Abstract:
A main spindle device of a machine tool includes a plurality of bearings that rotatably support a main spindle of the machine tool, and that are placed inside a housing with a pressure receiving member interposed between the bearings and the housing. The pressure receiving member is capable of moving in a direction perpendicular to an axial direction of the main spindle. A pressure chamber, which a pressure medium pressing the pressure receiving member toward the bearings in the perpendicular direction is supplied to, is formed in the housing. A plurality of the pressure chambers are independently formed in the housing so as to correspond to the bearings, respectively. The main spindle device further includes a plurality of adjustment units provided independently so as to correspond the pressure chambers, respectively, and each capable of independently adjusting a pressure of the pressure medium for a corresponding one of the pressure chambers.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This application claims the benefit of Japanese Patent Application Number 2012-245497 filed on Nov. 7, 2012, the entirety of which is incorporated by reference. 
       TECHNICAL FIELD 
       [0002]    The present invention relates to main spindle devices of machine tools in which a plurality of bearings rotatably supporting a main spindle of the machine tool are placed inside a housing with a pressure receiving member interposed between the bearings and the housing. A pressure chamber, which a pressure medium pressing the pressure receiving member toward the bearings is supplied to, is provided in the housing. 
       BACKGROUND ART 
       [0003]    For example, Japanese Patent Application Publication No. H05-138408 (JP H05-138408 A) discloses a main spindle device of a machine tool in which clearance between a main spindle housing and each of outer races of bearings rotatably supporting a main spindle can be controlled by a simple configuration to prevent reduction in rigidity of the main spindle in a low speed rotation range. In the main spindle device of JP H05-138408 A, a pressure chamber is formed between the main spindle housing and an outer race spacer by a recessed portion of a thin ring-shaped member and the main spindle housing. The thin ring-shaped member is elastically deformed by pressure fluid supplied to the pressure chamber, and thus eliminates the clearance between the outer race of each bearing and the main spindle housing. Accordingly, reduction in rigidity of the main spindle in the low speed rotation range can be prevented. 
         [0004]    Japanese Utility Model Application Publication No. H06-21803 (JP H06-21803 U) discloses a main spindle device of a machine tool which includes a preload adjustment device that applies preload to a plurality of bearings rotatably supporting a main spindle in a housing, in order to prevent reduction in rigidity of the main spindle. In the main spindle device of JP H06-21803 U, an annular pressing body that applies external force in the central axis direction of the main spindle is provided outward of outer races of the bearings. During low speed rotation of the main spindle, pressure oil is supplied to each pressurizing chamber formed in the annular pressing body so as to face the outer race of a corresponding one of the bearings. The annular pressing body deformed by the supply of the pressure oil presses the outer race of each bearing. This pressing force acts in the central axis direction of the main spindle and presses each bearing against the main spindle, whereby reduction in rigidity of the main spindle can be prevented. 
         [0005]    However, in the main spindle device of JP H05-138408 A, the pressure chamber is formed between the main spindle housing and the outer race spacer, has a shape that is long in the axial direction of the main spindle, and a large capacity. This hinders uniform control of the pressure of the pressure fluid. In this case, the thin ring-shaped member subjected to the pressure of the pressure fluid cannot be uniformly elastically deformed. Accordingly, if the clearance is left in any region between the main spindle housing and the outer races of the bearings, the bearing cannot be sufficiently pressed against the main spindle in this region, and rigidity of the main spindle may not be increased. 
         [0006]    In the main spindle device of JP H06-21803 U, the pressure oil is supplied to each pressurizing chamber through each branch oil passage that branches off from a single oil passage. The oil pressure of the pressure oil may vary between the pressurizing chamber located close to a pressure oil supply source and the pressurizing chamber located far away from the pressure oil supply source. In this case, the bearings are not uniformly pressed against the main spindle by the oil pressure. Thus, rigidity of the main spindle may not be increased in a region where the bearing is pressed by a low oil pressure. 
       SUMMARY OF THE INVENTION 
       [0007]    In view of such a situation, it is an object of the present invention to provide a main spindle device of a machine tool which is capable of increasing rigidity of a main spindle by uniformly pressing each bearing against the main spindle. 
