Abstract:
A ball screw in a continuously variable speed transmission has an axially immovable pulley half ( 2 ) and axially movable pulley half ( 3 ) both mounted on a rotary shaft ( 4 ). The axially movable pulley half ( 3 ) is moved by a ball screw ( 20 ) so as to infinitely vary the belt wrapping radius and thus the transmission speed. A nut ( 22 ), forming one part of the ball screw ( 20 ), is immovably secured in both axial and rotational directions. A screw shaft ( 21 ), forming the other part of the ball screw ( 20 ), is movable in both axial and rotational directions. The screw shaft  21  is connected to the axially movable pulley half ( 3 ). The axially movable pulley half ( 3 ) is axially moved by rotating the screw shaft ( 21 ).

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
         [0001]    This application claims priority to Japanese Patent Application Nos. 2001-353600 filed Nov. 19, 2001, which application is herein expressly incorporated by reference.  
         FIELD OF THE INVENTION  
         [0002]    The present invention relates, generally to a ball screw, and more particularly to a ball screw used as an actuator for driving an axially movable pulley half of a Continuously Variable speed Transmission (hereinafter referred to as a “CVT”) for example used in an automobile.  
         BACKGROUND OF THE INVENTION  
         [0003]    The ball screw comprises a screw shaft formed with a screw groove around the outer circumferential surface. A nut is formed with a screw groove on the inner circumferential surface. A plurality of balls are contained within a raceway formed by the opposite screw grooves of the screw shaft and the nut. The ball screw is used to convert a rotational force of the screw shaft (or the nut) to a thrust force of the nut (or the screw shaft) via the balls.  
           [0004]    The ball, screw has a very high transmission efficiency due to the rolling contact of balls between the screw shaft and the nut. Thus, it is possible to convert the rotational motion to translational motion with a driving torque of about ⅓ that of the sliding screw. It is therefore possible to obtain a large thrust force by applying a small torque.  
           [0005]    The CVT of the prior art is schematically shown in FIG. 5. The CVT comprises a pulley  50  on the input side and a pulley  51  on the output side, and a steel belt  52  wrapped around and extending between the pulleys  50  and  51 . The input and output pulleys  50  and  51  are formed, respectively, by axially immovable pulley halves  50 a and  51   a  and axially movable pulley halves  50   b  and  51   b.  The continuous speed variation can be achieved by axially shifting the axially movable pulley halves  50   b  and  51   b  and thus varying the belt wrapping radials of the input and output pulleys  50  and  51 .  
           [0006]    An actuator to shift the axially movable pulley halves  50   b  and  51   b  is formed by ball screws  53 . As shown in FIG. 6( a ), each ball screw  53  includes a screw shaft  54  and a nut  55  mounted thereon via a plurality of balls  58 . The balls  58  are contained in a raceway formed by opposite screw grooves  56  and  57  so that they are infinitely circulated. These balls  58  are all load supporting balls having the same diameter as shown in FIG. 6( b ).  
           [0007]    The screw shaft  54  of the ball screw  53  is supported by a supporting member (not shown) formed integrally with a casing (not shown) so that the screw shaft  54  cannot be moved in both a rotational and axial direction. The nut  55  is supported movably in both a rotation and axial direction. Accordingly, the axially movable pulley halves  50   b  and  51   b  connected to the nuts  55  via bearings (not shown) can be translated along the screw shaft  54  by rotating the nuts  55  (see Japanese Patent Publication No. 33170/1996).  
           [0008]    When an automobile provided with the CVT is running on a town street, the CVT is, in usual, frequently shifted in a narrow range between Lo-speed side and Hi-speed side. In this narrow range, the shifting range of the movable pulley halves  50   b  and  51   b  is very short. [ 0009 ] Under the circumstances, the balls  58  suffer from friction and damage due to lack of lubrication in local regions especially between surfaces of adjacent balls  58  rotating in “counter” directions as shown by arrows in FIG. 6( b ). This causes relative slippage between contacting points of adjacent balls. Thus, this lowers the mechanical efficiency of the ball screw  53  and diminishes the smooth speed change of an automobile.  
           [0009]    In addition, since the nuts  55  have to be rotated in the CVT of the prior art to shift the axially movable pulley halves, ball circulating portions (not shown) formed in the nuts  55  are also rotated together with the nuts  55 . Since a gap between the balls  58  and a raceway in the ball circulating portion is larger than that of the raceway formed by the screw grooves  56  and  57 , the balls  58  cannot support the moment load and the radial load acting on the ball screw  53  when they are in the raceway in the ball circulating portion.  
