Abstract:
A rotary vacuum pump ( 101; 201 ) comprising displacement sensors ( 121 A- 121 F;  221 A- 221 F), 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 relates to a structure for and a method of balancing a rotary pump.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention concerns a rotary vacuum pump and a structure and a method for 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 U.S. Pat. No. 6,158,986 or U.S. Pat. No. 5,688,106.  
         [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 a range 2×10 4  to 9×10 4  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 center of gravity or the rotation axis and thus is not balanced, forces of interior are generated within the pump and are transmitted through the housing to the outside of the pump. Such forces of interior 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 center 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 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 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.  
       SUMMARY OF THE INVENTION  
       [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]     Due 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 for 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 for 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. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [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:  
         [0025]      FIG. 1  is a diagrammatic view of the displacement sensor;  
         [0026]      FIG. 2  is a diagram of the electronic circuitry of the displacement sensor;  
         [0027]      FIG. 3   a  is a cross-sectional view of a first embodiment of a vacuum pump according to the present invention;  
         [0028]      FIG. 3   b  is a cross-sectional view of a second embodiment of a vacuum pump according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0029]     Referring to  FIG. 3   a , a first turbomolecular rotary pump  101  is schematically shown.  
         [0030]     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 the rotating shaft  105 .  
         [0031]     As clearly shown in  FIG. 3   a , according to the construction design of pump  101 , the 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).  
         [0032]     In  FIG. 3   a  the pump is shown during the balancing phase and hence rotor  113  is not located inside the pump housing, which is equipped with stator discs, but inside a vacuum-tight stationary bell  119  specifically intended for the balancing of said rotor  113 . Vacuum in the bell is achieved by means of an ancillary pumping system, not shown.  
         [0033]     According to the present invention, a plurality of displacement sensors (four in the disclosed embodiment)  121 A- 121 D are directly mounted in basement  103  of pump  101 , close to rotor  113  and to rotating shaft  105  thereof. Each sensor faces the shaft  105  or the rotor  113  so that changes, if any, in the distance between the rotor and the sensor during rotation of the rotor can be detected.  
         [0034]     More particularly, in the case depicted in  FIG. 3   a , a first pair of sensors  121 A,  121 B face rotating shaft  105  and are turned towards it, whereas a second pair of sensors  121 C,  121 D face internal wall  113   a  of rotor  113  and are turned towards such wall.  
         [0035]     According to present invention, eddy current displacement sensors are advantageously employed.  
         [0036]     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 .  
         [0037]     By using an impedance-to-voltage converter, such as that shown in  FIG. 2 , a voltage signal D, 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 .  
         [0038]     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 D that is a function of distance “a”.  
         [0039]     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.  
         [0040]     More particularly, in the described embodiment, a variation of the distance of internal wall  113   a  of rotor  113  from facing sensors  121 C,  121 D, caused by an unbalance in rotor  113 , will cause a measurable impedance variation in the 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.  
         [0041]     The process in case of a distance variation between rotating shaft.  105  and sensors  121 A,  121 B is similar.  
         [0042]     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.  
         [0043]     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.  
         [0044]     Further in accordance with the invention, and still referring to  FIG. 3   a , a third pair of displacement sensors  121 E,  121 F is provided, which sensors are arranged close to external wall  113   b  of rotor  113 , between a pair of said rotor discs, and are turned towards the wall. The sensors  121 E,  121 F are cantilevered on a vertical support  120  adjacent to a wall of outer bell  119 .  
         [0045]     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,  121 F integral with bell  119  will be removed. On the contrary, sensors  121 A- 121 D mounted in basement  103  of pump  101  will remain inside the pump even during operation thereof, and they could be advantageously used to carry out measurements on the rotor balance conditions during normal pump operation.  
         [0046]     Turning now to  FIG. 3   b , a second embodiment of the invention is partially depicted.  
         [0047]     A turbomolecular pump  201  differs from that previously disclosed with reference to  FIG. 3   a  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 are located in a pump region that is axially separated from the pumping region where rotor  213  is located.  
         [0048]     Such arrangement is generally used for small and medium size turbomolecular pumps (rotor diameter smaller than about 160 mm).  
         [0049]     Similarly to what is described above, according to the present invention a pair of displacement sensors  221 A,  221 B is provided in basement  203  of pump  201 , opposite rotating shaft  205  and at opposite sides of rotor  209  of electric motor  211 .  
         [0050]     Also in that second embodiment, said displacement sensors are preferably eddy current sensors.  
         [0051]     Like in the previous embodiment, further displacement sensors  221 C,  221 D and  221 E,  221 F are provided, which are integral with outer bell  219  and face rotor  213 .  
         [0052]     More particularly, in the embodiment shown, a second pair of sensors  221 C,  221 D is provided close to internal wall  213   a  of rotor  213 , whereas a third pair of sensors  221 E,  221 F is provided close to external wall  213   b  of rotor  213 . These sensors are turned towards the rotor so that any variation in the distance between the rotor and the sensor during rotation of the same rotor can be detected.  
         [0053]     In order to properly locate the second pair of sensors  221 C,  221 D, bell  219  is advantageously equipped with a central cylindrical projection  219   a  penetrating into central bore  213   c  of rotor  213 .  
         [0054]     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  221 E,  221 F.  
         [0055]     Similar to previous embodiment, pump  201  also has multiple threaded bores  223  with axes lying in planes orthogonal to the rotation axis of rotor  213  to allow locating and displacing additional masses.  
         [0056]     Also in this case, threaded dowels located in radial bores instead of tangentially oriented bores can be used.  
         [0057]     When, at the end of the balancing phase, bell  219  and support  220 , if present, will be removed, displacement sensors  221 C,  221 D and  221 E,  221 F will be removed as well, whereas sensors  221 A,  221 B 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.  
         [0058]     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.  
         [0059]     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.