TURBO-MOLECULAR PUMP

Abstract: The present invention provides a turbo-molecular (1) pump in which an exhaust action is achieved by virtue of the combination of stationary blades (4) rotor blades adapted to rotate at high speed and to cooperate with each other to impart momentum to gaseous molecules rushing in against the blades under an extremely low-pressurized condition. Disposed inside the casing are a fixed shaft (2) and the rotor (3), the latter being movably supported by a pair of active radial magnetic bearings. A motor (6) is interposed between said pair of magnetic bearings. A dynamic damper (10) is provided at one end of the fixed shaft to inhibit the occurrence of vibrations.

Description: The present invention relates to a turbo-molecular pump. Turbo-molecular pumps are known wherein an exhaust action is effected by virtue of the combination of stationary blades and rotor blades adapted to rotate at high speed. Both blades are adapted to cooperate with each other to impart momentum to gaseous molecules rushing in against the blades under a low-pressurized condition wherein the incidence of collisions between gaseous molecules is negligible. The adoption of magnetic bearings in such a turbo-molecular pump enables a non-contact support of the rotating shaft of the pump, and thus the life of the bearing can be beneficially made semipermanent. Referring to Fig. 5, a prior art turbo-molecular pump having magnetic bearings will now be described. A fixed shaft 2 is provided in a casing 1, and a rotor 3 is placed over the shaft 2. Stator blades 4 are provided on the inner wall of the casing 1. The fixed shaft 2 is provided thereon with active radial magnetic bearings 5, 5 and a motor 6, by means of which the rotor 3 can rotate at high speed in a non-contact manner. In Fig. 5, reference numerals 7, 7, respectively, denote touch-down bearings adapted to mechanically support the rotor 3 and a rotor cover 8A fixed to the rotor 3 in case of failure of the magnetic bearings 5. In a turbo-molecular pump of the type described above, vibrations caused by the residual unbalance that subsists during rotation, torque generated by the motor 6, shocks exerted on the casing 1 and so on disturb the control system of the magnetic bearings 5 as disturbance. And thus these disturbances act to vibrate the fixed shaft 2 at the natural frequency of the fixed shaft 2. This sometimes generates bending vibrations in the fixed shaft itself, which is shaped like in cantilever fashion, as shown in Fig. 6. The relationship between the amplitude and forced frequency ratio of these bending vibrations is shown in Fig. 7, and a simplified model of this structural system can, as shown in Fig. 8, be defined as having one degree of freedom. These vibrations can constitute an adverse factor that makes the control system unstable, and self-excited or forced vibrations are sometimes generated in the pump main body. This serves to generate excessive vibrations in the pump main body and then to make it impossible for the magnetic bearings 5, 5 to support the rotor 3 in a stable manner, the functional reliability of the turbo-molecular pump is thus being lowered. FR-A-2534980 and DE-A-23 49 033 relate to turbo-molecular pumps comprising magnetic bearings, touch-down bearings and dampening means to overcome vibrations. In the case of FR-A-2534980 annular elements made of copper or another conducting material are used for creating Foulcaut currents for dampening purposes In addition hydraulic dampening means are used. DE-A-23 49 033 uses axial as well as radial magnetic bearings. FR-A-1476267 describes in connection with electric power line cables a dampening means using a resilient annular element together with an annular dampening mass. Similarly, GB-A-474 585 discloses in connection with means for dampening the osscillations on a road vehicle a web of resilient material and an appropriate mass. The present invention has been contrived to solve the above stated drawbacks inherent to prior art pumps, and an object thereof is to provide a turbo-molecular pump that generates a reduced level of vibration during operation and that is reliable in functional terms. In accordance with the present invention a turbo-molecular pump as set forth in claim 1 is provided. Preferred embodiments are disclosed in the dependent claims. In the practice of the present invention, the dynamic damper includes a weight and a resilient member which acts as spring and damping components. The weight is preferably made in the form of a ring and of a material having great mass ratio and specific gravity (for instance, tungsten) in order to minimize the size thereof as well as to maintain the amplitude of the fixed shaft at as low a level as possible. The resilient member is interposed between the ring-shaped weight and the fixed shaft and is preferably made of a rubber having low temperature dependency (for instance, an EPT or fluorine system rubber). In order to adjust the dynamic damper to its optimum state, the spring constant of the resilient member adapted to act as a spring is preferably determined by giving consideration to such factors as the characteristics and size of the resilient member and the mass of the weight so that it constitutes a natural frequency estimated to be the optimum one on the basis of the mass ratio. This spring constant may be selected in such a manner that two fixed points through which the amplitude curve passes independently of damping become equal in height. The hardness of the resilient member regulates the damping capability of the dynamic damper, and the hardness is preferably selected to be such that the amplitude curve passes through a maximum value at the two fixed points. It should be noted, however, that the spring constant should be prevented from deviating from the optimum natural frequency of the dynamic damper since the hardness of the resilient member also has an effect on the spring constant. A dynamic damper having a spring constant defined as described above allows the amplitude curve of the structural system of the present invention to have its maximum amplitude points at the two fixed points, thus making it possible to hold the amplitude of resonance to a minimum level. With the turbo-molecular pump of the present invention configured as described above, a rotary member can be supported in a non-contact manner and hence can be rotated at high speed by means of the active radial magnetic bearings provided on the pump. In addition, in the present invention, the dynamic damper is mounted on one end of the fixed shaft, and this enables a simplified model of the structural system to be defined as having two degrees of freedom, thus making it possible to hold the amplitude of the natural frequency of the fixed shaft to a remarkably low level. With this configuration, the rotary member can be stably supported by the magnetic bearings, and furthermore, adverse factors that would act on the control system of the magnetic bearings can be eliminated, thus making it possible to provide a turbo-molecular pump which generates almost no vibration during operation. Fig. 1 is a front view in section showing one embodiment of the present invention, Fig. 2 is a front view in section showing in detail the dynamic damper, Fig. 3 is a diagram showing a simplified model of the structural system of the present invention, Fig. 4 is a characteristic diagram of amplitude/frequency ratio showing advantageous effects of the present invention, Fig. 5 is a front view in section showing a prior art turbo-molecular pump, Fig. 6 is a diagram showing a mode in which bending vibrations are generated in the fixed shaft of the prior art turbo-molecular pump, Fig. 7 is a characteristic diagram of amplitude/ frequency ratio resulting in the mode shown in Fig. 6, and Fig. 8 is a diagram showing the simplified model of the structural system according to the prior art. A preferred embodiment of the present invention will now be described with reference to the accompanying drawings. In Fig. 1, like reference numerals are used to denote constituent members that are like those shown in Fig. 5. As shown in Fig. 1, disposed on the upper end portion of a fixed shaft 2 is a dynamic damper 10, which is covered by a rotor cover 8. As shown in Fig. 2, the dynamic damper 10 comprises a ring-shaped weight 11 and a ring-shaped rubber member 12 which is secured to the weight 11 and interposed between the weight 11 and a spindle 2a projecting from the end portion of the fixed shaft 2. This weight 11 is made of a material, for instance, tungsten, which has a great mass ratio and specific gravity in order that it can hold the amplitude of the fixed shaft 2 to as low a level as possible and also so as to minimize its size. The rubber member 12 is made of an EPT or fluorine system rubber which has low temperature dependency. The spring constant and damping capability of the dynamic damper 10 is regulated not only by the size but also by the hardness of the rubber used for the rubber member 12; the hardness of the rubber is therefore so selected that the two fixed points P and Q on the amplitude curve become equal in height in its optimum state and that the amplitude curve passes through a maximum value on these two fixed points P and Q in the same state. A simplified model of the structural system of the turbo-molecular pump equipped with a dynamic damper configured as described above is, as shown in Fig. 3, defined as having two degrees of freedom, and the relationship between its amplitude and forced frequency ratio becomes as shown in Fig. 4. As clear from Fig. 4, the fixed points P and Q correspond to the maximum amplitude point of the amplitude curve, thus making it possible to hold resonance of the structural system to as a low level as possible. With the present invention configured as described above, the rotor can be stably supported in a non-contact manner by the magnetic bearings. Moreover, the amplitude of the fixed shaft can be made remarkably low, and generation of self-excited and forced vibrations during high-speed rotation can in turn be prevented. Thus, the present invention provides a turbo-molecular pump which generates a reduced level of vibration and is hence highly reliable.

