Patent Publication Number: US-6220831-B1

Title: Turbomolecular pump

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a turbomolecular pump which enables exhaust of a gas by interaction between rotary blades and stationary blades and/or a threaded rotor rotating at a high speed and a stator. 
     A general structure of a conventional turbomolecular pump is illustrated in FIG.  6 . The conventional turbomolecular pump comprises a rotor R including a main shaft  10  and a rotary cylinder  12  fixed to the main shaft for rotating integrally therewith, a stator S including a fixed cylinder  14  surrounding the shaft  10 , and a cylindrical casing  16  surrounding the rotary cylinder  12 , which are assembled on a base B. A conductance adjusting valve  100  and a gate valve  110  are provided in the space between the turbomolecular pump and an apparatus A to be evacuated provided on the upstream side of the turbomolecular pump. 
     In the conventional turbomolecular pump as described above, however, driving mechanisms  101  and  111  for the individual valve units  100  and  110  are provided adjacent to the turbomolecular pump and in the proximity of the valves. This has posed a problem of scaling-up of the valve units and results in a larger overall structure of the turbomolecular pump including these valves. It is conceivable, on the other hand, to form the valve unit integrally with the turbomolecular pump, but this may lead to contamination of the apparatus to be evacuated by particles arising from the valve driving mechanism. 
     SUMMARY OF THE INVENTION 
     The present invention was made to solve the problem described above, and has its object the provision of a turbomolecular pump which has a compact overall construction including valve units, and which is able to prevent contamination by the valve driving mechanism. 
     To accomplish the above object, according to a first aspect the invention, a turbomolecular pump is provided which comprises a rotor and a stator housed in a casing and forming an exhaust channel therebetween, and a suction port and an exhaust port formed in said casing, wherein the turbomolecular pump has a valve body for opening and closing the suction port, the valve body is movable in an axial direction of the turbomolecular pump, a valve driving mechanism for driving the valve body via a valve body supporting member which extends through a throughhole formed in the rotor and/or the stator, and bearing units for supporting the valve body supporting member within the throughhole. This arrangement enables a compact construction of the entire pump apparatus including the valve unit, since the valve unit is integrally formed with the pump. 
     The said bearing unit comprises a magnetic bearing unit for non-contactingly supporting the valve body supporting member. This arrangement permits prevention of contamination by particles arising from the supporting mechanism while stably supporting the valve body, because the valve body is driven by the valve driving mechanism while being non-contactingly supported by the magnetic bearing units via the valve body supporting member. 
     According to a second aspect of the invention, in a turbomolecular pump according to the first aspect, the rotor is non-contactingly supported by a rotor magnetic bearing, and a screw thread sealing mechanism which inhibits gas flow into the rotor magnetic bearing is provided between the rotor and the stator. This makes it possible to prevent corrosive exhaust gas from flowing into the rotor magnetic bearing, thus preventing corrosion of these members, and hence achievement of a turbomolecular pump having high durability can be accomplished. 
     According to a third aspect of the invention in a turbomolecular pump according to the first aspect, a gas feeding channel for feeding an inert gas is provided at a prescribed position between the rotor and the stator for inhibiting a gas flow into the bearing units by the inert gas. This provides a turbomolecular pump which prevents a corrosive exhaust gas from flowing into the rotor magnetic bearing while maintaining an inert atmosphere around the rotor magnetic bearing and, hence, has high durability. 
     According to a fourth aspect of the invention, in a turbomolecular pump according to the first aspect, there is provided gas deposition preventing means which prevents deposition of gas components at a contact portion between the suction port and the valve body by heating the suction port and/or the valve body. This permits maintenance of air-tightness of the valve body, thus ensuring safe operation. 
     The above and other objects, features and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which preferred embodiments of the present invention are shown by way of illustrative examples. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view illustrating a turbomolecular pump according to a first embodiment of the invention; 
     FIG. 2 is a sectional view illustrating a turbomolecular pump according to a second embodiment of the invention; 
     FIG. 3 is a sectional view illustrating a turbomolecular pump according to a third embodiment of the invention; 
     FIG. 4 is an enlarged sectional view illustrating a main portion of a turbomolecular pump shown in FIG. 3; 
     FIG. 5 is a sectional view illustrating a turbomolecular pump according to a fourth embodiment of the invention; and 
     FIG. 6 is a sectional view illustrating a conventional turbomolecular pump. 
