Abstract:
A variable-throttle hydrostatic bearing includes a diaphragm that faces a protruding portion via a predetermined gap and a discharge port formed in the protruding portion so as to communicate with a hydrostatic pocket, and adjusts a throttle amount based on the size of a gap between the diaphragm and the protruding portion. The variable-throttle hydrostatic bearing further includes a piston that contacts, at its first end, a surface on the opposite side of the diaphragm from a surface that faces the protruding portion, a cylinder that holds the piston such that the piston is slidable and forms a fluid chamber along with the piston, and a coil spring housed in the fluid chamber to press a second end of the piston.

Description:
INCORPORATION BY REFERENCE 
       [0001]    The disclosure of Japanese Patent Application No. 2014-260917 filed on Dec. 24, 2014 including the specification, drawings and abstract, is incorporated herein by reference in its entirety. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates to a variable-throttle hydrostatic bearing including a diaphragm-type variable throttle. 
         [0004]    2. Description of Related Art 
         [0005]    According to a related art, a variable-throttle hydrostatic bearing with a diaphragm-type variable throttle includes a variable throttle portion in a central portion of a surface of a diaphragm that is perpendicular to a direction in which the diaphragm is movable in order to enhance a vibration damping effect of the diaphragm to damp vibration of a fluid circuit including a hydrostatic pocket and the variable throttle. In the variable-throttle hydrostatic bearing, a gap representing a small clearance is formed between an outer peripheral portion of the diaphragm and a diaphragm holding member. The gap is filled with a hydrostatic fluid to suppress vibration of the diaphragm (see FIG. 7 of Japanese Patent Application Publication No. H10-196655 (JP H10-196655 A)). 
         [0006]    Displacement of the diaphragm is highest in its central portion and low in its peripheral portion, and thus, in the related art described in JP H10-196655 A, the displacement in the peripheral portion of the diaphragm, which contributes to suppressing vibration, is low, possibly making production of a sufficient damping effect difficult. 
       SUMMARY OF THE INVENTION 
       [0007]    An object of the present invention is to provide a variable-throttle hydrostatic bearing with a diaphragm-type variable throttle that allows a desired damping capability to be easily achieved by arranging a damping mechanism at a desired position. 
         [0008]    In an aspect of the present invention, a variable-throttle hydrostatic bearing includes: 
         [0009]    a hydrostatic pocket formed in a bearing surface; 
         [0010]    a fluid supply apparatus that supplies a fluid to the hydrostatic pocket; 
         [0011]    a fluid channel forming a channel for a fluid, which extends from the fluid supply apparatus to the hydrostatic pocket; and 
         [0012]    a variable throttle that is provided in a middle of the fluid channel and throttles a flow rate of the fluid to introduce the fluid into the hydrostatic pocket. 
         [0013]    The variable throttle includes a fluid storage chamber, a fluid supply chamber with a protruding portion in its central portion, a diaphragm that partitions the fluid supply chamber from the fluid storage chamber and in which a surface of the diaphragm orthogonal to a thickness direction of the diaphragm faces the protruding portion via a predetermined gap, and a channel that is provided in the protruding portion and communicates with the hydrostatic pocket. The variable throttle adjusts a throttle amount using an opening degree of the gap between the diaphragm and the protruding portion. 
         [0014]    The variable-throttle hydrostatic bearing further includes: 
         [0015]    a piston that contacts, at its first end, a surface on the opposite side of the diaphragm from the surface that faces the protruding portion; 
         [0016]    a cylinder that houses the piston such that the piston is slidable and forms a fluid chamber along with the piston; and 
         [0017]    an elastic member that presses a second end of the piston and that is housed in the cylinder. 
         [0018]    In the variable-throttle hydrostatic bearing in the above-described aspect, the cylinder may be closed at its first end, and a fluid in the fluid chamber flows into and out from the cylinder via a gap between the piston and an inner peripheral surface of the cylinder. 
         [0019]    In the variable-throttle hydrostatic bearing in the above-described aspect, viscous resistance of the fluid flowing out from the fluid chamber hinders motion of the diaphragm in a direction away from the protruding portion. Thus, when the fluid circuit with the hydrostatic pocket and the variable throttle vibrates and the diaphragm vibrates in a thickness direction thereof, a damping effect is provided to prevent vibration of the diaphragm. The piston is separated from the diaphragm and can thus be arranged at a desired position on the surface on the opposite side of the diaphragm from the surface that faces the protruding portion. This allows a variable-throttle hydrostatic bearing that facilitates setting of the desired damping effect to be achieved. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    The foregoing and further features and advantages of the invention will become apparent from the following description of example embodiments with reference to the accompanying drawings, wherein like numerals are used to represent like elements and wherein: 
           [0021]      FIG. 1  is a schematic diagram depicting a general configuration of a table feeding apparatus in the present embodiment; 
           [0022]      FIG. 2  is a sectional view taken along line A-A in  FIG. 1 ; 
           [0023]      FIG. 3  is a detailed diagram of a variable throttle in a portion B in  FIG. 2 ; 
           [0024]      FIG. 4  is a detailed diagram of a piston portion; 
           [0025]      FIG. 5  is a detailed diagram of a variable throttle in a variation; and 
           [0026]      FIG. 6  is a diagram of the variable throttle as viewed in a direction of arrow C in  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0027]    An embodiment of the present invention will be described taking, as an example, a case where the present invention is used for a table feeding apparatus. 
