Abstract:
A vacuum pump has a casing having an interior space, an inlet port for introducing gas molecules into the interior space, and an outlet port for discharging the gas molecules from the interior space. A rotor shaft extends into the interior space of the casing for undergoing rotation about a rotational axis. A stator is connected to the casing and has stator blades extending into the interior space of the casing. A rotor is disposed between the casing and the rotor shaft. The rotor has a preselected number of rotor blades disposed at an uppermost stage thereof and alternately disposed between the stator blades for undergoing rotation with the rotor shaft to direct gas molecules toward the outlet port. A rotational member is disposed between the inlet port and the rotor for undergoing rotation with the rotor shaft about the rotational axis. The rotational member has a generally conical-shaped surface gradually decreasing toward the inlet port. Guiding blades are disposed on the conical-shaped surface of the rotational member for undergoing rotation with the rotational member about the rotational axis to impart movement to the gas molecules in the interior space of the casing in a radial direction relative to the rotational axis.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a vacuum pump and, more specifically, to a vacuum pump with blades for discharging gas molecules arranged on an inlet port side of the vacuum pump. 
     2. Description of the Related Art 
     Vacuum pumps are widely used in, for example, a device for discharging a gas in a chamber of an apparatus for manufacturing semiconductors to thereby bring the chamber into a vacuum state. Among these vacuum pumps, there are ones composed only of blades, ones composed of a combination of a blade portion and a screw groove portion, etc. 
     FIGS. 12 and 13 illustrate the structure of a conventional vacuum pump. FIG. 12 is a diagram showing a part of the top view of the pump, and FIG. 13 is a diagram showing a part of the section thereof. 
     This vacuum pump is provided with stator blades  50  fixed to a casing  10  that has an inlet port  16 , and a rotor  41  having rotor blades  40  that are rotated while being fixed to a rotating rotor shaft  18 . The respective stator blades  50  and the rotor blades  40  are arranged in the axial direction in multi-stages, and constitute an exhaust system  13  for taking from the inlet port  16  gas molecules A into a space between the rotor  41  and the casing  10  to discharge the gas molecules A. 
     Such vacuum pump accomplishes vacuumizing (exhaustion) by rotating with a motor the rotor shaft  18  at a high speed of ten to ninety thousand rpm under the normal state. 
     There is known a measure in which the outer diameter of the rotor blades  40  is increased in order to increase the peripheral speed of the rotor blades  40  and enhance the discharging ability. However, this causes a decrease in rigidity of the rotor blades  40 , and hence the measure also includes enlargement of the inner diameter of the rotor blades  40 . Due to this structure, while the gas molecules A enter with the same extent that the inlet port  16  has, the flow of the gas molecules A is interrupted in a dead space defined by the inner diameter of the uppermost stage of the rotor blades  40  facing the inlet port  16  (a space around the top of the rotor shaft  18 ) where there are no blades. The existence of this dead space is practically equivalent to a lowering of the effective area of the inlet port, which reduces the conductance as well as the amount of gas molecules entering into spaces between the rotor blades  40 . This leads to a problem of decreased exhaust efficiency. 
     As countermeasures against that dead space in the center of the inlet port, there is proposed a vacuum pump in which a conic inducer  19  is attached to the upper end of the rotor shaft  18  as shown in FIG.  14 . This proposed pump can give an outward motion component in radial direction to the gas molecules A that collide against the wall surface of the inducer  19 . 
     However, the gas molecules A in a molecule flow region obey the law of cosines to head off in the normal direction with respect to the collision face, as shown in FIG. 14, and thus gains not only the outward motion component but also the upward (in the direction of the inlet port) motion component, resulting in an insufficient exhaust efficiency. 
     SUMMARY OF THE INVENTION 
     The present invention has been made to solve the above problems that the conventional vacuum pumps suffer from and, therefore, an object of the present invention is to provide a vacuum pump with enhanced exhaust efficiency achieved by increasing the amount of gas molecules that enter into spaces between rotor blades. 
     The present invention attains the above object through a vacuum pump comprising: a casing with an inlet port; a rotor shaft housed in the casing; an exhaust means arranged between the rotor shaft and the casing such that it can rotate together with the rotor shaft, the exhaust means discharging gas molecules, which are taken in through the inlet port, by rotating along with the rotation of the rotor shaft; and guiding blades arranged between the rotor shaft and the inlet port such that it can rotate together with the rotor shaft, the guiding blades imparting an outward motion component in radial direction to the gas molecules, which are taken in through the inlet port, by rotating along with the rotation of the rotor shaft. 
