US2492075A - Vacuum pump - Google Patents

Description

Dec. 20, 1949 c. M. VAN ATTA 2,492,075

7 VACUUM rum? 1 Filed Oct. 30, 1945 6 Sheets-Sheet 1 I as 'la v 80 r i .L O a:

l.1- le see e4 I as 1271212825502 Ciaesfierilt Vitae/1%,

Dec. 20, 1949 c. M. VAN ATTA VACUUM PUMP 6 Sheets-Sheet 2 Filed Oct. 30, 1945 Dec. 20, 1949 c. M. VAN ATTA VACUUM PUMP 6 Sheets-Sheet 4 Filed Oct. 30, 1945 E aw mu 00 Q hum n0 mR Q Dec. 20, 1949 c, M, VAN ATTA 2,492,075

VACUUM PUMP Filed'oct. 30, 1945 6 Sheets-Sheet 5 64 me lol- Dec. 20, 1949 c. M. VAN ATTA 2,492,075

VACUUM PUMP Filed Oct. \:50, 1945 s Sheets-Sheet e I lea V ge f huv -qa I Q Z 4 I I l I I I I I l I I I I l l I l I I I I I I I I I I l I I I I I l l l I I l Patented Dec. 20, 1949 VACUUM PUMP Chester M. Van Atta, Berkeley, Calii'., assignor to Kinney Manufacturing Company,

Jamaica Plain, Mass., a corporation of Massachusetts Application October 30, 1945, Serial No. 625,508

16 Claims. (Cl. 230-2) My present invention relates to vacuum pumps of the type in which first and second stage pumping units cooperate to produce a high vacuum.

Vacuum pumps in which a second stage pumping unit operates continuously as a backing pump for the first stage pumping unit have been in use for some time. While the first and second stage pumping units cooperate to produce a higher vacuum or lower pressure than either could establish by itself, the operation of such vacuum pumps may be characterized as entailing excessive power consumption.

In accordance with my invention, I provide vacuum pumps, each of which has an inlet chamher, a first stage pumping unit, a second stage pumping unit and an interstage chamber with which the outlet of the first stage pumping unit and the inlet of the second stage pumping unit are in communication. The inlet of the first stage pumping unit is connected to the inlet chamber and a conduit, controlled by a valve, preferably connects the inlet and interstage chambers directly.

I provide a separate drive for each pumping unit so that a vacuum pump in accordance with my invention may be operated with the conduit valve open and only the second stage pumping unit in service to establish a predetermined reduced pressure in the inlet chamber. When such a reduced pressure has been established, the valve is closed and the first stage pumping unit operated so that the first and second stage pumping unit cooperate to increase the pumping speed of my vacuum pump to makepossible the production of the desired high vacuum.

While any driving means may be employed that permits my vacuum pumps to be operated with or without the first stage pumping units in service, I prefer to use a separate motor to drive each pumping unit. With the combination and mode of operation provided by my invention, the applied load is only a fraction of that required in a vacuum pump wherein the two stages have a common drive or are started simultaneously to pump down a system from atmosphere. My invention therefore ensures power economy.

In my high vacuum pumps, the first stage pumping unit is of substantially greater theoretical volumetric capacity than the second stage pumping unit. While the first and second stage pumping units may be of any desired type, I prefer that the first stage pumping unit be of any simple, rotary positive displacement type which is not oil sealed and has its pumping capacity a function of its speed. Such pumping units are adapted to operate at high speeds and do con-. siderable work at high volumetric efiiciency when -operated in a low pressure region and with a low outment than the first stage pumping unit. Such a. I

pump has an efiective pumping speed over a wide range of vacuum.

With first and second stage pumping units of these types, high vacuum may be established in vacuum pumps in accordance with my invention without excessive power consumption by bringing the first stage pumping unit into service when a predeterminedreduced pressure has been established in the inlet chamber. My invention makes it possible to provide motors for the first stage pumping unit of relatively low power for the very large ultimate displacement of the first stage pumping unit. Maximum power economyresults when the first stage pumping unit of the preferred type is operated only in such a low pressure region and with a low pressure differential across its inlet and the outlet that it may have an effective pumping capacity.

