Rotary vacuum pump, structure and method for the balancing thereof

Abstract

 A rotary vacuum pump (101; 201) comprising displacement sensors (121A - 121F; 221A - 221F), variously coupled to the pump basement (103; 203) and arranged close to the pump rotor (113; 213) and/or to the rotating shaft (105; 205) thereof, the sensors being turned towards it (them) and being perpendicular thereto, in order to detect non-homogeneous distributions, if any, of masses of said rotor (113; 213) with respect to its rotation axis. The invention also concerns a structure for and a method of balancing a rotary pump.

Description

[0001] The present invention concerns a rotary vacuum pump and a structure and a method for the balancing thereof.

[0002] In the field of rotary vacuum pumps, it is known that either mechanical bearings, such as ball or roller bearings, or magnetic bearings can be used for supporting the rotating pump shaft.

[0003] The present invention concerns a rotary vacuum pump of the kind equipped with mechanical bearings.

[0004] More particularly, the present invention concerns a turbomolecular rotary vacuum pump of the kind disclosed for instance in EP-A-0962264 or EP-A-0773367.

[0005] As known, rotary pumps, and especially turbomolecular rotary pumps, are machines equipped with a rotating portion, including a rotating shaft to which a set of parallel rotor discs are secured, and co-operating with a stationary portion, generally a set of stator discs, in order to obtain gas pumping from an inlet port to an outlet port of the pump.

[0006] Depending on the kind of pump, higher or lower vacuum degrees can be obtained. For instance, a turbomolecular pump can generate a vacuum of the order of 10^-7 mbar (10^-5 Pa) with a shaft rotation speed in the range 2x10⁴ to 9x10⁴ rpm.

[0007] A vacuum pump is thus a machine with a mass that is rotated at extremely high speed. In a vacuum pump, such a rotating mass generally includes a rotating shaft, the rotor of the electric motor driving said shaft into rotation, the set of rotor discs and the inner rings of the rolling bearings rotatably supporting the pump shaft.

[0008] When the rotating mass is not arranged with its centre of gravity on the rotation axis and thus is not balanced, forces of inertia are generated within the pump and are transmitted through the housing to the outside of the pump. Such forces of inertia cause unwanted stresses and vibrations, which are sources of noise and lead to an early wear of the rolling bearings.

[0009] Moreover, in some specific applications, for instance where the pump is connected to a precision measuring instrument, such as in mass spectrometry, vibrations are sources of disturbances altering the operation of the measuring instrument and therefore they cannot be tolerated.

[0010] One of the problems encountered in designing a rotary vacuum pump equipped with mechanical bearings is thus how to reduce the vibrations produced by the pump due to unbalance of the rotating masses.

[0011] Generally, it is known that balancing of a rotating mass can be obtained by means of further additional rotating masses, coupled to the main mass so that the centre of gravity of the overall mass is brought again on the rotation axis (static balancing) and the rotation axis coincides with a main axis of inertia (dynamic balancing). A dynamically balanced rotor does not transmit stresses to the supports and it is therefore an optimum solution.

[0012] In the field of rotary vacuum pumps, and in particular of turbomolecular ones, the pump rotor is dynamically balanced through an iterative process in which measuring steps of the vibrations transmitted by the pump to an external structure alternate with adjusting steps of the position of one or more additional masses placed on the rotor, until the optimum conditions are attained.

[0013] The main problems related to the rotor balancing step are, on the one hand, the definition of the mathematical model used in order to relate the vibrations measured during the balancing step to the rotor unbalance and, consequently, to the arrangement of the correcting masses, and, on the other hand, the choice of the kind of vibration sensors and the arrangement thereof.

[0014] In the field of rotary vacuum pumps, the sensors generally used during the rotor balancing step are accelerometers, that is sensors capable of transforming the acceleration of a moving body to which they are secured into an electric signal, the intensity of which is just a function of the acceleration the sensor is being submitted to.

[0015] According to the prior art, the dynamic balancing of a vacuum pump rotor is performed by placing the pump, without stator discs, inside a bell-shaped casing onto which at least two accelerometers, for instance piezoelectric accelerometers, are located. Once the rotor is rotated at high speed, the accelerometers located onto the stationary bell allow measuring the vibrations induced by unbalances, if any, of the rotating masses.

