US2755005A

US2755005A - Method of exhausting and filling electric gaseous discharge devices

Info

Publication number: US2755005A
Application number: US329018A
Authority: US
Inventors: Brooks Joseph Horace; Kimball Lawrence Woodruff
Original assignee: Sylvania Electric Products Inc
Current assignee: GTE Sylvania Inc
Priority date: 1952-12-31
Filing date: 1952-12-31
Publication date: 1956-07-17
Anticipated expiration: 1973-07-17

Description

July 17, 1956 J. H. BROOKS ET AL METHOD OF EXHAUSTING AND FILLING ELECTRIC GASEOUS DISCHARGE DEVICES Filed Dec. 31, 1952 5e 75 Pol/6H l/Acuu Pump Z- 3 Sheets-Sheet l z IbGAsANDAIR SUPPLY 7b FINE Wlcuum RJMP 6 32 64/260 link JOSEPH H. BRooKs INVENTORS. LAWRENCE W. KIMBALL Arm/mix y 7, 1956 J. H. BROOK ET AL 2,755,005

METHOD OF EXHAUSTING AN ING ELECTRIC GASEOUS DISCHARGE ICES Filed Dec. 51, 1952 s Sheets-Sheet 2 1 I 9 40 H i i :J

6 I 5 3 60 fl/j M INVENTOR$-.

JOSEPH H. BROOKS LAWRENCE W. KIMBALL AWUE/VE/ y 17, 1956 J. H. BROOKS ET AL 2,755,005

METHOD OF EXHAUSTING AND FILLING ELECTRIC GASEOUS DISCHARGE DEVICES Filed Dec. 51, 1952 3 Sheets-Sheet 3 IZO-V-UNE I84 |82A I 6/ 40 J U 50 50 I T |7 2A Fig.6

IN VENTORS JOSEPH H. BRooKs LAWRENCE W. KIMBALL ATTORNEY METHOD OF EXHAUSTING AND FILLING ELEC- TRIC GA'SEOUS DISCHARGE DEVICES Joseph Horace Brooks, West Peabody, and Lawrence Woodrufi Kimball, Bedford, Mass., assignors to Sylvauia Electric Products Inc., Salem, Mass., :1 corporation of Massachusetts Application December 31, 1952, Serial No. 329,018

'10 Claims. (Cl. 226-19) This invention relates to the manufacture of electric gaseous discharge devices and more particularly to the exhausting and filling operations performed on these devices during their manufacture.

In the manufacture of electric gaseous discharge devices, such as fluorescent lamps for example, it has been the general practice heretofore to perform the exhausting and filling operations through a slender tube, commonly called an exhaust tube, projecting from a lamp mount at One end of the lamp tube and in communication with the interior thereof. The addition of longer lamps to the fluorescent lamp line has served to accentuate the time factor involved in the performance of these operations. For example, it takes considerably longer to properly evacuate and fill a fluorescent lamp 96 inches in length than it does to perform these same operations on a fluorescent lamp 48 inches in length. The longer the lamp, the more time-consuming the evacuation operation becomes.

An object of this invention is to effect a marked reduction in the amount of time required to perform the exhausting and filling operations in the manufacture of electric gaseous discharge devices.

Another object of this invention is to provide a method of and apparatus for evacuating and filling electric gaseous discharge devices at a speed substantially greater than the speeds heretofore attainable.

A further object is to provide a method of and apparatus for evacuating fluorescent lamps more thoroughly and more quickly than heretofore.

In one embodiment of our invention, an electric gaseous discharge device, such as a fluorescent lamp for example, is provided with a tubulation at each end thereof and in communication with the interior thereof. The exhausting and filling apparatus on which this lamp is treated has a pair of heads into which the tubulations projecting from the ends of the lamp are inserted. One or more lines of communication are established to the ends of the tubulations within the heads. These lines of communication connect one or the other or both ends of the lamp as desired to one or more vacuum pumps, a source of flushing gas, a source of filling gas, and a device for dispensing mercury. One of the features of the method and apparatus of our invention is the introduction of a flushing gas into the lamp through a tubulation at one end thereof and evacuation of the lamp through the tubulation at the other end thereof.

