US2958454A

US2958454A - Vacuum seal

Info

Publication number: US2958454A
Authority: US
Inventors: David G Hummer
Original assignee: General Dynamics Corp
Current assignee: General Dynamics Corp
Priority date: 1958-03-12
Filing date: 1958-03-12
Publication date: 1960-11-01
Anticipated expiration: 1977-11-01

Description

D. G. HUMMER Nov. 1, 1960 VACUUM SEAL Filed March 12, 1958 FORE PUMP

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United States Patent VACUUM SEAL David G. Hummer, Pittsburgh, Pa., assignor to General Dynamics Corporation, New York, N.Y., a corporation of Delaware Filed Mar. 12, 1958, Ser. No. 721,037

6 Claims. (Cl. 230-1) The invention relates to improvements in vacuum systems and has its chief application, at the present time, in connection with the maintenance of relatively low absolute pressure with a vacuum chamber in which there is a moving part actuated by a motor.

Where it is desired to operate a motor driven moving part, for example, a shutter, in a vacuum, where it is desired to evacuate to a pressure of the order of millimeters of mercury, it has been found that the vapor pressure of the motor windings and the lubricant is so high that if the motor is located within the vacuum chamber, it is not practical to reduce the absolute pressure in the chamber to the degree of evacuation which is desired. Hence, it is not practical to have within the vacuum chamber an electric motor having the usual types of windings and employing a conventional lubricant.

I have discovered a method and apparatus by the use of which conventional motors with conventional windings and conventional lubricants can be employed for the purpose of actuating a moving part, for example, a shaft which which must be located within the main vacuum chamber in order to operate a mechanism therein.

To accomplish this result, I employ, in addition to the main system pump, a fore pump which serves to evacuate an auxiliary enclosure associated with the main vacuum chamber and serving to enclose the motor, there being no connection between said enclosure and the main vacuum chamber except the opening through which a shaft extends from the motor into the main chamber. With this arrangement, provided that a proper seal to prevent excessive leakage from the motor enclosure into the main vacuum chamber is maintained, the perfection of the vacuum in the main chamber will not be seriously impaired by the fact that the vapor pressure of the winding and the lubricant of the motor is substantially higher than the absolute pressure desired in the main vacuum system.

It will be understood that in order to obtain the maximum advantages from the foregoing arrangement, especially where the main vacuum system is to be kept at an extremely low absolute pressure, an efficient seal should be maintained between the shaft and the wall of the opening through which the shaft extends from the motor into the main vacuum chamber. It is true that by means of a fore pump, the motor enclosure can be evacuated down to a pressure of the order of a few microns of mercury, but whereit is desired to obtain a pressure of about 10" millimeters of mercury, or less, in the main vacuum chamber, it is quite desirable to have a highly efficient seal around the motor shaft.

Although various types of seals might be employed for this purpose, I have found it especially advantageous not to depend upon mechanical seal or gland. Instead, I prefer to use a dynamic seal in the form of a pump motivated by the shaft itself, the inlet of which communicates with the main chamber and the outlet of which surrounds the shaft itself so that the discharge of the pump 2,958,454 Patented Nov. 1, 1960 will be directed only into the opening or gland through which the shaft passes.

A pump such as I have last described may be a modiiied form of Siegbahn pump, described on pages 159 inclusive in the Dushman book, Vacuum Technique, published by John Wiley & Sons, 1949. The Siegbahn pump is an inward flow, rotary volute pump having a tapered disc revolving close to the face of a stationary member having a spiral groove in its face, leading from the exterior or periphery of the face towards the axis of the disc and decreasing in its cross sectional area as it approaches the opening through which the shaft passes.

In the drawings accompanying this application,

Figure l is a diagrammatic view of a motor located in accordance with the present invention in an auxiliary enclosure contained within a main vacuum chamber;

Figure 2 is an enlarged view, partly in section, showing the motor enclosure and the principal features of a Siegbahn inward flow mechanical pump;

Figure 3 is an end view taken on the line 33 of Figure 2; and

Figure 4 is a modification of the arrangement shown in Figure 1.

Referring to the drawings, in Figure l, 10 is the main chamber which is evacuated by the system pump 11 through the conduit 12, in the present instances, to an absolute pressure of the order of 10* millimeters of mercury.

Within this main vacuum chamber 10, it may be desired to operate a rotary shutter or other device 13 by the shaft 14. The electrical motor 15' is provided with a seal 16 through which the shaft 14 extends and is mounted in an enclosure or can 17 which is connected by means of a conduct 18 to a fore pump 19. The pipe 18 is welded into a wall of the chamber 10 and into a wall of the can or enclosure 17 so that there is no leakage between the pipe 18 and the main chamber 10. The fore pump 19 is capable of lowering the pressure in the can 17 to about l0 millimeters of mercury.

As has been previously stated, when working at extremely low absolute pressures in the main chamber 10, I prefer to make the seal 16 in the form of an inward flow, volute pump, which in the present instance, is shown as one-half of a pump of the Siegbahn type. This pump is shown best in Figures 2 and 3 of the drawings.