While the valve in the conduit connecting the inlet and interstage chambers may be manually operated, automatic control of it and the drive for the first stage pumping unit in relation to the pressure in the inlet chamber is desirable. I accomplish this result by utilizing a solenoid operated valve with its solenoid and the motor' of the first stage drive in a circuit controlled by a switch actuated by a device responsive to pressure changes in the inlet chamber. With such a control, the valve is closed or opened and the motor for the first stage pumping unit is started or stopped depending on whether the pressure is below or above that pressure determined as ensuring the most economical production of the high vacuum.

In the accompanying drawings I have shown an illustrative embodiment of a vacuum pump in accordance with my invention from which these and other of its novel features and advantages will be readily apparent.

In the drawings:

Fig. 1 is a schematic View of a vacuum pump in accordance with my invention.

Fig. 2 is a top plan view of a preferred embodiment of my invention.

Fig. 3 is a section along the lines 3-3, of Fig. 2.

The general features and advantages of my invention will be readily understood from a consideration of Fig. 1 in which I have schematically shown a convenient arrangement of the elements of vacuum pumps in accordance with my invention.

I have indicated generally at l5 a chamber,

hereinafter referred to as the interstage chamber with which the outlet of the first stage pumping unit 18 and the inlet of the second stage pumping unit H are in communication. The inlet of the first stage pumping unit I8 is shown as connected to an'inlet chamber l8. At I8, I have indicated a conduit, controlled by a valve 28, which conduit may conveniently connect the interstage and inlet chambers. As I have indicated in Fig. 1, the first stage pumping unit I 6 is of substantially greater capacity than the second stage pumping unit H and each pumping unit has a separate drive.

By this arrangement of the pumping units relative to the interstage chamber, the second stage pumping unit may operate by itself, when the valve 20 in the conduit I8 is open, to establish a predetermined reduced pressure in the inlet chamber l8. After such a pressure has been produced, the valve 20 is closed and the first stage pumping unit It brought into service so that the second stage pumping unit I 1 becomes a backing pump for the first stage pumping unit -l8 and the pumping units. in cooperation with each other, increase the pumping speed of my vacuum pump and establish a high vacuum in the inlet chamber l8.

My invention, as thus broadly outlined, has the important advantage of economy in operation when the first stage pump is employed only when a predetermined reduced pressure exists in the inlet chamber.

While the first and second stage pumping units may be of any desired type, I prefer to use, for the second stage, a rotating plunger, oil sealed pump. While such a pumping unit is of relatively slow speed, it has the advantage that it has an effective pumping speed over a wide range of vacuum.

With such a second stage pumping unit, the first stage pumping unit may be of any simple, positive rotary displacement unit of large theoretical displacement compared to that of the second stage pumping unit. The capacity of such a pumping unit is a function of its speed because its clearances are not all sealed and because of the resistance of molecules to fiow through these clearances at low pressure, it has an effective pumping capacity when operated in a low pressure region with a low pressure differential across its inlet and outlet.

These factors contribute to making my vacuum pump capable of producing an extremely high vacuum and to making possible economies in manufacture and in operation such, for example,

pumping unit that is oil sealed.

Another economy is that where each drive includes a separate motor the motor for the first stage pumping unit may be of relatively low power forits displacement. To prevent the operation of the first stage pumping unit under conditions demanding excessive power from its motor and to provide advantages of automatic operation, I provide a control for the first stage pumping unit and the valve which is responsive to the pressure. in the inlet chamber 18.

While such automatic operation of my vacuum pumps may be effected by other means, I have shown schematically in Fig. 1 a control that is both convenient and satisfactory.

I have shown the first stage pumping unit It as being driven by a motor 2| and the second stage pumping unit as having a drive 22 including a motor 22. A switch controlled circuit 24 includes the second stage motor 23 and a relay operated switch in parallel therewith which, when de-energized, connects the solenoid 28 in the circuit 24 and when energized connects the first stage motor 2| with the circuit 24. I

The relay operated switch 25 is energized whenever the switch 21 is closed by the pressure actuated device 28 which is responsive to the pressure on the'high vacuum side of the pump. Illustrative examples of the device 28 are shown in Figs. 7 and 8.

