed on the second side surface is positioned on the opposite side of the first annular groove  51  in the main gear  5  in the rotation axis direction. The second annular groove  52  is overlapped with the first annular groove  51  when viewed from the rotation axis direction. Moreover, the second annular groove  52  includes the second inner circumference surface  5   b   4  provided on the rotation axis side in the radial direction with respect to the rotation axis; and the second outer circumference surface  5   a   4  which is provided on the teeth portion  5   a  side in the radial direction with respect to the rotation axis, and which has the axial depth (the depth in the rotation axis direction) greater than the depth of the second inner circumference surface  5   b   4 . A second bottom portion is positioned in the curved surface portion  52   c  connecting the second inner circumference surface  5   b   4  and the second outer circumference surface  5   a   4 . An angle formed by the second bottom portion and the second inner circumference surface  5   b   4  is obtuse. 
     A distance from the end surface portion  5   a   2  of the teeth portion  5   a  in the second annular groove  52  to the bottom portion of the second annular groove  52  is represented by L2. A distance from the end surface portion  5   b   1  of the base portion  5   b  in the first annular groove  51  to the bottom portion of the first annular groove  51  is represented by L1. L1 is longer than L2 (L1&gt;L2). That is, the lengths to the deepest bottom portions of the first annular groove  51  and the second annular groove  52  in the rotation axis direction are different from each other (the depths of the first annular groove  51  and the second annular groove  52  are different from each other). 
     In this way, the shapes of the first annular groove  51  and the second annular groove  52  are different from each other when viewed in the section taken along the rotation axis in this way. 
     Moreover, the first annular groove  51  and the second annular groove  52  are at least partially overlapped with each other in the radial direction with respect to the rotation axis. That is, L1 and L2 has an overlapping portion of ΔL. 
     Next, the operation of the main gear  5  including the first annular groove  51  and the second annular groove  52  are explained with reference to  FIG. 13A ,  FIG. 13B ,  FIG. 14A , and  FIG. 14B .  FIG. 13A  and  FIG. 14A  are sectional views for explaining the actuation of the main gear and the balancer drive shaft according to the first embodiment of the present invention.  FIG. 13B  and  FIG. 14B  are views showing the relationship of the force acted to the main gear according to the first embodiment of the present invention. 
     The balancer drive shaft  6  is integrally provided with the semi-circular balancer weight  6   c  (first balancer weight). When the balancer drive shaft  6  is rotated, the balancer drive shaft  6  is curved by the centrifugal force toward the side on which the balancer weight  6   c  is mounted. In  FIG. 13A , the central portion of the balancer drive shaft  6  is curved toward the lower side. In  FIG. 14A , the central portion of the balancer drive shaft  6  is curved toward the upper side. 
     The teeth of the teeth portion  5   a  of the main gear  5  according to this embodiment is a helical gear having a predetermined torsion angle θ with respect to the rotation axis. The helical gear has contact areas greater than those of a spur gear. The main gear  5  is engaged with the crank gear  3 . The driving force of the crank gear  3  is transmitted to the main gear  5 . In this embodiment, the main gear  5  is the helical gear. Accordingly, in the state where the central portion of the balancer shaft  6  is curved toward the lower side ( FIG. 13A ), a direction of the input Fo from the crank gear  3  is a direction perpendicular to the surface of the inclined tooth of the main gear  5 , as shown in  FIG. 13B . That is, the direction of the input Fo from the crank gear  3  is a direction inclined from the circumferential direction of the main gear  5  (the rotation direction of the main gar  5 ). The input Fo from the crank gear  3  in the teeth of the main gear  5  is divided into the force Fy in the circumferential direction of the main gear  5  (the rotation direction of the main gear  5 ), and the thrust Fx in the axial direction (the direction along the axial direction of the rotation axis of the main gear  5 ). In the state of  FIG. 13A , the thrust Fx (positive thrust) is acted to the main gear  5  toward the outer side in the axial direction. 
     The main gear  5  according to this embodiment includes the first annular groove  51  formed in the first side surface which is in the direction of the thrust force received by the teeth portion  5   a ; the second annular groove  52  formed in the second side surface which is on the side opposite to the first side surface on which the first annular groove  51  is provided; and the small thickness portion  5   c . When the thrust Fx is acted to the main gear  5  in the axially outward direction, the teeth portion  5   a  of the main gear  5  is moved to narrow the space of the first annular grove  51 , and to expand the space of the second annular groove  52 . That is, in  FIG. 13A , the teeth portion  5   a  of the main gear  5  is moved in the clockwise direction. 
     Moreover, in a state where the central portion of the balancer drive shaft  6  is curved toward the upper side ( FIG. 14A ), the direction of the input Fo from the crank gear  3  is a direction perpendicular to the surface of the inclined tooth of the main gear  5 , as shown in  FIG. 14B . In this case, the force opposite to the rotation direction is acted. That is, the direction of the input Fo from the crank gear  3  is the direction inclined from the circumferential direction (the counter-rotation direction) of the main gear  5 . The input Fo from the crank gear  3  in the teeth of the main gear  5  is divided into the force Fy of the circumferential direction (the counter-rotation direction) of the main gear  5 , and the thrust Fx in the axial direction. In the state of  FIG. 14A , the thrust Fx (the negative thrust) is acted to the main gear  5  in the axially inward direction. 
     When the thrust Fx is acted to the main gear  5  in the axially inward direction, the teeth portion  5   a  of the main gear  5  is moved to narrow the space of the second annular grove  52 , and to expand the space of the first annular groove  51 . That is, in  FIG. 14A , the teeth portion  5   a  of the main gear  5  is moved in the counterclockwise direction. 
