Abstract:
The present invention provides a ball screw having spacers which can assuredly be retained between balls and prevent direct contact between the balls. The ball screw includes a plurality of spacers ( 1 ) provided between respective adjacent pairs of balls. The spacers ( 1 ) each include a base plate ( 2 ) having front and back surfaces, and at least three projections ( 4, 6 ) provided on each of the front and back surfaces of the base plate ( 2 ). The projections ( 4 ) on the front surface is offset from the projections ( 6 ) on the back surface. The spacers ( 1 ) are each provided perpendicularly to a traveling direction of the balls with the projections ( 4, 6 ) on the front and back surfaces resiliently abutting against the balls.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a ball screw for use in a feed mechanism of a machine tool and, more particularly, to a ball screw having spacers interposed between respective adjacent pairs of balls.  
           [0003]    2. Description of the Prior Art  
           [0004]    Ball screws of the aforesaid type for use in a feed mechanism generally have a construction as shown in FIG. 6. As shown, the ball screw  100  includes a screw shaft  101  having a helical ball rolling groove  102  formed in an outer peripheral surface thereof, a ball nut  103  having a helical ball rolling groove  105  formed in an inner peripheral surface thereof in an opposed relation with the ball rolling groove  102  and ball circulation channels  106 ,  108 ,  110  provided therein to connect opposite ends of the ball rolling groove  105 , and a plurality of balls  111  provided in a channel defined between the ball rolling grooves  102  and  105  and the ball circulation channels  106 ,  108 ,  110 . The ball nut  103  includes a cylindrical nut body  104  and end caps  107 ,  109  provided at opposite ends of the nut body  104 . The nut body  104  is formed with the ball rolling groove  105  and the ball circulation channel  106 . The end caps  107  and  109  are formed with the ball circulation channels  108  and  110 , respectively.  
           [0005]    The ball screw  100  shown in FIG. 6 is of a so-called end cap type. Other exemplary types include a return tube type and a guide plate type, depending on the arrangement of the ball circulation channels.  
           [0006]    In the ball screw  100  having the aforesaid construction, however, the balls  111  have slight variations in revolving speed during the rolling movement thereof due to variations in the profile of the ball rolling grooves  102 ,  105 . If an upstream ball  111  revolves at a higher revolving speed than a downstream ball  111  with respect to a traveling direction of the balls  111 , the upstream and downstream balls  111  collide and push against each other, so that a compression force is exerted on a contact between the balls  111  pushing against each other. The compression force acting on the contact between the balls  111  causes the balls  111  sliding contact in a direction such as to prevent the rolling of the balls  111 . This develops a great resistance to prevent the rolling of the balls  111 , thereby varying or significantly increasing the dynamic torque of the ball screw  100 . Further, clogging with the balls  111  may occur.  
           [0007]    To solve the aforesaid problem, a ball screw has been proposed in which spacers are interposed between respective adjacent pairs of balls  111 . Examples of the spacers are shown in FIGS. 7 and 8.  
           [0008]    A spacer  120  shown in FIG. 7 is a hollow cylindrical member having opposite end faces  121 ,  121  of a concave spherical shape. The spacer  120  is interposed between each adjacent pair of balls  111 ,  111  with the opposite end faces  121 ,  121  in contact with the balls  111  to prevent direct contact between the balls  111 ,  111 . The concave spherical end faces  121 ,  121  of the spacer  120  entirely contact the balls  111 ,  111 , so that a great contact resistance is developed during the rolling of the balls  111 . Therefore, dynamic torques of the balls  111  are increased, resulting in clogging with the balls.  
           [0009]    A spacer  130  shown in FIG. 8 has opposite end faces  131 ,  131  of a generally concave spherical shape, which are each entirely formed with a plurality of projections  132 . With this arrangement, areas of contact between balls  111  and the end faces  131  are reduced, so that a contact resistance occurring therebetween is reduced to solve the aforesaid problem.  
