Patent Publication Number: US-2005115350-A1

Title: Motor with reduction mechanism and power seat motor with reduction mechanism

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a motor with a reduction mechanism suitable for use as, for example, a wiper motor or a power window motor and to a power seat motor with a reduction mechanism for moving up and down a seat.  
      2. Related Art  
      There exist a vehicle seat to which a motor with a reduction mechanism of the sort mentioned above is applied and a power seat motor as shown in  FIG. 8  and  FIG. 9 , respectively. See JP-A-2003-56674 (FIG. 1 and FIG. 5, page 1), for example.  
      As shown in  FIG. 8 , a vehicle seat  1  is mounted on the floor F in the interior of a vehicle via a displacement mechanism  2  and the seat o is driven to move up and down by a power seat motor  3  used in the displacement mechanism  2 .  
      As shown in  FIG. 9 , the power seat motor  3  has a motor shaft  3   b  projecting from a motor case  3   a  and rotatably supported by the worm-mounting cylindrical portion  4   a  of a gear case  4 , a worm  6  coupled to the front end portion of the motor shaft  3   b  via a square column-like coupling shaft  5 , a worm wheel  7  rotatably supported in the worm wheel mounting recess  4   b  of the gear case  4  and meshing with the worm  6 , an output shaft  8  concentric and integral with the worm wheel  7 , and a leaf spring  9  detachably mounted in the front-end opening portion of the worm-mounting cylindrical portion  4   a  and used for press-urging the semispherical front end portion  6   a  of the worm  6  toward the motor shaft  3   b.    
      Further, the seat elevating mechanism (not shown) of the displacement mechanism  2  is coupled to the output shaft  8 . Consequently, while the motor shaft  3   b  is rotating forward or reversely, the forward or reverse rotation of the worm  6  and the worm wheel  7  is reduced before being transmitted to the output shaft  8 , so that the seat elevating mechanism is driven to move up or down the seat  1   a.    
      In the conventional power seat motor  3  above, it has been arrange to prevent an unusual sound (noise) from being produced between the worm  6  and the worm wheel  7  by press-urging the semispherical front end portion  6   a  of the worm  6  toward the motor shaft  3   b  with the leaf spring  9  to eliminate the play in the thrust direction of the worm  6  due to a backlash between the worm  6  and the worm wheel  7 . However, because the leaf spring  9  is detachably mounted in the front-end opening portion of the worm-mounting cylindrical portion  4   a , the worm-mounting cylindrical portion  4   a  becomes longer axially, thus making the whole body of the power seat motor  3  greater in size.  
      In the case of a motor with a double-reduction mechanism that has widely been used in recent years, having a pair of worms formed in the vicinity of the front end of a motor shaft with the thread directions of screws oriented opposite to each other, a pair of counter gears facing each other with the pair of worms held therebetween and respectively meshing with the pair of worms, and an output gear meshing with the pair of the counter gears, the direction of the thrust load of the motor shaft, produced by meshing the worm on one side with the counter gear on one side and the direction of the thrust load of the motor shaft produced by meshing the worm on the other side with the counter gear on the other side are oriented opposite to each other, whereby the directions thereof are canceled out each other. Consequently, the motor shaft is exempted from the play in the thrust direction even though there exist a backlash between the worm and the counter gear and a backlash between the counter gear and an output gear.  
      When a great fluctuation in load acts on a motor like the power seat motor with the reduction mechanism using a two-speed mechanism in particular, an unusual sound may be produced because the lateral tooth side of each tooth part is hit by a great impact force due to play resulting from a tooth-to-tooth backlash between tooth parts in each tooth intermeshing portion in a case where the load acting on a motor shaft changes from a plus load (the load hindering the reverse rotation of the motor shaft) to a minus load (the load aiding the reverse rotation of the motor shaft) as in a case where the passenger&#39;s weight is added to a seat while the seat is moving down.  
     SUMMARY OF THE INVENTION  
      An object of the invention made to solve the foregoing problems is to provide a small-sized motor with a reduction mechanism and a power seat motor with a reduction mechanism, which motors are simple in construction and capable of preventing an unusual sound from being generated from each tooth intermeshing portion in a case that the motor adopts a double-reduction mechanism having an output gear meshing with each large-diameter gear of a pair of counter gears respectively meshing with a pair of worms formed on a motor shaft with the thread directions of screws oriented opposite to each other and even in a case that a great fluctuation in load acts on the motor shaft such that the load changes from one load (a plus load) hindering the rotation of the motor shaft to the other load (a minus load) aiding the rotation of the motor shaft.  