These and other objects, advantages and features of our invention will be apparent to those skilled in the art from the following description of a specific embodiment thereof, a specific embodiment of the apparatus being shown in the accompanying drawings in which:

Figure 1 is a plan view of an exhaust apparatus embodying the principles of our invention, as applied to a fluorescent lamp.

Figure 2 is a transverse sectional view of the apparatus of Figure 1 taken along the line 22 thereof.

Figure 3 is an elevational view partly in section of one nited States Patent Phtented July 17, 1956 of the lamp-supporting heads and the mercury dispenser associated therewith.

Figure 4 is a perspective view of a lamp exhaust schedule control mechanism employed in conjunction with the apparatus of Figures 1, 2 and 3.

Figure 5 is a schematic diagram of one of the circuits employed in conjunction with the mechanism of Figure 4 to control actuation of some of the members of the ap paratus of Figures 1, 2 and 3.

Figure 6 is a schematic diagram of a portion of the circuit employed to activate the lamp electrodes.

Referring now to Figures 1 and 2, the apparatus shown therein, with which the method of our invention may be employed, comprises an oven 10 resting on and supported by angle-iron ways 12 on table 14, and a pair of lamp-supporting

heads

16 and 18 on table 14. Head 18 is reciprocable in slide 19. The oven 10 comprises a metal case 20 provided with a lining 22 of insulating material and a burner manifold 24 extending longitudinally thereof adjacent to the junction of the bottom and the rear walls thereof. The burner manifold 24 is connected through a flexible tube 26 to a gas and air source, not shown. A rack 28 is attached to the underside of the oven 10 adjacent to each end thereof. Each rack meshes with a pinion 30 mounted on shaft 32 journaled in the sides of the angle-iron ways 12. The shaft 32 is provided with a crank handle 34.

The oven 10 is provided with an opening 36 in the front wall thereof and an elongated slot 38 in each end wall thereof for lamp clearance purposes. The lamp 4% comprises a lamp tube 42 with a lamp mount 44 sealed thereto at each end thereof. Each lamp mount 44 has a tubulation 46 formed integral therewith, projecting therefrom and in communication with the interior of the lamp tube 42, and a .pair of lead-wires 48, on the inner ends of which a filamentary electrode 50 is supported, the outer ends thereof serving as the means through which the electrodes 50 may be connected to a source of electrical energy.

A wall 52, of insulating material, on table 14 closes the opening 36 in the front wall of the oven 10 when the oven 10 is disposed in engagement therewith as in Figure 2. The wall 52 also serves as a mounting surface for two pairs of terminal posts 54 which extend therethrough. One end of each of the four terminal posts 54 is connected through a wire 56 to a source of electrical energy and the other end thereof supports a flexible metal finger 58 which engages and overlies a lamp lead-wire 43 which has been bent back along the lamp tube 42.

Referring now to Figure 3, the lamp supporting head 18, which is reciprocable in slide 19, comprises a block 60 having a chamber 62 therein in which a rubber washer 64 for gripping one of the tubulations 4-6 is disposed, a bore 66 through which communication is established between the chamber 62 and a port 68, and a mercury dispenser 70 in communication with the bore 66.

One end of the lamp 40 is positioned in the head 18 by threading tubulation 46 into the rubber washer 64. The tubulation 46 is securely held by the rubber washer 64 and an air-tight seal is effected within the head 13 by the application of a compressing force to the washer 64. The rubber washer 64 is compressed tightly around the tubulation 46 and seals off the chamber 62 by the pressure of a metal washer 72 thereagainst. The metal washer 72 and the ball bearings 65 in engagement therewith are moved by the cap 67 which is threaded onto the block 6%) and is turned 'by means of arm 69 thereon to increase or release the pressure on the rubber washer 64.