           [0010]    Accordingly it is necessary, in the CVT of the type of nut rotation, to increase the load supporting capacity or the rigidity of the ball screw  53  by enlarging the size of the balls in order to compensate for a deficiency of load supporting capacity. This diminishes a reduction of the weight and size of the CVT and makes a reasonable and fit design difficult.  
         SUMMARY OF THE INVENTION  
         [0011]    It is, therefore, an object of the present invention to provide a ball screw that enables the reduction of weight and size of a CVT without reduction of the transmission efficiency as well as a CVT in which such a ball screw is incorporated.  
           [0012]    According to the present invention, a ball screw includes a screw shaft formed with a screw groove around the outer circumferential surface. A nut is formed with a screw groove on the inner circumferential surface. A plurality of balls are contained within a raceway formed by the opposite screw grooves of the screw shaft and the nut. The screw shaft is formed as a hollow cylindrical member, and one end of the screw shaft is covered by one end of the nut.  
           [0013]    The structure that the screw shaft is formed as a hollow cylindrical member, and one end of the screw shaft is covered by one end of the nut makes it possible to suppress a splash of lubricant due to the rotation of the screw shaft and to prevent a drop of the transmission efficiency. In addition it is possible to limit an amount of axial shift of the screw shaft with a light-weight and compact structure.  
           [0014]    The dimension of the diameter “d” of the ball and the outer diameter “D” of the screw shaft is defined as a ratio of d/D≦{fraction (1/15)}. This can reduce the entire length of the nut and makes it possible to provide a light-weight and compact ball screw without reducing the load supporting capacity and the transmission efficiency. Especially in the CVT used in an automobile, it is possible to achieve a light-weight and compact CVT of a reasonable and fit design.  
           [0015]    The lead angle of the screw groove is smaller than 2°. This enables further reduction of the entire length of the nut without lowering the mechanical efficiency.  
           [0016]    The balls comprise load supporting balls and spacer balls. One spacer ball is arranged between at least two load supporting balls. This makes the rotational directions of adjacent balls “following”. Thus, this eliminates the relative slippage between contacting points of adjacent balls.  
           [0017]    The diameter of the spacer balls is 10˜100 μm smaller than that of the load supporting ball. This enables the gap between the balls and the screw grooves to be neither too much nor too less and thus brings smooth rolling of the spacer balls.  
           [0018]    A separating seat is interposed between the balls. Each of the opposite surfaces of the separating seat is formed as a conical surface so as to contact the ball at a predetermined contacting angle. This causes the ball rotation “following” without substantially reducing the load supporting capacity. Thus, this eliminates the relative slippage of contacting points between adjacent balls and increase the efficiency of the ball screw. In addition since the lubricant is held by the separating seat, it is possible to reduce an amount of supply of the lubricant and to improve the lubricating characteristics of the ball screw.  
           [0019]    An axially extending through aperture is formed between the opposite conical surfaces of the separating seat. This makes the minimum thickness of the separating seat large and further increases the lubricant holding capacity.  
           [0020]    Further according to the present invention, a continuously variable speed transmission (CVT) of belt type includes an axially immovable pulley half and axially movable pulley half both mounted on a rotary shaft. The axially movable pulley half is moved by a ball screw so as to infinitely vary the belt wrapping radius. Thus, the transmission is characterized in that the ball screw is formed by a nut forming one part of the ball screw which is immovably secured in both axial and rotational directions. A screw shaft forms the other part of the ball screw and is movable in both axial and rotational directions. The screw shaft is connected to the axially movable pulley half. The axially movable pulley is axially moved by rotating the screw shaft.  
           [0021]    Accordingly, the continuously variable speed transmission arranges the ball circulating portion of nut side of lesser load supporting ability at a no load supporting side when the axially movable pulley halves are axially shifted. Accordingly it is possible to prevent a ball vibration due to a gap increase within the ball circulating portion and due to translation of balls from the screw groove to the ball circulating portion or from the ball circulating portion to the screw grooves.  
           [0022]    The axially movable pulley half is connected rotatably to the screw shaft via an interposed bearing. This makes the axial dimension of the CVT compact without reducing the rigidity of the ball screw itself.  
           [0023]    One end of the screw shaft is secured to a connecting member. The connecting member has a flange portion radially outwardly extending beyond the outer diameter of the nut and a cylindrical portion axially extending around the outer circumferential surface of the nut. A gear for transmitting rotationally driving force to the screw shaft is secured to the outer circumferential surface of the cylindrical portion of the connecting member. This can reduce the entire length of the screw shaft by extending the thread length of the screw shaft and thus make the axial dimension of the CVT compact.  