         [0008]    According to a first aspect of the present invention, a main spindle device of a machine tool includes a plurality of bearings that rotatably support a main spindle of the machine tool, and that are placed inside a housing with a pressure receiving member interposed between the bearings and the housing. In the main spindle device, the pressure receiving member is capable of moving in a direction perpendicular to an axial direction of the main spindle, a pressure chamber, which a pressure medium pressing the pressure receiving member toward the bearings in the perpendicular direction is supplied to, is formed in the housing, and a plurality of the pressure chambers are independently formed in the housing so as to correspond to the bearings, respectively. The main spindle device of the machine tool further includes a plurality of adjustment units provided independently so as to correspond the pressure chambers, respectively, and each capable of independently adjusting a pressure of the pressure medium for a corresponding one of the pressure chambers. 
         [0009]    According to a second aspect of the present invention, in the first aspect, the plurality of bearings are arranged at different positions from each other in the axial direction so as to face the pressure chambers in the perpendicular direction, respectively, with the pressure receiving member interposed between the bearings and the pressure chambers. An entire length of each of the pressure chambers in the axial direction is equal to or larger than that of a corresponding one of the bearings in the axial direction. 
         [0010]    According to a third aspect of the present invention, in the first or second aspect, a circulating path in which a bearing cooling medium circulates is formed in the housing at positions close to each of the bearings. 
         [0011]    In the main spindle device of the machine tool according to the first aspect of the present invention, the pressure medium having its pressure adjusted by the adjustment units is supplied to each pressure chamber. Then, the pressure receiving member can be uniformly pressed toward the bearings by the pressure of the pressure medium in each pressure chamber. The pressure receiving member can thus uniformly press each bearing against the main spindle, and rigidity of the main spindle can be increased. 
         [0012]    According to the second aspect of the present invention, the pressure of the pressure medium in each pressure chamber can be uniformly applied over the entire length of each bearing in the axial direction of the main spindle via the pressure receiving member. Each bearing can thus be uniformly pressed against the main spindle over the entire length of the bearing. 
         [0013]    According to the third aspect of the present invention, generation of heat by, e.g., friction between the main spindle and each bearing during rotation of the main spindle can be suppressed by the hearing cooling medium circulating in the circulating path. This can suppress thermal expansion of each bearing, and can prevent seizure of each bearing. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a configuration diagram of a main part of a machining center including a main spindle device of an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0015]    An embodiment of the present invention will be described with reference to  FIG. 1 .  FIG. 1  shows a main spindle device  2  of a horizontal machining center  1 . The main spindle device  2  includes a main spindle  3 , a pressure receiving member  4 , rolling bearings  5 ,  6 , a housing  7 , and a lid member  8 . The machining center  1  is an example of a machine tool of the present invention. 
         [0016]    As shown in  FIG. 1 , the main spindle  3  has a hollow shape extending in the longitudinal direction of the machining center  1  (horizontal direction in  FIG. 1 ), and a tool (not shown) is detachably attached to the front end of the main spindle  3 . The pressure receiving member  4  is a hollow cylindrical shape, and is placed coaxially with the main spindle  3  so as to surround the main spindle  3 . The pressure receiving member  4  has a thin cylindrical portion  4 A on its front side and a thick cylindrical portion  4 B on its rear side which is continuous with the thin cylindrical portion  4 A. A stepped portion  10  is provided at the base end of the thin cylindrical portion  4 A. The thick cylindrical portion  4 B is provided with an annular protruding portion  11  integral with the thick cylindrical portion  4 B and protruding outward of the thick cylindrical portion  4 B. The rolling bearings  5 ,  6  are arranged at an interval in the axial direction X of the main spindle  3  so as to be interposed between the outer peripheral surface of the main spindle  3  and the inner peripheral surface of the pressure receiving member  4  (thin cylindrical portion  4 A) at different positions from each other. In the present embodiment, a small amount of clearance (12 μm to 15 μm) is provided between an outer race  5 B,  6 B of each rolling bearing  5 ,  6  and the pressure receiving member  4  in the state where no pressure oil is supplied to pressure chambers  20 ,  21  described below. As described below, when pressure oil is supplied to the pressure chambers  20 ,  21 , the pressure receiving member  4  moves toward the rolling bearings  5 ,  6  in the direction Y perpendicular to the axial direction X in response to the oil pressure of the pressure oil. An inner race spacer  12  is mounted between an inner race  5 A of the rolling bearing  5  and an inner race  6 A of the rolling bearing  6  in the axial direction X, and an outer race spacer  13  is mounted between the outer race  5 B of the rolling bearing  5  and the outer race  6 B of the rolling bearing  6  in the axial direction X. A collar  14  in  FIG. 1  is mounted on the outer peripheral surface of the main spindle  3  and contacts the inner race  6 A. The pressure oil is an example of the pressure medium in the present invention. 
