Abstract:
An object is to provide a linear motion guide device which has a high rigidity, a good linear motion guiding accuracy, and as little rattling as possible. The linear motion guide device does not allow sand, water or other contaminants from entering its actuator main body, and features low maintenance requirement. A linear motion member is constituted by a plurality of axially divided segments. Each of these segments supports a plurality of guided members which make contact with two, mutually paired guide surfaces. A preload can be applied between the guide surfaces and the guided member. The housing has a sealed structure, with a pressure inside the housing higher than an external pressure, for preventing external sand and water from entering the housing.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to linear motion guide devices used in linear motion guides for machine tools, measuring apparatuses, transport apparatuses, etc., and more specifically to a linear motion guide device which is suitable to guiding a linear motion member of a solar orbital tracking mechanism in a solar photovoltaic/thermal power generation apparatuses. 
       BACKGROUND ART 
       [0002]    A linear motion guide used in a positioning section of, e.g., a machine tool, may include a component as shown in  FIG. 16A  and  FIG. 16B , which is called linear rail guide (also called linear motion rolling bearing). The component is an assembly of a linear rail  30  and a sliding unit  31  which slides along the linear rail  30 . The linear rail guide has a large variety in its size and shape, etc., and a wide range of the product is available from a number of manufacturers according to specific application requirements such as expected load, mounting attitude, etc. 
         [0003]    Linear rail guides which are currently used widely can be divided into two types: One is called circulation type, in which rolling elements (not illustrated) such as balls and rollers in the linear rail guide circulate inside the sliding unit  31 . The other type is called finite stroke type, in which rolling elements called cross roller guides stay captured by a retainer. For increased durability, both types make use of hardened steel such as a bearing steel for their linear rail  30  which provides a guide surface and for the rolling elements. As understood from the example in  FIG. 16A ,  FIG. 16B , a ball screw mechanism and a linear rail guide are combined in parallel with each other, so that the ball screw mechanism will not come under a direct radial or moment load. This arrangement provides highly durable and accurate linear motion guides. 
         [0004]    There are linear motion guides which do not make use of the linear guide rail: Patent Literatures 1 through 4 disclose techniques for implementing such linear motion guides by using vertical grooves and vertical guide surfaces formed on, e.g., an inner surface of a housing, etc., and a cam follower or other bearings which move along these vertical grooves and vertical guide surfaces. Also, Patent Literature 5 discloses a technique of implementing a linear motion guide by arranging a plurality of bearings in a circumferential direction of a housing, so that outer ring surfaces of these bearings make contact with a linear motion member disposed at a center. 
       CITATION LIST 
     Patent Literature 
       [0005]    Patent Literature 1: JP-A-2007-216280 
         [0006]    Patent Literature 2: JP-A-H2-186157 
         [0007]    Patent Literature 3: JP-A-2010-179323 
         [0008]    Patent Literature 4: JP-A-2007-333046 
         [0009]    Patent Literature 5: JP-A-2001-221229 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0010]      FIG. 17  shows a cylinder linear motion actuator  7  driven by a motor  6 . Such an actuator is being introduced as a replacement for air cylinders in production/transportation facilities for energy saving purposes. This cylinder type linear motion actuator makes use of a linear motion guide mechanism as a critical component, and by far this linear motion guide mechanism determines an outlook, cost and performance of the cylinder linear motion actuator. 
         [0011]    Although the linear motion guide mechanism implemented by a linear rail guide offers high rigidity and superb linearity, it requires, as shown in  FIG. 16A ,  FIG. 16B , a large number of fasteners  32  such as bolts in order to fix a straight rail  30 , and this poses a hurdle in space saving and cost reduction. 