The pressure responsive device 28 shown in more detail in Fig. 7 has a bellows unit 28 which is subjected to the pressure on the high vacuum side of the pump and which is exhausted to such an extent that when the pressure in the work is in excess of a predetermined value, the switch 21 is opened with the result that the solenoid 26 which is in control of the valve 20 maintains that valve open so that the interstage and inlet chambers are connected. When the pressure in the high vacuum side of the pump falls below the predetermined value, the device 28 expands and effects the closing of the switch 21 with the result that the solenoid 26 is de-energized and the motor 2| is energized. The valve 20 is then closed by atmospheric pressure on the bellows seal 28 for the valve 20.

,By this arrangement, automatic operation requires only that the second stage pumping unit be brought into operation by closing the circuit 24. Whenever the pressure on the high vacuum side of the pump is at or below a pre determined value, the valve 20 is automatically closed and the first stage pumping unit brought into service. Whenever the pressure on the high vacuum side of the pump is above the predetermined value, the valve 20 is automatically held open and operation of the first stage pumping unit is impossible. Any means for securing this sequence of operation is embraced by my invention.

The pump construction detailed in the drawings In order to to ensure maximum efiiciency and economy in operation, it is desirable that the several elements of vacuum pumps in accordance with my invention be combined as one unit.

In Figs. 2-6, 9 and 10, I have shown in detail a particularly satisfactory pump construction. As may be most conveniently seen in Fig. 3, I have indicated generally at 3| a body, preferably in the form of a single casting and having a flanged port 32 to be connected to the system to be exhausted, a top wall 33, a bottom wall 34, end walls and 36 and side walls 31. The casting 3| has a partition 38 extending from the end wall 35 towards the port 32 in parallel with the top wall and joining a partition 39. The partition 39 is shaped to define with a similarly shaped part 40 of the end wall 36, a casing for the first stage pumping unit IS. The space between the port 32 and the inlet port of the casing for the first stage pumping unit constitutes the inlet chamber I8.

The bottom wall 34, a partition 4|, and the lower part 42 of the end wall 35 are adapted to establish a cylindrical housing for the second stage pumping unit H, the inlet port of which is defined by the end 43 of the partition 4|, and the intermediate portion 44 of the end wall 35. The space between the casings of the first and second stage pumping units, defined by said casings, the partition 38, the bottom wall 34, and the end walls 35 and 38 is the interstage chamber l5.

The space between the top wall 33 and the partition 38 establishes a conduit 45 similar in function to the conduit l9 of Fig. 1 and opening as at 45 into the inlet chamber l8 and closed at its other end by the end wall 35. The partition 38 and the top wall 33 have vertically alined ports for the valve seat 41 and the valve 48, respectively. The valve 4|! differs from the valve 2|] only in that the valve 48 is shown, for convenience, as manually operated. Like the valve 20, the function of the valve 48 is to connect the conduit 45 to or disconnect it from the interstage chamber i 5.

The portion 44 of the end wall 35 that is shaped to establish part of the inlet of the second stage pumping unit I1 is within a plane inclusive of the flange 49 and the portion 42 of the end wall 35. Bolted to the end wall 35 is a plate 5|! establishing with the portion 44 a chamber 5| and carrying a discharge pipe 52 extending into a housing 53 bolted to the fiange 49 and to the end wall 35 to establish a separator tank, generally indicated at 54. The separator tank has baille plates 55 and 55 disposed between the upwardly directed mouth of the discharge pipe 52 and the exhaust port 51. The tank has a drainage plug 58 and ribs 59.

To each side 31 of the casting 3|, I bolt heads 58 and 6|. The heads are apertured to receive the shafts 62 and 63 of the first stage pumping unit I8 and the heads 6| are apertured to receive the shaft 84 of the second stage pumping unit l1.

Fast on each of the shafts 82 and 53 are rotors 65. These are shaped to intermesh without contact as they rotate within the casing and without contact therewith (see Fig. 4).