     According to the present invention, the guiding blades are formed on a forming surface that is formed into a conic shape the diameter of which is gradually decreased toward the inlet port. 
     According to the present invention, the guiding blades are formed such that the front thereof in the rotation direction is perpendicular to the forming surface. 
     According to the present invention, the guiding blades are formed such that the front thereof in the rotation direction is sloped down to the rear rotation direction with respect to the radial direction with the axis of rotation as its center. 
     According to the present invention, the exhaust means comprises at least a plurality of blades, and the number of the guiding blades is set by multiplying the number of rotor blades, which are arranged in the uppermost stage of the above plurality of blades, by its divisor or by an integer. 
     According to the present invention, the guiding blades are formed at positions corresponding to a casing&#39;s decreased diameter portion on a casing inner wall the diameter of which is gradually decreased toward the inlet port. 
     According to the present invention, the exhaust means comprises a blade portion or a screw groove portion, or comprises a combination of the blade portion and the screw groove portion. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings: 
     FIG. 1 is a sectional view showing a vacuum pump according to an embodiment of the present invention; 
     FIG. 2 is a diagram showing a forming surface of the present invention; 
     FIG. 3 is a diagram showing another forming surface of the present invention; 
     FIG. 4 is a diagram showing still another forming surface of the present invention; 
     FIG. 5 is a perspective view showing guiding blades and a forming surface of the present invention; 
     FIG. 6 is a sectional view showing the guiding blades; 
     FIG. 7 is a sectional view showing an example in which a reflecting surface is attached at an acute angle to the forming surface; 
     FIG. 8 is a sectional plan view showing guiding blades; 
     FIG. 9 is an enlarged view showing an angle of elevation of the guiding blades; 
     FIG. 10 is a perspective view showing another embodiment of the present invention; 
     FIG. 11 is a plan view showing the embodiment illustrated in FIG. 10; 
     FIG. 12 is a plan view showing a conventional vacuum pump; 
     FIG. 13 is a vertical sectional view showing the conventional vacuum pump; and 
     FIG. 14 is a vertical sectional view showing another conventional vacuum pump. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinbelow a description will be given in detail of preferred embodiments of the present invention with reference to the drawings. 
     FIG. 1 is a diagram showing in section the whole structure of a vacuum pump according to an embodiment of the present invention. 
     This vacuum pump denoted by reference numeral  1  is installed in, for example, an apparatus for manufacturing semiconductors and discharges process gas from a chamber or the like. The vacuum pump  1  is provided with a turbomolecular pump unit T and a screw groove pump unit S, the unit T transferring the process gas from the chamber or the like downstream with stator blades  72  and rotor blades  62 , the unit S transferring further the process gas from the unit T with a screw groove pump to discharge the gas. 
     As shown in FIG. 1, the vacuum pump comprises a cylindrical casing  10 , a columnar rotor shaft  18  arranged in the center of the casing  10 , a rotor  60  that is fixedly placed on the rotor shaft  18  and rotates along with the rotor shaft  18 , and a stator  70 . 
     The casing  10  has at its upper end a flange  11  elongated outward in the radial direction. This flange  11  is fastened to the apparatus for manufacturing semiconductors or the like with, for example, a bolt to connect an inlet port  16  formed inside the flange  11  to an outlet port of a vessel, e.g., the chamber, so that the interior of the vessel is communicated with the interior of the casing  10 . 
     The rotor  60  includes a rotor body  61  shaped like an inverted letter U and arranged on the outer periphery of the rotor shaft  18 . This rotor body  61  is attached to the top of the rotor shaft  18  with bolts  19 . In the turbomolecular pump unit T, the rotor body  61  has multi-stages of rotor blades  62  formed on its outer periphery. The rotor blades are a plurality of blades which are open on the outer side. 
     The stator  70  is provided with, in the turbomolecular pump unit T, spacers  71  and stator blades  72  that are arranged between the respective stages of the rotor blades  62  while each of the stator blades is supported on its outer periphery side between two adjacent spacers  71 . In the screw groove pump unit S, the stator  70  is provided with screw groove portion spacers  200  that are formed as a continuation of the spacers  71 . 
     The spacers  71  have a cylindrical shape with a stepped portion and are stacked inside the casing  10 . The length in the axial direction of the stepped portion positioned on the inner side of each of the spacers  71  is determined in accordance with the interval between two adjacent rotor blades  62 . 
