Patent Publication Number: US-2016238114-A1

Title: Seal for Ball Screw Device

Description:
TECHNICAL FIELD 
     The present invention relates to a seal used for a ball screw device. 
     BACKGROUND ART 
     Conventionally, a ball screw device has been known as a mechanism that converts rotational motion to linear motion. A ball screw device is provided with a threaded shaft in which a helical rolling groove is formed on its outer peripheral surface, and a nut in which a helical rolling groove, which faces the rolling groove of the threaded shaft, is formed on its inner peripheral surface. The nut is screwed into an outer peripheral side of the threaded shaft through a number of balls interposed between both of the rolling grooves, and is able to move in an axis direction of the threaded shaft by rotating the threaded shaft. Then, in such a ball screw device, a seal member (herein after, sometimes referred to as a seal) is often provided in both end parts of the nut in an axis direction. This seal member is fitted onto the nut, prevents dust and so on from entering between the nut and the threaded shaft from outside, and also prevents lubricant, which is present between the nut and the threaded shaft, from leaking outside. 
     Here, since the rolling groove is formed on the outer peripheral surface of the threaded shaft, a sectional shape of the threaded shaft in a direction perpendicular to the axis direction is not a simple circular shape. Therefore, due to an error in shape accuracy or a mounting error of the seal member, a gap is likely to be generated between the threaded shaft and the seal member, and, in order to prevent this, enormous efforts are required for management and adjustment of precision of the seal member. When a tightening margin of the seal member with respect to the threaded shaft is formed to be large, it is possible to absorb an error in shape accuracy and a mounting error and restrain generation of such a gap, but contact pressure of the seal member to the threaded shaft becomes high at the same time. This means that frictional resistance of the seal member to the threaded shaft is increased. Then, operation performance of the ball screw device could be deteriorated. Further, heat generation due to friction is increased, which becomes one of causes of a decrease in accuracy of positioning by the ball screw device. 
     Thus, in order to improve sealability of lubricant, a seal member is proposed, in which an inner peripheral surface is made into a noncircular shape that resembles a sectional shape of a threaded shaft (a range that is in contact with a rolling groove and a range that is in contact with a land part between the rolling grooves) (see Patent Literature 1 and Patent Literature 2). Further, in order to satisfy sealability in a case where a rolling groove of a threaded shaft is made into a gothic arc shape (not a single circular arc), a technique is proposed to improve sealability while a ball screw device is operating, by making a shape of a tip part (lip) of an inner peripheral side of a seal member into a spherical shape. 
     CITATION LIST 
     Patent Literatures 
     PTL 1: JP H9-303517 A 
     PTL 2: JP 2006-300192 A 
     SUMMARY OF INVENTION 
     Technical Problem 
     As stated above, a conventional seal member tends to provide a solution by a shape of the seal member, for example, by considering its inner peripheral shape and a shape of its tip part on an inner peripheral side. 
     However, since operations of arranging the seal member in the nut and matching a phase with that of a threaded shaft are extremely time-consuming, the shape of the inner peripheral surface of the seal member is made into a non-circular shape, and, as a result, there were some instances where a mounting error and mounting failure of the seal member happened. 
     Specifically, it is difficult for an operator to ascertain the non-circular shape of the inner peripheral surface of the seal member, and phase precision when the seal member is combined with the threaded shaft was dependent on a sense of an operator. 
     Therefore, there has been a room for improvement because a mounting error and mounting failure of the seal member could cause damage to the seal member and lubrication failure such as leakage of grease. 
     Thus, the present invention has been accomplished by focusing on the above-mentioned problem, and its object is to provide a seal for a ball screw device, which is able to restrain damage to a seal member and leakage of grease. 
     Solution to Problem 
     One aspect of a seal for a ball screw device in order to achieve the above-mentioned object is an annular seal for a ball screw device provided in a ball screw device having 
     a threaded shaft extending in an axis direction and having a helical rolling groove provided in an outer peripheral surface, and 
     a cylindrical shaped nut having a rolling groove, which faces the rolling groove, in an inner peripheral surface, and screwed into the threaded shaft through a plurality of balls arranged between the rolling groove and the rolling groove of the threaded shaft, 
     the seal for a ball screw device being provided on an inner diameter side of the nut, which seals a gap between the nut and the threaded shaft by sliding contact of as an inner peripheral edge part with an outer peripheral surface of the threaded shaft, wherein 
     in at least one of end surfaces in the axis direction, a guide part is provided, which has a projection or a recessed part in the axis direction as a dimension in a radial direction is gradually increased and gradually decreased in a circumferential direction within 280° in the circumferential direction in a portion corresponding to the inner peripheral edge part that is in sliding contact with the rolling groove of the threaded shaft or a land part between the rolling grooves, and 
     minimum value of a width of the guide part in the radial direction is within 1/20 of an outer diameter of the threaded shaft. 
