Abstract:
A linear actuator. On a guide block that configures a guide mechanism, a pair of ball-circulating grooves is formed on the lower surface that faces a cylinder body and multiple balls are loaded in the ball-circulating grooves. Paired cover blocks are respectively mounted on the two ends of the guide block. Paired ball clips, which are formed as arms, are engaged with the cover blocks when the clips are inserted in the ball-circulating grooves and are holding the balls. Paired cover plates are respectively mounted on the end faces of the cover blocks, and arm-shaped cover clips are mounted so as to hold the cover plates. The cover plates, the cover blocks and the guide block are thereby integrally linked.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a linear actuator in which a slide table is made to move reciprocally along an axial direction of a cylinder main body. 
       BACKGROUND ART 
       [0002]    Heretofore, a linear actuator, for example, which is made up of a fluid pressure cylinder or the like, has been used as a means for transporting workpieces under the supply of a pressure fluid. As disclosed in Japanese Patent No. 3795968, the present applicants have proposed a linear actuator, which is capable of transporting a workpiece that is loaded onto a slide table by causing the slide table to move reciprocally in a straight line along a cylinder main body. However, with the aforementioned linear actuator, in recent years, there has been a demand to reduce manufacturing costs and to simplify the structure of the apparatus. 
       SUMMARY OF INVENTION 
       [0003]    A general object of the present invention is to provide a linear actuator, which makes it possible to reduce production costs and to simplify the structure of the linear actuator. 
         [0004]    The present invention is characterized by a linear actuator in which a slide table is made to move reciprocally along an axial direction of a cylinder main body, comprising: 
         [0005]    a guide mechanism including a guide block attached to the cylinder main body and in which circulation grooves are formed through which a plurality of rolling bodies roll and circulate, and a cover member disposed on an end of the guide block, the guide mechanism guiding the slide table along an axial direction of the cylinder main body, and 
         [0006]    a retainer installed on the guide block, for retaining the rolling bodies freely circulatable in the circulation grooves, and for retaining the cover member with respect to the guide block, 
         [0007]    wherein the circulation grooves are formed to open in the guide block along a longitudinal direction thereof, and the retainer is disposed detachably with respect to the guide block. 
         [0008]    According to the present invention, the guide mechanism that constitutes the linear actuator is equipped with the circulation grooves, which open along the longitudinal direction of the guide block, and the plural rolling bodies, which are circulated through the circulation grooves, are retained by the retainer. The retainer retains the plural rolling bodies so as to be freely circulatable in the circulation grooves of the guide block, and the rolling bodies are further retained by the cover member, which is disposed on the end of the guide block. 
         [0009]    Accordingly, in comparison with a situation in which a cover member is attached with respect to the guide block by bolts or the like, the cover member can be simplified in structure, together with being more easily assembled through use of the retainer. 
         [0010]    Further, on the guide block, instead of providing penetrating through holes in which the rolling bodies are circulated, since circulating grooves are formed that open along the axial direction, compared to a case of forming such through holes, the number of process steps and processing costs can be reduced. As a result, production costs for the linear actuator are reduced, and hence the linear actuator can be manufactured inexpensively. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0011]      FIG. 1  is an exterior perspective view of a linear actuator according to a first embodiment of the present invention; 
           [0012]      FIG. 2  is an exploded perspective view of the linear actuator shown in  FIG. 1 ; 
           [0013]      FIG. 3  is an exploded perspective view of the linear actuator of  FIG. 2  as seen from a different direction; 
           [0014]      FIG. 4  is an overall vertical cross sectional view of the linear actuator of  FIG. 1 ; 
           [0015]      FIG. 5  is a cross sectional view taken along line V-V of  FIG. 4 ; 
           [0016]      FIG. 6  is a cross sectional view taken along line VI-VI of  FIG. 4 ; 
           [0017]      FIG. 7  is a cross sectional view taken along line VII-VII of  FIG. 4 ; 
           [0018]      FIG. 8  is an exterior perspective view of a guide mechanism that constitutes part of the linear actuator of  FIG. 1 ; 
           [0019]      FIG. 9  is an exterior perspective view of the guide mechanism of  FIG. 8  as seen from a different direction; 
           [0020]      FIG. 10  is an exploded perspective view of the guide mechanism shown in  FIG. 8 ; 
           [0021]      FIG. 11  is an exterior perspective view of a linear actuator according to a second embodiment of the present invention; 
           [0022]      FIG. 12  is a cross sectional view taken along line XII-XII of  FIG. 11 ; 
           [0023]      FIG. 13  is an exploded perspective view of a linear actuator according to a third embodiment of the present invention; 
           [0024]      FIG. 14  is an exterior perspective view of a guide mechanism that constitutes part of the linear actuator of  FIG. 13 ; 
           [0025]      FIG. 15  is an exterior perspective view of the guide mechanism of  FIG. 14  as seen from a different direction; 
           [0026]      FIG. 16  is an exploded perspective view of the guide mechanism shown in  FIG. 14 ; 
           [0027]      FIG. 17  is an exterior perspective view of a ball clip that is used in the guide mechanism of  FIG. 14 ; 
           [0028]      FIG. 18  is a cross sectional view taken along line XVIII-XVIII of  FIG. 14 ; 
           [0029]      FIG. 19  is a cross sectional view showing a direction perpendicular to the longitudinal direction of the linear actuator of  FIG. 13 ; and 
           [0030]      FIG. 20  is an exterior perspective view showing a ball clip according to a modification. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0031]    As shown in  FIGS. 1 through 7 , a linear actuator  10  comprises a cylinder main body  12 , a slide table  14  disposed on an upper portion of the cylinder main body  12  and which undergoes reciprocal motion in a straight line along a longitudinal direction (the directions of arrows A and B), a guide mechanism  16  disposed to intervene between the cylinder main body  12  and the slide table  14  for guiding the slide table  14  in the longitudinal direction (the directions of arrows A and B), and a stopper mechanism  18 , which is capable of adjusting a displacement amount of the slide table  14 . 
         [0032]    The cylinder main body  12  has a rectangular cross section and has a predetermined length along the longitudinal direction (the directions of arrows A and B). A recess  20  having a sunken arcuate shape in cross section is formed roughly in the center on the upper surface of the cylinder main body  12  (see  FIG. 2 ). The recess  20  extends along the longitudinal direction (the directions of arrows A and B), and in the recess  20 , a pair of penetrating bolt holes  24  is provided, through which connecting bolts  22  are inserted for connecting the cylinder main body  12  with the guide mechanism  16 . 
