Abstract:
In a fixing structure for a rolling bearing in which a bearing fixing ring is incorporated in a device and a rotating member is fixed to a bearing rotating ring using bolts, fastening areas with the bolts are set so that the natural frequency of the device as a whole is larger than the frequency of vibration resulting from the rotation of the rolling bearing. This prevents generation of noise or vibration in the rolling bearing.

Description:
BACKGROUND OF THE INTENTION 
     a. Field of the Invention 
     The invention relates to a fixing structure for an ultra thin rolling bearing for use, for example, in an industrial robot, a machine tool, and medical equipment. 
     b. Related Art 
     A gantry bearing in a CT scanner is used as a rotating body bearing provided with an X-ray tube and photographing equipment typically used for image processing. The bearing is an ultra large size device having a bearing outer size of about 1 m and therefore typically coupled to the main body frame of the CT scanner using bolts when it is fixed to the frame. 
       FIG. 9  shows an example of the CT scanner used as a kind of medical equipment. As shown, in the CT scanner, X-rays generated by an X-ray tube device  50  are directed upon a subject  56  through a wedge filter  52  that equalizes the intensity distribution and a slit  54  that limits the intensity distribution. The X-rays passed through the subject  56  are received at a detector  58  and converted into an electrical signal for transmission to a computer that is not shown. The elements such as the X-ray tube device  50 , the wedge filter  52 , the slit  54 , and the detector  58  are mounted to a rotating base  64  in a substantially cylindrical shape rotatably supported at a fixed base  62  via a bearing  60 , and rotated around the subject  56  as the rotating base  64  rotates. In the CT scanner device, the X-ray tube device  50  and the detector  58  opposing each other rotate around the subject  56 , so that projection data covering every angle in every aspect in a plane of the subject  56  for examination is obtained, and a tomogram is obtained based on the data using a preprogrammed restructuring program. 
     The rotating base  64  of the device is coupled to the width side of the rotating ring (hereinafter referred to as “bearing rotating ring”) among the inner and outer rotating rings of the bearing  60 . The dominant frequency for vibration resulting from the rotation of the bearing  60  is determined based on the number of bolts or the number of areas fastened with the bolts and the number of revolutions of the bearing  60 . Meanwhile, the frame  62  fixed to the non-rotating ring (hereinafter referred to as “bearing fixed ring”) among the inner and outer rings of the bearing  60  tends to have a relatively low natural frequency because the rigidity of the structure is reduced to satisfy a need for a more compact and simple device. Therefore, the frequency of vibration resulting from the rotation of the bearing  60  matches the natural frequency of the structure, and resonance is caused. 
     SUMMARY OF THE INVENTION 
     It is therefore a main object of the invention to reduce or prevent the resonance when the rolling bearing rotates. 
     The dominant frequency for vibration or associated noise occurs corresponding to a bolt number order component in the rotating speed (number of revolutions) of the bearing. So-called multi-angular distortion corresponding to the number of the fixing bolts is generated. When, for example, the bearing is fixed in seven positions, a septenary component in the rotating frequency is excited. The frequency determined based on the number of revolutions and the bolt number order component must not coincide with the natural frequency of the structure in order to reduce the resonance. According to the invention based on this finding, the rotating member is fixed using bolts (or areas fastened with the bolts) whose number does not cause resonance with the natural frequency of the structure. More specifically, according to the invention, in a fixing structure for a rolling bearing in which a bearing fixing ring is incorporated in a device and a rotating member is fixed to a bearing rotating ring using bolts, fastening areas with the bolts are set so that the natural frequency of the device as a whole is larger than the frequency of vibration resulting from the rotation of the rolling bearing. 
     As a gantry bearing in a CT scanner, an ultra thin rolling bearing includes: an outer member having a raceway at an inner circumference thereof; an inner member having a raceway at an outer circumference thereof; a plurality of rolling elements interposed between the raceways of the inner and outer members; and a cage holding the rolling elements at prescribed intervals. One of the outer member and the inner member is fixed to a rotating base of the CT scanner rotating around a subject, and the other is fixed to a fixed base of the CT scanner. In this way, the rotating base of the CT scanner is supported rotatably to the fixed base. 
     According to the invention, the dominant frequency band changes and resonance is reduced. 