Bolted to the heads 6|] are supports 66 which house the spring pressed shaft seals 61 and serve as oil reservoirs. The supports 68 are apertured to receive the shaft supporting bearings. One end of each of the shafts 62 and 53 is supported by bearings 68 seated in one of the supports 55 by a cap 59 having an oil cup 18. The shafts 82 and G3, at their other ends, are supported by bearings 1| seated in the other of the supports by a head 12 to which a gear box 13 is bolted.

The gear box 13 is filled with oil to the desired level to ensure the lubrication of the intermeshing gears fast on the shafts 62 and 63, respectively. Of these gears, only the gear 14 is shown. The shaft 53 extends through the gear box 13 and has a connection 15 for the drive shaft of the motor 2| of the first stage pumping unit "5.

The housing for the second stage pumping unit H (see Figs. 3 and 6) establishes a cylindrical passage transversely of the casting 31| and in this I insert a cylindrical casing 15. The casing 16 has a partition 11 apertured to receive the shaft 84 and establishes a pair of cylinders 18 and 19. Fast 0n the shaft 54 is a cam 80 in each of the cylinders. The cams 80 are preferably disposed at an angle of 180 relative to each other.

Each cam 80 fits within a cylindrical piston 8| havingan arm 82 formed with channels 83 and slidably supported by a pin 84 rotatably mounted in the casing 16. As the shaft 64 rotates, the pistons 8| are given a rotating plunger motion tangent to the cylinder wall causing oscillation of the pins 84 and the slidin of the arm 82 therein so that the channels 83 are alternately connected to or closed from the cylinders. Discharge ports open through the casing 16 and the end wall into the chamber 5| and are closed by spring tensioned valves 85 mounted in the plate 50.

The heads 6| are recessed to accommodate shaft supportin bearings 85 (see Figs. 5 and 6). The bearings 85 at one end of the shaft 54 are seated by a cap 81 bolted to the head GI and at its other end the shaft 54 extends through a housing 88 for the spring pressed shaft seal 89. The housing 88 positions the bearing seating cap 98 and serves as a reservoir for sealing oil. The shaft 64 has a pulley 9| as indicative of the drive 22 for the second stage pumping unit l1.

One of the factors contributing to the efficiency of the second stage pumping unit shown in the drawings is that it is oil sealed. Oil in the separator tank 56 (see Figs. 2 and 3) flows to each of the bearings 86 and into the housing 88. At 92, I have indicated a support bolted to the housing 53 and having a conduit 93 in communication with a port 94 in the end wall 95 of the housing 53. Conduits 96 in the support 92 are connected to the conduit 93 by a valve 91 opened when its operating solenoid 98 is energized. The solenoid 9B is preferably in series with a motor. 23 so that when the second stage pumping unit is not in service, the valve 91 is automatically closed to prevent the fiooding of the second stage pumping unit I! with sealing oil.

The support 92 is grooved as at 99 to establish when it is bolted to the end wall 95 of the housing 53, a pair of passages connecting each of the conduits 96 with one of the conduits |ll8 extending through the ribs 59 with which the side walls of the housing 53 are formed and into the end wall 35 where the conduits Hill are in communication with passages |0| extending into the heads 6|. Conduits I02, controlled by adjustable valves 13 provide for the regulated delivery of the oil to the bearings 86. A conduit Hi4 leads from the separator tank to the housing 88.

In order to ensure satisfactory pump operation, it is desirable to eliminate the possibility of leakage into the first stage pumping unit I6. To accomplish that result, I provide that any leakage between the heads 80 and the sides 31 of the cast ing 3| or any leakage into the bearing supports 6'3 is drawn into the interstage chamber |5 by the second stage pumping unit H.

To this end I form the face of each of the heads 60 that engages the sides of the casting 3| with a groove Hi5 that registers with an aperture H58 in each side wall 31 of the casting 3| (see Figs. 3,

' I and the side wall 2] oi the casting 3 I, it is very slight. By maintaining the interior of. the supports I. and the groove Ill continuously under the exhausting influence oi. the second stage pumping unit II, any such slight leakage cannot interfere with the function of the first stage pumping unit I I.

