Patent Publication Number: US-2007096598-A1

Title: Drive device for ultrasonic linear motor

Description:
TECHNICAL FIELD  
      This invention relates to a drive device of an ultrasonic linear motor in which with the vibration of ultrasonic vibrators as a drive source a rail and a base body move relatively with respect to a length direction.  
     BACKGROUND ART  
      An ultrasonic linear motor of this kind is made up of a base body having a piezoelectric device as a drive source in its center and having sliding parts at its ends, and a rail supporting the sliding parts of this base body. An ultrasonic linear motor in which the sliding parts of the base body are abutted with this rail and the base body moves relative to the rail is disclosed for example in JP-A-6-6989. The ultrasonic linear motor disclosed in this publication will be described on the basis of  FIG. 10 .  
      The ultrasonic linear motor shown in  FIG. 10  is made up of a rail  101 , a base body  103  made of an elastic material and placed on the rail  101 , piezoelectric devices  104 ,  104  mounted on the top of the base body  103  with a predetermined spacing, and sliding parts  105 ,  105  provided on the left and right of the base body  103 .  
      The base body  103  includes the piezoelectric devices  104 ,  104  and the sliding parts  105 ,  105 . The base body  103  doubles as an ultrasonic vibrator  112 .  
      Guiding of the base body  103  with respect to the rail  101  is carried out by the guide mechanism  110 .  
      The guide mechanism  110  is made up of groove parts  106 ,  106  formed in the sides of the rail  101 , a linear guide member  107  provided slidably along the groove parts  106 , projections  108  projecting from the linear guide member  107 , projections  109  projecting from the base body  103 , and springs  111  extending between the projections  108  and the projections  109 .  
      When a voltage having a predetermined frequency is applied to the piezoelectric devices  104 ,  104  provided on the base body  103 , the piezoelectric devices  104 ,  104  vibrate. Vibration occurring at the left and right sliding parts  105 ,  105  causes the base body  103  (the ultrasonic vibrator  112 ) to move along the rail  101  in the length direction.  
      However, because the load of the base body  103  (the ultrasonic vibrator  112 ) is born by the sliding parts  105 ,  105 , with use over a long period wear of the sliding parts becomes large. In particular, in an environment such as a factory where a lot of foreign matter such as dust floats around, there is the problem that with use over a long period wear proceeds quickly, and lifespan becomes short.  
      Consequently, maintenance such as regularly replacing the base body  103  (the ultrasonic vibrator  112 ) becomes necessary. Accordingly, an ultrasonic linear motor with superior durability has been awaited.  
     DISCLOSURE OF THE INVENTION  
      The present invention provides a drive device of an ultrasonic linear motor in which a rail and a base body are driven movably relative to one another by a driving part interposed between the rail and the base body, the driving part including: at least a left-right pair of rollers making contact with side faces of the rail; at least a left-right pair of ultrasonic vibrators for applying a turning force individually to each of the pair of rollers; and urging means for urging the ultrasonic vibrators and the rollers toward the side faces of the rail.  
      Because in this invention the driving parts are interposed between the rail and the base body like this, the driving forces produced by the driving parts act from the base body toward the side faces of the rail. Because the urging means urge the ultrasonic vibrators and the rollers toward the side faces of the rail, by adjusting the urging force it is possible to set the urging force on the rail to an optimal value, and an optimal driving torque can be obtained. Also, because the ultrasonic vibrators constituting the drive source are provided left-right symmetrically about the width-direction center of the base body, the driving force can be made strong stably, the base body can be driven with good left-right balance, and the base body can be moved strongly and smoothly.  
      In the drive device of the invention, preferably, an ultrasonic vibrator and rollers and urging means are received en bloc in a holding frame and fitted to the base body via this holding frame, and the rollers are removably mounted to the holding frame. When the rollers are provided removably on the holding frame like this, even when as a result of use in an environment such as a factory where a lot of foreign matter such as dust floats around the frequency of replacement of rollers becomes high due to wear and deterioration, the rollers can be replaced easily. As a result, the maintainability of the rollers can be increased.  
      The rail preferably has an upper face for bearing the load of the base body and sloping side faces formed on its left and right sides, the base body has opposing faces facing the sloping side faces, and the rail is gripped by rollers provided on the opposing faces and making contact with the sloping side faces and by the bottom face of the base body. Because the rollers are directed diagonally upward into contact with the sloping side faces by the urging means like this and the load of the base body is born by the top face side of the rail, the rail is gripped by the rollers and the bottom face of the base body. Consequently, looseness of the base body with respect to the rail in the up-down direction and the left-right direction is prevented from arising.  