      (1) A motor with a reduction mechanism according to the invention, comprising: 
          a shaft having an armature fixed in a vicinity of a first end of the shaft and supported in a motor case so as to be rotatable;     a pair of worms formed in a vicinity of a second end of the shaft having opposite thread directions to each other;     a pair of counter gears opposed to each other with respect to the shaft, each of which is provided with a large-diameter gear meshing with the corresponding worm and a smaller diameter gear concentric with the large-diameter gear so as to be integrally rotatable with the large-diameter gear; and     an output gear meshing with the small-diameter gears so that thrust bearings for supporting both end faces of the motor shaft are not necessary;     wherein a spring member and a slide member are housed in a recess extending in an axial direction of the shaft from an end face of the first end; and     the slide member is urged by the spring member to contact with an inner face of the motor case so that the thrust force is always generated toward the second end of the shaft by a resilient force of the spring member.        

      (2) In the invention, a front end portion of the slide member may be shaped into a semisphere and a semisolid oil lubricant may be disposed between a semispherical top portion of the front end portion and the inner face of the motor case.  
      (3) A power seat motor with a reduction mechanism of the invention, comprises a motor shaft which has an armature fixed to the vicinity of the back end of the motor shaft and is supported in a motor case such that the motor shaft is rotatable forward or reversely, a pair of worms formed in the vicinity of the front end of the motor shaft with the thread directions of screws oriented opposite to each other, a pair of counter gears formed opposite to each other with the motor shaft held therebetween, having large-diameter gears respectively meshing with the pair of worms and small-diameter gears which are concentric with the large-diameter gears and rotate integrally with the large-diameter gears, and an output gear meshing with small-diameter gears, wherein thrust bearings for supporting both end faces of the motor shaft are not necessary; and an output shaft coupled to the output gear is driven so that a seat is moved up or down when the motor shaft is rotated forward or reversely, characterized in that: a recess is formed in the axial direction of the motor shaft from the end face of the back end of the motor shaft; a spring member elastically deformable in the axial direction of the motor shaft is housed in the recess; a slide member is slidably housed in the recess; the front end portion of the slide member is pressed to contact the inner face of the end portion of the motor case by the elastic force of the spring member; and thrust force oriented in the direction of the front end of the motor shaft is always generated in the motor shaft by the resilient force of the spring member.  
      As set forth above, the motor with the reduction mechanism according to the invention is configured such that the recess is formed in the axial direction of the motor shaft from the end face of the back end of the motor shaft; the spring member elastically deformable in the axial direction of the motor shaft is housed in the recess; the slide member is slidably housed in the recess; the front end portion of the slide member is pressed to contact the inner face of the end portion of the motor case by the elastic force of the spring member; and the thrust force oriented in the direction of the front end of the motor shaft is always generated in the motor shaft by the resilient force of the spring member. Consequently, even when a great fluctuation in load ranging from a plus load to a minus load acts on the motor, the lateral tooth side of each tooth part becomes free from being hit by a great impact force due to the backlash between the large-diameter gears of the pair of counter gears meshing with the pair of worms and the tooth parts of each tooth intermeshing portion of the output gear meshing with the small-diameter gears of the pair of counter gears or undergoes a largely eased impact force to ensure that even in the case of a double-reduction mechanism, a tooth-to-tooth unusual sound between the tooth parts of each tooth intermeshing portion can be eliminated by the motor simple in construction, thus making it feasible to reduce the size of the whole construction.  
      With the motor with the reduction mechanism, as the semi solid oil lubricant is disposed between the top portion of the semispherical front end portion and the inner face of the end portion of the motor case, the spring member and the slide member together with the motor shaft can smoothly be rotated by means of a simple construction moreover stable thrust force is applicable to the motor shaft in the direction of the front end of the armature shaft.  
      With the power seat motor with the reduction mechanism according to the invention, as the thrust force oriented in the direction of the front end of the motor shaft is always generated in the motor shaft, even though the minus load aiding the rotation of the motor shaft acts in the course of moving down the seat by the seat elevating mechanism, the lateral tooth side of each tooth part becomes free from being hit by a great impact force due to the backlash between the large-diameter gears of the pair of counter gears meshing with the pair of worms and the tooth parts of each tooth intermeshing portion of the output gear meshing with the small-diameter gears of the pair of counter gears or undergoes a largely eased impact force to ensure that even in the case of a double-reduction mechanism, a tooth-to-tooth unusual sound between the tooth parts of each tooth intermeshing portion can be eliminated by the motor simple in construction, thus making it feasible to reduce the size of the whole construction. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a plan view of a motor with a reduction mechanism according to the embodiment of the invention;  
       FIG. 2  is a sectional view of the motor with the reduction mechanism;  
       FIG. 3  is a plan view of a state in which the gear case of the motor with the reduction mechanism has been removed;  
       FIG. 4  is an enlarged sectional view of the principal part of the motor with the reduction mechanism;  
       FIG. 5  is a diagram illustrating a state in which a motor shaft for use in the motor with the reduction mechanism is unrotated;  
       FIG. 6  is a diagram illustrating a state in which the motor shaft for use in the motor with the reduction mechanism is rotating forward;  
       FIG. 7  is a diagram illustrating a state in which the motor shaft of the motor with the reduction mechanism is rotating reversely;  
       FIG. 8  is a schematic diagram of a vehicle seat to which a conventional motor with a reduction mechanism is applied;  
       FIG. 9  is a sectional view of the conventional motor with the reduction mechanism. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
      An embodiment of the invention will now be described by reference to the drawings.  