The automatic control of the first stage pump Where it is desired that my vacuum pumps be automatic in their operation as to the use of the first stage pumping units, a wide range of devices are available to open andclose the switch 21 in response to pressure changes in the inlet chamber I t. Such devices have to be reliable and accurate in their operation and as schematically illustrative of different types I have shown in Figs. 7 and 8 two diflerent forms of pressure responsive control devices that may be employed in the circuit 2| of Fig. 1 or in any other circuit providing for the automatic control of the valve 20 and the first stage pumping unit.

The device of Fig. 7 may conveniently consist of a vacuum tight housing II I mounted on the wall 33 of the casting :I' and having a conduit I I2 connecting it to the conduit 45 so that it will be responsive to pressure in inlet chamber I8. Within the housing I I I, I mount the bellows unit 29 secured to the flange I II of a plunger I slidably supported in the bore I I! of a stop I I6.

Within the housing III, I position a normally open snap action switch 21. The switch 21 is mounted in the housing I II for adjustment by means of the adjusting screw II'I.

The bellows unit 29 is exhausted and sealed so that it is compressed by pressures in the intake chamber It above a predetermined pressure. The flange. II3 of the plunger I04 then seats on the stop I I5. Below that predetermined pressure, the bellows unit 2! acts as a weak spring to carry the plunger I04 against the plunger I II of the switch 21 to hold the switch closed.

crease in the conductivity oi the gas as its prusure rises, the temperature 0! the tungsten wire I23 decreases and it accordingly contracts causing the switch arm I24 to open the switch 21 or to hold it opened. Due to the decrease in its conductivity as the pressure of the gas drops. the temperature 01 the wire increases and it accordingly expands permitting the switch 21 to close.

This control permits the switch 21 to be operated within a very closely established pressure range and with very low power consumption.

The insulated conductor I2I may conveniently be the means by which the device of Pig. 8 isadjusted. To that end, I have shown the housing II! as having an end wall I2! having a hole Illl therethrough to receive the conductor element m. Rubber washers I I2 and Ill are compressed against opposite surfaces of the end wall- I22 to establish a vacuum tight seal by nuts I and I" respectiyely. Metal washers are hull-- cated at I 38 and at I31 .I have shown a lock nut which may also conveniently serve to anchor the lead of the circuit I25 thereto.

A separate housing for each pressure responsive device and the switch which it operates is preferred as it may be easily removed and replaced it service isnecessary.

Operation advantages will be apparent.

When the inlet chamber I8 is connected to the system to be evacuated, the second stagepumping unit I! is operated withthe valve 20 open to eilect a pressure reduction. When the predetermined pressure is reached, the valve 20 is closed and the first stage pumping unit It is brought into service to increase the pumping speed at. my vacuum pump. The particular pressure at which the valve 20 is closed and the first stage'pumping .unit It operated depends to a considerable extent on the nature and requirements oi. the system. Where the operation of the first stage pumping unit is controlled by a device responsive to a predetermined pressure, the operation of the first stage pumping unit is prevented until the pressure in the inlet chamber drops below the predetermined value. This ensures against its operation with any appreciable pressure diflerential across its inlet and outlet and consequently avoids its use under conditions demanding excessive If desired, the "hot wire" control schematically illustrated in Fig. 8 may be used. As shown in Fig. 8, a vacuum tight housing II! is mounted on the vacuum pump with a conduit I20 connecting it to the interior-of the pump so that pressure in the housing I I9 rise and fall with the pressure in the system. Within the housing I I9, I mount a normally open snap action switch 21 having its plunger indicated at III.

The housing I I9 has a pair of insulated conductors indicated generally at I2I and I22. I have shown a section of tungsten wire I23 as connected to the conductor I2I while a switch operating arm I24 mounted on the switch 21 in engagement with the plunger III is connected to the conductor I 22. At their opposite ends, the switch arm I24 and the tungsten wire I23 are interconnected to constitute part of the circuit I25. A suitable power source is indicated at I28 and I have shown the circuit I25 as including a resister I21 and a rheostat I22.

when the circuit I2! is energized. Due tothe inpower from its motor.