     The screw groove portion spacers  200  are arranged inside the casing  10  and are formed, as a continuation of the spacers  71 , below the spacers  71  and the stator blades  72 . The screw groove portion spacers  200  have such a thickness that their inner diameter walls jetty to the extent that the walls are close to the outer peripheral surface of the rotor body  61 . A helical screw groove  81  with plural threads is formed on the inner diameter walls of the spacers  200 . The screw groove  81  is communicated with a path between the stator blades  72  and the rotor blades  62 , and gas transferred through the path between the stator blades  72  and the rotor blades  62  is introduced into the screw groove  81  and is further transferred through the screw groove  81  by the rotation of the rotor body  61 . 
     Although the screw groove  81  is formed on the stator  70  side in this embodiment, the screw groove  81  may be formed on the outer diameter wall of the rotor body  61 . Alternatively, the screw groove  81  may be formed both on the screw groove portion spacers  200  and on the outer diameter wall of the rotor body  61 . 
     The vacuum pump  1  further comprises a magnetic bearing  20  for supporting the rotor shaft  18  by a magnetic force and a motor  30  for generating a torque in the rotor shaft  18 . 
     The magnetic bearing  20  is a 5-axes-control type magnetic bearing, and is provided with: radial electromagnets  21 ,  24  for generating a magnetic force in the radial direction of the rotor shaft  18 ; radial sensors  22 ,  26  for detecting the position of the rotor shaft  18  in the radial direction; axial electromagnets  32 ,  34  for generating a magnetic force in the axial direction of the rotor shaft  18 ; an armature disc  31  upon which the magnetic force in the axial direction, generated by the axial electromagnets  32 ,  34 , acts; and an axial sensor  36  for detecting the position of the rotor shaft  18  in the axial direction. 
     The radial electromagnets  21  include two pairs of electromagnets arranged such that one pair is perpendicular to the other pair. The electromagnets in each pair are arranged at positions above the motor  30  on the rotor shaft  18  so as to face one another with the rotor shaft  18  interposed between two of the electromagnets. 
     Above the radial electromagnets  21 , two pairs of radial sensors  22  are arranged such that two sensors in each pair faces one another with the rotor shaft  18  interposed therebetween. Two pairs of the radial sensors  22  are arranged so that one pair is perpendicular to the other, corresponding to two pairs of radial electromagnets  21 . 
     At positions below the motor  30  on the rotor shaft  18 , two pairs of the radial electromagnets  24  are similarly arranged so that one pair is perpendicular to the other pair. 
     Below the radial electromagnets  24 , similarly, two pairs of the radial sensors  26  are arranged adjacent to the radial electromagnets  24 . 
     An excitation current is supplied to these radial electromagnets  21 ,  24  to float the rotor shaft  18  with a magnetic force. The control of the excitation current is made, when the shaft is floated with a magnetic force, in response to position detection signals sent from the radial sensors  22 ,  26 , to thereby keep the rotor shaft  18  at a predetermined position in the radial direction. 
     The armature disc  31  made of a magnetic member and shaped like a disc is fixed to a lower part of the rotor shaft  18 . A pair of axial electromagnets  32  and a pair of axial electromagnets  34  are also arranged on a lower part of the rotor shaft  18 , one electromagnet facing its counterpart with the armature disc  31  interposed therebetween. The axial sensor  36  is arranged on the lower end of the rotor shaft  18 . 
     The excitation current flowing through the axial electromagnets  32 ,  34  is controlled in response to a position detection signal sent from the axial sensor  36 , to thereby keep the rotor shaft  18  at a predetermined position in the axial direction. 
     The magnetic bearing  20  is provided with a not-shown magnetic bearing control unit as a control system  45 . The magnetic bearing control unit feedback-controls the excitation current flowing through the radial electromagnets  21 ,  24  and the axial electromagnets  32 ,  34  on the basis of detection signals sent from the radial sensors  22 ,  26  and a detection signal sent from the axial sensor  36 , respectively, thereby floating the rotor shaft  18  with a magnetic force. 
     In this way, it is possible for the vacuum pump  1  according to this embodiment to be driven in a clean environment, for the employment of the magnetic bearing eliminates any mechanical contacts to produce no dust, and dispenses the pump of oils such as a sealing oil to generate no gas. The vacuum pump as such is suitable for an application in which a high cleanness is required as in manufacture of semiconductors. 
     The vacuum pump  1  according to this embodiment also has touch down bearings  38 ,  39  arranged on an upper part and on a lower part of the rotor shaft  18 , respectively. 