       FIG. 1  is a plan view of a motor with a reduction mechanism according to the embodiment of the invention;  FIG. 2 , a sectional view of the motor;  FIG. 3 , a plan view of a state in which the gear case of the motor has been removed;  FIG. 4 , an enlarged sectional view of the principal part of the motor;  FIG. 5 , a diagram illustrating a state in which a motor shaft for use in the motor is unrotated;  FIG. 6 , a diagram illustrating a state in which the motor shaft for use in the motor is rotating forward; and  FIG. 7 , a diagram illustrating a state in which the motor shaft of the motor is rotating reversely. Incidentally, the vehicle seat (power seat)  1  shown in  FIG. 8 , presented for explaining the vehicle sheet having the conventional motor is also used for explaining the present invention.  
      As shown in  FIG. 1 ,  FIG. 2  and  FIG. 3 , a power seat motor  10  with a reduction mechanism (a motor with a reduction mechanism) has a substantially cylindrical yoke (motor case)  11  with one end side opened, and a gear case  21  with a flange portion  11   b  around the opening end  11   a  of the yoke  11  being fixedly tightened via machine screws.  
      As shown in  FIG. 20 , a pair of magnets  12  and  12  are secured to the inner peripheral face  11   c  of the yoke  11  with an adhesive agent or the like. Further, an armature shaft (motor shaft)  14  is rotatably supported by a radial bearing  13   a  fitted into a closed-end cylindrical portion  11   d  at the other end of the yoke  11  and radial bearings  13   b  and  13   c  fitted into the vicinity of both ends of the shaft hole  22  of the gear case  21 .  
      The armature shaft  14  has a first worm (worm)  15  and a second worm (worm)  150  formed in the vicinity of the front end  14   a  of the armature shaft with the thread directions of screws oriented opposite to each other. The first worm  15  and the second worm  150  are used to form the pair of worms. An armature  16  is mounted in a position opposite to the pair of magnets  12  and  12  of the armature shaft  14 . The armature  16  is fixed to the vicinity of the back end  14   b  of the armature shaft  14  and has an armature core  16   a  having coil-winding portions  16   b  with a predetermined number of slots and an armature coil  16   c  wound on the coil-winding portions  16   b  of the armature core  16   a.    
      A commutator  17  is fixed to a position opposite to the boundary portion between the yoke  11  of the armature shaft  14  and the gear case  21 . The commutator  17  has commutator bars  17   a  equal in number to the coil-winding portions  16   b  of the armature core  16   a , and each of the commutator bars  17   a  is electrically connected to the armature coil  16   c.    
      The opening end of the shaft hole  22  of the gear case  21  forms a large-diameter hole portion  22   a , and a pair of brushes  19  and  19  are mounted to a position opposite to the commutator  17  in the large-diameter hole portion  22   a  so that the pair of brushes are brought into contact with the respective commutator bars  17   a . Each of the brushes  19  is electrically connected to a motor control circuit (not shown). Switching the on-off of each switch out of a pair of switches of the motor control circuit causes an electric current to flow into the armature  16 , so that the armature shaft  14  is rotated forward or reversely.  
      As shown in  FIG. 2  and  FIG. 3 , the shaft hole  22  is formed substantially in the center of the gear case  21  and a depressed reduction-mechanism housing portion  23  is so formed as to communicate with the shaft hole  22 . Cylindrical bosses (thrust bearings for counter gears)  24  and  24 ′ are formed in a projected condition integrally in a predetermined position where the pair of worms  15  and  150  on the bottom wall of the reduction-mechanism housing portion  23  are sandwiched. Moreover, circular recesses  25  and  25 ′ are formed in the center of and in the respective cylindrical bosses  24  and  24 ′. The lower parts of metal pin-like pivots  26  and  26 ′ are press-fitted into the respective recesses  25  and  25 ′. A first counter gear (counter gear)  30  is rotatably supported by the pivot  26  and a second counter gear  300  is rotatably supported by the pivot  26 ′. Further, a circular hole  27   a  is as shown in  FIG. 3  formed in a position a little to the right of the front end of the worm  15  of the bottom wall of the reduction-mechanism housing portion  23 . A substantially annular rib  27   b  is formed in a projected condition integrally therewith around the circular hole  27   a . The lower end of the cylindrical portion  41  of an output gear  40  is rotatably supported in the substantially annular rib  27   b  via a radial bearing  28   a.    
