Abstract:
In order to provide a vacuum pump motor which is free from rotation imbalance, there is provided a vacuum pump motor  10  which is connected directly to a pump main shaft  21  of a vacuum pump  20 , including a motor rotor  120  which is attached directly or indirectly to the pump main shaft  21  and balance rings  127  which are attached directly or indirectly to the pump main shaft  21  at ends thereof which are spaced apart from end portions of a rotor core  123 , wherein the balance rings  127  contain a material having anticorrosion properties.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority to Japanese Application Number 2012-182203, filed Aug. 21, 2012, the entire content of which is hereby incorporated by reference. 
     TECHNICAL FIELD 
     The present invention relates to a vacuum pump motor and a vacuum pump including the same motor and more particularly to a vacuum pump motor in which balance rings are provided at both ends of a motor rotor in the direction of a rotating axis thereof and a vacuum pump including the same motor. 
     BACKGROUND ART 
     For example, vacuum pumps are used in semiconductor fabrication systems, and a motor as a drive source is provided thereto. As such a motor used in a vacuum pump, there is a motor having a structure in which a motor rotor is disposed directly on a pump main shaft. In the case of the motor having this structure, the motor rotor is exposed to a corrosive gas used in a semiconductor fabrication process. In addition, a sealing structure is realized to maintain a vacuum (low pressure) state in which a can is provided between a motor stator and the motor rotor (refer to Patent Document 1). 
     In addition, end rings are provided at both end portions of a rotor core of the motor rotor in the direction of a rotating axis thereof (refer to Patent Document 2). These end rings form an induction circuit together with the rotor core. However, since the motor rotor rotates at high speeds, in case there exists a rotation imbalance, large stress is applied to the pump main shaft, causing a deflection in the pump main shaft. In addition, there may be a case where the pump main shaft is caused to vibrate largely. As an approach to eliminate such a rotation imbalance, it is practiced that balance rings are attached to the pump main shaft so that the rotation of the pump main shaft is balanced by removing a little portion of the balance rings (refer to Patent Document 2). 
     CITATION LIST 
     Patent Document 
     [Patent Document 1] Japanese Patent No. 3443248 
     [Patent Document 2] Japanese Patent Public Disclosure No. 11-27911 
     SUMMARY OF INVENTION 
     Even if the can is provided between the motor rotor and the motor stator, the motor rotor is exposed to the corrosive gas as described above. Because of this, the motor rotor is corroded by the corrosive gas unless a countermeasure is taken. A surface treatment using an anticorrosion coating is often applied to the motor rotor in order to avoid the corrosion of the motor rotor. However, in the case of a motor in which only end rings are provided, in order to eliminate the rotation imbalance described above, little portions of the end rings is removed to keep the balance in rotation. This means that the anticorrosion coating is removed at the portions of the end rings where the end rings are partially removed, whereby the end rings are corroded at the portions where the end rings are partially removed. 
     It is also considered that the anticorrosion coating is applied after the end rings are partially removed. However, since the dimensions of the portions where the end rings are partially removed (correction holes) are very small, there often occurs a case where a coating of the anticorrosion material cannot be formed uniformly on the portions where the end rings are partially removed. Further, even when balance rings are attached, in the event that the little portions of the balance rings are removed, as in the case with the end rings, the problem remains unsolved that the coating of the anticorrosion material becomes uneven at the portions where little portions of the balance rings are removed. 
     The invention of this patent application has been made in view of the problem described above, and hence, an object thereof is to provide a vacuum pump motor which is free from rotation imbalance and a vacuum pump including the same motor. According to a first aspect of the invention, there is provided a vacuum pump motor which is coupled directly to a pump main shaft of a vacuum pump, including a motor rotor which is disposed directly or indirectly on the pump main shaft and balance rings which are disposed directly or indirectly on the pump main shaft at ends thereof which are spaced apart from end portions of a rotor core, wherein the balance rings contain a material having anticorrosion properties. 
     In addition, according to a second aspect of the invention, a configuration is adopted in which end rings are provided at end portions of the motor rotor and a predetermined gap is defined between the end rings and the balance rings. 
     Additionally, according to a third aspect of the invention, a configuration is adopted in which the balance rings each have an L-shaped cross-section and are disposed on a sleeve which is attached to the pump main shaft or disposed directly on the pump main shaft, having a function to position the motor rotor. 
     Further, according to a fourth aspect of the invention, a configuration is adopted in which a surface treatment involving anticorrosion coating or anticorrosion painting is applied to a surface of the motor rotor. 
     Furthermore, according to a fifth aspect of the invention, there is provided a vacuum pump including the motor according to any of the first to fourth aspects of the invention. 
     According to the invention, the corrosion-resistant motor rotor can be provided, and the problem of correcting the rotation imbalance can be solved to thereby realize an increase in performance. At the same time, by providing the axial gaps between the balance rings and the end rings, the interference of the balance rings with the end rings which expand as a result of thermal expansion can be avoided to thereby prevent the application of load in the direction of a rotating axis, providing an advantage that superiority in mechanical design can be realized. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a sectional view showing a motor according to an embodiment of the invention. 