           [0024]    The length of the cylindrical portion of the connecting member is longer than the axial stroke of the nut. This makes it possible to cover the outer circumferential side of an annular space made vacant due to the relative movement of the nut. This prevents splash of the lubricant.  
           [0025]    Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]    The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:  
         [0027]    [0027]FIG. 1 is a longitudinal section view of one embodiment of a CVT using a ball screw of the present invention;  
         [0028]    [0028]FIG. 2 is a diagrammatic view showing a relation between the transmission efficiency and the lead angle of a ball screw;  
         [0029]    [0029]FIG. 3 is a longitudinal section view of another embodiment of a CVT using a ball screw of the present invention;  
         [0030]    [0030]FIG. 4 is a partially enlarged section view of a ball screw of the present invention;  
         [0031]    [0031]FIG. 5 is a schematic view of a CVT of the prior art;  
         [0032]    [0032]FIG. 6 ( a ) is a section view of a ball screw used in a CVT of the prior art; and  
         [0033]    [0033]FIG. 6 ( b ) is a partially enlarged schematic section view of the ball screw of FIG. 6 ( a ). 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0034]    The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.  
         [0035]    [0035]FIG. 1 is a longitudinal section view of one embodiment of a CVT using a ball screw of the present invention. The CVT has a rotary shaft  4  which mounts an axially immovable pulley half  2  and an axially movable pulley  3 . Belt receiving surface  2   a  and  3   a  receive power transmission belt  1 . The surfaces  2   a  and  3   a  are opposite one another. The belt wrapping radius of the power transmission pulley  1  can be varied continuously or infinitely by axially shifting the movable pulley half  3  with a ball screw  5 . In the illustrated embodiment, although it is shown that the immovable pulley half  2  is formed integrally with the rotary shaft  4 , it may be possible to form the immovable pulley half  2  separately from the rotary shaft  4  and secure it on the rotary shaft  4  by any way known in the art.  
         [0036]    A nut  7  is adapted to be secured to a housing  8  of the CVT. The nut has a radially extending connecting portion  7   b.  The connecting portion  7   b  covers one end of a screw shaft  10  and limits an amount of shift of the screw shaft  10  in one direction in order to prevent fall out of balls  11  from the raceway and splash of lubricant adhered to the screw shaft  10 . The nut  7 , secured on the housing  8 , is also supported on the rotary shaft  4  rotatable relative to the shaft  4  via a ball bearing  6  fitted in the inner circumferential surface of the connecting portion  7   b    
         [0037]    The axially movable pulley half  3  has a cylindrical portion  9  on its back side mounted on the rotary shaft  4 , via a slide key. The pulley half  3  can be axially moved along the shaft  4 , however, it cannot be rotated relative to the shaft  4 . The slide key may be replaced by any other means having lesser sliding resistance such as a linear rolling bearing.  
         [0038]    The ball screw  5  includes the screw shaft  10  formed with a helical screw groove  10   a  on its outer circumferential surface. The nut  7  is adapted to fit around the screw shaft  10 . The nut  7  is formed with a helical screw groove  7   a  on its inner circumferential surface. A plurality of balls  11  are contained within a raceway formed between the opposite screw grooves  10   a  and  7   a  of the screw shaft  10  and the nut  7 . The screw shaft  10  is a hollow cylinder. The screw shaft  10  is supported on the cylindrical portion  9  of the axially movable pulley half  3  via a ball bearing  12  fitted in the inner circumferential surface of the screw shaft  10 .  
         [0039]    An annular connecting member  13  is securely fitted on a stepped portion  10   b  of the screw shaft  10 . A knurl may be formed on the outer circumferential surface of the stepped portion  10   b  to increase the caulking force of the connecting member  13 .  
         [0040]    The connecting member  13  may be press formed of steel sheet and includes a fitting portion  13   a,  fitted on the stepped portion  10   b;  a flange portion  13   b,  radially outwardly extending from the fitting portion  13   a;  and a cylindrical portion  13 c, axially extending from the flange portion  13   b  to cover the outer circumferential surface of the nut  7 . The nut  7  is received within an annular space  14  formed by the inner circumferential surface of the cylindrical portion  13   c  of the connecting member  13  and the outer circumferential surface of the screw shaft  10 .  
