Patent Document

BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a linear actuator, and particularly a linear actuator to convert a rotational movement into a linear movement. 
         [0003]    2. Description of the Related Art 
         [0004]    A linear actuator is conventionally provided which, for example, converts a rotational movement of a motor into a linear movement of an output shaft. 
         [0005]    For example, in a linear actuator disclosed in Japanese Patent Application Laid-Open No. 2002-122203, the outer periphery of an output shaft is provided with a screw thread while the inner periphery of a nut as a mating member fixedly attached to a rotor of a motor is provided with a screw thread, wherein both of the screw threads engage with each other whereby the rotational movement of the rotor is converted into the linear movement of the output shaft in the axial direction. 
         [0006]    Also, in a linear actuator disclosed in Japanese Patent Application Laid-Open No. 2002-372117, a ball screw system is provided between a rotor and an output shaft, whereby the rotational movement of the rotor is converted into the linear movement in the axial direction. 
         [0007]    In the linear actuator disclosed in Japanese Patent Application Laid-Open No. 2002-122203, the rotational movement of the rotor is transmitted to the output shaft by means of a screw system which has a high friction resistance, and therefore the transmission efficiency is low thus preventing the torque of the motor from being transmitted sufficiently. 
         [0008]    On the other hand, in the linear actuator which is disclosed in Japanese Patent Application Laid-Open No. No. 2002-372117, and which uses a ball screw system, the output shaft is provided with a ball screw, and also a place corresponding to a nut is provided with a ball groove, wherein balls must be circulated without running off from the screw portion, thus resulting in a complicated structure. 
       SUMMARY OF THE INVENTION 
       [0009]    The present invention has been made in light of the problems described above, and it is an object of the present invention to provide a linear actuator which has a simple structure, and at the same time in which a large thrust force is raised even with a small torque. 
         [0010]    In order to achieve the object described above, according to an aspect of the present invention, there is provided a linear actuator which includes a motor and a screw as an output shaft and in which the rotational movement of the motor is converted into the linear movement of the screw, wherein at least one retainer to hold a plurality of balls is disposed at the inner periphery of a hollow rotor of the motor, a ball screw is formed at the outer periphery of the screw, the balls are engaged with the ball screw such that the screw is set coaxial to the rotor, and wherein the screw provided with the ball screw is moved linearly by means of the rotation of the balls disposed circumferentially at the inner periphery of the rotor. 
         [0011]    In the aspect of the present invention, the linear actuator may include a plurality of retainers disposed to be located apart from each other in the axial direction of the screw. 
         [0012]    In the aspect of the present invention, the linear actuator may include one retainer, and the plurality of balls held by the one retainer may be arranged in a plurality of rows in the axial direction of the screw. 
         [0013]    In the aspect of the present invention, the liner actuator may include a rotation preventing member functioning to prevent the screw from rotating, which functions also to define forward and rearward moving ends of the linear movement of the screw. 
         [0014]    In the aspect of the present invention, the rotation preventing member may be a block which is disposed around the screw and which has an axial cross section having a polygonal shape. 
         [0015]    In the aspect of the present invention, the rotation preventing member may be a pin which is disposed on the screw and which is oriented substantially perpendicular to the axial direction of the screw. 
         [0016]    In the aspect of the present invention, the retainer may have a ring shape. 
         [0017]    And, in the aspect of the present invention, the retainer may have a ribbon shape. 
         [0018]    According to the present invention, a linear actuator can be provided which has a simple structure and in which a large thrust force is raised even with a small torque. 
         [0019]    Specifically, the linear actuator according to the present invention is structured such that a retainer to hold balls is attached to the inner periphery of a rotor without forming a ball groove at the inner periphery of the rotor, whereby a large thrust force is raised even with a small torque. 
         [0020]    Also, the linear actuator according to the present invention includes a force transmission mechanism which incorporates a combination of a screw and balls wherein the friction resistance can be reduced by means of the balls rolling, in comparison to a force transmission mechanism which is conventionally constituted by a screw and nut engagement thus involving a high friction resistance, whereby the torque efficiency can be improved. 
         [0021]    Also, the linear actuator according to the present invention, while maintaining a high precision, can be assembled with a reduced number of component members and with reduced man hours. 
