Abstract:
The present invention relates to a reclining device that includes an external teeth gear and an internal teeth gear and in which the rotation axis of one of the external teeth gear and the internal teeth gear revolves around the rotation axis of the other. The present invention aims to provide a reclining device of which the operation force is small and the operation force varies little. To achieve the aim, one of (a) the plane of each of the external teeth  31   b  that engages with the internal teeth  32   b  and (b) the plane of each of the internal teeth  32   b  that engages with the external teeth  31   b  is arranged to be flat, and the other of (a) and (b) is arranged to be convex.

Description:
The disclosure of Japanese Patent Application No. JP 2006-160198 filed on Jun. 8, 2006 including the specification, drawings and abstract is incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a reclining device that includes an external teeth gear and an internal teeth gear and in which the rotation axis of one of the external teeth gear and the internal teeth gear revolves around the rotation axis of the other, the external teeth gear having external teeth on the outer circumferential surface thereof and being disposed with one of a member on the seat cushion side and a member on the seat back side, and the internal teeth gear having internal teeth of which the number of teeth is larger than that of the external teeth of the external teeth gear and that are configured so as to engage with the external teeth and being disposed with the other of the member on the seat cushion side and the member on the seat back side. 
     2. Description of the Related Art 
     One example of reclining devices that allow continuous adjustments is a reclining device that includes an external teeth gear and an internal teeth gear and in which the rotation axis of one of the external teeth gear and the internal teeth gear revolves around the rotation axis of the other, the external teeth gear having external teeth on the outer circumferential surface thereof and being disposed with one of a member on the seat cushion side and a member on the seat back side, and the internal teeth gear having internal teeth of which the number of teeth is larger than that of the external teeth of the external teeth gear and that are configured so as to engage with the external teeth and being disposed with the other of the member on the seat cushion side and the member on the seat back side. 
     The external teeth of the external teeth gear and the internal teeth of the internal teeth gear that are included in such a reclining device are shaped so as to be involute teeth, as shown in  FIG. 13 . To be more specific, as shown in the drawing, the plane, within the range B, of each of the external teeth  3  of the external teeth gear  1  that engages with the internal teeth  7  of the internal teeth gear  5  is convex and protrudes in the direction of the internal tooth  7 . On the other hand, the plane, within the range A, of each of the internal teeth  7  of the internal teeth gear  5  that engages with the external teeth  3  of the external teeth gear  1  is concave and is recessed in the direction away from the external tooth  3  (For example, see Japanese Examined Patent Publication (Kokoku) No. H7-79740 (FIG. 3). 
     SUMMARY OF THE INVENTION 
     The internal teeth gear and the external teeth gear included in such a reclining device are each manufactured by pressing and shaping a thick steel sheet by squeezing it in the thickness direction. Thus, the plane at which each of the internal teeth of the internal teeth gear engages with an external tooth and the plane at which each of the external teeth of the external teeth gear engages with an internal tooth have surface roughness (unevenness) as large as approximately 0.01 mm. In the case where an external tooth is engaging with an internal tooth, the range within which the distance between the plane of the external tooth engaging with the internal tooth and the plane of the internal tooth engaging with the external tooth is equal to or smaller than the surface roughness (e.g. 0.01 mm) is the range C shown in  FIG. 13 . 
     If there is a bump due to the surface roughness in a portion of an external tooth that engages with an internal tooth or in the vicinity thereof or in a portion of an internal tooth that engages with an external tooth or in the vicinity thereof, when the external tooth engages and the internal tooth engage with each other, the bump hits the engaging teeth. This causes a phenomenon where the revolution orbit of the external teeth gear deviates from a predetermined course, and thus the operation force becomes larger. 
     In addition, if there is a bump due to the surface roughness, the internal tooth and the external tooth engage with each other somewhere within the range C. When the length of the range C is large, the point at which an internal tooth and an external tooth engage with each other varies largely in the radial direction of the internal teeth gear and the external teeth gear. This causes a problem where the operation force varies. 
     In view of the problems described above, it is an object of the present invention to provide a reclining device that has a small operation force and with which the operation force varies little. 