The mercury dispenser 70 comprises a mercury reservoir 74 on one end of block 60, a mercury dispensing chamber 76 formed in the top of block 60, and a mercury dispensing pin 78 disposed in chamber 76 and having a tapered end 80 which normally is seated in the fapered bottom of the chamber 76. Communication between mercury reservoir 74 and mercury dispensing chamber 76 is established by bore 82; and communication between mercury dispensing chamber 76 and bore 66 is established by bore 84. The mercury dispensing chamber 76 formed in the top of block 60 is covered by a cap 86 with a rubber diaphragm 88 disposed therebetween. The cap 86 is provided with a chamber 90 therein in register with mercury dispensing chamber 76. The mercury dispensing pin 78, which is disposed in

chambers

76 and 90, has a collar 92 attached thereto, and a spring 94 concentric therewith and disposed between the collar 92 and the tapered roof 96 of chamber 90.

A solenoid 98, mounted on a bracket 100 on slide 19, is mechanically connected to the mercury dispensing pin 78 by means of a lever 102 and a spring finger lever extension 102a, one end of which lies beneath and is engagcd by pin 106 of solenoid plunger 104. The other end of lever 102 is provided with an aperture therein through which the pin 78 extends. The pin 78 is provided with a head 108. A projection 110, on the top of cap 86, serves as a fulcrum for lever 102. A screw 112 extending through a bracket 114, and a spring 116 concentric therewith and having one end thereof attached to lever 102, serve as a stop for lever 102 when the mercury dispensing pin 78 is moved to its dispensing position.

The lamp-supporting head 16, disposed opposite head 18 on table 14, is similar in construction to lamp-supporting head 18, except that it does not have a mercury dispenser, i. e., the head 16 is provided with a rubber Washer which grips the tubulation 46, and a port and bore in communication therewith.

Referring now to Figure 1, tube 118 connects the port 68 of the lamp-supporting head 18 to a source of inert gas such as argon, solenoid valve 120 being disposed in the tube 118 between the head and the gas source to permit control of the flow of gas to the head. Tube 122 connects lamp-supporting head 16 to

tubes

124 and 126. Tube 124 is connected to one vacuum pump and tube 126 is connected to another vacuum pump, the

tubes

124 and 126 being provided with

solenoid valves

128 and 130 re spectively for control purposes hereinafter described.

Actuation of

valves

120, 128 and 130, energization of solenoid 98 to actuate the mercury dispenser 70, and heating of the filamentary electrodes 50 of the lamp 40 are all controlled by the lamp exhaust schedule control mechanism shown in Figure 4, through the circuits shown in Figures and 6.

The lamp exhaust schedule control mechanism shown in Figure 4 comprises a copper program plate 132 with a sequence layout designated generally by the reference number 134 disposed on a face thereof, a plurality of stationary finger switches designated generally by the reference number 136 engageable by the sequence layout 134, and means, designated generally by the reference numbr 138, for moving the program plate 132 with respect to the stationary finger switches 136 to bring the sequence layout 134 on the program plate 132 into engagement with the finger switches 136. The program plate 132, which is slidably disposed on a channel-like base 140 has a rack 142 depending from a longitudinal edge thereof which meshes with a pinion 144 on a shaft 146 journaled in the base 140. The shaft 146 is driven by a motor 148 through an adjustable gear reduction mechanism 150, pulleys 152 and belt 154. The sequence layout 134 comprises a plurality of strips of insulating tape 156 of pre-determined length atfixed to the program plate 132 at pre-determined locations thereon in alignment with the finger switches 136. The plurality of finger switches 136, with lead-wires 158 extending therefrom, are mounted on a bracket 160, and insulated from one another and the bracket.

Since the lamp exhaust schedule control mechanism,

as illustrated in Figure 4, is arranged to control five different operations, five separate electrical circuits are provided to initiate and terminate these operations. Since the circuits for controlling the operation of

solenoid valves

120, 128 and and mercury dispenser solenoid 98 are identical in nature, only one of these four is illustrated, this being shown in Figure 5, and only one of them will be described. The power source for this circuit is a transformer 162 with a 120 volt primary and a 12 volt secondary. One end of the secondary is connected by line 164 to the base on which the program plate 132 is disposed and the other end of the secondary is connected through line 166 and coil 168 of relay 170 to one of the lead-wires 158 connected to one of the finger switches 136. One side of the pri mary of transformer 162 is connected by line 172 through a coil 174 (of

solenoid valve

120, 128 or 130, or mercury dispenser solenoid 98, as the case may be) and line 182 to an electrical contact 176 of relay 170; and the other side of the primary is connected by line 178 to a springloaded finger 180 of relay 170, the finger 180 being normally closed with respect to contact 176.