         [0012]    The linear motion guide mechanisms which do not employ linear rail guides also have problems: Specifically, the disclosures in Patent Literatures 1 through 4 may be divided into two categories; one (Patent Literatures 1, 2) with a primary object of preventing rotation of a linear movement member used in a mechanism for converting rotating movement into linear movement, and the other (Patent Literatures 3, 4) with a primary object of making the mechanism capable of bearing an amount of load. However, even the latter ones are still limited in the direction of the load due to the number and locations of bearings. In particular, they are not applicable to loads exerted from oblique directions. 
         [0013]    Patent Literature 5 discloses a technique for bearing a load from various directions; however, complicated procedures have to be followed to fix bearings and to adjust pressures at which the bearings are pressed onto a linear motion member. 
         [0014]    When a linear motion guide device is utilized in solar orbital tracking components in solar photovoltaic/thermal power generation apparatuses, low maintenance features are essential and the device must be highly capable of preventing sand and water from entering actuator main bodies because the power generation apparatuses are usually installed in a huge number and in extreme environmental conditions such as in desert areas. 
         [0015]    It is therefore an object of the present invention to provide a linear motion guide device which has high rigidity and high linear motion guiding accuracy with as little rattling as possible. 
         [0016]    Another object of the present invention is to provide a linear motion guide device which requires as little maintenance as possible by preventing sand, water and others from entering an actuator main body. 
       Solution to Problem 
       [0017]    A linear motion guide device according to the present invention includes: a housing; a threaded shaft supported by the housing rotatably about a center axis but axially immovably; and a linear motion member including a nut threaded around the threaded shaft, for movement axially of the threaded shaft by rotation of the threaded shaft; In this device, the housing includes a plurality of guide surfaces along the axial direction of the threaded shaft; the guide surfaces are provided by pairs of two surfaces; the two guide surfaces in each pair are not parallel with each other but face away from each other; the linear motion member includes a plurality of segments arranged axially of the member and connected to each other; and each segment supports a plurality of guided members for contact with respective guide surfaces. 
         [0018]    The segments are connected to each other so as to give a preload between the guide surfaces and the guided members. 
         [0019]    By making the pressure inside the housing higher than the external pressure, it is possible to prevent external sand and water from entering the housing. 
         [0020]    In order to reduce pressure difference between a compressing side and an expanding side caused by a reciprocating movement of the linear motion member, it is preferable that the housing has a vent filter. 
         [0021]    A buffer member may be provided between the housing and the linear motion member. 
         [0022]    Alternatively, a spring mechanism which maintains a constant pressure may be provided between the housing and the linear motion member. 
       Advantageous Effects of Invention 
       [0023]    A linear motion guide device according to the present invention includes a linear motion member which is constituted by an axially divided plurality of segments, and each of these segments supports a plurality of guided members making contact with two paired guide surfaces. Since the arrangement makes it possible to give a preload between the guide surface and the guided member, the device provides increased rigidity and good linear motion guiding accuracy while reducing rattling. 
         [0024]    Also, the linear motion guide device according to the present invention is highly capable of preventing sand and water from entering its actuator main body, and therefore does not require high maintenance. Hence, the device can be used appropriately as a linear motion member for a solar orbital tracking system in solar photovoltaic/thermal power generation apparatuses. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0025]      FIG. 1A  is a partially broken side view of a linear motion actuator which includes a linear motion guide device according to an embodiment of the present invention. 
           [0026]      FIG. 1B  is a sectional view taken in lines IB-IB in FIG.  1 A. 
           [0027]      FIG. 2  is a partially enlarged view of  FIG. 1A . 
           [0028]      FIG. 3  is a partially enlarged view of  FIG. 1B . 
           [0029]      FIG. 4  is a sectional view showing a different example of a guide surface of the linear motion guide device. 
           [0030]      FIG. 5  is a sectional view showing still another different example of the guide surface. 
           [0031]      FIG. 6  is a sectional view showing still another different example of the guide surface. 
           [0032]      FIG. 7  is a sectional view showing still another different example of the guide surface. 
           [0033]      FIG. 8  is a sectional view showing a different example of a guided member of the linear motion guide device. 