     The invention defined in aspect 1 presents a reclining device that includes an external teeth gear and an internal teeth gear and in which the rotation axis of one of the external teeth gear and the internal teeth gear revolves around the rotation axis of the other, the external teeth gear having external teeth on the outer circumferential surface thereof and being disposed with one of a member on the seat cushion side and a member on the seat back side, and the internal teeth gear having internal teeth of which the number of teeth is larger than that of the external teeth of the external teeth gear and that are configured so as to engage with the external teeth and being disposed with the other of the member on the seat cushion side and the member on the seat back side, the reclining device being characterized in that one of (a) the plane of each of the external teeth that engages with the internal teeth and (b) the plane of each of the internal teeth that engages with the external teeth is flat, whereas the other of (a) and (b) is convex. 
     When one of the external teeth gear and the internal teeth gear rotates, the external teeth gear having the external teeth on the outer circumferential surface thereof and being disposed with one of the member on the seat cushion side and the member on the seat back side, and the internal teeth gear having the internal teeth of which the number of teeth is larger than that of the external teeth of the external teeth gear and that are configured so as to engage with the external teeth and being disposed with the other of the member on the seat cushion side and the member on the seat back side, the rotation axis of one of the external teeth gear and the internal teeth gear revolves around the rotation axis of the other, and therefore the angle between the seat back and the seat cushion changes. 
     The invention defined in aspect 2 presents the reclining device according to aspect 1 characterized in that the plane of each of the internal teeth that engages with the external teeth is flat, and the external teeth are involute teeth. 
     According to the invention defined in aspect 1 and aspect 2, one of (a) the plane of each of the external teeth that engages with the internal teeth and (b) the plane of each of the internal teeth that engages with the external teeth is flat, whereas the other of (a) and (b) is convex. Thus, the distance between the plane of each of the external teeth that engages with the internal teeth and the plane of each of the internal teeth that engages with the external teeth becomes wider more rapidly than in the case where both the internal teeth and the external teeth are involute teeth (having convex planes and concave planes), like in an example according to the conventional technique. 
     As a result, if the reclining device according to the present invention and the reclining device according to the conventional technique each have a bump due to the surface roughness that has an equal height and an equal distance from the engagement position, the reclining device according to the present invention has a smaller deviation in the revolution orbit of the external teeth caused by the bump, and also has a smaller operation force, than in the reclining device according to the conventional technique. 
     In addition, when the external teeth and the internal teeth engage with one another, the range within which the distance between the plane of each of the external teeth that engages with the internal teeth and the plane of each of the internal teeth that engages with the external teeth is equal to or smaller than the surface roughness is smaller than the one according to the conventional technique. Thus, the point at which an internal tooth and an external tooth engage with each other varies less in the radial direction of the internal teeth gear and the external teeth gear, and therefore the operation force also varies less. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a drawing for explaining an inventive portion of an exemplary embodiment of the present invention; 
         FIG. 2  is a vertical cross-sectional view of the exemplary embodiment of the present invention; 
         FIG. 3  is a left side view of the exemplary embodiment shown in  FIG. 2  (The upper arm and the lower arm are not shown); 
         FIG. 4  is a right side view of the exemplary embodiment shown in  FIG. 2  (The upper arm and the lower arm are not shown); 
         FIG. 5  is an exploded perspective view of a principal part of the exemplary embodiment shown in  FIG. 2 ; 
         FIG. 6  is a drawing for explaining the connection between a gear mechanism portion and the upper arm and the lower arm; 
         FIG. 7A  and  FIG. 7B  are drawings of an external teeth gear;  FIG. 7B  is a cross-sectional view at the line A-A in  FIG. 7A ; 
         FIG. 8A  and  FIG. 8B  are drawings of an internal teeth gear;  FIG. 8B  is a cross-sectional view at the line B-B in  FIG. 8A ; 
         FIG. 9A  and  FIG. 9B  are drawings of a rotation shaft;  FIG. 9B  is a cross-sectional view at the line C-C in  FIG. 9A ; 
         FIG. 10A ,  FIG. 10B , and  FIG. 10C  are enlarged views of a wedge-shaped member;  FIG. 10A  is a plan view;  FIG. 10B  is a front view; and  FIG. 10C  is a perspective view; 
         FIG. 11A  and  FIG. 11B  are drawings of a spring;  FIG. 11A  is a plan view; and  FIG. 11B  is a front view; 
         FIG. 12  is a drawing for explaining an embodiment example of the present invention; and 
         FIG. 13  is a drawing for explaining an example according to a conventional technique. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following explains an exemplary embodiment of the present invention, with reference to the drawings. In  FIG. 2  to  FIG. 6 , a lower arm  20  is a member on the seat cushion side. As shown in  FIG. 6 , the lower arm  20  has a plurality of (i.e. six) fitting holes  21  around the tilting center position of the seat back (i.e. the portion that intersects the tilting center axis of the seat back) that are positioned so as to be on the circumference of a circle. 
