US3057546A

US3057546A - Rotary vacuum pumps

Info

Publication number: US3057546A
Application number: US711861A
Authority: US
Inventors: Power Basil Dixon; Runki Alfred Aristides
Original assignee: Edwards High Vacuum Ltd
Current assignee: Edwards High Vacuum Ltd
Priority date: 1957-02-07
Filing date: 1958-01-29
Publication date: 1962-10-09
Anticipated expiration: 1979-10-09

Description

Oct. 9, 1962 B. D. POWER ETAL 3,057,546

ROTARY VACUUM PUMPS Filed Jan. 29, 1958 26 H542 FIG.3 BASIL DPGWER ALFRED A. Rum"! I INVENTORS Patented Oct. 9, 1962 3,057,546 RUTARY VACUUM PUMP Basil Dixon Power and Alfred Aristides Runlri, Langley Green, (Jrawley, England, assignors to Edwards High Vacuum Limited, (Irawiey, England, a liritish company Filed .lan. 29, W58, er. No. 711,861 Claims priority, application Great firitain Feb. 7, 1957 8 Claims. (Cl. 230-407) This invention relates to rotary vacuum pumps employing oil as a lubricating and/or sealing medium and incorporating a reservoir, or the equivalent, for oil storage.

It is a characteristic of such pumps that, when they are stopped under vacuum, oil is sucked from the reservoir through the oil feed ducts, and also past the pump outlet valve if that valve is an oil covered and non sealing type, the oil then passing into the system being evacuated where it may cause serious contamination. When the reservoir has been sufiiciently emptied, the oil is followed by atmospheric air which may scatter the oil about the system being evacuated and finally raise the pressure in the system to atmospheric.

Many methods have previously been suggested to overcome the difliculty referred to but they suffer from the disadvantage, either of adding to the expense of the pump out of proportion to the degree to which they overcome the problem, or they only partially solve the problem.

A possible arrangement or system for preventing oil suck-back is to draw the oil from the reservoir and to drive it along a single main oil way to the pump by means of a positive displacement oil pump driven by the prime mover of the vacuum pump itself. The main oil way is closed by a spring or gravity closed shut off valve when the pumps are not in action and this valve must be capable of resisting an atmospheric pressure difference without leaking. When the pumps are started however the oil pump is able to produce a high enough oil pressure in the main oil way to force open the valve and to supply oil to the vacuum pump. The oil flow required by the vacuum pump may vary greatly with operating temperature and with the tolerance of an individual pump and it is therefore necessary to select an oil pump capable of supplying an excess of oil at all times and to provide a spring or gravity loaded by-pass valve from the pressure side of the oil pump back to the reservoir to accommodate the excess flow of oil. The by-pass valve must be adjusted to open always after the main valve has been forced open, but it is usually undesirable to set it to open at a pressure greatly exceeding that at which the main valve operates, both because the load on the small oil pump is increased and also because it is often undesirable for the oil feed to the vacuum pump to be supplied at too high a pressure. The system described can provide a complete safeguard against oil suck-back, but it is both complex, involving, as it does two adjustable valves and the adjustment necessary to ensure that the valves open always in the right sequence and that the oil feed to the vacuum pump is always supplied at the most suitable pressure may be complicated and sometimes a little uncertain.

The object of the present invention is to provide a system for preventing oil suck-back having the merits of the system referred to, without its defects.

According to the present invention, a positive displacement oil pump preferably driven by the same prime mover as the main vacuum pump and preferably built in as an integral part of the main pump, is arranged to draw oil from the reservoir and to pump it along a main oil way to a single composite control valve constructed and arranged to permit the supply to all the various oil ways leading to the parts of the pump requiring oil from the main oil way at a desired steady pressure whatever the operating condition and to by-pass back into the main reservoir any excess oil delivered from the main oil way.

According to a feature of the invention, the composite control valve is constructed so that, when the oil pump and the vacuum pump are stopped due to any cause, the interior of the vacuum pump is sealed both from the main oil way and from the oil reservoir with the result that, provided an outlet valve of a sealing type is used as the exhaust valve of the main vacuum pump, neither oil nor air can be forced back into the main vacuum pump, not through the latter into the system to be evacuated.

According to a further feature of the invention, certain parts associated with the composite valve are so constructed that, should the valve fail to seal securely due, for example, to the presence of dirt or grit between the sealing faces, a minimum quantity of oil and, thereafter, only air will be sucked back through the valve into the vacuum system, thus ensuring minimum contamination of the vacuum system even if it should be raised to atmospheric pressure.

