Abstract:
A coupling portion is provided on either a rotary shaft or a worm shaft in an integrally rotational manner, said coupling portion for connecting the rotary shaft to the worm shaft and having a coupling hole therein. The coupling projection provided on the other of the rotary shaft and the worm shaft is inserted in the coupling hole. A circumferential clearance and a radial clearance are formed between the inner surface of the coupling hole and the coupling projection. Circumferential buffer portions are interposed in the circumferential clearance, and radial buffer portions are interposed in the radial clearance. The inner surface of the coupling hole and the coupling projection are arranged such that the circumferential buffer portions are compressed to deform in the circumferential direction whereby to allow the buffer portions to be brought into contact with each other when the rotary shaft is rotating.

Description:
RELATED APPLICATIONS 
     The present is a National Phase entry of PCT Application No. PCT/JP2012/074404, filed Sep. 24, 2012, which claims priority from Japanese Patent Application No. 2011-217245, filed Sep. 30, 2011, the disclosures of which are hereby incorporated by reference herein in their entirety. 
     FIELD OF THE INVENTION 
     The present disclosure relates to a motor having a speed reduction mechanism that includes a worm shaft and a worm wheel. 
     BACKGROUND OF THE INVENTION 
     Conventionally, this type of motor includes a worm shaft, which is coaxially coupled to a rotary shaft, or a drive shaft, of a motor body to rotate integrally with the drive shaft. For example, a motor disclosed in Japanese Laid-Open Patent Publication No. 2009-11077 includes a rotary shaft and a worm shaft, which are made of metal. The worm shaft includes a coupling portion formed at an axial end. A coupling hole is formed in the coupling portion. A coupling projection at a distal end of the rotary shaft is inserted into the coupling hole, so that the rotary shaft and the worm shaft are directly coupled to each other. 
     In the above described motor, for example, when the rotary shaft and the speed reduction mechanism are driven to rotate in the forward direction, the load from the output portion may stop, or locks, the rotation. At this time, the teeth of the worm shaft, or the worm portion, may bind with the teeth of the worm wheel. If the rotary shaft and the speed reduction mechanism begin rotating in the reverse direction from this state, torque for undoing the bound state is required in addition to the torque for rotating the worm shaft. 
     To solve this problem, clearances may be provided between the inner surface of the coupling hole of the worm shaft and the coupling projection of the rotary shaft both in the circumferential direction and in the radial direction. In a case in which such clearances are provided, when the rotary shaft starts rotating in the reverse direction from the bound state, that is, when the rotary shaft starts rotating in the reverse direction, the rotary shaft is rotated freely for a certain period of time. In this period, the rotary shaft gains an inertial force, or a rotational momentum. After rotating freely due to the clearances, the rotary shaft violently strikes the inner surface of the worm shaft coupling hole with the inertial force. This releases the bound teeth of the worm shaft and the worm wheel from each other in a favorable manner. 
     SUMMARY OF THE INVENTION 
     Problems that the Invention is to Solve 
     In the above described motor, the clearances in the circumferential direction and the radial direction are formed between the inner surface of the worm shaft coupling hole and the rotary shaft coupling projection. Therefore, when the above described motor is operating, for example, the worm shaft may move uncontrollably in the circumferential direction and the radial direction due to, for example, imperfect alignment. Uncontrollable movements of the worm shaft may cause the inner surface of the coupling hole to strike the coupling projection, resulting in noises. 
     Accordingly, it is an objective of the present disclosure to provide a motor that suppresses the generation of noises caused by uncontrollable movements of a worm shaft. 
     Means for Solving the Problems 
     In accordance with one aspect of the present disclosure, a motor is provided that includes a motor body having a rotary shaft, a speed reduction mechanism, which includes a worm shaft and a worm wheel meshed with the worm shaft, and a coupling portion, which has a coupling hole and couples the rotary shaft and the worm shaft with each other so that the rotary shaft the worm shaft are integrally rotational. The coupling portion is provided to be rotational integrally with one of the rotary shaft and the worm shaft. A coupling projection is provided on the other one of the rotary shaft and the worm shaft. The coupling projection is inserted in the coupling hole. A circumferential clearance, which is a clearance in a circumferential direction, and a radial clearance, which is a clearance in a radial direction, are formed between an inner surface of the coupling hole and the coupling projection. A circumferential buffer portion is located in the circumferential clearance. A radial buffer portion is located in the radial clearance. The inner surface of the coupling hole and the coupling projection are configured to be contactable with each other in the circumferential direction while compressing and deforming the circumferential buffer portion in the circumferential direction when the motor body is driven to rotate the rotary shaft. 