      In addition, because most of the load of the base body is born by the top face of the rail, the load of the base body does not readily act on the ultrasonic vibrators of the driving parts. As a result, wear of the ultrasonic vibrators of the driving parts is greatly reduced and the durability of the ultrasonic vibrators increases. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a sectional view showing a drive device of an ultrasonic linear motor according to a first embodiment of the invention;  
       FIG. 2  is a sectional view on the line  2 - 2  in  FIG. 1 ;  
       FIG. 3  is a perspective view of one driving part of the first embodiment shown in  FIG. 1 ;  
       FIG. 4A ,  FIG. 4B  and  FIG. 4C  are operation views of the driving part shown in  FIG. 3 ;  
       FIG. 5  is a perspective view showing a drive device of an ultrasonic linear motor according to a second embodiment of the invention;  
       FIG. 6  is a sectional view on the line  6 - 6  in  FIG. 5 ;  
       FIG. 7  is an exploded perspective view of a driving part of the second embodiment shown in  FIG. 6 ;  
       FIG. 8  is a view illustrating loads of a driving part on a rail of the second embodiment shown in  FIG. 6 ;  
       FIG. 9  is a sectional view showing a variation of the drive device of the second embodiment shown in  FIG. 6 ; and  
       FIG. 10  is a view showing an ultrasonic linear motor of related art. 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION  
       FIG. 1 ,  FIG. 2  and  FIG. 3  show a linear motor drive device  10  and driving parts  20  according to a first embodiment of the invention. The drive device  10  of the first embodiment is made up of a rail  11 , a base body  12  movably mounted on the rail  11 , and left and right driving parts  20 ,  20  provided on the base body  12 .  
      The base body  12  is sectionally U-shaped and has left and right leg parts  12   a ,  12   a  and surrounds the top face  11   a  and the side faces  11   b ,  11   c  of the rail  11  and is loaded on the top face  11   a  of the rail  11  via multiple bar-shaped bearings  13 . The leg parts  12   a ,  12   a  of the base body  12  have holding holes  14 ,  14  for holding the driving parts  20 ,  20 . The holding holes  14 ,  14  have open sides facing the side faces  11   b ,  11   c  of the rail and are formed in the length direction of the base body  12 . The holding holes  14  are made up of first holding holes  14   a  positioned in the inner sides of the leg parts  12   a  of the base body  12  and second holding holes  14   b  positioned nearer the outer side faces of the leg parts  12   a.    
      The multiple bearings  13  are disposed on the bottom face  12   b  of the base body so as to have a predetermined spacing in the length direction, and make the movement of the base body  12  relative to the rail  11  smooth. The rail  11  bears the load of the base body  12  on the rail top face  11   a  via the bearings  13 .  
      Although the ultrasonic linear motor drive device  10  of the first embodiment shown in  FIG. 1  and  FIG. 2  is an example wherein driving parts  20 ,  20  are provided on a base body  12  and the base body  12  is movable with respect to the rail  11 , alternatively the base body  12  may be fixed and the rail  11  made movable with respect to the base body  12 . That is, the rail  11  and the base body  12  may be constructed so as to be movable relative to each other.  
      The driving parts  20 ,  20  create driving forces for driving the base body  12 . These driving parts  20 ,  20  are interposed between the left and right side faces  11   b ,  11   c  of the rail  11 , which constitute guide faces, and the leg parts  12   a ,  12   a  of the base body  12 . Each of the driving parts  20  has an ultrasonic vibrator  21 , an urging member  22 , and multiple rollers  25 .  
      Each ultrasonic vibrator  21  is made up of a piezoelectric device  21   a  and a vibrating elastic member  21   b , and produces a vibration and applies a turning force to the rollers  25 . This ultrasonic vibrator  21  is received in a sectionally U-shaped holder member  23  as shown in  FIG. 3 . The piezoelectric device  21   a  and the vibrating elastic member  21   b  are affixed and disposed so that the piezoelectric device  21   a  is on the side of the side wall  23   a  of the holder member  23 .  
      The holder members  23  are received in the first holding holes  14   a  of the holding hole parts  14  formed in the leg parts  12   a  of the base body  12 . The urging members  22 ,  22  are interposed between the holder members  23  and opposing faces  12   c ,  12   d  (base body side faces  28 ) of the base body forming the second holding hole parts  14   b . The holder members  23  are urged by the urging members  22  so that they press against the guiding side faces  11   b ,  11   c  of the rail  11 .  