           [0010]    However, the spacer  120  of FIG. 7 and the spacer  130  of FIG. 8 suffer from the following problems. There are dimensional variations such as variations in the effective diameter of the channel defined between the ball rolling grooves  102  and  105 , the diameter of the balls  110  and the total length of the ball circulation channels  106 ,  108 ,  110  due to machining errors. Even if the balls  111  and the spacers  120  ( 130 ) are tightly inserted in an alternating relation in the channel defined between the ball rolling grooves  102  and  105  and the ball circulation channels  106 ,  108 ,  110 , a gap inevitably occurs between an initially inserted ball  111  and a finally inserted spacer  120  ( 130 ) due to the dimensional variations. Therefore, when the screw shaft  101  (or ball nut  103 ) is rotated, the dynamic torque of the ball screw  100  is varied or significantly increased, or the spacers  120  ( 130 ) are inclined, thereby preventing the circulation of the balls  111 .  
           [0011]    One conceivable approach to this problem is to prepare several types of spacers  120  ( 130 ) having different lengths and select a spacer  120  ( 130 ) having a length appropriate for filling the gap when it is finally inserted in the channel. However, the production, maintenance and assembly of such spacers are troublesome with little practical advantage.  
           [0012]    In view of the foregoing, it is an object of the present invention to provide a ball screw having spacers which can assuredly be retained between balls and prevent direct contact between the balls.  
         SUMMARY OF THE INVENTION  
         [0013]    In accordance with the present invention to achieve the aforesaid object, there is provided a ball screw which comprises: a screw shaft having a helical ball rolling groove formed in an outer peripheral surface thereof; a ball nut having a helical ball rolling groove formed in an inner peripheral surface thereof in an opposed relation with the ball rolling groove of the screw shaft, and a ball circulation channel provided therein to connect opposite ends of the ball rolling groove thereof; a plurality of balls provided in a channel defined between the ball rolling grooves and the ball circulation channel; and a plurality of spacers interposed between respective adjacent pairs of balls; wherein the spacers each comprise a base plate having front and back surfaces, and at least three projections provided on each of the front and back surfaces of the base plate, the projections on the front surface being offset from the projections on the back surface; wherein the spacers are each provided perpendicularly to a traveling direction of the balls with the projections on the front and back surfaces resiliently abutting against the balls adjacent thereto.  
           [0014]    In the ball screw according to the present invention, the spacers each have the base plate and the at least three projections provided on each of the front and back surfaces of the base plate, and the projections on the front surface are offset from the projections on the back surface. The spacers are provided perpendicularly to the traveling direction of the balls between the balls. When an action force is exerted on an adjacent pair of balls to urge the balls toward each other, the projections on the front and back surfaces of a spacer provided between the balls abut against the balls, and receive reaction forces from the balls. At this time, the reaction forces act on different points on the front and back surfaces of the base plate, because the projections on the front surface are offset from the projections on the back surface. The base plate is planar and, hence, is more liable to be resiliently deformed than the cylindrical member of the conventional spacer. In addition, the reaction forces act on the different points on the front and back surfaces. Therefore, the spacer can easily absorb the reaction forces by the resilient deformation thereof to permit reduction of a spacing between the balls.  
           [0015]    Thus, the spacers according to the present invention per se are resiliently deformed to adjust the spacing between the balls. Even if there is a gap between an initially inserted ball and a finally inserted spacer under no load due to dimensional variations such as variations in the effective diameter of the channel defined between the ball rolling grooves, the diameter of the balls and the length of the ball circulation channel which may occur due to machining errors, the gap can be filled by insertion of one more ball by resiliently deforming the spacers under a load applied to the spacers. The resilient deformation of the spacers provided between the balls makes it possible to firmly retain the spacers between the balls with no gap between the balls and the spacers.  
           [0016]    The aforesaid function of the spacers makes it possible to prevent the problems associated with the prior art, i.e., the variation or significant increase of the dynamic torque of the ball screw or the ball circulation failure due to the inclination of the spacers, which may otherwise occur due to gaps between the balls and the spacers.  
