Abstract:
A molecular vacuum pump ( 1 ) has a stator unit ( 9 ) and a rotor unit ( 8 ) disposed within a housing ( 2 ). A narrow gap is maintained during operation between the stator unit ( 9 ) and the rotor unit ( 8 ). The stator unit ( 9 ) and the rotor unit ( 8 ) are coupled together with respect to vibration to form a rotor/stator system ( 3 ). Elastic vibration elements ( 4, 5 ) mount the rotor/stator system in the housing such that the rotor/stator system rotates as a unit relative to the housing. Because the rotor and stator units vibrate together rather than relative to each other, very small, precise tolerances are maintained between them.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a molecular vacuum pump, which has a stator, a rotor, and a housing, and in which, during operation, a narrow gap is maintained between the stator and the rotor. The pumping characteristics of such pumps depend significantly on the size of the gap between the rotor and the stator. 
     Molecular vacuum pumps are typically constructed using an elastic connection between the stator and the rotor to prevent transfer of vibrations between the stator and the rotor. The shaft bearings are typically supported by elastomer rings in the housing. For example, German publication DE-U-80 27 697 discloses a rotor equipped with a spindle bearing, and completely supporting the rotor/spindle assembly in the housing by O-rings. 
     The stator/rotor gap of prior art molecular vacuum pumps did not fall below a few tenths of a millimeter, because this is the minimum tolerance practically obtainable using the elastomer coupling of the prior art. 
     The present invention contemplates a new molecular pump design which accommodates tighter stator/rotor gaps than is possible by the prior art methods. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the invention, a stator and a rotor are technically coupled, relative to vibration, and the coupled stator and rotor are jointly fastened in a housing via vibration elements. “Technically coupled, relative to vibration” shall mean that the rotor unit and the stator unit undergo essentially identical vibrations, so that significantly smaller gaps are practical between the stator and the rotor components than has been achieved by prior art designs. The joint vibrations of the coupled stator and rotor are absorbed by the vibration elements, by means of which the coupled stator and rotor are supported in the housing. 
     Preferably, the coupling between the rotor and the stator is a rigid coupling, whereby the size of the gap between the rotor and the stator is limited solely by the tolerances of the extruded and machine-produced components. These tolerances are sufficiently low that substantially lower gap tolerances are obtainable versus prior art designs which utilized elastomers. 
     For reasons of functional efficiency, it is frequently not possible to realize a rigid coupling. In such instances, there are preferably one or more vibration elements arranged between the stator unit and the rotor unit, whereby relative vibrational movements therebetween are permitted. The maximum amplitude of such vibrational movements is, however, substantially reduced compared with prior art designs, because the determinative joint vibrational movements of the stator and the rotor are absorbed by the vibration elements supporting the stator and rotor combination in the housing. A drastic reduction in the gap between the stator and the rotor components versus the prior art designs is therefore still achieved. 
     For example, the O-rings between the stator and the rotor units can be significantly more rigid than the outer vibration elements. Taking into consideration the respective vibrating masses, a vibrational amplitude ratio of 20:80 is achieved thereby. 
     Other advantages of the invention will be evident to those of ordinary skill in the art from the examples described in the following detailed description and the attached drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for the purposes of illustrating preferred embodiments and are not to be construed as limiting the invention. 
     FIG. 1 shows a longitudinal sectional view taken through a turbo-molecular pump formed in accordance with a first embodiment of the invention; and 
     FIG. 2 shows a sectional view taken through a molecular vacuum pump formed in accordance with a second embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to FIGS. 1 and 2, the two illustrated embodiments of a pump  1  include an outer housing  2 . Arranged within the housing  2  is a rotor/stator system.  3 , which is supported in the housing  2  by a first vibrational element  5  and by a second vibrational element  4 . A connection flange  6  is disposed on the suction side of the housing  2 , and a connection lid  7  is disposed on the pressure side of the housing  2 . The rotor/stator system  3  includes a rotor unit  8  and a stator unit  9 . 