         [0022]    Further, the retainer of the linear actuator according to the present invention can be formed in various configurations thus resulting in a high productivity. 
         [0023]    Still further, the balls can be made of a wide variety of materials, such as metal, ceramic, resin and the like thus resulting in a high productivity. 
         [0024]    And moreover, the linear actuator according to the present invention is structured such that the rotation preventing member functioning to prevent the output shaft from rotating functions also as a stopper member to define the forward and backward moving ends of the output shaft, which contributes to making the assembly easier, and therefore which results in reduced assembly man hours and in a higher productivity. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]      FIG. 1  is a partial axial cross-sectional view of a linear actuator according to a first embodiment of the present invention, taken at a relevant portion of an output shaft (screw); 
           [0026]      FIG. 2  is a radial cross-sectional view of the linear actuator of  FIG. 1 , taken along line II-II; 
           [0027]      FIG. 3  is a perspective view of one of retainers of the linear actuator of  FIG. 1  holding balls in place: 
           [0028]      FIGS. 4A and 4B  are explanatory views of a portion of the linear actuator of  FIG. 1  located in a vicinity of the retainers, wherein  FIG. 4A  shows a side view of the screw and the retainers, and  FIG. 4B  shows an axial cross section of the screw and the retainers taken along an axis center of the screw; 
           [0029]      FIG. 5  is a partial axial cross-sectional view of a linear actuator according to a second embodiment of the present invention, taken at a relevant portion of an output shaft (screw); 
           [0030]      FIG. 6  is a radial cross-sectional view of the linear actuator of  FIG. 5 , taken along line VI-VI; 
           [0031]      FIG. 7  is a partial axial cross-sectional view of a linear actuator according to a third embodiment of the present invention, taken at a relevant portion of an output shaft (screw); 
           [0032]      FIG. 8  is a radial cross-sectional view of the linear actuator of  FIG. 7 , taken along line VIII-VIII; 
           [0033]      FIG. 9  is a perspective view of one of ribbon retainers of the linear actuator of  FIG. 7  holding balls in place; and 
           [0034]      FIGS. 10A and 10B  are explanatory views of a portion of the linear actuator of  FIG. 7  located in a vicinity of the ribbon retainers, wherein  FIG. 10  shows a side view of the screw and the ribbon retainers, and  FIG. 10B  shows an axial cross section of the screw and the ribbon retainers taken along an axis center of the screw. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0035]    Exemplary embodiments of the present invention will hereinafter be described with reference to the accompanying drawings. 
         [0000]    A first embodiment of the present invention will be described with reference to  FIGS. 1 to 4B . 
         [0036]      FIG. 1  shows a partial axial cross section of a linear actuator  100  according to the first embodiment of the present invention, taken at a relevant portion of an output shaft. 
         [0037]    Referring to  FIG. 1 , the linear actuator  100  according to the first embodiment includes a pair of stator units  6  which in combination form a stator assembly, has a hollow cylindrical shape and each of which includes: a coil bobbin  3 ; a coil  5  which is wound around the coil bobbin  3 ; terminal elements  4  through which electric power is supplied to the coil  5 ; and cylindrical stator yokes which are made of a soft magnetic steel sheet and which are formed by sheet-metal processing, and which each have pole teeth arranged at the inner periphery thereof. 
         [0038]    The stator assembly composed of the pair of stator units  6  is fixedly sandwiched between a front housing  1  and a rear housing  11 . 
         [0039]    The linear actuator  100  further includes a rotor which is arranged in the hollow of the stator assembly and which includes: a cylindrical rotor sleeve  9  connected to the front and rear housings  1  and  11  via a pair of bearings  8 , respectively, so as to be rotatable with respect to the stator units  6  of the stator assembly; and cylindrical ring magnets (field magnets)  7  fixedly attached at the outer periphery of the rotor sleeve  9  by, for example, insert fitting. 
         [0040]    In the hollow of the rotor sleeve  9 , a screw  10  as an output shaft is provided coaxially to the rotor sleeve  9 . A ball screw  10   a  is formed at the outer periphery of the screw  10 . Retainers  12 , which have a cylindrical ring shape and which are adapted to hold a plurality of balls  13  having a spherical shape, are fixedly attached to the inner periphery of the rotor sleeve  9  by, for example, insert fitting. 
