Title:
VACUUM PUMPS
United States Patent 3749528


Abstract:
The vertical rotating assembly of a vacuum pump (comprising a turbomolecular rotor and the rotating member of an electric driving motor located under the rotor) is centered by a magnetic bearing located above the pump rotor and is supported by a ball bearing assembly located under the rotor and the motor. The ball bearing assembly is so constructed or connected to the pump housing as to tolerate radial deflection of the rotor.



Inventors:
Rousseau, Marcel-gustave (Saint-Denis, FR)
Berthoumieux, Robert (Paris, FR)
Application Number:
05/127951
Publication Date:
07/31/1973
Filing Date:
03/25/1971
Assignee:
SOC NAT D ETUDE ET CONSTRUCTION DE MOTEURS D AVIATION,FR
COMMISSARIAL A L ENERGIE ATOMIQUE,FR
Primary Class:
Other Classes:
310/90.5, 415/90, 415/199.5
International Classes:
F04D19/04; F16C23/08; F16C39/06; (IPC1-7): F04B17/00; F01D1/36; F16C35/00
Field of Search:
417/423,424 415
View Patent Images:
US Patent References:
3107310Magnetic coupling having a magnetic bearing1963-10-15Carriere et al.
2436939Magnetic coupling and bearing1948-03-02Schug
1402053Elastic-fluid turbine1922-01-03Dake



Foreign References:
CA530369A
FR1475765A
Primary Examiner:
Raduazo, Henry F.
Claims:
We claim

1. A vacuum pump for producing and maintaining a pressure difference between an outlet adapted to be connected to a primary vacuum source and an inlet, said vacuum pump comprising : a housing provided with said inlet and outlet ; a rotor carried by a vertical shaft and separating said inlet and outlet ; motor means under said rotor for rotating said shaft ; and means for supporting and centering said shaft, including a mechanical axial thrust-bearing located under said rotor and constructed and arranged to permit angular movement of said shaft about said thrust-bearing means and centering magnetic bearing means located above said rotor and centering the shaft with respect to said housing.

2. A pump in accordance with claim 1, wherein said thrust-bearing means comprises a bearing ring secured to the shaft, a ring non-rotatably connected to the pump housing by a swivel connection and bearing members between said rings.

3. A pump in accordance with claim 2, having first and second bearing rings secured to the shaft and separated by a spacer member, first and second stationary rings and a universal-joint assembly providing a swivel connection between said stationary rings and the casing, said bearing rings being formed with raceways located in register with raceways formed in said stationary rings.

4. A pump in accordance with claim 3, having resilient means compressed between the casing and the lower of said first and second stationary rings and biasing it upwardly.

5. A pump in accordance with claim 1, wherein said mechanical thrust-bearing comprises a bearing ring secured to the shaft and formed with raceways, a stationary bearing ring secured to the casing and formed with raceways, bearing elements disposed between the rings and rolling on said raceways, said element and raceways being so constructed as to permit angular displacement of said rings with respect to each other.

6. A pump in accordance with claim 1, wherein said magnetic bearing means comprises a rotary plate secured to the shaft, two stationary plates, each of the two end faces of said rotary plate having centering teeth which cooperate with similar teeth carried by said stationary plates, and a box secured to said housing, carrying said stationary plates and provided with magnets for producing a magnetic flux which is closed by air-gaps between oppositely-facing teeth of said rotary plate and said stationary plates.

7. A pump in accordance with claim 6, wherein the box is connected to the pump housing by small columns which are parallel to the axis of the shaft and having a flexibility sufficient for them to follow high-frequency oscillations of the box transversal to the axis.

8. A pump in accordance with claim 7, having friction members located between the box and the pump housing for damping the oscillations of the box.

9. A pump in accordance with claim 7, having a vertical rod, a weight at the lower end of said rod and means for securing the upper end of said rod, axially in said shaft at a location close to the magnetic bearing means for counteracting the oscillations of the shaft.

10. A pump in accordance with claim 2, having means for maintaining an oil film between the components of the universal-joint assembly.

11. A pump in accordance with claim 1, having a universal joint assembly comprising a stationary member secured to the casing, a swiveling member carrying said thrust bearing means and having a tail-piece, and resilient means between said tail-piece and housing adapted to damp angular oscillations of said shaft.

12. A vacuum pump in accordance with claim 1, having a turbomolecular pump rotor comprising a stack of disks, wherein said thrust-bearing is located beneath the rotor and in the primary vacuum whilst said magnetic bearing means is above thr rotor in a secondary vacuum.