     According to this configuration, the circumferential buffer portion and the radial buffer portion are provided in the circumferential clearance and the radial clearance between the inner surface of the coupling hole and the coupling projection, respectively. This reduces uncontrollable movements of the worm shaft when rotating. As a result, the generation of noise caused by the coupling hole inner surface striking the coupling projection is suppressed. Further, when the rotary shaft is driven to rotate in the reverse direction, that is, when the rotary shaft starts rotating in the reverse direction with the worm shaft bound with the worm wheel, the rotary shaft rotates while compressing and deforming the circumferential buffer portion between the coupling hole inner surface and the coupling projection. During this time, the rotary shaft gains inertial force, or the momentum of rotation. Thereafter, with the circumferential buffer portion compressed, the coupling hole inner surface and the coupling projection contact each other in the circumferential direction while receiving the inertial force of the rotary shaft. This releases the worm shaft and the worm wheel, which have been bound to each other, from each other. According to this configuration, as described above, the circumferential buffer portion and the radial buffer portion suppress uncontrollable movements of the worm shaft, while ensuring the inertial force of the rotary shaft using the clearance between the coupling projection and the inner surface of the coupling hole. 
     According one form of the disclosure, the rotary shaft and the worm shaft are made of metal, and the coupling portion is a plastic member formed separately from the rotary shaft and the worm shaft. 
     According to this configuration, the inner surface of the coupling hole, into which the coupling projection of the worm shaft or the rotary shaft is inserted, is also made of plastic. Thus, when the motor body rotates the rotary shaft, noise caused by the coupling hole inner surface striking the coupling projection in the circumferential direction is suppressed. This further suppresses the generation of noise. 
     According one form of the disclosure, the circumferential buffer portion and the radial buffer portion are provided in the coupling portion. 
     According to this configuration, the circumferential buffer portion and the radial buffer portion are formed in the coupling member, which is a plastic member separate from the rotary shaft and the worm shaft. The circumferential buffer portion and the radial buffer portion can therefore be formed. 
     According one form of the disclosure, the circumferential buffer portion and the radial buffer portion are made of rubber. 
     According to this configuration, since the circumferential buffer portion and the radial buffer portion are made of rubber, uncontrollable movements of the worm shaft are suppressed in a favorable manner. 
     According one form of the disclosure, an axial buffer portion is located between the coupling projection and a bottom surface of the coupling hole in the axial direction. 
     According to this configuration, due to the characteristics of the worm shaft and the worm wheel of the speed reduction mechanism, reversal of rotation of the worm shaft generates a load in the axial direction. The load may cause the coupling projection to strike the bottom surface of the coupling hole in the axial direction or may cause the worm shaft to strike the rotary shaft in the axial direction. However, such possible striking is suppressed by the axial buffer portion. As a result, the generation of noise caused by such striking is suppressed. 
     According one form of the disclosure, the rotary shaft and the worm shaft are made of metal. The coupling portion is a plastic member formed separately from the rotary shaft and the worm shaft, and the axial buffer portion is provided in the coupling portion. 
     According to this configuration, the axial buffer portion is formed in the coupling member, which is a plastic member separate from the rotary shaft and the worm shaft. The axial buffer portion can therefore be formed easily. 
     According one form of the disclosure, the axial buffer portion is made of rubber. 
     According to this configuration, since the axial buffer portion is made of rubber, the coupling projection is prevented from striking the bottom surface of the coupling hole in the axial direction, and the worm shaft is prevented from striking the rotary shaft in the axial direction in a favorable manner. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a motor; 
         FIG. 2A  is a plan view of a joint as viewed from a worm shaft; 
         FIG. 2B  is a cross-sectional view of the joint; and 
         FIG. 3  is an explanatory plan view of operation of the motor. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, a preferred embodiment according to the present disclosure will be described. 