         [0041]    In the second embodiment, two of the retainers  12  are provided respectively at two places of the screw  10  located apart from each other in the axial direction. The number of the retainers  12 , however, is not limited to two and may be three or more. The balls  13  held by the retainers  12  engage with the ball screw  10   a  of the screw  10 . 
         [0042]    A pin  2  is inserted in the screw  10  so as to be oriented substantially perpendicular to the axial direction of the screw  10 . 
         [0043]      FIG. 2  shows a radial cross section of the linear actuator  100  of  FIG. 1 , taken along line II-II. 
         [0044]    As shown in  FIG. 2 , the pin  2  inserted in the screw  10  radially protrudes from the outer periphery of the screw  10 , and on the other hand the front housing  1  has an inner diameter larger than the outer diameter of the screw  10  and at the same time has an inner radius smaller than the dimension which is defined between the axis center of the screw  10  and the top end of the pin  2  radially protruding from the outer periphery of the screw  10 . 
         [0045]    Further, a slit la is arranged at a place of the inner periphery of the front housing  1  located corresponding to the pin  2 . The slit  1   a  has a radial cross-sectional geometry substantially analogous to the longitudinal shape of the pin  2  with a slightly larger dimension and has an elongated axial dimension. With the arrangement described above, the pin  2  is prevented from coming off from the slit  1   a  and therefore the screw  10  is not allowed to rotate thus moving only forward and backward in the axial direction. 
         [0046]    The slit la is provided with front and rear stopper members which are located at the front and rear ends of the slit  1   a  and which are constituted respectively by portions of the front housing  1  and the rotor sleeve  9  against which the pin  2 , when moving forward and backward, hits, whereby the forward and backward moving ends of the screw  10  are defined. That is to say, the pin  2  as a rotation preventing member which prevents the screw  10  from rotating functions also as a stopper member to define the forward and backward moving ends of the linear movement of the screw  10 . 
         [0047]    In this connection, a spring, a washer or the like may be provided at the hitting places to thereby prevent the output shaft from getting stuck at the forward and backward moving ends. 
         [0048]    Description will now be made of the retainer  12  shown in  FIG. 1 . 
         [0049]      FIG. 3  perspectively shows the retainer  12  shown in  FIG. 1  holding the balls  13  in place. 
         [0050]      FIGS. 4A and 4B  illustrate a portion of the linear actuator  100  of  FIG. 1  located in the vicinity of the retainers  12 , wherein  FIG. 4A  shows the side view of the screw  10  and the retainers  12 , and  FIG. 4B  shows the axial cross section of the screw  10  and the retainers  12  taken along the axis center of the screw  10 . 
         [0051]    Referring to  FIG. 3 , a plurality of through-holes  12   a  are provided at each of the retainers  12  in such a manner as to be arranged equidistantly from one another in the circumferential direction and located so as to correspond to the ball screw  10   a.  Thus, the through-holes  12   a  are arranged in a spiral manner on the circumference of the retainer  12 . 
         [0052]    When the balls  13  are put in the through-holes  12   a  from the outer periphery of the retainer  12 , the inner tip of the ball  13  protrudes inwardly from the inner periphery of the retainer  12 , and a portion of the ball  13  protruding inwardly is adapted to engage with the ball screw of  10   a  of the screw  10 . 
         [0053]    The retainer  12  holds the balls  13  in place, wherein after the retainer  12  is attached to the rotor sleeve  9 , it does not happen that the balls  13  come off. With the structure that the retainer  12  is fixedly attached to the rotor sleeve  9 , the rotation of the rotor sleeve  9  is transmitted to the screw  10  via a point contact of the ball  13 . 
         [0054]    In the first embodiment, the retainer  12  is provided with seven of the through-holes  12   a,  which corresponds to the number of the balls  13 . The present invention, however, is not limited to this arrangement, wherein it is desirable that the balls  13  support the outer periphery of the ball screw  10   a  of the screw  10  without backlash. 