[0016] Yet such a solution has some drawbacks, of which the main is that the point where vibrations are measured, i.e. the area where the accelerometer is located, is relatively far from the source of said vibrations, i.e. the rotor.

[0017] The provision of a set of masses placed between the rotor and the accelerometer, and comprising members that in part are very rigid and in part are resilient and damping, makes it complex to define a reliable mathematical model relating the vibrations to their cause, i.e. the unbalance of the rotor and the other moving masses.

[0018] Consequently, the iterative balancing process may need several pump stopping and starting phases in order to apply the correcting masses, and this results in a considerable increase of the time required to reach the optimum conditions and hence in a considerable slowing down of the production.

[0019] It is the main object of the present invention to solve the problem of how effectively and quickly to balance the rotating masses of a rotary vacuum pump, more particularly a pump equipped with mechanical bearings such as a turbomolecular vacuum pump.

[0020] The above and other objects are achieved by means of a vacuum pump and a balancing method as claimed in the appended claims.

[0021] Thanks to the positioning of displacement sensors close to the rotating masses of the pump, it is possible to obtain a more direct measurement of the rotor vibrations and hence to make the proper balancing thereof simpler and quicker.

[0022] According to the invention, the vibration measurement is not affected by the presence of other pump components, which allows a considerable simplification of the mathematical model relating the measured displacements to the rotor unbalance inducing them.

[0023] Advantageously, the provision of displacement sensors permanently located inside the pump allows measuring the rotating mass unbalance also during steady state operation of the same pump, that is when the pump has been completed with the stator part, assembled and delivered to the customer.

[0024] Two embodiments of the invention, given by way of non-limiting example, will be described hereinbelow with reference to the accompanying drawings, in which:

• Fig. 1 is a diagrammatic view of the displacement sensor;
• Fig. 2 is a diagram of the electronic circuitry of the displacement sensor;
• Fig. 3a is a cross-sectional view of a first example of a vacuum pump according to the invention;
• Fig. 3b is a cross-sectional view of a second example of a vacuum pump according to the invention.

[0025] Referring to Fig. 3a, a first turbomolecular rotary pump 101 according to the invention is schematically shown.

[0026] Said pump 101 comprises a stationary portion and a rotating portion. The stationary portion comprises a basement 103 on which the rotating portion is mounted. The latter comprises a rotating shaft 105 supported by rolling bearings 107, for instance ball bearings. Rotor 109 of electric motor 111 (the stator of which has not been shown for sake of simplicity) used to rotate shaft 105, and pump rotor 113, equipped with smooth or finned discs 115, are mounted on said rotating shaft 105.

[0027] As clearly shown in Fig. 3a, according to the construction design of pump 101, said pump rotor 113 has a bell-shaped cavity 117 housing rotating shaft 105 of the pump and electric motor 111, in order to make the pump axially more compact. Such an arrangement is generally used for big turbomolecular pumps (rotor diameter of about 250 mm).

[0028] In fig. 3a the pump is shown during the balancing phase and hence rotor 113 is not located inside the pump housing, which, as known, is equipped with stator discs, but inside a vacuum-tight stationary bell 119 specifically intended for the balancing of said rotor 113. Vacuum in said bell is made by means of an ancillary pumping system, not shown.

[0029] According to the invention, a plurality of displacement sensors (four in the disclosed embodiment) 121A - 121D are directly mounted in basement 103 of pump 101, close to rotor 113 and to rotating shaft 105 thereof. Each sensor faces said shaft 105 or said rotor 113 so that changes, if any, in the distance between the rotor and the sensor during rotation of the rotor can be detected.

[0030] More particularly, in the case depicted in Fig. 3a, a first pair of sensors 121A, 121B face rotating shaft 105 and are turned towards it, whereas a second pair of sensors 121C, 121D face inner wall 113a of rotor 113 and are turned towards such wall.

[0031] According to the invention, eddy current displacement sensors are advantageously employed.