     The rotor unit  8  includes a central shaft  11  which supports an essentially bell-shaped rotor  12 . On the pressure side, the central shaft  11  has an armature  13  of the driving motor arranged thereon. The housing  2  supports a stator  14  of the driving motor. 
     The stator unit  9  includes three sleeve components: a first sleeve  15 , a second sleeve  16 , and a third sleeve  17 . The first sleeve  15  is arranged on the pressure side. The second and the third sleeves  16  and  17  are arranged on the suction side. The second sleeve  16  is arranged inside of a wall  18  of the bell-shaped rotor  12 , while the third sleeve  17  is arranged outside of the wall  18 . The end of the sleeve  15  closest to the pressure side is equipped with an inwardly oriented edge  21 , whose interior side forms a sliding fit  22  for a first shaft bearing  23  which is located on the pressure side of the shaft  11 . Additionally, the edge  21  of the sleeve  15  forms a receiving region which receives a first O-ring  24 , which is preferably made of an elastomer material. A corresponding receiving region is formed at the connection lid  7 , and the connection lid  7  is in turn connected to the housing  2 . The receiving regions include grooves, angles, or the like which are preferably designed so that the first O-ring  24  serves both as a sealing element and as the first vibration element  5 . That is, the first vibration element  5  which supports the rotor/stator system  3  on the pressure side is preferably one and the same element as the first sealing O-ring  24 , as shown in FIGS. 1 and 2. Of course, other elements such as radial packing rings, flat rings, or piston seals may be substituted for the first O-ring  24 . 
     The first sleeve  15  also includes, on the suction side, an outwardly oriented edge  26  which cooperates with the second and the third sleeves  16  and  17  to form an interior vacuum-tight housing. A cover nut  27  is inserted over the pressure side of the first sleeve  15  and threads onto the third sleeve  17 . The cover nut  27  braces together the outwardly oriented edge  26  of the first sleeve  15  and an outer edge  28  of the second sleeve  16 . 
     The connection flange  6  includes, on the suction side, an inwardly directed step  31  which receives a second O-ring  32 . A corresponding receiving region is disposed on the suction side of the third sleeve  17  for receiving the second O-ring  32 . In addition to providing sealing, the second O-ring  32  is preferably one and the same element as the second vibration element  4  which supports the rotor/stator system  3  in the housing  2 . The housing  2  forms, in cooperation with the lid  7  and the connection flange  6 , a clamping shell that tightens together the rotor/stator system  3 . With appropriate dimensioning, the housing  2  and the connection flange  6  may be integrally formed as a single piece. The second sleeve  16  is supported therewithin on a step-like ledge  29  of the first sleeve  15 . 
     The suction end of the second sleeve  16  is equipped with an inwardly oriented edge  34 , whose interior side forms a sliding fit  35  for a second shaft bearing  36  which is located on the suction side of the shaft  11 . In addition, there is arranged in this area an annular spring  37  which generates the required bearing pitch forces. 
     In the embodiments shown in FIGS. 1 and 2, the rotor unit  8  and the stator unit  9  are rigidly coupled together by the first and second bearings  23  and  36  and by the sliding fits  22  and  35 . This rigid coupling provides the desired reduction in play between the stator and the rotor. The rotor/stator system  3  is supported in the housing  2  by the first and second vibration elements  5  and  4 . In the embodiments shown, vibration elements  4 ,  5  are advantageously O-rings which simultaneously assume sealing functions, cooperatively forming a vacuum-tight separation between gas compartments inside the housing  2  and the outside atmosphere. Preferably, a third O-ring  38  surrounds the outer circumference of the outer edge  28  of the second sleeve  16 , so that vacuum sealing is assured also in the area of the cover nut  27 . Thus, the stator unit  9  forms, for all practical purposes, a second interior housing which is vacuum-tight. As a result, the housing  2  can be fitted with air slots  39 . 
     Having discussed the common features of the exemplary embodiments of FIGS. 1 and 2, reference is made next particularly to the first embodiment shown in FIG.  1 . 