         [0033]    Further, on one side surface of the cylinder main body  12 , first and second ports  26 ,  28  for supply and discharge of a pressure fluid are formed perpendicularly to the longitudinal direction of the cylinder main body  12 . The first and second ports  26 ,  28  communicate with a pair of penetrating holes  30   a ,  30   b  to be described later. Furthermore, on the opposite side surfaces of the cylinder main body  12 , sensor attachment grooves  32   a ,  32   b  are formed, respectively, at positions along the longitudinal direction (the directions of arrows A and B), which have sensors (not shown) mounted therein. 
         [0034]    On the bottom surface of the cylinder main body  12 , the pair of bolt holes  24  is formed centrally in the widthwise direction on the axial line. The connecting bolts  22  are inserted through the bolt holes  24  from below. Additionally, the ends of the connecting bolts  22  project from the upper surface of the cylinder main body  12 , and are connected mutually by threaded engagement with a guide block  76  of the guide mechanism  16 . 
         [0035]    On the other hand, inside the cylinder main body  12 , a pair of penetrating holes  30   a ,  30   b  is formed, which penetrates along the longitudinal direction (the directions of arrows A and B), the one penetrating hole  30   a  and the other penetrating hole  30   b  being disposed substantially in parallel with each other and separated by a predetermined distance. Inside the penetrating holes  30   a ,  30   b , cylinder mechanisms  42  are provided, respectively, each including a piston  38  on which a sealing ring  34  and a magnet  36  are installed on the outer circumference thereof, and a piston rod  40  connected to the piston  38 . The cylinder mechanisms  42  are constituted by installation of the pair of pistons  38  and the piston rods  40 , respectively, in the pair of penetrating holes  30   a ,  30   b.    
         [0036]    As shown in  FIG. 5 , the penetrating holes  30   a ,  30   b  are closed and sealed at one end thereof by caps  44 , whereas other ends of the penetrating holes  30   a ,  30   b  are sealed hermetically by rod holders  46 , which are retained via locking rings. 
         [0037]    Furthermore, one of the penetrating holes  30   a  communicates respectively with the first and second ports  26 ,  28 , whereas the other penetrating hole  30   b  also communicates mutually with the one penetrating hole  30   a  via a pair of connecting passages  48  formed between the one penetrating hole  30   a  and the other penetrating hole  30   b . More specifically, the pressure fluid is supplied to the first and second ports  26 ,  28  and introduced into the one penetrating hole  30   a , and then the pressure fluid is introduced into the other penetrating hole  30   b  through the connecting passages  48 . The connecting passages  48  are formed perpendicularly to the direction of extension (the directions of arrows A and B) of the penetrating holes  30   a ,  30   b.    
         [0038]    The slide table  14  comprises a table main body  50 , and an end plate  52  connected to another end of the table main body  50 . The end plate  52  is connected perpendicularly with respect to the table main body  50 . 
         [0039]    The table main body  50  is made up from a base member  54  that extends along the longitudinal direction (the directions of arrows A and B) with a predetermined thickness, and a pair of guide walls  56   a ,  56   b  that extend downward perpendicularly from both sides of the base member  54 . On inner surfaces of the guide walls  56   a ,  56   b , first ball guide grooves  60  are formed for guiding balls (rolling bodies)  58  of the guide mechanism  16 , to be described later. The first ball guide grooves  60  are recessed with substantially semicircular shapes in cross section. 
         [0040]    Further, on one end of the table main body  50 , a holder portion  64  of the later-described stopper mechanism  18  is fixed by a pair of bolts  62   a . Further, on another end of the table main body  50 , an end plate is fixed thereto by another pair of bolts  62   b.    
         [0041]    Four workpiece retaining holes  66  are formed in the base member  54 , which are separated mutually by predetermined distances. The workpiece retaining holes  66  are used for fixing a workpiece (not shown), for example, which is mounted on the slide table  14 . 
         [0042]    The end plate  52  is fixed to the other end of the table main body  50 , and is disposed to face toward an end surface of the cylinder main body  12 . The end plate  52  also is fixed to respective ends of the piston rods  40 , which are inserted through a pair of rod holes  68  formed in the end plate  52 . Owing thereto, the slide table  14  including the end plate  52  is displaceable together with the piston rods  40  along the longitudinal direction (the directions of arrows A and B) of the cylinder main body  12 . 
         [0043]    Further, a damper  70  is mounted through a damper installation hole substantially in the center of the end plate  52 . The damper  70  is made from an elastic material such as rubber or the like, and is mounted such that an end portion thereof projects outwardly from the end surface of the end plate  52 , so that upon displacement of the slide table  14 , the damper  70  comes into abutment against an end surface of the cylinder main body  12 . 
         [0044]    The stopper mechanism  18  includes the holder portion  64  disposed on a lower surface of one end of the table main body  50 , a stopper bolt  72  screw-engaged with respect to the holder portion  64 , and a lock nut  74  for regulating advancing and retracting movements of the stopper bolt  72 . The stopper mechanism  18  is disposed so as to face toward an end surface of the guide mechanism  16 , which is disposed on the cylinder main body  12 . 
         [0045]    The holder portion  64  is formed in a block-like shape and is fixed by two bolts  62   a  with respect to the base member  54  of the table main body  50  of the slide table  14 . In a center part of the holder portion  64 , a screw hole is formed in which the stopper bolt  72  is screw-engaged. 
         [0046]    The stopper bolt  72 , for example, is made from a shank-shaped stud bolt engraved with threads on the outer peripheral surface thereof, and is screw-engaged in a screw hole of the holder portion  64 . The lock nut  74  is screw-engaged with the stopper bolt  72  at a region projecting from an end surface of the holder portion  64 . 
         [0047]    Additionally, by threaded rotation of the stopper bolt  72  with respect to the holder portion  64 , the stopper bolt  72  is displaced along the axial direction (the directions of arrows A and B), so as to approach and separate away from the guide mechanism  16 , and by threaded rotation of the lock nut  74 , advancing and retracting movements of the stopper bolt  72  are regulated. 