     Usually, the rotor unit comprising the rotor shaft  18  and the parts attached to the shaft is, while being rotated by the motor  30 , axially supported by the magnetic bearing  20  without coming into contact with the bearing. The touch down bearings  38 ,  39  are bearings for protecting the entire pump by axially supporting the rotor unit instead of the magnetic bearing  20  when the touch down takes place. 
     Accordingly, the touch down bearings  38 ,  39  are arranged so that their inner rings do not come into contact with the rotor shaft  18 . 
     The motor  30  is arranged almost in the middle between the radial sensors  22  and  26 , inside the casing  10 , in the axial direction of the rotor shaft  18 . The motor  30  is energized to rotate the rotor shaft  18  as well as the rotor  60  and the rotor blades  62 , which are attached to the shaft. The number of revolutions thereof is detected by a revolution sensor  41 , and the rotation is controlled by the control system on the basis of a signal from the revolution sensor  41 . 
     An outlet port  17  for discharging to the outside the air transferred from the screw groove pump unit S is arranged in a lower part of the casing  10  of the vacuum pump  1 . 
     The vacuum pump  1  is connected to the control system through a connector and a cable. 
     What is special to the present invention is that, as shown in FIG. 1, guiding blades  80  for imparting, to the gas molecules A taken in from the inlet port  16 , an outward motion component in the radial direction and toward the entrance of the exhaust system  13  are integrally attached to the upper end of the rotor  60 . The guiding blades  80  are formed to be integrated with the rotor  60  or, alternatively, are formed of separate pieces that are separate from the rotor  60 . The example illustrated in FIG. 1 shows the guiding blades  80  formed of the separate pieces. 
     To be specific, the guiding blades  80  are formed on a conic boss portion  90  whose diameter is gradually reduced toward the inlet port  16 , so that the guiding blades  80  rotate, through the boss portion, in unison with the rotor  60  in the same direction that the rotor  60  rotates. An engagement groove  91  open to the inlet port  16  is formed on the rotor body  61 , and an engagement projection  92  for engaging with the engagement groove  91  is formed on the bottom of the boss portion  90  so as to project on the rotor body  61  side. A bolt  93  is inserted through the boss portion  90  and is screwed into the upper end of the rotor shaft  18 , to thereby fix the guiding blades  80  including the boss portion  90  to the rotor body  61 . 
     An outward motion component in the radial direction is thus imparted to the gas molecules A taken in from the inlet port  16  and drawn into the upstream to the rotor  60  by the guiding blades  80  that rotates in unison with the rotor  60 . As a result, the gas molecules A are forcedly guided to the entrance of the exhaust system  13 . The gas molecules entered into the exhaust system are therefore increased in number, enhancing the exhaust efficiency of the exhaust system  13 . 
     FIGS. 2,  3 ,  4  illustrate a rotational member having a forming surface  100  on which the guiding blades  80  are formed according to other embodiments of the invention. The forming surface  100  in each drawing is formed into a conic shape whose diameter is gradually decreased toward the inlet port  16 . 
     More specifically, shown in FIG. 2 is a forming surface  101  formed into a conic shape that is trapezoid in section and is decreased in diameter linearly from the downstream side to the upstream side. Shown in FIG. 3 is an example in which a forming surface  102  formed into a conic shape whose diameter is decreased in the radial direction inwardly. FIG. 4 shows a forming surface  103  formed into a conic shape that is increased in diameter in the radial direction outwardly, i.e., larger diameter on the downstream side, and is semicircular in section. 
     Guiding blades  81 ,  82 ,  83  each have an angle of elevation in accordance with the diameter of the conic shapes of the forming surfaces  101 ,  102 ,  103 . 
     The peripheral speed is increased as the distance from the axis of rotation is increased from the upstream side to the downstream side in any of the forming surfaces  101 ,  102 ,  103 . Therefore, when an outward motion component in the radial direction is imparted, a reflection speed distribution of the gas molecules A which has a shape similar to the shape of the forming surface  101 ,  102  or  103  is obtained, increasing the amount of gas entered into the exhaust system  13 . In order to increase the number of the gas molecules entered into the exhaust system  13 , it is desirable to, for example, set α base angle a of the forming surface to 15 to 60°. 
     In FIG. 5, the guiding blades  80  are formed on the forming surface  100  formed into a conic shape along the periphery thereof with equal gaps, and each of the guiding blades  80  has a reflecting surface  110  for reflecting the gas molecules A on its front in the rotation direction. 
     This reflecting surface  110  is formed so as to stand vertically to the forming surface  100  and is sloped down to the rear rotation direction with respect to the radial direction of the forming surface  100  with the axis of rotation as its center. FIGS. 6,  8 ,  9  are enlarged views each showing an important part of the guiding blades  80  and the reflecting surface  110  formed on each of the blades  80 . 
