Stator assembly for a turbomolecular pump
BACKGROUND OF INVENTION 
     The present invention relates to a stator for a turbomolecular pump. 
     In a turbomolecular pump the rotor impellers are interleaved with stator discs fixed to spacer rings located against the inner wall of the pump housing. Although it is possible to precise the machine the inner wall of the pump housing, the periphery of the stator disks and the spacer rings, it is almost impossible to prevent formation of interstices between the outer surface of the stator and the inner wall of the pump housing. Because of the large pressure difference existing between the suction side (high vacuum side) and the delivery side (low vacuum side) of a turbomolecular pump, a back streaming gas flow can be established through such interstices from the delivery side to the suction side; that is, a stream opposite to the pumping direction which, although involving tiny gas amounts, results in a considerable worsening of the pump performance. Several approaches have been proposed to overcome this problem. 
     For example, in U.S. Pat. No. 4,832,564 there are provided radial ducts in the spacer rings in order to establish communication between the outside of the stator group and the inner space thereof where the pumping takes place. Since the gas conductance of these ducts is greater than that of the interstices existing between the stator group and the pump housing, the back streaming gas has a high probability of passing through the stator inner space, and thus to be pumped away. 
     In German Patent Application No. 2,214,702 the gas back streaming directed towards the suction side is blocked by annular seal gaskets fitted between the pump housing and the stator group. 
     Nevertheless these solutions are not without disadvantages due, in the first case, to the necessity of special machining of the spacer rings, and in the second case to possible sealing defects of the annular gaskets fitted between the pump housing and the stator group. 
     Further, the known stator discs for turbomolecular pumps comprise substantially flat structures that render troublesome the assembling between the spacer rings and do not allow of an easy positioning of each stator disc at the desired intermediate location between two adjacent impellers of the rotor. Finally, because of the construction as two separate parts of each stator disc, misalignments are possible along the diametrical division line. 
     The object of the present invention is to eliminate or at least to reduce the above drawbacks of the known stators for turbomolecular pumps, by providing stators that are capable of eliminating or minimizing, through simple and reliable means, the back streaming of gas from the delivery side to the suction side of the pump. 
     Another object of the invention is to provide stator groups wherein the assembly of each stator disc between the spacer rings is easy. 
     An additional, object of the invention is to provide stator discs adapted to be positioned at the optimum intermediate locations between adjacent rotor impellers. 
     SUMMARY OF THE INVENTION 
     The above and other objects and advantages of the invention will become evident from the following description are achieved through a stator group for a turbomolecular pump comprising a plurality of stator discs with blades, each having a circular peripheral edge and formed in two parts, which in turn are fastened by means of spacer rings that are interleaved with said stator discs, said peripheral edges of the stator discs being equipped with a series of radial projections for the fastening between said spacer rings. 
     According to another characteristic of the invention the overall shape of each stator disc is frusto-conical. 
     According to a further characteristic of the invention the two separate parts making up each stator disc are joined together by means of tongues which keep them aligned. 
     A preferred embodiment of the invention will now be described, as a non-limiting example, with reference to the attached drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a top plane view of a stator disc according to the invention, with the two halves shown as separated; 
     FIG. 2 is a top plane view of the stator disc of FIG. 1, with the two halves joined together; 
     FIG. 3 is a cross-section view along line III--III in FIG. 2; 
     FIG. 4 is a cross-section view of a portion of the stator group mounted in a turbomolecular pump; and 
     FIG. 5 is a view showing an enlarged detail of FIG. 4. 
    
    
     DESCRIPTION OF A PREFERRED EMBODIMENT 
     With reference to FIGS. 1, 2 and 3 there is shown a stator disc for a turbomolecular pump made of two separate parts 1 and 2 having a semicircular shape and adapted to be joined together upon assembling the stator group as shown in FIG. 2. Each part comprises a plurality of blades 4 or 4a--in a known manner--ending at opposite sides with semicircular edges, 6, 8 and 6a, 8a. 
     According to a characteristic of the invention, each outer edge 6, 6a is provided with projections or tabs 10, 10a, respectively, radially extending outwardly from the disc along its whole circumference, in such a way as to form the fastening members for each stator disc as will be described below. 
     It is seen in FIG. 3, the stator disc according to the invention has a frusto-conical shape determined by the oblique or tilted arrangement of the blades 4, 4a with respect to the outer edges 6, 6a and the tabs 10, 10a lying in a single plane that is perpendicular to the stator axis. The inner edges 8, 8a lie in a plane that is parallel to that of the outer edges 6, 6a. Each part 1, 2 is equipped at the edge thereof with tongues 12, 12a adapted to join together the two parts 1 and 2 at their inner zone and to keep them coplanar along a diametrical line of division. 
     FIG. 4 schematically and partially illustrates the stator group of the invention assembled in a turbomolecular pump. 
     Reference numeral 13 designates the cylindrical housing of the turbomolecular pump within which a rotor is contained which comprises a self-supporting shaft 15 integral with a series of bladed impellers 16, in a known manner. Between each pair of adjacent rotor impellers there is inserted one of the above described stator discs. Each stator disc is fastened between two spacer rings 18 by means of radial tabs 10, 10a. The construction of the stator disc with a frusto-conical shape makes the assembly of the stator group quite easy and allows for minimizing the gaps between the impellers. 
     In FIG. 5 there is shown an enlarged detail of FIG. 4, illustrating how the stator of the invention facilitates pumping away from the inside of the pump, the backstreaming gas coming from the delivery side. Reference numeral 20 designates the interstitial gap (shown exaggeratedly large) that may exist between the stator group and the wall 13 of the pump. Pumping takes place along the direction shown by the arrow A, that is from the high vacuum (upper) side to the delivery or fore-vacuum (lower) side. Backstreaming gas is assumed to be coming from the delivery side and directed as indicated by arrow B, that is towards the suction side; such back streaming results in a worsening of the high vacuum level achieved by the pump. As a result of the disclosed construction for the stator discs, the back streaming gas can pass through the gaps defined between each tab or radial projection 10, 10a, as shown by arrow C in FIG. 5, and thus reach the inner space of the pump where it is again pumped away along the direction of arrow A. 
     Therefore, as a result of the improved structure of the stator discs, the stator according to the present invention achieves advantages both in respect to the pump performance and the assembling thereof. 
     A preferred embodiment of the invention has been described, but of course this latter can be subjected to several modifications and changes all coming within the scope of the attached claims.