US3168977A

US3168977A - Turbomolecular vacuum pump

Info

Publication number: US3168977A
Application number: US213414A
Authority: US
Inventors: Garnier Henri Louis; Desroches Louis Jacques; Marchal Raymond Hippoly Firmin; Meulien Louis Paul Henri
Original assignee: Commissariat a lEnergie Atomique CEA; SNECMA SAS
Current assignee: Safran Aircraft Engines SAS; Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Priority date: 1962-01-23
Filing date: 1962-07-30
Publication date: 1965-02-09
Anticipated expiration: 1982-02-09
Also published as: FR81075E; DE1728487A1; DE1728487B2

Description

Feb. 9, 1965 H. 1.. GARNIER ETAL 3,168,977

TURBOMOLECULAR VACUUM PUMP Filed July so, 1962 2 Sheets-Sheet 1 FIG.!

I INVENTORS HENRI LOUIS GARNIER Lsomos RUBET LOUIS JACQUES DESROCHE'S RAYMOND HIPPOLYTE FIRMIN MARCHAL LOUIS PAUL HENRI MEULIEN uJnlL-u), C04,, M Mud ATTORNEY 5 1965 H. 1.. GARNIER ETAL 3,168,977

TURBOMOLECULAR VACUUM PUMP Filed July 30, 1962 2 Sheets-Sheet 2 INVENTORS Y HENRI LOUIS GARNIER LEONIDE RUBET LOUIS JACQUES DESROCHES RAYMOND HIPPOLYTE FIRMIN MARCHAL LOUIS PAUL HENRI MEULIEN UM ATTORNEYS United States Patent 8 Claims. a. 23)-117) The present invention relates to turbomolecular vacuum pumps based on the principle of a compressor required to operate in a very low pressure range corresponding to the molecular system, the delivery of the gas being obtained by an assembly of successive discs, alternately fixed and rotating, in which there are formed apertures such that the probability of passage of the molecules is greater in one direction than in the other. A vacuum pump of this type has been described in US. patent application 189,021 filed by the applicant on May 15, 1961 and entitled Turbomolecular Vacuum Pump.

An essential characteristic of the present invention resides in the fact that the pump is of single flow type, the suction on the high vacuum being effected through an orifice of large conductance located on one side of the assembly of successive discs, while the delivery towards the preliminary vacuum is effected on the opposite side, the molecules of rarefied gas thereby having a mean path substantially without any bend and generally in the direction of the axis of rotation, the rotor of the pump being supported by means excluding any lubricated bearing on the high vacuum side.

According to one embodiment of the invention, the rotor is mounted on a main bearing disposed close to its centre of gravity and having great bending strength, a secondary bearing associated with the main hearing being placed at a certain distance from the centre of gravity and requiring a lesser bending strength. The mechanical transmission to the frame of the pump of the stresses or forces originating from the rotor through the bearings is effected entirely on one and the same side or end of the pump and more precisely on the side opposite to the high vacuum, so that it can be said that the rotor is in effect mounted so as to be overhung in the direction of the high vacuum.

The turbornolecular pump of the present invention has other important characteristics, the most interesting of which appear to be the following:

The main bearing positioned close to the centre of gravity of the rotor is connected to the assembly of rotating discs through the medium of a sleeve integral with a plate to which the assembly is fastened and which is located in a transverse plane off-set with respect to that of said hearing;

The bearings have their rotating races on the inside and their fixed races on the outside and are interposed between a central driving shaft and a drum surrounding the latter and fast with the frame of the pump, the driving shaft projecting at the end of the drum and being connected at this level to the rotor through the medium of a sleeve in turn surrounding the drum;

The lubrication circuit of the pump, and more generally the space in which the particles of lubricant are likely to circulate, are isolated in fluid-tight manner from the region of high vacuum;

The pump has a vertical axis with the suction orifice at the top, the lubricant accumulating by gravity towards the ,i. cut

bottom of the pump in a central region apart from the upstream peripheral region adjacent the high vacuum;

A cold screen is interposed around the space containing the lubricant for the purpose of ensuring condensation of oil vapour which may form and preventing the vapour ascending as far as the level of the high vacuum;

The heating of the pump in order to promote the emission and elimination of the gaseous particles adsorbed by the Walls is effected in the presence of the abovementioned cold screen and when the pump is rotating, for example at half speed, by heating externally, the suction flange and the upstream stages of the pump being advantageously heated in a preferential manner.