           [0017]    Since the balls abut against the projections of the spacer, spaces are defined between the base plate of the spacer and the balls. Therefore, a grease can be retained in the spaces for improved lubrication.  
           [0018]    Preferably, the projections are equidistantly arranged circularly around the center of the spacer on each of the front and back surfaces. This makes it possible to evenly resiliently deform the spacer to retain the spacer between the balls with an increased retention force.  
           [0019]    The spacers may each comprise a plate member which has front and back surfaces, and includes at least three projections and at least three indentations provided circularly around the center thereof in an alternating relation on each of the front and back surfaces, wherein the projections and the indentations on the front surface respectively positionally correspond to the indentations and the projections on the back surface. The spacers are each provided perpendicularly to the traveling direction of the balls between the balls with the projections on the front and back surfaces resiliently abutting against the balls adjacent thereto.  
           [0020]    With this arrangement, when an action force is exerted on an adjacent pair of balls to urge the balls toward each other, the projections on the front and back surfaces of a spacer provided between the balls abut against the balls, and receive reaction forces from the balls. At this time, the reaction forces act on different points on the front and back surfaces of the spacer, because the projections and the indentations on the front surface of the spacer respectively positionally correspond to the indentations and the projections on the back surface. The spacer is of a plate shape and, hence, is more liable to be resiliently deformed than the cylindrical member of the conventional spacer. In addition, the reaction forces act on the different points on the front and back surfaces of the spacer. Therefore, the spacer can easily absorb the reaction forces by the resilient deformation thereof to permit reduction of a spacing between the adjacent pair of balls.  
           [0021]    Thus, the spacers per se are resiliently deformed to adjust the spacing between the balls. Even if there is a gap between an initially inserted ball and a finally inserted spacer under no load due to dimensional variations such as variations in the effective diameter of the channel defined between the ball rolling grooves, the diameter of the balls and the length of the ball circulation channel which may occur due to machining errors, the gap can be filled by insertion of one more ball by resiliently deforming the spacers under a load applied to the spacers. The resilient deformation of the spacers provided between the balls makes it possible to firmly retain the spacers between the balls with no gap between the balls and the spacers.  
           [0022]    The aforesaid function of the spacers makes it possible to prevent the problems associated with the prior art, i.e., the variation or significant increase of the dynamic torque of the ball screw or the ball circulation failure due to the inclination of the spacers, which may otherwise occur due to gaps between the balls and the spacers.  
           [0023]    Since the balls abut against the projections of the spacer, spaces are defined between the indentations of the spacer and the balls. Therefore, a grease can be retained in the spaces for improved lubrication. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]    [0024]FIG. 1 is a front view illustrating a spacer according to one embodiment of the present invention;  
         [0025]    [0025]FIG. 2 is a sectional view of the spacer as seen in an arrow direction I - I in FIG. 1;  
         [0026]    [0026]FIG. 3 is a sectional view for explaining the function of the spacer according to the embodiment;  
         [0027]    [0027]FIG. 4 is a front view illustrating a spacer according to another embodiment of the present invention;  
         [0028]    [0028]FIG. 5 is a sectional view of the spacer as seen in an arrow direction II-II in FIG. 4;  
         [0029]    [0029]FIG. 6 is a sectional view illustrating the construction of a conventional ball screw;  
         [0030]    [0030]FIG. 7 is a sectional view of a spacer according to the prior art; and  
         [0031]    [0031]FIG. 8 is a sectional view of another spacer according to the prior art. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0032]    With reference to the attached drawings, the present invention will hereinafter be described by way of specific embodiments thereof. FIG. 1 is a front view illustrating a spacer according to one embodiment of the present invention, and FIG. 2 is a sectional view of the spacer as seen in an arrow direction I-I in FIG. 1. The ball screw according to this embodiment has substantially the same basic construction as the ball screw  100  shown in FIG. 6, except that spacers each having a construction as shown in FIGS.  1  to  3  are interposed between respective adjacent pairs of balls  111 . Therefore, a reference is made to FIG. 6 for the basic construction of the ball screw according to this embodiment, and no detailed explanation will be given thereto. In the following explanation, like components are denoted by like reference characters.  