     The main gear  5  according to this embodiment includes the small thickness portion  5   c  between the first annular groove  51  and the second annular groove  52 , in addition to the first annular groove  51  and the second annular groove  52 . Accordingly, when the thrust Fx is acted to the main gear  5 , the small thickness portion  5   c  is bent in accordance with the direction of the thrust Fx. It is possible to absorb the deviation of the engagement between the main gear  5  and the crank gear  3 , which is generated by the curvature of the balancer drive shaft  6 . With this, it is possible to suppress the teeth hitting noise. 
     Moreover, the teeth hitting noise is transmitted from the teeth portion  5   a  through the small thickness portion  5   c  and the base portion  5   b  to the balancer drive shaft  6 . In this embodiment, the small thickness portion  5   c  connecting the teeth portion  5   a  and the base portion  5   b  of the main gear  5  is disposed to be inclined with respect to the line perpendicular to the rotation axis. Accordingly, it is possible to lengthen the transmission path of the noise transmitted from the teeth portion  5   a  to the base portion  5   b , without increasing the size of the main gear  5 . It is possible to suppress the noise transmitted to the balancer drive shaft  6 . 
     Furthermore, in this embodiment, the lengths to the deepest bottom portions of the first annular groove  51  and the second annular groove  52  in the rotation axis direction are different from each other. Accordingly, it is possible to lengthen the transmission path of the noise transmitted from the teeth portion  5   a  to the base portion  5   b , without increasing the size of the main gear  5 . It is possible to suppress the noise transmitted to the balancer drive shaft  6 . 
     Moreover, the first annular groove  51  and the second annular groove  52  have the different shapes when viewed in a section taken along the rotation axis. Accordingly, it is possible to control the bending direction of the teeth portion  5   a , and the flexibility of the teeth portion  5   a.    
     Furthermore, the first annular groove  51  is entirely overlapped with the second annular groove  52  when viewed from the rotation axis direction. Accordingly, it is possible to alternately form the first annular groove  51  and the second annular groove  52  in the side surface portion of the main gear  5 . It is possible to decrease the size of the main gear  5 . 
     Besides, in this embodiment, the small thickness portion  5   c  is inclined so that the side opposite to the balancer weight  6   c  side in the rotation axis direction directs the radially outer side. Conversely, the small thickness portion  5   c  may be inclined so that the balancer weight  6   c  side directs the radially outer side. In this case, the relationship among the first outer circumference surface  5   a   3 , the first inner circumference surface  5   b   3 , the second outer circumference surface  5   a   4 , and the second inner circumference surface  5   b   4  is reversed. 
     That is, the first annular groove  51  includes the first inner circumference surface  5   b   3  provided on the rotation axis side of the balancer drive shaft  6  in the radial direction with respect to the rotation axis; and the first outer circumference surface  5   a   3  which is provided to confront the first inner circumference surface  5   b   3 , and which has the axial depth (the depth in the rotation axis direction) longer than that of the first inner circumference surface  5   b   3 . Furthermore, the second annular groove  52  includes the second inner circumference surface  5   b   4  provided on the rotation axis side of the balancer drive shaft  6  in the radial direction with respect to the rotation axis; and the second outer circumference surface  5   a   4  which is provided to confront the second inner circumference surface  5   b   4 , and which has the axial depth (the depth in the rotation axis direction) longer than that of the first inner circumference surface  5   b   3 . 
     In the first embodiment, the present invention is applied to the main gear  5 . However, the present invention is applicable to the crank gear  3 .  FIG. 15A  is a perspective view showing a crank gear according to this embodiment of the present invention, when viewed from the pump side.  FIG. 15B  is a perspective view showing the crank gear according to the embodiments of the present invention, when viewed from the opposite pump side.  FIG. 15C  is a plan view showing the crank gear according to the embodiment of the present invention, when viewed from the opposite pump side.  FIG. 15D  is a sectional view taken along a direction XVD-XVD in  FIG. 15C . 
     The crank gear  3  includes an opening portion  3   d  which is formed at the central portion of the crank gear  3 , and though which the crank shaft  2  passes. The teeth portion  3   a  of the crank gear  3  has a predetermined torsion angle with respect to the rotation axis. The teeth portion  3   a  of the crank gear  3  is engaged with the teeth portion  5   a  of the main gear  5 . 
     The crank shaft  2  is inserted into the opening portion  3   d  of the crank gear  3 . The base portion  3   b  of the crank gear  3  is fixed to the crank shaft  2 . 
     The crank gear  3  includes a first annular groove  61  and a second annular groove  62  which are formed on both side surfaces of the crank gear  3  in the rotation axis direction of the crank shaft  2 , and which are a plurality of annular grooves. The second annular groove  62  is overlapped with the first annular groove  61  when viewed from the direction of the rotation axis. In addition, the second annular groove  62  is partially overlapped with the first annular groove  61  (the bottom portions of the grooves are overlapped with each other) in the radial direction with respect to the rotation axis. A small thickness portion  3   c  is formed between the first annular groove  61  and the second annular groove  62 . The small thickness portion  3   c  is disposed so as to be inclined to the opposite pump side (the inside) of the rotation axis of the crank shaft  2  from the base portion  3   b  of the crank gear  3  toward the teeth portion  3   a.    
     The first annular groove  61  and the second annular groove  62  have the configurations identical to those of the above-described main gear  5 . With this, it is possible to obtain the identical effects. 
     Moreover, in a case where the present invention is applied to both the crank gear  3  and the main gear  5 , it is possible to further absorb the deviation of the engagement of the main gear  5  and the crank gear  3 . It is possible to further suppress the teeth hitting noise relative to a case where the present invention is applied to one of the crank gear  3  and the main gear  5 . 