         FIG. 2  is a perspective view showing some constituent components of the motor shown in  FIG. 1 . 
         FIG. 3  is a sectional view showing a vacuum pump including the motor shown in  FIG. 1 . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Next, an embodiment of the invention of this patent application will be described based on the accompanying drawings. 
     [Summary of Overall Structure of Motor] 
     Firstly, based on  FIGS. 1 and 2 , a motor  100  according to an embodiment of the invention will be described. The motor  100  according to the embodiment is intended to drive to rotate rotors  30  (refer to  FIG. 3 ) of a vacuum pump. The motor  100  includes a motor stator  110 , a motor rotor  120  and a motor frame  140 . 
     The motor frame  140  includes a frame main body  141  and an end plate  142 . The frame main body  141  has a cylindrical shape, and an interior space is formed in the frame main body  141  so as to extend along an axis of a pump main shaft  21 . The end plate  142  is attached to an open end of the frame main body  141  with a required attaching mechanism. The motor frame  140  can be formed of iron or aluminum, for example. A motor stator  110  and a motor rotor  120  are accommodated in the interior space in the motor frame  140 . 
     [Motor Stator] 
     The motor stator  110  has a configuration in which coils  111   b  are attached to a stator core  111   a . Specifically, in the motor stator  110 , the coils  111   b  protrudes axially outwards at both ends thereof in the direction of a rotating axis C of the pump main shaft  21 . The motor stator  110  is fixed to the motor frame  140  concentrically with respect to the rotating axis C of the pump main shaft  21  by the stator core  111   a  being fitted in an inner surface of the frame main body  141  of the motor frame  140 . The stator core  111   a  can be formed by laminating silicone steel plates, for example. 
     [Motor Rotor] 
     The motor rotor  120  is disposed concentrically with respect to the rotating axis C of the pump main shaft  21  in an interior of the motor stator  110  and is connected directly to the pump main shaft  21  of a vacuum pump  20 . The motor rotor  120  includes a sleeve  121  which is attached to an outer circumferential surface of the pump main shaft  21 , a rotor core  123  which is fixedly fitted on the sleeve  121 , end rings  125  which are disposed at both ends of the rotor core  123  in the direction of the rotating axis C of the pump main shaft  21  and balance rings  127  which are disposed at ends of an outer circumferential surface of the sleeve  121  which are situated further axially outwards than the end rings  125 . 
     The sleeve  121  is attached to a diametrically reduced portion of the pump main shaft  21  which is formed at a distal end portion thereof and is fixed in place by a washer member  122  and a bolt  124 . It is desirable to provide a rotation preventive or locking mechanism made up of a key and a key groove in a boundary area between the pump main shaft  21  and the sleeve  121  so as to prevent a relative rotation therebetween. Additionally, the rotor core  123  is fixed to the outer circumferential surface of the sleeve  121 . The rotor core  123  receives a magnetic force from the motor stator  110  to thereby produce a rotating force in the motor rotor  120 . 
     The end rings  125  which are attached individually to the ends of the rotor core  123  (the ends of the rotor core  123  in the direction of the rotating axis of the pump main shaft) connect to the rotor core  123  so as to form an induction circuit and are made up of a conductor made of a material having a high electric conductivity such as aluminum or copper. The end rings  125  are integrated with the motor rotor  120 . The end rings  125  of this embodiment are each formed into a simple ring member having a rectangular cross section. Aluminum or copper is corroded by a corrosive gas used in the semiconductor fabrication process, and therefore, a surface treatment involving anticorrosion coating or anticorrosion painting is applied to the surfaces of the rotor core  123  and the end rings  125 . 
     The balance rings  127  are attached to the outer circumferential surface of the sleeve  121  at the ends thereof which are situated further axially outwards than the end rings  125 . The balance rings  127  are intended to eliminate a rotation imbalance produced in the motor rotor  120 . The balance rings  127  each have an L-shaped section. Of two side portions which make up the L-shaped section, one side portion is in abutment with the sleeve  121 , and the other side portion rises in a direction which is at right angles to the rotating axis C of the pump main shaft  21 . In addition to the function of eliminating the rotation imbalance, the balance rings  127  having the L-shaped section also have a function to position the rotor core  123 . In addition, the balance rings  127  are made of an anticorrosion material such as stainless steel or a material which contains such an anticorrosion material at least partially. 
     In this embodiment, a predetermined gap is defined in the direction of the rotating axis C between the end ring  125  and the balance ring  127 , and a predetermined gap is also defined therebetween in the direction which is at right angles to the rotating axis C. Because of this, even in the event that the end ring  125  and the balance ring  127  are made of materials having different thermal expansion coefficients, there is produced no stress between the two members by thermal expansion. In addition, an end portion of the one side portion of the balance ring  127  is in abutment with the rotor core  123 . Since it is necessary to prevent a relative rotation between the balance ring  127  and the sleeve  121 , it is desirable to provide a rotation preventive or locking mechanism made up of a key and a key groove in a boundary area between the two members. 
