Abstract:
An object of the present invention is to provide a spindle device for stabilizing a retainer with a minimum quantity of air to be supplied to the bearing. A spindle device includes: supplying unit which supplies air from three or more points spaced in a circumferential direction between outer races and inner races of bearings supporting a spindle; and control unit which controls a supply quantity of air supplied by the supplying unit in such a manner as to independently vary the supply quantity at each of the supplying points.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a spindle device in a machining tool such as a machining center and, more particularly, to a spindle device which stably rotates a retainer for retaining a rolling element in a rolling-element bearing for supporting a spindle. 
         [0003]    2. Description of the Related Art 
         [0004]    A spindle in a machining tool is rotatably supported by a plurality of rolling-element bearings. The rolling-element bearing includes an inner race, an outer race, a rolling element and a retainer. The inner race is press-fitted to the spindle, to be rotated together with the spindle. In the meantime, the outer race is incorporated in a housing, and further, is securely pressed by a presser cap in an axial direction. The plurality of rolling-elements are movably incorporated between the inner race and the outer race, to be retained by the retainer in such a manner as to be held at equal intervals. The retainer is guided on the rolling-element or at an inner circumferential surface of the outer race, to be rotated together with the rolling-element. A retainer to be guided at the inner circumferential surface of the outer race is often used in most of bearings, each of which is rotated at as high a speed as a Dn value more than 1,500,000. The bearing incorporating therein the retainer to be guided at the inner circumferential surface of the outer race undergoes influences of a surface roughness of the inner circumferential surface of the outer race, a shape precision of the inner circumferential surface of the outer race, a surface roughness of an outer peripheral surface of the retainer, a shape precision of the retainer, a weight of the retainer, a shape precision of the rolling element, a dimensional error of the rolling element incorporated in the bearing, a clearance defined between the inner circumferential surface of the outer race and the outer peripheral surface of the retainer, and an oil film quantity between inner circumferential surface of the outer race and the outer peripheral surface of the retainer. When the spindle is rotated in the state in which the above-described conditions cannot be properly kept, the retainer is unstably rotated. As a result, an abnormal noise or vibration occurs, and therefore, a surface to be machined undergoes an adverse affect, thereby inducing damage on the bearing at the worst. 
         [0005]    A device for improving a spindle in the above-described state is exemplified by a bearing in which a pressure fluid flows into a retainer through a plurality of holes formed at an outer race in the bearing (see Japanese Utility Model Application Publication No. 45697/1994). 
         [0006]    Otherwise, there has been known a spindle device provided with a device for supplying a lubricant filled into a through hole in a spindle through a plurality of holes formed at an inner race in a bearing to a retainer through a hole formed at the spindle (see Japanese Patent Application Laid-open No. 166548/1999). 
         [0007]    In the conventional spindle device, the fluid need be supplied all the time in order to keep a constant clearance between the inner circumferential surface of the outer race of the bearing and the outer peripheral surface of the retainer, thereby increasing the consumption of the fluid. When the fluid to be supplied is the lubricant, the oil film can keep the constant clearance, but heat may be abnormally generated due to an excessive quantity of lubricant in the bearing. In the case of a low circularity of the outer diameter of the retainer or non-uniform deformation caused by the rotation, it may be difficult to constantly keep the clearance between the inner circumferential surface of the outer race of the bearing and the outer peripheral surface of the retainer over the entire circumference. Even if the lubricant can constantly keep an oil film quantity in the clearance, a frictional force locally occurs in the retainer, thereby inducing the fear of occurrence of vibrations. 
       SUMMARY OF THE INVENTION 
       [0008]    An object of the present invention is to provide a spindle device, in which a retainer can be stabilized with a minimum quantity of fluid to be supplied, and further, a flow rate and direction of fluid to be supplied to a retainer can be varied based on a rotational speed of a spindle, an attitude of the spindle and a value from a sensor fixed to the spindle. 
         [0009]    A spindle device according to the present invention includes: supplying unit which supplies fluid from three or more points spaced in a circumferential direction between an outer race and an inner race of a bearing supporting a spindle; and control unit which controls a supply quantity of fluid supplied by the supplying unit in such a manner as to independently vary the supply quantity at each of the supplying points. 
         [0010]    In the spindle device according to the present invention, even in the case where, for example, a retainer is rotated off balance, force can be exerted in a direction in which an imbalance is cancelled by regulating the flow rate of fluid through the three or more holes capable of independently supplying the fluid. As a consequence, the imbalance in the retainer can be reduced with a low flow rate of fluid. 