         [0055]    In this connection, it is not possible to unlimitedly increase the number of the balls  13  in view of the provision of the retainer  12 , so it is preferable to use an odd number of the balls  13  in order to achieve a high precision and a stable load with the least number of the balls  13 . 
         [0056]    If an even number of the balls  13  are used, when a weight is applied laterally (perpendicularly to the shaft), a symmetric position appears, and it can happen that a resultant load is focused on only one of the balls  13 . 
         [0057]    On the other hand, if an odd number of the balls  13  are used, the load is borne by two or more of the balls  13  and thus can be dispersed. Since it is not possible for three of the balls  13  to duly bear the load, and since the load may be focused on one of the balls  13  if four of the balls  13  are used, it is preferable to use at least five of the balls  13  or a larger odd number thereof. 
         [0058]    According to the first embodiment, since the retainer  12  can be fixedly attached in a unified manner, by means of adhesion, insert fitting or a like method, inside the hollow rotor which includes a magnet  7  as well as the rotor sleeve  9  made of metal (stainless steel, aluminum, or the like), the only thing to be done is to set the outer periphery of the magnet  7  coaxial to the inner periphery of the rotor sleeve  9 . Consequently, the linear actuator  100  can be assembly with an increased precision and with reduced man hours. 
         [0059]    It is preferable that the retainer  12  to hold the balls  13 , though not limited in terms of material, be made of metal or abrasion-resistant resin, such as polyacetal (POM) or polyphenylene sulphide (PPS), and also be structured to prevent the balls  13  from falling inside. With such a structure, the assembly can be done easily. 
         [0060]    A second embodiment of the present invention will be described with reference to  FIGS. 5 and 6 . 
         [0061]      FIG. 5  shows a partial axial cross section of a linear actuator  1100  according to the second embodiment of the present invention, taken at a relevant portion of an output shaft. 
         [0062]    Referring to  FIG. 5 , the linear actuator  1100  according to the second embodiment is substantially the same as the linear actuator  100  according to the first embodiment shown in  FIG. 1  except in that one retainer  112  is provided in place of the two retainers  12  and in that a block  14  is provided in place of the pin  2 . So, identical parts and similar have the same reference numbers as in  FIG. 1 , and a detailed description thereof will be omitted. 
         [0063]    In the hollow of a rotor sleeve  9 , a screw  10  as an output shaft is arranged coaxial to the rotor sleeve  9 . A ball screw  10   a  is formed at the outer periphery of the screw  10 . The aforementioned retainer  112 , which has a circular cylindrical shape and which is adapted to hold a plurality of balls  113  having a spherical shape, is fixedly attached to the inner periphery of the rotor sleeve  9  by, for example, insert fitting. The balls  113  held by the retainer  112  engage with a ball screw  10   a  of the screw  10 . 
         [0064]    While each retainer  12  according to the first embodiment shown in  FIG. 3  is structured to hold the balls  13  in such a manner that the balls  13  are arranged in one row running around the outer periphery of the screw  10 , the retainer  112  according to the second embodiment is structured to hold the balls  113  in such a manner that the balls  113  are arranged in a plurality of rows running around the outer periphery of the screw  10 . In the embodiment example shown in  FIG. 5 , the retainer  112  holds the balls  113  provided with seven of such rows. 
         [0065]    Also, in the second embodiment, the aforementioned block  14  is provided in place of the pin  2  as described above. The block  14  has a polygonal (quadrangular in the second embodiment) cross section taken along a direction substantially perpendicular to the axial direction, and is fixedly attached around the screw  10 . 
         [0066]      FIG. 6  shows a radial cross section of the linear actuator  1100  of  FIG. 5 , taken along line VI-VI. 
         [0067]    Referring to  FIG. 6 , the block  14  attached around the screw  10  protrudes radially outwardly from the outer periphery of the screw  10 . A front housing  101 , inside which the screw  10  is housed, has a hollow  101   a  which has a polygonal radial cross-sectional geometry analogous to and slightly larger than the radial cross section of the block  14  and the wall surface of which is located radially outwardly of the outer periphery of the screw  10 . 
         [0068]    With the arrangement described above, the block  14 , while prohibited from coming off from the polygonal hollow  101   a,  is prevented from moving in the circumferential direction, and therefore the screw  10  having the block  14  fixedly attached therearound is not allowed to rotate and thus allowed only to move forward and backward in the axial direction. 
