Compact vacuum pump

Abstract: A vacuum pump (11) comprises a body (1), a pumping rotor (9) having externally a plurality of rotor disks (12), coupled to corresponding stator rings integral to the body (1), and internally an axial bell-shaped cavity (13) and a rotating shaft (15), and an electric motor (7, 8), contained into the axial bell-shaped cavity (13) for rotating at high speed said rotating shaft (15) on slewing supporting means (5a, 5b) integral to the body (1), said motor comprising a stator (7) integral to the body (1) of the pump and a rotor (8) coupled to the internal surface of the axial bell-shaped cavity (13) of the pumping rotor (9).

Description: The present invention refers to a vacuum pump. More particularly the invention refers to a vacuum pump of the turbo molecular type, driven by a direct current electric motor. It is well known that a vacuum pump comprises schematically an external housing inside which are housed the gas pumping stages. The gas pumping stages are generally formed by the co-operation of stator rings integral to the pump body and rotor disks integral to a slewing shaft operated by the motor of the pump. The rotor disks can be flat disks or be provided with slanting and close fins. Some vacuum pumps, generally the turbo molecular ones, comprise both flat disks and disks having fins and allow to reach pressures of approximately 10 -8 Pa, with very high rotating speeds, even 100.000 revolutions per minute. The shaft of the pump rotor and the shaft of the motor normally coincide in one rotating shaft, supported by suitable slewing supporting means. Generally the shaft is supported by bearings that can be rolling bearings, having balls or rolls, or magnetic bearings, and guarantee to the shaft a free rotation and a precise balancing. A first type of well-known vacuum pump provides for a couple of rolling bearings placed on the rotating shaft between the electric motor and the pumping section. Such configuration however, although having some advantages as the simple construction and easy maintenance, in that the motor, the bearings and the pumping section are completely separate, does not allow to realise pumps having compact dimensions, especially in the axial direction. A second example of turbo molecular vacuum pump, axially more compact than that described above, is disclosed in document EP 0 408 791. Said document describes a vacuum pump having a bell-shaped pumping rotor, that is having, internally, a cylindrical cavity in which are housed the electric motor and the bearings, in addition to the rotating shaft of the pumping rotor. In particular the motor is placed between the bearings and its shaft coincides with the rotating shaft of the pump. This solution allows to render the pump more compact than the first type of pump described above, however the distance between bearings can never be smaller than the length of the motor. Document DE 37 08 663 discloses a vacuum pump comprising a cylindrical casing with a number of stator discs, a rotor shaft supporting a bell-shaped pumping rotor, and an electric motor having an annular rotor mounted on the radially inner surface of the rotor body and a stator mounted on the casing at a point lying within the annular rotor. The rotor shaft rotates in bearings, placed at opposite ends of the shaft, in the cylindrical casing. Document US 5,695,318 discloses a diagonal fan having a bell-shaped fan wheel, enclosing an electrical drive motor, mounted on a rotating shaft. The rotating shaft of the fan wheel is mounted, by means of two bearing means, on a bearing tube integral to the base structure of the fan. US 5,663,604 refers to a brushless motor, for use as a fan motor or the like, in which the overall size of the motor can be reduced while maintaining a desired mounting area for a drive control circuit. A first object of the present invention is therefore to allow the realisation of a vacuum pump axially compact and structurally very simple. These and other objects are reached by the vacuum pump according to the invention, as claimed in the enclosed claims. The vacuum pump according to the invention can be advantageously used in all that applications in which it is necessary to reduce to a minimum the dimensions of the pump, without renouncing to high performances. The aforesaid and other objects of the invention will become more evident from the description of a preferred embodiment with reference to the attached drawing in which it is shown a vacuum pump realised according to the present invention. With reference to the figure now will be described a vacuum pump 11 according to an embodiment of the present invention. With the reference 1 it is shown a body of the pump, normally made of metal, having a base portion and a cylindrical hollow portion 14, serving as basement and support for other elements f the pump. In particular in the figure a pumping rotor 9 is visible, having a plurality of rotor disks 12, coupled to corresponding stator rings integral to the body 1 of the pump, not shown in figure. The co-operation between stator rings and flat rotor disks 12 allows to realise gas pumping stages of different kinds. Some stages for example could provide flat rotor disks, others rotor disks having slanting and close fins, according to desired characteristics. The pumping rotor 9 has, internally, an axial bell-shaped cavity 13 in the centre of which is placed a rotating shaft 15. A part of the body 1, in particular the cylindrical hollow portion 14, penetrates into the axial bell-shaped cavity 13 of the pumping rotor 9 and houses internally slewing supporting means 5a, 5b for the rotating shaft 15. In general the slewing supporting means can be rolling bearings, having balls or rolls, or magnetic bearings coupled to safety ball bearings which intervene in case of sudden malfunctioning of magnetic bearings, for avoiding damages in the pump itself. In particular, in the embodiment shown, a first 5a and a second 5b rolling bearing are positioned into the cylindrical hollow portion 14. Each bearing has an outer ring, integral to the internal surface of the hollow cylindrical portion 14, and an inner ring integral to the rotating shaft 15 of the pumping rotor 9. Between the two rings are placed a plurality of rolling balls or rolls. Two rubber rings 4 are placed between the ball bearings and the internal surface of the cylindrical hollow portion 14. Advantageously both bearings 5a, 5b have seat in the basement portion of the pump, corresponding to the cylindrical hollow portion 14. This permits to simplify further on the structure of the pump allowing a better precision and consequently avoiding complex balancing and centering operations of the bearings otherwise necessary for a correct rotation of the pump shaft. Between the two rolling bearings is present a spacing bar 6, having a shape substantially cylindrical, that maintains constant the distance between the bearings. The bearings 5a and 5b are kept in position by an axial containment ring 2b fixed on the top of the cylindrical hollow portion 14, by a cover 2a fixed to the base of the body 1 and by a pre-loading spring 3 placed between the cover 2a and the bearing 5b. A direct current electric motor 7, 8, comprised into the axial bell-shaped cavity 13, comprises a stator 7, integral to the body 1 of the pump, and a rotor 8, coupled to the internal surface of the axial bell-shaped cavity 13 of the pumping rotor 9. The rotor 8 of the motor is made of an annular permanent magnet, having north and south poles alternating on its circumference, and is keyed into the axial bell-shaped cavity 13 of the pumping rotor 9. Alternatively the rotor 8 can be made of a plurality of permanent magnets, coupled to the internal surface of the axial bell-shaped cavity 13 of the pumping rotor 9, arranged to form as a whole a magnetic ring having alternating polarities along its circumference. The magnet or the magnets can be placed into a recess obtained into the axial bell-shaped cavity 13 of the pumping rotor 9 so that they are coplanar with the internal surface of the bell. In this way the space taken by rotor-stator assembly of the motor can be further on reduced. The stator 7, having annular shape, is fixed to the external surface of the cylindrical hollow portion 14 of the body 1, so that it is integral to the body 1 of the pump. The use of a direct current electric motor having a permanent magnet incorporated into the pumping rotor 9 allows a remarkable simplification of the geometry of the pump body in the bearing housing area. The distance between the supporting bearings can be therefore reduced to the minimum necessary for guarantee a correct balancing of the shaft, without being limited by the physical length of the motor. The motor rotor is in fact keyed into the cavity 13 of the pumping section 9 and does not take space on the rotating shaft of the pump where are placed the bearings. The distance between the rolling bearings 5a, 5b along the rotating shaft 15 is shorter then the axial length of the motor 7, 8. It is therefore obtained a remarkable constructive simplicity, a better compactness especially in the axial direction, and a better bending rigidity that simplifies the balancing operations of the rotating parts.