[0032] Referring to Fig. 1, there is schematically shown a generic displacement sensor 51 comprising a coil 53, which is wound on a core 55 and in which a high frequency AC current generating a main magnetic field flows. The variation of distance "a" between coil 53 and an electrically conducting body R, for instance the pump rotor or the shaft thereof, causes a corresponding variation of the magnetic field induced and consequently of impedance Z measured in the coil of sensor 51.

[0033] By using an impedance-to-voltage converter, such as that shown in Fig. 2, a voltage signal U, the value of which depends on impedance Z and hence on the distance of the metal body from the sensor, can be obtained at the output from sensor 51.

[0034] More precisely, the circuit shown in Fig. 2 comprises a high frequency oscillator 65, an impedance 67 in series and a demodulator 63. Impedance 67 must be sufficiently high to obtain a high sensitivity. Demodulation of voltage signal u outgoing from the sensor allows obtaining a voltage signal U that is a function of distance "a".

[0035] Eddy current displacement sensors are capable of measuring distance variations of the order of 1 nm and are perfectly suitable for use in balancing turbomolecular pump rotors.

[0036] More particularly, in the described case, a variation of the distance of internal wall 113a of rotor 113 from facing sensors 121C, 121D, caused by an unbalance in rotor 113, will cause a measurable impedance variation in said sensors. By measuring such an impedance variation, it is possible to obtain the distance variation, and hence the unbalance having generated it, and to correct such unbalance.

[0037] The process in case of a distance variation between rotating shaft 105 and sensors 121A, 121B is similar.

[0038] To correct the unbalance of rotor 113, cylindrical threaded bores 123 are provided in rotor 113 and are arranged with their axes lying in a plane orthogonal to the rotation axis of the rotor and tangentially relative to the same rotor. Additional masses consisting of threaded dowels can be located and displaced in said bores.

[0039] As an alternative, other balancing methods comprise the insertion of masses consisting of threaded dowels to be screwed into bores with axes radially arranged relative to the rotor.

[0040] Further in accordance with the invention, and still referring to Fig. 3a, a third pair of displacement sensors 121E, 121F is provided, which sensors are arranged close to external wall 113b of rotor 113, between a pair of said rotor discs, and are turned towards said wall. Said sensors 121 E, 121F are cantilevered on a vertical support 120 adjacent to a wall of outer bell 119.

[0041] It is clear that, at the end of the balancing phase, bell 119 and support 120, if provided, will be removed and replaced by pump housing 121 with the stator integral thereto, so that the pump will be ready for being sent to the customer and used. Consequently, at the end of the balancing phase, displacement sensors 121 E, 121F integral with bell 119 will be removed. On the contrary, sensors 121A - 121D mounted in basement 103 of pump 101 will remain inside said pump even during operation thereof, and they could be advantageously used to carry out measurements on the rotor balance conditions during normal pump operation.

[0042] Turning now to Fig. 3b, a second embodiment of the invention is partly depicted.

[0043] A turbomolecular pump 201 differs from that previously disclosed with reference to Fig. 3a in that rotor 213 has no bell-shaped cavity receiving rotating shaft 205 and electric motor 211. Shaft 205 is instead supported by a pair of rolling bearings 207, for instance ball bearings, and is driven by an electric motor 211, the bearings and the motor being located in a pump region that is axially separated from the pumping region where rotor 213 is located.

[0044] That arrangement is generally used for small and medium size turbomolecular pumps (rotor diameter smaller than about 160 mm).

[0045] Similarly to what described above, according to the invention a pair of displacement sensors 221A, 221B is provided in basement 203 of pump 201, opposite rotating shaft 205 and at opposite sides of rotor 209 of electric motor 211.

[0046] Also in that second example, said displacement sensors are preferably eddy current sensors.

[0047] Like in the previous embodiment, further displacement sensors 221C, 221D and 221E, 221F are provided, which are integral with outer bell 219 and face rotor 213.

[0048] More particularly, in the embodiment shown, a second pair of sensors 221C, 221D is provided close to inner wall 213a of rotor 213, whereas a third pair of sensors 221E, 221F is provided close to outer wall 213b of rotor 213. Said sensors are turned towards said rotor so that any variation in the distance between the rotor and the sensor during rotation of the same rotor can be detected.

