Abstract:
There is provided a ball screw that is used to apply a preload to balls and that has a configuration of achieving a balance between the cooling efficiency and ease of assembly and disassembly. To this end, the ball screw includes a screw shaft, and a plurality of nuts movable relative to the screw shaft in an axial direction. Each of the nuts includes flow channels, which are cooling mechanisms and which respectively serve as independent channels to allow a cooling medium to pass through them. These flow channels are provided symmetrically with respect to a space between the nuts, and are configured so that the cooling medium can independently circulate in each of the nuts.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to ball screws, more particularly to a ball screw having a function to cool a nut. 
       BACKGROUND ART 
       [0002]    There has heretofore been known a ball screw which includes a screw shaft, and a nut that is screwed to the screw shaft via plural rolling elements (e.g., balls) and in which the screw shaft and the nut are relatively rotatable. In this ball screw, frictional heat resulting from a point contact or surface contact is generated between the screw shaft and the nut during rotation. Hence, a cooling portion to reduce the frictional heat is provided in some cases. 
         [0003]    Types of installing the cooling portion in the conventional ball screw include a shaft center cooling type and a nut cooling type. The shaft center cooling type is a configured such that the screw shaft is a cooling target so that the cooling portion is provided in the screw shaft. As an example of this shaft center cooling type, the screw shaft is hollow to allow a cooling medium to flow through the screw shaft. The nut cooling type is configured such that the nut is a cooling target and the cooling portion is provided in the nut. 
         [0004]    Here, the shaft center cooling type may have a drawback in cost for making a hollow hole in the screw shaft, when the shaft center cooling type is used in a large and long ball screw device. Therefore, the nut cooling type is often used. 
         [0005]    The technique disclosed in Patent Literature 1 is given as the ball screw using such a nut cooling type is. Specifically, according to the technique in Patent Literature 1, a cooling medium is made to pass through a flow channel provided in a nut in an axial direction to cool the nut. 
       CITATION LIST 
     Patent Literature 
       [0006]    PTL 1: JP 2010-133556 A 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0007]    The mainly used ones of recent large and long ball screws are ball screws manufactured by using double nut preload. In such ball screws, it is important to efficiently cool plural nuts, and prevent leakage of the cooling medium during disassembly, assembly, and maintenance. 
         [0008]    However, even when the ball screw disclosed in Patent Literature 1 is applied to the ball screw manufactured by using the double nut preload, there is room for improvement in providing a ball screw which achieves a balance between efficient cooling of plural nuts and the prevention of the leakage of the cooling medium. 
         [0009]    The present invention has been made to address the above drawbacks, and has an object to provide a ball screw, which achieves a high cooling efficiency, which prevents the leakage of a cooling medium even during disassembly, assembly, and maintenance, and which achieves a well balance between the cooling efficiency and the prevention of leakage. 
       Solution to Problem 
       [0010]    To achieve the above object, a ball screw according to one embodiment of the present invention includes: a screw shaft in which a spiral groove is arranged on an outer circumferential surface; 
         [0011]    two nuts, each of which includes a spiral groove facing the spiral groove of the screw shaft, and which are screwed to the screw shaft via a plurality of balls disposed between the spiral grooves of the two nuts and the spiral groove of the screw shaft; and 
         [0012]    a preload application member configured to apply a preload to the balls, 
         [0013]    wherein cooling portions are independently provided in the two nuts, respectively, 
         [0014]    wherein the cooling portions are respectively provided in the nuts to be symmetric with respect to a space between the two nuts, 
         [0015]    wherein each of the cooling portions is a flow channel to pass a cooling medium, 
         [0016]    wherein the flow channel comprises at least one of an axial flow channel extending in an axial direction or a circumferential flow channel provided to intersect perpendicularly to the axial direction, 
         [0017]    wherein the preload application member is a spacer which is coaxial with the two nuts and which is disposed between the two nuts, 
         [0018]    wherein the axial flow channel is provided through each of the two nuts in the axial direction, and 
         [0019]    wherein a seal portion is provided coaxially with the two nuts, the seal portion being provided at an end of each of the two nuts and having one surface forming the axial flow channel and the circumferential flow channel and another surface in contact with the spacer. 