           [0034]      FIG. 9  is a sectional view showing still another different example of the guided member. 
           [0035]      FIG. 10  is a sectional view showing still another different example of the guided member. 
           [0036]      FIG. 11  is a partially unillustrated, partially broken side view of a linear motion guide device according to a different embodiment of the present invention. 
           [0037]      FIG. 12  is a sectional view of the embodiment shown in  FIG. 11 . 
           [0038]      FIG. 13  is a sectional view of an embodiment including a housing having an inner surface formed with five track grooves.  FIG. 14  is a partially broken side view of a linear motion actuator which includes a linear motion guide device according to another embodiment of the present invention. 
           [0039]      FIG. 15  is a partially broken side view of a linear motion actuator which includes a linear motion guide device according to another embodiment of the present invention. 
           [0040]      FIG. 16A  is a partially broken side view of a conventional linear motion guide device. 
           [0041]      FIG. 16B  is a sectional view taken in lines XVIB-XVIB in  FIG. 16A . 
           [0042]      FIG. 17  is a partially broken side view of a conventional linear motion actuator. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0043]    An embodiment of the present invention will be described with reference to  FIGS. 1A ,  1 B through  FIG. 3 . 
         [0044]    Referring to  FIG. 1A  and  FIG. 1B , a linear motion guide device  1  includes a linear motion member  5  which has a housing  2 ; a threaded shaft  3  supported by the housing  2  rotatably around the center axis but immovably along the axis; and a nut  4  threaded around the threaded shaft  3 . The linear motion guide device  1  constitutes a linear motion actuator  7  together with a motor  6  which serves as a driving source. 
         [0045]    The housing  2  has a cylindrical linear motion guide section  2   a ; a threaded-shaft support section  2   b  which is connected to one of two left-right ends of the linear motion guide section  2   a ; a coupling storage section  2   c  which is connected to an end of the threaded-shaft support section  2   b ; a shaft support section  2   d  which is connected to the other of the left-right ends of the linear motion guide section  2   a ; and a cap section  2   e  which is connected to an end of the shaft support section  2   d.    
         [0046]    The threaded shaft  3  has a ball screw section  3   a  for the nut  4  to thread around; a cylindrical surface section  3   b  which continues from the ball screw section  3   a  toward a base end; and a male-thread section  3   c . The threaded shaft  3  is supported at the cylindrical surface section  3   b  rotatably by a multiple-row support bearing  8  which is fitted to an inner circumference of the threaded-shaft support section  2   b  of the housing  2 . The support bearing  8  is provided by, e.g., a rolling bearing such as an angular contact ball bearing. The male-thread section  3   c  is threaded with a retaining nut  9 , so the threaded shaft  3  is axially immovable. The threaded shaft  3  is connected to a rotation shaft  6   a  of the motor  6  which is disposed outside of the housing  2 , via a coupling  10  placed inside the coupling storage section  2   c.    
         [0047]    The nut  4  is provided, e.g., by a ball nut which includes balls (not illustrated) that make circulating movement along a contact surface with the ball screw section  3   a  of the threaded shaft  3 . The ball screw section  3   a  and the nut  4  constitute a ball screw mechanism  11 . In the ball screw mechanism  11 , the linear motion member  5  including the nut  4  makes axial movement as the threaded shaft  3  is rotated. 
         [0048]    The linear motion member  5  includes the nut  4 , a moving bracket  12  fitted around an outer circumference of the nut  4 , and a shaft  13  extending axially of the threaded shaft  3  from the moving bracket  12 . The nut  4  and the moving bracket  12  are connected to each other in a mutually non-rotatable fashion by means of splines for example, and are made axially immovable with respect to each other by unillustrated retaining means. 
         [0049]    The moving bracket  12  is an assembly of an axially divided plurality of segments  12   a ,  12   b . The embodiment shown in  FIGS. 1A and 1B  through  FIG. 3  is an example in which the bracket is divided into two segments. 
