US3924962A

US3924962A - Molecular pumps of the drum type

Info

Publication number: US3924962A
Application number: US503126A
Authority: US
Inventors: Louis Maurice
Original assignee: Alcatel CIT SA
Current assignee: Alcatel CIT SA
Priority date: 1973-09-14
Filing date: 1974-09-04
Publication date: 1975-12-09
Anticipated expiration: 1992-12-09
Also published as: BE819441A; SU580850A3; CH592818A5; DE2443727A1; JPS5055911A; IT1020719B; FR2244370A5; NL7412212A; GB1473713A

Abstract

Molecular pump of the high-vacuum type provided with a cylindrical drum on which helical grooves having a depth which decreases from the suction end to the discharge end, are cut, separated by low walls whose thickness increases from a point where the compression ratio becomes sufficiently high to cause great lateral leakages. Channels for draining that flux, blind on the suction side, are cut in these walls.

Description

Umted States Patent 11 1 1111 3,924,962

Maurice Dec. 9, 1975 [5 MOLECULAR PUMPS OF-THE DRUM TYPE 1,448,079 3/1923 Noeggerath 415/90 1,869,106 7/1932 Marchant 415/76 [75] Pans France 1,902,439 3/1933 Foss 415/76 [73] A i compagnie l i u des 3,794,449 2/1974 Brouwer 415/90 Telecommunications l, FOREIGN PATENTS OR APPLICATIONS 1 Pans France 1,213,459 11/1959 France 415/71 [22] Filed: Sept. 4, 1974 Primary ExaminerHenry F. Raduazo [21] Appl' Attorney, Agent, or FirmSughrue, Rothwell, Mion,

Zinn & Macpeak [30] Foreign Application Priority Data Sept. 14, 1973 France 73.33097 [57] ABSTRACT Molecular pump of the high-vacuum type provided [52] US. Cl. 415/90 with a cylindrical drum on which helical grooves hav- [51] Int. Cl. F03D 5/00 ing a depth which decreases from the suction end to [58] Field of Search 415/71, 73, 74, 75, 76, the discharge end, are cut, separated by low walls 415/90 whose thickness increases from a point where the compression ratio becomes sufiiciently high to cause [56] References Cited great lateral leakages. Channels for draining that flux,

UNITED S S PATENTS blind on the suction side, are cut in these walls.

1,053,637 2/1913 OReilly 415/76 4 Claims, 4 Drawing Figures MOLECULAR PUMPS OF THE DRUM TYPE BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention concerns an improvement to drum type molecular pumps.

2. Description of the Prior Art It is known that molecular pumps constituted by a cylindrical drum rotating at a high speed with a slight clearance inside a stator having a cylindrical face with the same axis of symmetry as the drum have been produced for nearly 50 years. On one of the two cylindrical walls facing each other, a thread has been cut in the form of a helix whose cross-section decreases from the part subjected to the highest vacuum towards the zone where the vacuum is slightest. The discharge of these first pumps reached only a few liters per second and that type of pump was abandonned for a certain time.

About years ago, a great improvement was made by cutting on the internal cylindrical face of the stator or on the cylindrical surface of the rotor, several parallel threads. It was thus made possible to produce pumps with multi-thread drums supplying a discharge in the order of 100 liters per second while remaining within reasonable limits of bulk. Nevertheless, when that type of pump is required to supply a high discharge, the compression ratiois lowered in a prohibitive may and, that type of pump loses all its advantages.

Very recent tests have shown that according to theory, there is, for a given active surface of a pump, having optimum characteristics, an opposition between the characteristic of the discharge and that of the compression ratio. It has thus been observed that for a given active surface defined by the diameter and length of the drum and for a determined rotation speed, a multithread pump according to the art known up till then, actually gives a nitrogen discharge of 120 liters per second but with a compression ratio of 10", this clearly being insufficient. It has also been observed that if the pump is designed to provide a compression ratio in the order of 10 for nitrogen, its discharge is no more than 40 liters per second.

It has therefore been observed that for a given gap between the drum and the stator, it is impossible, with the known art hereabove, to obtain high compression ratios for a pump designed, moreover, for a high discharge. That impossibility is explained by the increase in the leakages between two parallel threads in the part of the pump where the discharge is effected and in which the compression ratio per unit length of drum reaches a high value.

To reduce these leakages, various measures have been considered.

The first consists in reducing the clearance between the rotor and the stator, but that measure leads to the considerable increasing of the cost of machining and reduces appreciably the operational safety of such a pump. Indeed, the least foreign body, the least expansion of a fixed or rotating part and any mechanical deformation cause irremediable seizing of the pump, as happened during the manufacturing of the first pumpt of the Holweck type.

Another solution allowing a gap which may easily be formed between the drum and the stator, makes it possible, nevertheless, to reduce the leakages and to improve, consequently, the compression ratio without reducing the discharge. It has been experimented recently by the Inventor. It consists in increasing the number of threads from the suction end towards the dischargeand by subdividing each of the grooves comprised between two successive low walls into several separate channels. That method leads therefore to the producing of a high vacuum molecular pump comprising a cylindrical drum having several helical grooves cut at least on one of the faces opposite to each other according to the technique known since fifteen years ago, but according to that solution, each groove is subdivided over a part of its length at a certain distance from the input, into several narrower channels, by low parallel walls. As they progress on the drum from the suction end to the discharge end, that is, from the high vacuum zone to the primary vacuum zone, the number of grooves is increased in the zones where the compression ratio is sufficiently high for the leakages to become appreciable. A greater number of grooves is therefore observed on the discharge side than on the suction side. As the thickness of the low walls between the channels remains substantially constant, there is, according to this method, an increase in the total space kept for the low walls going from the suction end towards the distance end. In this way, from an overall point of view, the transversal impedence afforded to the fluid between the various parallel channels is increased and consequently, the leakages are reduced in the same proportions.