         [0017]    As shown in  FIG. 1 , the housing  7  is made of a metal and has a hollow cylindrical shape having an opening  15  at its front end. The housing  7  is mounted on the outer peripheral surface of the pressure receiving member  4  coaxially with the main spindle  3 . As shown in  FIG. 1 , the rolling bearings  5 ,  6  rotatably supporting the main spindle  3  are thus arranged in the hollow cylindrical housing  7 . Moreover, the lid member  8  includes a cylinder portion  16  and an annular flange portion  17  provided along the front end of the cylinder portion  16 . The lid member  8  is attached to the front end of the housing  7  and the front end of the pressure receiving member  4 . As shown in  FIG. 1 , in the state where the lid member  8  is attached to the front end of the housing  7  and the front end of the pressure receiving member  4 , the cylinder portion  16  is fitted in the opening  15  with the main spindle  3  being inserted through the cylinder portion  16 . In this state, the cylinder portion  16  presses the inner race spacer  12  and the outer race spacer  13  against the rolling bearing  6  via the rolling bearing  5 . The rolling bearing  6  is thus pressed against the stepped portion  10 . The flange portion  17  contacts the front surface of the housing  7  and presses the housing  7  toward the rear of the housing  7 . The rear end face of the housing  7  is thus brought in to abutment with the protruding portion  11 . 
         [0018]    As shown in  FIG. 1 , the pressure chambers  20 ,  21  and circulating paths  22 ,  23 ,  24  are formed in the housing  7 . The pressure chamber  20  is formed by a space surrounded by the pressure receiving member  4  and an annular groove  25  formed in the inner peripheral surface of the housing  7  along the entire circumference on the front side in the axial direction X (left side in  FIG. 1 ). The pressure chamber  21  is formed by a space surrounded by the pressure receiving member  4  and an annular groove  26  formed in the inner peripheral surface of the housing  7  along the entire circumference on the rear side in the axial direction X (right side in  FIG. 1 ). The interval between the central portion of the annular groove  25  and the central portion of the annular groove  26  in the axial direction X is the same as that between the central portion of the rolling bearing  5  and the central portion of the rolling bearing  6  in the axial direction X. Thus, as shown in  FIG. 1 , the pressure chamber  20  is placed to face the rolling bearing  5  with the thin cylindrical portion  4 A of the pressure receiving member  4  interposed between the pressure chamber  20  and the rolling bearing  5  in the perpendicular direction Y. The pressure chamber  21  is placed to face the rolling bearing  6  with the thin cylindrical portion  4 A interposed between the pressure chamber  21  and the rolling bearing  6  in the perpendicular direction Y. Moreover, in the present embodiment, the entire length A of the pressure chamber  20  in the axial direction X is equal to or larger than the entire length B of the rolling bearing  5  in the axial direction X. The entire length C of the pressure chamber  21  in the axial direction X is equal to or larger than the entire length D of the rolling bearing  6  in the axial direction X. O-rings  27 ,  28  that seal the pressure oil in the pressure chamber  20  and O-rings  29 ,  30  that seal the pressure oil in the pressure chamber  21  are provided between the pressure receiving member  4  and the housing  7 . 
