Patent Publication Number: US-2021162889-A1

Title: Seat slide mechanism

Description:
TECHNICAL FIELD 
     The present invention relates to a seat slide mechanism to be provided to a vehicle. 
     BACKGROUND ART 
     A vehicle seat traditionally includes a seat cushion, a seat back, a headrest, and a seat slide mechanism. The seat cushion forms a seating surface for an occupant. The seat back forms a backrest surface for the occupant. The headrest holds the head of the occupant. The seat slide mechanism enables adjustment of a seat position relative to front-rear directions of the vehicle, in accordance with the physique of the occupant seated on the seat. 
     Specifically, as disclosed in Patent Document 1, the seat slide mechanism includes a lower rail, a slider, and a plurality of balls (steel balls). The lower rail is fixed to a floor pan of a vehicle body and extends in the front-rear directions of the vehicle. The slider is slidable in the lower rail, and allows a seat to be attached thereabove. The balls are disposed between the lower rail and the slider to allow sliding of the slider along the lower rail. 
     CITATION LIST 
     Patent Document 
     PATENT DOCUMENT 1: Japanese Unexamined Patent Publication No. 2016-196262 
     SUMMARY OF THE INVENTION 
     Technical Problem 
     Such a vehicle seat is to be sat by an occupant of the vehicle, and requires an ability to absorb and damp vibration input to the seat, for the sake of improved comfort. 
     Specifically, of the input vibration from outside the vehicle, the vibration from the road surface that is input, through the tires, to the floor pan serving as a vehicle body is transmitted to the seat through the above-described seat slide mechanism, and sensed by the occupant. To address this, there is a need for absorbing and damping the vibration input from outside the vehicle to improve the noise, vibration, and harshness (NVH) performance of the vehicle. 
     In view of the foregoing background, it is an object of the present invention to provide a seat slide mechanism capable of effectively reducing vibration to be sensed by an occupant seated on a seat of the vehicle, and thus improving the NVH performance of the vehicle. 
     Solution to the Problem 
     To achieve the above object, the present invention is directed to a seat slide mechanism to be provided in a vehicle. The seat slide mechanism includes: a lower rail fixed to a floor pan constituting a part of a vehicle body of the vehicle and extending in front-rear directions of the vehicle; a slider slidable in the lower rail and allowing a seat to be mounted thereabove; and a slide-allowing member arranged between the lower rail and the slider and enabling sliding of the slider along the lower rail. The slide-allowing member has a spring constant set to be smaller than a spring constant of the lower rail or a spring constant of the slider. 
     In the above structure, the spring constant of the slide-allowing member is set to be smaller than the spring constant of the lower rail or the spring constant of the slider. This achieves the following effects. 
     Namely, vibration can be effectively damped by the slide-allowing member by reducing the spring constant of the slide-allowing member of the seat slide mechanism that has a large mass and is arranged closely to a vibration input point (floor pan) in a vibration transmission pathway extending from the vibration input point to an occupant. This consequently enables effective reduction of vibration to be sensed by the occupant seated on the seat, and improves the NVH performance of the vehicle. 
     This is based on a finding that, to achieve a better NVH performance of the vehicle, it is most effective, in reducing vibration, to improve the seat slide mechanism having a large mass and arranged very closely to a vibration input point in a vibration transmission pathway extending from the floor pan, which serves as the vibration input point, to the occupant. 
     Further, the above-described structure achieves the vibration reduction effect with a simple modification essentially to the slide-allowing member, with respect to an existing seat slide mechanism. 
     An embodiment of the above seat slide mechanism is such that the slide-allowing member is structured by a tubular hollow member or a solid member having a pillar shape and is arranged between the lower rail and the slider in such a manner as to extend along the lower rail and the slider. 
     This structure enables easy control of the spring constant of the slide-allowing member by modifying the length and the like of the tubular hollow member or the solid member having a pillar shape. The hollow member may have a cylindrical shape or a shape of a square tube. The solid member may have a columnar shape or a shape of a square column. 
     Another embodiment of the above seat slide mechanism is such that the slide-allowing member is structured by a spheric member, and that the spheric member is a hollow spheric member whose inside is hollowed out, or a solid spheric member having an outer shell and an interior member whose spring constant is set to be smaller than a spring constant of the outer shell. 
     With this structure, the spring constant of the slide-allowing member made of a spheric member can be easily reduced. Further, since the slide-allowing member is structured by the spheric member, it is possible to achieve the above-described effect of reducing the vibration without significant modification to a typical existing seat slide mechanism having a plurality of balls that allow sliding of the slider along the lower rail. 