     [Can Construction] 
     A can  130  is provided in the interior of the motor  100 . The can  130  is intended to separate a space where the motor rotor  120  resides and a space where the motor stator  110  resides. This is because in the case of the motor  100  used in the vacuum pump  20  (refer to  FIG. 3 ), a vacuum (or low pressure) state needs to be maintained in the space where the pump main shaft  21  and the motor rotor  120  reside. Additionally, the vacuum pump  20  is often used in a semiconductor fabrication facility, and a countermeasure for preventing the leakage of a process gas used in fabrication of semiconductors needs to be taken in the vacuum pump  20 . Thus, the can  130  plays this role. 
     The can  130  of this embodiment is a cylindrical member which is opened at both ends thereof and is made of a metallic material. As shown in  FIG. 1 , the can  130  is held by the frame main body  141  and the end plate  142  therebetween at one end thereof and is fixed to an inner circumferential surface of a ring-shaped sealing member  132  at the other end (a right end in  FIG. 1 ). By adopting this configuration, although the space where the motor rotor  120  resides is allowed to communicate with the vacuum pump  20 , the space where the motor stator  110  resides is isolated from the vacuum (low pressure) state. 
       FIG. 2  is a perspective view intended to help a reader to understand the invention and shows some constituent components of the motor  100 . Specifically,  FIG. 2  shows the pump main shaft  21 , the sleeve  121 , the balance rings  127 , the washer member  122  and the bolt  124 . As shown therein, the balance rings  127  are fixed onto the sleeve  121 , and the sleeve  121  is attached to the pump main shaft  21 . Then, the bolt  124  is screwed into the pump main shaft  21  via the washer member  122 . In the motor  100  of this embodiment, although the rotor core  123  is placed on the outer circumferential surface of the sleeve  121 , in this invention, since the sleeve  121  is not an indispensable constituent element, it is possible to omit the sleeve  121 . 
     [Function] 
     Next, the function of the motor  100  configured as has been described heretofore will be described. At the time of fabrication of a motor  100 , with a sleeve  121 , a rotor core  123  and end rings  125  assembled to a pump main shaft  21 , a rotation imbalance is produced as a result of a fabrication tolerance. In order to eliminate the rotation imbalance so produced, it is considered, for example, to remove part of the end rings  125 . However, the anticorrosion coating applied thereto is also removed together the part of the end rings  125  so removed. Thus, the portions of the end rings  125  where the end rings  125  are partially removed are corroded by the process gas. 
     To counteract this, in this embodiment, the balance rings  127  are provided. Namely, with the balance rings  127  which are made of the anticorrosion material attached to the sleeve  121 , a rotation imbalance is measured. Then, the balance rings  127  are removed partially in order to eliminate the rotation imbalance. Even in the event that the balance rings  127  are partially removed, the balance rings  127  are made of the anticorrosion material, and therefore, the balance rings  127  are not corroded. 
     [Vacuum Pump] 
     Next, the vacuum pump  20  will be described based on  FIG. 3  which includes the motor  100  according to the embodiment.  FIG. 3  shows a section of the vacuum pump  20  taken along a portion which contains the rotating axes C of two pump main shafts  21  provided in the vacuum pump  20 . As shown in the figure, the vacuum pump  20  includes a pair of rotors  30 . In the motor  100  of this embodiment, each rotor  30  includes a first stage rotor  31 , a second stage rotor  32 , a third stage rotor  33  and a fourth stage rotor  34 . The pump main shafts  21  which support these rotors  30  are supported by bearings  51 ,  61  near both end portions thereof. In addition, the rotors  30  are accommodated within a casing  40 . 
     The rotors  30  are driven by the motor  100 . A pair of timing gears  70  which mesh with each other is fixed to the other end of the pump rotors  30 . By doing so, the pump main shafts  21  and the pump rotors  30  are made to rotate in opposite directions to each other at the same speed. 
     When the motor  100  is driven, the pump rotors  30  rotate in opposite directions without contacting each other with slight gaps held between an inner surface of the casing  40  and the pump rotors  30  and between the pump rotors  30  themselves. A gas at an induction side is shut in between the pump rotors  30  and the casing  40  and is then moved towards a discharge end as the pair of pump rotors  30  rotates. The gas induced from an induction port (whose illustration is omitted) is compressed and moved towards a discharge port (whose illustration is omitted) by the fourth stage rotors  34  to thereby be discharged from the discharge port. 
     It should be noted that in the embodiment, the invention is described as a combination of the individual constituent elements. In this invention, as long as the sealing construction can be realized, arbitrary combinations of the individual constituent elements are understood to fall within the scope of the invention. For example, although the invention is described as incorporating all constituent elements A, B, and C, in the event that those skilled in the art determine that the invention can be established even by a combination of A and B, it should be understood that the combination of A and B falls within the scope of the invention. 
     INDUSTRIAL APPLICABILITY 
     The invention of this patent application can be applied to eliminate the rotation imbalance of the vacuum pump motor. 
     REFERENCE SIGNS LIST 
       20  vacuum pump;  21  pump main shaft;  100  motor;  110  motor stator;  120  rotor;  121  sleeve;  123  rotor core;  125  end ring;  127  balance ring;  130  can.