     An upper arm  10  is a member on the seat back side. As shown in  FIG. 6 , the upper arm  10  has a plurality of (i.e. six) fitting holes  11  around the tilting center position of the seat back (i.e. the portion that intersects the tilting center axis of the seat back) that are positioned so as to be on the circumference of a circle. 
     As shown in  FIG. 2  in particular, a gear mechanism  30  is attached between the upper arm  10  and the lower arm  20  and is configured so as to adjust the tilting angle of the upper arm  10 . An external teeth gear  31  and an internal teeth gear  32  that are included in the gear mechanism  30  respectively have a plurality of cylindrical-column-shaped projections  31   a  and  32   a  that are configured so as to fit into the fitting holes  11  in the upper arm  10  and the fitting holes  21  in the lower arm  20 . 
     As shown in  FIG. 7A , the external teeth gear  31  is substantially in the shape of a circular disc. The external teeth gear  31  has external teeth  31   b  on the outer circumferential surface thereof and has, at the center, a cylinder portion  31   f  that extends in the direction toward the internal teeth gear  32 . The positioning of the external teeth gear  31  is defined by fitting the plurality of (i.e. six) projections  31   a  provided on the lateral face thereof into the fitting holes  11  in the upper arm  10 . After the positioning is determined in this manner, the external teeth gear  31  is welded to the upper arm  10 . 
     As shown in  FIG. 8A , the internal teeth gear  32  is also substantially in the shape of a circular disc. The internal teeth gear  32  has internal teeth  32   b  of which the number of teeth is larger, at least by one, than the number of teeth of the external teeth  31   b  in the external teeth gear  31  and that are configured so as to internally touch the external teeth  31   b . Also, the internal teeth gear  32  has, at the center, a circular-shaped through hole  32   f . The cylinder portion  31   f  of the external teeth gear  31  is inserted in the through hole  32   f . The positioning of the internal teeth gear  32  is defined by fitting the plurality of (i.e. six) cylindrical-column-shaped projections  32   a  provided on the lateral face thereof into the fitting holes  21  in the lower arm  20 . After the positioning is determined in this manner, the internal teeth gear  32  is welded to the lower arm  20 . 
     Of the opposing faces of the external teeth gear  31  and the internal teeth gear  32 , the opposing face of the external teeth gear  31  has a concave portion  31   c  so that a substantially arc-shaped space is formed, whereas the opposing face of the internal teeth gear  32  has a convex portion  32   c  that projects into the concave portion  31   c . Because the internal teeth gear  32  included in the gear mechanism  30  is configured so as to adjust the tilting angle of the seat back, the internal teeth gear  32  naturally rotates relative to the external teeth gear  31 , but it is also necessary to regulate the rotation range of the external teeth gear  31  to a predetermined range. The concave portion  31   c  and the convex portion  32   c  function as the stoppers to regulate the rotation range. The rotation of the external teeth gear  31  is regulated to the predetermined range by having the convex portion  32   c  abut against a lateral wall  31   d  of the concave portion  31   c.    
     A neck portion  33   a  of a rotation shaft  33  with a flange, as shown in  FIG. 9A  and  FIG. 9B , is rotatably fitted into the cylinder portion  31   f  of the external teeth gear  31 . The rotation shaft  33  has a flange portion (a striker)  33   b , the neck portion  33   a , an annular-shaped groove  33   d  formed in the neck portion  33   a , a first serration portion  33   i  to which a gear  100  shown in  FIG. 2  is to be attached, and a second serration portion  33   j  to which a connection rod (not shown in the drawing) that transfers rotations to the rotation shaft in the reclining device on the other side is to be attached. The flange portion  33   b  has an arc-shaped cutout portion  33   c . The rotation shaft  33  is driven and rotated when the tilting angle of the seat back is adjusted. 