     Here, in the seal for a ball screw device, the guide part may be provided in both end surfaces of the seal member in the axis direction. 
     Further, in the seal for a ball screw device, the guide part may be provided continuously in a circumferential direction of the threaded shaft. Yet further, in the seal for a ball screw device, the guide parts may be provided in a plurality of lines in the circumferential direction of the threaded shaft. 
     Advantageous Effects of Invention 
     According to one aspect of the present invention, it is possible to provide a seal for a ball screw device, which is able to restrain damage to a seal member and leakage of grease. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a partial sectional view of a main part, illustrating a structure of a ball screw device in an embodiment of a seal for a ball screw device; 
         FIGS. 2A and 2B  are views illustrating a structure in an embodiment of a seal for a ball screw device, where  FIG. 2A  is a front view and  FIG. 2B  is a sectional view; 
         FIGS. 3A to 3D  are views explaining a shape of an inner peripheral edge in an embodiment of a seal for a ball screw device, where  FIG. 3A  is a sectional view of a threaded shaft,  FIG. 3B  is a sectional view of the threaded shaft,  FIG. 3C  is a front view of a seal member in this embodiment, and  FIG. 3D  is a front view of a conventional seal member; 
         FIGS. 4A to 4F  are views illustrating a structure in another embodiment of a seal for a ball screw device, where  FIG. 4A  to  FIG. 4F  are partial sectional views; and 
         FIG. 5  is a view explaining proximity in another embodiment of a seal for a ball screw device. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     In the detailed explanation below, a number of specific details are described so as to provide complete understanding of embodiments of the present invention. However, without such specific details, it will be obvious that one or more embodiments can be carried out. In addition, in order to simplify the drawings, known structures and devices are illustrated in outline views. 
     Herein below, embodiments of a seal for a ball screw device according to the present invention are explained with reference to the drawings. In this specification, a longitudinal direction in which a threaded shaft extends is regarded as an axis direction, and a direction orthogonal to the axis direction is referred to as a radial direction. Also, for a nut and a seal member, these directions are similar in a state of being assembled to a threaded shaft. 
       FIG. 1  is a partial sectional view of a main part, illustrating a structure in an embodiment of a seal for a ball screw device. 
     Structure of a Ball Screw Device 
     As illustrated in  FIG. 1 , a seal for a ball screw device according to this embodiment is provided in a ball screw device  1  that is provided with a threaded shaft  10 , a nut  20 , and balls  30 . The nut  20  is fitted to the threaded shaft  10  with a given gap, and an inner diameter of the generally cylindrical nut  20  is larger than an outer diameter of the threaded shaft  10 . 
     In an outer peripheral surface of the threaded shaft  10 , a helical rolling groove  11  having a generally semicircular shaped section is provided. Further, in an inner peripheral surface of the nut  20 , a helical rolling groove  21  having a generally semicircular shaped section is provided using the same lead as the rolling groove  11 . Either of these semicircular shapes has a curvature radius larger than a radius of the ball  30 . Then, the balls  30  are interposed, so as to be able to roll, inside a raceway that has a generally circular shaped section and is formed by these rolling grooves  11 ,  21 , and the nut  20  is screwed into the threaded shaft  10  through these balls  30 . By forming such a structure, the ball screw device is able to move the nut  20  in the axis direction of the threaded shaft  10  by rotating the threaded shaft  10 . 
     Seal for a Ball Screw Device 
     In both end parts of the nut  20  in the axis direction, circular shaped recessed parts  22  for mounting the seal members are formed, respectively ( FIG. 1  only illustrates the recessed part  22  on one side). On an inner diameter side of each of the recessed parts  22 , an annular seal member  40  is provided, which is fitted onto the threaded shaft  10  and used for sealing a gap between the nut  20  and the threaded shaft  10 . 