     Similarly, in a case where the present invention is applied to both the balancer drive gear  7  and the balancer driven gear  8 , it is possible to further absorb the deviation of the engagement of the balancer drive gear  7  and the balancer driven gear  8 . It is possible to further suppress the teeth hitting noise relative to a case where the present invention is applied to one of the balancer drive gear  7  and the balancer driven gear  8 . 
     Moreover, in a case where the present invention is applied to both the oil pump drive gear  21  and the oil pump driven gear  43 , it is possible to further absorb the deviation of the engagement of the oil pump drive gear  21  and the oil pump driven gear  43 . It is possible to further suppress the teeth hitting noise relative to a case where the present invention is applied to one of the oil pump drive gear  21  and the oil pump driven gear  43 . 
     Second Embodiment 
     Next, a second embodiment is explained with reference to  FIG. 16 .  FIG. 16  is a sectional view showing a main gear according to the second embodiment of the present invention. Configurations identical to those of the first embodiment have identical symbols. Detailed explanations thereof are omitted. 
     The main gear  5  includes the first annular groove  51  and the second annular groove  52  which are formed on both side surfaces of the main gear  5  in the rotation axis direction of the balancer drive shaft  6 , and which are a plurality of annular grooves. The small thickness portion  5   c  is formed between the first annular groove  51  and the second annular groove  52 . The small thickness portion  5   c  is disposed and inclined from the base portion  5   b  of the main gear  5  to the teeth portion  5   a  of the main gear  5  in the extension direction (the outside) of the rotation axis of the balancer drive shaft  6 . 
     A distance from the end surface portion  5   a   1  of the teeth portion  5   a  (the end surface portion  5   b  of the base portion  5   b ) in the first annular groove  51  to the bottom portion of the first annular groove  51  is represented by L1. A distance from the end surface portion  5   a   2  of the teeth portion  5   a  in the second annular groove  52  to the bottom portion of the second annular groove  52  is represented by L2. In a range between the end surface portion  5   a   1  to the end surface portion  5   a   2  of the teeth portion  5   a , L1 and L2 are not overlapped with each other in the radial direction with respect to the rotation axis. That is, the bottom portion of the first annular groove  51  and the bottom portion of the second annular groove  52  are positioned on the line extending in the radial direction perpendicular to the rotation axis. Moreover, L1+L2 is identical to a distance between the end surface portion  5   a   1  and the end surface portion  5   a   2 . 
     Moreover, a distance from a boundary portion between the first outer circumference surface  5   a   3  and the linear line portion  51   b  in the first annular groove  51  to a boundary portion between the second outer circumference surface  5   a   4  and the curved surface portion  52   a  is represented by x1. A distance from a boundary portion between the curved surface portion  52   c  and the second inner circumference surface  5   b   4  of the base portion  5   b  to a boundary portion between the first inner circumference surface  5   b   3  and the curved surface portion  51   a  is represented by x2. That is, x1 is an axial width (a width in the rotation axis direction) of the small thickness portion  5   c  on the teeth portion  5   a  side. X2 is an axial width (a width in the rotation axis direction) of the small thickness portion  5   c  on the base portion  5   b  side. In this embodiment, x1 and x2 are overlapped by Δx with each other in the rotation axis direction. Moreover, the distance x1 is identical to the distance x2 (x1=x2). That is, in the small thickness portion  5   c  in this embodiment, the axial width (x1) (the width in the rotation axis direction) of the portion connected to the teeth portion  5   a  side is identical to the axial width (x2) (the width in the rotation axis direction) of the portion connected to the base end portion  5   b  side. Moreover, x1 is partially overlapped with x2. That is, x1 is overlapped with x2 by Δx. 
     The main gear  5  according to this embodiment includes the small thickness portion  5   c  between the first annular groove  51  and the second annular groove  52 , in addition to the first annular groove  51  and the second annular groove  52 . Accordingly, when the thrust Fx is acted to the main gear  5 , the small thickness portion  5   c  is bent in accordance with the direction of the thrust Fx. It is possible to absorb the deviation of the engagement between the main gear  5  and the crank gear  3 , which is generated by the curvature of the balancer drive shaft  6 . With this, it is possible to suppress the teeth hitting noise. 
     Moreover, the teeth hitting noise is transmitted from the teeth portion  5   a  through the small thickness portion  5   c  and the base portion  5   b  to the balancer drive shaft  6 . In this embodiment, the small thickness portion  5   c  connecting the teeth portion  5   a  and the base portion  5   b  of the main gear  5  is disposed to be inclined with respect to the line perpendicular to the rotation axis. Accordingly, it is possible to lengthen the transmission path of the noise transmitted from the teeth portion  5   a  to the base portion  5   b , without increasing the size of the main gear  5 . It is possible to suppress the noise transmitted to the balancer drive shaft  6 . 
     Furthermore, in this embodiment, the first annular groove  51  is not overlapped with the second annular groove  52  in the radial direction with respect to the rotation axis when viewed in the section taken along the rotation axis. Accordingly, the small thickness portion  5   c  is easy to be bent. 
     Moreover, in this embodiment, the small thickness portion  5   c  has the overlapped portion of Δx which is a part when viewed from the radial direction perpendicular to the rotation axis. Accordingly, it is possible to ensure the strength against the load in the radial direction. 
     Third Embodiment 
     Next, a third embodiment is explained with reference to  FIG. 17 .  FIG. 17  is a sectional view showing a main gear according to the third embodiment of the present invention. Configurations identical to those of the first embodiment and the second embodiment have identical symbols. Detailed explanations thereof are omitted. 
