US 3826588 A
In a turbomolecular vacuum pump which has axially spaced pump stages each formed of a stator blade wheel and a cooperating rotor blade wheel, the pitch angle of the blades in at least one pump stage increases in a direction radially outwardly from the blade ends attached to the pump rotor or pump stator for maintaining the compression ratio in that pump stage at least approximately constant in the radial direction.
Description (OCR text may contain errors)
United StatesPatent [1 1 Frank [111 3,826,588 [451 July 30, 1974 TURBOMOLECULAR VACUUM PUMP  Inventor: Riidiger Frank, Cologne, Germany [731 Assigneer igae drlis aeve-Ysma lv GrnbH, Cologne-Bayental,
Germany  Filed: June 19, '1973  Appl. No.: 371,372
 Foreign Application Priority Data June 19; 1972 Germany 2229724 521U.s..c1. ..415/90,4l5/199R s1 Int.Cl. F0ld 1/36, F03b 5/00, FOld 1/02  FieldoiSearch ..415/90,93, 103,101,97;
 I References Cited UNITED STATES PATENTS 3,748,055 7/1973 Becker 415/90 Primary Examiner- Carlton R. Croyle Assistant ExaminerLouis .l. Casaregola Attorney, Agent, or Firm-Spencer & Kaye [57 1 ABSTRACT In a turbomolecular vacuum pump which has axially spaced pump stages each formed of a stator blade wheel and a cooperating rotor blade wheel, the pitch angle of the blades in at least one pump stage increases in a direction radially outwardly from the blade ends attached to the pump rotor or pump stator for maintaining the compression ratio in that pump stage at least approximately constant in the radial direction. I
4 Claims, 9 Drawing Figures PATENTED 3.826.588
SHEU 1 BF 2 FIG. 1
sum 2 or 2 FIG. 50
BACKGROUND OF THE INVENTION This invention relates to a turbomolecular vacuum pump of the type that has a stator to which there are affixed stator blade wheels and a rotor carrying rotor blade wheels arid in which the stator blade wheels are in an interleavingrelationship with the rotor blade wheels. The pitch angle of the blades, defined by their inclination with respect to the plane of the blade wheels, changes in the radial direction. The blade wheels of the rotor which may be driven with a high rpm, cooperate as runner wheels in a turbine-like manner with the stationary blade wheels of the stator which is affixed to the pump housing.
In turbomolecular vacuum pumps of the aforeoutlined type various difficulties are encountered in the selection of appropriate blading. Because of the high rpms which are often in excess of 40,000, the arrangement has to be considered in the first place from the point of view of sufficient strength with respect to the centrifugal forces generated during operation. Further,
to ensure a sufficient suction output, it is necessary to avoid internal losses in the flow path of the gas moleculesbetween the pump stages each formed of a coopcrating rotary and stationary blade wheel. For improving the mechanical strength in a simplified manufacture of these structures, it has already been known to make straight-bladed blade wheels in such a manner that the pitch angle defined by the surface of the blade and the plane of the blade wheel decreases from the location of attachment of the bladeat the rotor to the edge of the blade. In this manner the stress imparted by the centrifugal forces is sought to be reduced.
An examination of the flow conditions within one pump stage shows that the suction output and the compression ratio are essentially functions of the circumferential velocity, the pitch angle and the-degree of overlap (that is, the ratio of the distance between two blades of the same wheel to the blade width). Since, however, the circumferential velocity linearly increases along the wheel radius, there are obtained different values of suction output and compression ratio along the length of the blades belonging to the same pump stage. Consequently, in the peripheral zones there prevails a higher suction output and a higher compression ratio, causing a pressure equalization flow towards the zone where the blades are attached and where the circumferential velocity is smaller. In case of relatively long blades of the type which have to be used in the high vacuum stage for obtaining a sufficiently large suction gap, the difference between the circumferential velocitiesand its effect on' the pump characteristics are no longer negligible. The equalization flow between the zone of a relatively high compression ratio at the blade edge and the zone ofa relatively small compression ratio at the blade foot (locus of attachment) hinders the obtainment of an optimal pump effect.
SUMMARY OF THE INVENTION It is therefore an object of the invention to provide an improved turbomolecular vacuum pump in which an internal equalization flow normally generated by the different compression ratios radially along the blades is prevented and the suction output of the pump is improved.
This object and-others to become apparent as the specification progresses are accomplished by the invention according to which, briefly stated, the pitch angle of the blades in at leastone stage formed of a rotor blade wheel and a stator blade wheel increases radially outwardly from the attached end of the blades in such a manner that the compression ratio of that pump stage remains at least approximately constant at radially different locations. Such an arrangement prevents the generation of undesired equalization flows and improves the efficiency of the pump.
BRIEF DESCRIPTION OF THE DRAWINGS along the cylinder planes I, II and III, respectively, of
FIG. 1, of three circumferentially successive blades belonging to the same rotor blade wheel.
FIGS. 5a and 5b illustrate plan views of parts of rotor blades with different blade distances.
FIGS. 6, 7 and 8 illustrate developed sections as in FIGS. 2, 3, and 4, wherein the blade widths are radially constant.