      As shown in  FIG. 1 , further, an opening at one end of the reduction-mechanism housing portion  23  of the gear case  21  is covered with a substantially triangular platelike plastic gear case cover  29  securely tightened with machine screws  20   b . Circular recesses  29   a  and  29   a ′ are formed in positions opposite to the respective recesses  25  and  25 ′ of the reduction-mechanism housing portion  23  of the gear case cover  29 . The upper part of the pivot  26  is press-fitted into the recess  29   a  and the upper part of the pivot  26 ′ is press-fitted into the recess  29   a ′. Further, a circular hole  29   b  is formed in a position opposite to the circular hole  27   a  of the reduction-mechanism housing portion  23  of the gear case cover  29 . The upper end of the cylindrical portion  41  of the output gear  40  is rotatably supported in the circular hole  29   b  via a thrust-cum-radial bearing  28   b . The pair of worms  15  and  150  and the pair of counter gears  30  and  300  and the output gear  40  are housed in the reduction-mechanism housing portion  23  of the gear case  22  to form a double-reduction mechanism.  
      As shown in  FIG. 2 ,  FIG. 5 ,  FIG. 6  and  FIG. 7 , the first counter gear  30  is formed of a large-diameter plastic gear  31  and a first small-diameter metal gear  35  concentric with the large-diameter gear  31 . A tooth part  32  meshing with the first worm  15  is formed on the outer periphery of the large-diameter gear  31  and an inside spline  33  is formed on the inner periphery of the large-diameter gear  31 . Further, a tooth part  36  meshing with the tooth part  42  of the output gear  40  and an outside spline  37  meshing with the inside spline  33  of the large-diameter gear  31  are formed on the outer periphery of the first small-diameter gear  35  are integrally formed in the axial direction in a concentric, difference-in-level form. In this case, fixing the large-diameter gear  31  relatively to the first small-diameter gear  35  is made by insert molding when the large-diameter plastic gear  31  is formed by molding. Similarly, the second counter gear  300  is formed of a large-diameter plastic gear  310  and a second small-diameter metal gear  350  concentric with the large-diameter gear  310 . A tooth part  320  meshing with the second worm  150  is formed on the outer periphery of the large-diameter gear  310 , and the inside spline  33  is formed on the inner periphery of the large-diameter gear  310 . Further, a tooth part  360  meshing with the tooth part  42  of the output gear  40  and the outside spline  37  meshing with the inside spline  33  of the large-diameter gear  310  are formed on the outer periphery of the second small-diameter gear  350  are integrally formed in the axial direction in a concentric, difference-in-level form. In this case, fixing the large-diameter gear  310  relatively to the second small-diameter gear  350  is made by insert molding when the second large-diameter plastic gear  310  is formed by molding.  
      As shown in  FIG. 2 ,  FIG. 5 ,  FIG. 6  and  FIG. 7 , an output shaft  43  is fixed in the cylindrical portion  41  of the output gear  40 , and the seat elevating mechanism (not shown) of the displacement mechanism  2  of a vehicle seat  1  is coupled to a portion projected outside from the gear case  21  of the output shaft  43 , whereby the seat elevating mechanism is driven to move a seat  1   a  up or down when the armature shaft  14  is rotated forward or reversely. In other words, the output shaft  43  coupled to the output gear  40  is driven to move up the seat  1   a  when the armature shaft  14  is rotated forward and to move down the seat  1   a  when the armature shaft  14  is rotated reversely.  
      As shown in  FIG. 2  and  FIG. 4 , a cylindrical recess  14   c  circular in section is formed from the end face  14   f  of the back end  14   b  of the armature shaft  14  in the axial direction of the armature shaft  14 , and a metal helical compression spring  51  as a spring member elastically deformable in the axial direction of the armature shaft  14  is housed in the cylindrical recess  14   c , so that one end portion of the helical compression spring  51  is made to contact the base  14   d  of the cylindrical recess  14   c , a plastic columnar slide member  52  being housed in the cylindrical recess  14   c  as well. The front end portion  52   b  of the slide member  52  is projected outside from the opening end  14   e  of the cylindrical recess  14  by the elastic force of the helical compression spring  51  disposed between the base  14   d  of the cylindrical recess  14   c  formed in the armature shaft  14  and the end face  52   a  of the back end portion of the slide member  52  and pressed to contact the base portion (inner face of the end portion of the motor case)  11   e  of the closed-end cylindrical portion  11   d  of the yoke  11 , whereby the thrust force directed to the front end  14   a  of the armature shaft  14  is always generated in the armature shaft  14 . The front end portion  52   b  of the slide member  52  is made semispherical in configuration and grease (semisolid oil lubricant)  53  is disposed between the top portion  52   c  of the semispherical front end portion  52   b  and the base portion  11   e  of the closed-end cylindrical portion  11   d.    
