A stator for a turbo-molecular pump

Abstract: A stator for a turbo-molecular pump wherein each stator disc has a frusto-conical shape for the tilted orientation of the blades (4, 4a), is divided into two semicircular parts (1, 2) having fastening tongues (12, 12a) at their inside, and is equipped along its circular outer edges (6, 6a) with projecting radial tabs (10, 10a) for fastening the stator disc between two adjacent spacer rings (18). Thanks to this characteristic the gas back streaming that may be estabished through the interstices (20) between the stator and the pump housing (13) and directed towards the suction side of the pump, can reach the inside space of the stator and thus to be pumped away.

Description: The present invention relates to a stator for a turbo-molecular pump according to the pre-characterizing part of claim 1 and as disclosed in US-A- 4832564. In a turbo-molecular pump the rotor impellers are interleaved with stator discs fixed to spacer rings located against the inner wall of the pump housing. Although the inner wall of the pump housing and the periphery of the stator discs and of the spacer rings can be precisely machined, it is practically impossible that interstices do not form between the outer surface of the stator group and the inner wall of the pump housing. Because of the large pressure difference existing between the suction side and the delivery side of a turbo-molecular pump, through such interstices a back streaming gas flow can be established which is directed from the delivery side (low vacuum side) to the suction side (high vacuum 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. To overcome this inconvenience several suggestions have been proposed. For example, in US patent 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 gases have 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 on the contrary 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 a 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 turbo-molecular pumps have substantially flat structures that render troublesome the assembling between the spacer rings and do not allow for 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, misalignements are possible along the diametral division line. The object of the present invention is to eliminate or at least to reduce the above drawbacks of the known stators for turbo-molecular pumps, by providing stators that are capable to eliminate or minimize through simple and reliable means the back streaming of gas from the delivery side to the suction side. Another object of the invention is that of providing stator groups wherein the assembling of each single stator disc between the spacer rings is easy. An additional object of the invention is that of providing stator discs adapted to be positioned at the optimun intermediate locations between two adjacent rotor impellers. The above and other objects and advantages of the invention that will become evident from the following of the description are achieved through a stator group for a turbo-molecular pump comprising a plurality of stator discs with blades, each having a circular peripheral edge and formed by two parts, fastened by means of spacer rings that are interleaved with said stator discs, characterized in that said peripheral edges of said stator discs are 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 keeping them always aligned. A preferred embodiment of the invention will now be described, as a non-limiting example, with reference to the attached drawings in which: 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 turbo-molecular pump; and Fig. 5 is a view showing an enlarged detail of Fig. 4. With reference to Figures 1, 2 and 3, there is shown a stator disc for a turbo-molecular pump made up by 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 the disc along the whole circumference of this latter, in such a way as to form the fastening members for each stator disc as will be illustrated later on. As 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 is 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 diametral line of division. Fig. 4 schematically and partially illustrates the stator group of the invention assembled in a turbo-molecular pump. Numeral reference 13 designates the cylindrical housing of the turbo-molecular 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 illustrated 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 renders quite easier the assembly of the stator group 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 allows for the possible gas back streaming coming from the delivery side to be pumped away from the inside of the pump. Numeral reference 20 designates the interstitial gap (shown as 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. Anyhow, a gas back streaming can be established coming from the delivery side and directed as indicated by arrow B, that is towards the suction side, such back streaming resulting in a worsening of the high vacuum degree achieved by the pump. Thanks to the disclosed construction for the stator discs, the back streaming gas can nevertheless pass through the gaps defined between each tabs or radial projection 10, 10a, as shown by arrow C in Fig. 5, and thus reach the inner space of the pump where they are again pumped away along the direction of arrow A. This way, thanks to the improved structure of the stator discs only, the stator according to the invention achieves advantages both in respect of 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 invention as defined in the claims.

Claims: A stator group for a turbo-molecular pump comprising a plurality of stator discs with blades (4, 4a), each having a circular peripheral edge (6, 6a) and formed by two parts (1, 2) fastened by means of spacer rings (18) that are interleaved with said stator discs, characterized in that said peripheral edges (6, 6a) of said stator discs are equipped with a series of radial projections (10, 10a) for the fastening between said spacer rings (18). A stator group as claimed in claim 1, characterized in that the fastening of said stator discs to said spacer rings (18) is accomplished through the positioning of said radial projections (10, 10a) only between said spacer rings. A stator group as claimed in claim 1, characterized in that the overall shape of each stator disc is frusto-conical. A stator group as claimed in claim 1, characterized in that each of said two parts (1, 2) making up each stator disc is connected to the other, in the assembled state of the stator group, through tongues (12, 12a) provided at the coupling edges (8, 8a) of the parts themselves (1, 2).

Inventors: CASARO FAUSTO, INSERRA DARIO, LEVI GIAMPAOLO, PELLIZZARI PAOLO, CASARO, FAUSTO, INSERRA, DARIO, LEVI, GIAMPAOLO, PELLIZZARI, PAOLO
Applicants: VARIAN SPA, VARIAN S.P.A.
Application year: 1991
Main classification: F04D19
IPC codes: F04D19, F04D29