         [0050]    The segments  12   a ,  12   b  are connected and fixed to each other by set screws  27  so that their relative rotation angle around the threaded shaft  3  can be varied. 
         [0051]    As shown in  FIG. 2 , the shaft  13  is cylindrical, and into its hollow space, a tip portion of the threaded shaft  3  extending out of the nut  4  is inserted. The shaft  13  is supported axially slidably along the threaded shaft  3  by a linear motion sliding bearing  14  which is fitted to an inner circumference of the shaft support section  2   d  of the housing  2 . The moving bracket  12  and the shaft  13  may be integral with each other or separate from each other. 
         [0052]    As shown in  FIG. 1B , the linear motion guide section  2   a  of the housing  2  has a substantially square section when cut perpendicularly to the axial direction of the threaded shaft  3 , and is hollow, having an inner surface formed with a plurality of guide surfaces  15  ( 15 A through  15 D) of a consistent width axially of the threaded shaft  3 . Each guide surface  15  is for guiding a guided member  16 , which will be described later, axially of the threaded shaft  3 . In the illustrated example, a total of four guide surfaces  15 , i.e., a pair on the left side and another pair on the right side, are formed. These guide surfaces  15  are grouped in two pairs of an upper and a lower surfaces: Namely, the guide surface  15 A and the guide surface  15 B make a pair, whereas the guide surface  15 C and the guide surface  15   d  make the other pair. In each pair, the guide surfaces  15  are not in parallel with each other and are facing away from each other. In other words, in each pair, the guide surfaces  15  would cross each other if they are extended. In each pair, the guide surfaces  15  are slanted with respect to a flat bottom surface F of the housing  2 , by a degree of plus 45° and minus 45° respectively (θA, θB, θC and θD=45°). 
         [0053]    Each of the segments  12   a ,  12   b  of the moving bracket  12  in the linear motion member  5  has guided members  16  which make contact with the guide surfaces  15 . Each guided member  16  is provided by a guide bearing which includes a trunnion shaft  17  extending radially of the threaded shaft  3  from an outer surface of the respective segments  12   a ,  12   b  which constitutes the moving bracket  12 ; and a rolling bearing  18  mounted to the trunnion shaft  17  so that the bearing&#39;s outer circumferential surface makes rolling contact with the guide surface  15 . In each guided member  16 , the trunnion shaft  17  has its center axis P crossing a center axis O of the threaded shaft  3 . In this embodiment, the rolling bearing  18  is provided by a deep groove ball bearing. 
         [0054]    The segments  12   a ,  12   b  are connected and fixed to each other with set screws  12   c  so that their relative rotation angle around the threaded shaft  3  can be varied. Therefore, it is possible to vary a contact pressure between the guided member  16  and the guide surface  15 . This allows preload adjustment between the guided member  16  and the guide surface  15 . 
         [0055]    In the linear motion actuator  7  which makes use of the linear motion guide device  1 , the motor  6  drives and rotates the threaded shaft  3 , whereby the linear motion member  5  including the nut  4  is moved axially of the threaded shaft  3 . In this movement, the plurality of guided members  16  on the moving bracket  12  of the linear motion member  5  maintain their contact with a corresponding one of the guide surfaces  15  formed in the housing  2 , thereby accurately guiding the linear motion member  5  axially of the threaded shaft  3 . 
         [0056]    Since these guide surfaces  15  are made into pairs, and since two guide surfaces  15  in each of the two pairs are not in parallel with each other and are facing away from each other, a load exerted onto the linear motion member  5  is spread to all of the guide surfaces  15 . Therefore, it is possible to receive loads from various directions and/or a big load. Also, the two guided members  16  which make contact with the two guide surfaces  15  which face away from each other can be press-fitted onto their respective guide surfaces  15  by rotating the segments  12   a ,  12   b  relatively from each other around the threaded shaft  3  and then fixing to each other by using the set screws  12   c . Since this results in a structure that the guided members  16  grasp a portion of the housing  2  which is sandwiched by the two guide surfaces  15 , in a compressing manner, this improves rigidity of the housing  2 . Since the guide surfaces  15  are arranged in such a manner that they are in pairs, and are on both sides of the threaded shaft  3  in terms of a diametrical direction, it is possible to support the load exerted onto the linear motion member  5  in a well balanced fashion with the pairs of guide surfaces  15 . Since the guide surfaces  15  are formed on an inner surface of the housing  2  and it is not necessary to separately provide rails for guiding the guided members  16 , it is possible to make the device compact. 