Other characteristics of the present invention will be clear from the following description given by way of exampie with reference to the accompanying drawings, in which:

FIGURE 1 is an axial half section of a turbomolecular vacuum pump in accordance with the invention having a horizontal axis of rotation, and

FIGURE 2 is a similar view of a modified construction with a vertical axis of rotation.

The turbomolecular vacuum pumps illustrated in the drawings each comprise a series of rotating discs 1 between which are interposed fixed discs 2 fixed to the casing 5 of the pump. Through the discs 1 and 2 suitable apertures for the passage of the rarefied gas molecules are formed over opposite annular zones 3 and 4 The arrangement and form of these apertures are not shown in the drawings for reasons of simplification and also because they do not appear to be essential for a good understanding of the present invention; in this connection it would be possible to use with advantage the arrang ments forming the subject-matter of copending patent ap plication Serial No. 213,416, filed on July 30, 1962, now abandoned.

The rotating discs 1 each ha e an annular portion 6 of extra thickness with plane parallel faces; the thickness of this portion 6 is in fact equal to the pitch or axial distance of recurrence of the discs 1 or 2, so that the rotating discs 1 are stacked without any clearance between the thickened portions 6, as shown clearly in the drawings. In this way, scaling is obtained between a peripheral space A in which the molecules of the rarefied gas circulate and an inner space E in the hub region of the rotating discs 1. The hubs 11 of the discs 1 are likewise of extra thickness, but are arranged with a slight clearance between them so as to avoid the creation of a large number of small sealed-off chambers between the discs 1.

The discs 1 are assembled by means of tie rods 7 parallel to the axis of rotation X-Y and distributed around a pitch circle centred on that axis, so as to preserve symmetry and equilibrium of the assembly. The tie rods 7 extend through the extra thickness portions 6 of the discs 3.

In the embodiment shown in FIGURE 1, the rods 7 also extend through a plate 9 located in front of the adjacent end disc 1 and a plate 10 occupying an intermediate position towards the rear of the stack or set of discs 1. The plates 9 and it are of extra thickness adjacent the peripheries at 9a and 10a.

The plate 10, which is of annular form and formed with holes 12 providing communication between the space B and a space C, is also formed integrally with a coaxial sleeve 13 on which there is fixed, by means of screws 14, a solid circular end cover 15 adjacent the plate 9. The sleeve 13 is slidably engaged at the front end 13a over a circular rib 16 on the plate 9, while a slight clearance is left between the inner periphery of the rotating discs 1 and the outer surface of the sleeve 13, again with the aim of avoiding local sealed-off areas in the space B.

On the other hand, the sleeve 13 isolates an inner space D from the space E, another space E being defined by the end cover 15, the plate 9 and the end 13a of the sleeve 13. The solid circular plate 9 isolates an upstream space F, which communicates with the chamber to be evacuated and is therefore under a high vacuum, from the inner downstream spaces B and E which are in turn isolated from the space D inside the sleeve.

The sleeve 13 is fitted over a hollow shaft 17 which is of large diameter over the major part of its length but terminates in a journal 17a of small diameter. The shaft 17 is fixed relatively to the casing of the pump and its connection with the rotary sleeve 13 is effected through the medium of ball bearings 18 and 19; the outer race of the former is housed in the cover and the outer race of the latter in an annular part 13!) fast with the sleeve l3, while the inner races are fixed to the journal 17a of the fixed shaft 17 and linked by means of a spacing sleeve 24.