         [0011]    Furthermore, in the spindle device, the supply quantity of fluid controlled by the control unit may be determined based on the rotational speed of the spindle. 
         [0012]    If the spindle device is equipped with a function of determining the flow rate and position of the fluid to be supplied to the retainer based on the rotational speed of the spindle, a set value can be determined per rotational speed, so that the retainer can be stably held even if the retainer is deformed or vibrated by an influence of the rotational speed. For example, the rotational speed region of the spindle is divided into low, middle and high speed regions, in each of which an optimum flow rate of the fluid is determined, so that the retainer can be stably held in all of the speed regions. 
         [0013]    Moreover, in the spindle device, the supply quantity of fluid controlled by the control unit may be determined based on an inclination angle of the spindle. 
         [0014]    If the spindle device is equipped with the function of determining the flow rate and position of the fluid to be supplied to the clearance between the retainer and the outer race based on the attitude of the spindle, the retainer can be stably held even if the behavior of the retainer by an influence of the weight of the spindle per se or the weight of the retainer per se is varied with a variation in attitude of the spindle. For example, the flow rate of the fluid is determined based on an inclination of the spindle with respect to a reference position in a machine capable of rotating a spindle device at an arbitrary angle. 
         [0015]    Additionally, the spindle device may include a sensor which detects vibration of the spindle, wherein the supply quantity of fluid controlled by the control unit may be determined based on a value detected by the sensor. 
         [0016]    If the spindle device is equipped with a function of extracting a value within a predetermined frequency range from information obtained by one or more sensors attached to the spindle device so as to vary the flow rate and position of the fluid to be supplied to the clearance between the retainer and the outer race based on the value, the fluid is supplied at a proper flow rate when the value from the sensor satisfies a condition as the result of a real-time analysis. In the case of this device, every condition can be set according to the characteristics of the sensor. For example, the fluid is supplied at a minimum vibration within a set frequency range by the use of an acceleration sensor in machining with high precision. 
         [0017]    According to the present invention, the flow rates and positions of the plurality of fluid supplying holes can be independently set, so that the retainer can be stably rotated in spite of the variation of the rotational speed or the attitude of the spindle. In addition, the flow rate and position of the fluid can be regulated in such a manner as to optimize the value from the sensor attached to the spindle device. Thus, it is possible to produce an effect in finishing requiring for a spindle rotational accuracy, and further, to produce an effect in preventing damage on the bearing caused by abrasion of the retainer. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  is a vertically cross-sectional view showing a spindle device according to the present invention; 
           [0019]      FIG. 2  is a laterally cross-sectional view showing the spindle device; 
           [0020]      FIG. 3  is a cross-sectional view showing a part of  FIG. 1  in enlargement; 
           [0021]      FIG. 4  is a cross-sectional view showing a modification of a part shown in  FIG. 3 , being equivalent to  FIG. 3 ; 
           [0022]      FIG. 5  is a table illustrating an air supplying quantity; and 
           [0023]      FIG. 6  is a diagram explanatory of inclination angles of a spindle. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0024]    A detailed description will be given below of a preferred embodiment according to the present invention in reference to the attached drawings. 
         [0025]    In the description below, a left side in reference to  FIG. 1  is referred to as the left, and further, a side opposite to the left side is referred to as a right. In addition, the left is a front side while the right is a rear side. 
         [0026]    A spindle device is provided with a hollow spindle  11  having an axis in a horizontal direction, a horizontally cylindrical sleeve  12  fitted around the spindle  11 , a first bearing  21  and a second bearing  22  which support the spindle  11  on the left side thereof with an axial interval, a third bearing  23  which supports the spindle  11  on the right side thereof, a left housing  24  which surrounds the first bearing  21  and the second bearing  22  and is fixed to an inner surface of the sleeve  12 , and a right housing  25  which surrounds the third bearing  23  and is fixed to the inner surface of the sleeve  12 . 
         [0027]    At an outer surface of the spindle  11  are formed a large-diameter portion  31 , a middle-diameter portion  32  and a small-diameter portion  33  in sequence via steps from left to right. 
         [0028]    A stator  35  for a motor  34  is secured to the inner surface of the sleeve  12  between the second bearing  22  and the third bearing  23 . Furthermore, a rotor  36  for the motor  34  is secured to the outer surface of the spindle  11  in such a manner as to correspond to the stator  35 . 