         [0057]    In the present embodiment, the guided member  16  is provided by a guide bearing which includes a trunnion shaft  17  and a rolling bearing  18 , and the rolling bearing  18  has its outer ring  18   a  making rolling contact with the guide surface  15 . Therefore, friction resistance between the guide surface  15  and the guided member  16  is small, and it is possible to move the linear motion member  5  smoothly. Also, the rolling bearing  18  is provided by a deep groove ball bearing, which is easy to assemble, widely available and is low cost. 
         [0058]    As exemplified in  FIG. 4 , the guide surface  15  may be a curved surface which has an arc-like convex section in a sectional view which is taken vertically to the center axis O of the threaded shaft  3 . In this case, the arrangement prevents the rolling bearing  18  from making contact on an edge of an outer circumferential surface of the outer ring  18   a  with respect to the guide surface  15 . 
         [0059]    Also, as shown in  FIG. 5 , an axially extending oil storage groove  20  may be formed within a width of the guide surface  15  which makes contact with the rolling bearing  18 . In this case, the arrangement prevents lack of lubrication oil on the outer circumferential surface of the outer ring  18   a  in the rolling bearing  18 , leading to improved durability of the rolling bearing  18 . 
         [0060]    As shown in  FIG. 6 , a platy member  21  which has a higher surface hardness than the guide surface  15  may be placed between the guide surface  15  and the rolling bearing  18  for wear resistance. The platy member  21  may be adhesively provided on a surface of the guide surface  15  for example. In this case, the arrangement improves durability of the guide surface  15  against wear from rolling contact by the rolling bearing  18 . 
         [0061]    Instead of using the platy member  21 , a surface hardening treatment may be provided to the guide surface  15 . This also improves durability of the guide surface  15 . Alternatively, the entire housing  2  may be heat treated for increased hardness. This also improves durability of the guide surface  15 . 
         [0062]    As shown in  FIG. 7 , the housing  2  may be constituted by a housing main body  2 A and a guide surface formation member  22  which has a guide surface  15  and is fixed to the housing main body  2 A. The housing main body  2 A and the guide surface formation member  22  are fixed with bolts or other appropriate methods. In this case, the arrangement makes it easy to form the guide surface  15 . 
         [0063]    As shown in  FIG. 8 , the rolling bearing  18  may be provided by a plurality of angular contact ball bearings which are assembled back-to-back or face-to-face, axially around the trunnion shaft  17 . These angular contact ball bearings should desirably be preloaded. The rolling bearings  18  provided by multiple angular contact ball bearings make the device capable of receiving not only a load vertical to the guide surface  15  but also a load from a width direction of the guide surface  15 . Also, a preload eliminates a gap between the bearings, and increases rigidity. 
         [0064]    As shown in  FIG. 9 , instead of a combination of the trunnion shaft  17  and the rolling bearing  18 , the guide bearing constituting the guided member  16  may be provided by a cam follower  24  which includes a shaft  24   a  and a roller  24   c  attached around an outer circumference of the shaft via rolling elements  24   b  and functioning also as an outer ring. In the cam follower  24 , its outer ring, i.e., the roller  24   c , makes wider contact than in the rolling bearings  18  provided by a deep groove ball bearing or an angular contact ball bearing, so it is possible to increase a load capacity in a single component. If the rolling elements  24   b  are provided by rollers such as cylindrical rollers, it is possible to increase the load capability further. Also, since the cam follower  24  does not have an inner ring, it can be disposed in a diametrically tight space. 