     Advantages of the Invention 
     According to a seat slide mechanism of the present invention described above, vibration is damped by a slide-allowing member of the seat slide mechanism having a large mass and arranged closely to a vibration input point (floor pan) in a vibration transmission pathway extending from the vibration input point to the occupant. This can effectively reduce the vibration to be sensed by the occupant seated on the seat, and improve the NVH performance of the vehicle. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a vehicle seat attached to a floor pan of a vehicle via a pair of right and left seat slide mechanisms according to an exemplary embodiment, which vehicle seat is for an occupant of the vehicle to sit thereon. 
         FIG. 2  is a cross-sectional view providing an enlarged view of a main part of one of the pair of right and left seat slide mechanisms, as viewed from the front of the vehicle. 
         FIG. 3  is a perspective view showing slide-allowing members and retainers in a pair of right and left units provided in one of the seat slide mechanisms. 
         FIG. 4  is a cross-sectional view of slide-allowing members of a first embodiment, each of which is a cylindrical hollow member. 
         FIG. 5  is an explanatory diagram showing a vibration model of the vehicle seat. 
         FIG. 6  is a cross-sectional view of slide-allowing members of a second embodiment, each of which is a solid member having a columnar shape. 
         FIG. 7  is a cross-sectional view of slide-allowing members of a third embodiment, each of which is a solid member having a columnar shape. 
         FIG. 8  is a diagram of a fourth embodiment corresponding to  FIG. 3 , showing a pair of right and left units (slide-allowing members and retainers). 
         FIG. 9  is a cross-sectional view of slide-allowing members of the fourth embodiment, each of which is a spheric member with its inside being hollowed out. 
         FIG. 10  is a cross-sectional view of slide-allowing members of a fifth embodiment, each of which is a spheric member having an outer shell and an interior member. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Exemplary embodiments will now be described in detail with reference to the drawings. 
     First Embodiment 
       FIG. 1  illustrates a vehicle seat (hereinafter, seat)  10  attached to a floor pan  1  of a vehicle via a pair of right and left seat slide mechanisms  15  according to an exemplary first embodiment, which seat  10  is for an occupant of the vehicle to sit thereon.  FIG. 2  shows an enlarged view of a main part of one of the pair of right and left seat slide mechanisms  15 . The following description of the seat  10  and the seat slide mechanisms  15  assumes that the seat  10  and the seat slide mechanisms  15  are mounted on the vehicle. The front, rear, left, right, top, and bottom of the vehicle refer to the front, rear, left, right, top, and bottom of the seat  10  mounted on the vehicle, respectively. The front, rear, left, right, top, and bottom of the vehicle are hereinafter simply referred to as “front,” “rear,” “left,” “right,” “top,” and “bottom,” respectively. 
     In  FIG. 1 , the seat  10  includes a seat cushion  11 , a seat back  12 , and a headrest  13 . The seat cushion  11  forms a seating surface for an occupant to sit on the seat  10 . The seat back  12  forms a backrest surface for the occupant and is capable of reclining. The headrest  13  holds the head of the occupant and the height of the headrest  13  is adjustable. 
     The seat cushion  11  includes a seat portion  11   a  and a pair of right and left side support portions  11   b  (so-called projected portions), which are integrally formed. The seat portion  11   a  is located in a middle portion relative to the right-left directions. The side support portions  11   b  project upward from both right and left side portions of the seat portion  11   a , respectively, and extend in the front-rear directions. The seat cushion  11  has a seat cushion frame  14 , a seat cushion spring (not shown), and a urethane member (not shown). The seat cushion frame  14  has a right end portion and a left end portion ( FIG. 1  only shows the left end portion) and a connecting portion. The right end portion and the left end portion are positioned above the pair of right and left seat slide mechanisms  15 , which will be described later, respectively. The connecting portion connects the right end portion with the left end portion. 
     The seat back  12  includes a backrest  12   a  and a pair of right and left side support portions  12   b  (so-called projected portions), which are integrally formed. The backrest  12   a  is positioned in a middle portion relative to the right-left directions. The side support portions  12   b  project forward from both right and left side portions of the backrest  12   a , respectively, and extend in the up-down direction. Although illustration is omitted, the seat back  12  has a seat back frame, a seat back spring, and a urethane member. 
     In the present embodiment, the seat  10  is a bucket seat (synonym to a separate seat). The seat  10  is configured so that its position is adjustable according to the physique of the occupant. 