13. A vertical turbomolecular pump comprising : a stationary housing provided with an inlet and an outlet ; a driving motor having a stationary part and a part rotatable about a vertical axis ; a turbomolecular pump rotor located coaxially with said motor, separating said inlet and outlet ; a transmission shaft drivably connecting said rotor and rotatable part, and means for supporting and centering said rotor, shaft and rotating part, with respect to said housing, including magnetic centering means located above said rotor and mechanical thrust bearing means located under said rotor.

Description:
BACKGROUND OF THE INVENTION

The invention relates to vacuum turbomolecular pumps of the type described in French patent specification 1,304,689, having a vertical shaft carrying a rotor which is rotated at high speed within a housing and is provided with a stack of disks which draw residual gas from an end of the rotor and move it toward the other end of the rotor connected to a source of primary vacuum (in the 10-3 torr range) : such pumps are adapted to create a vacuum in the 10-11 torr range.

It is one of the main difficulties in the construction of turbomolecular pumps that the rotor (rotating at speeds in the 10,000 rpm range) should be adequately supported and centered. The problem is still rendered more difficult by the requirement that contamination of the vacuum by lubricant vapors should be avoided in order to retain one of the advantages of turbomolecular pumps, namely the production of a clean vacuum; the latter requirement prevents from locating ball bearings in the secondary vacuum.

In the past, several attempts were made to solve the problem ; one of them involves the use of gas bearings : but the rate of flow across gas bearings adapted to operate correctly is so high that it has an adverse effect on the performances of the pump. Another solution described in French patent No. 1,475,765 of one of the assignees of the present invention makes use of electromagnetic bearings for supporting the rotor and centering it. The regulation of such a system requires an elaborate servo loop.

SUMMARY

It is an object of the invention to provide a vertical axis vacuum pump in which the mechanism supporting and centering the rotating assembly is improved with respect to the prior art mechanisms particularly in that it is simple and rugged in design and provides satisfactory performance and long life. It is a more particular object of the invention to provide a pump in which the rotor is centered by a magnetic bearing located on one side of the rotor, while the weight of the complete rotating assembly of the pump is taken by mechanical bearing means located on the other side of the rotor and so mounted as to leave the rotating assembly free to tilt to a certain extent.

In the case of a turbomolecular pump, the thrust-bearing may preferably be placed under the rotor, where the primary vacuum is maintained, in order that degassing be lessened and in order to avoid any danger of contaminating the secondary vacuum with lubricant vapors.

In a preferred embodiment of the invention, the thrust-bearing is then placed between the lower end of the shaft and the pump housing whilst the magnetic centering bearing retains the upper end of the shaft.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood from the following description of particular embodiments of the invention given as examples and without any implied limitation, reference being made in the description to the accompanying drawings, in which

FIG. 1 is a very diagrammatic sectional view taken along the vertical mid-plane of a turbomolecular pump which constitutes a first embodiment of the invention ;

FIG. 2 is a detail view on a large scale showing the lower portion of FIG. 1 and in particular the thrust-bearing ;

FIG. 3 is a view on a large scale showing the upper portion of FIG. 1 and in particular the magnetic bearing for centering the shaft ;

FIG. 4 which is similar to FIG. 2 shows a second embodiment of the invention ;

FIGS. 5 and 6 which are also similar to FIG. 2 show two further embodiments .

The turbomolecular pump which is illustrated in FIGS. 1 and 3 comprises a housing 10 formed of a number of assembled parts and having an upper opening 11 which is connected to a pipe for drawing the gases from the enclosure to be evacuated (not shown in the figures) and a lower opening 12 which provides a connection with a pipe 14 for discharging to a primary vacuum source (vane pump or oil vapor pump, for example). A rotor 16 having a vertical axis is placed within the cylindrical pump housing and provides a separation between the suction and discharge pipes. Said rotor is made up of stacked discs 18 which are mounted on a sleeve 20 and clamped elastically by means of a terminal socket 22 and tie-rods 24 against a flange 26 of the sleeve 20. The pump housing is also provided with stacked circular discs 28 which are interposed between the rotary discs 18 and clamped between an annular shoulder 30 of the pump housing and resilient washers 32, the pressure applied by said washers being adjustable by means of adjusting screws 34 which produce action on thrust members 36 (as shown in FIG. 2). The longitudinal and radial clearances which are necessary for operation are provided between the rotary discs 18 and the stationary discs 28.

In accordance with the invention, the rotor is supported vertically by a thrust-bearing 38 which would permit angular displacement of the rotor if this latter were alone (FIGS. 1 and 2) and is centered by a magnetic bearing 40 (shown in FIGS. 1 and 3).