    
    
     EMBODIMENT OF THE INVENTION 
     Preferred embodiments of the present invention will now be described with reference to the drawings. The turbomolecular pump according to a first embodiment of the invention is shown in FIG. 1, which comprises a rotor R including a main shaft  10  and a rotary cylinder  12  fixed to the main shaft for rotating integrally therewith, a stator S including a stationary cylinder  14  surrounding the main shaft  10 , and a cylindrical casing  16  fixed to the stator S and surrounding the rotary cylinder  12 , which are assembled on a base B. A disk shaped valve body  20  is provided at a suction port  18  of the casing  16  for opening and closing the suction port  18 . 
     A driving motor  22  for rotating the rotor R at high speed is provided between the main shaft  10  and the stationary cylinder  14 . An upper radial bearing  24  and a lower radial bearing  26  are provided on the upper and lower sides of the driving motor  22 , respectively for non-contactingly supporting the rotor R. In the lower portion of the main shaft  10 , a target disk  28  is provided at the lower end of the main shaft, and an axial bearing  32  including upper and lower coils  30  is provided on the stator S, so that the rotor R rotates at high speed under active control along  5  axis with driving of the driving motor  22 . 
     Rotary blades  34  are formed integrally with the rotary cylinder  12  on the outer periphery of the upper portion thereof so as to form impellers  36 . On the inner surface of the casing  16 , on the other hand, stationary blades  38  are provided alternately with the rotary blades  34  with a spacer interposed therebetween. There is accordingly formed a blade exhaust portion  40  in which gas exhaust action is accomplished through interaction between the rotary blades  34  rotating at high speed and the stationary blades  38 . 
     A screw thread portion  42  is provided on the rotary cylinder  12  so as to extend downwardly therefrom surrounding the outer periphery of the stationary cylinder  14 , and screw thread  44  is provided on the outer peripheral surface of the screw thread portion  42 . A spacer  46  surrounding the outer periphery of the screw thread portion  42  is provided on the stator S. As a result, a screw-thread exhaust portion  48  which performs gas exhaust action under drag action caused by the screw thread  44  of the screw thread portion  42  rotating at a high speed is provided between the blade exhaust portion  40  and an exhaust port  49 . 
     A throughhole  52  for receiving a valve rod  50  of the valve body  20  is formed in the main shaft  10 , the rotary cylinder  12  and the base B. An actuator  54  for driving the valve body  20  in the axial direction via the valve rod  50  is provided at the lower portion of the casing  16 . A flange  17  of the casing  16  at the suction port  18  is provided with an O-ring  56  for air-tightly closing the suction port  18  by the valve body  20 . A sealing mechanism (not shown) is provided at the connecting portion between the casing  16  and the actuator  54 . The actuator  54  itself has an air tight structure. 
     The valve rod  50  is up and down movably supported by an upper and a lower magnetic radial bearings  70  and  72  provided in the suction port  18  and on the base B, respectively. The upper magnetic bearing  70  is supported by a supporting members  76  provided at inner ends of a plurality of arms  74  radially extending from inner surface of the casing  16  toward the center portion of the suction port  18 . At the center portion of the suction port  18 , a recess  78  is formed on the top surface of rotary cylinder  12  of the rotor R, and the supporting member  76  is accommodated in the recess  78 . 
     In the embodiment shown, the valve rod  50  is stably supported by the upper and the lower magnetic bearings  70  and  72  so as to ensure smooth opening/closing of the valve body  20  without causing positional shift thereof. Because the magnetic bearings  70  and  72  can non-contactingly support the valve rod  50 , particles are hardly generated by friction, and, thus, the apparatus to be evacuated is not contaminated by the particles. 
     The valve body is opened or closed by the operation of the actuator  54 , and conductance can be adjusted by adjusting opening of the valve body  20  or opening it to prescribed positions. The turbomolecular pump can directly be attached to a duct  58  or the like of an apparatus to be evacuated without interposing a valve unit therebetween as shown in FIG.  4 . Because the actuator  54  drives the valve body  20  for opening/closing it in the axial direction of the main shaft of the rotor or a turbomolecular pump, the structure of axial the valve unit and the driving mechanism can largely be simplified. It is therefore possible to provide a compact turbomolecular pump as a whole, and to effectively utilize a narrow space such as a clean room. 
     FIG. 2 illustrates a second embodiment of the present invention, wherein screw thread sealing portions  80  and  82  are formed between the outer surface of the supporting member  76  and the inner surface of the upper recess  78  in the rotary cylinder, and between the inner surface of the screw thread portion  42  of the rotary cylinder  12  and the outer surface of the stationary cylinder  14 . These screw thread sealing portions  80  and  82  serve to prevent a gas from entering the central throughhole  52  and a space between the rotary cylinder  12  and the stationary cylinder  14  upon rotation of the rotor R. 