[0049] In order to properly locate the second pair of sensors 221C, 221D, bell 219 is advantageously equipped with a central cylindrical projection 219a penetrating into central bore 213c of rotor 213.

[0050] A removable vertical support 220 is provided adjacent to one of the walls of external bell 219 for the cantilevering of the third pair of displacement sensors 221E, 221F.

[0051] Like in the previously disclosed pump, also pump 201 has multiple threaded bores 223 with axes lying in planes orthogonal to the rotation axis of rotor 223 to allow locating and displacing additional masses.

[0052] Also in this case, threaded dowels located in radial bores instead of tangentially oriented bores can be used.

[0053] When, at the end of the balancing phase, bell 219 and support 220, if present, will be removed, displacement sensors 221C, 221D and 221E, 221F will be removed as well, whereas sensors 221A, 221B mounted in basement 203 of pump 201 will remain inside said pump even during operation thereof, and they could be advantageously used to carry out field measurements.

[0054] It is clear that the turbomolecular pump according to the invention attains the intended aims, since using displacement sensors directly mounted inside the pump, close to the rotor or the rotating shaft thereof, allows using simpler and more precise mathematical models to determine the rotor unbalance. Consequently, the balancing phase might be carried out in quicker manner and with better results.

[0055] It is also clear that the above description has been given only by way of non-limiting example and that several modifications are possible without departing from the scope of the invention.

Claims

1. A rotary vacuum pump (101; 201) comprising a stationary portion (103; 203) and a portion rotating relative to said stationary portion, said rotating portion comprising a rotating shaft (105; 205) equipped with a rotor assembly (113; 213) co-operating with a stator assembly for gas pumping, said rotating shaft being driven by an electric motor (111; 211) and being supported by at least one rolling mechanical bearing (107; 207) relative to said stationary portion, characterised in that at least one displacement sensor (121A-121D; 221A-221B), capable of generating an electrical signal varying with the distance between said stationary portion and said rotating portion during the rotation of said shaft and said rotor assembly, is provided between said stationary portion and said rotating portion.

2. A pump as claimed in claim 1, wherein said pump (101) comprises a basement (103) on which the rotating shaft (105) supported by a pair of rolling bearings (107) is mounted, the rotor (109) of the pump electric motor (111) used to rotate the shaft (105) and the pump rotor (113) being mounted on said rotating shaft (105).

3. The pump as claimed in claim 2, wherein said pump rotor (113) has a bell-shaped cavity (117) housing the pump rotating shaft (105) and the electric motor (111).

4. The pump as claimed in claim 3, wherein said pump comprises at least one pair of displacement sensors (121A - 121D) mounted in the basement (103) of the pump (101), close to the rotor (113) and/or to the rotating shaft (105) thereof, each sensor facing said shaft (105) or said rotor (113) so that the variation, if any, in the distance between the rotor and the sensor during rotation of the rotor can be measured.

5. The pump as claimed in claim 4, wherein said pump comprises a first pair of sensors (121A, 121B) facing the rotating shaft (105) and turned towards it, and a second pair of sensors (121C, 121D) facing the inner wall (113a) of the rotor (113) and turned towards such wall.

6. The pump as claimed in claim 1, wherein said rotor (113) comprises at least one threaded cylindrical bore (123) arranged with its axis lying in a plane orthogonal to the rotation axis of the rotor (113) and tangentially relative to said rotor, in which bore additional masses consisting of threaded dowels can be located and displaced in order to reduce the unbalance of said rotating portion.

7. The pump as claimed in claim 1, wherein said rotor (113) comprises at least one threaded cylindrical bore (123) arranged with its axis lying in a plane orthogonal to the rotation axis of the rotor (113) and radially relative to said rotor, in which bore additional masses consisting of threaded dowels can be located and displaced in order to reduce the unbalance of said rotating portion

8. The pump as claimed in claim 1, wherein the rotating shaft (205) is supported by a pair of rolling bearings (207) and is driven by an electric motor (211), the bearings and the motor being located in a pump region that is axially separated from the pumping region where the rotor (213) is housed.

9. The pump as claimed in claim 8, wherein a pair of displacement sensors (221A, 221B) is provided in the basement (203) of the pump (201), opposite the rotating shaft (205) thereof and at opposite sides of the rotor (209) of the electric motor (211).