      Each of the holder members  23  has multiple support plates  26  fixed cantilever-style to an upper wall  23   b  and a lower wall  23   c  by flush screws  32 ,  32  as shown in  FIG. 3 . The support plates  26  extend toward the guiding side faces  11   b ,  11   c  of the rail  11  so as to project from the edges of the upper wall  23   b  and lower wall  23   c.    
      The rollers  25  are rotatably mounted between the upper and lower support plates  26 ,  26  on shaft members  27 . These rollers  25  can be removed from the upper and lower support plates  26 ,  26  by the flush screws  32 ,  32  being removed. That is, the rollers  25  are removable and replaceable with respect to the holder members  23 . The rollers  25  are pressed into contact with the guiding side faces  11   b ,  11   c  of the rail  11  by the urging means  22  via the holder members  23 . Accordingly, the base body  12  moves in a straight line along the guiding side faces  11   b ,  11   c  of the rail  11  on the multiple rollers  25  rotating with the vibration of the left and right ultrasonic vibrators  21 ,  21  as drive sources.  
      The holder members  23 ,  23  holding the ultrasonic vibrators  21 ,  21  are slidable with respect to the first holding hole parts  14   a  but are stopped by stoppers  29 ,  29  provided at the length-direction ends of the base body  12  as shown in  FIG. 2 . The stoppers  29 ,  29  are fixed to the ends of the base body  12  by multiple bolts  29   a.    
      As explained above, on the rail  11 , the face that bears the load of the base body  12  and the faces that make the base body  12  driveable are different. In other words, the load of the base body  12  is just supported by the rail top face  11   a  via the multiple bearings  13 , and this load does not act on the guiding side faces  11   b ,  11   c  of the rail  11 .  
      The reference letter E denotes an encoder for detecting the position of the base body  12  with respect to the rail  11 .  
      The material of the rollers  25  is preferably steel, but there is no limitation to this and alternatively they may be aluminum or a resin material.  
      Although in this embodiment as the urging means  22  for urging the ultrasonic vibrators  21  the example of springs has been shown, they may alternatively be elastic members such as rubber members or resin members.  
      Although in the first embodiment an example has been shown wherein the holder member  23  having received the ultrasonic vibrator  21  is fitted directly in the holding hole  14  formed in the base body  12 , alternatively a sectionally U-shaped holding frame  54 , which will be described as a second embodiment with reference to  FIG. 6 , may be interposed between as it is assembled to the holding hole  14 .  
      A piezoelectric device  21   a  for forward use and a piezoelectric device  21   a  for reverse use are combined to make one segment. As shown in  FIG. 4A , the ultrasonic vibrator  21  is made up of four segments, and in  FIG. 2  vibrating elastic members  21   b ,  21   b ,  21   b ,  21   b  constituting four segments are shown. Four rollers  25  are provided so as to be adjacent to the vibrating elastic members  21   b ,  21   b ,  21   b ,  21   b  constituting four segments. The ultrasonic vibrators  21  are connected to a control circuit not shown in the drawings.  
      Although in the example shown in  FIG. 1  an example in which the ultrasonic vibrators  21  are mounted to the base body  12  via holder members  23  has been shown, alternatively the ultrasonic vibrators  21 ,  21  may be fitted in a base body  12  that doubles as a holding frame with the left and right leg parts  12   a ,  12   a  of the base body  12  as the holding frame, and the rollers  25  rotatably mounted adjacent to the respective ultrasonic vibrators  21 ,  21 .  
      When a single-phase a.c. voltage is applied to an ultrasonic vibrator  21 , the ultrasonic vibrator  21  vibrates in the form of a wave, and an elliptical motion accompanying this vibration and the progress of the wave arises. The rollers  25  are rotated by this elliptical motion of the ultrasonic vibrator  21 . The direction of the rotation of the rollers  25  is made forward or reverse by the direction of the single-phase a.c. voltage applied to the piezoelectric devices  21   a  (see  FIG. 1 ) being changed via the control circuit not shown.  
      The driving part  20  shown in  FIG. 3  drives the base body  12  shown in  FIG. 1 .  
      Because as explained above the rollers  25  are removable and replaceable with respect to the holder members  23  holding the ultrasonic vibrators  21 , when the device is used in an environment such as a factory where a large amount of foreign matter such as dust floats around, even if wear and deterioration of the rollers  25  becomes severe and the frequency of replacement of the rollers  25  becomes high, they can be replaced easily. As a result, the maintainability of the rollers  25  can be greatly increased.  