Apart from power economy, a vacuum pump utilizing the preferred types of first and second stage Dumping units is particularly efl'ective even where the first stage pumping unit is operated continuously with the second stage pumping unit. This results from the fact that the preferred type of first stage pumping unit is of relatively large capacity and is not oil sealed so that-its pumping capacity is a function of its speed. Even it run simultaneously with the second stage pumping unit, it becomes Operative in a low pressure region and with a low pressure diil'erential across its inlet and outlet to increase the pumping speed of a vacuum pump in accordance with my invention in cooperation with the second stage pumping unit. .1

In general, however, power economy is an important consideration and maximum economy oi power consumption is attained by not operating the first stage p mp ng unit before the second. stage Dumping unit has become comparatively. ineflicient at which time the first stage pumping stage chamber and accordingly comparatively eiiicient operation of the second stage pumping unit through a longer period results. Whenthe control of the operation of the valve 20 and the motor 2| is automatic at the pressure predetermined as ensuring most satisfactory operation, this may be done by any suitable accurate and reliable pressure responsive device of which the types suggested in Figs. 7 and 8 are but illustrative.

The schematic arrangement of the elements of my invention shown in Fig. 1 illustrates in general features and advantages. It is not, of course, necessary that the conduit I9 connect the interstage and inlet chambers, but such is preferable to a separate line from the system to the inlet of the second stage pumping unit H.

The pump construction detailed in Figs. 2-6 is a preferred embodiment of my invention since it ensures compactness, most economical, operation, minimizes the possibility of leakage, and safeguards against any such leakage interfering with performance.

What I therefore claim and desire to secure by Letters Patent is:

1. A vacuum pump for evacuating a system, said pump comprising first and second stage pumping units each including an inlet and outlet, an interstage chamber with which the outlet of the first stage pumping unit and the inlet of the second stage pumping unit are in communication, valve controlled means in communication with the inlet of said second stage pumping unit with its inlet in parallel with the inlet of the first stage pumping unit so that when said means are open the inlets of both pumping units are in parallel with respect to the system and a separate drive for each pumping unit so that th vacuum pump may be operated with only the second stage pumping unit in service and the valve controlled means open to establish a reduced pressure in the system and in the interstage chamber or with both pumping units in service and the valve controlled means closed to increase the pumping speed of the vacuum pump.

2. A vacuum pump for evacuating a system, said pump comprising first and second stage pumping units each including an inlet and an outlet, an interstage chamber with which the outlet of the first stage pumping unit and the inlet of the second stage pumping unit are in communication, a conduit in communication with the inlet of said second stage pumping unit with its inlet in parallel with the inlet of the first stage pumping unit so that when said conduit is open the inlets of both pumping units ar in parallel with respect to the system, a valve in said conduit, a separate drive for each pumping unit so that the vacuum pump may be operated with only the second stage pumping unit in service and the valve controlled conduit open to establish a predetermined reduced pressure in the system and in the interstage chamber or with both pumping units in service and the valve controlled conduit closed to increase the pumping speed of the vacuum pump, and means responsive to the pressure in the work to cause the closing of the valve and the operation of the drive for the first stage pumping unit whenever the pressure is at or below the predetermined pressure.

3. A vacuum pump for evacuating a system, said pump comprising first and second stage pumping units each including an inlet and anoutlet, an interstage chamber with which the outlet of the first stage pumping unit and the inlet of the second stage pumping unit are in communication, the second stage pumping unit being of a positive displacement type having an effective pumping speed over a wide range of vacuum and the first stage pumping unit being of a positive displacement type having a large theoretical volumetric displacement in relation to that of the second stage pumping unit and having an actual efiective pumping capacity when operated on a low pressure region with a low pressure differential across the inlet and outlet, valve controlled means in communication with the inlet of the second stage pumping unit with its inlet in parallel with the inlet of the first stage pumping unit so that when said means are open, the inlets of both pumping units are in parallel with respect to the system and a separate drive, for each pumping unit so that the vacuum pump may be operated with only the second stage pumping unit in service and the valve controlled means open to establish a reduced pressure in the system and in the interstage chamber or with the valve controlled means closed and both pumping units in service, and cooperating to increase the pumping speed of the vacuum pump.