One form of vacuum pump embodying the invention and incorporating an oil suck-back reducing or eliminating valve and an alternative form of valve, will now be described in greater detail by way of example with reference to the accompanying drawings, in which:

FIGURE 1 is an elevation, in section, of such a pump;

FiGURE 2 is a view of the valve to an enlarged scale; and

FIGURE 3 shows one alternative form of valve.

Referring to FIGS. 1 and 2 of the drawings, a rotary vacuum pump 1 is mounted within a tank of main reservoir 2 containing oil, the normal level of which is indicated by the line 3. The rotor 4 of the pump is mounted on a shaft 5 which carries a driving pulley wheel 6. The front bearing block 7 of the pump shaft is formed with a recess or bore 8 which is slightly eccentric with respect to the centre of the main bearing indicated at 9. The walls of the bore 8 constitute the stator of a small oil pump P of the well known vane type. The rotor 10 of the oil pump P is mounted on the main vacuum pump shaft 5 and is driven thereby, and a pair of blades ltib are fitted in known manner in slots of the rotor 10 the blades being spring urged against the inner wall of the bore 8 and serving to sweep out a crescent shaped pumping chamber as the shaft 5 rotates. Any other convenient type of small oil pump may be used.

Concentric with the bearing 9 is a further recess 11 which accommodates an elastomer O ring 12 to seal off the oil pump from the main or vacuum pump. A rectangular duct 13, formed by a milling cut in the front face of the bearing block 7 and completed by the rear surface of the front plate 14 of the tank reservoir, communicates with the oil in the reservoir providing the intake 13!) and outlet of the small oil pump. An 0 ring is accommodated in a recess in the plate 14 and clamped in position by a screw held plate 16, seals the oil pump from-the outside.

The upper part of the duct 13 which serves as the outlet of the oil pump, communicates through a main oil way 13a with a circular recess 17 indicated by an arrow lead line and forming a small chamber which constitutes a container of predetermined limited capacity which lies higher than the oil level in the reservoir and which is formed in a head box 17a at the upper end of bearing block 7. From this chamber the main pump receives all its lubricating and sealing oil by suitable oil ways.

The recess 17 has a flat bottom formed with two

concentric grooves

18 and 19. An elastomer disc 20 forming a closure member is placed over the

grooves

18 and 19 and is covered by a metal plate 21 of the same size. A spring 22 held in place by a screw 23 keeps the elastomer disc, with the aid of the metal disc, pressed evenly against the flat bottom of the recess. It will be seen from FIGURE 2 that the clamping screw 23 is formed with a shoulder or step 24 which provides a definite stop position limiting the downward travel of the screw. A second step 25 is formed by deeply undercutting the screw as shown, the step 25 serving to clamp the disc 20 near its centre but with a controlled amount of compression.

The groove 18 communicates with the duct 13 on the discharge side of the oil pump via the main oil way 13a and the groove 19 communicates with the oil ways 26 supplying the main vacuum pump. The assembly comprising the recess 17, the

grooves

18, 19, the discs 20, 2.1, the spring 22 and screw 23, together constitute a composite control valve.

When the vacuum pump is started the gas being exhausted enters the main pump through an inlet spout 27 and the oil pump will immediately start to deliver oil into the groove 18. After this groove is filled with oil, the pressure of the oil will raise the elastomer disc 20 and the oil will flow over and into the outer groove 19. The oil pump is designed so that it will deliver more oil than is required for the main pump. Thus the outer groove 19 will also fill up and overflow through a circular gap formed between the periphery of the elastomer disc 20 and the bottom of the recess 17 by the lifting of the disc, although a certain amount of the oil is constantly sucked away by the main pump.

As the groove 19 is very close to the periphery of the elastomer disc which is forced downward by the spring 22, and as only a light spring is normally employed the oil is only under slightly higher pressure than atmospheric. The excess oil not required by the main pump will fill up the whole of the recess 17 and eventually flow over its rim and back into the main reservoir of the pump.

If during operation the main pump be stopped for any reason, the oil supply to the inner groove 18 will cease, while the vacuum of the main pump will cause the elastomer disc 20 to be forced down by atmospheric pressure on to the outer groove 19, thus closing the way for further oil and, consequently for any air to enter the main pump, provided the exhaust valve 28 of the main pump is of the sealing type. The return of the elastomer disc 20 to the closed position is accelerated by the spring 22. The oil remaining above the elastomer disc 20 after closing, acts as sealing fluid.

If for any cause, such as the presence of foreign matter such as dirt or grit or imperfection of the elastomer valve disc 20, this should fail to seal completely, then the only oil to enter the pump will be the small quantity contained in the recess 17 and as soon as this recess is empty, air will enter the pump. The very small quantity of oil from the recess will fall to the bottom of the pump stator, and only air will get through to the system beyond the pump.