     A bush  34  that is in the shape of a cylinder and has an anti-abrasion characteristic is fitted into the through hole  32   f  of the internal teeth gear  32  and is fixed. Further, a pair of wedge-shaped members, namely a wedge-shaped member (a first wedge-shaped member)  35  and a wedge-shaped member (a second wedge-shaped member)  36 , are inserted between the inner circumferential surface of the bush  34  and the outer circumferential surface of the cylinder portion  31   f  of the external teeth gear  31 , in such a manner that the first and the second wedge-shaped members  35  and  36  are in contact with the bush  34  and the cylinder portion  31   f.    
     The wedge-shaped members  35  and  36  are shaped so as to be plane symmetrical. The shapes of the wedge-shaped members  35  and  36  are shown in  FIG. 5  and  FIGS. 10A ,  10 B, and  10 C. The inner circumferential surface  35   a  and the inner circumferential surface  36   a  of the wedge-shaped member  35  and the wedge-shaped member  36  each have an inside diameter that is substantially the same as the outer circumferential surface of the cylinder portion  31   f  of the external teeth gear  31 . The outer circumferential surface  35   b  and the outer circumferential surface  36   b  of the wedge-shaped member  35  and the wedge-shaped member  36  each have an outside diameter that is substantially the same as the inside diameter of the bush  34 . In each of the wedge-shaped member  35  and the wedge-shaped member  36 , because the central axes of the inner circumferential surface  35   a  and the inner circumferential surface  36   a  do not coincide with the central axes of the outer circumferential surface  35   b  and the outer circumferential surface  36   b , the thickness varies in the manner of a wedge. 
     Further, the wedge-shaped member  35  and the wedge-shaped member  36  respectively have a fin portion  35   e  and a fin portion  36   e  that are configured so as to abut against an end face of the cylinder portion  31   f  of the external teeth gear  31 . 
     According to the present exemplary embodiment, the outer circumferential surface of the cylinder portion  31   f  of the external teeth gear  31  that opposes the inner circumferential surface  35   a  and the inner circumferential surface  36   a  of the wedge-shaped member  35  and the wedge-shaped member  36  is arranged to be at substantially the same position, in the axial direction of the rotation shaft  33 , as the outer circumferential surface of the bush  34  (i.e. the inner circumferential surface of the through hole  32   f  in the internal teeth gear  32 ) that opposes the outer circumferential surface  35   b  and the outer circumferential surface  36   b  of the wedge-shaped member  35  and the wedge-shaped member  36 . 
     As shown in  FIG. 3 , into the space between a lateral end face  35   c  of the wedge-shaped member  35  on the thinner side and a lateral end face  36   c  of the wedge-shaped member  36  on the thinner side, the flange portion  33   b  that serves as a striker and is integrally formed with the rotation shaft  33  protrudes. With this arrangement, when the rotation shaft  33  rotates counter-clockwise in  FIG. 3 , a lateral wall  33   g  of the cutout portion  33   c  in the rotation shaft  33  abuts against the lateral end face  35   c  of the wedge-shaped member  35 . On the contrary, when the rotation shaft  33  rotates clockwise in  FIG. 3 , a lateral wall  33   h  of the cutout portion  33   c  in the rotation shaft  33  abuts against the lateral end face  36   c  of the wedge-shaped member  36  on the opposite side. 
     The wedge-shaped member  35  and the wedge-shaped member  36  are disposed between the inner circumferential surface of the bush  34  and the outer circumferential surface of the cylinder portion  31   f  of the external teeth gear  31  in such a manner that the thicker sides of the wedge-shaped members  35  and  36  oppose each other. Thus, the internal teeth gear  32  is arranged so as to be eccentric with respect to the external teeth gear  31 , so that the internal teeth  32   b  engage with the external teeth  31   b . Because of the wedge-shaped member  35  and the wedge-shaped member  36 , as described later, the rotation shaft  33  is able to arrange so that the internal teeth  32   b  and the external teeth  31   b  engage with one another and also one of the external teeth gear  31  and the internal teeth gear  32  revolves using the gear axis of the other as the revolution center. The wedge-shaped members  35  and  36  receive energizing forces in directions that separate them from each other, from a spring  37  that serves as an energizing unit as shown in  FIG. 11A  and  FIG. 11B . 