         [0028]    As depicted in  FIG. 1 , a table feeding apparatus  1  includes a table  2  slidably mounted on a slide portion of a base  10  and a pair of back plates  5  attached to lower portions of opposite ends of the table  2  such that the table  2  is movable only in an X axis direction. 
         [0029]    As depicted in  FIG. 2 , two hydrostatic pockets  2   a  are formed in a bearing surface of the table  2  that faces the base  10  such that the hydrostatic pockets  2   a  open downward. A pair of hydrostatic pockets  2   c  facing each other in a lateral direction is also formed in a bearing surface of the table  2 . Variable throttles  3  are in communication with the hydrostatic pockets  2   a  and  2   c.  An oil feed line  4  is in communication with each of the variable throttles  3 . A pump  11  (fluid supply apparatus) is coupled to the oil feeding line  4  to supply a fluid. 
         [0030]    The back plates  5  also include hydrostatic pockets  5   a  open upward. The variable throttles  3  are in communication with the hydrostatic pockets  5   a.  The oil feed line  4  is in communication with each of the variable throttles  3 . 
         [0031]      FIG. 3  depicts the variable throttle  3  in detail. The variable throttle  3  includes a variable throttle base  31  with a fluid supply chamber  31   a  and a cap  32  with a fluid storage chamber  32   a  such that the fluid supply chamber  31   a  and the fluid storage chamber  32   a  face each other and an outer peripheral portion of the diaphragm  33  is sandwiched between the variable throttle base  31  and the cap  32 . The variable throttle base  31  includes a protruding portion  31   b  and a discharge port  31   c  both located in a central portion of the fluid supply chamber  31   a.  The cap  32  includes a cylinder  32   b  that is a cylindrical blind hole, in a central portion of the fluid storage chamber  32   a.  A piston  34  is slidably housed inside the cylinder  32   b.  A coil spring  35  is compressively arranged inside a fluid chamber  32   c  including the piston  34  and the cylinder  32   b  such that the piston  34  is pressed toward the diaphragm  33 . This pressing force presses the piston  34  against the diaphragm  33 . 
         [0032]    When the diaphragm  33  is in a neutral position, the protruding portion  31   b  and the diaphragm  33  face each other via a gap t 2 . The oil feed line  4  is in communication with the fluid storage chamber  32   a  through a channel  32   d  formed in the cap  32 . The oil feed line  4  is in communication with the fluid supply chamber  31   a  through a channel  31   d  formed in the variable throttle base  31  and the channel  32   d.  The discharge port  31   c  is in communication with the hydrostatic pocket  2   a  through an inflow passage  2   b  in the table  2 . 
         [0033]    Details of the piston  34  will be described based on  FIG. 4 . 
         [0034]    The piston  34  has a small-diameter portion  34   b  and large-diameter portions  34   a  arranged at opposite ends of the small-diameter portion  34   b  in its axial direction. The present embodiment includes two large-diameter portions  34   a.  The piston  34  further includes an end  34   c  that contacts the diaphragm  33 . The two large-diameter portions  34   a  have a diameter D 2 . The small-diameter portion  34   b  has a diameter set at approximately 80% of the diameter D 2  of the large-diameter portions  34   a.  The end  34   c  has a diameter set equal to or less than 50% of the diameter D 2  of the large-diameter portions  34   a.  The large-diameter portion  34   a  farther from the end  34   c  has a length L 1 , and the large-diameter portion  34   a  closer to the end  34   c  has a length L 2 . The small-diameter portion  34   b  has a length L 3 . 
         [0035]    Operations of the variable-throttle hydrostatic bearing will be described based on  FIG. 3 . 
         [0036]    When a fluid is supplied through the line  4 , the fluid storage chamber  32   a  is filled with the fluid having flown through the channel  32   d.  Moreover, the fluid chamber  32   c  is filled with the fluid having flown through the gap (throttle) between fitting portions of the cylinder  32   b  and the piston  34 . On the other hand, the fluid supply chamber  31   a  is filled with the fluid having flown through the channel  32   d  and the channel  31   d.  The fluid in the fluid supply chamber  31   a  flows into the hydrostatic pocket  2   a  via a gap between the diaphragm  33  and the protruding portion  31   b  and the discharge port  31   c.  The fluid in the hydrostatic pocket  2   a  flows out through a gap between the hydrostatic pocket  2   a  and the base  10 , which represents a clearance t 1 . 
         [0037]    This also occurs in the hydrostatic pockets  2   c  open in a horizontal direction of the table  2  and in the hydrostatic pockets  5   a  in the back plate  5 . As a result, the base  10  and the table  12  are held with the clearance t 1  defined by each of the hydrostatic pockets  2   a.    
