Abstract:
According to a first aspect of the invention, there is provided a bearing structure including: a resin bush having a flange formed to be brought into contact with a plate face around a bearing hole from one side, a locking edge formed to be locked with the plate face around the bearing hole from the other side, and a circular through-hole formed to bear the support shaft inserted therethrough, wherein, upon receiving a load applied to the support shaft, the entire resin bush is flexibly displaced so that an inner circumferential surface of the through-hole is aligned with a hole edge of the bearing hole and the support shaft is born by the hole edge of the bearing hole.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application claims priority from Japanese Patent Application No. 2009-297669 filed on Dec. 28, 2009, the entire contents of which are incorporated herein by reference. 
     BACKGROUND 
     1. Field of the Invention 
     The present invention related to a bearing structure in which a resin bush is inserted into a bearing hole formed in one constituent member, and a support shaft protruding from the other constituent member is inserted into the bearing hole through the resin bush so as to be rotatbly supported thereon. 
     2. Description of the Related Art 
     JP-2005-287724-A discloses an armrest provided on a side of a vehicle seat. In JP-2005-287724-A, a support shaft extends from a backside frame of the vehicle seat, bearing holes are formed in facing frames of the armrest at a base portion, and the support shaft is inserted into the bearing holes to support the armrest in the width direction. The armrest is stored at a side of a seat back in an erected state and forwardly reclined from the erected state with the support shaft as a pivot. 
     In the armrest, an arc-like guide hole is formed in a plate face of the armrest frame with the support shaft as a center, a stopper pin for regulating a rotation range of the armrest is protruded from a plate face of the backside frame at a position apart from the support shaft and inserted into the guide hole, and a hook-like lock cam is provided to the backside frame to be engageable with the stopper pin to thereby store the armrest at the side of the seat back in the erected state. 
     In the armrest, metal bushes are mounted onto the stopper pin to smoothen engagement/disengagement of the lock cam with/from the stopper pin. 
     In the metal bushes, a cylindrical portion (main body) is fitted with the stopper pin, a flange is formed at one end of the cylindrical portion, an inner cylindrical surface of the cylindrical portion and a flange surface of the flange faced to the armrest frame are coated with a resin having a low frictional coefficient (see JP-2005-287724-A). 
     Here, when further bushes for the support shaft are provided in addition to the bushes for the stopper pin, a rotational movement of the armrest can be further smoothened. 
     However, when the metal bushes are mounted into bearing holes of the armrest frames for inserting the support shaft therethrough, since an excessive load is applied to the support shaft during the use of the armrest, the resin coating of the bushes may be easily abraded. Further, since the bushes are formed of metal, it is necessary to process the cylinder end of the cylindrical portion or the shaft end of the support shaft so as to prevent the bushes from dropping from the bearing holes, and workability for the assembly is deteriorated. 
     By using a resin bush instead of the metal bush, the above-mentioned drop-out prevention process may be omitted with a simple structure. However, when the resin bushes are mounted into the bearing holes for inserting the support shaft to support the armrest, since an excessive load is applied to the support shaft during the use of the armrest as described above, the bushes may be crushed and destroyed. 
     SUMMARY 
     One object of the invention is to provide, for example, in an armrest of a vehicle seat, a bearing structure using a resin bush for a support shaft to support the armrest while preventing the resin bush from being crushed and destroyed even when an excessive load is applied to the support shaft during the use of the armrest. 
     According to a first aspect of the invention, there is provided a bearing structure inserted into and fixed to a bearing hole formed in one constituent element for bearing a support shaft protruded from the other constituent element, the bearing structure including: a resin bush having a flange, at one end, formed to be brought into contact with a plate face of the one constituent member around the bearing hole from one side, a locking edge, at the other end, formed to be locked with the plate face of the one constituent member around the bearing hole from the other side, and a circular through-hole, at a enter, formed to bear the support shaft inserted therethrough, wherein, upon receiving a load applied to the support shaft, the entire resin bush is flexibly displaced so that an inner circumferential surface of the through-hole is aligned with a hole edge of the bearing hole and the support shaft is born by the hole edge of the bearing hole. 