         [0020]    That is, in a ball screw according to one embodiment of the present invention including a screw shaft in which a spiral groove is arranged on an outer circumferential surface; two nuts, each of which includes a spiral groove facing the spiral groove of the screw shaft, and which are screwed to the screw shaft via a plurality of balls disposed between the spiral grooves of the two nuts and the spiral groove of the screw shaft; and a preload application member configured to apply a preload to the balls, 
         [0021]    cooling portions (cooling mechanisms) are independently provided in the two nuts, respectively, and 
         [0022]    the cooling portions are respectively provided in the nuts to be symmetric with respect to a space between the two nuts. The above-described two nuts are incorporated by coupling, connecting, or adjacently arranged. 
         [0023]    Here, in the ball screw, each of the cooling portions is a flow channel to pass a cooling medium, and the flow channel may include at least one of an axial flow channel extending along the axial direction and a circumferential flow channel provided so as to intersect at right angles with the axial direction. That is, each of the cooling portions has a flow channel to pass the cooling medium as an independent channel. Each of these flow channels includes at least one of an axial flow channel and a circumferential flow channel. The axial flow channel is a flow channel axially provided between the inner circumference of the nut and the outer circumference of the nut. The circumferential flow channel is a flow channel provided between the inner circumference of the nut and the outer circumference of the nut in a direction that intersects at right angles with the axis of the nut. The cooling portions are configured so that the cooling medium can independently circulate in each of the nuts. 
         [0024]    The ball screw may be configured so that each of the nuts is provided with an inflow opening which is coupled to the flow channels and into which the cooling medium flows, and a discharge opening which is coupled to the flow channels and which discharges the cooling medium. That is, each of the nuts may be configured to have at least one pair of an inflow opening which is an inflow portion for the cooling medium and a discharge opening which is an outflow portion. 
         [0025]    The ball screw may be configured so that the preload application member is a spacer which is coaxial with the two nuts and which is disposed between the two nuts. 
         [0026]    As the preload application member, it is possible to use a configuration which uses, for example, fixed position preload produced by a positional adjustment between the nuts, constant pressure preload produced by an elastic body such as a spring, or variable control preload to set a predetermined preload load by using fluid pressure or an actuator such as a piezoelectric element. More specifically, it is possible to use a configuration in which the above configuration of the preload is disposed between the nuts so that a plurality of nuts press or pull one another. 
       Advantageous Effects of Invention 
       [0027]    According to one aspect of the present invention, it is possible to provide a ball screw, which achieves a high cooling efficiency, which prevents leakage of a cooling medium even during disassembly, assembly, and maintenance, and which achieves a well balance between the cooling efficiency and the prevention of leakage. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0028]      FIG. 1  is a side view of a configuration of a ball screw in one embodiment of the present invention; 
           [0029]      FIG. 2  is a partial sectional view of a configuration of the ball screw in one embodiment of the present invention; and 
           [0030]      FIG. 3  is a side view of a configuration of the ball screw in one embodiment of the present invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0031]    Embodiments of the present invention will now be described with reference to the drawings. 
         [0032]      FIG. 1  is a side view of a configuration of a ball screw in one embodiment of the present invention.  FIG. 2  is a partial sectional view of a configuration of the ball screw in one embodiment of the present invention.  FIG. 3  is a side view of a configuration of the ball screw in one embodiment of the present invention, and flow channels are highlighted by hatching. 
         [0033]    As illustrated in  FIG. 1 , a ball screw  1  in one embodiment of the present embodiment includes two nuts (which are a first nut  100  and a second nut  200 ), a screw shaft  300 , and a preload application member which is provided between the two nuts and which is configured to apply preload to the ball. 