         [0048]    As shown in  FIGS. 8 through 10 , the guide mechanism  16  includes the wide flat guide block  76 , a pair of cover blocks (cover members)  78   a ,  78   b  disposed on opposite ends of the guide block  76 , plural balls  58  that circulate in the longitudinal direction of the guide block  76 , and a pair of ball clips (retaining members)  82   a ,  82   b  that retain the balls  58  in respective ball circulation grooves  80  of the guide block  76 . Further, the guide mechanism  16  includes a pair of cover clips (other retaining members)  84   a ,  84   b  that serve to retain the cover blocks  78   a ,  78   b  with respect to the guide block  76 , and two pairs of cover plates  86   a ,  86   b  which are mounted respectively on the cover blocks  78   a ,  78   b.    
         [0049]    The guide block  76 , is formed, for example, from a metal material such as stainless or carbon steel, and is formed with second ball guide grooves  88  on opposite side surfaces thereof along the longitudinal direction (the directions of arrows A and B), and with the pair of ball circulation grooves  80 , in which the balls  58  are installed, formed on a bottom surface thereof along the longitudinal direction (the directions of arrows A and B). More specifically, the second ball guide grooves  88  and the ball circulation grooves  80  are formed substantially in parallel with each other. Moreover, the second ball guide grooves  88  are formed with semicircular shapes in cross section in the same manner as the first ball guide grooves  60 . 
         [0050]    Additionally, when the slide table  14  is arranged on an upper part of the guide mechanism  16 , the second ball guide grooves  88  are formed at positions confronting the first ball guide grooves  60 , and the ball circulation grooves  80  are formed in facing relation to the upper surface of the cylinder main body  12 . 
         [0051]    Further, on the upper surface of the guide block  76 , in a central part thereof, an upwardly bulging projection  90  is formed that extends in the longitudinal direction. The projection  90  is formed with a trapezoidal shape in cross section becoming slightly narrower in an upward direction. 
         [0052]    The cover blocks  78   a ,  78   b , for example, are made from a resin material such as nylon or the like, each of which includes a main body portion  92 , and a cutout portion  94 , which is cutout substantially centrally in the widthwise direction of the main body portion  92 . 
         [0053]    The main body portion  92  is formed in a divided manner in left and right widthwise directions about the cutout portion  94 , with one end surface thereof that abuts against the guide block  76  being formed in a planar shape, and the other end surface thereof opposite from the one end surface being formed in a stepped shape. 
         [0054]    Further, substantially in the center in the widthwise direction of the main body portion  92 , an arcuate section  96  is formed, which projects downwardly with an arcuate shape in cross section. The arcuate section  96  is inserted into the recess  20  when the guide mechanism  16  is connected to the upper part of the cylinder main body  12 . 
         [0055]    Furthermore, when the main body portions  92  are placed in abutment against end surfaces of the guide block  76 , the ends of the ball circulation grooves  80  are closed, and since the cutout portions  94  thereof penetrate from the one end surface to the other end surface of the cover blocks  78   a ,  78   b , the end surfaces of the guide block  76  are exposed through the cutout portions  94 . In addition, upon displacement of the slide table  14 , displacement of the slide table  14  is regulated by abutment of the stopper bolt  72  of the stopper mechanism  18  against one end surface of the guide block  76 . 
         [0056]    Further, on one end surface of the main body portion  92 , a pair of return guides  98 , which serve to reverse the direction in which the balls  58  are circulated, is disposed respectively via installation holes  100 . The return guides  98  are equipped with groove-shaped guide portions  102  formed with semicircular shapes in cross section, through which the balls  58  roll along an outer circumferential surface thereof. In addition, when the cover blocks  78   a ,  78   b , in which the return guides  98  are installed, are mounted on opposite end surfaces of the guide block  76 , one of the ends of the return guides  98  are connected to the ball circulation grooves  80 , whereas other ends thereof are connected to the second ball guide grooves  88 . 
         [0057]    More specifically, the ball circulation grooves  80  and the second ball guide grooves  88  are connected by the return guides  98 , such that, in the return guides  98 , the balls  58  roll continuously while the direction of movement thereof is converted 180° from the ball circulation grooves  80  to the first and second ball guide grooves  60 ,  88  via the guide portions  102 . 
         [0058]    On the other hand, on the other side surface of the main body portion  92 , a pair of retaining holes  104   a ,  104   b , which are separated mutually by a predetermined distance, is formed about the cutout portion  94 . Positioning pins  106   a ,  106   b  that project in the longitudinal direction are formed respectively on outer sides in the widthwise direction with respect to the retaining holes  104   a ,  104   b . The retaining holes  104   a ,  104   b  are formed with a predetermined depth toward the one end surface side of the main body portion  92 . 
         [0059]    Further, on the other end surface of the main body portion, regions thereof having the positioning pins  106   a ,  106   b  are formed to project out in a direction away from the one end surface with respect to the region thereof in which the retaining holes  104   a ,  104   b  are formed. 
         [0060]    More specifically, on the other end surface of the main body portion  92 , outer sides thereof in the widthwise direction are formed with stepped portions, which project at a predetermined height in a direction away from the one end surface, with respect to a central vicinity alongside the cutout portion  94 . 
         [0061]    Furthermore, clip grooves  108 , in which portions of the ball clips  82   a ,  82   b  are inserted, are formed on sides of the retaining holes  104   a ,  104   b . The clip grooves  108  are formed with L-shapes in cross section, so as to connect with the lower surface from the other end surface of the main body portion  92 , and end portions of the clip grooves  108  communicate with the ball circulation grooves  80 , whereas on other end portions thereof, clip holes (not shown) are formed, which are recessed toward the one end surface side of the main body portion  92 . The clip grooves  108  are formed with a width dimension that corresponds to the thickness of the ball clips  82   a ,  82   b , to be described below. 
         [0062]    The ball clips  82   a ,  82   b , for example, are formed from a metal material such as stainless steel or the like, each of which comprises a first straight portion  112  formed in a straight line, and a pair of first bent portions  114   a ,  114   b  formed on opposite ends of the first straight portion  112 . The first bent portions  114   a ,  114   b  are formed perpendicularly with respect to the first straight portion  112 . One of the first bent portions  114   a  and the other of the first bent portions  114   b  are bent in the same direction with respect to the first straight portion  112 , and are formed substantially in parallel mutually with one another. 
         [0063]    Further, on ends of the first bent portions  114   a ,  114   b , engagement parts  116  are formed, respectively, which are folded again perpendicularly from the ends and extend in directions to approach one another mutually. The engagement parts  116  are formed substantially in parallel with the first straight portion  112 , and with a predetermined length with respect to the first bent portions  114   a ,  114   b.    