     According to a second aspect of the invention, there is provided the bearing structure, wherein the resin bush further has a cylindrical body defining circular through-hole while being inserted into the bearing hole, the flange being formed at one end of the cylindrical body, the locking edge being formed at the other end of the cylindrical body, a partition cut formed to open through the locking edge, the cylindrical body and the flange, and thin-walled portions formed to extend from both sides of the partition cut along an outer circumferential surface of the cylindrical body with a reduced thickness, wherein, in the one constituent element, a protrusion is formed at the hole edge of the bearing hole to be positioned within the partition cut and not to reach the inner circumferential surface of the through-hole, and wherein, upon receiving the load applied to the support shaft, the cylindrical body is flexibly displaced at the thin-walled portions so that the inner circumferential surface of the through-hole is aligned with a protrusion end surface of the protrusion and the support shaft is born by the protrusion end surface of the protrusion. 
     According to a third aspect of the invention, there is provided the bearing structure, wherein the resin bush further has a cutout formed to open the cylindrical body except for the flange or the locking edge at a position apart from the partition cut. 
     According to a fourth aspect of the invention, there is provided the bearing structure, wherein the resin bush further has a bearing piece formed on the flange to rise from the hole edge of the through-hole so as to face the support shaft inserted through the through-hole, the locking edge being continuously extended from a protrusion end of the bearing piece, and a loophole formed in the flange around a rising base of the bearing piece, wherein, in the one constituent element, the bearing hole is formed so that the hole edge thereof does not reach a hole edge of the through-hole and a groove is formed around the hole edge of the bearing hole to receive the bearing piece, and wherein, upon receiving the load applied to the support shaft, the bearing piece is flexibly displaced into the groove through the loophole so that the inner circumferential surface of the through-hole is aligned with the hole edge of the bearing hole and the support shaft is born by the hole edge of the bearing hole. 
     According to a fifth aspect of the invention, there is provided the bearing structure, wherein the resin bush further has guide blades formed on both sides of the bearing piece to slidingly guide the flexible displacement of the bearing piece along both edges of the groove. 
     According to a sixth aspect of the invention, there is provided an armrest of a vehicle seat, using the above-mentioned bearing structure, wherein the one constituent element is one of an armrest frame of the armrest and a backside frame of the vehicle seat, and the other constituent element is the other of the armrest frame and the backside frame. 
     In the above-mentioned bearing structure and the above-mentioned armrest, a resin bush is used. In the resin bush, a flange coming in contact with the plate face of one constituent member around one hole edge of the bearing hole formed in the one constituent member (one of the armrest frame and the backside frame) is formed at one end thereof, a locking edge locked to the plate face of the one constituent member around the other hole edge of the bearing hole is formed at the other end thereof, and a circular through-hole for inserting the support shaft protruding from the other constituent member (the other of the armrest frame and the backside frame) therethrough is formed at the center thereof. As a result, the drop-out of the resin bush from the bearing hole is prevented by the flange and the locking edge. 
     According to the first aspect of the invention, the bush is inserted into and fixed to the bearing hole formed in one constituent member and the support shaft protruding from the other constituent member is inserted through the through-hole of the bush so as to rotatably bear the support shaft. Accordingly, by assembling the resin bush in which the entire bush is flexibly displaced with the load applied to the support shaft, the inner circumferential surface of the through-hole is aligned with the hole edge of the bearing hole, and the support shaft is born by the hole edge of the bearing hole, the bush can be assembled without being crushed and destroyed even when the resin bush bears the support shaft. 
     According to the second aspect of the invention, the resin bush includes a cylindrical body inserted into the bearing hole and defining the circular through-hole formed at the center between the flange and the locking edge. The bush is inserted into and fixed to the bearing hole and the support shaft is inserted through the through-hole of the bush so as to rotatably bear the support shaft. A partition cut opened through the locking edge, the cylindrical body and the flange and thin-walled portions with a reduced thickness of the cylindrical body extending from both sides of the partition cut to the outer circumferential surface of the cylindrical body are formed in the resin bush, a protrusion is formed at the hole edge of the bearing hole to be positioned within the partition cut and not to reach the inner circumferential surface of the through-hole, the cylindrical body is flexibly displaced at the thin-walled portions with the load applied to the support shaft, the inner circumferential surface of the through-hole is aligned with the protrusion end surface of the protrusion, and the support shaft is born by the protrusion end surface of the protrusion. Accordingly, the bush can be assembled without being crushed and destroyed even when the resin bush bears the support shaft. By providing the partition cut and the thin-walled portions, the entire bush can be made to be flexible and thus can be assembled into the bearing hole. Since the partition cut is formed correspondingly with the protrusion, the positioning is carried out well when inserting the bush into the bearing hole, thereby easily determining the assembly direction. 