         [0038]    When the diaphragm  33  is displaced away from the protruding portion  31   b,  the diaphragm  33  pushes the piston  34 , which is then pushed into the cylinder  32   b  to reduce the volume of the fluid chamber  32   c.  Consequently, the fluid flows out from the fluid chamber  32   c  via the gap (throttle) between the fitting portions of the piston  34  and the cylinder  32   b.  Thus, the piston  34  is subjected to a force that reduces a displacement rate of the piston  34  due to the viscous resistance of the fluid flowing between the fitting portions. The force is transmitted to the diaphragm  33 , and a displacement rate of the diaphragm  33  is also reduced. 
         [0039]    When a fluid circuit including the hydrostatic pockets and the variable throttles vibrates, the diaphragm  33  acts to vibrate, but the viscous resistance acts on the piston  34  so as to prevent the vibration. That is, the variable throttles provide a damping effect. 
         [0040]    On the other hand, when the diaphragm  33  is displaced closer to the protruding portion  31   b,  a decelerating force acting on the piston  34  is not transmitted to the diaphragm  33 . That is, the displacement rate of the diaphragm  33  is not reduced. 
         [0041]    To enhance the damping effect, it is effective that when diaphragm  33  is displaced away from the protruding portion  31   b,  the diaphragm  33  and the piston  34  constantly contact each other, maximizing the time for which damping occurs. To achieve this, even when the diaphragm  33  is displaced closer to the protruding portion  31   b,  the piston  34  needs to be displaced in time for the displacement of the diaphragm  33 . In this case, the vibration frequency of a vibration system including the piston  34  and the coil spring  35  may be set higher than the vibration frequency of the diaphragm  33 , or the mass of the piston  34  and the pressing force of the coil spring  35  may be set such that the acceleration of the piston  34  is higher than the maximum acceleration of vibration of the diaphragm  33 . 
         [0042]    In the present embodiment, as depicted in  FIG. 4 , the small-diameter portion  34   b  having a diameter that is approximately 80% of the outer diameter D 2  of the large-diameter portions  34   a  is provided in the center of the piston  34  in the axial direction, and the large-diameter portions  34   a,  producing a throttling effect, are arranged at the opposite ends of the small-diameter portion  34   b.  Thus, a desired throttling characteristic is achieved, and the operation of the piston  34  is made more stable. The throttling characteristic is determined using, as parameters, the size D 1 -D 2  of a gap that is the difference between the bore diameter D 1  of the cylinder  32   b  and the outer diameter D 2  of the large-diameter portions  34   a  of the piston  34  and the sum L 1 +L 2  of the lengths of the large-diameter portions  34   a.  On the other hand, when the piston  34  is tilted with respect to the cylinder  32   b,  the opposite ends of the large-diameter portions  34   a  come into contact with an inner wall of the cylinder  32   b.  A large tilt causes the piston to bite into the cylinder to preclude smooth motion of the cylinder. The degree of the tilt decreases with an increase in distance L between the opposite ends of the large-diameter portions  34   a.  The desired throttling characteristic and the stability of the operation of the piston  34  can both be achieved by setting the value of L 3  such that L=L 1 +L 2 +L 3 , which is determined by the lengths L 1 +L 2  of the large-diameter portions  34   a  at which the appropriate throttling characteristic can be achieved and the acceptable value of the tilt of the piston  34 . 
         [0043]    The diameter of the end  34   c  of the diaphragm  33  that contacts the diaphragm  33  is set equal to 50% or less of the diameter D 2  of the large-diameter portions  34   a.  A small sectional area of the end  34   c  contributes to increasing the surface pressure of a contact portion between the piston  34  and the diaphragm  33  (the portion where the piston  34  and the diaphragm  33  contact each other), hindering formation of an oil film in the contact portion. The presence of an oil film causes a reduction in a force transmitted from the piston  34  to the diaphragm  33 , degrading the damping effect. Thus, it is effective to reduce the diameter of the end  34   c  for preventing the damping effect from being degraded. 
         [0044]    In the above-described embodiment, the fluid is supplied to the fluid supply chamber  31   a  via the channel  31   d.  As depicted in  FIG. 5 , channels  330   a  may be formed in portions of the diaphragm  33 , which do not face a protruding portion  310   b,  such that the fluid is fed from a fluid storage chamber  320   a  to a fluid supply chamber  310   a  via the channels  330   a.  As depicted in  FIG. 6 , the channels  330   a  may be arranged on a circumference at regular intervals. This allows a channel in a variable throttle base  310  to be abolished, simplifying the structure. 
         [0045]    In the above-described embodiment, the piston  34  is pressed by the coil spring  35 . However, another elastic member such as rubber or an air spring may be used. 
         [0046]    Reference numerals  310 ,  310   a,    310   b,    320 ,  320   a,    320   d,  and  330  in  FIG. 5  and  FIG. 6  correspond to reference numerals  31 ,  31   a,    31   b,    32 ,  32   a,    32   d,  and  33  in  FIG. 3 .