4. The vacuum pump of claim 3, in which the second stage pumping unit is of the rotating plunger, oil sealed type.

5. A vacuum pump, comprising an inlet chamber, an interstage chamber and an outlet, a first stage pumping unit having its inlet in communication with said inlet chamber, and its outlet in communication with said interstage chamber, a second stage pumping unit having its inlet in communication with said interstage chamber and its outlet in communication with the atmosphere, a separate drive for each pumping unit, a conduit connecting said chambers, and a valve in said conduit so that when a predetermined reduced pressure has been established in said inlet chamber by said second stage pumping unit, said valve may be closed and said drive for said first stage pumping unit operated to bring said first stage pumping unit into service.

6. A vacuum pump, comprising a body having top, bottom, side and first and second end walls, said body including a flanged inlet port extending upwardly from said top wall, a transverse partition extending from said first end wall and spaced from said top wall to define a conduit, and including a downwardly disposed portion, said portion and the upper part 'of said second end wall being shaped to establish with said side walls a first pump casing having an inlet and outlet and spaced below said inlet port to establish an inlet chamber with which said conduit and said inlet are in communication, a second transverse partition extending upwardly from said bottom wall, said second partition, the lower part of said first end wall and the included part of said bottom wall being shaped to establishv with said side walls a second pump casing and an inlet therefor and said lower part of said second wall having an outlet port, said partitions, and the intermediate portions of said end and side walls establishing an interstage chamber with which the outlet of said first casing and the inlet of said second casing are in communication, a valve controlled port intermediate said conduit and said interstage chamber, shaft supported 11 pumping means in each of said casings, and sealed shaft supporting means attached to said body at the opposite ends of each of said casings.

7. The vacuum pump of claim 6, in which there are a pair of ports in each side wall of the interstage chamber adjacent the first pump casing, each sealed shaft supporting means has a conduit in communication with one'of the ports and its interior and a groove establishing in the assembled vacuum pump a conduit to entrap any leakage between the shaft supporting means and the adjacent pump side and disposed to communicate with the interstage chamber through the other port.

8. A vacuum pump for evacuating a system, said pump comprising first and second stage pumping units each including an inlet and an outlet, said first stage pumping unit having a large theoretical volumetric displacement in relation to that of the second stage pumping unit, an interstage chamber with which the outlet of the first stage pumping unit and the inlet of the second stage pumping unit are in communication, a conduit in communication with the inlet of said second stage pumping unit and disposed so that the inlets of both pumping units are in parallel with respect to the system, a valve in said conduit, a separate drive for each pumping unit so that the vacuum pump may be operated with only the second stage pumping unit in service and the valveopen to establish a reduced pressure in the system and in the interstage chamber or with both pumping units in service and the valve closed to increase the umping speed of the vacuum pump, the drive for the first stage pumping unit including an electric motor, a solenoid to actuate said valve, a circuit including said motor and said solenoid, a switch in control of said circuit and having a first position in which said motor is inoperative and said solenoid actuated valve is open and a second position in which said motor is energized and said solenoid actuated valve is closed, and means responsive to the pressure at the inlet side of said vacuum pump to close and open said switch when the pressure therein is below or above a predetermined pressure.

9. The vacuum pump of claim 8, in which the pressure operated means consists of a plunger to operate the switch, a support and a bellows unit connected to the plunger and to the support and being evacuated so that it expands and contracts in response to pressure variations at the inlet side of the vacuum pump.

'10. The vacuum pump 01' claim 8, in which the pressure operated means consists of a support, a switch operating plunger slidable in the support into or out of a seated position, and a bellows unit connected to the plunger and to the support and being evacuated so that it expands as the pressure at the inlet side of the vacuum pump decreases to carry the plunger from its seated position into a switch operating position.

11. The vacuum pump of claim 8 in which the pressure operated means consists of a housing connected to the vacuum pump so that the pressure in the housing varies with the pressure at the inlet side of the vacuum pump, a switch within the housing, a circuit including a pair of conductors extending through the walls of the housing, a section of tungsten wire connected to one of the conductors, and a switch arm connected to the tungsten and the other of the conductors.