10. The pump as claimed in claim 1, wherein said displacement sensors are eddy current displacement sensors.

11. The pump as claimed in claim 10, wherein said sensors comprise a coil (53) in which a high frequency AC current generating a variable magnetic field flows.

12. The pump as claimed in claim 11, wherein said sensors comprise an impedance-to-voltage converter (61), such that a variation in the voltage level of an output signal of said converter (61) corresponds to an impedance variation in the coil of said sensor.

13. The pump as claimed in claim 12, wherein said sensors (121A - 121D; 221A, 221B) provide, during pump operation, a signal representative of the displacement of the rotating portion relative to the stationary portions.

14. The pump as claimed in claim 1, wherein said pump is a turbomolecular pump.

15. A structure for balancing a rotary vacuum pump of the kind comprising a stationary portion (103; 203) and a portion rotating relative to said stationary portion, said rotating portion comprising a rotating shaft (105; 205) equipped with a rotor assembly (113; 213) co-operating with a stator assembly for gas pumping, said rotating shaft being driven by an electric motor (111; 211) and being supported by at least one rolling mechanical bearing (107; 207) relative to said stationary portion, said structure comprising a vacuum-tight bell (119; 219) in which the pump can be housed during balancing, characterised in that at least one displacement sensor (121A-121D; 221A-221B), capable of generating a electrical signal varying with the distance between said bell and said rotating portion during the rotation of said shaft and said rotor assembly, is provided between said bell and said rotating portion.

16. The structure as claimed in claim 15, wherein said pump (101; 201) comprises a basement (103; 203) on which the rotating shaft (105; 205) supported by a pair of rolling bearings (107; 207) is mounted, the rotor (109; 209) of the pump electric motor (111; 211) used to rotate the shaft (105; 205) and the pump rotor (113; 213) being mounted on said rotating shaft (105; 205).

17. The structure as claimed in claim 16, wherein said pump rotor (113; 213) has a bell-shaped cavity (117) where the pump rotating shaft (105; 205) and the electric motor (111; 211) are housed.

18. The structure as claimed in claim 17, wherein said pump comprises a plurality of displacement sensors (121A - 121D; 221A, 221B) mounted in the pump basement (103; 203), close to the rotor (113; 213) and/or to the rotating shaft (105; 205) thereof, each sensor facing said shaft (105; 205) or said rotor (113; 213) so that a variation, if any, in the distance between the rotor and the sensor during rotation of the rotor can be detected.

19. The structure as claimed in claim 18, wherein said pump comprises a first sensor pair (121A, 121B; 221A, 221B) facing the rotating shaft (105; 205) and turned towards it, and a second sensor pair (121C, 121D; 221C, 221D) facing the inner wall (113a; 213a) of the rotor (113; 213) and turned towards such wall.

20. The structure as claimed in claim 19, wherein, in order the second sensor pair (221C, 221D) can be properly located, the bell (219) is equipped with a central cylindrical projection (219a) penetrating into the pump rotor (213).

21. The structure as claimed in claim 19, wherein a third pair of displacement sensors (121E, 121F; 221E, 221F) is provided, which sensors are arranged close to the outer wall (113b; 213b) of the pump rotor (113; 213), between a pair of said rotor discs (115; 215), and are turned towards said wall.

22. The structure as claimed in claim 21, wherein said sensors (121E, 121F; 221E, 221F) of said third pair of displacement sensors are cantilevered on a vertical support (120; 220) adjacent to one of the walls of the outer bell (119; 219).