     That is, the pair of right and left seat slide mechanisms  15  are provided between the seat cushion frame  14  of the seat cushion  11  of the seat  10  and the floor pan  1  constituting a part of the vehicle body of the vehicle. The pair of right and left seat slide mechanisms  15  are formed to have the same structure. The seat  10  is attached to the floor pan  1  so as to be relatively movable in the front-rear directions by the pair of right and left seat slide mechanisms  15 . 
     Each seat slide mechanism  15  includes a lower rail  16 , a slider  17 , and a plurality of slide-allowing members  30  (see  FIG. 2  and  FIG. 3 ). The lower rail  16  is fixed to the floor pan  1  and extends in the front-rear directions. The slider  17  is slidable in the lower rail  16 . Above the slider  17 , the seat  10  is attached (specifically, the seat cushion  11 ). The plurality of slide-allowing members  30  are arranged between the lower rail  16  and the slider  17 , and allow the slider  17  to slide along the lower rail  16 . 
     As shown in  FIG. 1 , an upper portion (a later-described top part  17   a  (see  FIG. 2 )) of the slider  17  constituting the seat slide mechanism  15  and the left end portion or the right end portion of the seat cushion frame  14  are coupled to each other at two positions, that is, at a front position and at a rear position. For the coupling at the front position, a lower bracket  18  and an upper bracket  19  are used. For the coupling at the rear position, a lower bracket  20  and an upper bracket  21  are used. 
     As shown in  FIG. 2 , the plurality of slide-allowing members  30  of each seat slide mechanism  15  are arranged between the lower rail  16  and the slider  17 , while being supported by a plurality of retainers  22 . 
     As shown in  FIG. 2  and  FIG. 3 , each retainer  22  supports the slide-allowing member  30  in each of its upper end part and its lower end part. In the upper end part of each retainer  22 , a relatively small slide-allowing member  30  is supported. This slide-allowing member  30  is also referred to as an “upper slide-allowing member  30 U.” In the lower end part of the retainer  22 , a relatively large slide-allowing member  30  is supported. This slide-allowing member  30  is also referred to as a “lower slide-allowing member  30 L.” The upper and lower slide-allowing members  30 U and  30 L are combined as a unit by the retainer  22 , and each seat slide mechanism  15  has a plurality of such units U 1  and U 2 . 
     The unit U 1  is arranged on the left side of the seat slide mechanism  15 , whereas the unit U 2  is arranged on the right side of the seat slide mechanism  15 . The left unit U 1  and the right unit U 2  are arranged substantially symmetrically with respect to the center of the right-left directions of the lower rail  16 . The respective structures of the left unit U 1  and the right unit U 2  are substantially symmetrical with respect to the center of the right-left directions. These units U 1  and U 2  having structures that are substantially symmetrical to each other in the right-left directions are paired, and the present embodiment includes a plurality of pairs of units U 1  and U 2  aligned in the longitudinal direction (corresponding to the front-rear directions) of the lower rail  16  and the slider  17 , between the lower rail  16  and the slider  17 . 
     As shown in  FIG. 2 , the lower rail  16  has a bottom part  16   a , two standing parts  16   c , two connecting parts  16   e , and two interior parts  16   g  which are integrally formed by bending. The bottom part  16   a  is to be fixed to the floor pan  1 . The two standing parts  16   c  rise upward from rounded corner parts  16   b , on both ends of the bottom part  16   a  relative to a vehicle width direction (corresponding to the right-left directions). The two connecting parts  16   e  extend obliquely upward from arched corner parts  16   d  arranged at the respective upper ends of the two standing parts  16   c . The two interior parts  16   g  extend downward from arched corner parts  16   f  arranged at the respective upper ends of the two connecting parts  16   e . It is possible to say that the lower rail  16  has an outline in a shape of a groove open upward. 
     As shown in  FIG. 2 , the slider  17  has a top part  17   a , two standing parts  17   c , two connecting parts  17   e , two lower exterior portions  17   f , and two upper exterior portions  17   g  which are integrally formed by bending. To the top part  17   a , the seat  10  is attached through brackets  18  to  21  (see  FIG. 1 ). The two standing parts  17   c  extend downward from corner parts  17   b , on both ends of the top part  17   a  relative to the vehicle width direction (corresponding to the right-left directions). The two connecting parts  17   e  extend outward and obliquely upward from corner parts  17   d  arranged at the respective lower ends of the two standing parts  17   c . The two lower exterior portions  17   f  extend upward from the respective outer ends (upper ends) of the two connecting parts  17   e . The two upper exterior portions  17   g  extend upward from arched corner parts  17   h  arranged at the respective upper ends of the two lower exterior portions  17   f . The slider  17  is positioned in the groove of the lower rail  16  except for its top part  17   a , and the top part  17   a  projects upward from the opening of the groove. 