       FIGS. 2A and 2B  are views illustrating a structure of a seal member  40 , where  FIG. 2A  is a front view (an external view seen in the axis direction of the threaded shaft  10 ), and  FIG. 2B  is a sectional view along  2   b - 2   b  in  FIG. 2A . 
     As illustrated in  FIG. 2A and 2B , the seal member  40  has an annular core material  41  on an outer diameter side, an annular body part  42  fixed to an inner peripheral side of the core material  41 , and a lip part  43  that is formed to project inwardly in a radial direction from an inner diameter part of the body part  42 . The lip part  43  is formed across an entire circumference in a circumferential direction. An inner diameter side of the lip part  43  is an opening, and the threaded shaft  10  is inserted into the opening. 
     The core material  41  is made from metal or hard synthetic resin, and is formed into a circular shape. A projecting part  41   a  projecting inwardly in the radial direction is formed in the core material  41 , and a recessed part  42   a  corresponding to this projecting part  41   a  is formed in an outer peripheral surface of the body part  42 , and, the body part  42  is fixed to the core material  41  as these projecting part  41   a  and the recessed part  42   a  are fitted to each other. The body part  42  and the lip part  43  are formed from a soft elastic material such as elastomer and rubber. The body part  42  and the lip part  43  are formed integrally from the same material. The seal member  40  structured like this is fixed to the nut  20  by fitting an outer peripheral surface side of the core material  41  to the inner peripheral surface of the recessed part  22  of the nut  20 . 
     As illustrated in  FIG. 2B , the lip part  43  is structured from a seal piece  44  that is inclined and extends outwardly in the axis direction of the nut  20  from the inner diameter part of the body part  42 , and a sliding contact part  45  that is formed in an end part of the seal piece  44  on an inner diameter side. The sliding contact part (inner peripheral edge part)  45  is a part that comes into sliding contact with the outer peripheral surface of the threaded shaft  10 . 
     The seal piece  44  is formed into a shape that resembles the shape of a truncated conical side surface part, and is formed across the entire circumference in the circumferential direction. A sectional shape of the sliding contact part  45  is formed into a generally circular shape, and is formed across the entire circumference of the end part of the seal piece  44  on the inner diameter side. Therefore, the sliding contact part  45  forms an annular part that continues generally annularly. The seal member  40  seals a gap between the nut  20  and the threaded shaft  10  as the sliding contact part  45  of the lip part  43  comes into sliding contact with the outer peripheral surface of the threaded shaft  10 . In particular, by making the sectional shape of the sliding contact part  45  into a generally circular shape, it is possible to maintain a contact state between the sliding contact part  45  and the threaded shaft  10  constant regardless of a contact position of the sliding contact part  45  with the threaded shaft  10  for example, a contact position with a groove surface of the rolling groove  11  inside the rolling groove  11  of the threaded shaft  10 ). 
     As illustrated in  FIG. 2B , a diameter dimension of the section of the sliding contact part  45  is formed to be larger than a thickness dimension of the end part of the seal piece  44  on the inner diameter side. By forming the sliding contact part  45  as stated above, the sliding contact part  45  in a state of sliding against the threaded shaft  10  is restrained from being deformed. Further, the diameter dimension of the section of the sliding contact part  45  is formed to be smaller than a width dimension of the rolling groove  11  of the threaded shaft  10 . By forming the sliding contact part  45  as stated above, a contact area of the sliding contact part  45  with the threaded shaft  10  is reduced, thereby suppressing frictional resistance of the seal member  40  to the threaded shaft  10  to be small. 
     By arranging the seal member  40  while e making the lip part  43  inclined in the direction illustrated in  FIG. 1 , namely, outwardly in the axis direction of the nut  20  (synonymous with the axis direction of the threaded shaft  10 ), entry of dust and so on from outside into the gap between the nut  20  and the threaded shaft  10  is effectively prevented. When the seal member  40  is arranged while making the lip part  43  inclined inwardly in the axis direction of the nut  20  from the body part  42 , it is possible to effectively prevent leakage of lubricant in a case where the lubricant cant is interposed between the nut  20  and the threaded shaft  10 . 
     As illustrated in  FIG. 2A , a shape of an inner peripheral edge of the seal member  40  seen in the axis direction, namely, a shape of the sliding contact part  45  of the lip part  43  seen in the axis direction is a non-circular shape (a circular shape different from a generally perfect circle) in which a plurality of arcs projecting outwardly in the radial direction is combined. The neighboring arcs smoothly continue from each other. In the following explanation, the shape of the sliding contact part  45  of the lip part  43  seen in the axis direction is referred to as a “shape of an inner peripheral edge of the seal member  40 ”. 