         [0019]    Moreover, as shown in  FIG. 1 , a pressure-oil supply path  32  that allows the pressure chamber  20  to communicate with the outside of the housing  7  is formed in the housing  7 . A pressure-oil supply path  33  that allows the pressure chamber  21  to communicate with the outside of the housing  7  is formed in the housing  7  independently of the pressure-oil supply path  32 . The pressure-oil supply path  32  is connected through a pressure reducing valve  35  to a hydraulic unit  36  capable of supplying pressure oil. The hydraulic unit  36  is also connected to the pressure-oil supply path  33  through a pressure reducing valve  37 . 
         [0020]    A control device  38  is connected to the hydraulic unit  36  and the pressure reducing valves  35 ,  37 . A main-spindle-rotational-speed detection device  39 , a temperature detection sensor  40 , and a storage device  41  are connected to the control device  38 . The main-spindle-rotational-speed detection device  39  detects the rotational speed of the main spindle  3  based on a rotational speed command value that is set by the operator of the machining center  1  operating a control panel (not shown). The temperature detection sensor  40  is accommodated in the pressure receiving member  4  and detects the temperature of the pressure receiving member  4 . This temperature detection sensor  40  indirectly detects the temperature of both rolling bearings  5 ,  6  from the detected temperature of the pressure receiving member  4 . The storage device  41  prestores, for each pressure chamber  20 ,  21 , data of the oil pressure of the pressure oil that is supplied to each pressure chamber  20 ,  21  corresponding to the rotational speed of the main spindle  3  detected by the main-spindle-rotational-speed detection device  39  and the temperature of the pressure receiving member  4  detected by the temperature detection sensor  40 . The control device  38  selects the data of the oil pressure from the storage device  41  based on the rotational speed of the main spindle  3  and the temperature of the pressure receiving member  4 . The control device  38  sends an operation command signal according to the selected data to the hydraulic unit  36 , and sends an open/close control signal according to the selected data to each pressure reducing valve  35 ,  37 . The oil pressure in the pressure chamber  20  and the oil pressure in the pressure chamber  21  are thus adjusted to a predetermined oil pressure selected from the storage device  41 , as described below. 
         [0021]    As shown in  FIG. 1 , the circulating paths  22  to  24  are formed in the housing  7  and circulate cooling oil for cooling each rolling bearing  5 ,  6 . The circulating path  22  is formed in the housing  7  so as to adjoin the pressure chamber  20  on the front side of the pressure chamber  20  in the axial direction X. This circulating path  22  is formed by a space surrounded by the pressure receiving member  4  and an annular groove  44  formed in the inner peripheral surface of the housing  7  along the entire circumference on the front side of the annular groove  25  (see  FIG. 1 ) in the axial direction X. An O-ring  47  is provided between the pressure receiving member  4  and the housing  7 . The cooling oil is sealed in the circulating path  22  with this O-ring  47  and the O ring  27  (see  FIG. 1 ). As shown in  FIG. 1 , the circulating path  22  adjoins the pressure chamber  20  facing the rolling bearing  5  with the pressure receiving member  4  interposed between the pressure chamber  20  and the rolling bearing  5 . Therefore, the circulating path  22  can he formed in the housing  7  at a position close to the rolling hearing  5 . 
         [0022]    The circulating path  23  is formed in the housing  7  so as to adjoin the pressure chamber  21  on the rear side of the pressure chamber  21  in the axial direction X. This circulating path  23  is formed by a space surrounded by the pressure receiving member  4  and an annular groove  45  formed in the inner peripheral surface of the housing  7  along the entire circumference on the rear side of the annular groove  26  (see  FIG. 1 ) in the axial direction X. An O-ring  48  is provided between the pressure receiving member  4  and the housing  7 . The cooling oil is sealed in the circulating path  23  with this O-ring  48  and the O ring  30  (see  FIG. 1 ). As shown in  FIG. 1 , the circulating path  23  adjoins the pressure chamber  21  facing the rolling bearing  6  with the pressure receiving member  4  interposed between the pressure chamber  21  and the rolling bearing  6 . Therefore, the circulating path  23  can be formed in the housing  7  at a position close to the rolling bearing  6 . 