       FIG. 1  is a diagram for use in illustration of the basic concept of the invention. When the natural frequency of the structure such as a frame is 11.5 Hz, and the number of revolutions of the bearing is 180 rpm at maximum, the bearing rotating ring and the rotating member holding a heavy subject are connected with bolts in three positions in the circumferential direction, so that the peak frequency of the vibration resulting from the rotation of the bearing is not more than the natural frequency of the structure such as the frame, and resonance can be prevented. More specifically, in the example in  FIG. 1 , the peak frequencies of 60 rpm, 120 rpm, and 180 rpm are shown as an example. If the number of fastening areas with the bolts are three, the peak frequency of vibration resulting from the rotation of the bearing and the natural frequency of the structure do not match in any of these three regions of 60 rpm to 180 rpm. Therefore, the resonance can be reduced. 
     Now, an embodiment of the invention will be described in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a graph for use in illustration of an embodiment of the invention; 
         FIG. 2  is a front view of a rolling bearing; 
         FIG. 3  is an enlarged sectional view of a rolling bearing; 
         FIG. 4  is a front view of a cage; 
         FIG. 5  is a development plan view of a segment forming a cage; 
         FIGS. 6A to 6C  are power spectra when fixing is carried out in seven positions with bolts; 
         FIGS. 7A to 7C  are power spectra when fixing is carried out in three positions with bolts; 
         FIGS. 8A to 8C  are power spectra when fixing is carried out in two positions with bolts; and 
         FIG. 9  is a sectional view of a CT scanner. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Now, an ultra thin rolling bearing shown in  FIGS. 2 and 3  will be described. In this example,  FIG. 3  corresponds to a sectional view of the bearing  60  for use in the CT scanner in  FIG. 9 . The bearing  60  includes an outer member  10 , an inner member  18 , rolling elements  26 , and a cage  28  as essential elements. The outer member  10  is in a ring shape and has a single raceway  12  at its inner circumference. The inner member  18  is in a ring shape and provided concentrically at the inner circumferential side of the outer member  10  and has a single raceway  20  at its outer circumference. A plurality of rolling elements  26  are interposed in a single row between the raceways  12  and  20  of the outer and inner members  10  and  18 . The cage  28  holds the rolling elements  26  at prescribed intervals in the circumferential direction. The balls are shown as the rolling elements  26  by way of illustration, but rollers may be used instead. The reference numeral  48  refers to a seal device that seals each of the openings at both ends of the bearing in a non-contact state. 
     The bearing  60  is an ultra thin bearing and the value φ of the ratio D B /PCD is not more than 0.03 (φ≦0.03) in which D B  represents the diameter of the ball  26  and PCD represents the pitch circle diameter. This is normally applied to a large size bearing whose PCD is about in the range from 500 mm to 1500 mm. More specifically, when the ball size is ½ inches (12.7 mm) and PCD is 1041.4 mm, φ is 0.012. 
     The cage  28  is made of a resin material unlike a conventional metal material. As shown in  FIG. 4 , the resin cage  28  is a split type having a plurality of resin segments  30  having a circular sectional shape connected in the circumferential direction to be in an annular form. As shown in  FIG. 5 , a raised fitting portion  32  and a recessed fitting portion  34  are formed on the ends of each of the segments  30 . The recessed fitting portion  34  and the raised fitting portion  32  at ends of corresponding segments  30  are coupled with each other to couple the segments with each other, so that the annular cage  28  is formed. The segments  30  as shown each include a circular base portion  36  produced by dividing an annular member in a plurality of positions in the circumferential direction, pillar portions  38  extending in one direction in the axial direction from the base portion  36 , and a plurality of pockets  40  and  42  provided between adjacent pillar portions  38 . 
     The pockets  40  and  42  as shown are in different shapes. More specifically, the first pockets  40  have a retaining function for the ball  26  (including the capability of providing the balls at equal intervals) and the second pockets  42  have only the capability of providing the balls at equal intervals. The cage  28  according to the embodiment has these two kinds of pockets  40  and  42  alternately provided at equal intervals in the circumferential direction. The shapes or structures of the pockets  40  and  42  are only by way of illustration, and the pockets may have, for example, a single shape. In other words, pockets having various shapes and structures may be employed depending on the conditions of how the bearing is used. 