      With the power seat motor  10  including the reduction mechanism, since the large-diameter gears  31  and  310  of the pair of counter gears  30  and  300  are made to mesh with the pair of worms  15  and  150  formed in the vicinity of the front end  14   a  of the armature shaft  14  with the thread directions of screws oriented opposite to each other in order to make the motor shaft rotate forward or reversely, the direction of the thrust load of the armature shaft  14  oriented by causing the first worm  15  to mesh with the first counter gear  30  and the direction of the thrust load of the armature shaft  14  oriented by causing the second worm  150  to mesh with the second counter gear  301  are oriented opposite to each other and canceled out each other. Thus, thrust bearings for pivotably supporting both edges faces  14   a   1  and  14   f  of the armature shaft  14  are not necessary, so that such thrust bearings as to rotatably support the solid first counter gear  30  and the solid second counter gear  300  with precision can also be dispensed with. Moreover, play in the thrust direction of the armature shaft  14  of the motor  10  due to a backlash between the tooth parts of each tooth intermeshing portion is eliminated, so that the armature shaft  14  can smoothly be rotated forward or reversely.  
      As shown in  FIG. 5 , since the front end portion  52   b  of the slide member  52  is pressed to contact the base portion (inner face of the end portion of the motor case)  11   e  of the closed-end cylindrical portion  11   d  of the yoke  11  by the elastic force applied by the compression of the helical compression spring  51  housed in the cylindrical recess  14   c  of the armature shaft  14  while the armature shaft  14  is unrotated, the resilient force of the helical compression spring  51  allows thrust force in the direction of an arrow F that is the direction in which the front end  14   a  of the armature shaft  14  is oriented to always act on the armature shaft  14 . Then the end face  14   f  of the back end  14   b  of the armature shaft  14  is located at a position A, so that a predetermine gap is secured between the end face  14   a   1  of the front end  14   a  of the armature shaft  14  and the base  13   c   1  of a radial bearing  13   c . Further, a position B is a position to which the end face  14   f  of the back end  14   b  of the armature shaft  14  is movable when force opposite in direction to the direction F is applied from the outside while the armature shaft  14  is unrotated. The distance from the position A to the position B corresponds to the backlash produced in each tooth part. Even when the end face  14   a   1  of the front end  14   a  of the armature shaft  14  is moved to the position B, the distance above is set so that the end face is never brought into contact with the base portion  11   e  of the closed-end cylindrical portion  11   d  of the yoke  11 .  
      While the armature shaft  14  is unrotated, the state in which the first worm  15  and the first counter gear  30  are meshing with each other is such that the lateral tooth side  32   a  on one side of the tooth part  32  of the first counter gear  30  is in contact with the first worm  15  and the state in which the second worm  150  and the second counter gear  300  are meshing with each other is such that the lateral tooth side  320   a  on one side of the tooth part  320  of the second counter gear  300  is in contact with the second worm  150 . Further, the state in which the first small-diameter gear  35  and the output gear  40  are meshing with each other is such that the lateral tooth side  42   a  on one side of the tooth part  42  of the output gear  40  is in contact with the first small-diameter gear  35  and the state in which the second small-diameter gear  350  and the output gear  40  are meshing with each other is such that the lateral tooth side  42   b  on the other side of the tooth part  42  of the output gear  40  is in contact with the second small-diameter gear  350 .  
       FIG. 6  illustrates a state in which the armature shaft  14  is rotating forward. The first counter gear  30 , the first small-diameter gear  35 , the second counter gear  300  and the second small-diameter gear  350  are rotated counterclockwise as in the direction of the arrow by rotating the armature shaft  14  forward and the output shaft  43  is also rotated clockwise as in the direction of the arrow whereby to move up the seat elevating mechanism (not shown) coupled to the output shaft  43 . Then the armature shaft  14  moves to the left in  FIG. 6  from the position of  FIG. 5  showing the unrotated condition of the armature shaft  14  while resisting the resilient force of the helical compression spring, so that the end face  14   f  of the back end  14   b  of the armature shaft  14  moves to a position C as the midposition between the position A and the position B.  
      While the armature shaft  14  is rotating forward, the state in which the first worm  15  and the first counter gear  30  are meshing with each other is such that like the case where the armature shaft  14  is unrotated as shown in  FIG. 5  the lateral tooth side  32   a  on one side of the tooth part  32  of the first counter gear  30  is in contact with the first worm  15 . On the other hand, the state in which the second worm  150  and the second counter gear  300  are meshing with each other is such that unlike the case where the armature shaft  14  is unrotated as shown in  FIG. 5  the lateral tooth side  320   b  on the other side of the tooth part  320  of the second counter gear  300  is in contact with the second worm  150 . Further, the state in which the first small-diameter gear  35  and the output gear  40  are meshing with each other is such that like the where the armature shaft  14  is unrotated as shown in  FIG. 5  the lateral tooth side  42   a  on one side of the tooth part  42  of the output gear  40  is in contact with the first small-diameter gear  35 . On the other hand, the state in which the second small-diameter gear  350  and the output gear  40  are meshing with each other is such that unlike the case where the armature shaft  14  is unrotated as shown in  FIG. 5  the lateral tooth side  42   a  on one side of the tooth part  42  of the output gear  40  is in contact with the second small-diameter gear  350 .  
