Patent Publication Number: US-2006016284-A1

Title: Ball screw apparatus

Description:
The present invention claims foreign priority to Japanese patent application no. 2004-184191, filed on Jun. 22, 2004, the contents of which is incorporated herein by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      This invention relates to an improved ball screw apparatus for converting a rotational motion into a linear motion.  
      2. Description of the Background Art  
      A ball screw apparatus, used in a feed mechanism of a machine tool or the like, has a screw shaft, a nut having a ball screw groove which is formed in an inner peripheral surface thereof and is opposed to a ball screw groove formed in an outer peripheral surface of the screw shaft, and a number of balls which roll in a ball load rolling passageway, formed between the ball screw grooves of the nut and the screw shaft, in accordance of the rotation of the screw shaft or the nut.  
      In such a ball screw apparatus, when the balls are caused to roll along the ball load rolling passageway, for example, by the rotation of the screw shaft, the nut makes a linear motion relative to the screw shaft in an axial direction thereof. In order to continue the relative linear motion of the nut, it is necessary to circulate the balls rolling along the ball load rolling passageway so that the balls can repeatedly roll along the ball load rolling passageway. However, in the type of ball screw apparatus using a ball circulation tube as a part for circulating the balls, there has been encountered a problem that noises, vibration and others are liable to develop as the speed of revolution of the balls increases.  
      Therefore, there has been proposed a construction in which a ball return through hole for circulating the balls is formed in the nut, and extends in an axial direction of the screw shaft; and ball turning members, each having a ball turning passageway communicating with both of a ball load rolling passageway and the ball return through hole, are provided at opposite end portions of the nut, respectively (see Japanese Utility Model Examined Publication JP-UM-B-3034052).  
      In such the ball screw apparatus, the development of noises, vibration and others can be suppressed as compared with the type of ball screw apparatus employing the ball circulation tube. However, when the diameter of the ball return through hole is too large relative to the diameter of the balls, the play of the balls in the ball return through hole (that is, a gap between the ball return through hole and the balls) is large. As a result, the balls are liable to be arranged in a staggered manner in the ball return through hole. In this case, when the screw shaft or the nut is rotated at high speed, the balls in the ball return through hole advances therealong while impinging on the inner surface of the ball return through hole. Accordingly, there is a problem that an acoustic performance is lowered.  
     SUMMARY OF THE INVENTION  
      This invention has been made in view of this problem, and one of objects of the invention is to provide a ball screw apparatus in which a good acoustic performance can be obtained even when the screw shaft or the nut is rotated at high speed.  
      In order to achieve the above object, according to a first aspect of the present invention, there is provided a ball screw apparatus comprising:  
      a screw shaft having a first ball screw groove formed on an outer peripheral surface thereof;  
      a nut including: 
          a second ball screw groove which is formed on an inner peripheral surface thereof so as to oppose to the first ball screw groove;     a ball return through hole for circulating the balls extended in an axial direction of the screw shaft; and     ball turning members having: 
            a ball turning passageway communicating with both of the ball load rolling passageway and the ball return through hole, and provided at opposite end portions of the nut, respectively; and    
               