Claims: Vacuum pump (11) comprising: a body (1) made of at least a base portion and a cylindrical hollow portion (14) integral to the base portion and having an internal surface and an external surface, a pumping rotor (9) having a plurality of rotor disks (12) coupled to corresponding stator rings for forming a plurality of pumping stages, and having an axial bell-shaped cavity (13) extending along a portion of said rotor (9) which partially encloses the cylindrical hollow portion (14) of the body (1), and a rotating shaft (15) which enters co-axially into the cylindrical hollow portion (14), an electric motor (7, 8), coupled to the external surface of said hollow cylindrical portion (14), comprising a stator (7) integral to the external surface of the cylindrical hollow portion (14) and a rotor (8) coupled to the internal surface of the axial bell-shaped cavity (13) of the pumping rotor (9), a pair of slewing supporting means (5a, 5b), characterised in that the pair of slewing supporting means (5a, 5b) is arranged internally to the cylindrical hollow portion (14) of the body, having a stationary part integral to the internal surface of the hollow cylindrical portion (14) and a slewing part coupled to the rotating shaft (15) of the pumping rotor (9) and in that said supporting means (5a, 5b) are spaced apart by means of a spacing bar (6) provided along said rotating shaft (15). Vacuum pump according to claim 1, in which said base portion has an opening, closed by a cover (2a), for accessing the internal of the cylindrical hollow portion (14). Vacuum pump according to claim 2, wherein the distance between said slewing supporting means (5a, 5b) along said rotating shaft is shorter than the axial development of the electric motor (7, 8). Vacuum pump according to claim 1 or 2, wherein the electric motor is a direct current electric motor. Vacuum pump according to claim 4, wherein the rotor (8) of the electric motor is an annular permanent magnet, having north and south poles alternating on its circumference, and is keyed into the axial bell-shaped cavity (13) of the pumping rotor (9). Vacuum pump according to claim 4, wherein the rotor (8) of the electric motor is made of a plurality of permanent magnets coupled to the internal surface of the axial bell-shaped cavity (13) of the pumping rotor (9). Vacuum pump according to one of the claims 5 or 6, wherein the rotor (8) of the electric motor is placed into a recess obtained into the axial bell-shaped cavity (13) of the pumping rotor (9). Vacuum pump according to one of the claims 5, 6 or 7, wherein the stator (7) of the electric motor has an annular shape and is fixed to the external surface of the cylindrical hollow portion (14) of the body (1), corresponding to the rotor (8) of the electric motor. Vacuum pump according to claim 1 or 2, wherein said slewing supporting means (5a, 5b) comprise a couple of rolling bearings, having balls or rolls, each having an outer ring, integral to the internal surface of the hollow cylindrical portion (14) of the body (1), and an inner ring integral to the rotating shaft (15) of the pumping rotor (9). Vacuum pump according to claim 9, comprising some rubber rings (4) placed between the outer rings of said rolling bearings and the internal surface of the hollow cylindrical portion (14) of the body (1). Vacuum pump according to claim 10, wherein said slewing supporting means (5a, 5b) are kept in position by an axial containment ring (2b), fixed on the top of the cylindrical hollow portion (14), and by said cover (2a) fixed to the base of the body (1), a pre-loading spring (3) being placed between said cover (2a) and an adjacent slewing supporting means (5b). Vacuum pump according to claim 1 or 2, wherein said slewing supporting means (5a, 5b) comprise a couple of magnetic bearings.

Inventors: CARETTO RAFFAELLA, CASARO FAUSTO, CARETTO, RAFFAELLA, CASARO, FAUSTO
Applicants: VARIAN SPA, VARIAN S.P.A.
Application year: 1999
Main classification: F04D19
IPC codes: F04D19, F04D25