     According to the third aspect of the invention, in the resin bush, a cutout is opened in the cylindrical body and one of the flange and the locking edge, at a position apart form the partition cut. Accordingly, the entire bush is allowed to be deformed at the cutout in accordance with the flexible displacement of the cylindrical body at the thin-walled portions, thereby further promoting the flexible displacement of the cylindrical body so that the bush is surely assembled without being crushed and destroyed. 
     According to the fourth aspect of the invention, a bearing piece faced to the support shaft inserted through the through-hole is formed on the flange of the bush so as to rise from the hole edge of the through-hole, a loophole is formed in the flange around the rising base of the bearing piece, and the through-hole of the resin bush having the hole edge not reaching the hole edge of the bearing hole and the groove recessed from the hole edge of the bearing hole to receive the bearing piece are formed. The bearing piece is flexibly displaced to the deep side of the groove through the loophole, the inner circumferential surface of the through-hole is aligned with the hole edge of the bearing hole, and the support shaft is born by the hole edge of the bearing hole. Accordingly, the bush can be assembled without being crushed and destroyed even when the resin bush bears the support shaft. By providing the flexible bearing piece, the entire bush can be made to be flexible and can be easily assembled into the bearing hole. Since the bearing piece is provided correspondingly with the groove, the positioning is carried out well when inserting the bush into the bearing hole, thereby easily determining the assembly direction. 
     According to the fifth aspect of the invention, guide blades are formed on both sides of the bearing piece for sliding along both edges of the groove when the bearing piece flexibly displaces. Accordingly, the bush can be assembled so that the bearing piece can be stably flexibly displaced to the loophole by the guide blades. 
     According to the sixth aspect of the invention, the armrest of the vehicle seat includes the bearing structure using the above-mentioned resin bush. Accordingly, when the resin bush is inserted into and fixed to the bearing hole formed in the backside frame and the support shaft of the armrest is inserted through the resin bush to be born, the bush can be prevented from being crushed and destroyed even when an excessive load is applied to the support shaft during the use of the armrest. 
    
    
     
       DRAWINGS 
         FIG. 1  illustrates the frame structure of an armrest of a vehicle seat assembled with a bearing structure using a resin bush according to a first embodiment. 
         FIG. 2  is an exploded perspective view of the bearing structure using a resin bush shown in  FIG. 1 . 
         FIG. 3A  is a front view of the resin bush shown in  FIG. 2 . 
         FIG. 3B  is a sectional view of the resin bush shown in  FIG. 3A . 
         FIG. 4  is a front view of a bearing hole into which the bush shown in  FIG. 3B  is inserted. 
         FIG. 5  is a sectional view of the bearing structure using the resin bush shown in  FIG. 1 . 
         FIG. 6  illustrates the bearing structure using the resin bush shown in  FIG. 5 . 
         FIG. 7A  partially illustrates the bearing structure using the resin bush shown in  FIG. 5 . 
         FIG. 7B  illustrates a state where a load is applied to the bearing structure using the resin bush shown in  FIG. 7A . 
         FIG. 8A  illustrates a first modified example of the bearing structure using the resin bush shown in  FIG. 7A . 
         FIG. 8B  illustrates a state where a load is applied to the bearing structure using the resin bush shown in  FIG. 8A . 
         FIG. 9A  illustrates a second modified example of the bearing structure using the resin bush shown in  FIG. 7A . 
         FIG. 9B  illustrates a state where a load is applied to the bearing structure using the resin bush shown in  FIG. 9A . 