     The main gear  5  includes the first annular groove  51  and the second annular groove  52  which are formed on both side surfaces of the main gear  5  in the rotation axis direction of the balancer drive shaft  6 , and which are a plurality of annular grooves. The small thickness portion  5   c  is formed between the first annular groove  51  and the second annular groove  52 . The small thickness portion  5   c  is disposed and inclined from the base portion  5   b  of the main gear  5  to the teeth portion  5   a  of the main gear  5  in the extension direction (the outside) of the rotation axis of the balancer drive shaft  6 . 
     A distance from the end surface portion  5   a   1  of the teeth portion  5   a  (the end surface portion  5   b  of the base portion  5   b ) in the first annular groove  51  to the bottom portion of the first annular groove  51  is represented by L1. A distance from the end surface portion  5   a   2  of the teeth portion  5   a  in the second annular groove  52  to the bottom portion of the second annular groove  52  is represented by L2. In a range between the end surface portion  5   a   1  to the end surface portion  5   a   2  of the teeth portion  5   a , L1 and L2 are not overlapped with each other. L1 is apart from L2 by ΔL. That is, the bottom portion of the first annular groove  51  is apart from the bottom portion of the second annular groove  52  by ΔL in the radial direction perpendicular to the rotation axis. 
     Moreover, a distance from the end surface portion  5   b   1  of the base portion  5   b  (the end surface portion  5   a   1  of the teeth portion  5   a ) in the first annular groove  51  to a boundary portion between the second outer circumference  5   a   4  and the curved surface portion  52   a  is represented by x1. A distance from a boundary portion between the second inner circumference surface  5   b   4  (the end surface portion  5   a   2  of the teeth portion  5   a ) and the curved surface portion  52   c  to a boundary portion between the first inner circumference surface  5   b   3  and the curved surface portion  51   a  is represented by x2. That is, x1 is an axial width (a width in the rotation axis direction) of the small thickness portion  5   c  on the teeth portion  5   a  side. X2 is an axial width (a width in the rotation axis direction) of the small thickness portion  5   c  on the base portion  5   b  side. In this embodiment, x1 and x2 are overlapped by Δx with each other in the rotation axis direction. Moreover, the distance x1 is identical to the distance x2 (x1=x2). That is, in the small thickness portion  5   c  in this embodiment, the axial width (x1) (the width in the rotation axis direction) of the portion connected to the teeth portion  5   a  side is identical to the axial width (x2) (the width in the rotation axis direction) of the portion connected to the base end portion  5   b  side. 
     The main gear  5  according to this embodiment includes the small thickness portion  5   c  between the first annular groove  51  and the second annular groove  52 , in addition to the first annular groove  51  and the second annular groove  52 . Accordingly, when the thrust Fx is acted to the main gear  5 , the small thickness portion  5   c  is bent in accordance with the direction of the thrust Fx. It is possible to absorb the deviation of the engagement between the main gear  5  and the crank gear  3 , which is generated by the curvature of the balancer drive shaft  6 . With this, it is possible to suppress the teeth hitting noise. 
     Moreover, the teeth hitting noise is transmitted from the teeth portion  5   a  through the small thickness portion  5   c  and the base portion  5   b  to the balancer drive shaft  6 . In this embodiment, the small thickness portion  5   c  connecting the teeth portion  5   a  and the base portion  5   b  of the main gear  5  is disposed to be inclined with respect to the line perpendicular to the rotation axis. Accordingly, it is possible to lengthen the transmission path of the noise transmitted from the teeth portion  5   a  to the base portion  5   b , without increasing the size of the main gear  5 . It is possible to suppress the noise transmitted to the balancer drive shaft  6 . 
     Furthermore, in this embodiment, the first annular groove  51  is not overlapped with the second annular groove  52  in the radial direction with respect to the rotation axis when viewed in the section taken along the rotation axis. Accordingly, the small thickness portion  5   c  is easy to be bent. 
     Moreover, in this embodiment, the small thickness portion  5   c  has the overlapped portion of ΔL on the teeth portion  5   a  side, and the overlapped portion of Δx on the base portion  5   b  side when viewed in the radial direction perpendicular to the rotation axis. Accordingly, it is possible to ensure the strength against the load in the radial direction. 
     Fourth Embodiment 
     Next, a fourth embodiment is explained with reference to  FIG. 18 .  FIG. 18  is a sectional view showing a main gear according to the fourth embodiment of the present invention. Configurations identical to those of the first embodiment to the third embodiment have identical symbols. Detailed explanations thereof are omitted. 
     The main gear  5  includes the first annular groove  51  and the second annular groove  52  which are formed on both side surfaces of the main gear  5  in the rotation axis direction of the balancer drive shaft  6 , and which are a plurality of annular grooves. The small thickness portion  5   c  is formed between the first annular groove  51  and the second annular groove  52 . The small thickness portion  5   c  is disposed and inclined from the base portion  5   b  of the main gear  5  to the teeth portion  5   a  of the main gear  5  in the extension direction (the outside) of the rotation axis of the balancer drive shaft  6 . 
     A distance from the end surface portion  5   a   1  of the teeth portion  5   a  (the end surface portion  5   b  of the base portion  5   b ) in the first annular groove  51  to the bottom portion of the first annular groove  51  is represented by L1. A distance from the end surface portion  5   a   2  of the teeth portion  5   a  in the second annular groove  52  to the bottom portion of the second annular groove  52  is represented by L2. In a range between the end surface portion  5   a   1  to the end surface portion  5   a   2  of the teeth portion  5   a , L1 is overlapped with L2 by AO. That is, the bottom portion of the first annular groove  51  is overlapped with the bottom portion of the second annular groove  52  by AO in the radial direction perpendicular to the rotation axis. 