The thrust-bearing 38 which is illustrated in FIG. 2 is interposed between a shaft 39 which forms a downward extension of the sleeve 20 and a casing 42 which is secured to the base 44 of the pump housing 10 by means of screws 46. Said thrust-bearing comprises a ball-bearing made up of an inner ring 48 which is attached to the lower end of the shaft 39, a stationary outer ring 52 and balls 50 (which could in any case be replaced by rollers or needles). The stationary ring 52 is connected to the casing 42 by means of a ball-joint coupling which permits oscillation of the shaft 39 about a point located at the intersection of its axis with the central plane of the bearing-ring 48. Said ball-joint coupling comprises a semi-spherical member 44 in which is forcibly fitted the outer ring 52 and two half-cages 56 which are secured in position between circlips 58, said circlips being mounted in grooves of the casing 42. The ball-joint coupling has only a small degree of play and the contacting parts are preferably impregnated with lubricant. Although it does not usually prove necessary to take this precaution, the semi-spherical member 54 can be secured against rotation in order to prevent this latter from being driven by the shaft across the ball-bearing.

The magnetic centering bearing 40 which is illustrated in FIGS. 1 and 3 is disposed between the cover 60 (in which is formed the suction opening 11) and the socket 22. Said bearing comprises a portion which is secured to the cover 60 and a rotary portion which is attached to the socket. The portion which is secured to the cover comprises a flange plate 62 (as shown in FIG. 3) which can be rigidly fixed to the cover 60 by means of threaded rods 64. The terminal portions of a number of small columns 66 which are parallel to the axis of the rotor and may be three in number, for example, are fixed at uniform intervals around the periphery of the flange plate 62. The other end of each small column is attached to a box 68 which is formed of a plurality of assembled components. The main portion of the box 68 which is of cylindrical shape contains a block 70 of material having high magnetic permeability in which are placed magnets 72. The magnetic flux of said magnets is closed in a loop by means of a magnetic circuit which comprises in addition to the block 70 two magnetic plates 74 and 76 provided with oppositely-facing centering teeth (tha plate 76 forms the cover of the casing 68).

The rotary portion of the centering bearing 40 is constituted by a magnetic plate 78 provided on both faces with teeth which are intended to be placed in alignment with the teeth of the magnetic plates 74 and 76. Said rotary plate 78 is keyed by means of a screw 80 on an extension 82 of the socket 22. The toothed magnetic bearings constitute elements which are now well known and the constructional make-up and operation of these latter do not call for any extended description here ; only the method of assembly of the centering bearing 40 will accordingly be mentioned. The small columns 66 which are formed of material having a high degree of elasticity (non-magnetic spring steel, for example) facilitate transition to the critical speeds of the rotor and compensate to a certain extent for the low degree of stiffness of the magnetic bearings. For example, a beater 84 which is shown in chain-dotted lines in FIG. 1 can be substituted for the columns 66 for the purpose of damping oscillations and is placed within the sleeve.

The magnetic bearing and the columns 66 exert a restoring force on the rotor in the centered position and damping of angular oscillations is carried out at least to a partial extent as a result of the friction which is set up between the half-cages 56 and the semi-spherical member 54 of the ball-joint coupling. As shown in FIG. 3, elastic supporting elements can be provided for the box 68 of the centering bearing 40. Each elastic element comprises a spring 86 interposed between a stud 88 which is applied against the box and an adjusting screw 90. In practice, these elements are necessary only when the rotor is of large size. In the case of pumps of small size, said elements should preferably not be provided particularly as consideration must be given to the potential danger of jamming resulting from degassing of the constituent materials of said elements which work under a secondary vacuum (pressure within the range of 10-8 and 10-11 torr).

The motor which serves to drive the rotor is placed between the thrust-bearing and the stack of discs and therefore works under a primary vacuum. The motor which is illustrated in FIGS. 1 and 2 comprises a stator winding 92 carried by the casing 42 and supplied with current through a sealed lead-in bushing 96 and a rotor winding 94 which is mounted on the shaft 39.

The embodiment which is illustrated in FIG. 2 comprises a circulation system 98 for cooling the casing 42 and the stator 92. Apart from its intended function of removing the heat given off by the electric motor, the circulation system 98 serves to condense the vapor on the casing 42 (said vapor being liable to emanate from the oil vapor pump which produces the primary vacuum) and thus has a contributory function in maintaining the purity of the secondary vacuum.

A turbomolecular pump in accordance with the foregoing description has been constructed and has rotated for a period of several hundred hours at its normal speed of 27,000 rpm without any incident. It has also been possible to increase the pump speed to 30,000 rpm without giving rise to any major difficulty.