         [0041]    A gear  16  is secured to the connecting member  13  around the outer circumferential surface of the cylindrical portion  13   c.  The gear  16  slidably mates with a gear  15  driven by a motor (not shown). The screw shaft  10  is driven by the motor, via the gears  15  and  16 , and the connecting member  13 . The gear  16  may be made of plastic material having a wear resistance and a low coefficient of friction. Thus, its teeth are smoothly slidable against those of the gear  15 .  
         [0042]    When driven by the motor, the screw shaft  10 , rotatably supported by the ball bearing  12 , is rotated around the axially movable pulley  3  and simultaneously axially moved relative to the secured nut  7  to shift the axially movable pulley half  3  toward the axially immovable pulley half  2 . Accordingly, the distance between the belt receiving surface  3   a  of the movable pulley half  3  and the opposed belt receiving surface  2   a  of the immovable pulley half  2  is reduced. Thus, the belt wrapping radius of the belt  1  is increased. When the screw shaft  10  is reversely rotated by the motor, the distance between the belt receiving surfaces  2   a  and  3   a  is spread and thus the belt wrapping radius is reduced.  
         [0043]    In the ball screw  5 , it is generally necessary to have the lead angle of the screw grooves  7   a  and  10   a  substantially large in order to keep a preferable working performance. Since the transmission efficiency of the ball screw  5  is suddenly lowered, as shown in FIG. 5, if the lead angle of the screw grooves is set smaller than about  2 , the lead angle is usually set at an angle larger than  2 .  
         [0044]    It is necessary to increase the diameter of balls  11  when the diameter of the screw shaft  10  is increased since the torque variation is increased as well as the working torque performance is lowered when the number of the balls arranged within one circulation passage is increased. Ordinarily, the diameter “d” of the ball  11  and the outer diameter “D” of the screw shaft  10  is experientially set as having a relation “d/D&gt;1/15”.  
         [0045]    In the ball screw  5  of the CVT shown in FIG. 1, not only the entire length of but the outer diameter of the nut  7  are increased since the outer diameter “d” of the balls  11  as well as the lead angle are increased when the outer diameter “D” of the screw shaft  10  is increased. Accordingly, further improvements should be done in order to reduce the weight and size of the CVT.  
         [0046]    [0046]FIG. 3 is a longitudinal section view of the other embodiments of a CVT using a ball screw of the present invention. Same reference numerals as those used in FIG. 1 are also used in FIG. 3 as to same elements used in FIG. 1. The ball screw  20  used in the CVT includes a screw shaft  21  formed with a helical screw groove  21   a  on its outer circumferential surface. A nut  22  is fitted around the screw shaft  21  and has a helical screw groove  22   a  on its inner circumferential surface. A plurality of balls  23  are contained within the raceway formed between the opposite screw grooves  21   a  and  22   a  of the screw shaft  21  and the nut  22 . The screw shaft  21  is formed by a hollow cylinder and is rotatably supported on the cylindrical portion  9  of the axially movable pulley half  3  via the ball bearing  12 .  
         [0047]    The nut  22  is secured to the housing  8  of the CVT and has a radially extending connecting portion  22   b  for covering one end of the screw shaft  21 . The nut  22 , secured on the housing  8 , is also supported on the rotary shaft  4  rotatable relative to the shaft  4  via the ball bearing  6  fitted in the inner circumferential surface of the connecting portion  22   b.    
         [0048]    In this embodiment, the ball  23  is formed as having a remarkably small diameter “d′” and a relation “d′/D′≦1/15” relative to an outer diameter “D′” of the screw shaft  21 . The reduction of the size of the ball  23  enables the reduction of the lead angle and thus it is set smaller than 2°  
         [0049]    Although it causes a problem of lowering the transmission efficiency of the ball screw  20  when reducing the lead angle smaller than 2°, this problem can be solved by forming the balls  23  from two kinds of balls. Load supporting balls  23   a  support the bearing load and spacer balls  23   b,  having a smaller diameter than that of the load supporting balls  23   a,  are arranged so that one of the spacer balls  23   b  is interposed between at least two load supporting balls  23   a.  This arrangement, of one spacer balls  23   b  between at least two load supporting balls  23   a,  brings a “following” rotation (not a “counter” direction) between mutually adjacent balls  23   a  and  23   b.  Thus, this eliminates the relative slippage between contacting points of the balls  23   a  and  23   b.  Accordingly, it is possible to increase the efficiency of the ball screw.  