         FIG. 10  illustrates the frame structure of an armrest of a vehicle seat assembled with a bearing structure using a resin bush according to a second embodiment. 
         FIG. 11  is an exploded perspective view of the bearing structure using a resin bush shown in  FIG. 10 . 
         FIG. 12A  is a front view of the resin bush shown in  FIG. 11 . 
         FIG. 12B  is a side view of the resin bush shown in  FIG. 12A . 
         FIG. 12C  is a sectional view of the resin bush shown in  FIG. 12A . 
         FIG. 13  is a front view of a bearing hole into which the bush shown in  FIG. 12C  is inserted. 
         FIG. 14  is a sectional view of the bearing structure using the resin bush shown in  FIG. 12C . 
         FIG. 15  illustrates the bearing structure using the resin bush shown in  FIG. 14 . 
         FIG. 16A  partially illustrates the bearing structure using the resin bush shown in  FIG. 12A . 
         FIG. 16B  illustrates a state where a load is applied to the bearing structure using the resin bush shown in  FIG. 16A . 
         FIG. 17  is a front view of a modified example of the resin bush shown in  FIG. 12A . 
         FIG. 18  is a front view of a bearing hole into which the bush shown in  FIG. 17  is inserted. 
         FIG. 19A  illustrates the bearing structure using the resin bush shown in  FIG. 17 . 
         FIG. 19B  illustrates a state where a load is applied to the bearing structure using the resin bush shown in  FIG. 19A . 
     
    
    
     DETAILED DESCRIPTION 
     An embodiment may be described with an armrest of a vehicle seat with a bearing structure using a resin bush as an example. In the embodiment, as shown in  FIG. 1 , a laterally-extending support shaft  1  is hanged and fixed between armrest frames  2   a  and  2   b  faced to each other, a bush bearing hole (no reference numeral is shown in  FIG. 1 ) to be described later is formed in bracket plates  4   a  and  4   b  of backside frames  3   a  and  3   b  located on one side of a seat back. Thus, the armrest can be stored at a side of the seat back in an erected state and forwardly reclined from the erected state. Further, arc-like guide holes  5   a  and  5   b  are formed in plate faces of the bracket plates  4   a  and  4   b  with the support shaft  1  as a center, and a stopper shaft  6  protruded from the plate faces of the armrest frames  2   a  and  2   b  at a position apart from the support shaft  1  are inserted into the guide holes  5   a  and  5   b , thereby regulating a rotation range of the armrest. 
     As shown in  FIGS. 1 and 2  (where only one side is shown), the bearing structure of the support shaft  1  using a resin bush is constructed by forming bush bearing holes  7  in the plate faces of the bracket plates  4   a  and  4   b  and inserting resin bushes  10   a  and  10   b  according to the first embodiment into the bearing holes  7  to be fixed thereto. The entire bushes  10   a  and  10   b  are formed of resins such as polyacetal, nylon, and polypropylene by molding. 
     In the first embodiment, each of the resin bushes  10   a  and  10   b  (hereinafter, the resin bush  10   a  will mainly be described) includes a cylindrical body  11  inserted into the bearing hole  7  (see  FIG. 2 ), a flange  12  disposed at one end of the cylindrical body  11 , a locking edge  13  disposed at the other end of the cylindrical body  11 , and a circular through-hole  14  extending from the flange  12  to the locking edge  13  through the cylindrical body  11 . The flange  12  has a disk-like shape and the locking edge  13  has a right-triangular sectional shape. 
     In the bush  10   a , a partition cut  15  opened from the locking edge  13  to the flange  12  through the cylindrical body  11 , and thin-walled portions  16   a  and  16   b  with a reduced thickness (see reference sign a&lt;b in  FIG. 3A ) of the cylindrical body  11  extending from both sides of the partition cut  15  to the outer circumferential surface of the cylindrical body  11  are formed. The partition cut  15  may have a V-shape in which the width decreases from the outer circumferential surface to the inner circumferential surface. The thin-walled portions  16   a  and  16   b  may be formed substantially over the hillside of the cylindrical body  11 . 