     As described above, the gas molecules A are reflected vertically by the wall surface from the law of cosines in the molecule flow region. Therefore, when the reflecting surface is formed perpendicular to the forming surface  100  as shown in FIG. 6, the gas molecules A can be reflected outward in the radial direction and toward the downstream (axial direction opposite to the inlet port  16 ) without colliding against the forming surface  100 . 
     That is, when the reflecting surface  110  is formed so as to slant to the forming surface  100  at an acute angle as shown in FIG. 7, the gas molecules A reflected by the reflecting surface  110  is collided with the forming surface  100 , and further is vertically reflected by the forming surface  100 . This makes it difficult to give the gas molecules A an outward motion component in the radial direction. 
     As shown in FIGS. 5,  8 ,  9 , the reflecting surface  110  is formed so as to slope down to the rear rotation direction at a given sweepback angle with respect to the radial direction of the forming surface  100  with the axis of rotation as its center. This sets the front of the guiding blades  80  outward in the radial direction, making it possible to give the gas molecules A a larger outward motion component in the radial direction. 
     The reflecting surface  110  formed on each of the guiding blades  80  has an angle of elevation of 15 to 60° with respect to the axial section cut at the right angle, as shown in FIGS. 5,  6 ,  8 ,  9 . 
     In this way, the number of the guiding blades  80  formed on the forming surface  100  along the periphery thereof in the rotation direction with equal gaps is set to a number obtained by multiplying the number of blades in the uppermost stage of the rotor  60  by its divisor or by an integer. Setting the number of the guiding blades  80  to such a number, the gas molecules A collide against the top surface of the rotor blades  62 , i.e., a surface facing the inlet port  16  at a lower rate, to thereby prevent the backward flow of the gas molecules A. 
     Furthermore, as shown in FIG. 1, the reflecting surface  110  is formed in the surface opposite to the guiding blades  80  at a position along the height of the casing&#39;s reduced diameter portion  12  where the inner wall of the casing is gradually decreased in diameter toward the inlet port  16 . Also the molecules collided with the casing are thus reflected toward the exhaust system  13 , increasing even more the amount of the gas molecules entered into the exhaust system  13  and enhancing the exhaust efficiency. 
     FIGS. 10 and 11 illustrate the reflecting surface  110  and the guiding blades  80  on which the reflecting surface is formed according to another embodiment of the invention. A reflecting surface  111  formed on each of guiding blades  84  stands vertically on a forming surface  104  that is disk-like and flat, and is gradually sloped down to the rear rotation direction with respect to the radial direction of the forming surface  104  with the axis of rotation as its center. Accordingly, the gas molecules A are vertically reflected by the reflecting surface  111  to be given with a motion component outward to the tangential direction. This increases the amount of gas molecules entered into the exhaust system  13  to enhance the exhaust efficiency, as in the previous embodiment. 
     As has been described in the above, the following effects can be obtained through the vacuum pump according to this embodiment: 
     (1) The guiding blades for imparting an outward motion component in the radial direction to gas molecules are attached to the upper end of the rotor unit, thereby increasing the amount of gas molecules entered into the exhaust system and enhancing the exhaust efficiency. 
     (2) The guiding blades are formed on the forming surface that is formed into a conic shape, thereby increasing the amount of gas molecules entered into the exhaust system and enhancing the exhaust efficiency. 
     (3) The reflecting surface of the guiding blades is formed so as to vertically stand on the forming surface, thereby imparting the gas molecules an outward motion component in the radial direction and increasing the amount of gas molecules entered into the exhaust system. 
     (4) The reflecting surface of the guiding blades is sloped down to the rear rotation direction with respect to the radial direction, thereby imparting a large outward motion component in the radial direction. 
     (5) The number of the guiding blades is set by multiplying the number of rotor blades in the uppermost stage of the rotor unit by its divisor or by an integer, thereby preventing the gas molecules from flowing backwards from the exhaust system to the upstream side. 
     (6) The casing is reduced in diameter on the surface opposite to the guiding blades, thereby increasing even more the amount of gas molecules entered into the exhaust system and enhancing the exhaust efficiency. 
     In conclusion, according to the present invention, the guiding blades are attached between the rotor shaft and the inlet port which rotate with the rotor shaft to impart an outward motion component in the radial direction to the gas molecules taken in from the inlet port. Thus the gas molecules from the inlet port can be efficiently guided to the exhaust means, enhancing the exhaust efficiency.