     With particular reference to FIG. 1, a single-stage turbo-molecular vacuum pump  1  is shown, with a single gas compartment  40  that tapers from the suction side toward the pressure side. Rows of stator blades  42  extend inwardly from the outer sleeve  17 , while rows of rotor blades  41  extend outwardly from the outer surface of the rotor wall  18 . The path of transported gases through the pump  1  is denoted by arrows  43 . Gases enter via the connection flange  6  into the gas compartment  40  which is equipped with the rotor blades  41  and the stator blades  42 . The gases then pass through openings  44  in the second stator sleeve  16 , travel along the shaft  11  and through openings  45  in the edge  21  of the first sleeve  15 , and finally arrive at the discharge opening  46 . 
     With reference now to FIG. 2, the second exemplary illustrated embodiment of the invention will now be described. FIG. 2 shows a three-stage molecular pump. The interior side of the third stator sleeve  17  is equipped with a threading  47 , and the exterior side of the second stator sleeve  16  is equipped with a threading  48 . Threadings  47  and  48  cooperate with the cylindrical rotor wall  18  to produce the desired gas transport, in the illustrated orientation downward through threads  47  and upward through threads  48 . The exterior side of the shaft  11 , which has an enlarged diameter in the region of the second stator sleeve  16 , is also equipped with a threading  49  and cooperates with the interior side of the second stator sleeve  16  to form the third pumping stage. 
     The path of transported gases is identified by arrows  51 . The transported gases enter into the outer first pumping stage via the connection flange  6 . Preferably, there exists ahead of the outer first pumping stage a filling stage  52  which includes a crown of blades. After passing through the outer first pumping stage, the gases enter the second pumping stage which is disposed between the rotor wall  18  and the second stator sleeve  16 . Gases flow in a transport direction which is opposite to the direction of flow in the first pumping stage. The gases experience another change of direction as they pass from the second pumping stage through openings  53  in the edge  34  of the second stator sleeve  16 . After passing through openings in the spring  37 , the gases enter the third pumping stage. Upon exiting the third pumping stage, the gases pass through openings  45  in the edge  21  of the first sleeve  15 , and finally arrive at the discharge opening  46 . 
     The embodiment shown in FIG. 2 can easily be converted to a single-stage molecular vacuum pump. The conversion involves removing the third stator sleeve  17 , the rotor bell  18 , and the cover nut  27 , whereupon only the third pumping stage would exist and be operative. In this single-stage design, the edges  26  and  28  as well as the threading  48  may preferably be eliminated. Also, the diameter of the vibration-and-sealing element  4 ,  32  and of the front of the second sleeve  16  would be made to essentially match so that the rotor/stator system  3  is supported elastically by housing  2  and connection lid  7 . 
     In the first exemplary embodiment shown in FIG. 1, the stator unit  9  and the rotor unit  8  are, relative to vibration, rigidly coupled with each other at slide fits  22  and  35 . In the second exemplary embodiment shown in FIG. 2, there is arranged between the second shaft bearing  36  and the interior side of the,edge  34  of the second stator sleeve  16  an O-ring  63 . O-ring  63  has a significantly smaller diameter as compared with the first and the second O-rings  24  and  32 . The O-ring  63  serves to accommodate play in the fit, and does not have any significant influence in determining the gap between the rotor and the stator unit. 
     The disclosed invention is particularly applicable to small turbo-molecular pumps. With decreasing pump size, there is an increase in undesirable gas backflow relative to the forward transport of gas through the pump, and this leads to a disproportionate deterioration in pumping performance. The disclosed invention enables reduction of the gaps between the rotor and the stator through improved pump designs in accordance with the invention, which will improve the pumping performance. Alternatively, the invention permits design of smaller pumps with performance equivalent to a larger pump built using the designs of the prior art. Another advantage of the invention is a reduction in the number of manufacturing parts. 
     The invention has been described with reference to the preferred embodiments. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.