     A motor  1  illustrated in  FIG. 1  is used, for example, as a drive source of a wiper device (not shown) mounted on a vehicle. The motor  1  includes a motor body  2  and a speed reducing portion  3 , which reduces the speed and increases the torque of the output rotation of the motor body  2  and transmits the rotation to the wiper device. 
     The motor body  2  also includes a yoke  4  as a yoke housing. The yoke  4  is made of conductive metal and shaped as a cylinder with a closed end. A flange  4   b  is formed at an opening  4   a  of the yoke  4 . The speed reducing portion  3  has a housing  6 . The flange  4   b  is coupled and fixed to a fixing portion  6   a  of the gear housing  6  with bolts B. 
     A brush holder  5 , which is made of an insulating material such as plastic, is attached to the opening  4   a  of the yoke  4  to close the opening  4   a . When the yoke  4  and the gear housing  6  are secured to each other with the bolts B, the brush holder  5  is tightly held by the flange  4   b  and the fixing portion  6   a  of the gear housing  6 . 
     Magnets MG are fixed to the inner surface of the yoke  4  to face each other. An armature (rotor)  7  is rotationally accommodated inward of the magnets MG which face each other. A rotary shaft  8  is fixed to the armature  7 . The proximal end of the rotary shaft  8  is rotationally supported by a bearing  9   a  provided on the inner bottom of the yoke  4 . A bearing  9   b  is attached to a center position of the brush holder  5  to rotationally support the distal end of the rotary shaft  8 , which protrudes into the gear housing  6 . 
     The brush holder  5  holds a pair of feeder brushes  5   a . The brushes  5   a  are configured to slide along segments SG of a commutator  7   a  of the armature  7  to feed currents to the segments SG. The brush holder  5  has a connector portion  5   b , which is connectable with an external connector (not shown). Currents from the external connector are supplied to the brushes  5   a  via terminals  5   c  provided in the connector portion  5   b.    
     The speed reducing portion  3  includes the gear housing  6  and a speed reduction mechanism  11  accommodated in the gear housing  6 . A joint accommodating portion  12  is formed in the gear housing  6  at the fixing portion  6   a , which is fixed to the yoke  4 . The joint accommodating portion  12  has an opening that faces the yoke  4 . The gear housing  6  has a worm shaft accommodating portion  13  and a worm wheel accommodating portion  14 . The worm shaft accommodating portion  13  extends from the joint accommodating portion  12  along an axis L 1  of the rotary shaft  8  and in a direction opposite from the yoke  4 . The worm wheel accommodating portion  14  is located beside the worm shaft accommodating portion  13 , or on the lower side as viewed in  FIG. 1 . 
     The rotary shaft  8  extends through the brush holder  5  and has a distal end  8   a , which protrudes into the joint accommodating portion  12 . The joint accommodating portion  12  accommodates a joint  16  in it. The joint  16  is a coupling portion that couples the distal end  8   a  of the rotary shaft  8  and a worm shaft  15  accommodated in the worm shaft accommodating portion  13  to each other. 
     The worm shaft  15  is arranged to be coaxial with the rotary shaft  8 . The worm shaft  15  has a proximal end, which is located at the right end as viewed in  FIG. 1  and is connected to the joint  16 . At the proximal end, that is, in the vicinity of a coupling projection  15   a , which will be discussed below, the worm shaft  15  is rotationally supported by a bearing  17   a  provided in the gear housing  6 . The distal end of the worm shaft  15  is rotationally supported by a bearing  17   b  provided in the worm shaft accommodating portion  13 . The worm shaft  15  has a screw-like worm portion  15   b  formed at center portion in the axial direction, or a section between the bearings  17   a ,  17   b . The worm shaft accommodating portion  13  has a thrust receiving plate  17   c  at a closed end in the axial direction. The thrust receiving plate  17   c  receives thrust load of the worm shaft  15 . 