         [0064]    Furthermore, on the ball clips  82   a ,  82   b , the distance in the longitudinal direction (the directions of arrows A and B) between one of the first bent portions  114   a  and the other of the first bent portions  114   b  is substantially the same or slightly smaller than the length in the longitudinal direction (the directions of arrows A and B) of the pair of cover blocks  78   a ,  78   b  and the guide block  76 . 
         [0065]    In addition, the first straight portions  112  of the ball clips  82   a ,  82   b  are installed on the lower surface side of the guide block  76 , and are inserted inside the ball circulation grooves  80 . In greater detail, the first straight portions  112  are arranged in the ball circulation grooves  80  on a central side of the guide block  76 , and at locations proximate to the lower surface of the guide block  76  (see  FIG. 7 ). 
         [0066]    More specifically, the first straight portions  112  retain the balls  58  by abutment against outer circumferential surfaces of the balls  58  in the ball circulation grooves  80 , and the balls  58  are retained in the ball circulation grooves  80  in a condition that prevents falling out of the balls  58  in a direction away from the ball circulation grooves  80 . 
         [0067]    Stated otherwise, the first straight portions  112  serve to guide the plural balls  58  in the ball circulation grooves  80  so as to be freely circulatable along the longitudinal direction (the directions of arrows A and B) of the guide block  76 . 
         [0068]    Further, in a condition in which the pair of cover blocks  78   a ,  78   b  is disposed on both end surfaces of the guide block  76 , opposite ends of the first straight portions  112  and the first bent portions  114   a ,  114   b  of the ball clips  82   a ,  82   b  are inserted into the clip grooves  108  of the cover blocks  78   a ,  78   b , together with the engagement parts  116  being inserted respectively into the clip holes. Consequently, simultaneously with the ball clips  82   a ,  82   b  being retained with respect to the guide block  76  and the cover blocks  78   a ,  78   b , the guide block  76  is sandwiched between the pair of cover blocks  78   a ,  78   b  while being biased by the ball clips  82   a ,  82   b  in a direction to be pulled mutually toward each other, so that the cover blocks  78   a ,  78   b  are retained in a state of abutment against the guide block  76 . 
         [0069]    Stated otherwise, the ball clips  82   a ,  82   b  are doubly equipped with a function to retain the plural balls  58  with respect to the guide block  76 , and with a function to retain the pair of cover blocks  78   a ,  78   b  with respect to both ends of the guide block  76 . 
         [0070]    The cover clips  84   a ,  84   b , for example, are formed from a metal material such as stainless steel or the like, each of which comprises a second straight portion  118  formed in a straight line, and a pair of second bent portions  120   a ,  120   b  formed on both ends of the second straight portion  118 . The cover clips  84   a ,  84   b , for example, are formed by wires having a thickness greater than that of the ball clips  82   a ,  82   b.    
         [0071]    The second bent portions  120   a ,  120   b  are formed substantially perpendicular to the second straight portion  118 , and include bulging portions  122  that bulge outwardly in the direction of extension (the directions of arrows A and B) of the second straight portion  118  in the vicinity of the ends thereof. One of the second bent portions  120   a  and the other of the second bent portions  120   b  are bent in the same direction with respect to the second straight portion  118 , and are formed substantially in parallel mutually with one another. 
         [0072]    The bulging portions  122 , for example, are arc-shaped in cross section, bulging outwardly such that the bulging portion  122  on one of the second bent portions  120   a  and the bulging portion  122  on the other of the second bent portions  120   b  approach one another mutually. 
         [0073]    Furthermore, on the cover clips  84   a ,  84   b , the distance in the longitudinal direction (the directions of arrows A and B) between one of the second bent portions  120   a  and the other of the second bent portions  120   b  is substantially the same or slightly smaller than the length in the longitudinal direction of the pair of cover blocks  78   a ,  78   b  and the guide block  76 . 
         [0074]    The cover plates  86   a ,  86   b , for example, are formed by press forming a plate-like member made from a metal material. The cover plates  86   a ,  86   b  are installed, as one pair each, respectively, on the other side surfaces of the main body portions  92  on one and the other of the cover blocks  78   a ,  78   b.    
         [0075]    The cover plates  86   a ,  86   b  are formed in stepped shapes corresponding to the other side surfaces of the cover blocks  78   a ,  78   b , and comprise insertion parts  124  formed on one portion thereof, which are inserted into the retaining holes  104   a ,  104   b , and pin holes  126  formed on another portion thereof, on outer sides in the widthwise direction of the cover plates  86   a ,  86   b  with respect to the insertion parts  124 . 
         [0076]    The insertion parts  124  project outwardly in semispherical shapes on one side surface in the thickness direction of the cover plates  86   a ,  86   b , and are recessed inwardly on the other side surface thereof. 
         [0077]    In addition, when the cover plates  86   a ,  86   b  are mounted on the cover blocks  78   a ,  78   b , the cover plates  86   a ,  86   b  are placed in abutment against the other end surfaces of the cover blocks  78   a ,  78   b , and the positioning pins  106   a ,  106   b  are inserted in the pin holes  126 , together with the insertion parts  124  being inserted into the retaining holes  104   a ,  104   b . Consequently, the pairs of cover plates  86   a ,  86   b  are mounted in a properly positioned condition with respect to the other end surface of the guide block  76 . 
         [0078]    Further, in a state in which the pair of cover blocks  78   a ,  78   b  have been mounted on both end surfaces of the guide block  76 , installation is carried out from the upper side of the guide block  76 . At this time, the second straight portions  118  of the cover clips  84   a ,  84   b  are placed in abutment against the upper surface of the guide block  76  in contact with the projection  90 . In addition, the bulging portions  122  are inserted respectively into the recesses formed on the other side surfaces of the insertion parts  124 . 
         [0079]    Consequently, in a state in which the cover plates  86   a ,  86   b  are in abutment against the cover blocks  78   a ,  78   b , and the cover blocks  78   a ,  78   b  are in abutment against end surfaces of the guide block  76 , the cover plates  86   a ,  86   b , the cover blocks  78   a ,  78   b , and the guide block  76  are retained by the pair of cover clips  84   a ,  84   b , so as not to be displaceable relatively in the longitudinal direction (the directions of arrows A and B). Stated otherwise, the cover clips  84   a ,  84   b , the cover plates  86   a ,  86   b , the cover blocks  78   a ,  78   b , and the guide block  76  are maintained in a connected condition in the longitudinal direction (the directions of arrows A and B) of the guide block  76 . 