In the embodiment which is illustrated in FIGS. 1 and 2, the angular displacements of the ball-joint coupling may possibly be braked as a result of friction forces developed between the moving parts. The embodiment which is illustrated in FIG. 4 differs primarily from the preceding form of construction in that provision is made for an elastic restoring device which serves to return the shaft to its centered position and which is associated with the ball-joint coupling. In FIG. 4, in which the components corresponding to those which have already been illustrated in FIG. 2 are designated by the same reference numerals to which is assigned the prime index, there is again shown a pump housing 10' whose end-wall 44' carries a casing 42' which is provided with a cooling circulation system 98'. The shaft 39' which forms a downward extension of the sleeve 20 carries the inner ring 48' of a ball-bearing, the outer ring 52' of which is clamped within a semi-spherical member 54'. As in the case of FIG. 2, said semi-spherical member is imprisoned between two half-cages 56'. Moreover, the member 54' is provided with a tubular tail-piece 100. A centering and damping device constituted by a toric ring 102 of deformable material (such as synthetic rubber, for example), said ring being placed within a circular groove of an annular member 103 which is secured to the casing 42', tends to bring the tail-piece 100 back to an orientation in which the axis of this latter coincides with the normal axis of rotation of the shaft 39'.

In the alternative embodiment which is illustrated in FIG. 5 (in which the components corresponding to those previously described are provided with the same reference numerals followed by the second index), the thrust-bearing comprises two ball-bearings between which the axial load is distributed. The inner rings 48" and 104 of the two ball-bearings are passed over the shaft 39". The ring 48" bears on an annular shoulder of the shaft and the ring 104 is applied by a lock-nut 106 against a tubular spacer member 108 which determines the distance between the two bearings. The outer rings 52" and 110 of the ball-bearings are carried by the semi-spherical member 54". The ring 52" is secured in position between an internal shoulder and a threaded sleeve 112. The lower ring 110 is slidably fitted within a bore of the semi-spherical member 54" and is supported by a spring 114, the compression of which is set by means of a threaded sleeve 116 at a value such that the lower ball-being carries approximately one-half of the axial loads.

The bearing which is illustrated in FIG. 5 comprises a circulation system for lubrication of the ball-joint under pressure ; said system comprises a supply duct 118 which extends through the pump housing and has its opening in a toric distribution recess 119 formed in the space between the sliding surfaces of the ball-joint coupling. Said space has a width of a few hundredths of a millimeter (3/100 mm, for example) and the oil under pressure which is contained therein attenuates the oscillations and facilitates transition to critical speeds. A bore 120 formed in the semi-spherical member 54" also supplies oil to the two ball-bearings and ensures lubrication of these latter. Oil losses flow into a sump 122 (only the top portion of which is shown in FIG. 5) and can be collected in said sump.

In the embodiment which is illustrated in FIG. 5, the shaft 39" is adapted to carry a deflector 124 which is located beneath the rotor 94" of the electric motor which serves to drive the shaft in rotation, with the result that any oil which may creep upwards along said shaft 39" is returned downwards by said deflector.

In the embodiment which is illustrated diagrammatically in FIG. 6 (in which the components corresponding to those which have already been described are followed by the same reference numeral to which is assigned the tierce index), the ball-type coupling or universal-joint assembly which permits oscillation of the shaft is constituted by a bearing consisting of a double row of balls. As in the embodiment of FIG. 5, provision is again made in this form of construction for two ball-bearings between which the axial load is distributed. The outer rings 52'" and 110'" of said ball-bearings are carried by an annular member 54'" which is connected to an end-plate 124 of the pump housing by means of the universal joint assembly. Said assembly is constituted by a ball-bearing comprising an inner cage 126 which is locked in position by means of a nut 126 against a shouldered portion of the annular member 54'", a cage 130 which is locked in position against an annular shoulder of the end-plate 124 by means of a threaded retaining ring 131 and a row of balls disposed in staggered relation. The ball-race profiles are such as to permit a slight angular displacement of the outer cage 130 with respect to the inner cage 126.

Further embodiments of this invention can evidently be contemplated and it must be understood that the alternative forms of either all or part of the arrangements herein described which come within the definition of equivalent mechanical means remain within the scope of this patent. In particular, the magnetic bearing can be of a type other than that which has been described and could consist of a more conventional arrangement instead of magnets in the form of separate studs. In some cases, the small columns could be omitted or replaced by shorter tie-rods in the event that damping of oscillations is carried out by means of different arrangements.