     The internal space of the worm shaft accommodating portion  13  is connected to the internal space of the worm wheel accommodating portion  14 . The worm wheel accommodating portion  14  rotationally accommodates a disk-like worm wheel  18  in it. The worm wheel  18  is meshed with the worm portion  15   b . The axis of the worm wheel  18  extends perpendicular to the axis of the worm shaft  15 , or in a direction perpendicular to the sheet of  FIG. 1 . The worm wheel  18  and the worm shaft  15  form the speed reduction mechanism  11 . An output shaft  19  is provided at the radial center of the worm wheel  18  to be rotational integrally with the output shaft  19  so that the output shaft  19  extends along the axis of the worm wheel  18 . The output shaft  19  has a distal end, which is coupled to the wiper device. 
     [Structure of Joint] 
     Hereinafter, the joint  16 , which couples the rotary shaft  8  of the motor body  2  and the worm shaft  15  of the speed reducing portion  3  to each other, will be described with reference to  FIGS. 2A and 2B . As shown in  FIG. 2B , the joint  16  has a plastic portion  20 , which is fixed to the distal end  8   a  of the rotary shaft  8  to rotate integrally with the rotary shaft  8 , first rubber members  21 , and second rubber members  22   a ,  22   b . The first and second rubber members  21 ,  22   a ,  22   b  are integrally formed with the plastic portion  20  by two-color molding. 
     The plastic portion  20  has a substantially cylindrical fixing portion  23 , which is fixed to the distal end  8   a  of the rotary shaft  8 . The distal end  8   a  of the rotary shaft  8  has a flat shape with a pair of parallel surfaces. The distal end  8   a  is press-fitted in a press-fitting hole  24 , which is formed to extend axially through a center portion of the fixing portion  23 . 
     The fixing portion  23  has a pair of rubber accommodating holes  25  located about, or radially outside of, the press-fitting hole  24 . The rubber accommodating holes  25  are arranged at a 180-degree interval in the circumferential direction. Each rubber accommodating hole  25  has one of the first rubber members  21 . An end of each first rubber member  21  that is closer to the worm shaft  15  serves as a projection  21   a , which protrudes in the axial direction from an end face  23   a  of the fixing portion  23  that is close to the worm shaft  15 . The end face  23   a  is a bottom surface of coupling hole  28  in the axial direction. The first rubber members  21  are axial buffer portions, which absorb the load in the axial direction received by the worm shaft  15  toward the motor body  2 . Each first rubber member  21  has at its ends in the axial direction a step portion  21   b . The step portions  21   b  are engaged with the rubber accommodating hole  25  in the axial direction to prevent the first rubber member  21  from coming off in the axial direction. 
     A worm shaft coupling portion  26  is integrally formed in a part of the fixing portion  23  that is close to the worm shaft  15 . The worm shaft coupling portion  26  has a cylindrical shape with a diameter larger than the fixing portion  23 . That is, in the present embodiment, the plastic portion  20  as a small diameter portion and the worm shaft coupling portion  26  as a large diameter portion form a plastic single member extending along the axis L 1 . A flange portion  27  is formed at an axial end of the worm shaft coupling portion  26  that is close to the motor body  2 . 
     As shown in  FIG. 2A , a coupling hole  28  is formed in a center portion of the worm shaft coupling portion  26 . The coupling hole  28  receives the coupling projection  15   a , which is formed at the proximal end of the worm shaft  15 . The coupling projection  15   a  of the worm shaft  15  has a flat shape with parallel flat surfaces  15   x ,  15   y , which extend along the axis L 1 . The coupling projection  15   a  is formed to be symmetrical about the axis L 1 . The coupling projection  15   a  has longitudinal-direction end faces  15   c ,  15   d , which bulge radially outward and are located between the flat surfaces  15   x  and  15   y . In contrast, the inner surface of the coupling hole  28  of the worm shaft coupling portion  26  has an elongated shape slightly larger than the shape of the coupling projection  15   a  when viewed in the axial direction. That is, the inner surface of the coupling hole  28  has a flattened shape and symmetrical about the axis L 1 . Clearances C 1 , C 2  are formed between the inner surface of the coupling hole  28  and the coupling projection  15   a . Specifically, circumferential clearances C 1  are formed between the inner surface of the coupling hole  28  and the flat surfaces  15   x ,  15   y , while radial clearances C 2  are formed between the inner surface of the coupling hole  28  and the longitudinal-direction end faces  15   c ,  15   d  at the longitudinal-direction ends of the coupling projection  15   a.    