Summary of the Invention Lastly, taking approximately equivalent considerations as a basis, the Inventor has perfected a new method making it possible to maintain a high compression ratio while allowing a greater gap between the rotating part and the stator entailing a very slight lowering of the discharge. A satisfactory performance is thus obtained from the point of view both of the discharge and of the compression ratio while simplifying the structure of that type of pump, since an increase in the clearance between the fixed part and the rotating part is allowed, this simplifying appreciably the machining of pumps according to the invention without weakening noticeably the performances thereof.

The object of the invention is therefore a high vacuum molecular pump comprising a cylindrical drum rotating at high speed with slight play in a stator whose internal face is cylindrical, comprising several helical grooves cut in one or the other of the faces opposite to each other, which are parallel to each other, separated by low walls which end at the output of the pump where the discharge is effected, characterized in that on the one hand, from the suction end to the discharge end, the width of the grooves decreases whereas the thickness of the low walls increases and in that, on the other hand, grooves which are blind on the suction side but open on the discharge side are cut starting from a certain distance from the suction end in the thickness of the low walls.

The improvement achieved due to the invention will be more easily understood on referring to the detailed description of two embodiments given with reference to the accompanying drawings in which:

Brief Description of the Drawings FIG. 1 is a developped view of a groove cut in the rotor (or the stator) according to a first variant of the invention;

rotor or the stator according to another variant of the invention.

Description of the Preferred Embodiment On referring to FIG. 1, showing a developped view of a groove cut in the rotor according to the first variant of the invention, the two low walls limiting the groove 3 on the suction side 4 may be seen at l and at 2, these two low walls extending parallel to each other over a certain length, for example half the height of the drum, up to the point 21 where the compression ratio becomes sufficiently high for a great leakage of fluid to occur between the groove 5 and the groove 3 separated by the low wall 2. The low wall 2 is then extended towards the left and enlarged. A groove 8 parallel to the groove 3 but blind on the suction side is cut in that enlarged low wall 7. That groove will scavenge a part of the leaks which occurred between the groove 5 and the groove 3 over the low wall 2 and the groove 8 will canalize the flow which it scavenges towards the discharge end 9. At the last third of the height of the groove, the enlarged low wall 7 is prolonged laterally by a portion, itself enlarged, provided with a blind groove 1 1, limited towards the left by the low wall 12. In this way, on the discharge side 9, the original groove 3 is replaced by a groove 14 which is clearly narrower, limited by the low walls 1 and 12, whereas two

blind grooves

8 and 11 on the suction side 4 lead out from the discharge side 9. The width of the low wall 2 is defined by the distance measured between the right-hand edge of the groove 3 and the left-hand edge of the groove 5.

FIG. 2 makes it easier to understand the development of the thickness of the grooves.

FIG. 2a shows more particularly a cutaway view of FIG. 1 through AB. The great height of the low walls 1 and 2 limiting the main groove 3 in the vicinity of the suction end will be observed.

0n referring to FIG. 2b corresponding to a cutaway view of FIG. 1 in the vicinity of the discharge end 9, it will be observed that the depth of the main groove 3 has considerably decreased and that the

grooves

11 and 8 which are blind on the suction side and open on the discharge side have an even slighter depth, their function being to effect the draining of the flow passing through the play 15 between the low wall 2 and the smooth cylindrical wall 16 of the stator.

In certain cases, for easier machining, the same depth is however imparted to them as to the main groove 3.

FIG. 3 shows a second example of embodiment comprising, as in the case of FIG. 1, on the suction side 4, a groove 5 and a groove 6 separated from each other by the low wall 2. The main groove 3 is limited, as previously, by the low walls 1 and 2.

The development of the low wall 2 in question is effected more progressively, starting from the point 21 then along the drum where the compression ratio is sufficient for a leakage to occur between the flow running along the groove 6 and the main groove 3 over the low wall 2.

The low wall 2 thick end progressively starting from the point 21 up to the point 22 situated downstream in the vicinity of the discharge end.

That low wall 23 which becomes progressively thicker comprises, as previously, two blind grooves 8' and 10' fulfilling, as previously, the function of draining channels.

Although the devices which have just been described appear to afford the greatest advantages for implementing the invention in the state of the art, it will be understood that various modifications entailing the implementing of elements fulfilling the same technical functions, may be made to the devices described hereinabove, without going beyond the scope of the invention.

I claim:

1. In a high vacuum molecular pump comprising a cylindrical drum rotating with a slight clearance in a stator whose internal face is cylindrical, comprising several parallel helical, main grooves separated by low walls and cut in at least one of the opposite cylindrical faces, wherein; the improvement starting from a certain point, from the suction end to the discharge end, on the one hand the thickness of the low walls increases and on the other hand, grooves which are blind on the suction side and open on the discharge side are cut in the thickness of the low wall.

2. The high vacuum molecular pump according to claim 1, wherein: on the suction side, each of the grooves has a greater depth than on the discharge side and the grooves which are blind on the suction side have a depth which is at the most, equal to that of the main grooves.

3. The high vacuum molecular pump according to claim 2, wherein: the thickness of the low walls increases suddenly starting from a certain point where the lateral leakages become large and, then subsequently undergoes a further sudden increase, downstream from that point.

4. The high vacuum pump according to claim 2, wherein: the thickness of the low walls increases progressively starting from a certain point where the lateral leakages become large.

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