A plug 20 closes the free end of the fixed shaft 17, which is slightly set in with respect to the rotary cover 115, while small ducts 21, 21 provide communication be tween the internal space G of the hollow shaft 17 and the ball-race bearings 18 and 19. This arrangement is designed to permit lubrication of the bearings by oil from the space G, return of the oil being effected through the space D inside the sleeve 13 and through ducts 22 formed in the hollow shaft 17 with a collecting bush 23 fixed to the latter.

It is to be observed that, as a result of the sealing between the space'D containing oil particles and the inner spaces B and E achieved by the cover 15 and the sleeve 13 and the sealing between these latter spaces and the upstream space F achieved by the plate 9, the ball-race bearings 18 and 19 and the lubricating system are completely isolated from the high vacuum. In fact, it is essential that no trace of oil should reach the chamber to be evacuated.

The driving of the rotary assembly constituted by the discs 1 and the support 9, l3, 13 to which they are connected by the tie rods 7 is effected by means of an electric motor incorporated in the pump. This motor, which is of the axial plane gap type comprises, on the one hand, a stator 25 in the form of a disc fixed to the hollow shaft 17 and carrying the motor windings which may be constituted by a flat coil with larnellar conductors and the connections to the external supply circuit and, on the other hand, a rotor which may be constituted by

toroidal magnets

26, 26 made, for example, of ferrite. The magnets are fixed to a cylindrical frame or casing 27 having a flange 23 against which the heads 29 of the tie rods '7 engage, while nuts 3% screwed on to the threaded ends 8 of the rods 7 bear against the terminal plate 9.

It is appropriate to observe the special arrangement of the whole of the rotary part of the pump which is overhung on a fixed shaft. This arrangement enables any risk of vibration to be eliminated under normal running conditions and also during acceleration. This result takes account of the fact that the critical speed of the assembly must be higher than the speed of rotation, in spite of the use of ball bearings of small diameter which is necessitated by the high speeds of rotation.

The solution of this problem has been facilitated by the great bending strength obtained by arranging the bearing 19 close to the centre of gravity of the rotating part. Immediately after this hearing, on the side where it is fitted in, the fixed shaft 17 exhibits a considerable increase in diameter, and the shaft 17 is moreover fixed to parts which are extremely rigid in themselves. The second bearing 18 is simply designed to withstand moments around the centre of gravity of the rotating part; the lesser rigidity of the journal 17a of the fixed shaft, between the two bearings 18 and 19, is nevertheless sufficient to ensure this latter function. With this arrangement, high natural frequencies have been obtained in spite of the small diameter of the ball bearings.

It is to be observed, moreover, that the rotary part of the pump remains centered without play both under normal running conditions and during starting up, owing to the absence of any radial expansion opposite the hearing 19. This result is obtained as a result of the arrangement of the plate It) in the form of a disc and which is off-set axially with respect to the bearing 19. Thus, the centrifugal force acting in the plane of the plate 10 scarcely has any further influence at the plane of the bearing 19 as a result of the elastic deformation of the cylindrical portion of the sleeve 13 which separates them.

As regards centring on the bearing 18, it is to be noted that the radial deformations are negligible on the bore corresponding to the outer diameter of that bearing as the cover 15 which supports it is solid.

It is to be noted, finally, that the rotating discs 1 are centered solely by the tie rods 7, the assembly of which forms a hub of large diameter, thus enabling any spacer to be dispensed with and, consequently, any relative slidingof the parts which do not undergo identical deformations to be avoided. Moreover, the plates 9 and 10 can be calculated in such manner that there is no'relative sliding between them and the rotating discs 1.

So as not to subject the bores in the discs 1 through which the tie rods 7 extend to too heavy ovalising stresses at high speeds of rotation, a certain balancing of the masses is obtained owing to the portions of extra thickness 11 on the inner periphery of the discs 1 and to the portions of extra thickness 9a and 10a on the outer periphery of the plates 9 and 10.