      a number of balls rolling in a ball load rolling passageway, which is formed between the first and second ball screw grooves, in accordance with a rotation of the screw shaft or the nut,  
      wherein  
      when a diameter of the balls is represented by Dw, and a diameter of the ball return through hole is represented by D 1 , D 1 −Dw is 3% of Dw or more and 10% of Dw or less.  
      According to a second aspect of the present invention, as set forth in the first aspect of the present invention, it is preferable that the ball screw apparatus according to claim  1 , wherein when a diameter of the ball turning passageway is represented by D 2 , D 2 −Dw is 10% of Dw or less.  
      According to a third aspect of the present invention, as set forth in the first aspect of the present invention, it is preferable that a spacer made of material which is softer than that of the balls is provided between any two adjacent balls.  
      According to a fourth aspect of the present invention, as set forth in the third aspect of the present invention, it is preferable that each of the spacers is formed into a disk-like shape, and is smaller in diameter than the balls.  
      According to a fifth aspect of the present invention, as set forth in the third aspect of the present invention, it is preferable that each of the spacers is formed into a spherical shape, and is smaller in diameter than the balls.  
      According to a sixth aspect of the present invention, as set forth in the second aspect of the present invention, it is preferable that a spacer made of material which is softer than that of the balls is provided between any two adjacent balls.  
      According to a seventh aspect of the present invention, as set forth in the sixth aspect of the present invention, it is preferable that each of the spacers is formed into a disk-like shape, and is smaller in diameter than the balls.  
      According to an eighth aspect of the present invention, as set forth in the sixth aspect of the present invention, it is preferable that each of the spacers is formed into a spherical shape, and is smaller in diameter than the balls.  
      In the ball screw apparatus of the present invention, when the diameter of the balls is represented by Dw, and the diameter of the ball return through hole is represented by D 1 , D 1 −Dw is not smaller than 3% of Dw and not larger than 10% of Dw. With this construction, the possibility that the balls are arranged in a stagger manner in the ball return through hole is greatly reduced, therefore a good acoustic performance is obtained even when the screw shaft or the nut is rotated at high speed. And besides, the competing of the balls with each other within the ball return through hole is suppressed, therefore the operability of the ball screw apparatus is enhanced. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a cross-sectional view of a first embodiment of a ball screw apparatus of the present invention;  
       FIG. 2  is a perspective view showing a ball turning member shown in  FIG. 1 ;  
       FIG. 3  is a cross-sectional view showing a spacer shown in  FIG. 1 ;  
       FIG. 4  is a diagram showing the relation between the amount of gap (defined between balls and a ball return through hole) and an acoustic level of the ball screw; and  
       FIG. 5  is a cross-sectional view of a second embodiment of a ball screw apparatus of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      A preferred embodiment of the present invention will now be described with reference to the drawings.  
       FIG. 1  is a cross-sectional view of a first embodiment of a ball screw apparatus of the invention. In this Figure, reference numeral  11  denotes a screw shaft of the ball screw apparatus of the first embodiment, and a ball screw groove  12  is formed in an outer peripheral surface of the screw shaft  11 . Reference numeral  13  denotes a nut having a ball screw groove  14  which is formed in an inner peripheral surface thereof, and is opposed to the ball screw groove  12  of the screw shaft  11 . A ball load rolling passageway  15  of a spiral configuration is formed between the ball screw groove  14  of the nut  13  and the ball screw groove  12  of the screw shaft  11 . There are provided a number of balls  16  which roll in and along the ball load rolling passageway  15  in accordance with the rotation of the screw shaft  11  or the nut  13 .  
      A ball return through hole  17  for circulating the balls  16  rollingly moving through the ball load rolling passageway  15  is formed in the nut  13 , and extends in an axial direction of the screw shaft  11 . Ball turning members  18  are incorporated in opposite end portions of the nut  13 , respectively. Each of the ball turning members  18  is made of a synthetic resin, and aba turning passageway  19  (see  FIG. 2 ) which communicates with both of the ball load rolling passageway  15  and the ball return through hole  17  is formed in each ball turning member  18 .  
      The balls  16  are made, for example, of steel, and a spacer  20  is provided between any two adjacent balls  16 . The spacer is made of material (for example, a synthetic resin) which is softer than the material of the balls  16 . Each spacer  20  is formed into a disk-like shape, and is smaller in diameter than the ball  16 . Concave spherical surfaces  21  (see  FIG. 3 ) are formed in opposite sides of each spacer  20 , respectively, and each concave spherical surface  21  has a curvature larger than the diameter of the ball  16 . The spacer  20  has a through hole  22  passing through central portions of the two concave spherical surfaces  21  thereof.  
      The ball return through hole  17  is formed in the nut  13  such that the following formula (1) is satisfied: 
 
( D   1   −Dw )≦0.1 Dw    (1) 
 
      where Dw represents the diameter of the ball  16 , and D 1  represents the diameter of the ball return through hole  17 .  
      The ball turning passageway  19  is formed in the ball turning member  18  such that the following formula (2) is satisfied: 
 