     More specifically, a screw thread  84  is formed on the outer surface of the supporting member  76 , so that the gas is exhausted from bottom to top in FIG. 2, upon rotation of the rotor R. This prevents a gas from the suction port  18  from entering the throughhole  52  and reaching the lower end portion of the rotor R via the throughhole  52 . Even when exhausting a corrosive gas, therefore, it is possible to prevent corrosion of the magnetic bearings  70  and  72 ,  24 ,  26  and  32  and the driving motor  22  provided there. 
     Similarly, a screw thread  84  is formed on the outer surface of the stationary cylinder  14 , so that the gas is exhausted from top to bottom in FIG. 2, in the lower screw thread sealing portion  82  upon rotation of the rotor R. This prevents the gas from the discharge port  49  from entering the space between the rotary cylinder  12  and the stationary cylinder  14  and reaching the magnetic bearings  24 ,  26 ,  32  and the driving motor  22 . While two screw thread sealing portions  82  and  84  are formed in this embodiment, only one of these screw thread sealing portions may be adopted as required. 
     FIG. 3 illustrates a third embodiment of the present invention. In this embodiment, purge gas feeding channels  86  and  88  are formed for preventing a corrosive gas from passing through the throughhole  52  and corroding the magnetic bearings  24 ,  26  and  32 , or the driving motor  22  of the turbomolecular pump. More particularly, the first feeding channel  86  extends from the casing  16  near the suction port  18  toward the supporting member  76  through the interior of the arm  74  and runs down the support member  76  to open at the lower surface of the supporting member  76  as shown in FIG.  4 . The second feeding channel  88  extends inwardly from the lower side surface of the stator S and, on the one hand, extends up through the stationary cylinder  14  to open at the top of the screw thread sealing portion  82  and extends down through the stator S to open at the axial bearing  32  on the other hand. Although the former opening is provided at the top of the screw thread sealing portion  82  in this embodiment, it may be provided at the middle or at the bottom of the screw thread sealing portion  82 . Also, the magnetic bearings  24 ,  26  and the motor  22 , may be directly purged. Further, the number of openings may be either single or plural. An inert gas supply piping, such as nitrogen gas or the like, is connected to the openings on the outer surfaces of these feeding channels  86  and  88 . 
     In this embodiment, it is possible to positively prevent a corrosive exhaust gas from flowing into the magnetic bearings  24 ,  26  and  32  or the driving motor  22  by supplying a purge gas or an inert gas into the paths leading from the suction port  18  or the discharge port  49  to the magnetic bearings  24 ,  26  and  32  or the motor  22 , assisted by the action of the aforementioned screw thread sealing portions  80  and  82 . While both the purge gas feeding channels  86  and  88  and the screw thread sealing portions  80  and  82  are provided in this embodiment, a purge gas feeding channels  86  and  88  alone may be provided. Further a purge gas feeding channel  86  or  88  alone may be provided. 
     FIG. 5 illustrates a fourth embodiment of the present invention, wherein gas deposit preventing means is provided to prevent deposit of gas components on the contact portion between the valve body  20  and the suction port  18  so as to ensure positive sealing of the suction port  18  by the valve body  20 . More specifically, a heater  90  for heating the contact surfaces is provided on the casing  16  near the suction port  18 . While an electric heater is adopted in this embodiment, any appropriate heater, e.g. supplying of a hot air or water, may be adopted. In this embodiment, the casing  16  and the flange  17  are heated by the operation of the heater  90 , thus preventing the components of the exhaust gas from being deposited in this area, or inhibiting such deposition. 
     In this embodiment, a heater  92  for heating the valve rod is further provided at a prescribed position of the actuator  54  of the valve driving unit. As a result, heat from the heater  92  is transmitted to the valve body  20  via the valve rod  50 , and further from the center to the edge of the valve body  20 , thus keeping the contact portion between the valve body  20  and the flange  17  at a prescribed temperature. This prevents components of the exhaust gas from being deposited at this portion, thus keeping stable or positive opening/closing operations of the valve body. 
     Although, in the illustrated embodiments, a throughhole for receiving the valve body supporting rod is formed in the rotor, it is possible to form the throughhole in the stator or in the stator and the rotor when the main shaft is provided as a stationary member at the center of the turbomolecular pump and the rotor is provided around the main shaft. 
     According to the present invention, as described above, it is possible to form the entire apparatus including the valve unit into a compact construction by integrally forming the valve unit and the turbomolecular pump. Also, it is possible to prevent contamination caused by particles arising from the supporting mechanism and to stably support the valve body by supporting and driving the rotor without contact. Thus, it is possible to provide a highly practicable turbomolecular pump which permits effective use of a small space such as a clean room.