Claims: A turbo-molecular pump comprising: a casing (1), a shaft (2) fixed inside said casing (1), a rotor (3) placed over said fixed shaft (2), a pair of active radial magnetic bearings (5, 5) for movably supporting said rotor (3), a motor (6) disposed between said pair of active radial magnetic bearings (5, 5), a dynamic damper (10) disposed at one end of said fixed shaft (2), said dynamic damper (10) comprising a ring-shaped weight (11) disposed in such a manner as to surround a spindle (2a) mounted at one end of said fixed shaft (2) and a ring-shaped resilient member (12) secured to said weight (11) and interposed between said weight (11) and said spindle (2a), wherein one end of said fixed shaft (2) is connected to one end of said cylindrical casing (1) in such a manner as to be supported thereat, while the other end of said shaft is made free, wherein touch-down bearings (7) for mechanically supporting said rotor (3) in case of failure of said magnetic bearings (5, 5) are provided on said fixed shaft (2) at a position close to the free end thereof and at a position close to the supported end thereof, and wherein and said dynamic damper (10) is provided at the free end of said fixed shaft (2). A turbo-molecular pump according to claim 1, wherein said rotor (3) is adapted to surround the portion of said fixed shaft (2) which is exposed to the outside and to axially extend over said fixed shaft (2), said rotor (3) being provided with a rotor cover (8) for covering said dynamic damper (10) and a row of rotary blades which are axially spaced from each other and each extend radially outwardly, and a row of stationary blades are provided on the inside of said cylindrical casing (1) in such a manner as to extend radially inwardly, respectively, into the related gaps formed between said row of rotary blades. A turbo-molecular pump of claim 1 or 2 wherein said casing (1) and said rotor (3) are cylindrical, and said rotor cover (8) closes one end of said cylindrical rotor (3).

Inventors: INANAGA, SATOSHI, WATANABE, KATSUHIDE
Applicants: EBARA CORPORATION, EBARA RESEARCH CO., LTD.
Application year: 1989
Main classification: F04D19
IPC codes: F04D29, F04D19, F16F7