The apparatus formed in this way has, at opposite ends, on the one hand an upstream orifice 31 of large conductance connected to the chamber to be evacuated in which it is desired to obtain the high vacuum and, on the other hand, a downstream orifice 32 connected to a conventional vacuum backing pump (not shown) designed to create a preliminary vacuum. Thus, the molecules flow in the direction indicated by the arrows, that is to say along a quasi-rectilinear mean path in the direction of the axis of rotation X-Y, from the suction end to the delivery end. In particular, the absence of bends at the suction end, and therefore at the end where the highest vacuum is located, is an appreciable advantage. With such an arrangement, the losses in the rate of flow or delivery in the known devices, which are due 'to constricted or throttled sections or to non-rectilinear paths, are eliminated.

The modified construction shown in FIGURE 2 is principally distinguished from the foregoing form of embodiment in that its axis of rotation X-Y is vertical, the flow of the molecules taking place in the general direction from top to bottom.

Moreover, the stack of rotating discs 1 is clamped, by means of the tie rods 7, between two

end plates

33 and 34. These plates, which are of annular form with peripheral edges 33a and 34a of extra thickness, are integral with twocoaxial sleeves the inner periphery of the rotating discs 1 and the outer surface of the

sleeves

35 and 36, again with the aim of avoiding local sealed-off areas in the space B. The latter is moreover placed in communication with the lower space H of the pump through ducts 37 formed through the plate 34. V

The sleeve 35 which is closed by an upper head or end 38 and the sleeve 36, which has no head or end, are connected to one another by a central disc 39 having orifices (not shown) placing the internal spaces I and J of the two sleeves in communication. These spaces are isolated from the space B outside the sleeves and which is itself isolated from the top space F under high vacuum.

In the central disc 39 there is housed the end of the driving shaft 40 of the pump. The shaft 40 rotates in

roller bearings

41 and 42 carried by a cylindrical drum 43 integral with a frusto-conical base 44. fixed rigidly to the casing 5. of the apparatus by means of a collar 45. The drum 43 is interposed, with the required clearances, between the shaft 40 and the sleeve 36 and has its upper end set in with respect to the central disc 39 for attachment of the rotor of the pump to said driving shaft 40. The effect of this arrangement is to cause the bearings to operate with their rotating races on the inside and their fixed races on the outside, which affords an appreciable advantage in view of the very high speeds of rotation and the severe rnechanical conditions of which the pump is the seat.

The upper bearing 41 is located in a transverse plane extending through the centre of gravity of the rotary assembly or in the immediate proximity of that plane. It is intended to have a great radial rigidity or strength and may as shown take the form of a roller bearing, the axial thrust due to the weight of the rotary assembly being transferred to the lower bearing 42 which may have an oblique contact bearing contact. The security of the assembly in the axial direction is ensured by a stop 46 which comes into play only in quite exceptional circumstances if the force directed upwardly and originating from the differential pressures should become preponderant over the weight of the rotor or if the bearing 42 should fail.

The pump is again driven by a built-in electric motor of any type, for example of the axial plane gap type and with a flat winding having lamellar conductors; this winding is formed on discs 47 which form part of the stator of the motor and are fixed to the base 44 of the drum, while the rotor 48, constiuted, for example, by a torus of magnetic ferrite, is fastened to a shaft 49 rotating in bearing (not shown) carried by the stator of the motor.

The shaft 49 of the motor is coupled to the shaft 40 of the vacuum pump through a splined joint 50 adapted to transmit a pure torque and permitting a certain freedom of relative movement of the rotors of the two machines. Moreover, this connection offers the possibility of removing one of the machines without affecting the other. In short, the vacuum pump and theelectric motor constitute two machines independent of one another, apart from the coupling such that the motor transmits a pure torque to the pump.