       Nuts 
       [0034]    The first nut  100  includes a body portion  100 A cylindrically formed to have an inside diameter larger than the outside diameter of the screw shaft  300 , a cap  110 , and tubes  120  and  121 . The cap  110  is attached to one end of the body portion  100 A by, for example, unillustrated screws via a seal material  111 . The tubes  120  and  121  are members for rolling element circulation provided in a flat portion  115  of the outer circumferential surface of the body portion  100 A, and are fixed by a holding plate  122  fastened to the first nut  100  by screws  123  to  125 . The first nut  100  is known as a flangeless type nut. 
         [0035]    The second nut  200  includes a body portion  200 A cylindrically formed to have an inside diameter larger than the outside diameter of the screw shaft  300 , a flange portion  201  provided at one end of the body portion  200 A, a cap  210 , and tubes  220  and  221 . The cap  210  is attached to an end of the body portion  200 A by, for example, unillustrated screws via a seal material  211 . The tubes  220  and  221  are members for rolling element circulation provided in a flat portion  215  of the outer circumferential surface of the body portion  200 A, and are fixed by a holding plate  222  fastened to the second nut  200  by screws  223  to  225 . 
         [0036]    Here, as illustrated in  FIG. 2 , a spiral groove  101  is formed on an inner circumferential surface  100   a  of the first nut  100  so as to face a spiral groove  301  spirally formed on an outer circumferential surface  300   a  of the screw shaft  300 . The spiral groove  201  is also formed on an inner circumferential surface  200   a  of the second nut  200  so as to face the spiral groove  301  of the screw shaft  300 . The first nut  100  and the second nut  200  are coaxially arranged in the axial direction of the screw shaft  300 , and are screwed with the screw shaft  300  by plural rolling elements B provided between the spiral groove  101  and the spiral groove  201 . Thus, the rolling elements B are capable of rolling through a rolling channel formed by the spiral groove  301  and the spiral grooves  101  and  201 , so that the screw shaft  300 , and the first nut  100 , and the second nut  200  can relatively move in the axial direction. 
       Preload Application Member 
       [0037]    As illustrated in  FIG. 1  and  FIG. 2 , a spacer  128  is provided coaxially with the first nut  100  and the second nut  200  between the other end of the first nut  100  and the other end of the second nut  200  via seal materials  129  and  130 . The first nut  100  and the second nut  200  are coupled to each other by a coupling member  127  via the spacer  128  and the seal materials  129  and  130 . Only one coupling member  127  is illustrated in  FIG. 1  and  FIG. 2 , but the first nut  100  and the second nut  200  may be coupled to each other at plural places by plural coupling members  127  as needed. 
         [0038]    Thus, the first nut  100  and the second nut  200  are coupled to each other by the coupling member  127  via the spacer  128 , so that the spacer  128  and the coupling member  127  function as preload application members, leading to a state in which what is known as fixed position preload is applied. In the ball screw  1  in one embodiment of the present invention, two-point contact preload in a pulling direction is applied to the first nut  100  and the second nut  200  to cancel an increase in preload torque caused by cooling the first nut  100  and the second nut  200 . This configuration enables efficient cooling of the first nut  100  and the second nut  200 . 
         [0039]    As the preload application member, it is possible to select a configuration which uses, for example, not only the fixed position preload produced by a positional adjustment between the above nuts  100  and  200  but also constant pressure preload produced by an elastic body such as a spring, fluid pressure preload, or variable control preload to set a predetermined preload by using an actuator such as a piezoelectric element. 
       Cooling Portions 
       [0040]    The first nut  100  and the second nut  200  respectively include cooling portions  150  and  250  which independently cool the nuts  100  and  200 . These cooling portions  150  and  250  are respectively arranged in the nuts  100  and  200  symmetrically with respect to a virtual area between the two nuts  100  and  200 . The area is a virtual plane A (see  FIG. 2 ) extending through the midpoint in the axial direction of the spacer  128  and passing perpendicularly to the axial direction. The provision of the cooling portions  150  and  250  keeps the weight balance of a ball screw device, and does not prevent smooth activation. Here, the configurations of the cooling portions  150  and  250  are not limited in particular, as long as the cooling portions  150  and  250  are provided symmetrically with respect to the virtual area between the two nuts  100  and  200 , and as long as the cooling portions  150  and  250  independently cool the first nut  100  and the second nut  200 , respectively. A suitable configuration can be selected depending on the purpose. 