     The spring  37  has an annular portion  37   a  of which the middle portion corresponds to one turn and end portions  37   b  and  37   c  that rise from the annular portion  37   a . The annular portion  37   a  is configured so as to be wound around the rotation shaft  33  in the part except for the space provided between the internal teeth gear  32  and the external teeth gear  31 , and especially in the present exemplary embodiment, in the part between the internal teeth gear  32  and the lower arm  20 . In addition, the end portion  37   b  is held in a groove portion  35   d  provided in the lateral end face of the wedge-shaped member  35  on the thicker side. The end portion  37   c  is held in a groove portion  36   d  provided in the lateral end face of the wedge-shaped member  36  on the thicker side. Further, the lower arm  20  is shaped so as to cover the internal teeth gear  32 . 
     The holding member  38  that is in the shape of a cylinder is fitted to the outer circumferential surface of the internal teeth gear  32 . Also, the end portions of the holding member  38  on either side protrude toward the center axis so as to sandwich the external teeth gear  31  and the internal teeth gear  32 . With this arrangement, it is possible to regulate the external teeth gear  31  and the internal teeth gear  32  so that they do not separate from each other in the axial direction. In addition, a ring  39  for the purpose of slip-off prevention is held in the annular-shaped groove  33   d  provided on the rotation shaft  33 . 
     According to the present exemplary embodiment, as shown in  FIG. 1 , the plane, within the range A′, of each of the internal teeth  32   b  of the internal teeth gear  32  that engages with the external teeth  31   b  of the external teeth gear  31  is arranged to be flat, whereas the plane, within the range B′, of each of the external teeth  31   b  of the external teeth gear  31  that engages with the internal teeth  32   b  of the internal teeth gear  32  is arranged to be convex and to protrude in the direction of the flat plane of the internal tooth  32   b , so that the external teeth are involute teeth. 
     Next, the operation according to the exemplary embodiment described above will be explained. When no rotation operation force is applied from an external source to the rotation shaft  33 , the spring  37  energizes the wedge-shaped members  35  and  36  in directions that separate them from each other and gives each of the wedge-shaped members  35  and  36  a force in the direction to put in the wedge. With this arrangement, the relative movement between the internal teeth gear  32  and the rotation shaft  33  is prohibited. Also, the gear mechanism  30  is in a locked state in which the external teeth  31   b  in the external teeth gear  31  engage with the internal teeth  32   b  in the internal teeth gear  32 . Accordingly, the seat back is locked in the present position. 
     When the gear mechanism  30  is in the locked state, if the rotation shaft  33  is rotated, for example, clockwise in  FIG. 3 , a force in the direction to pull the wedge-shaped member  36  out of the gap is applied from the lateral wall  33   h  of the rotation shaft  33  to the lateral end face  36   c  of the wedge-shaped member  36 . Accordingly, the rotation shaft  33  and the wedge-shaped member  36  rotate clockwise with respect to the internal teeth gear  32 . As a result, there is a gap between the wedge-shaped member  36  and the neighboring members, and the internal teeth gear  32  becomes movable. Consequently, the wedge-shaped member  35 , which is energized by the spring  37 , rotates clockwise so as to fill in the gap. 
     As a result of these movements in conjunction with one another, the wedge-shaped members  36  and  35  rotate clockwise, together with the rotation shaft  33 . The same applies to the counter-clockwise rotation. Accordingly, the internal teeth gear  32  is supported by the rotation shaft  33  at an eccentric position where the internal teeth  32   b  engage with the external teeth  31   b . Thus, the external teeth gear  31 , the internal teeth gear  32 , and the rotation shaft  33  structure the gear mechanism. 
     As described above, as the rotation shaft (i.e. the flange portion  33   b )  33  is rotated, the position at which the external teeth  31   b  of the external teeth gear  31  engage with the internal teeth  32   b  of the internal teeth gear  32  changes. Accordingly, it is possible to adjust the tilting angle of the seat back by tilting the upper arm  10  with respect to the lower arm  20 . 