         [0023]    The circulating path  24  is formed in the housing  7  so as to be interposed between the pressure chamber  20  and the pressure chamber  21  in the axial direction X. This circulating path  24  is formed by a space surrounded by the pressure receiving member  4  and an annular groove  46  formed in the inner peripheral surface of the housing  7  along the entire circumference between the annular groove  25  and the annular groove  26  in the axial direction X. As shown in  FIG. 1 , the lateral dimension of the annular groove  46  in the axial direction X is made slightly smaller than the interval between the rear wall of the pressure chamber  20  and the front wall of the pressure chamber  21  in the axial direction X. The front wall of the annular groove  46  in the axial direction X adjoins the O-ring  28  that seals the pressure oil in the pressure chamber  20 . The rear wall of the annular groove  46  in the axial direction X adjoins the O-ring  29  that seals the pressure oil in the pressure chamber  21 . The cooling oil can thus be sealed in the circulating path  24  (annular groove  46 ) with the O-ring  28  and the O-ring  29 . As shown in  FIG. 1 , since the front wall of the circulating path  24  in the axial direction X adjoins the rear wall of the pressure chamber  20  facing the rolling bearing  5  with the pressure receiving member  4  interposed between the pressure chamber  20  and the rolling bearing  5 , the circulating path  24  can be formed in the housing  7  at a position close to the rolling bearing  5 . Moreover, since the rear wall of the circulating path  24  in the axial direction X adjoins the front wall of the pressure chamber  21  facing the rolling bearing  6  with the pressure receiving member  4  interposed between the pressure chamber  21  and the rolling bearing  6 , the circulating path  24  can be formed in the housing  7  at a position close to the rolling bearing  6 . 
         [0024]    As shown in  FIG. 1 , a main cooling oil passage  50  is formed in the housing  7 . The main cooling oil passage  50  extends along the axial direction X and has its rear end opening to the rear end face of the housing  7 . Moreover, a first branch cooling oil passage  51 , a second branch cooling oil passage  52 , and a third branch cooling oil passage  53  are formed in the housing  7 . The first branch cooling oil passage  51  branches off from the main cooling oil passage  50  to communicate with the circulating path  22 . The second branch cooling oil passage  52  branches off from the main cooling oil passage  50  to communicate with the circulating path  23 . The third branch cooling oil passage  53  branches off from the main cooling oil passage  50  to communicate with the circulating path  24 . 
         [0025]    A cooling oil supply path  54  is formed in the protruding portion  11  of the pressure receiving member  4 . The front end of the cooling oil supply path  54  is connected to the rear end of the main cooling oil passage  50  in the abutting portion of the rear end face of the housing  7  on the protruding portion  11 . An O-ring  55  is provided between the rear end face of the housing  7  and the protruding portion  11  so as to be disposed on the outer periphery of the main cooling oil passage  50  and the outer periphery of the cooling oil supply path  54 . A cooling oil supply device  56  capable of supplying the cooling oil for cooling each rolling bearing  5 ,  6  is connected to the cooling oil supply path  54 , and the control device  38  is connected to the cooling oil supply device  56 . This cooling oil supply device  56  supplies the cooling oil to the main cooling oil passage  50  through the cooling oil supply path  54  in response to an operation command signal from the control device  38 . The cooling oil is supplied from the main cooling oil passage  50  to each circulating path  22  to  24  through each branch cooling oil passage  51  to  53 , and is then returned to the cooling oil supply device  56  through a cooling oil recovery path (not shown). Thereafter, the cooling oil supply device  56  repeats the operation of cooling the cooling oil and sequentially circulating the cooling oil in the cooling oil supply path  54 , the main cooling oil passage  50 , each branch cooling oil passage  51  to  53 , each circulating path  22  to  24 , and the cooling oil recovery path. The cooling oil is an example of the bearing cooling medium in the present invention. 