      Next, the operating principle by which the multiple rollers  25  adjacent to the ultrasonic vibrators  21  rotate will be explained, on the basis of  FIG. 4A ,  FIG. 4B  and  FIG. 4C .  
      As mentioned above, in the first embodiment, the piezoelectric devices  21   a  of the ultrasonic vibrator  21  constitute four segments. One segment has two polarized regions  21   ap ,  21   an.    
      Rectangular projecting parts  36  are provided on the surface  35  of the vibrating elastic member  21   b , made up of four segments, of the ultrasonic vibrator  21 . The reference symbol Pa shows the positions of the projecting parts  36  provided on the ultrasonic vibrator  21 .  
       FIG. 4A  shows a state in which no a.c. voltage is being applied to the segments of the ultrasonic vibrator  21 , i.e. the two polarized regions  21   ap ,  21   an  of the piezoelectric devices  21   a . In this case, the ultrasonic vibrator  21  does not vibrate and no rotational vibration arises in the projecting parts  36  provided on the surface  35  of the ultrasonic vibrator.  
      As shown in  FIG. 4B , when a single-phase a.c. voltage is applied to the polarized region  21   an  of each of the segments of the ultrasonic vibrator, the surface  35  of the ultrasonic vibrator starts to vibrate, and a wavelike face forms. A counter-clockwise elliptical rotational vibration Pb accompanying the progress of the waveform arises in the projecting parts  36  provided on the surface  35 . Because the projecting parts  36  are in contact with the circumferential faces  37  of the rollers  25 , a clockwise turning force is applied to the rollers  25  by this rotational vibration Pb. Consequently, rotation of the rollers  25  causes a force F 1  toward the right in  FIG. 4B  to act on the driving part  20 .  
      As shown in  FIG. 4C , when a single-phase a.c. voltage is applied to the other polarized region  21   ap  of each of the segments of the ultrasonic vibrator, the surface  35  of the ultrasonic vibrator vibrates in the form of a wave. The phase of the waveform produced in the surface  35  of the ultrasonic vibrator is the opposite phase to that of the waveform shown in  FIG. 4B . At the projecting parts  36  provided on the surface  35 , a clockwise elliptical turning vibration Pc arises accompanying the progress of the waveform. Consequently, the projecting parts  36  cause a counter-clockwise driving force to be applied to the rollers  25 . This driving force on the rollers  25  causes a force F 2  toward the left in  FIG. 4C  to act on the driving part  20 .  
      Thus, with the drive device of this embodiment, the turning direction F 1 , F 2  of the rollers  25  can be changed just by the polarized regions  21   an ,  21   ap  of the piezoelectric devices  21   a  to which the single-phase a.c. voltage is applied being switched.  
      Because as shown in  FIG. 1  the urging members  22 ,  22  are provided between the opposing faces  12   c ,  12   d  of the base body  12  and the holder member  23  on which the rollers  25 ,  25  are mounted, the urging force of the rollers  25 ,  25  on the rail  11  is set to an optimal value. As a result, by means of the driving torque created by the ultrasonic vibrator  21  the base body  12  can provide an optimal driving torque via the rollers  25 ,  25 .  
      Because the ultrasonic vibrators  21  constituting the drive sources are mounted so as to face the rail  11  orthogonally to the axial direction, the driving forces can be strengthened and the base body  12  can be driven with good left-right balance. As a result, strong and smooth movement of the base body  12  can be obtained.  
      Because the load of the base body  12  on the rail  11  is made up of the vertical load supported by the rail top face  11   a  and the urging forces of the urging members  22  of the driving part  20  on the guiding side faces  11   b ,  11   c  of the rail  11 , most of the load of the base body  12  acts on the bottom face  12   b  of the base body  12  only, and the load of the base body does not readily act on the ultrasonic vibrators  21  of the driving units  20 . Consequently, there is the merit that an unnecessary burden does not act on the ultrasonic vibrators  21 . As a result, the durability of the ultrasonic vibrators  21  increases.  
       FIG. 5  to  FIG. 8  show a drive device of an ultrasonic linear motor according to a second embodiment.  