       FIG. 7  illustrates a state in which the armature shaft  14  is rotating reversely. The first counter gear  30 , the first small-diameter gear  35 , the second counter gear  300  and the second small-diameter gear  350  are rotated clockwise as in the direction of the arrow by rotating the armature shaft  14  reversely and the output shaft  43  is also rotated counterclockwise as in the direction of the arrow whereby to drive the seat elevating mechanism (not shown) coupled to the output shaft  43  so as to move down the seat  1   a . Then the armature shaft  14  moves to the left in  FIG. 7  from the position of  FIG. 5  showing the unrotated condition of the armature shaft  14  while resisting the resilient force of the helical compression spring, so that the end face  14   f  of the back end  14   b  of the armature shaft  14  moves to the position C as the midposition between the position A and the position B as in the case where the armature shaft  14  is rotated forward as shown in  FIG. 6 .  
      While the armature shaft  14  is rotating reversely, the state in which the first worm  15  and the first counter gear  30  are meshing with each other is such that unlike the case where the armature shaft  14  is unrotated as shown in  FIG. 5  and where the armature shaft  14  is rotated forward as shown in  FIG. 6  the lateral tooth side on the other side of the tooth part  32  of the first counter gear  30  is in contact with the first worm  15 . On the other hand, the state in which the second worm  150  and the second counter gear  300  are meshing with each other is such that like the case where the armature shaft  14  is unrotated as shown in  FIG. 5  but unlike the case where the armature shaft  14  is rotated forward as shown in  FIG. 6  the lateral tooth side  320   b  on the other side of the tooth part  320  of the second counter gear  300  is in contact with the second worm  150 . Further, the state in which the first small-diameter gear  35  and the output gear  40  are meshing with each other is such that unlike the case where the armature shaft  14  is unrotated and unlike the case where the armature shaft  14  is rotated forward as shown in  FIG. 6  the lateral tooth side  42   b  on the other side of the tooth part  42  of the output gear  40  is in contact with the first small-diameter gear  35 . On the other hand, the state in which the second small-diameter gear  350  and the output gear  40  are meshing with each other is such that like the case where the armature shaft  14  is unrotated as shown in  FIG. 5  but unlike the case where the armature shaft  14  is rotated forward as shown in  FIG. 6  the lateral tooth side  42   b  of the tooth part  42  of the output gear  40  is in contact with the second small-diameter gear  350 .  
      In the middle of moving down the seat  1   a  by rotating the output shaft  43  counterclockwise as in the direction of the arrow in  FIG. 6  when the armature shaft  14  is rotated reversely to drive the seat elevating mechanism (not shown) of the displacement mechanism  2 , there may occur such a phenomenon a plurality of times in one descending operation that the load acting on the armature shaft  14  changes from a load (a plus load hindering the reverse rotation of the armature shaft  14 ) necessary for operating the seat elevating mechanism to a so-called minus load when the weight of a passenger sitting on the seat  1   a  is added to the seat  1   a , for example, whereby the load aiding the reverse rotation of the armature shaft  14  becomes greater than the load necessary for operating the seat elevating mechanism.  
      In such a state that the seat elevating mechanism (not shown) is unoperated, that is, when a switch for moving down a motor control circuit (not shown) is switched from off to on in order to move down the seat  1   a  by driving the seat elevating mechanism after the armature shaft  14  is set unrotated as shown in  FIG. 5 , the armature shaft  14  rotates reversely and is reduced to the plus load condition as shown in  FIG. 7 . When the load acting on the armature shaft  14  changes from the plus load condition to the minus load condition, the armature shaft  14  is reduced to an no-load condition in the course of the change. While the armature shaft  14  is in the no-load condition, the armature shaft  14  is moved to the front end  14   a  due to the resilient force of the helical compression spring  51 , and the end face  14   f  of the back end  14   b  of the armature shaft  14  moves from the position C up to the position A and is held in the state shown in  FIG. 5 . In other words, the state in which the first worm  15  and the first counter gear  30  are meshing with each other is such that the tooth part  32  of the first counter gear  30  in contact with the first worm  15  shifts from the other lateral tooth side to the one lateral tooth side. As the armature shaft  14  moves in the direction of the front end  14   a  during the shifting operation, the one lateral tooth side  32   a  comes into contact with the first worm  15  while the other lateral tooth side  32   b  is kept in contact with the first worm  15 , so that these lateral tooth sides are prevented from colliding with each other by a great impact force. Consequently, no unusual sound is generated between the first worm  15  and the tooth part  32  of the first counter gear  30 . While the first small-diameter gear  35  is meshing with the output gear  40 , though the tooth part  42  of the output gear  40  in contact with the first small-diameter gear  35  moves from the other lateral tooth side  42   b  to the one lateral tooth side  42   a , no unusual sound is not generated between the first small-diameter gear  35  and the tooth part  42  of the output gear  40  likewise. On the other hand, the state in which the second worm  150  and the second counter gear  300  are meshing with each other and the state in which the second small-diameter gear  350  and the output gear  40  are meshing with each other remain unchanged as shown in  FIG. 7 . In other words, the one lateral tooth side  320   a  of the tooth part  320  of the second counter gear  300  is in contact with the second worm  150 , whereas the other lateral tooth side  42   b  of the tooth part  42  of the output gear  40  is in contact with the small-diameter gear  350 .  