         [0033]    As shown in FIGS. 1 and 2, the spacer  1  according to this embodiment has a planar annular base plate  2 , three projections  4  provided on a front surface  3  of the base plate  2 , and three projections  6  provided on a back surface  5  of the base plate  2 . The projections  4  and  6  are of a semi-spherical shape, and equidistantly arranged circularly around the center of the spacer on the front and back surfaces  3  and  5 , respectively. The projections  4  on the front surface  3  are angularly offset by 60 degrees from the projections  6  on the back surface  5 .  
         [0034]    As shown in FIG. 2, the spacer  1  is provided perpendicularly to a traveling direction of the balls  111  between each adjacent pair of balls  111  and  111  with the projections  4 ,  6  abutting against the balls  111 ,  111 . As a result, spaces  7 ,  8  which serve as grease retaining spaces are defined between the base plate  2  and the balls  111 .  
         [0035]    With the spacer  1  thus provided between the adjacent pair of balls  111  and  111 , when an action force is exerted on the adjacent balls  111 ,  111  in an arrow direction to urge the balls toward each other as shown in FIG. 3, the projections  4 ,  6  provided on the front and back surfaces  3 ,  5  abut against the balls  111 ,  111 , and receive reaction forces from the balls  111 ,  111 . At this time, the reaction forces act on different points on the front and back surfaces  3 ,  5  of the base plate  2 , because the projections  4  on the front surface  3  are offset from the projections  6  on the back surface  5 . The base plate  2  is planar and, hence, is more liable to be resiliently deformed than the cylindrical member of the conventional spacer. In addition, the reaction forces act on the different points on the front and back surfaces  3 ,  5 . Therefore, the spacer  1  can easily absorb the reaction forces by the resilient deformation thereof to permit reduction of a spacing between the balls  111  and  111 . In FIG. 3, a state where no load acts on the spacer  1  is indicated by a chain line, and a state where a spacer  1  is resiliently deformed under a load is indicated by a continuous line.  
         [0036]    Thus, the spacers  1  according to this embodiment per se are resiliently deformed to adjust the spacing between the balls  111 . Even if there is a gap between an initially inserted ball  111  and a finally inserted spacer  1  under no load due to dimensional variations such as variations in the effective diameter of a channel defined between ball rolling grooves  102 ,  105 , the diameter of the balls  111  and the total length of ball circulation channels  106 ,  108 ,  110  which may occur due to machining errors, the gap can be filled by insertion of one more ball  111  by resiliently deforming the spacers  1  under a load applied to the spacers  1 . The resilient deformation of the spacers  1  provided between the balls  111  makes it possible to firmly retain the spacers  1  between the balls  111  with no gap between the balls  111  and the spacers  1 .  
         [0037]    The aforesaid function of the spacers  1  makes it possible to prevent the problems associated with the prior art, i.e., the variation or significant increase of the dynamic torque of the ball screw or the ball circulation failure due to the inclination of the spacers, which may otherwise occur due to gaps between the balls  111  and the spacers.  
         [0038]    Further, the grease retaining spaces  7 ,  8  which each have a greater volume are defined between the base plate  2  of the spacer  1  and the balls  111  for improved lubrication.  
         [0039]    The spacers  1  according to this embodiment can be produced by injection molding of a synthetic resin or sheet metal press working.  
         [0040]    In this embodiment, the numbers of the projections  4  and  6  are three, but not limited thereto. The projections  4 ,  6  may each be provided in any number not smaller than three. The shape of the projections is not limited to the semi-spherical shape. However, it is important to provide the projections  4  and  6  in an offset relation on the front and back surfaces of the spacer, respectively.  