         [0065]    In cases where the guide bearing is provided by the rolling bearing  18  or the cam follower  24  which makes rolling contact with the guide surface  15 , a resin coating with polyurethane for example, may be provided on an outer circumferential surface of the outer ring  18   a  or  24   c  of the guide bearing. This can improve slidability between the outer ring  18   a  or  24   c  and the guide surface  15 . 
         [0066]    As shown in  FIG. 10 , the guided member  16  may make sliding contact with the guide surface  15 . This guided member  16  includes a support member  25  extending radially outward from the moving bracket  12 , and a sliding contact member  26  fixed thereon for making sliding contact with the guide surface  15 . The guided member  16  which makes sliding contact with the guide surface  15  also can guide the linear motion member  5  accurately along the axial direction of the threaded shaft  3 . 
         [0067]      FIG. 11  shows an arrangement where the moving bracket  12  of the linear motion member  5  is constituted by axially arranged three segments  12   a ,  12   b ,  12   c , and each of the three segments  12   a ,  12   b ,  12   c  has three guided members  16  at an interval of 120° in a rotating direction of the threaded shaft  3 . 
         [0068]    In the embodiment shown in  FIG. 11 , axially extending three linear track grooves  28  are formed as shown in  FIG. 12 , in an inner surface of the housing  2 , and mutually opposing walls in each track groove  28  provide guide surfaces  15  for the guided member  16 . 
         [0069]      FIG. 15  shows an example in which axially extending five linear track grooves  28  are formed in an inner surface of the housing  2 , and mutually opposing walls in each track groove  28  provide guide surfaces  15  for the guided member  16 . 
         [0070]    When the linear motion guide device  1  is utilized in solar orbital tracking components in solar photovoltaic/thermal power generation apparatuses, low maintenance features are essential and the device must be highly capable of preventing sand and water from entering actuator main bodies because the generation apparatuses are usually installed in a huge number under extreme environmental conditions such as in desert areas. 
         [0071]    For this reason, it is preferable that the housing  2  has a sealed structure, and the housing  2  has its internal pressure kept higher than external pressure. By making the pressure inside the housing  2  higher than the external pressure, it becomes possible to prevent external sand and water from entering the housing  2 . The pressure inside the housing  2  can be made higher than the external pressure by, for example, connecting a pressurizing pump  29  to the housing  2  as shown in  FIG. 14 . 
         [0072]    If sealed structure is used for the housing  2 , the reciprocating linear motion member  5  creates pressure difference within the housing  2 , with air compressed on one side while expanded on the other side. In order to reduce the pressure difference, it is preferable to use a vent filter  30  in the housing  2  as shown in  FIG. 15 . 
         [0073]    Also, a buffer member may be provided between the housing  2  and the linear motion member  5 . 
         [0074]    Alternatively, a spring mechanism which maintains a constant pressure may be provided between the housing  2  and the linear motion member  5 . 
       REFERENCE SIGNS LIST 
       [0000]    
       
           1  Linear Motion Guide Device 
           2  Housing 
           2 A Housing Main Body 
           3  Threaded Shaft 
           4  Nut 
           5  Linear Motion Member 
           6  Motor 
           7  Linear Motion Actuator 
           12  Moving Bracket 
           12   a ,  12   b ,  12   c  Segments 
           15 ,  15 A,  15 B,  15 C,  15 D Guide Surface 
           16  Guided Member 
           17  Trunnion Shaft 
           18  Rolling Bearing 
           18   a  Outer Ring 
           20  Oil Storage Groove 
           21  Platy Member 
           22  Guide Surface Formation Member 
           24  Cam Follower 
           24   c  Roller (Outer Ring) 
           26  Sliding Contact Member 
           27  Set Screw 
           28  Track Groove 
           29  Pressurizing Pump 
           30  Vent Filter