     As shown in  FIG. 2 , the lower rail  16  and the slider  17  are each formed of steel. The lower rail  16  is formed symmetrically or substantially symmetrically with respect to the center in the left-right directions. The slider  17  is also formed symmetrically or substantially symmetrically with respect to the center in the left-right directions. The center of the slider  17  in the left-right directions substantially matches with the center in the left-right directions of the lower rail  16 . 
     As shown in  FIG. 3 , each retainer  22  has a retainer body  22   a . At an upper end part of the retainer body  22   a , a pair of front and rear supports  22   b  and  22   c  are formed. Between the pair of front and rear supports  22   b  and  22   c  at the upper end part of the retainer body  22   a , a recess  22   d  recessed downward is formed. The upper slide-allowing member  30 U is positioned within the recess  22   d  and supported by the supports  22   b  and  22   c.    
     Similarly to the upper end part of the retainer  22 , a pair of front and rear supports  22   e  and  22   f  are formed at a lower end part of the retainer body  22   a . Between the pair of front and rear supports  22   e  and  22   f  at the lower end part of the retainer body  22   a , a recess  22   g  recessed upward is formed. The lower slide-allowing member  30 L is positioned within the recess  22   g  and supported by the supports  22   e  and  22   f.    
     Note that in  FIG. 3 , the arrow F is directed forward, the arrow R is directed rearward, and the arrow UP is directed upward. 
     In each seat slide mechanism  15 , the spring constant of each slide-allowing member  30  is set to be smaller than the spring constant of a steel lower rail  16  or a steel slider  17 . 
     In the present embodiment, as shown in  FIG. 4 , each of the slide-allowing members  30  is structured by a tubular (cylindrical shape in the present embodiment) hollow member  31  to reduce the spring constant thereof, and is arranged between the lower rail  16  and the slider  17  in such a manner as to extend along the lower rail  16  and the slider  17  (i.e., extend in the front-rear directions). 
     Specifically, the hollow member  31  is structured by a cylindrical pipe member  32  made of metal (e.g., stainless steel) and an outer shell  33 . To be more specific, the outer circumferential face of the pipe member  32  is coated with a synthetic resin (e.g., ultralow friction fluororesin) to form the cylindrical outer shell  33  of the hollow member  31  integrally with the pipe member  32 . Inside the pipe member  32 , an internal space  32   a  is formed. 
     In this embodiment, substantially hemispherical portions  33   a  and  33   b  for supporting the hollow member  31  are formed integrally with both ends of the hollow member  31  relative to its longitudinal direction (both ends relative to the front-rear directions). 
     As shown in  FIG. 3  and  FIG. 4 , of the slide-allowing members  30  each structured by the hollow member  31 , the upper slide-allowing member  30 U positioned on the upper side has a size of approximately 10 mm in length and approximately 5 mm in diameter. 
     Further, of the slide-allowing members  30  each structured by the hollow member  31 , the lower slide-allowing member  30 L positioned on the lower side has a size of approximately 14 mm in length and approximately 7 mm in diameter. However, these numeric values of the length and the diameter are no more than examples, and are not intended to limit the scope of the invention. 
     The slide-allowing members  30  (upper and lower slide-allowing members  30 U and  30 L) each structured by the hollow member  31  (so-called a hollow roller) shown in  FIG. 4  are supported by the retainer  22  shown in  FIG. 3  as follows. That is, the front and rear substantially hemispherical portions  33   a  and  33   b  of the upper slide-allowing member  30 U are supported by the pair of front and rear supports  22   b  and  22   c  in the upper end part of the retainer  22 . The front and rear substantially hemispherical portions  33   a  and  33   b  of the lower slide-allowing member  30 L are supported by the pair of front and rear supports  22   e  and  22   f  in the lower end part of the retainer  22 . 
     To support the substantially hemispherical portions  33   a  and  33   b  of the upper slide-allowing member  30 U, the retainer  22  has recesses (not shown) formed on both opposing surface portions of the support  22   b  and the support  22   c . These recesses are hemispherically concaved to correspond to the substantially hemispherical portions  33   a  and  33   b . Further, to support the substantially hemispherical portions  33   a  and  33   b  of the lower slide-allowing member  30 L, the retainer  22  has recesses (not shown) formed on both opposing surface portions of the support  22   e  and the support  22   f . These recesses are hemispherically concaved to correspond to the substantially hemispherical portions  33   a  and  33   b.    