At the end of the shaft 49 of the electric motor there may be provided a small auxiliary lubricating pump 51, for example of the centrifugal type, exerting suction through an axial duct 52, in a cup or reservoir 53 fixed below the base 44, and delivering into a pipe 54 arranged to permit the wetting or spraying of the bearings. The return of the lubricant to the cup or reservoir 53 takes place under gravity through the inner space K which is maintained at the pressure of the preliminary vacuum by means of a breather pipe 55; any lubricant which has managed to infiltrate into the space I is collected at the bottom of the latter and reintroduced into the inner space K through one or more suitable bores 58.

The oil reservoir is cooled by circulation of water through a coil or a jacket 56 covering the frustoconical base 44. This therefore performs the function of a cold wall in relation to any oil vapour likely to arise and to infiltrate as far as the internal space H and prevents the oil vapour reaching a considerable pressure level. The conduits (not shown) for the inlet and outlet of cooling water extend through said space H and perform the function of a second cold-producing stage superimposed on the preceding one.

It is to be noticed that, as in the arrangement of FIG- URE 1, as a result of the sealing between the internal spaces I, J, K which are likely to contain particles of oil and the intermediate space B and of the sealing between the latter and the upstream space F, the

bearings

41, 42 and the lubricating system are completely isolated from the high vacuum.

Since the cooling of the electric motor cannot be effected by convection by reason of its position in the chamber K under vacuum, it is advisable to provide for cooling of the stator 47 by circulation of water; the rotor 48, constituted by a magnetic ferrite, does not require cooling.

The apparatus which has just been described lends itself remarkably to the usual baking or out-gassing operation which consists in heating it so as to promote the emission of the gaseous particules occluded in its walls, without runningthe latent risk of producing a vaporisation of oil such that the oil vapour may ascend as far as the upstream space F and the chamber to be evacuated.

This reliability is obtained by virtue of the general arrang ement of the pump, particularly the following features:

(a) Admission at the top and circulation of the gaseous molecules at the periphery of the pump, with oil reservoir at the bottom andlubricating zone in the centre of the P p;

(b) Complete sealing between the spaces through which particles ofoil are likely to pass and the region of high vacuum; and

(c) Cold wall preventing any possibleoilvapour from reaching a considerable pressure level.

, In order to carry out the out-gassing operation, it is possible, according to the invention, to utilise a heating resistance 57 disposedaround the casing 5 and covered with an asbestos jacket (not shown), a preferential heating being advantageously applied to the suction flange 31a and to the upstream stages of the pump where the elimination of the occluded gaseous molecules assumes a critical character. The heat is transmitted by conduction to the fixed outer parts of the pump and by radiation to the rotor. The temperature level reached in the lower central portion of the pump is relatively low and the presence of the cold screen 56 strongly reduces it further.

In the course of the out-gassing process, the assembly is at the pressure of the preliminary vacuum through the action of the conventional vacuum backing pump (not shown) permanently connected to the outlet orifice 32, the evacuation of the molecules emitted by the walls being ensured by operating the turbomolecular pump at low speed, for example at half speed, so as not to subject any parts in an overheated state to excessive mechanical stresses.

Once out-gassing has been completed, the current supplying the heating resistance 57 is cut off, the pump is allowed to cool and, finally, the

electric motor

47, 43 is started at full speed until the desired high vacuum is obtained.

As in the case of the embodiment of FIGURE 1, the modified embodiment shown in FIGURE. 2 enables extremely high speeds of rotation to be attained by reason of the great bending strength obtained by arranging the bearing 41 close to the centre of gravity of the rotating part, the second bearing 42 being'intended merely to withstand moments around the centre of gravity of the rotating part: the rigidity of the portion of the drum 43 between the two bearings is suflicient to ensure this latter function.