     According to the present exemplary embodiment, the plane of each of the internal teeth  32   b  of the internal teeth gear  32  that engages with the external teeth  31   b  of the external teeth gear  31  is arranged to be flat, whereas the plane of each of the external teeth  31   b  of the external teeth gear  31  that engages with the internal teeth  32   b  of the internal teeth gear  32  is arranged to be convex and to protrude in the direction of the flat plane of the internal tooth  32   b , so that the external teeth are involute teeth. Accordingly, compared to the example according to the conventional technique in which both the internal teeth and the external teeth are involute teeth (i.e. having convex planes and concave planes that are recessed away from the convex planes), the distance between the plane of each of the external teeth  31   b  that engages with the internal teeth  32   b  and the plane of each of the internal teeth  32   b  that engages with the external teeth  31   b  becomes wider more rapidly. Consequently, if the reclining device according to the present exemplary embodiment and the reclining device according to the conventional technique each have a bump due to the surface roughness that has an equal height and an equal distance from the engagement position, the reclining device according to the present exemplary embodiment has a smaller deviation in the revolution orbit of the external teeth  31   b  caused by the bump, and also has a smaller operation force, than in the reclining device according to the conventional technique. 
     Further, the plane of each of the internal teeth  32   b  of the internal teeth gear  32  that engages with the external teeth  31   b  of the external teeth gear  31  is arranged to be flat, whereas the plane of each of the external teeth  31   b  of the external teeth gear  31  that engages with the internal teeth  32   b  of the internal teeth gear  32  is arranged to be convex and to protrude in the direction of the flat plane of the internal tooth  32   b  so that the external teeth are involute teeth. Accordingly, the range (the range C′ shown in  FIG. 1 ) within which the distance between the plane of each of the external teeth  31   b  that engages with the internal teeth  32   b  and the plane of each of the internal teeth  32   b  that engages with the external teeth  31   b  is equal to or smaller than 0.01 mm, which corresponds to the surface roughness, is smaller than the case where both the internal teeth and the external teeth are involute teeth (cf.  FIG. 13 ). As a result, the point at which an internal tooth  32   b  and an external tooth  31   b  engage with each other varies less in the radial direction of the internal teeth gear  32  and the external teeth gear  31 , and therefore the operation force also varies less. 
     It should be noted that the present invention is not limited to the exemplary embodiment described above. For example, according to the exemplary embodiment described above, the plane of each of the external teeth that engages with the internal teeth is arranged to be convex, whereas the plane of each of the internal teeth that engages with the external teeth is arranged to be flat. However, alternatively, it is acceptable to have an arrangement in which the plane of each of the external teeth that engages with the internal teeth is flat, whereas the plane of each of the internal teeth that engages with the external teeth is convex. 
     Further, the shape of each of the teeth that forms a convex plane is not limited to that of an involute tooth. For example, the teeth may be cycloid teeth or the like. 
     In addition, according to the exemplary embodiment described above, the external teeth gear  31  is attached to the upper arm  10 , and the internal teeth gear  32  is attached to the lower arm  20 . However, the arrangement may be reversed. 
     The inventor of the present invention confirmed, by performing an experiment, that the reclining device that were configured as shown in  FIG. 2  to  FIG. 11  and in which each of the internal teeth  32   b  had a flat plane like in the exemplary embodiment had a smaller operation force than in a reclining device according to the conventional technique where the internal teeth were involute teeth. 
     More specifically, the electric current value in a motor was measured while the rotation shaft  33  was driven by the motor. In  FIG. 12 , the vertical axis shows the electric current value in the motor, whereas the horizontal axis shows the reclining angle of the seat back. 
     In the experiment, the seat back was tilted and moved from the maximum forward position to the maximum backward position, and then was tilted again from the maximum backward position to the maximum forward position, while the electric current value was measured. 
     With the reclining device in which each of the internal teeth  32   b  had a flat plane, the electric current value was measured for a case where a load of 30 kg was applied to the seat back side and for a case where no load was applied. 
     Also, with the reclining device in which each of the internal teeth  32   b  was an involute tooth, the electric current value was measured for a case where a load of 30 kg was applied to the seat back side and for a case where no load was applied. 
     As shown in the drawing, for both of the cases where the load of 30 kg was applied to the seat back side and where no load was applied, it was confirmed that the reclining device in which each of the internal teeth  32   b  had a flat plane exhibited a smaller electric current, and therefore a smaller operation force, than the reclining device in which each of the internal teeth  32   b  was an involute tooth.