12. A vacuum pump for evacuating a system, said pump comprising first and second stage pumping units each including an inlet and an outlet, said first stage pumping unit having a large theoretical volumetric displacement in relation to that of the second stage pumping unit, an interstage chamber with which the outlet of the first stage pumping unit and the inlet oi the second stage pumping unit are in communication, a conduit in communication with the inlet of said second stage pumping unit and disposed so that the inlets of. both pumping units are in parallel with respect to the system, a valve in said conduit, a separate drive for each pumping unit so that the vacuum pump may be operated with only the second stage pumping unit in service and the valve open to establish a reduced pressure in the system and in' the interstage chamber or with both pumping units in service and the valve closed to increase the pumping speed of the vacuum pump, the drive for each pumping unit including a motor, a solenoid to open said valve, means to close said valve, a circuit including the motor for the second stage pumping unit, a relay operated switch to connect the motor for the first stage pumping unit or said solenoid in series with the motor for the second stage pumping unit and having a normal position in which said solenoid is energized, and a switch in control of said relay, and means responsive to the pressure in the high vacuum side of the vacuum pump to close the relay controlling switch when the pressure in the high vacuum side is below a predetermined reduced pressureand to open the relay controlling switch when the pressure in the high vacuum side is above the predetermined pressure.

13. The vacuum pump of claim 12, in which the valve has a stem slidably extending through the wall of the conduit, and the means to close the valve comprises a bellows fastened to the stem and to the outer surface of the conduit wall to establish a seal for the stem.

14. A vacuum pump comprising an inlet chamber, an interstage chamber, first and second stage pumping units each including an inlet and outlet, the first stage pumping unit having its inlet in communication with the inlet chamber and its outlet in communication with the interstage chamber, the second stage pumping unit having its inlet in communication with the interstage chamber, a conduit connecting the chambers, a valve in control of the conduit, a solenoid to operate said valve, an electric motor to operate the first stage pumping unit, an electric circuit including a switch, the motor and the solenoid, a vacuum tight housing mounted on the vacuum pump, a conduit connecting the interior of the housing and the interior of the inlet chamber, the switch being mounted in the housing, and means in the housing responsive to pressure to actuate the switch to eifect the running of the motor and the closing of the valve whenever the pressure in the inlet chamber is at or below a predetermined pressure.

15. The vacuum pump of claim 14, and adjustable means to vary the operative relation of the switch and the pressure responsive means.

16. A vacuum pump comprising a casting having top, bottom, side and first and second end walls, said casting including a flanged inlet port extending upwardly from said top wall, a transverse partition extending from said first end wall and spaced from said top wall to definea conduit,

and including a downwardly disposed portion,

said portion and the upper part of said second end wall being shaped to establish with said side walls a first pump casing having an inlet and an outlet and spaced below said inlet part to establish an inlet chamber with which said conduit and said inlet are in communication, a second transverse partition extending upwardly from said bottom wall, said second partition, the lower part of said first end wall and the included .parts of said bottom and side walls bein shaped to establish a second pump casing and an inlet and said lower part of said second wall having an outlet, said partitions, and the included portions of said side and bottom walls establishing an interstage chamber with which the outlet of said first casing and the'iniet of said second casing are in communication, a port intermediate said conduit and said interstage chamber, a valve in control of said port, shaft supported pumping means in each of said casings, the pumping means in the second pump casing being of the oil sealed type, sealed shaft supporting means attached to said casting at the opposite ends of each of said casings, a casting attached to the first end wall and defining therewith an oil separator tank, and

oil conduits from said separator tank to the sealed shaft supporting means of the pumping means in the second pump casing.

CHESTER M. VAN A'I'IA.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,185,944 Schley June 6, 1916 83 Giesler Apr. 25, 1933 1,927,799 Mann Sept. 19, 1933 1,929,198 Crawford Oct. 3, 1933 1,936,935 Fitch Nov. 28, 1933 2,126,553 Hornberzer Aug. 9, 1938 FOREIGN PATENTS 20 Number Country Date 453,608 Great Britain Sept. 15, 1936

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