     Moreover, the first annular groove  51  is disposed radially outside the second annular groove  52  by LD1 in the radial direction with respect to the rotation axis. 
     By this embodiment, the first annular groove  51  is partially overlapped with the second annular groove  52  in the rotation axis direction when viewed in the section taken along the rotation axis. Accordingly, the small thickness portion  5   c  is easy to be bent. 
     The main gear  5  according to this embodiment includes the small thickness portion  5   c  between the first annular groove  51  and the second annular groove  52 , in addition to the first annular groove  51  and the second annular groove  52 . Accordingly, when the thrust Fx is acted to the main gear  5 , the small thickness portion  5   c  is bent in accordance with the direction of the thrust Fx. It is possible to absorb the deviation of the engagement between the main gear  5  and the crank gear  3 , which is generated by the curvature of the balancer drive shaft  6 . With this, it is possible to suppress the teeth hitting noise. 
     Fifth Embodiment 
     Next, a fifth embodiment is explained with reference to  FIG. 19 .  FIG. 19  is a sectional view showing a main gear according to the fifth embodiment of the present invention. Configurations identical to those of the first embodiment to the fourth embodiment have identical symbols. Detailed explanations thereof are omitted. 
     The main gear  5  includes the first annular groove  51  and the second annular groove  52  which are formed on both side surfaces of the main gear  5  in the rotation axis direction of the balancer drive shaft  6 , and which are a plurality of annular grooves. The small thickness portion  5   c  is formed between the first annular groove  51  and the second annular groove  52 . The small thickness portion  5   c  is disposed and inclined from the base portion  5   b  of the main gear  5  to the teeth portion  5   a  of the main gear  5  in the extension direction (the outside) of the rotation axis of the balancer drive shaft  6 . 
     A distance from the end surface portion  5   a   1  of the teeth portion  5   a  (the end surface portion  5   b  of the base portion  5   b ) in the first annular groove  51  to the bottom portion of the first annular groove  51  is represented by L1. A distance from the end surface portion  5   a   2  of the teeth portion  5   a  in the second annular groove  52  to the bottom portion of the second annular groove  52  is represented by L2. In a range between the end surface portion  5   a   1  to the end surface portion  5   a   2  of the teeth portion  5   a , L1 is overlapped with L2 by ΔO. That is, the bottom portion of the first annular groove  51  is overlapped with the bottom portion of the second annular groove  52  by AO in the radial direction perpendicular to the rotation axis. 
     Moreover, the second annular groove  52  is disposed radially outside the first annular groove  52  by ΔD2 in the radial direction with respect to the rotation axis. 
     By this embodiment, the first annular groove  51  is partially overlapped with the second annular groove  52  in the rotation axis direction when viewed in the section taken along the rotation axis. Accordingly, the small thickness portion  5   c  is easy to be bent. 
     The main gear  5  according to this embodiment includes the small thickness portion  5   c  between the first annular groove  51  and the second annular groove  52 , in addition to the first annular groove  51  and the second annular groove  52 . Accordingly, when the thrust Fx is acted to the main gear  5 , the small thickness portion  5   c  is bent in accordance with the direction of the thrust Fx. It is possible to absorb the deviation of the engagement between the main gear  5  and the crank gear  3 , which is generated by the curvature of the balancer drive shaft  6 . With this, it is possible to suppress the teeth hitting noise. 
     Moreover, in this embodiment, the second annular groove  52  is disposed radially outside the first annular groove  51  by ΔD2 in the radial direction with respect to the rotation axis. The small thickness portion  5  is further easy to be bent on the first annular groove  51  side positioned radially inside the second annular groove  52 . 
     Sixth Embodiment 
     Next, a sixth embodiment is explained with reference to  FIG. 20 .  FIG. 20  is a sectional view showing a main gear according to the sixth embodiment of the present invention. Configurations identical to those of the first embodiment to the fifth embodiment have identical symbols. Detailed explanations thereof are omitted. 
     In the first embodiment to the fifth embodiment, the balancer weight  6   c  is mounted to the balancer drive shaft  6 . Accordingly, the balancer drive shaft  6  is bent so that the thrust force becomes the positive region and the negative region. For example, when the balancer weight is not mounted to the shaft, the thrust force does not become the negative region. The thrust force is varied in the positive region. In the case of this shaft, it is possible to omit the second annular groove  52  provided to the main gear in the first embodiment to the fifth embodiment. In the sixth embodiment, the main gear  5  is the helical gear like the first embodiment to the fifth embodiment. The directions of the teeth of the main gear are identical to those of  FIG. 12B . 
     The main gear  5  includes the first annular groove  51  formed in the one surface in the rotation axis direction of the balancer drive shaft  6 . The first annular groove  51  including the small thickness portion  5   c  includes the curved surface portion  51   a  which is formed at the position confronting the first annular groove  51  in the rotation axis direction, which is recessed from the axial end surface portion  5   b   1  of the base portion  5   b  in the axially inward direction, and which includes the recessed bottom portion having the predetermined radius of the curvature. The end surface portion  5   b   1  and the curved surface portion  51   a  are connected by the first inner circumference surface  5   b   3  of the base portion  5   b . The first inner circumference surface  5   b   3  is provided on the rotation axis side. The linear line portion  51   b  is formed in the surface of the small thickness portion  5   c  linearly extending from the curved surface portion  51   a  in the radially outward direction. The small thickness portion  51   b  is connected to the first outer circumference surface  5   a   3  of the teeth portion  5   a  of the main gear  5 . The first outer circumference surface  5   a   3  is connected to the end surface portion  5   a   1 . The first inner circumference surface  5   b   3  confronts the first outer circumference surface  5   a   3 . The axial depth (the depth in the axial direction) of the first outer circumference surface  5   a   3  is smaller than that of the first inner circumference surface  5   b   3 . 