      Like the first embodiment, this linear motor is made up of a rail  42 , a base body  44  and driving parts  50 . The base body  44  is sectionally U-shaped with leg parts  44   a ,  44   a  at its sides. The leg parts  44   a  are formed so as to project outward and have holding hole parts  45  for holding the driving parts  50  in their inner side faces. Multiple bar-shaped bearings  46  are interposed between the top face  42   a  of the rail  42  and the bottom face  44   b  of the base body  44 , and the base body  44  can move smoothly with respect to the rail  42 . The side faces of the rail  42  have sectionally V-shaped guide grooves formed in them, and have left and right sloping side faces  42   b ,  42   c  of these guide grooves sloping downward. The driving parts  50 ,  50  are interposed between these sloping side faces  42   b ,  42   c  and opposing faces  44   c ,  44   d  (see  FIG. 6 ) forming the holding hole parts  45  formed in the left and right leg parts  44   a ,  44   a  of the base body  44  facing the sloping side faces  42   b ,  42   c.    
      In the figures, stoppers mounted at the ends of the base body  44  for fixing the driving parts  50  have been omitted.  
      As shown in  FIG. 6 , the rail  42  is gripped by the bottom face  44   b  of the base body  44  and left and right rollers  51 ,  51  of the driving parts  50 ,  50  mounted on the base body  44 .  
      An ultrasonic vibrator  52  is made up of piezoelectric devices  52   a  and vibrating elastic members  52   b , and has multiple segments as in the first embodiment. The piezoelectric devices  52   a  are made up of piezoelectric devices for forward and for reverse. Holder members  63 ,  63  receive the ultrasonic vibrators  52 ,  52  and have removable rollers  51 ,  51  in the same way as in the first embodiment.  
      Sectionally U-shaped holding frames  54 ,  54  are fitted in the holding hole parts  45 ,  45 . The holding frames  54 ,  54  hold the holder members  63 ,  63  together with urging members  53 ,  53 . The urging members  53 ,  53  urge the rollers  51 ,  51  via the holder members  63 ,  63  into contact with the sloping side faces  42   b ,  42   c . That is, the holding frames  54 ,  54  have the function of holding the rollers  51 ,  51 , the ultrasonic vibrators  52 ,  52 , the urging members  53 ,  53  to the base body  44 .  
      Because the base body  44  grips the rail  42  with the urging forces of the urging member  53 ,  53  urging the rollers  51 ,  51  to make contact facing diagonally upward with the sloping side faces  42   b ,  42   c  formed on the left and right side faces of the rail  42  and the base body  44  load acting on the rail top face  42   a  via the bearings  46 ,  46  like this, the occurrence of up-down and left-right looseness of the base body  44  with respect to the rail  42  is prevented without any other members being added. As a result, there is the merit that the number of parts can be reduced.  
      Also, because the driving parts  50  including the ultrasonic vibrators  52  constituting the drive sources for moving the base body  44  are provided on the inner sides of the leg parts  44   a ,  44   a  of the base body  44 , the drive sources can be disposed in a compact way. For example, whereas a drive mechanism made up of a ball and screw normally has a complicated mechanism and takes up space and is heavy, a drive mechanism of this kind becomes unnecessary. As a result, there is the merit that it is possible to make the drive mechanism light, compact and simple.  
      Although in the second embodiment an example was shown wherein the base body  44  was made movable with respect to the rail  42 , as in the first embodiment the base body  44  may alternatively be fixed and the rail  42  made movable with respect to the base body  44 .  
       FIG. 7  shows a driving part  50  of the second embodiment shown in exploded perspective view.  
      Referring to  FIG. 7 , an ultrasonic vibrator  52  divided into four segments is received in a holder member  63 . Support plates  55 ,  55  are fixed with flush screws  56 ,  56  to side faces of the holder member  63  in correspondence with the segments. Four urging members  53 ,  53  are disposed in four recesses  58  formed in the bottom  57  of the sectionally U-shaped holding frame  54 . The holder member  63  is disposed on these urging members  53 , and the four rollers  51  are supported on the upper ends  55   a ,  55   a  of the support plates  55 ,  55 .  
      The rollers  51  have shaft members  61  and are rotatable with respect to the shaft members  61 . Semi-circular shaft supporting parts  59 ,  59  are formed in the upper ends  55   a ,  55   a  of the support plates  55 ,  55 , and the shaft members  61 ,  61  are supported on the shaft supporting parts  59 ,  59  and the rollers  51  thereby removably supported on the support plates  55 .  
      A projecting part  64  is provided on each of the segments of the ultrasonic vibrator  52 , and the projecting parts  64  are disposed so as to make contact with the rollers  51  as shown in  FIG. 6 .  