It is of course desirable that the outlet valve arrangements be generally as illustrated diagrammatically, the valve 28 being of the sealing type and surrounded by a low wall 29 to form a chamber or the equivalent so that, in the case of the valve failing to seal for any reason only the small amount of oil contained in the chamber at the level indicated by the line 30 will be sucked into the pump. This small amount of oil will not be sufficient to fill up the pump and only air will get through to the system beyond the pump.

In the alternative form of oil suck-back preventing valve shown in FIGURE 3, a central recess 31 replaces the groove 18 of FIGURES 1 and 2 and the disc 20 may be free under a plate 32 which replaces the disc 21 of FIGURES 1 and 2, or the disc 20 may be bonded, clamped or otherwise secured to the plate 32. The function of the screw 23 and spring 22 combination shown in FIGURES 1 and 2 is, in FIGURE 3, performed by a spring 33 held under compression by a bridge piece 34 formed with a hole mid-way of its length and through which a stem 35 projects vertically from the plate 32 to permit raising and lowering of that plate and with it the disc 20 under varying oil pressure. The plate 34 is secured in position by screws 36.

The device affording protection against oil suck-back in the system described will operate whenever the pump stops for any cause and will exclude oil from the pump entirely while leaving both the pump and the system being evacuated by it under vacuum. Further the protective device will ensure that whenever the vacuum pump is operating, oil is fed to it always at the most suitable pressure which usually is at or a little above atmospheric pressure, whatever the temperature or viscosity of the oil used. Again, no adjustment or critical manufacture is necessary to ensure that any valves open and close in a desired sequence and, finally, it Will be seen that the device is compact and may be made cheaply enough to build into the smallest vacuum pump without undue increase in price.

We claim:

1. A vacuum pumping device, comprising, in combination:

(a) a housing for containing a quantity of lubricating and sealing oil;

([1) a small open head box disposed above the normal oil level in said housing and constituting an oil container of predetermined limited capacity;

(c) an oil pump in said housing having an intake arranged to draw oil from said housing and an outlet terminating in a main oil way delivering to said head box;

(d) a rotary vacuum pump in said housing including oil way means connected for receiving lubricating and sealing oil from said head box and for delivering such oil to moving parts of said vacuum pump; and

(e) valve means responsive to the delivery of oil from said oil pump to said head box for controlling comcunication between said head box and both said oil way means and said main oil way, and closing communication between said head box and both said oil way means and said main oil way in the absence of delivery of oil from said oil pump to said head box, and opening communication therebetween by the delivery of oil at predetermined pressure from said oil pump to said head box.

2. A device as defined in claim 1 wherein said head box has a bottom wall, said oil way means comprises an annular oil groove in said bottom wall, said main oil way enters said head box through said bottom wall in inwardly spaced relation to said annular oil groove, and said valve means comprises a closure member which in closed position interrupts communication between said oil groove and said head box, between said oil groove and said main oil way, and between said main oil way and said head box, and which is movable by the pressure of oil being delivered through said main oil way to open such communication.

3. Vacuum pumping means as defined in claim 2 and which comprises resilient means yieldingly pressing said closure member toward its closed position.

4. Vacuum pumping means as defined in claim 2 wherein said bottom wall is flat and said main oil way terminates in a recess in said bottom wall concentric with said annular oil groove, and said closure member is in the form of a flat sealing element which in closed position sealingly contacts said bottom wall in closing relation to said annular groove and said concentric recess.

5. Vacuum pumping means as defined in claim 4 in which said fiat sealing element is carried by a spring loaded rigid plate.

6. Vacuum pumping means as defined in claim 4 in which said closure member is annular, said concentric recess is annular, and a supporting post is carried by said bottom wall in concentrically spaced relation inwardly of said annular recess and which passes through said annular closure member.

7. Vacuum pumping means as defined in claim 6 in which said annular closure member comprises an annular sealing member and an annular rigid plate, said annular scaling element extends inwardly of said annular rigid plate, and said supporting post comprises a stop overlying and clamping the inner portions of said sealing member to said bottom wall inwardly of said annular recess.

8. Vacuum pumping means as defined in claim 7 in which a coil spring surrounds and is supported by said posts and bears against said rigid plate.

References Cited in the file of this patent UNITED STATES PATENTS Machlet Dec. 8, Case Feb. 5', Cuthbert Aug. 14, Money Nov. 29, Stoltz Sept. 17, Wishart June 17, Freeman June 9, Reed Feb. 27, Matricon Apr. 6, Boyce Nov. 1, Ziock J an. 29,

FOREIGN PATENTS Germany June 9, Great Britain Oct. 23,