     Shape of an Inner Peripheral Edge of a Seal Member 
     Here, the shape of the inner peripheral edge of the seal member  40  is explained in further detail with reference to  FIGS. 3A to 3D .  FIGS. 3A to 3C  are views explaining the shape of the inner peripheral edge of the seal member  40  in this embodiment, where  FIG. 3A  is a sectional view of the threaded shaft,  FIG. 3B  is a side view of the threaded shaft,  FIG. 3C  is a front view of the seal member of the this embodiment, and  FIG. 3D  is a front view of a conventional seal member. As illustrated in  FIG. 3C , since the shape of the inner peripheral edge of the seal member  40  is gradually shifted compared to the circular shape made by the core material  41 , it is hard to distinguish which part of the seal member  40  is a part that abuts on the roiling groove  11  of the threaded shaft  10 . 
     Here, in a case where the seal member  40  is inserted from an end part  10   a  (see  FIG. 3B ) of the threaded shaft  10 , unless the seal member  40  is inserted after matching the phases of the inner periphery of the seal member  40  and the section of the threaded shaft  10 , the seal member  40  is forced to be pushed into the threaded shaft  10 . This leads to damage to the seal member  40 . In a conventional seal member  140  illustrated in  FIG. 3D , since nothing can become a marker for matching phases of an inner periphery of a seal member  40  and a section of a threaded shaft  10 , it has been difficult to match the phases accurately when the seal member  140  is inserted into the threaded shaft  10 . In  FIG. 3D , reference numerals  141 ,  142 ,  143  are members that correspond to “the core material  41 ”, “the body part  42 ”, and “the lip part  43 ”, respectively, in  FIG. 2A  and  FIG. 3C . 
     Here, as illustrated in  FIG. 2B  and  FIG. 3C , in the seal member  40  in this embodiment, a guide part  50  is provided, which corresponds to an inner peripheral edge part (sliding contact part  45 ) that is in sliding contact with the rolling groove  11  of the threaded shaft  10  or a land part  12  between the rolling grooves  11 ,  11 . This guide part  50  is indicated by hatching in  FIG. 2A . 
     This guide part  50  is provided in at least one of end surfaces only one end surface in  FIG. 2B ) of the seal member  40  in the axis direction (parallel to the axis direction of the threaded shaft  10 ), and is provided in a shape projecting or recessed in the foregoing axis direction with respect to the end surface. For example, a range where the guide part  50  corresponding to the rolling groove  11  of the threaded shaft  10  is provided is at 280° or smaller in the circumferential direction of the seal member  40 , and preferably, at 240° or smaller in the circumferential direction of the seal member  40  as illustrated in  FIG. 2A . 
     As explained so far, as the guide part  50  provided to correspond to the rolling groove  11  is provided in the seal member  40 , an operator is able to match the phases of the threaded shaft  10  and the seal member  40  easily and surely while visually recognizing the guide part  50  from the axis direction. Further, since the seal member  40  is mounted on the threaded shaft  10  with accurate phases while visually recognizing the guide part  50  from the axis direction, it is less likely that the seal member  40  is broken. As a result, no more damage to the seal member, a mounting error and mounting failure happen, and it becomes possible to reduce grease leakage, wobble, and noise when the ball screw device is operating. 
     This guide part  50  is intentionally formed integrally with the seal member  40  (body part  42 ), and, for example, even when a joint between the core material  41  and the body part  42  ended up becoming a marker having a similar function as the guide part  50 , that is not regarded as the guide part. 
     Other Embodiments 
     Next, other embodiments of a seal for a ball screw device according to the present invention are explained with reference to the drawings. In the explanation of these embodiments, only the shape of the guide part  50  is different assuming that the guide part is provided in at least one of end surfaces of the seal member  40  in the axis direction in a portion corresponding to the inner peripheral edge part that is in sliding contact with the rolling groove  11  or the land part  12  of the threaded shaft  10 . Therefore, explanation of similar structure, actions, and effects to those of the foregoing embodiment are omitted. 