     As shown in  FIG. 2 , while the units U 1  and U 2  are arranged between the lower rail  16  and the slider  17 , the upper slide-allowing member  30 U of each of the units U 1  and U 2  is interposed between the corner part  16   d  of the lower rail  16  and the corner part  17   h  of the slider  17 , and the lower slide-allowing member  30 L is interposed between the rounded corner part  16   b  of the lower rail  16  and the connecting part  17   e  of the slider  17 . 
       FIG. 5  shows a vibration model of the seat  10 , from a vibration source  24  (vehicle wheels) to an occupant P seated on the seat  10 , via a vibration input point  25  (floor pan  1 ). 
     The vibration model of the seat  10  is roughly divided into three elements: a seat frame  26 , a seat spring  27 , and a urethane member  28  constituting the seat cushion member. 
     The seat frame  26  includes the seat cushion frame  14 , the seat back frame, and the seat slide mechanism  15 . The seat frame  26  has a mass m1, a spring constant k1, and a damping term c1 (synonym to a “damping coefficient”). 
     The seat spring  27  includes the seat cushion spring and the seat back spring. The seat spring  27  has a mass m2, a spring constant k2, and a damping term c2 (synonym to a “damping coefficient”). 
     The urethane member  28  includes the urethane member of the seat cushion  11  and the urethane member of the seat back  12 . The urethane member  28  has a mass m3, a spring constant k3, and a damping term c3 (synonym to a “damping coefficient”). 
     An acceleration rate A of the entire system including the seat frame  26 , the seat spring  27 , and the urethane member  28  can be approximated by the formula (1) below based on the second law of motion (so-called “motion equation”). 
       A≈α+[(c1·V+k1·x)/M]
 
     In the formula (1):
 
“A” is the acceleration of the entire system;
 
“α” is an acceleration rate, regarding the seat frame  26 , the seat spring  27 , and the urethane member  28  as one piece;
 
the “c1·V” is the product of the damping term c1 of the seat frame  26  (including, as an element, the seat slide mechanism  15  having the slide-allowing member  30 ) and the velocity V with respect to the vibration input point  25 , regarding the seat frame  26 , the seat spring  27 , and the urethane member  28  as one piece;
 
the “k1·x” is the product of the spring constant k1 of the seat frame  26  (including, as an element, the seat slide mechanism  15  having the slide-allowing member  30 ) and the displacement x with respect to the vibration input point  25 , regarding the seat frame  26 , the seat spring  27 , and the urethane member  28  as one piece; and “M” is the mass of the entire system, i.e., m1+m2+m3.
 
     According to the formula (1), the acceleration rate A of the entire system decreases with an increase in the mass M (=m1+m2+m3) of the entire system. Further, the acceleration rate A of the entire system decreases with a decrease in the spring constant k1. Therefore, by reducing the acceleration rate A of the entire system, the entire system (the seat frame  26 , the seat spring  27 , the urethane member  28 ) will not vibrate. 
     Thus, to achieve a better NVH performance of the vehicle, it is most effective, in reducing and absorbing vibration, to improve the seat frame  26  having a large mass and arranged closely to the vibration input point  25  (particularly, the seat slide mechanism  15  arranged very closely to the vibration input point  25 ) in a vibration transmission pathway extending from the floor pan  1 , which serves as the vibration input point  25 , to the occupant P. 
     Based on this finding, the present embodiment takes a measure to reduce vibration at the seat slide mechanism  15  having a large mass and arranged very closely to the vibration input point  25  (floor pan  1 ) in the vibration transmission pathway extending from the vibration input point  25  to the occupant P. 
     To reduce the spring constant k1 of the seat frame  26 , the present embodiment reduces the spring constant of each slide-allowing member  30  in the seat slide mechanism  15 , thereby effectively reducing vibration at the seat  10 . 
     In this embodiment, the spring constant of each slide-allowing member  30  is set to be smaller than the spring constant of the lower rail  16  or the slider  17 . That is, since each slide-allowing member  30  is structured by the hollow member  31  which has the pipe member  32  made of stainless steel and the outer shell  33  made of a resin, the spring constant of each slide-allowing member  30  is smaller than the spring constant of the lower rail  16  made of steel or the slider  17  made of steel. 
