Patent Publication Number: US-6905306-B2

Title: Mechanical kinetic vacuum pump with rotor and shaft

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to mechanical kinetic vacuum pumps. It finds particular applications to vacuum pumps in which the rotor and stator are connected by a shrink-fit or a screw-fit. 
   By definition gaseous ring vacuum pumps, turbo vacuum pumps (axial, radial) and molecular/turbomolecular pumps belong to the class of mechanical kinetic vacuum pumps. They are capable of mechanically transporting within the molecular flow range (pressures below 10 −3  mbar) the gas particles which are to be pumped. Moreover, molecular pumps are also capable of pumping gases within the Knudsen flow range (10 −3  to 1 mbar). Presently employed mechanical kinetic vacuum pumps frequently offer a turbomolecular pumping stage and a downstream molecular pumping stage (compound or hybrid pump), since such pumps are capable of compressing gases up in to the viscous flow range. 
   Pumps of the kind affected here, in particular turbomolecular vacuum pumps are operated at high rotational speeds up to 100,000 rpm. This requires a firm and tight joint between rotor and shaft which meets the requirements regarding rotor dynamics when passing through critical speeds. The joint commonly is provided by a shrink- or screw-fit. The shrink-fit joint is provided by joining the warm rotor and the cooled shaft, in that the shaft is introduced into a bore in the rotor. Generally steel is employed as the material for the shaft, since steel has a relatively high modulus of elasticity. For reasons of rotor dynamics a lighter material, preferably aluminium, is employed as the rotor material. Here aluminium alloys produced by melt metallurgy, e.g., casting, are well proven. However, in the instance of the material pair of steel/aluminium it is difficult to implement a joint between rotor and shaft which is free of backlash and settling since the coefficients of expansion of steel (about 11×10 −6 /K) and aluminium (about 22×10 −6 /K) differ. 
   From U.S. application Ser. No. 09/937,876, it is known to achieve freedom from backlash and settling at the joint between rotor and stator by providing reinforcement rings preventing an expansion of the aluminium rotor which would give rise to backlash. These measures are involved engineering-wise. 
   SUMMARY OF THE INVENTION 
   It is the task of the present invention to create a mechanical kinetic vacuum pump in which a firm joint between shaft and rotor is attained by more simple means. 
   This task is solved by alloying the rotor to have an expansion coefficient which essentially corresponds to the expansion coefficient of the shaft. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating a preferred embodiment and are not to be construed as limiting the invention. 
     The FIGURE is a side-sectional view of a vacuum pump in accordance with the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Aluminium alloys produced through powder metallurgy (for example, through spray forming) are basically known in other applications. These are manufactured such that the melt consisting of the alloy&#39;s constituents is sprayed by nozzles on to a cold surface. Compared to the melt metallurgical manufacture of aluminium materials, e.g., casting, the melt solidifies very rapidly through which the alloy attains a new structure with changed properties. Aluminium alloys manufactured by spray forming, with the principal constituent being silicon, may be so adjusted that their coefficient of expansion corresponds to that of steel. 
   In that there is no or only a slight difference between the coefficients of expansion of shaft and rotor, loosening of the shrink-fit joint between shaft and rotor under the influence of temperatures in the operational state is prevented. Equally a joint offering a reduced shrink-fit tension may be manufactured which is easier to join and which incurs less strain on the material. Also, the bore and shaft can be manufactured with greater tolerances which—just like the more simple joining process—causes manufacture to be less involved and thus less costly. 
   The present invention shall be explained in the following with reference to the pump of the kind affected here depicted in the FIGURE. The pump depicted has an outer housing  1  with a central bearing sleeve  2  penetrating inside the housing. The bearing sleeve  2  supports the shaft  3  by means of a spindle bearing arrangement  4 . Drive motor  5  and the rotor system  6 ,  7  are linked by shaft  3 . 
   The single-piece rotor has two rotor sections  6  and  7  differing in design. Rotor section  6  is cylindrical in shape with a smooth outer and inner surface  8 ,  9 . In the area of the surface  8 , the housing  1  is equipped on its inside with a thread  10 , thus at the same time forming the stator of a screw pumping stage. The surface  8  and the thread  10  are the active pumping surfaces of a screw pumping stage which is basically known and which pumps molecules entering into the pumping slot  11  towards the direction of the outlet  12 . 
   In the area of the inner surface  9  of the rotor section  6  the outside of the bearing sleeves  2  is equipped with a thread  13  and thus forms the stator of a further screw pumping stage. The thread  13  and the inner surface  9  are the active pumping surfaces of the further screw pumping stage with the pumping slot  14 . The gases being pumped from the bottom to the top through pumping slot  14  flow to the outlet  12  through bores  15  in the bearing sleeve  2 . 
   Located upstream of the screw pumping stage  8 ,  10  is a further pump stage. This has a rotor section  7  which consists of a cone-shaped hub component  23  and the ridges or vanes  24 . These ridges  24  form, with the stator wall  25  surrounding them in housing  1 , a pump stage  7 ,  25 . Gas molecules entering between the individual ridges  24  or into the slot  26  are pumped by the pump stage  24 ,  25  in the direction of pumping slot  11  of the molecular pumping stage  6 ,  10 . 
   The shaft  3  carries the rotor section  7  which in turn carries the rotor section  6 . The cylindrically shaped rotor section  6  may, but not must, consist of the same material as for rotor section  7 . The employment of cylinder sections containing carbon fibres, for example, in the rotors of molecular pumps is also possible. The joint between shaft  3  and rotor section  7  is produced by a shrink-fit. 
   If the shaft  3  is fabricated of steel and the rotor system  6 ,  7 —or at least rotor section  7 —is fabricated of the alloy in agreement with the present invention, then the coefficients of expansion of shaft  3  and rotor  6 ,  7  are equal or almost equal. Even in the instance of high temperature loads on the rotor, which occur in particular when employing the pumps affected here in the semiconductor industry, a secure joint of rotor and shaft is ensured. 
   Materials of the types according to the present invention are being offered on the market under the names of DISPAL (DISPAL A/S 230, DISPAL S241, A and S 250, for example). Besides aluminium they contain 16 to 22 percent in weight silicon as the main constituent as well as other alloy constituents like iron, nickel, copper, magnesium, and/or zircon at shares of between 0.3 and 8 percent in weight. 
   In a material of comparable properties, a different light material namely magnesium may be present instead of the aluminium base material. Thus the advantage detailed for alloys based on powder metallurgy may be also utilised for alloys based on magnesium. The coefficient of expansion may be adjusted through suitable additional constituents like Si, for example. 
   The invention has been described with reference to the preferred embodiment. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.