     The worm shaft coupling portion  26  has the second rubber members  22   a ,  22   b , which are embedded by two-color molding. Each of the second rubber members  22   a ,  22   b  is formed by a circumferential buffer portion  31 , a radial buffer portion  32 , and a coupling portion  33 , which couples the buffer portions  31 ,  32  to each other. The second rubber members  22   a ,  22   b  are formed symmetrically with respect to the axis L 1 . 
     The circumferential buffer portion  31  of each of the second rubber members  22   a ,  22   b  has a recess  28   a , which is formed by radially outwardly recessing the inner surface of the coupling hole  28 . The inner surface of the coupling hole  28  includes contact portion  28   b ,  28   e  on both sides of the circumferential buffer portion  31  of the second rubber member  22   a , and contact portions  28   d ,  28   c  on both sides of the circumferential buffer portion  31  of the second rubber member  22   b . The contact portion  28   b  and the contact portion  28   c  are symmetrical about the axis L 1 , and the contact portion  28   d  and the contact portion  28   e  are symmetrical about the axis L 1 . A part of the circumferential buffer portion  31  protrudes radially inward from the recess  28   a . In the state illustrated in  FIG. 2A , if the motor  1  is not operating, the circumferential buffer portions  31  of the second rubber members  22   a ,  22   b  contact the flat surfaces  15   x ,  15   y  of the coupling projection  15   a  at two positions that are symmetrical about the axis L 1  of the worm shaft  15 . That is, in  FIG. 2A , the circumferential buffer portion  31  of the left-hand second rubber member  22   a  contacts the flat surface  15   x  at two positions with the axis L 1  in between, and the right-hand second rubber member  22   b  contacts the flat surface  15   y  at two positions with the axis L 1  in between. The two positions at which one of the circumferential buffer portions  31  contacts the flat surface  15   x  and the two positions at which the other circumferential buffer portion  31  contacts the flat surface  15   y  are symmetrical about the axis L 1 . 
     The radial buffer portions  32  of the second rubber members  22   a ,  22   b  project radially inward from the inner surface of the coupling hole  28  and contact the longitudinal-direction end faces  15   c ,  15   d  of the coupling projection  15   a . A contact surface  32   a  of each radial buffer portion  32  contacts the coupling projection  15   a  and projects arcuately radially inward. As described above, in a direction perpendicular to the axis, that is, in the plane of the sheet of  FIG. 2A , the coupling projection  15   a  of the worm shaft  15  is supported from four directions, that is, from both sides in the longitudinal direction and from both sides in the transverse direction by the circumferential buffer portions  31  and the radial buffer portions  32  of the second rubber members  22   a ,  22   b.    
     Each of the second rubber members  22   a ,  22   b  includes an engaging portion  34 , which extends axially through the worm shaft coupling portion  26  and is engaged with the coupling portion  26  in the axial direction. The engaging portions  34  prevent the second rubber members  22   a ,  22   b  from falling off in the axial direction toward the worm shaft  15 . 
     Operation of the present embodiment will now be described. 
     When the motor body  2  drives the rotary shaft  8  to rotate, for example, in the forward direction, that is, clockwise as viewed in  FIG. 2A , the joint  16  rotates together with the rotary shaft  8 . Then, the circumferential buffer portion  31  of the second rubber member  22   a , which contacts the flat surface  15   x  of the coupling projection  15   a  of the worm shaft  15 , and the circumferential buffer portion  31  of the second rubber member  22   b , which contacts the flat surface  15   y  of the coupling projection  15   a , are compressed and deformed in the circumferential direction by the rotational driving force of the joint  16 , that is, the rotary shaft  8  as shown in  FIG. 3 . When the load at the output shaft  19  reaches or exceeds a predetermined value, the contact portions  28   b ,  28   c  on the inner surface of the coupling hole  28  contact the flat surfaces  15   x ,  15   y  of the coupling projection  15   a  in the circumferential direction, respectively, so that the rotation of the joint  16  is transmitted to the coupling projection  15   a . This causes the rotary shaft  8 , the joint  16 , and the worm shaft  15  to rotate integrally, and the rotation of the worm shaft  15  is transmitted to the worm wheel  18 . In the rotating state, since the coupling projection  15   a  of the worm shaft  15  is contacted and held by the circumferential buffer portions  31  and the radial buffer portions  32  of the second rubber members  22   a ,  22   b , the worm shaft  15  is prevented from moving uncontrollably in directions perpendicular to the axis due to, for example, imperfect alignment. 