         [0080]    At this time, the second straight portions  118  of the cover clips  84   a ,  84   b  are arranged in an exposed manner on an upper portion of the guide block  76 , whereas the first straight portions  112  of the ball clips  82   a ,  82   b  are arranged in an exposed manner on a lower portion of the guide block  76 . 
         [0081]    The linear actuator  10  according to the first embodiment of the present invention basically is constructed as described above. Next, an explanation shall be given concerning assembly of the guide mechanism  16  of the linear actuator  10 . 
         [0082]    At first, from the state shown in  FIG. 10 , after the plural balls  58  have been installed in the second ball guide grooves  88  and the ball circulation grooves  80  of the guide block  76 , the cover blocks  78   a ,  78   b  are placed in abutment against both end surfaces of the guide block  76 . In this case, the balls  58  installed in the second ball guide grooves  88  are retained to prevent their falling out from the guide block  76  by a non-illustrated jig or the like. 
         [0083]    Next, the pair of ball clips  82   a ,  82   b  is brought into proximity with respect to the guide block  76 , and after the first straight portions  112  thereof have been inserted inside the pair of ball circulation grooves  80 , the first bent portions  114   a ,  114   b  are inserted into the clip grooves  108 , together with the engagement parts  116  thereof being inserted respectively in the clip holes. As a result, the plural balls  58  in the ball circulation grooves  80  are retained by the first straight portions  112  of the ball clips  82   a ,  82   b , so that the balls  58  cannot fall out from the open ball circulation grooves  80 . 
         [0084]    Further, concerning the pair of cover blocks  78   a ,  78   b , due to the engagement parts  116  of the pair of ball clips  82   a ,  82   b  being engaged with the clip holes, and the first bent portions  114   a ,  114   b  being inserted into the clip grooves  108 , the cover blocks  78   a ,  78   b  are pressed respectively toward the sides of the guide block  76 . Owing thereto, the cover blocks  78   a ,  78   b  are retained against both end surfaces of the guide block  76  by the ball clips  82   a ,  82   b . Stated otherwise, the ball clips  82   a ,  82   b  pull on one of the cover blocks  78   a  and the other of the cover blocks  78   b  in directions to approach one another mutually. 
         [0085]    Lastly, the cover plates  86   a ,  86   b  are mounted respectively on the pair of cover blocks  78   a ,  78   b . In this case, first, the insertion parts  124  of the cover plates  86   a ,  86   b  are inserted into the retaining holes  104   a ,  104   b  of the cover blocks  78   a ,  78   b , and the positioning pins  106   a ,  106   b  are inserted into the pin holes  126 , whereby the respective pairs of cover plates  86   a ,  86   b  are positioned with respect to the cover blocks  78   a ,  78   b . In addition, the pair of cover clips  84   a ,  84   b  is arranged in proximity to the guide block  76  and the cover blocks  78   a ,  78   b  from a side opposite from the ball clips  82   a ,  82   b , the second straight portions  118  are placed in abutment against the vicinity of the projection  90 , and the bulging portions  122  are inserted into the insertion parts  124  of the cover plates  86   a ,  86   b.    
         [0086]    Consequently, by operation of both end parts of the pair of cover clips  84   a ,  84   b, a  state is maintained in which the cover plates  86   a ,  86   b  are mounted respectively on the cover blocks  78   a ,  78   b , and both end parts of the ball clips  82   a ,  82   b , which are inserted in the clip grooves  108  and the clip holes, are covered and held by the cover plates  86   a ,  86   b . Also, the cover plates  86   a ,  86   b , the cover blocks  78   a ,  78   b , and the guide block  76  are integrally fixed. 
         [0087]    As a result, in a state in which the cover plates  86   a ,  86   b  abut against the cover blocks  78   a ,  78   b , and the cover blocks  78   a ,  78   b  abut against end surfaces of the guide block  76 , the cover plates  86   a ,  86   b , the cover blocks  78   a ,  78   b , and the guide block  76  are retained together in an integrally connected fashion by the pair of cover clips  84   a ,  84   b , such that relative displacement thereof in the longitudinal direction (the directions of arrows A and B) cannot occur. 
         [0088]    In addition, the guide mechanism  16 , which is assembled as described above, is fixed to an upper part of the cylinder main body  12  through the connecting bolts  22 . 
         [0089]    In the foregoing manner, with the first embodiment, in the guide mechanism  16  that constitutes the linear actuator  10 , by using the pair of ball clips  82   a ,  82   b , the plural balls  58  can be retained in a freely circulatable condition with respect to the guide block  76 , and the pair of cover blocks  78   a ,  78   b , which are in abutment against both ends of the guide block  76 , can be fixed easily by the pair of cover clips  84   a ,  84   b . As a result, for example, compared to a case of being assembled mutually using bolts or the like, the guide mechanism  16  including the guide block  76  and the cover blocks  78   a ,  78   b , etc., can be assembled more easily, and the structure thereof can be simplified. Therefore, in a linear actuator  10  that includes the guide mechanism  16 , the number of manufacturing steps can be reduced. 
         [0090]    Further, in the guide block  76 , instead of providing penetrating holes through which the balls  58  circulate, a structure is provided in which ball circulation grooves  80 , which open downwardly, are formed, and the balls  58  are made to circulate through the ball circulation grooves  80 . Owing thereto, compared to the case of forming penetrating holes therein, the number of process steps as well as processing costs can be reduced. As a result, manufacturing costs for the linear actuator  10  can be reduced. 
         [0091]    Furthermore, since the ball clips  82   a ,  82   b  are elastic, in a state in which the guide block  76  and the cover blocks  78   a ,  78   b  are installed, the ball clips  82   a ,  82   b  are capable of being deformed in a follow-on manner with respect to the outer circumferential surfaces of the plural balls  58 , and the balls  58  can be retained reliably in a condition that enables circulation thereof. 
         [0092]    Further, by installation of the ball clips  82   a ,  82   b  in the clip grooves  108  formed on the cover blocks  78   a ,  78   b , positioning of the ball clips  82   a ,  82   b  can be carried out easily and reliably. Therefore, it is possible for the ball clips  82   a ,  82   b  to always be retained at the same position with respect to the plural balls  58 , and the balls  58  can be retained in a stable manner. 