     The annular groove  51  includes the first inner circumference surface  5   b   3  provided on the rotation axis side; and the first outer circumference surface  5   a   3  provided on the teeth portion  5   a  side, and which has the depth smaller than that of the first inner circumference surface  5   b   3 . The first bottom portion is positioned in the curved surface portion  51   a  connecting the first inner circumference surface  5   b   3  and the first outer circumference surface  5   a   3 . An angle formed by the first bottom portion and the first inner circumference surface  5   b   3  is acute. 
     The main gear  5  according to this embodiment includes the first annular groove  51 . Accordingly, when the thrust Fx is applied to the main gear  5 , the small thickness portion  5   c  is bent in accordance with the direction of the thrust Fx. It is possible to absorb the deviation of the engagement between the main gear  5  and the crank gear  3 . With this, it is possible to absorb the teeth hitting noise. 
     Moreover, in this embodiment, the first annular groove  51  is formed only in the one side of the main gear  5 . Accordingly, it is possible to readily form the main gear  5 . Furthermore, when the main gear  5  is cleaned, in a case where the main gear  5  is located to direct the first annular groove  51  side in the downward direction, it is possible to fasten the drying process without accumulating the cleaning solvent within the groove formed by the processing. 
     Besides, the present invention is not limited to the above-described embodiments. The present invention includes various variations. The above-described embodiments are explained in detail for the easy understanding of the present invention. The present invention is not limited to the configuration including the all explained components. 
     The embodiments according to the present invention include the balancer drive shaft  6  and the main gear  5  fixed to the balancer drive shaft  6 . Moreover, the present invention is applicable to a mere combination between a shaft and a gear (the effects in the respective embodiments). In recent years, it is required to further improve the fuel economy for improving the environment performance. For the improvement of the fuel economy, it is required to decrease the weight of the engine. It is necessary to avoid the size increase of the components constituting the engine. The embodiments are focused on the improvement to suppress the noise, and to suppress the size increase of the components. 
     In the embodiments, a main gear  5  (gear) configured to rotate as a unit with a balancer drive shaft  6  (shaft), the main gear  5  including: 
     a first annular groove  51  and a second annular groove  52  (a plurality of annular grooves) formed on both side surfaces of the main gear  5  (gear) in a direction of a rotation axis of the balancer drive shaft  6  (shaft), at least partially overlapped when viewed from the direction of the rotation axis of the balancer drive shaft  6 , and at least partially overlapped in a radial direction with respect to the rotation axis. 
     In this embodiment, it is possible to lengthen the transmission path of the noise. Accordingly, it is possible to provide a gear to suppress the noise generated by the engagement of the gears, and to suppress the size increase. 
     Moreover, in the above-described configuration in the embodiment, a first annular groove  51  is provided on one side surface of the both side surfaces of the main gear  5  (gear) in the direction of the rotation axis, and a second annular groove  52  is provided on a side opposite to the first annular groove  51 ; and depths of deepest bottom portions of the first annular groove  51  and the second annular groove  52  in the direction of the rotation axis are different from each other. 
     In this embodiment, it is possible to lengthen the transmission path of the noise. Accordingly, it is possible to provide a gear to suppress the noise generated by the engagement of the gears, and to suppress the size increase. 
     Moreover, in the above-described configuration in the embodiment, the shapes of the first annular groove  51  and the second annular groove  52  are different from each other. 
     In this embodiment, it is possible to control the gear bending direction, and to suppress the noise generated by the engagement of the gears. 
     Moreover, in the above-described configuration in the embodiment, the first annular groove  51  includes a first inner circumference surface  5   b   3  provided on the rotation axis side in the radial direction with respect to the rotation axis, and a first outer circumference surface  5   a   3  which confronts the first inner circumference surface  5   b   3 , and which has an axial depth smaller than an axial depth of the first inner circumference surface  5   b   3 ; and 
     the second annular groove  52  includes a second inner circumference surface  5   b   4  provided on the rotation axis side in the radial direction with respect to the rotation axis, and a second outer circumference surface  5   a   4  which confronts the second inner circumference surface  5   b   4 , and which has an axial depth greater than an axial depth of the second inner circumference surface  5   b   4 . 
     In this embodiment, the gear is easy to be bent on the first annular groove  51  side. It is possible to suppress the noise generated by the engagement of the gears. 
     Moreover, in the above-described configuration in the embodiment, the first annular groove  51  is deviated from the second annular groove  52  in the radial direction with respect to the rotation axis. 
     In this embodiment, the gear is easy to be bent on the first annular groove  51  side. It is possible to suppress the noise generated by the engagement of the gears. 
     Moreover, in the above-described configuration in the embodiment, the first annular groove  51  and the second annular groove  52  of the plurality of the annular grooves are entirely overlapped when viewed in the direction of the rotation axis. 
     In this embodiment, it is possible to lengthen the transmission path of the noise. Accordingly, it is possible to provide a gear to suppress the noise generated by the engagement of the gears, and to suppress the size increase. 
     Moreover, in the above-described configuration in the embodiment, the main gear  5  (gear) including the first annular groove  51  and the second annular groove  52  (the plurality of the annular grooves) has a Z shape when viewed in a section taken along the rotation axis. 
     In this embodiment, it is possible to lengthen the transmission path of the noise. Accordingly, it is possible to provide a gear to suppress the noise generated by the engagement of the gears, and to suppress the size increase. 