         [0041]    Alternatively, spacers each having a construction as shown in FIGS. 4 and 5 may be employed. FIG. 4 is a front view of the spacer. FIG. 5 is a sectional view of the spacer as seen in an arrow direction II-II in FIG. 4.  
         [0042]    As shown in FIGS. 4 and 5, the spacer  10  is an annular plate member having three projections  12  and three indentations  13  provided circularly around the center thereof in an alternating relation on a front surface  11  thereof and three projections  15  and three indentations  16  provided circularly around the center thereof in an alternating relation on a back surface  14  thereof. The projections  12  and the indentations  13  on the front surface  11  respectively positionally correspond to the indentations  16  and the projections  15  on the back surface  14 . For example, the front surface  11  shown in FIG. 4 has six equiangularly sectored regions, in which the projections  12  and the indentations  13  are alternately defined. The projections  12  each have a greatest height at the center of a corresponding sectored region, while the indentations  13  each have a greatest depth at the center of a corresponding sectored region.  
         [0043]    As shown in FIG. 5, the spacer  10  is provided perpendicularly to the traveling direction of the balls  111  between each adjacent pair of balls  111  and  111  with the projections  12  and  15  on the front and back surfaces  11  and  14  abutting against the balls  111 ,  111 . As a result, spaces  17 ,  18  which serve as grease retaining spaces are defined between the spacer  10  and the balls  111 .  
         [0044]    With the spacer  10  thus provided between the adjacent pair of balls  111  and  111 , when an action force is exerted on the adjacent balls  111 ,  111  in an arrow direction to urge the balls toward each other, the projections  12 ,  15  provided on the front and back surfaces  11 ,  14  abut against the balls  111 ,  111 , and receive reaction forces from the balls  111 ,  111 . At this time, the reaction forces act on different points on the front and back surfaces  11 ,  14  of the spacer  10 , because the projections  12  and the indentations  13  on the front surface  11  respectively positionally correspond to the indentations  16  and the projections  15  on the back surface  14 . The spacer  10  is of a plate shape and, hence, is more liable to be resiliently deformed than the cylindrical member of the conventional spacer. In addition, the reaction forces act on the different points on the front and back surfaces  11 ,  14 . Therefore, the spacer  10  can easily absorb the reaction forces by the resilient deformation thereof to permit reduction of a spacing between the balls  111 .  
         [0045]    Thus, the spacers  10  according to this embodiment per se are resiliently deformed to adjust the spacing between the balls  111  as the spacers  1 . Even if there is a gap between an initially inserted ball  111  and a finally inserted spacer  10  under no load due to dimensional variations such as variations in the effective diameter of the channel defined between the ball rolling grooves  102 ,  105 , the diameter of the balls  111  and the total length of the ball circulation channels  106 ,  108 ,  110  which may occur due to machining errors, the gap can be filled by insertion of one more ball by resiliently deforming the spacers  10  under a load applied to the spacers  10 . The resilient deformation of the spacers  10  provided between the balls  111  makes it possible to firmly retain the spacers  10  between the balls  111  with no gap between the balls  111  and the spacers  10 .  
         [0046]    The aforesaid function of the spacers makes it possible to prevent the problems associated with the prior art, i.e., the variation or significant increase of the dynamic torque of the ball screw or the ball circulation failure due to the inclination of the spacers, which may otherwise occur due to gaps between the balls  111  and the spacers.  
         [0047]    The grease retaining spaces  17 ,  18  which each have a greater volume are defined between the spacer  10  and the balls  111  for improved lubrication.  
         [0048]    The spacers  10  according to this embodiment can be produced by injection molding of a synthetic resin or sheet metal press working.  
         [0049]    In this embodiment, the numbers of the projections  12 ,  15  and the indentations  13 ,  16  provided on the front and back surfaces  11 ,  14  are three, but not limited thereto. The projections  12 ,  15  and the indentations  13 ,  16  may be provided in any numbers not smaller than three.