      Stoppers  66 ,  66  for stopping the holder member  63  from flying out of the holding frame  54  by more than a predetermined amount due to the urging members  53  are provided at the ends of the holding frame  54 . The stoppers  66 ,  66  have retaining flanges  67 ,  67  projecting toward each other. The holder member  63  and the ultrasonic vibrator  52  are stopped from flying out by these retaining flanges  67 ,  67 . The stoppers  66 ,  66  are fixed to the ends of the holding frame  54  by multiple bolts  68 .  
      Although in the second embodiment shown in  FIG. 6  and  FIG. 7  an example is shown in which the support plates  55 ,  55  are mounted on the holder member  63 , it is also possible to make a single part by providing the support plates  55 ,  55  integrally on the holding frame  54  side. In this case, the rollers  51  are removable and replaceable with respect to the holding frame  54 .  
      When the rollers  51 ,  51  are provided removably with respect to the holding frame  54  like this, in use in an environment such as a factory where a large amount of foreign matter such as dust floats around, even if due to wear and deterioration the frequency of replacement becomes high, the rollers  51 ,  51  can be replaced easily. As a result, the maintainability of the rollers  51 ,  51  can be greatly increased.  
      Although in the second embodiment the shaft members  61  of the rollers  51  were supported on shaft supporting parts  59 ,  59 , alternatively circular holes may be formed in the upper parts of the supporting members as shaft supporting parts, and the shaft members  61  inserted into these holes.  
      Although in the embodiment shown in  FIG. 7  an example was shown wherein the number of urging members  53 ,  53  for urging the ultrasonic vibrator  52  was made four, it may alternatively be two or three, and the number may be set freely. Also, when elastic rubber is used as the urging members  53 ,  53  just one may be used.  
       FIG. 8  is a schematic view illustrating the loads acting on the rail  42  from the base body. The rail  42  has been drawn as an inverted isosceles triangle with the sloping side faces  42   b ,  42   c  of the rail  42 , which are faces that bear loads, made sloping sides  72 ,  72 , and the rail top face  42   a  made a top side  71 .  
      As a result of the force of the load of the base body  44  shown in  FIG. 6  acting on the upper face  71 , which is the rail top face  42   a , through the bearings  46  and the urging forces of the left and right rollers  51 ,  51  acting on the left and right sloping sides  72 ,  72  due to the urging members  53 ,  53  shown in  FIG. 7 , a stable force acts on the rail  42 . Consequently, the base body  44  has good left-right balance and coupled with the bearings  46  moves smoothly with respect to the rail  42  without looseness arising.  
      Also, by the urging force of the urging members  53 ,  53  being adjusted an optimal pressing force (urging force) of the rollers  51 ,  51  on the sloping sides  72 ,  72  can be obtained, and an optimal driving force on the base body  44  can be secured.  
       FIG. 9  shows a variation of the ultrasonic linear motor drive device of the second embodiment shown in  FIG. 6 . Parts the same as in the second embodiment of  FIG. 6  have been given the same reference numerals and their description will be omitted. The point that differs from the second embodiment shown in  FIG. 6  is that instead of bar-shaped bearings  46  provided in the center of the bottom face  44   b  of the base body, ball-shaped bearings  46   a ,  46   a  are provided at the corners of the bottom face  44   b  of the base body. Specifically, ball-shaped bearings  46   a ,  46   a  are provided at or near the points of intersection  74 ,  74  of the bottom face  44   b  of the base body with the inside faces  44   e ,  44   e  of the left and right leg parts  44   a ,  44   a  of the base body  44 . The rest of the construction is the same as in the second embodiment shown in  FIG. 6 .  
      Thus, in the variation shown in  FIG. 9 , because bearings  46   a ,  46   a  are provided at the width-direction ends of the base body bottom face  44   b , the base body  44  is supported at four locations, the load of the base body  44  being born at the top left and right by concave parts  42   d ,  42   d  formed in the left and right corners of the rail top face  42   a  and by the left and right rollers  51 ,  51 , and consequently the occurrence of looseness is better prevented and the base body  44  can move smoothly with respect to the rail  42 . And, the load burden on the rollers  51 ,  51  is also lightened.  
     INDUSTRIAL APPLICABILITY  
      As described above, this invention can be used optimally in parts processing and assembly steps in a factory where a large amount of foreign matter such as dust floats around, in robot hands and jigs of production equipment used in carrying work to processing machines and assembly machines and attaching and removing workpieces, and is also usable in ordinary mechanical equipment such as construction machines and agricultural machines, and can also be used in automobiles and electrical appliances.