         [0069]    The polygonal hollow  101   a  is provided with front and rear stopper members which are located at the front and rear ends of the polygonal hollow  101   a  and which are constituted respectively by portions of the front housing  101  and the rotor sleeve  9  against which the block  14 , when moving forward and backward, hits, whereby the forward and rearward moving ends of the screw  10  are defined. That is to say, the block  14  as a rotation preventing member which prevents the screw  10  from rotating functions also as stopper members to define the forward and rearward moving ends of the linear movement of the screw  10 . 
         [0070]    The block  14  may be composed of, for example, two pieces and put together so as to fixedly enclose and grip the small diameter portion of the screw  10 , whereby the assembly can be performed easily while the screw  10  is prevented from rotating and also from axially coming off (that is to say, the forward and rearward moving ends of the linear movement are defined). 
         [0071]    In this connection, a spring, a washer or the like may be provided at the hitting places to thereby prevent the output shaft from getting stuck at the forward and rearward moving ends. 
         [0072]    A third embodiment of the present invention will be described with reference to  FIGS. 7 to 10B . 
         [0073]      FIG. 7  shows a partial axial cross section of a linear actuator  2100  according to the third embodiment of the present invention, taken at a relevant portion of an output shaft. 
         [0074]    Referring to  FIG. 7 , the linear actuator  2100  according to the third embodiment is substantially the same as the linear actuator  100  according to the first embodiment shown in  FIG. 1  except in that two ribbon retainer  212  are provided in place of the two retainers  12 . So, identical and similar parts have the same reference numbers as in  FIG. 1 , and a detailed description thereof will be omitted. 
         [0075]    In the hollow of a rotor sleeve  9 , a screw  10  as an output shaft is provided coaxially to the rotor sleeve  9 . A ball screw  10   a  is formed at the outer periphery of the screw  10 . Ribbon retainers  212  adapted to hold a plurality of balls  213  having a spherical shape are fixedly attached to the inner periphery of the rotor sleeve  9 . 
         [0076]    In the third embodiment, two of the ribbon retainers  212  are provided respectively at two places of the screw  10  located axially apart from each other. The number of the ribbon retainers  212  is not limited to two and may be three or more. The balls  213  held by the ribbon retainers  12  engage with the ball screw  10   a  of the screw  10 . 
         [0077]    A pin  2  is inserted in the screw  10  so as to be oriented substantially perpendicular to the axial direction of the screw  10 . 
         [0078]      FIG. 8  shows a radial cross section of the linear actuator  2100  of  FIG. 7 , taken along line VIII-VIII. 
         [0079]    As shown in  FIG. 8 , the pin  2  inserted in the screw  10  radially protrudes from the outer periphery of the screw  10 . On the other hand, the inner periphery of a front housing  1  has a larger diameter than the outer periphery of the screw  10  and at the same time has a radius smaller than the dimension which is defined between the axis center of the screw  10  and the top end of the pin  2  radially protruding from the screw  10 . 
         [0080]    Further, a slit  1   a  is arranged at a place of the inner periphery of the front housing  1  located corresponding to the pin  2 . The slit  1   a  has a radial geometry substantially same as the side shape of the pin  2  with a slightly larger dimension and has an elongated axial dimension. 
         [0081]    With the arrangement described above, the pin  2  is prevented from running off from the slit  1   a  and therefore the screw  10  having the pin  2  inserted therein is not allowed to rotate thus moving only forward and backward in the axial direction. 
         [0082]    The slit la is provided with front and rear stopper members which are located at the front and rear ends of the slit la and which are constituted respectively by portions of the front housing  1  and the rotor sleeve  9  against which the pin  2 , when moving forward and backward, hits, whereby the forward and rearward moving ends of the screw  10  are defined. That is to say, the pin  2  as a rotation preventing member which prevents the screw  10  from rotating functions also as stopper members to define the forward and rearward moving ends of the linear movement of the screw  10 . 
         [0083]    In this connection, a spring, a washer or the like may be provided at the hitting places to thereby prevent the output shaft from getting stuck at the forward and rearward moving ends. 