Referring to Figures 2 and 3, it will be observed that the right hand end of the motor can or enclosure is made in the form of a solid plate 20 to which the right hand end of the motor is attached by brackets 21. In the plate 20, there is bored an opening 22 through which the shaft 14 extends from the rotor of the motor into the interior of the main vacuum chamber 10. Outside of the plate 20, there is fixedly mounted upon the shaft 14, a hub 23 carrying a rotor or disc 24, the inner face of which is tapered or bevelled off and terminates in a sharp edge 25 registering substantially with the outer edge. of the stator, which is in the form of a plate 20. The plate 20 is formed with a smooth conical face having the same degree of taper as the taper of the rotary disc 24, there being a very slight space between the two surfaces of the rotor 24 and stator 20, this preferably being of the order of a few hundredths of a millimeter. The outer surface of the stator 20 is made with a spiral or volute groove 26 which, at the outer periphery of the stator, has a cross section of about 10 x 10 millimeters, and the depth of which gradually decreases to about one millimeter as the spiral groove approaches the axis or opening 22.

'It will be understood that the tapered disc 24 is rotated in the same direction as would be taken by a particle of material traveling from the peripheral end of the groove to the axis. There is sufiicient clearance between the shaft and the wall of the opening 22 so that the How of gas evacuated from the main chamber is not restricted In order to obtain the desired pumping action, the motor and shaft 14 should be driven at a speed of from about 4,000 to about 10,000 r.p.m.

In Figure 4 of the drawings, I have shown a slightly modified arrangement in which the motor can or enclosure is made in the form of a chamber projecting from the side of the main vacuum chamber.

Various of the features of the invention believed to be new are set forth in the appended claims.

I claim:

1. A vacuum system comprising means forming a chamber,

a pump connected to said chamber to a low pressure,

a motor having a shaft extending therefrom and into the interior of said chamber,

a closure containing said motor and having an opening therein through which said shaft extends,

a second pump connected to said enclosure for evacuating said enclosure while maintaining in said enclosure a pressure higher than that of the chamber, and

a seal for said opening.

2. A vacuum system comprising means forming a chamber,

a pump connected to said chamber for evacuating said chamber,

a second pump connected to said enclosure for evacuating said enclosure, and

a seal for said opening comprising a pump driven by said shaft and having an inlet communicating with the chamber and an outlet communicating with said opening.

3. A vacuum system comprising means forming a chamber,

a pump connected to said chamber for evacuating said chamber,

a second pump connected to said enclosure for evacuating said enclosure, and

a seal for said opening comprising a volute pump connected to and surrounding said opening.

4. A vacuum system comprising means forming a chamber,

a system pump connected to said chamber for evac uating said chamber to obtain therein an absolute pressure of the order of 10' millimeters of mer- 5. A vacuum system comprising means forming a chamber,

a pump connected to said chamber for evacuating said chamber to obtain therein an absolute pressure of the order of 10 millimeters of mercury,

a motor having a high speed rotary shaft extending therefrom and into the interior of said chamber,

:a second pump connected to said closure for evacuating said enclosure to a pressure of the order of 10 millimeters of mercury,

a seal for said opening comprising a disc on said shaft at one side of said opening,

said enclosure having a plate interposed between the motor and the disc slightly spaced from said disc, surrounding said opening and provided with a spiral groove therein facing said disc, the peripheral space between the plate and the disc communicating with the chamber and the space around the shaft communicating with said opening,

said groove decreasing in cross section as it approaches the axis in the direction of rotation of the disc, whereby said disc and said grooved plate serving as an inward-flow rotary pump drawing from the peripheral space and discharging into the opening, and thereby holding the pressure in the interior of the chamber substantially lower than the pressure within the enclosure.

. A vacuum system comprising main chamber and discharging it through said opening into said enclosure.

References Qited in the file of this patent cury, a.

a motor having a high speed rotary shaft extending UNITED STATES PATENTS therefrom and into the interior of said chamber, 1,586,160 Mauron et al May 25, 1926 l a closure containing said motor and having an open- 1,914,498 Gardner June 20, 1933 I 1 ing therein through which said shaft extends, 2,721,694 Van Atta Oct. 25, 1955 a UNITED STATES PATENT OFFICE CERTIFICATION OF CORRECTION Patent No. a /58,454 November 1, 1960 David G. Hummer,

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 3, line 16, after "chamber" insert for evacuating said chamber my Signed and sealed this. 2nd day of May 1961.

(SEAL) Attest:

ERNEST We SWIDER DAVID L LADD Attesting Officer Commissioner of Patents UNITED STATES PATENT OFFICE @ERTIFICATION OF CORRECTION Patent N0 2,958,454 November 1, 1960 David G, Hummer It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 3, line 16, after "chamber" insert for evacuating said chamber Signed and sealed this 2nd day of May 1961.,

(SEAL) Attest:

ERNEST We SWIDER DAVID L, LADD Attesting Officer Commissioner of Patents