         [0029]    At a left end of an inner surface of the left housing  24  is disposed a left inward annular projection  37 . In the meantime, at a right end of an inner surface of the right housing  25  is disposed a right inward annular projection  38 . 
         [0030]    The first to third bearings  21  to  23  have the same structure.  FIG. 3  particularly shows the second bearing  22 . The second bearing  22  includes an outer race  41  secured to the inner surface of the left housing  24 , an inner race  42  secured to the outer surface of the spindle  11 , a plurality of rolling elements  43  interposed between the outer race  41  and the inner race  42 , and a retainer  44  which is rotated together with the rolling elements  43  under a guidance of an inner surface of the outer race  41  so as to retain the rolling elements  43  at predetermined intervals. 
         [0031]    Referring to  FIG. 1  again, an outer race inter-seat  45  secured to the inner surface of the left housing  24  is interposed between the outer races  41  of the first bearing  21  and the second bearing  22 . In the meantime, an inner race inter-seat  46  secured to the outer surface of the spindle  11  is interposed between the inner races  42  of both of the bearings  21  and  22 . 
         [0032]    A left opening of the sleeve  12  is capped with a left presser cap  51 . The left presser cap  51  presses the outer races  41  of the first bearing  21  and the second bearing  22  toward the left inward annular projection  37  together with the outer race inter-seat  45 . A left pressing nut  52  is screwed on a right side of the second bearing  22 . The left pressing nut  52  presses the inner races  42  of the first bearing  21  and the second bearing  22  against the step of the large-diameter portion  31  and the middle-diameter portion  32  together with the inner race inter-seat  46 . A right opening of the sleeve  12  is capped with a right presser cap  53 . The right presser cap  53  presses the outer race  41  of the third bearing  23  toward the right inward annular projection  38 . A right pressing nut  54  is screwed on a right side of the third bearing  23 . The right pressing nut  54  presses the inner race  42  of the third bearing  23  against the step of the middle-diameter portion  32  and the small-diameter portion  33 . 
         [0033]    Referring to  FIG. 3  again, an inward opening annular groove  61  is formed at the right side surface of the outer race inter-seat  45  in such a manner as to face a clearance defined between the outer race  41  and the inner race  42  in the second bearing  22 . At a portion just a left side of the second bearing  22 , an outer air supplying hole  62  are formed at the sleeve  12  and an inner air supplying hole  63  are formed at the left housing  24 , respectively, in such a manner that an inner air supplying hole  64  is formed at the outer race inter-seat  45  continuously in alignment inward and outward. The annular groove  61  and a bottom of the inner air supplying hole  64  are connected to each other via a communication hole  65 . These outer air supplying holes  62 , inner air supplying holes  63 , inner air supplying holes  64  and communication holes  65  are formed in the same manner on a right side of the first bearing  21  and on a left side of the third bearing  23 , respectively. The outer air supplying hole  62 , inner air supplying hole  63 , inner air supplying hole  64  and communication hole  65  corresponding to each of the bearings  21  to  23  are formed at four points I to IV quartered on the sleeve  12 , the housing and the outer race inter-seat  45 , as shown in  FIG. 2 . 
         [0034]      FIG. 4  shows a modification of the annular groove  61 , the outer air supplying hole  62 , the inner air supplying hole  63 , the inner air supplying hole  64  and the communication hole  65 . In this modification, air is supplied directly to between the outer race  41  and the inner race  42  in the second bearing  22  without any connection between the annular groove  61  and the communication hole  65 . An outer air supplying hole  66 , an inner air supplying hole  67  and another inner air supplying hole  68  are formed in such a manner as to pass between the outer race  41  and the inner race  42  in the second bearing  22 . The inner air supplying hole  68  penetrates inward and outward of the outer race  41  in the second bearing  22 . 
         [0035]    Returning to  FIG. 1 , each of the outer air supplying holes  62  is connected to a compressor  72  in an air supplying apparatus via a flow rate regulator  71 . Each of the flow rate regulators  71  is controlled by a controller  73 . 
         [0036]    A rotational speed detecting sensor  74  is attached to a side surface of the right presser cap  53  in such a manner as to expose a right side end of the spindle  11 . In addition, an acceleration detecting sensor  75  is attached to an intermediate portion in a longitudinal direction of the outer surface of the sleeve  12 . 
         [0037]    Next, description will be made on an air supplying operation. 