     Furthermore, in this embodiment, a main gear  5  (gear) configured to rotate as a unit with a balancer drive shaft  6  (shaft), the main gear  5  (gear) including: 
     a teeth portion  5   a  which are provided in a circumferential direction with respect to the rotation axis of the balancer drive shaft (shaft), and which has a predetermined torsion angle with respect to the rotation axis; and 
     a first annular groove  51  which is formed on a first side surface of both side surfaces of the main gear  5  (gear) in a direction of the rotation axis, the first side surface being on a direction of a thrust force received by the teeth portion  5   a , the first annular groove  51  including;
         a first inner circumference surface  5   b   3  provided on the rotation axis side in a radial direction with respect to the rotation axis,   a first outer circumference surface  5   a   3  which is provided on the teeth portion  5   a  side in the radial direction with respect to the rotation axis, and which has an axial depth smaller than an axial depth of the first inner circumference surface  5   b   3 , and   a first bottom portion connecting the first inner circumference surface  5   b   3  and the first outer circumference surface  5   a   3 , and having an angle which is formed by the first bottom portion and the first inner circumference surface  5   b   3 , and which is acute.       

     In this embodiment, it is possible to lengthen the transmission path of the noise. Accordingly, it is possible to provide a gear to suppress the noise generated by the engagement of the gears, and to suppress the size increase. 
     Moreover, in this embodiment, the first annular groove  51  is formed only in the one side of the main gear  5 . Accordingly, it is possible to readily form the main gear  5 . Furthermore, when the main gear  5  is cleaned, in a case where the main gear  5  is located to direct the first annular groove  51  side in the downward direction, it is possible to fasten the drying process without accumulating the cleaning solvent within the groove formed by the processing. 
     Moreover, in the above-described configuration in the embodiment, the main gear  5  (gear) includes a second annular groove  52  that is formed on a second side surface of the both side surfaces of the main gear  5  (gear) in the direction of the rotation axis, the second side surface being opposite to the first side surface on which the first annular groove  51  is formed, and that is overlapped with the first annular groove  51  when viewed in the direction of the rotation axis; 
     the second annular groove  52  includes
         a second inner circumference surface  5   b   4  provided on the rotation axis side in a radial direction with respect to the rotation axis,   a second outer circumference surface  5   a   4  which is provided on the teeth portion  5   a  side in the radial direction with respect to the rotation axis, and which has an axial depth greater than an axial depth of the second inner circumference surface  5   b   4 , and   a second bottom portion connecting the second inner circumference surface  5   b   4  and the second outer circumference surface  5   a   4 , and having an angle which is formed by the second bottom portion and the second inner circumference surface  5   b   4 , and which is obtuse.       

     In this embodiment, it is possible to lengthen the transmission path of the noise. Accordingly, it is possible to provide a gear to suppress the noise generated by the engagement of the gears, and to suppress the size increase. 
     Moreover, in the above-described configuration in the embodiment, the first annular groove  51  is not′ overlapped with the second annular groove  52  in the radial direction with respect to the rotation axis when viewed in a section taken along the rotation axis. 
     In this embodiment, the main gear  5  is easy to be bent. It is possible to ensure the predetermined strength with respect to the load in the radial direction. 
     Moreover, in this embodiment, a balancer device including: 
     a balancer drive shaft  6  provided with a balancer weight  6   c;    
     a main gear  5  (drive gear) configured to rotate as a unit with the balancer drive shaft  6 , and to which a rotation force is transmitted from a crank shaft  2  through a crank gear  3 ; 
     a plurality of annular grooves (first annular groove  51  and second annular groove  52 ) which are formed on both side surfaces of the main gear  5  (drive gear) in a direction of a rotation axis of the balancer drive shaft  6 , which are at least partially overlapped when viewed in the direction of the rotation axis, and which are at least partially overlapped in a radial direction with respect to the rotation axis. 
     In this embodiment, it is possible to lengthen the transmission path of the noise. Accordingly, it is possible to provide the balancer device to suppress the noise generated by the engagement of the gears, and to suppress the size increase. 
     Moreover, in the above-described configuration in the embodiment, the overlapped portions of the plurality of the annular grooves (first annular groove  51  and second annular groove  52 ) in the radial direction with respect to the rotation axis is positioned on a side opposite to a side on which a thrust force is applied in the direction of the rotation axis. 
     In this embodiment, the side receiving the thrust force is easy to be gent. Accordingly, it is possible to provide the balancer device to suppress the noise generated by the engagement of the gears, and to suppress the size increase. 
     Moreover, in the above-described configuration in the embodiment, the plurality of the annular grooves includes a second annular groove  52  provided on the balancer weight  6   c  side in the direction of the rotation axis, and a first annular groove  51  provided on a side opposite to the second annular groove  52 ; and depths of deepest bottom portions of the first annular groove  51  and the second annular groove  52  in the direction of the rotation axis are different from each other. 
     In this embodiment, it is possible to lengthen the transmission path of the noise. Accordingly, it is possible to provide the balancer device to suppress the noise generated by the engagement of the gears, and to suppress the size increase. 
     Moreover, in the above-described configuration in the embodiment, the plurality of the annular grooves includes a second annular groove  52  provided on the balancer weight  6   c  side in the direction of the rotation axis, and a first annular groove  51  provided on a side opposite to the second annular groove  52 ; and shapes of the first annular groove  51  and the second annular groove  52  are different from each other when viewed in a section taken along the rotation axis. 
     In this embodiment, it is possible to control the gear bending direction, and to suppress the noise generated by the engagement of the gears. 