         [0050]    The diameter of the spacer ball  23   b  is smaller than that of the load supporting ball  23   a  preferably by 10˜100 μm, more preferably 20˜50 μm. If the difference of the diameter between the balls  23   a  and  23   b  is too small such as less than 10 μm, it is difficult to obtain the effect of eliminating the relative slippage. On the contrary, if the difference of the diameter between the balls  23   a  and  23   b  is too large such as more than 100 μm, the gap between the screw grooves  21   a  and  22   a  and the spacer balls  23   b  becomes too large to smoothly roll and thus it would be impossible to achieve smooth actuation of the ball screw  20 .  
         [0051]    In order to prevent the reduction of the load supporting capacity due to reduction of ball size, it is preferable to appropriately select a ratio of the spacer ball  23   b  and the load supporting ball  23   a , for example, 1:2 or 1:3 other than 1:1.  
         [0052]    The spacer balls  23   b  can be replaced by separating seats  25  interposed between balls  26  and  26  as shown in FIG. 4. The separating seats  25  are formed by a ring shaped configuration having concave surfaces  27  on opposite surfaces that contact the balls  26  and  26 . An axially extending through aperture  28  is formed between the concave surfaces  27 . Similarly to the spacer balls  23   b,  the separating seats  25  prevent the direct contact of balls  26  contained within the raceway  31  formed by screw grooves  29   a  and  30   a  of the screw shaft  29  and the nut  30 . The seats  25  also achieve the smooth rolling of balls  26 .  
         [0053]    The concave surface  27  of the separating seat  25  may be formed as either a conical surface or a concave spherical surface. In order to achieve stable pose of the separating seat  25  and smooth rolling of the balls  26 , the separating seat  25  should be contacted with the balls at a predetermined contacting angle ø. The contacting angle ø is an angle formed by a line connecting the center “0” of the ball  26  and a contacting point “A” and a line connecting centers “0-0” of the adjacent balls  26  and is preferably a range of 20˜−30°.  
         [0054]    The diameter of the through-aperture  28  is 30% or less of the diameter of the ball  26 . If the diameter of the through-passage  28  is larger than that range, the rigidity of the spacer seat  25  will be insufficient. On the contrary, if the diameter of the through-passage  28  is larger than that range, the wall thickness forming the through-passage  28  will become too thin to easily manufacture the spacer seat  25 . The provision of the through passage  28  enables holding of lubricant as well as reduction the distance between balls  26 . Accordingly, it is possible to increase the load supporting capacity of the ball screw as compared with the spacer ball  23   b  mentioned above.  
         [0055]    The outer diameter of the separating seat  25  is 50˜80% of the diameter of the ball  26 . If the outer diameter is out of this range, the pose of the separating seat  25  will become unstable and thus contact the screw grooves  29   a  and  30   a  due to the presence of a gap between the mutually adjacent balls  26 , which would cause the torque increase.  
         [0056]    The separating seat can be made of either sintered metal or plastic material having self-lubricating property. Such a material includes for example polyimide (PI) or polyamide (PA) reinforced by any member. The material having high lubricity includes solid self lubricating component such as ultra-high-molecular-weight polyolefin dispersed with lubricating components such as lubricating oil or grease. More particularly, such a material can be made by mixing 95˜1 weight % ultra-high-molecular-weight polyolefin powder and 5˜99 weight % lubricating component, introducing this mixture into a mold, heating the mixture in the mold to a temperature higher than the gelling temperature of ultra-high-molecular weight polyolefin, and finally cooling the mixture. The ultra-high-molecular-weight polyolefin powder includes powders of polyethylene, polypropylene, polybutene, or copolymers of these materials, or mixured powders of these materials each having average molecular weight of about 1×10 6 ˜5×10 6 .  
         [0057]    Ordinarily, the ball screw is provided with a ball circulating portion (not shown). There are several types of structures of ball circulating portion, for example, a return tube type where the ball circulating portion is formed on the outer circumferential portion of the nut; a guide plate type where the ball circulating portion is formed on the inner circumferential portion of the nut; a “top” type where the ball circulating portion is formed on the inner circumferential portion of the nut and the balls are returned within a raceway formed by connecting mutually adjacent screw grooves; and an end cap type where the ball circulating portion is formed on the end of the nut and the balls are translated in a reversed manner into an axially extending through passage of the nut. The present invention can be applied to any one of these types. However, the guide plate type or the “top” type which does not have any projection on the outer circumferential portion is preferable.  
         [0058]    The present invention has been described with reference to the preferred embodiment. Obviously, modifications and alternations will occur to those of ordinary skill in the art upon read and understanding the preceding detailed description. It is intended that the present invention be construed as including all such alternations and modifications insofar as they come within the scope of the appended claims or the equivalents thereof.