DESCRIPTION OF THE PREFERRED EMBODIMENTS Turning to FIG. 1, there is illustrated a stator 1 which is rigidly affixed to the pump housing and which surrounds a rotor 2'supported for rotation about a vertical axis 6. To the stator 1 there are affixed stator blade wheels 3 whereas the rotor 2 is provided with rotor blade wheels 4. To the housing, 1, at the high vacuum side of the pump, there is attached a coupling flange 5. Further technical details of this pump with regard to drive means, seals and rotor supports are considered conventional and are described, for example, in Garnier et al., US. Pat. No. 3,168,977.
Turning now to FIGS. 2, 3, and 4, it is seen that along the cylinder surface I (FIG. 2) the blades have, at their foot zones (that is, in the vicinity of their attachment) the small pitch angle a, of approximately 20; along the cylinder surface II (FIG. 3) the same blades have, approximately in their mid zone, an increased pitch angle a of approximately 35; and along the cylinder surface III (FIG. 4) the same blades have, in their peripheral end zones a maximum pitch angle a, of approximately 45. In order to achieve, in the structure of the illustrated embodiment, a degree of overlap (which is expressed by the ratio s:b, that is, the ratio of the distance s between two adjacent blades of the same blade wheel to the blade width b) which is approximately constant in the radial direction, the blade width b is increased radially outwardly (that is, b b b because of the radially outward increase of the blade distance s (that is, s s s It is to be understood, however, that the blades may have other shapes wherein the blade widths are radially constant.
In some instances it may be preferred to so construct the pump according to the invention that at least the blades of a high vacuum stage are, at least in their peripheral zone, not in an optical overlap, that is, s/b 1.
While maintaining the compression ratio constant in a radial direction according to the invention, it may be expedient to increase the pitch angle with respect to known embodiments and thus achieve an additional increase of the suction capacity.
In a further expedient embodiment of the invention only the high vacuum side stages of the turbomolecular vacuum pump are provided with radially changing blade pitch angles and thus with radially constant compression ratios. It has namely been found that in subsequent pump stages (viewed in the direction of flow), due to the precompression that already has set in, an arrangement according to the invention is less effective so that in those subsequent pump stages it may be sufficient to provide a conventional rotor and stator blading having radially changing compression ratios.
The change of the blade pitch angle may expediently be continuous at least along subsequent length portions in the radial direction.
FIG. A illustrates a part of a rotor blade having continuously changing blade distances s s and s and blade widths b b and b As it is observable in this fig ure, the dimensions s and b that have identical index numbers are measured along the same circle concentric with the blade wheel. For particular purposes, however, the blades may be advantageously made in such a manner that the change of the pitch angle is periodically discontinued. This may be accomplished, for example, by twisting the blade in different pitch angle zones. Such a rotor blade is shown in FIG. 512. Between the lines A, B, C, and D are zones with different in each zone constant pitch angle. The number of zones can be changed. Preferably both the stator and v the rotor blade wheels have radially outwardly increasing pitch angles.
,By using a blade structure according to the invention, there is accomplished, by virtue of a lowering of the compression ratio in the middle and peripheral zones of the blades, an elimination of the undesirable equalization flow of the fluid medium and there is thus achieved a significant improvement of the suction capacity.
The blade wheels may be made of circular, radially slotted stamped sheet metal according to known manufacturing methods.
It is to be understood that the invention is not limited to the single-flow turbomolecular vacuum pump with vertically arranged rotors in accordance with the described embodiment, but it may also find application in known two-flow structures in which the rotor in general is arranged to be rotated about a horizontal axis.
It will be understood that the above description of the present invention is susceptible to various modifications, changes and adaptations, and the same. are intended to be comprehended within the meaning and range of equivalents of the appended claims.
' 1. In a turbomolecular vacuum pump having an axis, a high vacuum side, a stator, a plurality of axially spaced stator blade wheels carried by said stator, each stator blade wheel being formed of a plurality of circumferentially spaced stator blades each having one end attached to said stator and each extending in a radial direction with respect to said axis, a rotor arranged axially coextensive with said stator, a plurality of axially spaced rotor blade wheels carried by said rotor, each rotor blade wheel being formed of a plurality of circumferentially spaced rotor blades each having one end attached to said rotor and each extending in a radial direction, each blade having a pitch angle defined by the inclination of the blade to the plane of the blade wheel, said stator blade wheels being in an interleaving relationship with said rotor blade wheels, each stator blade wheel forming a pump stage with an adjacent, cooperating rotor blade wheel, the improvement comprising the arrangement of the blades in at least one pump stage, wherein the pitch angle of the blades in said at least one pump stage increases in a direction radially outwardly from their respective attached end for maintaining the compression ratio in said at least one pump stage at least approximately constant in the radial direction.
2. A turbomolecular vacuum pump as defined in claim 1, wherein solely the blades of the pump stages at the high vacuum side of the pump have radially varying pitch angles.
3. A turbomolecular vacuum pump as defined in claim 1, wherein said radially outwardly increasing pitch angle varies continuously at least along subsequent length portions of the blade.
4. A turbomolecular vacuum pump as defined in claim 1, wherein the degree of overlap defined by the ratio of the distance between corresponding points on adjacent blades of the same blade wheel as measured along a circle concentric with said blade wheel, to the width of one of said blades as measured along said concentric circle, is approximately constant in the radial direction in at least one pump stage.