     As described, the slide-allowing members  30  each having a reduced spring constant (more flexible) damps vibration at the seat slide mechanism  15  having a large mass and arranged very close to the vibration input point  25  (floor pan  1 ) in the vibration transmission pathway extending from the vibration input point  25  to the occupant P. This configuration effectively reduces the vibration (particularly, vibration in the up-down directions) to be sensed by the occupant P, while the occupant P is seated on the seat  10 . 
     Further, the present embodiment achieves a vibration reduction effect with a simple modification essentially to the slide-allowing members (or the slide-allowing member and the retainers), with respect to an existing seat slide mechanism. 
     Further, in the present embodiment, each slide-allowing member  30  is structured by a tubular hollow member  31  and arranged between the lower rail  16  and the slider  17  so as to extends along the lower rail  16  and the slider  17 . Therefore, by changing the length, for example, of the tubular hollow member  31 , the spring constant of each slide-allowing member  30  can be easily controlled. 
     Second Embodiment 
       FIG. 6  shows an exemplary second embodiment, in which the structure of each slide-allowing member  30  is different from that of the first embodiment. The rest of the structure is similar to that of the first embodiment. 
     In the present embodiment, each of the slide-allowing members  30  is structured by a solid member  34  (so-called solid roller) having a pillar shape (columnar shape in the present embodiment) and is arranged between the lower rail  16  and the slider  17  in such a manner as to extend along the lower rail  16  and the slider  17  (i.e., extend in the front-rear directions). 
     Similarly to the above-described first embodiment, the spring constant of each slide-allowing member  30  in the present embodiment is set to be smaller than the spring constant of a steel lower rail  16  or a steel slider  17 . To achieve this, the solid member  34  is made of a resin in the present embodiment. 
     A resin adoptable for this member can be a polyamide having an excellent impact resistance, a low-friction coefficient, and a self-lubricating property. 
     Also in this embodiment, hemispherical portions  34   b  and  34   c  for supporting the solid member  34  at the retainer  22  (see  FIG. 3 ) are formed integrally with both ends of the columnar solid portion  34   a  of the solid member  34  relative to its longitudinal direction (both ends relative to the front-rear directions). 
     As described, in the second embodiment shown in  FIG. 6 , each of the slide-allowing members  30  is structured by a solid member  34  having a pillar shape (columnar shape in the present embodiment) and is arranged between the lower rail  16  and the slider  17  in such a manner as to extend along the lower rail  16  and the slider  17  (see  FIG. 2 ), similarly to the above-described first embodiment. 
     This structure enables easy control of the spring constant of each slide-allowing member  30  by modifying the length and the like of the solid member  34  having a pillar shape. 
     Third Embodiment 
       FIG. 7  shows an exemplary third embodiment, in which the structure of each slide-allowing member  30  is different from those of the first and second embodiments. The rest of the structure is similar to those of the first and second embodiments. 
     That is, in the present embodiment, each slide-allowing member  30  is structured by a solid member  36  having a two-layer structure (inner and outer layers) which is formed by surrounding the entire outer surface of the solid member  34  of the second embodiment shown in  FIG. 6  with an outer shell member  35 . 
     In this embodiment, the solid member  34  is made of a resin (e.g., polyamide), and the outer shell member  35  is made of a metal (e.g., stainless steel). 
     Also in the present embodiment, each slide-allowing member  30  is arranged between the lower rail  16  and the slider  17  in such a manner as to extend along the lower rail  16  and the slider  17  (see  FIG. 2 ), similarly to the above-described first and second embodiments. Further, similarly to the above-described first and second embodiments, the spring constant of each slide-allowing member  30  is set to be smaller than the spring constant of the lower rail  16  or the slider  17 . 
     As described, in the third embodiment shown in  FIG. 7 , each of the slide-allowing members  30  is structured by a solid member  36  having a pillar shape (columnar shape in the present embodiment) and is arranged between the lower rail  16  and the slider  17  in such a manner as to extend along the lower rail  16  and the slider  17  (see  FIG. 2 ), similarly to the above-described second embodiment. 
     This structure enables easy control of the spring constant of each slide-allowing member  30  by modifying the length and the like of the solid member  36  having a pillar shape. Further, in the present embodiment, the solid member  36  of each slide-allowing member  30  has a two-layer structure (inner and outer layers), and the outer layer is an outer shell member  35  made of stainless steel. This enables improvement of durability and antirust effect of each slide-allowing member  30 . 