     As shown in  FIG. 4 , in the hole edge of the bearing hole  7 , a protrusion  7   a  is formed to be positioned in the partition cut  15  of the bush  10   b . The protrusion  7   a  is formed not to reach the inner circumferential surface of the through-hole  14  of the bush  10   b  (see  FIG. 3A ). The projecting height of the protrusion  7   a  may substantially correspond to the thickness of the thin-walled portions  16   a  and  16   b  of the bush  10   a , and the protrusion end surface may have an arc shape substantially corresponding to the circumferential surface of the support shaft  1  (see  FIG. 1 ). 
     The protrusion  7   a  receives a load applied to the support shaft  1  as described later. The load applied to the support shaft  1  downwardly acts on the bush  10   a  due to the structure (see  FIG. 1 ) in which the stopper shaft  6  regulating a rotation range of the armrest is protruded from the plate face of the armrest frame  2   a  at apposition separated from the support shaft  1  and is inserted into the guide hole  5   a . Accordingly, the protrusion  7   a  is disposed in the lower hole edge of the bearing hole  7 . 
     As shown in  FIGS. 5 and 6 , the bush  10   a  is assembled into the bracket plate  4   a  without drop-out by engaging the cylindrical body  11  with the bearing hole  7 , bringing the flange  12  into contact with the plate face of the bracket plate  4   a  around one hole edge of the bearing hole  7 , locking the locking edge  13  to the plate face of the bracket plate  4   a  around the other hole edge of the bearing hole  7 , and inserting the support shaft  1  through the through-hole  14 . In the assembling, the bush  10   a  is squeezed using the partition cut  15 , and inserted into the bearing hole  7  from the locking edge  13 . Since the partition cut  15  is faced to the protrusion  7   a , the positioning for inserting the bush  10   a  into the bearing hole  7  can be carried out, thereby determining the assembly direction. 
     When the bush  10   a  is assembled, as shown in  FIG. 7A , the protrusion  7   a  is positioned within the partition cut  15  and does not reach the inner circumferential surface of the through-hole  14 . 
     As shown in  FIG. 7B , when a load (the downward straight arrow) applied to the support shaft  1  acts on the bush  10   a , the inner circumferential surface of the through-hole  14  is aligned with the protrusion end surface of the protrusion  7   a  and the bush  10   a  is flexibly displaced in the partition cut  15  so that the support shaft  1  is born by the protrusion end surface of the protrusion  7   a . Accordingly, even when the resin bush  10   a  bears the support shaft, the bush  10   a  can be assembled without being crushed and destroyed. 
     When the load is released from the support shaft  1  of the armrest, the cylindrical body  11  is spring-like restored at the thin-walled portions  16   a  and  16   b  and thus the support shaft  1  is born by the through-hole  14  of the cylindrical body  11 , as shown in  FIG. 7A . 
     In the first embodiment, as shown in  FIG. 8A , a cutout  17  may be further formed on the opposite side of the partition cut  15  to open the flange  12  and the cylindrical body  11  except for the locking edge  13 . While  FIG. 8   a  shows a case where the cutout  17  is formed from the flange  12  to the cylindrical body  11 , the cutout  17  may be formed from the locking edge  13  to the cylindrical body  11 . 
     By forming the cutout  17 , as shown in  FIG. 8B , the entire bush  10   a  is allowed to be displaced at the cutout  17  (in the upward arc arrow direction) in accordance with the flexible displacement of the cylindrical body  11  at the thin-walled portions  16   a  and  16   b , thereby further promoting the flexible displacement of the cylindrical body  11  so that the bush  10   a  is surely assembled without being crushed and destroyed. 
     As shown in  FIG. 9A , two cutouts  17   a  and  17   b  may be formed at an angle interval of about 120° with respect to the partition cut  15  interposed therebetween. In this case, as shown in  FIG. 9B , the entire bush  10   a  is allowed to be displaced (in the upward arc arrow direction) at the cutouts  17   a  and  17   b  in accordance with the flexible displacement of the cylindrical body  11  at the thin-walled portions  16   a  and  16   b.    
     Resin bushes  20   a  and  20   b  according to a second embodiment may be used instead of the resin bushes  10   a  and  10   b  according to the first embodiment. As shown in  FIGS. 10 and 11 , the armrest can be attached to the seat back by using the resin bushes  20   a  and  20   b  according to the second embodiment. The same elements as shown in  FIGS. 1 and 2  are referenced by like reference numerals and signs and the elements will not be described in detail. 