         [0041]    For example, as illustrated in  FIG. 1  and  FIG. 2 , the cooling portions  150  and  250  include plural flow channels  152  to  154  and  252  to  256  which are respectively pierced in the first nut  100  and the second nut  200  so that a cooling medium can pass through them. Each of these flow channels can include at least one of an axial flow channel extending in the axial direction and a circumferential flow channel provided to intersect perpendicularly to the axial direction. The aforementioned “axial flow channel” includes the axial flow channels  152  and  154  extending in the axial direction in the first nut  100 , and axial flow channels  253  and  255  extending in the axial direction in the second nut  200 . Among these axial flow channels  152 ,  154 ,  253 , and  255 , the axial flow channel  152  and the axial flow channel  253  are flow channels symmetrically provided between the two nuts  100  and  200 . The axial flow channel  154  and the axial flow channel  255  are flow channels symmetrically provided in the nuts  100  and  200 , respectively. 
         [0042]    The aforementioned “circumferential flow channel” includes the circumferential flow channel  153  provided in the circumferential direction to intersect perpendicularly to the axial direction in the first nut  100 , and the circumferential flow channels  252 ,  254 , and  256  provided in the circumferential direction to intersect perpendicularly to the axial direction in the second nut  200 . Among these circumferential flow channels  153 ,  252 ,  254 , and  256 , the circumferential flow channel  153  and the axial flow channel  254  are flow channels symmetrically provided in the nuts  100  and  200 , respectively. In this way, the cooling portions  150  and  250  are configured to allow the cooling medium to independently circulate in each of the nuts  100  and  200 . 
         [0043]    In the ball screw  1  in one embodiment of the present invention, an inflow opening  151  which is coupled to the flow channels  152  to  154  and into which the cooling medium flows, and a discharge opening  155  configured to discharge the cooling medium may be provided in the first nut  100 . Moreover, in the ball screw  1  in one embodiment of the present invention, an inflow opening  251  which is coupled to the flow channels  252  to  256  and into which the cooling medium flows, and a discharge opening  257  configured to discharge the cooling medium may be provided in the second nut  200 . That is, each of the nuts  100  and  200  may include at least one pair of the inflow opening  151  or  251 , which is an inflow portion for the cooling medium, and a discharge opening  155  or  257 , which is a discharge portion. 
         [0044]    Therefore, as illustrated in  FIG. 1  to  FIG. 3 , the cooling portion  150  of the first nut  100  includes the inflow portion  151  provided at the end of the cap  110  in the axial direction, the axial flow channel  152 , the circumferential flow channel  153 , the axial flow channel  154 , and the discharge portion  155  provided at the end of the cap  110  in the axial direction. That is, a single independent system of flow channel is formed in the first nut  100 . Two or more systems of flow channels may be provided as long as the flow channels are independently provided in each of the nuts  100  and  200 . 
         [0045]    As illustrated in  FIG. 1  to  FIG. 3 , the cooling portion  250  of the second nut  200  includes the inflow portion  251  provided in the outer circumference of the flange portion  201  in a direction that intersects perpendicularly to the axis, the circumferential flow channel  252 , the axial flow channel  253 , the circumferential flow channel  254 , the axial flow channel  255 , the circumferential flow channel  256 , and the outflow portion  257  provided in the outer circumference of the flange portion  201  in the direction that intersects perpendicularly to the axis. That is, a single independent system of flow channel is formed in the second nut  200 . Two or more systems of flow channels may be provided as long as the flow channels are independently provided in each of the nuts  100  and  200 . 
         [0046]    Here, tube taper screws for pipe fastening are provided in the inflow portions  151  and  251  and the discharge portions  155  and  257 , so that pipes are connected to these tube taper screws. This configuration enables supply and discharge of the cooling medium. 