      The armature shaft  14  changes from the no-load condition to the minus load condition; the minus load condition is similar to the state in which the armature shaft  14  is rotating forward, whereupon the armature shaft  14  is moved in the direction of the back end  14   b  and the end face  14   f  of the back end  14   b  of the armature shaft  14  moves from the position A up to the position C and is held in the state shown in  FIG. 6 . The state in which the first worm  15  and the first counter gear  30  are meshing with each other and the state in which the first small-diameter gear  35  and the output gear  40  are meshing with each other remain unchanged. The one lateral tooth side  32   a  of the tooth part  32  of the first counter gear  30  is kept in contact with the first worm  15 , and the one lateral tooth side  42   a  of the tooth part  42  of the output gear  40  is kept in contact with the first small-diameter gear  35 . On the other hand, with respect to the state in which the second worm  150  and the second counter gear  300  are meshing with each other, the tooth part  320  of the second counter gear  300  in contact with the second worm  150  shifts from the one lateral tooth side  320   a  to the other lateral tooth side  320   b  and with respect to the state in which the second small-diameter gear  350  and the output gear  40  are meshing with each other, the tooth part  42  of the output gear  40  in contact with the second small-diameter gear  350  shifts from the other lateral tooth side  42   b  to the one lateral tooth side  42   a.    
      In the course of the change from the no-load condition to the minus load condition of the armature shaft  14 , the thrust force is acting on the armature shaft  14  in the direction of the arrow F as shown in  FIG. 5  due to the resilient force of the helical compression spring  51 . When the tooth part  320  of the second counter gear  300  in contact with the second worm  150  shifts from the one lateral tooth side  320   a  to the other lateral tooth side  320   b , the helical compression spring  51  generating the thrust force in the direction of the arrow F functions as a damper, so that the other lateral tooth side  320   b  is prevented from colliding with the second worm  150  by a great impact force. When the tooth part  42  of the output gear  40  in contact with the second small-diameter gear  350  shifts from the other lateral tooth side  42   b  to the one lateral tooth side  42   a , moreover, the one lateral tooth side  42   a  is prevented from colliding with the second small-diameter gear  350  by a great impact force likewise. Consequently, the lateral tooth side of each tooth intermeshing portion becomes free from being hit by a great impact force due to the backlash between the tooth parts of each tooth intermeshing portion or undergoes a largely eased impact force, so that no unusual sound is generated between the tooth parts of each tooth intermeshing portion.  
      A description will now be given of a case where the load acting on the armature shaft  14  changes from the minus load condition to the plus load condition in the course of moving down the seat  1   a  next. When the load changes from the minus load condition to the plus load condition, the armature shaft  14  is in a no-load condition during the course above. In the no-load condition of the armature shaft  14 , the armature shaft  14  is moved in the direction of the front end  14   a  due to the resilient force of the helical compression spring  51  and the end face  14   f  of the back end  14   b  of the armature shaft  14  moves from the position C up to the position A and is held in the state shown in  FIG. 5 . In other words, the state in which the first worm  15  and the first counter gear  30  are meshing with each other and the state in which the first small-diameter gear  35  and the output gear  40  are meshing with each other remain unchanged. Then the one lateral tooth side  32   a  of the tooth part  32  of the first counter gear  30  remains in contact with the first worm  15 , and the one lateral tooth side  42   a  of the tooth part  42  of the output gear  40  remains in contact with the first small-diameter gear  35 . On the other hand, the state in which the second worm  150  and the second counter gear  300  are meshing with each other is such that the tooth part  320  of the second counter gear  300  in contact with the second worm  150  shifts from the other lateral tooth side  320   b  to the one lateral tooth side  320   a . As the armature shaft  14  moves in the direction of the front end  14   a  during the shifting operation, the one lateral tooth side  32   a  comes into contact with the first worm  15  while the other lateral tooth side  32   b  is kept in contact with the first worm  15 , so that these lateral tooth sides are prevented from colliding with each other by a great impact force. Consequently, no unusual sound is generated between the first worm  15  and the tooth part  32  of the first counter gear  30 . Further, the state in which the second small-diameter gear  350  and the output gear  40  are meshing with each other is such that the tooth part  42  of the output gear  40  in contact with the second small-diameter gear  350  shifts from the one lateral tooth side  42   a  to the other lateral tooth side  42   b , so that no unusual sound is generated between the first small-diameter gear  35  and the tooth part  42  of the output gear  40  like wise.  