         [0038]    First of all, the rotational speed detecting sensor  74  detects the rotational speed of the spindle  11 . Incidentally, the rotational speed may be detected by using a spindle control command value. Upon the detection of the rotational speed, an air flow rate is determined in reference to a previously created table, as illustrated in  FIG. 5 . The table shows supplying quantities per rotational speed (rpm) of the spindle at the positions I to IV in  FIG. 2  at the outer air supplying hole  62 , the inner air supplying hole  63  and the inner air supplying hole  64  corresponding to each of the bearings  21  to  23  on six levels 0 to 5. Although the supplying quantity is set per 2000 rpm in the table illustrated in  FIG. 5 , it may be further divisionally set. Otherwise, in the case of an intermediate rotational speed such as 0 to 2000 rpm or 2000 to 4000 rpm, 0 to 999 rpm, for example, is set to 0 rpm or 1000 to 1999 rpm, for example, is set to 2000 rpm. Set values in the table illustrated in  FIG. 5  are set such that an increased air flow rate in one direction keeps a balance since the vibration of the retainer  44  and the imbalance are liable to occur by the large clearance between the outer race  41  and the retainer  44  due to a small centrifugal force of the retainer  44  or a small thermal expansion during low-speed rotation of 0 to 6000 rpm. The air flow rate is set in such a manner as to become small since the vibration of the retainer  44  becomes small caused by the small clearance between the outer race  41  and the retainer  44  during high-speed rotation of 8000 rpm or higher. A command as to the set value is sent to each of the flow rate regulators  71  from the controller  73 , to thus regulate the air flow rate. 
         [0039]      FIG. 6  illustrates the attitude of the spindle  11 , that is, inclination angles θ1 to θ4. The inclination angles θ1 to θ4 indicate angles in reference to a vertically downward state of the spindle  11 . First, the inclination angles θ1 to θ4 of the spindle  11  are detected. The inclination angle may be detected based on a spindle angle command value or a detection value from an angle detecting sensor fixed to the spindle device. Upon the detection of the inclination angles θ1 to θ4, the air flow rate is determined in reference to a previously created table, not illustrated, in conformance with  FIG. 5 . 
         [0040]    In order to create the table, the following is taken into consideration. The attitude of the retainer  44  for guiding the outer race  41  is varied due to its own weight since the clearance is defined between the inner circumferential surface of the outer race  41  and the rolling element  43 . When the inclination angle of the spindle  11  is, for example, 90°, that is, θ2 or θ4, the center of the rotation of the retainer  44  is moved downward. If the rotation is continued as it is, imbalance occurs, thereby possibly generating an abnormal noise or an abnormal vibration. In order to prevent any occurrence of such abnormality, the air flow rate at each of the positions I to IV is set. The table may be created in consideration of the rotational speed. Alternatively, the table illustrated in  FIG. 5  created per rotational speed also may be used at the same time. 
         [0041]    Subsequently, in the case where the vibration generated in the spindle is detected and the air flow rate is set so as to suppress the vibration, a description will be given by way of one example in which the air flow rate at each of the positions I to IV is set by the use of the rotational speed detecting sensor  74  and the acceleration detecting sensor  75 . 
         [0042]    The acceleration detecting sensor  75  detects the axis of the sleeve  12  and a vertical acceleration. A frequency of a signal obtained from the acceleration detecting sensor  75  is analyzed at real time or a signal is stored in a memory, and then, its frequency is analyzed, so that only a multiple component of a rotational frequency is extracted. Multiple component to be extracted may be arbitrarily determined. The rotational speed detecting sensor  74  gives the rotational frequency component of the spindle  11 . In the case where the size of the signal indicating the extracted multiple component is greater than a predetermined threshold as a result of comparison, a phase having a larger vibration in the spindle rotational direction is specified by the rotational speed detecting sensor  74 , and then, the flow rate and direction of the air are determined in such a manner as to reduce the vibration of the phase. In the case of the consideration of both of the axis and the horizontal vibration, each of values may be set to become smaller by the use of the biaxial acceleration detecting sensor  75 . Furthermore, several kinds of air supplying patterns are prepared in order to reduce the vibration, and then, the flow rate and direction of the air may be determined by testing the patterns in sequence. Alternatively, the air flow rate may be manually regulated in such a manner as to reduce the value of the vibration sensor while monitoring the value of the vibration. 
         [0043]    Although the acceleration detecting sensor  75  is used as one example for obtaining the vibration in the present preferred embodiment, a sound pressure sensor and a displacement sensor may be used singly or in combination as unit for detecting information relating to the vibration.