       Cooling Medium 
       [0047]    Various gases and liquids can be used as a fluid to serve as the cooling medium. As a gas, not only air or compressed air but also nitrogen, an inert gas (e.g., argon), hydrocarbon (e.g., butane or isobutane), helium, ammonia, carbon dioxide, or a mixture of the above gases can be used. As a liquid, not only water but also a coolant in which antirust is added to water, a coolant in which various additives are added to water, or various oils as cooling medium oils can be used. Specifically, mineral oils, animal and vegetable oils, or synthetic oils can be used. A suitable one of the above may be selected depending on, for example, the use environment. Further, the temperature of the cooling medium may be managed and the flow volume may also be managed. The cooling medium may be used in a turbulent state. 
         [0048]    In one embodiment of the present invention, the temperature can be managed in each nut, and, for example, the preload can be controlled. 
         [0049]    Furthermore, the positions and sizes of the inflow portions and the discharge portions, and the sectional shape and sectional area of each flow channel can be suitably adjusted depending on the use condition. 
         [0050]    According to the ball screw in one embodiment of the present invention, the preload load can be higher, and the ball screw can therefore be suitably applied to what is known as a large-sized ball screw (i.e., the outside diametrical dimension of the screw shaft  300  is substantially 80 nm or more). 
         [0051]    According to the ball screw in one embodiment of the present invention, the cooling medium can independently circulate in each of the nuts. Thus, the cooling efficiency is high. Such a high cooling efficiency allows the nuts to be cooled firstly. Then, and the effects of the cooling are transmitted to the balls that are the rolling elements from the spiral grooves on the inner circumferences of the nuts and further transmitted to the spiral groove of the screw shaft, and also cools the screw shaft. The preload change and the deterioration of lubrication caused by heat generation are prevented, accordingly. The cooling effects are remarkably exhibited, when the preload load is relatively high and when the contact states between the spiral grooves and the rolling elements, and the spiral grooves and the rolling elements are maintained. Therefore, the ball screw according to the present invention is suitable to a relatively large-sized ball screw. 
         [0052]    According to the ball screw in one embodiment of the present invention, even a long (i.e., about 4 m or more) ball screw in which the use of what is known as the shaft center cooling type is difficult can be applied without the shaft center cooling, but the shaft center cooling can be used together when necessary. 
         [0053]    Furthermore, according to the ball screw in one embodiment of the present invention, the flow channel of the cooling medium for cooling is independently provided in each of the nuts. Therefore, for example, even at the time of maintenance in which some of the nuts need to be replaced, the leakage of the cooling medium during maintenance can be effectively prevented when piping to each of the cooling medium flow channels is blocked. 
         [0054]    Thus, the ball screw in one embodiment of the present invention can be suitably used as a ball screw which demands processing accuracy in particular and which is used in what is known as a linearly moving part of a large-sized machine tool that may be subject to the maintenance. 
         [0055]    The ball screw in one embodiment of the present invention achieves a high cooling efficiency. Hence, there are no significant changes in the preload and the length of the screw shaft, and excessive heat generation is prevented. Therefore, no significant deterioration of a lubricant can be found. Accordingly, there is no significant deterioration in positioning accuracy of the linearly moving part caused by heat generation in the ball screw, a stable operation in which what is known as torque variation is small can be maintained for a long period of time, and such advantages can be found at even a long screw shaft. Consequently, the ball screws in some embodiments of the present invention are particularly applicable as ball screws used in linearly moving parts of large-sized machine tools for high-precision processing. The ball screw in one embodiment of the present invention can be used together with the shaft center cooling. 
         [0056]    The ball screws in some embodiments of the present invention are applicable as ball screws used in linearly moving parts of large-sized machine tools for high-precision processing. 
         [0057]    While the embodiments of the present invention have been described above, the present invention is not limited thereto, and various modifications and improvements can be made. 
       REFERENCE SIGNS LIST 
       [0000]    
       
         
           
               1  ball screw 
               100  first nut 
               150  cooling portion 
               200  second nut 
               250  cooling portion 
               300  screw shaft