     Fourth Embodiment 
       FIG. 8  and  FIG. 9  show an exemplary fourth embodiment, in which the structure of each slide-allowing member  30  and the structure of each retainer  22  are different from those of the first to third embodiments. The rest of the structure is similar to those of the first to third embodiments. 
     That is, in the present embodiment, each slide-allowing member  30  is structured by a spheric member  41 . Also in the present embodiment, the plurality of slide-allowing members  30  of each seat slide mechanism  15  are arranged between the lower rail  16  and the slider  17 , while being supported by a plurality of retainers  22 , similarly to the above-described first to third embodiments. 
     Each retainer  22  supports a plurality of slide-allowing members  30  (two in this embodiment) in each of its upper end part and its lower end part. In the upper end part of each retainer  22 , relatively small slide-allowing members  30  are supported. These slide-allowing members  30  are also referred to as “upper slide-allowing members  30 U.” In the lower end part of the retainer  22 , relatively large slide-allowing members  30  are supported. These slide-allowing members  30  are also referred to as “lower slide-allowing members  30 L.” The upper and lower slide-allowing members  30 U and  30 L are combined as a unit by the retainer  22 , and each seat slide mechanism  15  has a plurality of such units U 1  and U 2 . 
     Also in the present embodiment, the unit U 1  is arranged on the left side of each seat slide mechanism  15 , whereas the unit U 2  is arranged on the right side of the seat slide mechanism  15 . The left unit U 1  and the right unit U 2  are arranged substantially symmetrically with respect to the center of the right-left directions of the lower rail  16 . The respective structures of the left unit U 1  and the right unit U 2  are substantially symmetrical with respect to the center of the right-left directions. These units U 1  and U 2  having structures that are substantially symmetrical to each other in the right-left directions are paired, and a plurality of pairs of units U 1  and U 2  are aligned in the longitudinal direction (corresponding to the front-rear directions) of the lower rail  16  and the slider  17 , between the lower rail  16  and the slider  17 . 
     As shown in  FIG. 8 , each retainer  22  has a retainer body  22   a . At an upper end part of the retainer body  22   a , a plurality of supports  22   h ,  22   i , and  22   j  are formed apart from one another in the front-rear directions. Between the supports  22   h  and  22   i , and between the supports  22   i  and  22   j , at the upper end part of the retainer body  22   a , recesses  22   k  and  22   l  recessed downward are formed, respectively. The two upper slide-allowing members  30 U are positioned within the recesses  22   k  and  22   l , and supported by the supports  22   h  and  22   i , and the supports  22   i  and  22   j , respectively. 
     Similarly to the upper end part of the retainer  22 , a plurality of supports  22   m ,  22   n , and  22   o  are formed at a lower end part of the retainer body  22   a  apart from one another in the front-rear directions. Between the supports  22   m  and  22   n , and between the supports  22   n  and  22   o  at the lower end part of the retainer body  22   a , recesses  22   p  and  22   q  recessed upward are formed respectively. The two lower slide-allowing members  30 L are positioned within the recesses  22   p  and  22   q , and supported by the supports  22   m  and  22   n , and the supports  22   n  and  22   o , respectively. 
     To support the two spheric upper slide-allowing members  30 U, the retainer  22  has spherically concaved recesses (not shown) formed on both opposing surface portions of the support  22   h  and the support  22   i , and both opposing surface portions of the support  22   i  and the support  22   j . Further, to support the two spheric lower slide-allowing members  30 L, the retainer  22  has spherically concaved recesses (not shown) formed on both opposing surface portions of the support  22   m  and the support  22   n , and both opposing surface portions of the support  22   n  and the support  22   o.    
     Also in the present embodiment, while the units U 1  and U 2  are arranged between the lower rail  16  and the slider  17 , the upper slide-allowing members  30 U of each of the units U 1  and U 2  are interposed between the corner part  16   d  of the lower rail  16  and the corner part  17   h  of the slider  17 , and the lower slide-allowing members  30 L are interposed between the rounded corner part  16   b  of the lower rail  16  and the connecting part  17   e  of the slider  17 . 
     As shown in  FIG. 9 , in the present embodiment, the spheric member  41  constituting each slide-allowing member  30  is a hollow spheric member with its inside being hollowed out (a hollow portion  42  is provided inside). 
     The spring constant of each slide-allowing member  30  is set to be smaller than the spring constant of a steel lower rail  16  or a steel slider  17 . To achieve this, the spheric member  41  is made of a resin in the present embodiment. 