     When the load at the output shaft  19  is less than the predetermined value, the rotation of the joint  16  is transmitted to the coupling projection  15   a  of the worm shaft  15  without the contact portions  28   b ,  28   c  on the inner surface of the coupling hole  28  contacting the flat surfaces  15   x ,  15   y  of the coupling projection  15   a . That is, in this case, the rotation of the joint  16  is transmitted to the coupling projection  15   a  via the circumferential buffer portions  31  of the second rubber members  22   a ,  22   b , so that the rotary shaft  8 , the joint  16 , and the worm shaft  15  are rotated integrally. In this rotating state also, since the coupling projection  15   a  of the worm shaft  15  is contacted and held by the circumferential buffer portions  31  and the radial buffer portions  32  of the second rubber members  22   a ,  22   b , the worm shaft  15  is prevented from moving uncontrollably in directions perpendicular to the axis due to, for example, imperfect alignment. 
     The motor  1  is configured such that, with the contact portions  28   b ,  28   c  of the coupling hole  28  contacting the flat surfaces  15   x ,  15   y  of the coupling projection  15   a , the circumferential center line L 2  of the radial buffer portion  32  of the second rubber member  22   a  does not go beyond a corner  35   a  defined by the flat surface  15   y  and the longitudinal-direction end face  15   c  of the coupling projection  15   a  in the forward rotation direction. The same is the case with the radial buffer portion  32  of the other second rubber member  22   b . That is, the motor  1  is configured such that the circumferential center line L 2  of the radial buffer portion  32  does not go beyond a corner  35   b  defined by the flat surface  15   x  and the longitudinal-direction end face  15   d  of the coupling projection  15   a  in the forward rotation direction. The corner  35   a  and the corner  35   b  are located at symmetrical positions with respect to the axis L 1 . The motor  1  is configured to act in the same manner when the rotary shaft  8  and the joint  16  rotate in the reverse rotation. Accordingly, when the rotary shaft  8 , the joint  16 , and the worm shaft  15  rotate integrally, the radial buffer portions  32  of the second rubber members  22   a ,  22   b  are prevented from coming off the coupling projection  15   a . As a result, for example, catching of the radial buffer portions  32  by the coupling projection  15   a  is prevented, which would be caused if the radial buffer portions  32  came off the coupling projection  15   a.    
     As described above, when a state in which the rotary shaft  8  and the speed reduction mechanism  11  are rotating in the forward direction is switched to, for example, a state in which the load of the output shaft  19  stops, that is, locks the rotary shaft  8 , the worm portion  15   b  of the worm shaft  15  and the teeth of the worm wheel  18  may be bound to each other. If the rotary shaft  8  is rotated in the reverse rotation in this state, the joint  16  is rotated in the reverse rotation so that the contact portion  28   d  of the coupling hole  28  contacts the flat surface  15   y  of the coupling projection  15   a , and the contact portion  28   e  of the coupling hole  28  contacts the flat surface  15   x  of the coupling projection  15   a . By the time the contact portions  28   d ,  28   e  contact the flat surfaces  15   y ,  15   x , the rotary shaft  8  and the joint  16  gain rotational inertial force. This releases the teeth of the worm shaft  15  and the teeth the worm wheel  18 , which are bound to each other, from each other in a favorable manner. Likewise, when the rotary shaft  8  is rotated in the forward direction after being stopped while rotating in the reverse rotation, the teeth of the worm shaft  15  and the teeth the worm wheel  18 , which are bound to each other, are released from each other in a favorable manner. As described above, according to the present embodiment, the worm shaft  15  is prevented from moving uncontrollably by the circumferential buffer portions  31  and the radial buffer portions  32  of the second rubber members  22   a ,  22   b , and inertial force of the rotary shaft  8  is ensured by the circumferential clearances C 1  and the radial clearances C 2  between the coupling projection  15   a  and the inner surface of the coupling hole  28 . 