         [0084]    Description will now be made in detail of the ribbon retainer  212  shown in  FIG. 7 . 
         [0085]      FIG. 9  perspectively shows the retainer  212  shown in  FIG. 7  holding the balls  213  in place. 
         [0086]    And,  FIGS. 10A and 10B  illustrate a portion of the linear actuator  2100  of  FIG. 7  located in the vicinity of the ribbon retainers  212 , wherein  FIG. 10A  shows the side view of the screw  10  and the ribbon retainers  212 , and  FIG. 10B  shows the axial cross section of the screw  10  and the ribbon retainers  212  taken along the axis center of the screw  10 . 
         [0087]    Referring to  FIG. 9 , the ribbon retainer  212  is composed of two retainer pieces  214  and  215 , which are formed by, for example, pressing a steel sheet. 
         [0088]    The retainer piece  214  includes: a plurality of ball holding portions  214   a  which are arranged in an equidistant manner and which are each have a curve formed according to the shape of the ball  213  so that the balls  213  are set around the screw  10  equiangularly; and a plurality of flat portions  214   b  which each connect between two adjacent ball holding portions  214   a.    
         [0089]    In the same way, the retainer piece  215  includes: a plurality of ball holding portions  215   a  which are arranged in an equidistant manner so as to oppose the ball holding portions  214   a  of the retainer piece  214  and which are each have a curve formed according to the shape of the ball  213  so that the balls  213  are set around the screw  10  equiangularly; and a plurality of flat portions  215   b  which each connect between two adjacent ball holding portions  215   a.  And when the two retainer pieces  214  and  215  described above are coupled to each other, the balls  213  can be held in place around the screw  10  equidistantly. 
         [0090]    The retainer pieces  214  and  215  are put together with the plurality (seven in the present embodiment) of balls  213  sandwiched between the respective ball holding portions  214   a  and  215   a,  and are fixed to each other by, for example, swaging. 
         [0091]    The balls  213  held by the ribbon retainer  212  are arranged to be located corresponding to the ball screw  10   a  of the screw  10 . That is to say, the ribbon retainer  212  is formed spirally in accordance with the spiral of the ball screw  10   a  of the screw  10 . 
         [0092]    In the third embodiment, as shown in  FIGS. 10A and 10B , a groove  15  shaped according to the ball  213  is formed at the inner periphery of the rotor sleeve  9  and located corresponding to the balls  213  held by the ribbon retainer  212 . The balls  213  are engaged with the groove  15 , and the ribbon retainer  212  is fixedly attached to the rotor sleeve  9  (the ribbon retainer  212  may be fixed to the rotor sleeve  9  by adhesive or the like), and the retainer  212  is caused to rotate in accordance with the rotation of the rotor sleeve  9 . 
         [0093]    In the third embodiment, the groove  15  is formed at the inner periphery of the rotor sleeve  9 , but the present invention is not limited to this arrangement, and the groove  15  may not be formed at the inner periphery of the rotor sleeve  9  wherein the ribbon retainer  212  to hold the balls  213  may be fixedly attached to the rotor sleeve  9  by adhesion, or the like. 
         [0094]    The ribbon retainer  212  is adapted to hold the balls  213  in place, and the balls  213  are prevented from coming off from the ribbon retainer  212 . While the ribbon retainer  212  stays fixedly with respect to the rotor sleeve  9 , the rotation of the rotor sleeve  9  is transmitted to the screw  10  via the point contact of the ball  213 . 
         [0095]    Seven of the ball holding portions  214   a  and  215   a  of the ribbon retainer  212  as well as seven of the balls  213  are used in the third embodiment, but the present invention is not limited to this arrangement, wherein it is desirable to use five or a larger odd number of balls  213 . 
         [0096]    According to the third embodiment in which the ribbon retainers  212  are employed, the balls  213  are duly held by the ball holding portions  214   a  and  215   a  and prevented from falling inside or outside. 
         [0097]    The present invention has been explained with respect to the specific embodiments thereof but is by no means limited thereto. It will be apparent to those skilled in the art that numerous modifications and combinations may be possible without departing from the spirit and scope of the present invention, and also various combinations of the compositions of each embodiment may be included in the present invention.

Technology Category: y