What is claimed is:

1. A rotary high-vacuum pump of the molecular type comprising a generally cylindrical casing with a substantially vertical axis, a high-vacuum end section with a large-conductance suction orifice at the top of said casing, a preliminary-vacuum end section with a discharge orifice toward the bottom of said casing, a plurality of superposed, generally horizontal rotor discs housed within said casing intermediate said end sections and provided with gas molecule passages in a peripheral zone of said pump adjacent said casing, a driving shaft of substantially vertical rotation axis connected with said rotor discs, bearing means for said shaft positioned in a central zone of said pump remote from said peripheral zone thereof, a lubricant container in a bottom central position with respect to said pump, means for sucking lubricant from said con tainer and circulating said lubricant within said central zone of the pump to lubricate said bearing means, lubricant passages in said central zone for the return of lubricant by gravity to said container, and means for fluidtightly separating said container and said central zone of the pump wherein particles of lubricant circulate from said high-vacuum end section and at least the peripheral zone of the pump adjacent thereto wherein rarefied gas molecules circulate. I

2. A pump as claimed in claim 1, comprising further a cold screen extending around at least a part of the central Zone of the pump wherein lubricant is present and adapted to exert a refrigerating action on lubricant vapor.

3. A pump as claimed in claim 2, comprising further heating means extending on an external portion of the pump for heating the same to promote emission of gaseous particles adsorbed by adjacent physical walls thereof.

4. A pump as claimed in claim 3, wherein the heating means comprise an electrical resistance surrounding the casing and arranged to effect privileged heating of the high-vacuum end section and upper peripheral portion adjacent thereto.

v 5. A pump as claimed in claim 1, wherein the bearing means comprise rotary inner races and stationary outer races.

6. A pump as claimed in claim 5, comprising further R L a stationary drum extending around the shaft and supporting the bearing means, the rotary inner races of which being fast With said shaft and the stationary outer races of which being fast with said drum, and a rotaary sleeve extending around said drum and mechanically connecting the rotor discs with an end of said shaft projecting upwardly from said drum, I

7. A pump as claimed in claim 6, comprising further an incorporated electric motor drivingly connected with an end of the shaft remote from the projecting end thereof, and a sealed inner casing integral with thedrum and inside which said motor is housed. a

8. A pump as claimed in claim '7, comprising further a breather pipe connecting the inside of the sealed inner casing with the preliminary-vacuum end section of the pump, and means for cooling the motor by fluid circulation.

References Cited in the file of this patent UNITED STATES PATENTS

Claims

1. A ROTARY HIGH-VACUUM PUMP OF THE "MOLECULAR" TYPE COMPRISING A GENERALLY CYLINDRICAL CASING WITH A SUBSTANTIALLY VERTICAL AXIS, A HIGH-VACUUM END SECTION WITH A LARGE-CONDUCTANCE SUCTION ORIFICE AT THE TOP OF SAID CASING, A PRELIMINARY-VACUUM END SECTION WITH A DISCHARGE ORIFICE TOWARD THE BOTTOM OF SAID CASING, A PLURALITY OF SUPERPOSED, GENERALLY HORIZONTAL ROTOR DISCS HOUSED WITHIN SAID CASING INTERMEDIATE SAID END SECTIONS AND PROVIDED WITH GAS MOLECULE PASSAGES IN A PERIPHERAL ZONE OF SAID PUMP ADJACENT SAID CASING, A DRIVING SHAFT OF SUBSTANTIALLY VERTICAL ROTATION AXIS CONNECTED WITH SAID ROTOR DISCS, BEARING MEANS FOR SAID SHAFT POSITIONED IN A CENTRAL ZONE OF SAID PUMP REMOTE FROM SAID PERIPHERAL ZONE THEREOF, A LUBRICANT CONTAINER IN A BOTTOM CENTRAL POSITION WITH RESPECT TO SAID PUMP, MEANS FOR SUCKING LUBRICANT FROM SAID CONTAINER AND CIRCULATING SAID LUBRICANT WITHIN SAID CENTRAL ZONE OF THE PUMP TO LUBRICATE SAID BEARING MEANS, LUBRI-