         [0026]    Next, operation of the main spindle device  2  will be described. A tool is mounted on the front end of the main spindle  3  shown in  FIG. 1 , and for example, a workpiece (not shown) is cut with the tool rotating together with the main spindle  3 . Any chatter vibrations caused during the cutting work can be suppressed by increasing the rigidity of the main spindle  3  as follows. If chatter vibrations are caused, the operator of the machining center  1  operates the control panel to send an oil pressure application command signal to the control device  38 . In response to the oil pressure application command signal, the control device  38  selects data of a predetermined oil pressure stored in the storage device  41 , based on the rotational speed of the main spindle  3  detected by the main-spindle-rotational-speed detection device  39  and the temperature of the pressure receiving member  4  detected by the temperature detection sensor  40 . Then, the control device  38  sends an operation command signal according to the data of the oil pressure to the hydraulic unit  36 , and sends an open/close control signal according to the oil pressure data to each pressure reducing valve  35 ,  37 . The pressure reducing valves  35 ,  37  are thus independently controlled to be opened or closed to control the oil pressure in the pressure oil supply passages  32 ,  33 , respectively. The oil pressure in the pressure chamber  20  communicating with the pressure oil supply path  32  and the oil pressure in the pressure chamber  21  communicating with the pressure oil supply path  33  are thus adjusted to the predetermined oil pressure selected from the storage device  41 . 
         [0027]    If the oil pressure in each pressure chamber  20 ,  21  increases to the predetermined oil pressure, the oil pressure in these pressure chambers  20 ,  21  uniformly presses the thin cylindrical portion  4 A of the pressure receiving member  4 , and the pressure receiving member  4  uniformly moves toward the outer races  5 B,  6 B in the perpendicular direction Y. The clearance between each outer race  5 B,  6 B and the pressure receiving member  4  is thus eliminated, and the oil pressure is uniformly applied to each outer race  5 B,  6 B through the pressure receiving member  4 . Each inner race  5 A,  6 A is thus uniformly pressed against the outer peripheral surface of the main spindle  3 . This can increase the rigidity or the main spindle  3 . In particular, in the present embodiment, since the entire length A of the pressure chamber  20  is made equal to or larger than the entire length B of the rolling bearing  5  in the axial direction X, the oil pressure in the pressure chamber  20  can be uniformly applied to the outer race  5 B in the axial direction X through the thin cylindrical portion  4 A. Moreover, since the entire length C of the pressure chamber  21  is made equal to or larger than the entire length D of the rolling bearing  6  in the axial direction X, the oil pressure in the pressure chamber  21  can be uniformly applied to the outer race  6 B in the axial direction X through the thin cylindrical portion  4 A. The pressure reducing valves  35 ,  37 , the hydraulic unit  36 , the control device  38 , the main-spindle-rotational-speed detection device  39 , the temperature detection sensor  40 , and the storage device  41  are an example of the adjustment units in the present invention. 
         [0028]    In addition, the present embodiment is advantageous in that the pressure receiving member  4  can be easily moved toward both outer races  5 B,  6 B because the oil pressure is applied to the thin cylindrical portion  4 A that is lighter than the thick cylindrical portion  4 B. Moreover, since the pressure receiving member  4  is made of a metal, it has higher mechanical strength than, e.g., a pressure receiving member molded by using an elastic material. The pressure receiving member  4  is thus advantageous in that it has high fatigue strength. 
         [0029]    When cutting the workpiece, the cooling oil supply device  56  repeats the operation of cooling the cooling oil and sequentially circulating the cooling oil in the cooling oil supply path  54 , the main cooling oil passage  50 , each branch cooling oil passage  51  to  53 , each circulating path  22  to  24 , and the cooling oil recovery path, in response to the operation command signal from the control device  38 . In the cutting work, heat generated by, e.g., friction between the main spindle  3  and each rolling bearing  5 ,  6  is conducted to the pressure receiving member  4  that contacts each rolling bearing  5 ,  6 . The cooling oil circulating in each circulating path  22  to  24  removes the heat, whereby generation of the heat can be suppressed. Thermal expansion of the rolling bearings  5 ,  6  can thus be suppressed. This can prevent, e.g., seizure between a rolling element  58  and the inner race  5 A and between a rolling element  59  and the inner race  6 A. In particular, in the present embodiment, the circulating paths  22  to  24  are formed in the housing  7  so as to contact the thin cylindrical portion  4 A. Accordingly, heat that is conducted through the thin cylindrical portion  4 A faster than through the thick cylindrical portion  4 B can be effectively removed by the cooling oil circulating in the circulating paths  22  to  24 . Generation of the heat can thus be effectively suppressed. 