     There is a clearance (pocket clearance) between the surface of the ball  26  and the pocket inner surfaces in the first and second pockets  40  and  42 . While the bearing rotates, the presence of the pocket clearance allows the cage  28  to move in the radial direction relatively to the ball  26 . The relative movement causes the cage  28  to contact one of the outer circumferential surface  22  of the inner member  18  or the inner circumferential surface  14  of the outer member  10 , so that the cage  28  is guided to rotate. In the shown embodiment, the outer circumferential surface  44  of the cage  28  is in contact with the inner circumferential surface  14  of the outer member  10 , and in this case, the cage  28  is driven by the driving force from the outer member  10  to rotate as it contacts the outer member  10 . Note that the inner circumferential surface  46  of the cage  28  may be contacted to the outer circumferential surface  22  of the inner member  18  to guide the cage  28  to rotate. 
     A screw hole  16  is formed on an end surface at one end (right side in  FIG. 3 ) of the outer member  10 , and fastening means such as a bolt (not shown) is screwed in the screw hole  16 , so that the outer member  10  is fixed to the rotating base  64  of the CT scanner (see  FIG. 9 ). A screw hole  24  is similarly formed on an end surface at the other end of the inner member  18 , and fastening means such as a bolt (not shown) is screwed in the screw hole  24 , so that the inner member  18  is fixed to the fixed base  62  (see  FIG. 9 ). In this case, the outer member  10  serves as a rotating member that rotates together with the rotating base  64 , and the inner member  18  serves as a non-rotating fixed member. Depending on the structure of the CT scanner, the outer member  10  may serve as the fixed side and the inner member  18  may serve as the rotating side that rotates together with the rotating base  64 . As shown in  FIG. 2 , the screw holes  16  are provided at equal intervals in the circumferential direction of the outer member  10 , and the screw holes  24  are provided at equal intervals in the circumferential direction of the inner member  18 . The fastening areas by the bolts are normally provided at equal intervals in the circumferential direction, and the number of bolts in each fastening area may be more than one, though an example with one bolt will be described for the ease of illustration. The structure of the CT scanner has low rigidity to reduce the weight of the scanner, and therefore its natural frequency is low. A peak frequency of vibration is generated when the gantry bearing in the CT scanner rotates. When the natural frequency of the structure and the peak frequency caused by the rotation are near, resonance is caused, which gives rise to noise or vibration. While the resonance could be prevented by changing the rigidity of the structure to change the natural frequency or by changing the number of revolutions of the bearing if possible, the number of bolts for fixing to the bearing rotating ring is changed in this example to prevent the resonance. 
     When, for example, the number of bolts is seven, and the number of revolutions of the bearing is 98 rpm (1.6 Hz), the vibration component is produced as 1.6 Hz×7=11.2≈11.5 Hz, which is maximum (see  FIG. 6B ). In this case, when the number of revolutions is 98 rpm (about 100 rpm) and the natural frequency of the structure such as the frame is, for example, 11.5 Hz, the natural frequency of the structure and the vibration component of the bearing are equal, and resonance is generated. When, for example, the number of bolts is changed to three, the vibration component of the bearing is produced as 1.6 Hz (98 rpm)×3=4.8 Hz≈4.9 Hz (see  FIG. 7B ), which is shifted from the natural frequency of the structure. Therefore, resonance is not generated. 
     Herein,  FIGS. 6A to 6C ,  FIGS. 7A to 7C , and  FIGS. 8A to 8C  each show a power spectrum or the root mean square of temporal or spatial fluctuation as a distribution of frequency components. The abscissa represents the frequency (Hz), and the ordinate represents the noise level (dB). In  FIGS. 6A to 6C , the bearing is fixed in seven positions in the circumference. Similarly in  FIGS. 7A to 7C , the bearing is fixed in three positions, and in  FIG. 8A to 8C , in two positions. The number of revolutions is 60 rpm (1 Hz) in  FIGS. 6A ,  7 A, and  8 A, 98 rpm (1.6 Hz) in  FIGS. 6B ,  7 B, and  8 B, and 120 rpm (2 Hz) in  FIGS. 6C ,  7 C, and  8 C. 
     As in the foregoing, in the bearing fixed to the structure in the fastening areas provided at equal intervals on the circumference, the number of bolts is increased or reduced by at least one so that the number of bolts is not approximated to the value produced by dividing the natural frequency of the structure by the number of revolutions.