     A resin adoptable for this member can be a polyamide having an excellent impact resistance, a low-friction coefficient, and a self-lubricating property. 
     As shown in  FIG. 8 , of the slide-allowing members  30  each structured by the spheric member  41 , the upper slide-allowing members  30 U positioned on the upper side are each approximately 5 mm in diameter, and the lower slide-allowing members  30 L positioned on the lower side are each approximately 7 mm in diameter. However, these numeric values of the diameter are no more than examples, and are not intended to limit the scope of the invention. 
     Note that in  FIG. 8 , the arrow F is directed forward, the arrow R is directed rearward, and the arrow UP is directed upward. 
     In this embodiment, the spring constant of each slide-allowing member  30  is set to be smaller than the spring constant of the lower rail  16  or the slider  17 . This achieves the following effects. 
     That is, the slide-allowing members  30  each having a small spring constant damps vibration at the seat slide mechanism  15  having a large mass and arranged closely to the vibration input point (floor pan  1 ) in the vibration transmission pathway extending from the vibration input point to the occupant. This effectively reduces the vibration to be sensed by the occupant, while the occupant is seated on the seat  10 , and hence can improve the NVH performance of the vehicle. 
     Further, since each slide-allowing member  30  of the present embodiment is structured by the spheric member  41  (hollow spheric member) having the hollow portion  42 , the present embodiment does not necessitate significant modification to a typical existing seat slide mechanism having a plurality of balls that allow sliding of the slider along the lower rail, and achieves the above-described effect of reducing vibration, simply by replacing the balls with the spheric member  41 . 
     Fifth Embodiment 
       FIG. 10  shows an exemplary fifth embodiment, in which the structure of each slide-allowing member  30  is different from that of the fourth embodiment. The rest of the structure is similar to that of the fourth embodiment. 
     That is, in the present embodiment, each slide-allowing member  30  is structured by a spheric member  43 . The spheric member  43  is a solid spheric member having an outer shell  44  and an interior member  45 . 
     Also in this embodiment, the spring constant of each slide-allowing member  30  is set to be smaller than the spring constant of the lower rail  16  or the slider  17 . To achieve this, the spring constant of the interior member  45  is set to be smaller than the spring constant of the outer shell  44 . Specifically, in the present embodiment, the outer shell  44  is made of a metal (e.g., stainless steel), and the interior member  45  is made of a resin (e.g., polyamide). 
     Since each slide-allowing member  30  of the present embodiment is structured by the spheric member  43 , the present embodiment achieves the above-described effect of reducing vibration without significant modification to a typical existing seat slide mechanism having a plurality of balls that allow sliding of the slider along the lower rail, similarly to the above-described fourth embodiment. 
     Further, each slide-allowing member  30  (spheric member  43 ) has the outer shell  44  made of, for example, stainless steel. This enables improvement of durability and antirust effect of each slide-allowing member  30 . 
     The present invention is not limited to the embodiment described above. Any substitution can be made within the scope of the claims. 
     For example, the above-described first embodiment deals with a case where each slide-allowing member  30  is structured by a cylindrical hollow member  31 , whereas the above-described second and third embodiments deal with a case where each slide-allowing member  30  is structured by a solid member  34 ,  36  having a columnar shape. However, each slide-allowing member  30  may be structured by a hollow member having a shape of a square tube or a solid member having a shape of a square column. 
     The foregoing embodiment is merely an example in nature, and the scope of the present invention should not be interpreted in a limited manner. The scope of the present invention is defined by the appended claims, and all variations and modifications belonging to a range equivalent to the range of the claims are within the scope of the present invention. 
     INDUSTRIAL APPLICABILITY 
     The present invention is applicable to a seat slide mechanism including: a lower rail fixed to a floor pan constituting a part of a vehicle body of a vehicle and extending in the front-rear directions of the vehicle; a slider slidable in the lower rail and allowing a seat to be mounted thereabove; and a slide-allowing member arranged between the lower rail and the slider and enabling sliding of the slider along the lower rail. 
     DESCRIPTION OF REFERENCE CHARACTERS 
     
         
           1  Floor Pan 
           10  Vehicle Seat 
           15  Seat Slide Mechanism 
           16  Lower Rail 
           17  Slider 
           30  Slide-Allowing Member 
           31  Hollow Member 
           34  Solid Member 
           35  Solid Member 
           41  Spheric Member (Hollow Spheric Member) 
           43  Spheric Member (Solid Spheric Member) 
           44  Outer Shell 
           45  Interior Member