23. The structure as claimed in claim 22, wherein said sensors are eddy current displacement sensors.

24. A method of balancing a rotary vacuum pump of the kind comprising a stationary portion (103; 203) and a rotating portion comprising a rotating shaft (105; 205) equipped with a rotor assembly (113; 213) co-operating with a stator assembly for gas pumping, said rotating shaft being driven by an electric motor (111; 211) and being supported by at least one rolling mechanical bearing (107; 207) relative to said stationary portion, the method comprising the steps of:

a) providing a vacuum-tight bell (119; 219) in which the pump can be housed during balancing;

b) coupling said pump, without said stator assembly, to said bell;

c) making vacuum in said bell;

d) driving said rotating pump portion into rotation;

e) measuring the displacement, at the rotation frequency, of said rotating portion relative to said stationary portion;

f) stopping said rotating portion;

g) balancing said rotating portion by means of additional masses;

h) repeating, if necessary, steps b) to g);

and being characterised in that said displacement measurement is obtained by means of at least one displacement sensor (121A - 121F; 221A - 221F), capable of generating an electrical signal varying with the distance between said stationary portion or said bell and said rotating portion during the rotation of said shaft (105; 205) and said rotor assembly (113; 213).

Amended claims

Amended claims in accordance with Rule 86(2) EPC.

1. A rotary vacuum pump (101; 201) comprising a stationary portion (103; 203) and a portion rotating relative to said stationary portion, said rotating portion comprising a rotating shaft (105; 205) equipped with a rotor assembly (113; 213) co-operating with a stator assembly for gas pumping, said rotating shaft being driven by an electric motor (111; 211) and being supported by at least one rolling mechanical bearing (107; 207) relative to said stationary portion, characterised in that at least two displacement sensors (121A-121D; 221A-221B), capable of generating an electrical signal varying with the distance between said stationary portion and said rotating portion during the rotation of said shaft and said rotor assembly, are provided between said stationary portion and said rotating portion.

2. A pump as claimed in claim 1, wherein said pump (101) comprises a basement (103) on which the rotating shaft (105) supported by a pair of rolling bearings (107) is mounted, the rotor (109) of the pump electric motor (111) used to rotate the shaft (105) and the pump rotor (113) being mounted on said rotating shaft (105).

3. The pump as claimed in claim 2, wherein said pump rotor (113) has a bell-shaped cavity (117) housing the pump rotating shaft (105) and the electric motor (111).

4. The pump as claimed in claim 3, wherein said pump comprises at least one pair of displacement sensors (121A - 121D) mounted in the basement (103) of the pump (101), close to the rotor (113) and/or to the rotating shaft (105) thereof, each sensor facing said shaft (105) or said rotor (113) so that the variation, if any, in the distance between the rotor and the sensor during rotation of the rotor can be measured.

5. The pump as claimed in claim 4, wherein said pump comprises a first pair of sensors (121A, 121B) facing the rotating shaft (105) and turned towards it, and a second pair of sensors (121C, 121D) facing the inner wall (113a) of the rotor (113) and turned towards such wall.

6. The pump as claimed in claim 1, wherein said rotor (113) comprises at least one threaded cylindrical bore (123) arranged with its axis lying in a plane orthogonal to the rotation axis of the rotor (113) and tangentially relative to said rotor, in which bore additional masses consisting of threaded dowels can be located and displaced in order to reduce the unbalance of said rotating portion.

7. The pump as claimed in claim 1, wherein said rotor (113) comprises at least one threaded cylindrical bore (123) arranged with its axis lying in a plane orthogonal to the rotation axis of the rotor (113) and radially relative to said rotor, in which bore additional masses consisting of threaded dowels can be located and displaced in order to reduce the unbalance of said rotating portion

8. The pump as claimed in claim 1, wherein the rotating shaft (205) is supported by a pair of rolling bearings (207) and is driven by an electric motor (211), the bearings and the motor being located in a pump region that is axially separated from the pumping region where the rotor (213) is housed.

9. The pump as claimed in claim 8, wherein a pair of displacement sensors (221A, 221B) is provided in the basement (203) of the pump (201), opposite the rotating shaft (205) thereof and at opposite sides of the rotor (209) of the electric motor (211).

10. The pump as claimed in claim 1, wherein said displacement sensors are eddy current displacement sensors.

11. The pump as claimed in claim 10, wherein said sensors comprise a coil (53) in which a high frequency AC current generating a variable magnetic field flows.

12. The pump as claimed in claim 11, wherein said sensors comprise an impedance-to-voltage converter (61), such that a variation in the voltage level of an output signal of said converter (61) corresponds to an impedance variation in the coil of said sensor.

13. The pump as claimed in claim 12, wherein said sensors (121A - 121D; 221A, 221B) provide, during pump operation, a signal representative of the displacement of the rotating portion relative to the stationary portions.