         [0093]    Furthermore, the bulging portions  122  of the cover clips  84   a ,  84   b  are inserted and placed in engagement with the insertion parts  124  of the cover plates  86   a ,  86   b , whereby falling out of the cover clips  84   a ,  84   b  with respect to the cover plates  86   a ,  86   b  is prevented. As a result, the cover plates  86   a ,  86   b , the cover blocks  78   a ,  78   b , and the guide block  76  can be kept in a connected state stably and reliably by the cover clips  84   a ,  84   b.    
         [0094]    Further still, at the same time that the pair of cover blocks  78   a ,  78   b  is retained by the pair of ball clips  82   a ,  82   b  with respect to the guide block  76 , the cover blocks  78   a ,  78   b  are retained by the pair of cover clips  84   a ,  84   b , which are formed with a wire thickness greater than that of the ball clips  82   a ,  82   b . Therefore, the cover blocks  78   a ,  78   b  can be fixed reliably and firmly with respect to both end surfaces of the guide block  76 . 
         [0095]    Still further, in the guide block  76 , since the ball circulation grooves  80  in which the balls  58  are installed are disposed so as to open on the lower surface side facing the cylinder main body  12 , the balls  58  are retained by the ball clips  82   a ,  82   b , and the ball clips  82   a ,  82   b  are retained by abutment thereof on the upper surface of the cylinder main body  12 . For this reason, even though the ball clips  82   a ,  82   b  may not have high structural integrity, the balls  58  can be retained reliably thereby in cooperation with the upper surface of the cylinder main body  12 . 
         [0096]    Next, operations and effects of the linear actuator  10 , which includes the guide mechanism  16  assembled in the foregoing manner, will be explained. A condition in which the end plate  52  of the slide table  14  abuts against the end surface of the cylinder main body  12 , as shown in  FIGS. 4 and 5 , will be referred to as an initial position. 
         [0097]    First, a pressure fluid from a non-illustrated pressure fluid supply source is introduced to the first port  26 . In this case, the second port  28  is placed in a condition of being open to atmosphere under the operation of a non-illustrated switching valve. 
         [0098]    Pressure fluid supplied to the first port  26  is supplied to one of the penetrating holes  30   a  and also is supplied to the other of the penetrating holes  30   b  through the connecting passages  48 , whereby the pistons  38  are pressed (in the direction of the arrow A) toward the rod holders  46 . Consequently, the slide table  14  is displaced together with the piston rods  40 , which are connected to the pistons  38 , in a direction to separate away from the cylinder main body  12 . 
         [0099]    At this time, the balls  58  of the guide mechanism  16  roll along the ball circulation passage accompanying displacement of the slide table  14 , whereby the slide table  14  is guided in the axial direction by the guide mechanism  16 . 
         [0100]    Then, the end of the stopper bolt  72 , which is provided at one end of the slide table  14 , abuts against the end surface of the guide block  76  of the guide mechanism  16 , and displacement of the slide table  14  is stopped, whereupon the slide table  14  reaches a displacement end position. 
         [0101]    After loosening the lock nut  74  to enable advancing/retracting movement of the stopper bolt  72 , the amount at which the stopper bolt  72  projects from the end surface of the holder portion  64  may be adjusted by threaded-rotation of the stopper bolt  72 , whereby the displacement amount of the slide table  14  can also be adjusted. 
         [0102]    On the other hand, in the case that the slide table  14  is displaced in a direction opposite to the above direction, i.e., in a direction away from the displacement end position, the pressure fluid, which was supplied to the first port  26 , is supplied with respect to the second port  28 , whereas the first port  26  is placed in a state of being open to atmosphere. As a result, by means of the pressure fluid, which is supplied into the pair of penetrating holes  30   a ,  30   b  from the second port  28 , the pistons  38  are displaced in a direction to separate away from the rod holders  46  (in the direction of the arrow B), and the slide table  14  is displaced through the pistons  38  together with the piston rods  40  in a direction to approach the cylinder main body  12 . In addition, by abutment of the damper  70 , which is disposed on the end plate  52  of the slide table  14 , against the end surface of the cylinder main body  12 , the initial position is restored. 
         [0103]    With the above-described guide mechanism  16 , although a structure is provided that is equipped, respectively, with the ball clips  82   a ,  82   b  for retaining the plural balls  58  and the pair of cover blocks  78   a ,  78   b , and the cover clips  84   a ,  84   b  for retaining the cover blocks  78   a ,  78   b  and the cover plates  86   a ,  86   b , the guide mechanism  16  is not limited to such a structure. For example, the guide mechanism  16  may be of a structure in which the balls  58  are retained together with the cover blocks  78   a ,  78   b  only by the ball clips  82   a ,  282   b , and which does not include the cover plates  86   a ,  86   b  and the cover clips  84   a ,  84   b . In this case, the number of parts of the guide mechanism  16  and the number of assembly steps can be reduced. 
         [0104]    Next, a linear actuator  150  according to a second embodiment is shown in  FIGS. 11 and 12 . Constituent elements thereof, which are the same as those found in the linear actuator  10  according to the aforementioned first embodiment, are denoted by the same reference characters, and detailed description of such features is omitted. 
         [0105]    As shown in  FIGS. 11 and 12 , the linear actuator  150  according to the second embodiment differs from the linear actuator  10  according to the first embodiment, in that, in the cylinder main body  12  and the guide mechanism  16 , lubricating oil supply portions  152  are provided, which are capable of supplying lubricating oil, for example, grease or the like, to the balls  58  of the ball circulating grooves  80 . 
         [0106]    The lubricating oil supply portions  152  are constituted from first supply grooves  154  formed in the upper surface of the cylinder main body  12 , and which extend in a direction perpendicular to the longitudinal direction (the directions of arrows A and B) of the cylinder main body  12 , and second supply grooves  156 , which communicate with the first supply grooves  154  and are formed in a lower surface of the guide block  76  of the guide mechanism  16 . 
         [0107]    The first supply grooves  154 , for example, are disposed in plurality (e.g., four grooves) and are separated at predetermined intervals along the longitudinal direction (the directions of arrows A and B) of the cylinder main body  12 . The first supply grooves  154  are formed, respectively, on opposite sides centrally about the recess  20  of the cylinder main body  12 . Additionally, the first supply grooves  154  are recessed at a predetermined depth with respect to the upper surface of the cylinder main body  12 , and extend from both side surfaces of the cylinder main body  12  to positions facing the ball circulation grooves  80  of the guide block  76 . 