     In the second embodiment, as shown in  FIGS. 12A to 12C , each of the resin bushes  20   a  and  20   b  (hereinafter, the resin bush  20   a  will mainly be described) each includes a flange  22  having a circular through-hole  21  formed at a center thereof for inserting a support shaft  1  (see  FIG. 10 ) therethrough and three bearing pieces  23   a  to  23   c  formed at regular intervals around the through-hole  21  on the flange  22  as a base. The entire bushes  20   a  and  20   b  are formed of a resin such as polyacetal or nylon by molding. 
     The bearing pieces  23   a  to  23   c  are provided to face the support shaft  1  inserted through the through-hole  21  and arranged at predetermined intervals around the through-hole  21  so as to rise to be flush with the hole edge of the through-hole  21 . From the protrusion ends of the bearing pieces  23   a  to  23   c , locking edges  24   a  to  24   c  are continuously extended obliquely outward toward the flange  22  to each have an inversed fingertip shape with a stepped end. 
     In the flange  22 , loopholes  25   a  to  25   c  are formed around rising bases of the bearing pieces  23   a  to  23   c . Both sides of the bearing pieces  23   a  to  23   c  are provided with guide blades  26   a  and  26   b  to  28   a  and  28   b  which can slide along both edges of grooves formed in a bearing hole  70  to be described later. 
     The bearing hole  70  is formed in the plate faces of the bracket plates  4   a  and  4   b . The hole edge of the bearing hole  70  has a circular shape as shown in  FIG. 13 , and grooves  71   a  to  71   c  are formed around the bearing hole  70  so as to be recessed from the hole edge correspondingly with the interval of the bearing pieces  23   a  to  23   c . The grooves  71   a  to  71   c  allow the bearing pieces  23   a  to  23   c  to be flexibly displaced in the depth direction. 
     As shown in  FIG. 14 , the bearing hole  70  has such a diameter that the hole edge of the through-hole  21 , through which the support shaft  1  is inserted, of the bush  20   a  does not protrude into the hole. Specifically, it is set to satisfy the diameter φ 1  of the support shaft  1 &lt;the diameter φ 2  of the through-hole  21 &lt;the diameter φ 3  of the bearing hole  70 . 
     The grooves  71   a  to  71   c  of the bearing hole  70  receive the guide blades  26   a  and  26   b  to  28   a  and  28   b  of the bearing pieces  23   a  to  23   c  and have such a depth that the stepped ends of the locking edges  24   a  to  24   c  can be engaged with the deep edges. 
     As shown in  FIGS. 14 and 15 , the bush  20   a  is assembled into the bearing hole  70  by bringing the flange  22  into contact with one surface of the bracket plate  4   a , causing the bearing pieces  23   a  to  23   c  to protrude from the grooves  71   a  to  71   c  of the bearing hole  70  to the other surface of the bracket plate  4   a , and engaging the stepped ends of the locking edges  24   a  to  24   c  with the deep edges of the grooves  71   a  to  71   c . The support shaft  1  is inserted through the through-hole of the flange  22  within the space between the bearing pieces  23   a  to  23   c  to be born with the bush  20   a.    
     In the bearing structure using the bushes  20   a  and  20   b  according to the second embodiment, in a normal state, as shown in  FIG. 16A , the support shaft  1  is born by the hole edge of the through-hole  21  and the bearing pieces  23   a  to  23   c  so that the armrest smoothly rotates about the support shaft  1 . 
     When the armrest is forwardly reclined from the side of the seat back, the stopper shaft  6  comes in contact with the upper hole edge of the guide hole  5   a , and an excessive load is applied to the support shaft  1  of the armrest. On this occasion, the bearing pieces  23   b  and  23   c  are downwardly pressed by the support shaft  1  from the hole edge of the through-hole  21  and flexibly displaced (in the arrow direction) into the loopholes  25   b  and  25   c  as shown in  FIG. 16B , whereby the support shaft  1  can be born by the hole edge of the bearing hole  70 . 