( D   2   −Dw )≦0.1 Dw    (2) 
 
      where Dw represents the diameter of the ball  16 , and D 2  represents the diameter of the ball turning passageway  19 .  
      As described above, when the diameter of the ball  16  is represented by Dw, and the diameter of the ball return through hole  17  is represented by D 1 , D 1 −Dw is not larger than 10% of Dw. With this construction, the play of each ball  16  in the ball return through hole  17  is reduced. As a result, the possibility that the balls  16  are arranged in a stagger manner in the ball return through hole  17  is greatly reduced. Therefore, therefore a good acoustic performance is obtained even when the screw shaft  11  or the nut  13  is rotated at high speed. And besides, the competing of the balls  16  with each other within the ball return through hole  17  is suppressed, and therefore the operability of the ball screw apparatus is enhanced.  
      Further, in the above embodiment, when the diameter of the ball  16  is represented by Dw, and the diameter of the ball turning passageway  19  is represented by D 2 , D 2 −Dw is not larger than 10% of Dw. With this construction, the play of each ball  16  in the ball turning passageway  19  is reduced. As a result, the possibility that the balls  16  are arranged in a stagger manner in the ball turning passageway  19  is greatly reduced, and therefore the good acoustic performance is obtained even when the screw shaft  11  or the nut  13  is rotated at high speed. And besides, the competing of the balls  16  with each other within the ball turning passageway  19  is suppressed, and therefore the operability of the ball screw apparatus is enhanced.  
      Furthermore, in the above embodiment, the spacer  20  (which is made of the material softer than the material of the balls  16 ) is provided between any two adjacent balls  16 ,  16 , and by doing so, the following ball  16  is prevented from impinging on the ball  16  moving ahead thereof, and therefore the development of noises, vibration and others due to the impingement of the balls on each other is suppressed.  
      In order to confirm the above effects, the inventor of the present invention carried out an acoustic measurement test for the ball screw apparatus in which a microphone was installed at a level of height spaced upwardly 400 mm from the axis of a screw shaft, and the following test conditions were adopted.  
      Testing Machine used: Ball Screw Acoustic Measurement Testing Machine manufactured by NSK;  
      Preload: 1800 N;  
      Testing Load (Acceleration Load): 1800 N;  
      Maximum Revolution Number: 3000 min −1 ;  
      Stroke: 800 mm;  
      Lubrication: VG#68 (Idemitsu Kosan);  
      Testing Ball Screw Apparatus: Ball Screw Number: 40×12×1000−C5 manufactured by NSK:  
      Results of the test are shown in  FIG. 4 .  
      In  FIG. 4 , the abscissa axis represents the amount S of gaps (=D 1 −Dw, D 2 −Dw) between the ball return through hole  17  and the balls  16  and also between the ball turning passageway  19  and the balls  16 , and the ordinate axis represents an acoustic ratio obtained when the level of a sound pressure, produced when a conventional ball screw apparatus is operated, is expressed as 1. Lines TP 1 , TP 2  and TP 3  show results of the acoustic measurement test carried out, using the ball screw apparatus of the above specification.  
      As shown in  FIG. 4 , the acoustic ratio much decreases when the gap amount S exceeds Dw/8, and it will be appreciated that when the gap amount S is not larger than Dw/8 and preferably not larger than Dw/10, the decrease of the acoustic ratio is small. Therefore, by setting the amount S of the gap between the ball return through hole  17  and the balls  16  and also between the ball turning passageway  19  and the balls  16  to not larger than Dw/8 and preferably not larger than Dw/10, noises and vibration can be further reduced.  
      When the gap amount S is smaller than 3% of Dw, the balls  16  are liable to be somewhat caught because of an error in the mounting of the ball turning members  18  in the nut  13 , and therefore it is preferred that the gap amount S should be not smaller than 3% of Dw and not larger than 10% of Dw.  
      The present invention is not limited to the above embodiment. For example, in the above embodiment, although each of the spacers  20  is formed into a disk-like shape, and is smaller in diameter than the balls  16 , there can be used spacers  20  of a spherical shape which are smaller in diameter than the balls  16 , as shown in  FIG. 5 .  
      While there has been described in connection with the preferred embodiments of the present invention, it will be obvious to those skilled in the art that various changes and modification may be made therein without departing from the present invention, and it is aimed, therefore, to cover in the appended claim all such changes and modifications as fall within the true spirit and scope of the present invention.