         [0108]    The second supply grooves  156  are disposed in plurality similar to the first supply grooves  154 , and extend perpendicularly to the longitudinal direction (the directions of arrows A and B) of the guide block  76 , such that when the guide mechanism  16  is assembled on the cylinder main body  12 , the second supply grooves  156  are formed along straight lines together with the first supply grooves  154 . Further, the second supply grooves  156  extend from both side surfaces of the guide block  76  to the pair of ball circulation grooves  80 , and ends of the second supply grooves  156  are formed in facing relation to the balls  58  that are installed in the ball circulation grooves  80 . 
         [0109]    By supply of lubricating oil from ends of the first supply grooves  154  under operation of a non-illustrated lubricating oil supply means (e.g., a grease injection tool), the lubricating oil is introduced and directed along the first supply grooves  154  to the side of the guide mechanism  16 . Also, part of the oil is introduced from the first supply grooves  154  to the second supply grooves  156 . Consequently, lubricating oil is supplied to the interior of the ball circulation grooves  80  and the plural balls  58  are lubricated. 
         [0110]    More specifically, by supply of lubricating oil on a regular basis to the lubricating oil supply portions  152 , lubrication of the plural balls  58  can be carried out easily and reliably, and by ensuring that the balls  58  are circulated smoothly, the slide table  14  can be displaced in a smooth manner, and durability of the balls  58  can be enhanced. 
         [0111]    Further, when lubrication of the balls  58  is carried out, since such lubrication can be carried out easily through the lubricating oil supply portions  152  without requiring disassembly of the linear actuator  150 , ease of maintenance on the linear actuator  150  can be enhanced. 
         [0112]    Furthermore, in the guide block  76 , since the ball circulation grooves  80  in which the balls  58  are installed are disposed so as to open on the lower surface side facing the cylinder main body  12 , by disposing the first supply grooves  154  on the upper surface side of the cylinder main body  12 , lubricating oil can be supplied easily to the ball circulation grooves  80 . 
         [0113]    Further still, the guide mechanism  16  having the aforementioned structure is not limited to a case in which a fluid pressure cylinder is used that drives the slide table  14  through the cylinder mechanisms  42  and the guide mechanism  16 , the cylinder mechanisms  42  being displaced by a pressure fluid supplied to the cylinder main body  12 , as in the linear actuators  10 ,  150  according to the first and second embodiments. For example, the principles of the present invention may be applied to an electric actuator comprising the above-described guide mechanism  16 , and in which the slide table  14  is displaced along the cylinder main body  12  by a drive source such as a motor or the like. 
         [0114]    Next, a linear actuator  200  according to a third embodiment is shown in  FIGS. 13 through 19 . Constituent elements thereof, which are the same as those found in the linear actuator  10  according to the aforementioned first embodiment, are denoted by the same reference characters, and detailed description of such features is omitted. 
         [0115]    The linear actuator  200  differs from the linear actuator  10  according to the first embodiment, in that a guide mechanism  206  is used in which the cover plates  86   a ,  86   b  are dispensed with, and which is equipped with ball clips  202   a ,  202   b  having a pair of straight portions  204   a ,  204   b  capable of retaining the balls  58 . 
         [0116]    As shown in  FIGS. 13 through 16 , the guide mechanism  206  includes a wide flat guide block  76 , a pair of cover blocks  208   a ,  208   b  disposed on opposite ends of the guide block  76 , plural balls  58  that circulate in the longitudinal direction of the guide block  76 , the pair of ball clips  202   a ,  202   b  that retain the balls  58  in the ball circulation grooves  80  of the guide block  76 , and a pair of cover clips  84   a ,  84   b  that serve to retain the cover blocks  208   a ,  208   b  with respect to the guide block  76 . 
         [0117]    As shown in  FIG. 17 , the ball clips  202   a ,  202   b  each include the pair of straight portions  204   a ,  204   b  constituted from a wire material, for example, made from a metal material such as stainless steel or the like, and which are formed along straight lines substantially in parallel with each other, a connecting portion  210  that connects ends of the straight portions  204   a ,  204   b , and curved portions  212   a ,  212   b , which are formed on other ends of the straight portions  204   a ,  204   b.    
         [0118]    The straight portions  204   a ,  204   b , for example, are separated at an interval which is substantially the same as the widthwise dimension of the ball circulation grooves  80 , and are formed with a length in the longitudinal direction (the directions of arrows A and B), which is the same or slightly smaller than the length in the longitudinal direction (the directions of arrows A and B) of the pair of cover blocks  208   a ,  208   b  and the guide block  76 . 
         [0119]    The connecting portion  210  is formed with a U-shape in cross section so as to connect an end on one of the straight portions  204   a  and an end on the other of the straight portions  204   b , and the connecting portion  210  is formed perpendicularly to the longitudinal direction of the straight portions  204   a ,  204   b.    
         [0120]    The curved portions  212   a ,  212   b  are formed in a curved fashion with respect to the other ends of the straight portions  204   a ,  204   b  so as to bend toward the one end side (in the direction of the arrow A). One of the curved portions  212   a  and the other of the curved portions  212   b  are formed mutually in parallel. The ball clips  202   a ,  202   b  may be formed, for example, by bending a single wire material by way of press forming or the like. 
         [0121]    The cover blocks  208   a ,  208   b  are disposed respectively on one end side and the other end side of the guide block  76 . On the main body portions of the cover blocks  208   a ,  208   b , respective first clip grooves  214  are formed at positions along a straight line with the ball circulating grooves  80  of the guide block  76 . Together therewith, at positions on the side of the arcuate section with respect to the first clip grooves  214 , a pair of second clip grooves  216  are formed for engagement with the cover clips  84   a ,  84   b.    
         [0122]    The first clip grooves  214  are recessed toward the side of the guide block  76  with respect to the end surfaces of the cover blocks  208   a ,  208   b , with upper portions thereof being inclined toward the side of the guide block  76 . More specifically, as shown in  FIG. 18 , from a lower end to an upper end thereof, the first clip grooves  214  are inclined at a predetermined angle toward the side of the guide block  76 . 
         [0123]    The first clip grooves  214  are separated by a predetermined distance and extend upwardly, and between the first clip grooves  214 , projections  218  are provided with which the connecting portions  210  of the ball clips  202   a ,  202   b  are engaged. Upper ends of the projections  218  are formed in a semicircular shape, for example, and project outwardly at a predetermined height with respect to the first clip grooves  214  toward end surface sides of the cover blocks  208   a ,  208   b.    