     In the present embodiment, due to the characteristics of the worm shaft  15  and the worm wheel  18 , reversal of rotation of the worm shaft  15  generates a load in the axial direction. At this time, the load in the axial direction, which acts toward the motor body  2  and is received by the worm shaft  15 , is received by the projections  21   a  of the first rubber members  21 , which contact the coupling projection  15   a  of the worm shaft  15  in the axial direction. Thus, the first rubber members  21  prevent collision of the coupling projection  15   a  against the bottom surface of the coupling hole  28  in the axial direction, that is, against the end face  23   a  of the fixing portion  23  that is close to the worm shaft  15 , and collision of the coupling projection  15   a  against the distal end  8   a  of the rotary shaft  8 . As a result, the generation of noise caused by such collisions is prevented. 
     The present embodiment has the following advantages. 
     (1) The joint  16  is rotational integrally with the rotary shaft  8 . The joint  16  has the coupling hole  28 , into which the coupling projection  15   a  of the worm shaft  15  is inserted such that the coupling projection  15   a  has the circumferential clearances C 1  and the radial clearances C 2 . The circumferential buffer portions  31  and the radial buffer portions  32  are located in the circumferential clearances C 1  and the radial clearances C 2  between the inner surface of the coupling hole  28  and the coupling projection  15   a , respectively. The motor  1  is configured such that, when the rotary shaft  8  is rotated by the motor body  2 , the inner surface of the coupling hole  28  and the coupling projection  15   a  compresses and deforms the circumferential buffer portions  31  in the circumferential direction, while contacting each other in the circumferential direction. This reduces uncontrollable movements of the worm shaft  15  when the motor  1  is operating and stopped. As a result, the generation of noise caused by the inner surface of the coupling hole  28  striking the coupling projection  15   a  is suppressed. Further, when the rotary shaft  8  is driven to rotate in the reverse direction with the worm shaft  15  bound with the worm wheel  18 , that is, when the rotary shaft  8  starts rotating in the reverse direction, the rotary shaft  8  rotates while compressing and deforming the circumferential buffer portions  31  between the inner surface of the coupling hole  28  and the coupling projection  15   a . During this time, the inertial force, or the momentum of rotation, of the rotary shaft  8  is gained. Thereafter, with the circumferential buffer portions  31  compressed, the inner surface of the coupling hole  28  and the coupling projection  15   a  contact each other in the circumferential direction while receiving the inertial force of the rotary shaft  8 . This releases the teeth of the worm shaft  15  and the teeth of the worm wheel  18 , which have been bound to each other, from each other. As described above, according to the present disclosure, the worm shaft  15  is prevented from moving uncontrollably by the circumferential buffer portions  31  and the radial buffer portions  32 , and inertial force of the rotary shaft  8  is ensured by the circumferential clearances C 1  and the radial clearances C 2  between the coupling projection  15   a  and the inner surface of the coupling hole  28 . 
     (2) The rotary shaft  8  and the worm shaft  15  are made of metal, and the joint  16  is made of plastic and formed separately from the rotary shaft  8  and the worm shaft  15 . Accordingly, the inner surface of the coupling hole  28 , into which the coupling projection  15   a  of the worm shaft  15  is inserted, is made of plastic. Thus, when the motor body  2  rotates the rotary shaft  8 , noise caused by the inner surface of the coupling hole  28  striking the coupling projection  15   a  in the circumferential direction is suppressed. This further suppresses the generation of noise. 
     (3) The circumferential buffer portions  31  and the radial buffer portions  32  are provided in the joint  16 , which is a plastic member separate from the rotary shaft  8  and the worm shaft  15 . The circumferential buffer portions  31  and the radial buffer portions  32  are therefore formed easily. 
     (4) Since the circumferential buffer portions  31  and the radial buffer portions  32  are made of rubber, the worm shaft  15  is prevented from moving uncontrollably in a favorable manner. 
     (5) The first rubber members  21  are located between the coupling projection  15   a  and the bottom surface of the coupling hole  28  in the axial direction, that is, the end face  23   a  of the fixing portion  23 , which is close to the worm shaft  15 . Reversal of rotation of the worm shaft  15  generates a load in the axial direction. The load may cause the coupling projection  15   a  of the worm shaft  15  to strike the end face  23   a  of the fixing portion  23 , which is close to the worm shaft  15  and the distal end  8   a  of the rotary shaft  8 . However, such possible striking is suppressed by the first rubber members  21 . As a result, the generation of noise caused by such striking is suppressed. 