       Effects of the Embodiment 
       [0030]    In the main spindle device  2  of the present embodiment, the oil pressure adjusted to a predetermined oil pressure is supplied to each pressure chamber  20 ,  21 , whereby the pressure receiving member  4  can be uniformly pressed toward both rolling bearings  5 ,  6  by the oil pressure in each pressure chamber  20 ,  21 . The pressure receiving member  4  can thus uniformly press each rolling bearing  5 ,  6  against the main spindle  3 , and the rigidity of the main spindle  3  can be increased. 
         [0031]    Since the entire length A of the pressure chamber  20  is made equal to or larger than the entire length B of the rolling bearing  5  in the axial direction X, the oil pressure in the pressure chamber  20  can be uniformly applied to the outer race  5 B in the axial direction X through the thin cylindrical portion  4 A. Moreover, since the entire length C of the pressure chamber  21  is made equal to or larger than the entire length D of the rolling bearing  6  in the axial direction X, the oil pressure in the pressure chamber  21  can be uniformly applied to the outer race  6 B in the axial direction X through the thin cylindrical portion  4 A. This allows the rolling bearing  5  and the rolling bearing  6  to be uniformly pressed against the main spindle  3  over the entire length B of the rolling bearing  5  and the entire length D of the rolling bearing  6 . 
         [0032]    Moreover, heat that is generated by friction between the main spindle  3  and each rolling bearing  5 ,  6  during, e.g., rotation of the main spindle  3  can be suppressed by the cooling oil circulating in each circulating path  22  to  24 . This can suppress thermal expansion of the rolling bearings  5 ,  6 , and thus can prevent seizure between the rolling element  58  and the inner race  5 A and between the rolling element  59  and the inner race  6 A. 
         [0033]    The present invention is not limited to the above embodiment, and may be carried out by partially modifying the configuration as appropriate without departing from the spirit and scope of the invention. In the above embodiment, the pressure receiving member  4  is moved toward both outer races  5 B,  6 B by the oil pressure. Alternatively, however, the pressure receiving member  4  may be moved toward both outer races  5 B,  6 B by, e.g., the pressure of compressed air that is supplied by an air supply device. In the above embodiment, generation of the heat during rotation of the main spindle  3  is suppressed by the cooling oil. Alternatively, however, generation of the heat may be suppressed by, e.g., cooling water or coolant. 
         [0034]    In the above embodiment, the temperature of each rolling bearing  5 ,  6  is indirectly detected by the temperature detection sensor  40  provided in the pressure receiving member  4 . Alternatively, the temperature of each rolling bearing  5 ,  6  may be directly detected by a temperature detection sensor attached to each rolling bearing  5 ,  6 . Moreover, in the above embodiment, the oil pressure in each pressure chamber  20 ,  21  is adjusted by automatically controlling the hydraulic unit  36  and the pressure reducing valves  35 ,  37  based on the operation command signal and the open/close control signal which are sent from the control device  38 . Alternatively, for example, a display device that displays data of a predetermined oil pressure selected from the storage device  41  by the control device  38  may be provided, and the operator may manually operate a hydraulic pump or the pressure reducing valves to adjust the oil pressure in each pressure chamber  20 ,  21  to the predetermined oil pressure. Moreover, the clearance dimension between the outer race of each rolling bearing  5 ,  6  and the pressure receiving member  4  is not limited to 12 μm to 15 μm, and may be set to an appropriate value. 
         [0035]    It is explicitly stated that all features disclosed in the description and/or the claims are intended to be disclosed separately and independently from each other for the purpose of original disclosure as well as for the purpose of restricting the claimed invention independent of the composition of the features in the embodiments and/or the claims. It is explicitly stated that all value ranges or indications of groups of entities disclose every possible intermediate value or intermediate entity for the purpose of original disclosure as well as for the purpose of restricting the claimed invention, in particular as limits of value ranges.