         [0124]    The second clip grooves  216  are formed, respectively, to incline upwardly in directions away from the first clip grooves  214 . The second bent portions  120   a ,  120   b  of the cover clips  84   a ,  84   b  are inserted and engaged, respectively, in the second clip grooves  216 . 
         [0125]    Additionally, when the ball clips  202   a ,  202   b , and the cover clips  84   a ,  84   b  are mounted on the guide mechanism  206 , initially, in a state in which the guide block  76  is placed with the ball circulation grooves  80  oriented upwardly, and the plural balls  58  are installed in the ball circulation grooves  80 , the straight portions  204   a ,  204   b  of the ball clips  202   a ,  202   b  are placed in openings  220  of the ball circulation grooves  80  in abutment against outer circumferential surfaces of the balls  58  (see  FIG. 19 ). 
         [0126]    Next, the cover blocks  208   a ,  208   b  are disposed respectively on both end surfaces of the guide block  76 , and the second bent portions  120   a ,  120   b  of the cover clips  84   a ,  84   b  are engaged respectively in the second clip grooves  216 , whereby the pair of cover blocks  208   a ,  208   b  is retained on both ends of the guide block  76 . Simultaneously, the connecting portions  210  of the ball clips  202   a ,  202   b  are made to engage with the projections  218  on one of the cover blocks  208   a , and the curved portions  212   a ,  212   b  are made to engage with the first clip grooves  214  on the other of the cover blocks  208   b.    
         [0127]    Consequently, the pair of ball clips  202   a ,  202   b  is retained on the pair of cover blocks  208   a ,  208   b  via the connecting portions  210  and the curved portions  212   a ,  212   b , and the plural balls  58  are kept in a retained state in the ball circulation grooves  80  by the straight portions  204   a ,  204   b . That is, an assembled condition is brought about, in which the pair of ball clips  202   a ,  202   b  and the cover clips  84   a ,  84   b  are attached with respect to the guide mechanism  206 . 
         [0128]    In the foregoing manner, with the linear actuator  200  according to the third embodiment, since the cover plates  86   a ,  86   b , which are used in the linear actuator  10  according to the aforementioned first embodiment, are unnecessary, the number of parts can be reduced and the linear actuator  200  can be made smaller in scale. Together therewith, by reducing the number of assembly steps, ease of manufacturing is enhanced and manufacturing costs can be reduced. 
         [0129]    Further, since the balls  58 , which are installed in the ball circulation grooves  80  of the guide block  76 , are retained in a movable state along the direction of travel of the balls by the pair of straight portions  204   a ,  204   b , the balls  58  can be retained securely and can reliably be prevented from dropping out from the ball circulation grooves  80 . Furthermore, due to retention of the balls  58  by the pair of straight portions  204   a ,  204   b , for example, compared to a situation in which the balls  58  are retained with respect to the ball circulation grooves  80  by a single straight portion, since sliding resistance of the balls  58  can be decreased, upon movement of the slide table  14 , the balls  58  can be made to roll more smoothly along the ball circulation grooves  80 . 
         [0130]    Furthermore, since the balls  58  are retained reliably in the ball circulation grooves  80  by the ball clips  202   a ,  202   b , the openings  220  of the ball circulation grooves  80  can be formed with a large size, and ease of manufacturing can be enhanced, for example, in the case that the guide block  76  equipped with the ball circulation grooves  80  is formed by a drawing process or by forging or the like. 
         [0131]    Further, the aforementioned ball clips  202   a ,  202   b  are not limited to the case of being formed from a wire material as shown in  FIG. 17 . According to a modification, as shown in  FIG. 20 , a ball clip  230  may be formed by press forming a plate-like member, which is made from a metal material or a resin material, for example. 
         [0132]    The ball clip  230  comprises a ball retaining portion  232  having a predetermined width and which extends in the longitudinal direction for retaining the balls  58 , and a pair of respective bent portions  234   a ,  234   b , which are bent at substantially right angles on opposite ends of the ball retaining portion  232 . The ball retaining portion  232  is formed, for example, with a flat shape, and filling grooves  236 , which are filled with lubricating oil such as grease or the like, are formed in a central part of the ball retaining portion  232 . The filling grooves  236  are disposed in plurality separated mutually in the longitudinal direction of the ball retaining portion  232 , and are recessed a predetermined depth with respect to the surface of the ball retaining portion  232 . 
         [0133]    Upper ends of the respective bent portions  234   a ,  234   b  are formed with semicircular shapes in cross section, and in the centers thereof, frame-like shapes are provided in which engagement holes  238  are formed. The bent portions  234   a ,  234   b  are formed with substantially the same widthwise dimension as the ball retaining portion  232 . 
         [0134]    In addition, when the aforementioned ball clips  230  are installed on the guide block  76  and the cover blocks  208   a ,  208   b , in a state in which the balls  58  are installed in the ball circulation grooves  80  of the guide block  76 , after the surfaces of the ball retaining portions  232 , i.e., the surfaces on the side of the filling grooves  236 , have been arranged to face toward the balls  58 , the bent portions  234   a ,  234   b  are inserted respectively into the first clip grooves  214  and the second clip grooves  216  of the cover blocks  208   a ,  208   b , and the engagement holes  238  are engaged with the projections  218 . 
         [0135]    As a result, a condition is brought about in which the ball clips  230  are retained on the pair of cover blocks  208   a ,  208   b  through the pair of bent portions  234   a ,  234   b , and the plural balls  58  are retained inside the ball circulation grooves  80  by the ball retaining portions  232 . In addition, because the filling grooves  236 , which are filled with lubricating oil, are arranged in positions facing toward the plural balls  58 , when the balls  58  roll along the ball circulation grooves  80 , the balls  58  can be lubricated suitably by the lubricating oil. 
         [0136]    In this manner, as a result of forming the ball clips  230  by press forming a plate-like material, the production cost of the ball clips  230  can be reduced, and ease of manufacturing thereof can be enhanced. Further, by forming the ball retaining portions  232  as flat plates, the filling grooves  236 , which are capable of being filled with lubricating oil, can be formed therein, and thus the ball retaining portions  232  can be equipped with a lubricating function for the balls  58 . 
         [0137]    The linear actuator according to the present invention is not limited to the embodiment described above, but various alternative or additional features and structures may be adopted without deviating from the essence and scope of the invention as set forth in the appended claims.