     Moreover, in the above-described configuration in the embodiment, the first annular groove  51  includes a first inner circumference surface  5   b   3  provided on the rotation axis side of the balancer drive shaft  6  in the radial direction with respect to the rotation axis, and a first outer circumference surface  5   a   3  which confronts the first inner circumference surface  5   b   3 , and which has an axial depth smaller than an axial depth of the first inner circumference surface  5   b   3 ; and 
     the second annular groove  52  includes a second inner circumference surface  5   b   4  provided on the rotation axis side of the balancer drive shaft  6  in the radial direction with respect to the rotation axis, and a second outer circumference surface  5   a   4  which confronts the second inner circumference surface  5   b   4 , and which has an axial depth greater than an axial depth of the first inner circumference surface  5   b   4 . 
     In this embodiment, it is possible to lengthen the transmission path of the noise. Accordingly, it is possible to provide the balancer device to suppress the noise generated by the engagement of the gears, and to suppress the size increase. 
     Moreover, in the above-described configuration in the embodiment, the first annular groove  51  includes a first inner circumference surface  5   b   3  provided on the rotation axis side of the balancer drive shaft  6  in the radial direction with respect to the rotation axis, and a first outer circumference surface  5   a   3  which confronts the first inner circumference surface  5   b   3 , and which has an axial depth greater than an axial depth of the first inner circumference surface  5   b   3 ; and 
     the second annular groove  52  includes a second inner circumference surface  5   b   4  provided on the rotation axis side of the balancer drive shaft  6  in the radial direction with respect to the rotation axis, and a second outer circumference surface  5   a   4  which confronts the second inner circumference surface  5   b   4 , and which has an axial depth smaller than an axial depth of the first inner circumference surface  5   b   4 . 
     In this embodiment, it is possible to lengthen the transmission path of the noise. Accordingly, it is possible to provide the balancer device to suppress the noise generated by the engagement of the gears, and to suppress the size increase. 
     Moreover, in the above-described configuration in the embodiment, the main gear  5  (drive gear) includes a small thickness portion  5   c  formed between the first annular groove  51  and the second annular groove  52  of the plurality of the annular grooves; and the small thickness portion  5   c  is inclined so that a side opposite to the balancer weight  6   c  side in the direction of the rotation axis directs in a radially outward direction. 
     In this embodiment, it is possible to lengthen the transmission path of the noise. Accordingly, it is possible to provide the balancer device to suppress the noise generated by the engagement of the gears, and to suppress the size increase. 
     Furthermore, in this embodiment, a balancer device including: 
     a main gear  5  (drive gear) engaged with a crank gear  3  (input gear) to which a rotation force is transmitted from a crank shaft  2 ; 
     a balancer drive shaft  6  to which the rotation force is transmitted from the main gear  5  (drive gear), and which includes a balancer weight  6   c  (first balancer weight); 
     a balancer drive gear  7  configured to rotate as a unit with the balancer drive shaft  6 ; 
     a balancer driven gear  8  engaged with the balancer drive gear  7 ; 
     a balancer driven shaft  9  which is configured to rotate as a unit with the balancer driven gear  8 , and which includes a balancer weight  9  (second balancer weight); and 
     a plurality of annular groove (first annular groove and second annular groove) which are formed in both side surfaces of at least one of the main gear  5  (drive gear), the balancer drive gear  7 , and the balancer driven gear  8 , in a rotation axis of the balancer drive shaft  6 , which are at least partially overlapped when viewed in the direction of the rotation axis, and which are at least partially overlapped in a radial direction with respect to the rotation axis. 
     In this embodiment, it is possible to lengthen the transmission path of the noise. Accordingly, it is possible to provide the balancer device to suppress the noise generated by the engagement of the gears, and to suppress the size increase. 
     Moreover, in the above-described configuration in the embodiment, the balancer device includes an oil pump  4  including an oil pump drive gear  21  provided to the balancer driven shaft  9 , and an oil pump driven gear  43  engaged with the oil pump drive gear  21 ; and 
     a plurality of annular groove (first annular groove and second annular groove) which are formed in both side surfaces of at least one of the main gear  5  (drive gear), the balancer drive gear  7 , the balancer driven gear  8 , the oil pump drive gear  21 , and the oil pump driven gear  43  in a rotation axis of the balancer drive shaft  6 , which are at least partially overlapped when viewed in the direction of the rotation axis, and which are at least partially overlapped in a radial direction with respect to the rotation axis. 
     In this embodiment, it is possible to lengthen the transmission path of the noise. Accordingly, it is possible to provide the balancer device to suppress the noise generated by the engagement of the gears, and to suppress the size increase. 
     EXPLANATION OF SYMBOLS 
       1  . . . balancer device,  2  . . . crank shaft,  3  . . . crank gear (input gear),  3   a  . . . teeth portion,  3   b  . . . base portion,  3   c  . . . small thickness portion,  4  . . . oil pump,  5  . . . main gear (drive gear),  5   a  . . . teeth portion,  5   a   1  . . . end surface portion,  5   a   2  . . . end surface portion,  5   a   3  . . . first outer circumference surface,  5   a   4  . . . second outer circumference surface,  5   b  . . . base portion,  5   b   1  . . . end surface portion,  5   b   2  . . . end surface portion,  5   b   3  . . . second inner circumference surface,  5   b   4  . . . second inner circumference surface,  5   c  . . . small thickness portion,  6  . . . balancer drive shaft,  6   c  . . . balancer weight,  7  . . . balancer drive gear,  8  . . . balancer driven gear,  9  . . . balancer driven shaft,  9   c  . . . balancer weight,  21  . . . oil pump drive gear,  43  . . . oil pump driven gear,  51  . . . first annular groove,  51   a  . . . curved surface portion,  51   b  . . . linear line portion,  52  . . . second annular groove,  52   a  . . . curved surface portion,  52   b  . . . curved surface portion,  61  . . . first annular groove,  62  . . . second annular groove