     Accordingly, even when the support shaft  1  is born by the bushes  20   a  and  20   b  according to the second embodiment, the bushes  20   a  and  20   b  can be assembled without being crushed and destroyed. 
     In addition, since the bearing pieces  23   a  to  23   c  protrude from the grooves  71   a  to  71   c  of the bearing hole  70  and the locking edges  24   a  to  24   c  are engaged with the deep edges of the grooves  71   a  to  71   c , the positioning of the centers of the bushes  20   a  and  20   b  with the hole center of the bearing hole  70  can be carried out, and the drop-out of the bushes  20   a  and  20   b  from the bearing hole  70  can be prevented. Since the bearing pieces  23   a  to  23   c  are formed to be flexible, the entire bush can be flexed and easily assembled into the bearing hole  70 . Since the bearing pieces  23   a  to  23   c  are faced to the grooves  71   a  to  71   c , the positioning for inserting the bushes  20   a  and  20   b  into the bearing holes  70  is carried out, thereby easily determining the assembly direction. 
     When the bearing pieces  23   b  and  23   c  are flexibly displaced into the loopholes  25   b  and  25   c  from the hole edge of the through-hole  21  through which the support shaft  1  is inserted, the guide blades  27   a ,  27   b ,  28   a , and  28   b  formed on both sides of the bearing pieces  23   b  and  23   c  slide along both edges of the grooves  71   b  and  71   c . That is, the guide blades  27   a ,  27   b ,  28   a , and  28   b  guide the flexible displacement of the bearing pieces  23   b  and  23   c  into the loopholes  25   b  and  25   c  from the hole edge of the through-hole  21  through which the support shaft  1  is inserted. 
     When the load is released from the support shaft  1  of the armrest, the locking edges  24   b  and  24   c  pressed by the deep edges of the grooves  71   b  and  71   c  and flexed toward the bearing pieces  23   b  and  23   c  are restored spring-like, and thus the support shaft  1  can be born by the hole edge of the through-hole  21  and the bearing pieces  23   a  to  23   c  as shown in  FIG. 16A . 
     As shown in  FIG. 17 , two bearing pieces  23   a  and  23   b  and two locking edges  24   a  and  24   b  may be provided on both sides, or four bearing pieces and four locking edges (not shown) may be provided to be located oblique in the vertical direction at intervals of about 90°. In this case, as shown in  FIG. 18 , the grooves  71   a  and  71   b  corresponding to the number of the bearing pieces and the locking edges can be formed around the bearing hole  70 . 
     In this case, in a normal state, as shown in  FIG. 19A , the support shaft  1  is born by the hole edge of the through-hole  21  and the bearing pieces  23   a  and  23   b  so that the armrest can smoothly rotates about the support shaft  1 . 
     When an excessive load is applied to the support shaft  1 , the bearing pieces  23   a  and  23   b  are pressed downwardly by the support shaft  1  from the hole edge of the through-hole  21  through which the support shaft  1  is inserted and are flexibly displaced (in the arrow direction) into the loopholes  25   a  and  25   b , as shown in  FIG. 19B , thereby bearing the support shaft  1  with the hole edge of the bearing hole  70 . Accordingly, even when the support shaft  1  is born by the bushes  20   a  and  20   b  according to this modified example, the bushes  20   a  and  20   b  can be assembled without being crushed and destroyed. 
     In the above-mentioned embodiments, the support shaft is fixed to the armrest frame and the bearing hole is formed in the plate faces of the bracket plates. However, the support shaft may be fixed to protrude laterally from the plate faces of the bracket plates and the bearing hole may be formed in the plate faces of the armrest frames. 
     While the examples where the bush is applied to the support shaft of the armrest is described, the bearing structure may be applied also to a case where a rear side of a seat cushion is axially attached to a base bracket fixed to a vehicle floor, a case where a link bar of a height adjusting device of a vehicle seat is axially attached, and the like. 
     The bearing structure can be applied not only to the vehicle but also to various mechanisms in which a resin bush is inserted into and fixed to a bearing hole and a support shaft is inserted through and born by the resin bush. And, the invention can be also applied to a case where a load applied to the support shaft acts upward.