      The armature shaft  14  changes from the no-load condition to the plus load condition; the plus load condition is similar to the state in which the armature shaft  14  is rotating reversely, whereupon the armature shaft  14  is moved in the direction of the back end  14   b  and the end face  14   f  of the back end  14   b  of the armature shaft  14  moves from the position A up to the position C and is held in the state shown in  FIG. 7 . The state in which the second worm  150  and the second counter gear  300  are meshing with each other and the state in which the second small-diameter gear  350  and the output gear  40  are meshing with each other remain unchanged. The one lateral tooth side  320   a  of the tooth part  320  of the second counter gear  300  is kept in contact with the second worm  150 , and the other lateral tooth side  42   b  of the tooth part  42  of the output gear  40  is kept in contact with the second small-diameter gear  350 . On the other hand, with respect to the state in which the first worm  15  and the first counter gear  30  are meshing with each other, the tooth part  32  of the first counter gear  30  in contact with the first worm  15  shifts from the one lateral tooth side  32   a  to the other lateral tooth side  32   b  and with respect to the state in which the first small-diameter gear  35  and the output gear  40  are meshing with each other, the tooth part  42  of the output gear  40  in contact with the first small-diameter gear  35  shifts from the one lateral tooth side  42   a  to the other lateral tooth side  42   b.    
      In the course of the change from the no-load condition to the plus load condition of the armature shaft  14 , the thrust force is acting on the armature shaft  14  in the direction of the arrow F as shown in  FIG. 5  due to the resilient force of the helical compression spring  51 . When the tooth part  32  of the first counter gear  30  in contact with the first worm  15  shifts from the one lateral tooth side  32   a  to the other lateral tooth side  32   b , the helical compression spring  51  generating the thrust force in the direction of the arrow F functions as a damper, so that the other lateral tooth side  32   b  is prevented from colliding with the second worm  15  by a great impact force. When the tooth part  42  of the output gear  40  in contact with the first small-diameter gear  35  shifts from the one lateral tooth side  42   a  to the other lateral tooth side  42   b , moreover, the other lateral tooth side  42   b  is prevented from colliding with the first small-diameter gear  35  by a great impact force likewise. Consequently, the lateral tooth side of each tooth intermeshing portion becomes free from being hit by a great impact force due to the backlash between the tooth parts of each tooth intermeshing portion or undergoes a largely eased impact force, so that no unusual sound is generated between the tooth parts of each tooth intermeshing portion.  
      As set forth above, the front end portion  52   b  of the slide member  52  is pressed to contact the base portion  11   e  of the yoke  11  by the elastic force of the helical compression spring  51  housed in the cylindrical recess  14   c  of the armature shaft  14  and the thrust force in the direction of the front end  14   a  of the armature shaft  14  is always generated by the resilient force of the helical compression spring  51 . Therefore, even though the minus load condition occurs a plurality of times in one descending operation in the course of moving down the seat  1   a  by the seat elevating mechanism, the lateral tooth side of each tooth part becomes free from being hit by a great impact force due to the backlash between the large-diameter gears  31  and  310  of the pair of counter gears  30  and  300  meshing with the pair of worms  15  and  150  and the tooth parts of each tooth intermeshing portion of the output gear  40  meshing with the small-diameter gears  35  and  350  of the pair of counter gears  30  and  300  or undergoes a largely eased impact force to ensure that even in the case of the double-reduction mechanism, a tooth-to-tooth unusual sound between the tooth parts of each tooth intermeshing portion can be eliminated by the motor simple in construction.  
      As it has been arranged that the helical compression spring  51  and the slide member  52  are housed in the cylindrical recess  14   c  formed in the back end  14   b  of the armature shaft  14 , the helical compression spring  51  and the slide member  52  are not substantially projected outside, whereby the whole power seat motor can be reduced in size. Further, as grease is disposed between the top portion  52   c  of the semispherical front end portion  52   b  of the slide member  52  and the base portion  11   e  of the yoke  11 , the helical compression spring  51  and the slide member  52  together with the armature shaft  14  are made rotatable smoothly by a simple construction provided so that the helical compression spring  51  and the slide member  52  are housed in the cylindrical recess  14   c  formed at the back end  14   b  of the armature shaft  14  and moreover stable thrust force is applicable to the armature shaft  14  in the direction of the front end  14   a  of the armature shaft  14 .  
      Although the motor with the reduction mechanism has been described as a power seat motor with a reduction mechanism for a motor vehicle according to the embodiment of the invention, the embodiment thereof is needless to say applicable any other motor such as wiper motors and power window motors with a reduction mechanism.