14. The pump as claimed in claim 1, wherein said pump is a turbomolecular pump.

15. A structure for balancing a rotary vacuum pump of the kind comprising a stationary portion (103; 203) and a portion rotating relative to said stationary portion, said rotating portion comprising a rotating shaft (105; 205) equipped with a rotor assembly (113; 213) for gas pumping when co-operating with a stator assembly, said rotating shaft being driven by an electric motor (111; 211) and being supported by at least one rolling mechanical bearing (107; 207) relative to said stationary portion, said structure comprising a vacuum-tight bell (119; 219) in which the pump can be housed during balancing, characterised in that at least two displacement sensors (121A-121D; 221A-221B), capable of generating a electrical signal varying with the distance between said bell and said rotating portion during the rotation of said shaft and said rotor assembly, are provided between said bell and said rotating portion.

16. The structure as claimed in claim 15, wherein said pump (101; 201) comprises a basement (103; 203) on which the rotating shaft (105; 205) supported by a pair of rolling bearings (107; 207) is mounted, the rotor (109; 209) of the pump electric motor (111; 211) used to rotate the shaft (105; 205) and the pump rotor (113; 213) being mounted on said rotating shaft (105; 205).

17. The structure as claimed in claim 16, wherein said pump rotor (113; 213) has a bell-shaped cavity (117) where the pump rotating shaft (105; 205) and the electric motor (111; 211) are housed.

18. The structure as claimed in claim 17, wherein said pump comprises a plurality of displacement sensors (121A - 121D; 221A, 221B) mounted in the pump basement (103; 203), close to the rotor (113; 213) and/or to the rotating shaft (105; 205) thereof, each sensor facing said shaft (105; 205) or said rotor (113; 213) so that a variation, if any, in the distance between the rotor and the sensor during rotation of the rotor can be detected.

19. The structure as claimed in claim 18, wherein said pump comprises a first sensor pair (121A, 121B; 221A, 221B) facing the rotating shaft (105; 205) and turned towards it, and a second sensor pair (121C, 121D; 221C, 221D) facing the inner wall (113a; 213a) of the rotor (113; 213) and turned towards such wall.

20. The structure as claimed in claim 19, wherein, in order the second sensor pair (221C, 221D) can be properly located, the bell (219) is equipped with a central cylindrical projection (219a) penetrating into the pump rotor (213).

21. The structure as claimed in claim 19, wherein a third pair of displacement sensors (121E, 121F; 221E, 221F) is provided, which sensors are arranged close to the outer wall (113b; 213b) of the pump rotor (113; 213), between a pair of said rotor discs (115; 215), and are turned towards said wall.

22. The structure as claimed in claim 21, wherein said sensors (121E, 121F; 221E, 221F) of said third pair of displacement sensors are cantilevered on a vertical support (120; 220) adjacent to one of the walls of the outer bell (119; 219).

23. The structure as claimed in claim 22, wherein said sensors are eddy current displacement sensors.

24. A method of balancing a rotary vacuum pump of the kind comprising a stationary portion (103; 203) and a rotating portion comprising a rotating shaft (105; 205) equipped with a rotor assembly (113; 213) for gas pumping when co-operating with a stator assembly, said rotating shaft being driven by an electric motor (111; 211) and being supported by at least one rolling mechanical bearing (107; 207) relative to said stationary portion, the method comprising the steps of:

a) providing a vacuum-tight bell (119; 219) in which the pump can be housed during balancing;

b) coupling said pump, without said stator assembly, to said bell;

c) making vacuum in said bell;

d) driving said rotating pump portion into rotation;

e) measuring the displacement, at the rotation frequency, of said rotating portion relative to said stationary portion;

f) stopping said rotating portion;

g) balancing said rotating portion by means of additional masses;

h) repeating, if necessary, steps b) to g);

and being characterised in that said displacement measurement is obtained by means of at least two displacement sensors (121A - 121F; 221A - 221F), capable of generating an electrical signal varying with the distance between said stationary portion or bell and said rotating portion during the rotation of said shaft (105; 205) and said rotor assembly (113; 213).

Drawing