       FIGS. 4A to 4F  are views illustrating a structure of a seal for a ball screw device in other embodiments, where  FIGS. 4A to 4F  are sectional views of a part where the inner peripheral surface of the seal member is in contact with the threaded shaft  10 . In the foregoing embodiment, as illustrated in  FIG. 2B , the recessed part is formed on the inner peripheral side of the body part  42  as the guide part  50 , but the form of the guide part  50  provided in the seal member  40  may be forms illustrated in  FIGS. 4A to 4F . 
     Specifically, as illustrated in  FIG. 4A , such a form may be possible that a recessed part is formed in an outer peripheral side of the body part  42  as the guide part  50 . Further, as illustrated in  FIG. 4B , such a form may be possible that a recessed part (groove shape) is formed as the guide part  50  near a central part of the body part  42 . Moreover, as illustrated in  FIG. 4C , such a form is possible that a projecting part is formed as the guide part  50  near the center part of the body part  42 . 
     Moreover, as illustrated in  FIGS. 4D, 4E , such a form is possible that a recessed part (cutout) is formed as the guide part  50  in both end surfaces  42   b,    42   c  of the body part  42  in the axis direction. This form is in contrast to the form where the guide part  50  illustrated in  FIG. 2B  and  FIGS. 4A to 4C  are provided in only one of end surfaces  42   b  of the body part  42  in the axis direction. (parallel to the axis direction of the threaded shaft  10 ). As stated above, by providing the guide part  50  on both end surfaces  42   b,    42   c  of the body part  42  in the axis direction, it is not necessary to consider an installing direction of the seal member  40  in the axis direction, and an effect of improving versatility of the seal member  40  is obtained. The form illustrated in  FIG. 4D  is a form in which the form of the guide part  50  illustrated in  FIG. 2B  is combined in both end surfaces  42   b ,  42   c  of the body part  42  in the axis direction. Further, the form illustrated in  FIG. 4E  is a form in which the form of the guide part  50  illustrated in  FIG. 2B  and the form of the guide part  50  illustrated in  FIG. 4A  are combined on both end surfaces  42   b ,  42   c  of the body part  42  in the axis direction. 
     Further, as illustrated in  FIG. 4E , such a form is possible that the guide part  50  is provided by forming a visible minute projection in one of end surfaces  42   b  of the body part  42  in the axis direction. 
     Moreover, in the foregoing embodiment, the guide part  50  is provided in the body part  42  by using the rolling groove  11  as a reference, but similar effects are obtained even when the guide part  50  is provided by using the land part  12  as a reference. 
     Furthermore, as a form of the guide part  50  provided in the seal member  40 , the guide part  50  having a recessed shape or a projecting shape may be provided on either side in the axis direction, the guide part  50  may be provided continuously in the circumferential direction, and a plurality of separated guide parts  50  may be provided in a line in the circumferential direction. 
     Thus, a noncircular shape of the seal member  40 , and a portion to be aligned with the positions of the rolling groove  11  and the land part  12 , which could not be visually clear, become clear, thereby making it easier to match phases when the seal member  40  is inserted into the threaded shaft  10 . 
     Proximity 
     Here, for example, the guide part  50  illustrated in  FIG. 2 ( b )  has the same shape as the shape of the inner peripheral edge part (sliding contact part  45 ) of the seal member  40 , but the guide part  50  is not necessarily provided so as to be in adjacent to the inner peripheral edge of the seal member  40 . As illustrated in  FIG. 5 , a minimum value of a gap (proximity) d between the guide part  50  and the inner peripheral edge of the seal member  40  is preferably within. 1/20 of an outer diameter of the threaded shaft  10 . 
     The seal for a ball screw device according to this embodiment is suitable for a ball screw device used in, for example, an actuator and so on. Further, for example, the seal for a ball screw device may be a seal member used for a roller screw device instead of the foregoing ball screw device. 
     Although the present invention has been explained so far with reference to specific embodiments, the explanation is not intended to limit the present invention. With reference to the explanation of the present invention, other embodiments as well as various modified examples of the disclosed embodiments are obvious to a person skilled in the art. Therefore, it should be understood that the scope of the patent claims covers these modified examples and embodiments included in the scope and the gist of the present invention. 
     REFERENCE SIGNS LIST 
     
         
           10  threaded shaft 
           11  rolling groove 
           20  nut 
           21  rolling groove 
           30  ball (rolling element) 
           40  seal member 
           50  guide part