     (6) The joint  16 , which is a plastic member separate from the rotary shaft  8  and the worm shaft  15 , has the first rubber members  21 . The first rubber members  21  are therefore made easily. 
     (7) Since the first rubber members  21  are made of rubber, the coupling projection  15   a  is prevented from striking the bottom surface of the coupling hole  28  in the axial direction, and the coupling projection  15   a  is prevented from striking the distal end  8   a  of the rotary shaft  8  in the axial direction in a favorable manner. 
     (8) The circumferential buffer portions  31  are located radially inward of the contact portions  28   b ,  28   c ,  28   d ,  28   e  on the inner surface of the coupling hole  28 , which contact the coupling projection  15   a . According to this configuration, it is possible to ensure the space for the circumferential buffer portions  31  while maintaining the length of the arms for transmitting torque from the rotary shaft  8  to the worm shaft  15 , that is, while maintaining a long measurement between the axis L 1  of the rotary shaft  8  and the contact portions  28   b ,  28   c ,  28   d ,  28   e.    
     The embodiments of the present disclosure may be modified as follows. 
     In the above illustrated embodiment, the circumferential buffer portions  31  and the radial buffer portions  32  of the second rubber members  22   a ,  22   b  are connected to each other by the coupling portions  33 . However, the circumferential buffer portions  31  and the radial buffer portions  32  may be formed separately from each other. 
     In the above illustrated embodiment, the circumferential buffer portions  31  and the radial buffer portions  32  of the second rubber members  22   a ,  22   b  are provided in the joint  16 . That is, the circumferential buffer portions  31  and the radial buffer portions  32  are supported by the rotary shaft  8 . In a modification, the circumferential buffer portions  31  and the radial buffer portions  32  may be formed on the flat surfaces  15   x ,  15   y  of the coupling projection  15   a  of the worm shaft  15 . That is, the circumferential buffer portions  31  and the radial buffer portions  32  may be supported by the worm shaft  15 . 
     In the above illustrated embodiment, the first rubber members  21  are supported by the rotary shaft  8 . That is, the first rubber members  21  are provided on the plastic portion  20  located about the distal end  8   a  of the rotary shaft  8 . In a modification, the first rubber members  21  may be supported by the worm shaft  15 . For example, the first rubber members  21  may be formed on an axial end face of the coupling projection  15   a  of the worm shaft  15 . 
     In the above illustrated embodiment, the plastic portion  20  of the joint  16  is fixed to the rotary shaft  8 , and the coupling projection  15   a  of the worm shaft  15  is inserted into the coupling hole  28  of the plastic portion  20 . However, in a modification, the relationship may be reversed. That is, the plastic portion  20  may be fixed to the worm shaft  15 , and the distal end  8   a  of the rotary shaft  8  may be used as a coupling projection to be inserted into the coupling hole  28  of the plastic portion  20 . 
     In the above illustrated embodiment, the joint  16  has the plastic portion  20 , which is separate from the metal rotary shaft  8 , and the plastic portion  20  is fixed to the rotary shaft  8 . The embodiment is not limited to this configuration. In a modification, the joint  16  may be made of the same material as and formed integrally with the rotary shaft  8 . Further, the joint  16 , which functions as a coupling portion, may be provided in the worm shaft  15 . The joint  16  may be made of the same material as and formed integrally with the worm shaft  15 . 
     DESCRIPTION OF THE REFERENCE NUMERALS 
       1  . . . Motor,  2  . . . Motor body,  8  . . . Rotary Shaft,  11  . . . Speed Reduction Mechanism,  15  . . . Worm Shaft,  15   a  . . . Coupling Projection,  16  . . . Joint as Coupling Portion,  18  . . . Worm Wheel,  20  . . . Plastic Portion,  21  . . . First Rubber Portions (Axial Buffer Portions),  22   a ,  22   b  . . . Second Rubber Portions,  28  . . . Coupling Hole,  31  . . . Circumferential Buffer Portions,  32  . . . Axial Buffer Portions, C 1  . . . Circumferential Clearances, C 2  . . . Radial Clearances.