The circuit illustrated in Figure 6 is a portion of the circuit employed to effect heating of the filamentary elec trodes 50 of the lamp 40, the remainder of the circuit being the same as the circuit of Figure 5, with the lamp circuit of Figure 6 substituted for the coil 174 of Figure 5. One leg of each of the filamentary electrodes 50 is connected through line 172a to one side of the 120 volt pri mary and the other leg of each of the filamentary electrodes 50 is connected through a ballast 184, such as an incandescent lamp for example, and line 182a to a relay 170.

The method of our invention, as practiced on the apparatus shown in the accompanying drawings and described above, will now be described. Referring first to Figures 1, 2 and 3, lamp 40, which is a 40 watt fluorescent lamp, 48 inches in length and 1% inches in diameter, for example, is positioned on the subject apparatus by threading tubulation 46 at one end of lamp 40 into the aperture in the rubber washer 64 in lampsupporting head 16. Lamp-supporting head 18 is retracted on its slide 19 a distance sufficient to permit the tubulation 46 at the other end of the lamp 40 to be threaded into the rubber washer 64 therein. The head 18 is then advanced to the position shown in Figure 3 and secured in this position by manipulation of nut 21 on screw 23. The rubber washers 64 in the

heads

16 and 18 are deformed by manipulation of arms 69 to securely grip the tubulations 46 and effect an air-tight seal in the

heads

16 and 18. The lamp lead-wires 48 are then bent back over the periphery of the lamp and beneath the flexible metal fingers 58. The oven 10 is moved forward from the position shown in Figure 1 to the position shown in Figure 2 by rotation of the crank handle 34. Rotation of the crank handle 34 is translated into linear movement of the oven 10 through shaft 32 on which the handle 34 is mounted, pinions 30 mounted on the shaft 32, and racks 28 attached to the underside of the oven 10, the oven 10 sliding on the angle-iron ways 12. As the oven 10 approaches the wall 52, the opening 36 in the oven 10 provides clearance for the lamp tube 42 and the elongated slots 38 in the end walls of the oven 10 provide clearance for the tubulations 46 projecting from the ends of the lamp tube.

With the lamp 40 in the position shown in Figure 2, and the oven 10 at a temperature high enough to insure a tube temperature during exhaust of about 200 C. to 350 C. for example, the lamp exhausting operation is started. Initiation of the lamp exhausting operation is effected by energization of the motor 148 which drives the program plate 132 of the lamp exhaust schedule control mechanism shown in Figure 4. As the program plate 132 moves from left to right in Figure 2, the

sequence layout 134 thereon is carried into and out of engagement with the stationary finger switches 136 which, through circuits like the one shown in Figure 5, control actuation of

solenoid valves

120, 128 and 130 (Fig. 1) and mercury dispenser 70 (Fig. 3), and, through the circuits of Figures 5 and 6, control heating of the filamentary electrodes 50 of the lamp 40.

When the transformer 162 is connected to a source of electrical energy, current flows through the several 12 volt circuits like the one shown in Figure 5. Each of these circuits comprises line 164, program plate 132, one of the finger switches 136, line 158, coil 168 of one of the relays 170, and line 166. When current flows in the 12 volt circuits, the coil 168 of each of the several relays 170 effects and maintains a break in each of the 120 volt circuits by drawing the finger 180 away from its normally closed position with respect to contact 176.

When the program plate 132 reaches a point in its travel from left to right in Figure 2 where insulating tape 156a comes into contact with finger switch 136a, this 12 volt circuit is broken and thus this 120 volt circuit closes with the return of spring-loaded finger 180 of relay 170 to engagement with electrical contact 176. Closing of the 120 volt circuit and energization of coil 174 of solenoid valve 128 (Fig. 1) effects an opening of this valve and thereby places the lamp 40 in communication with a rough vacuum pump. Rough pumping continues until this 120 volt circuit is broken by the movement of insulating tape 156a out of engagement with finger switch 136a.

Shortly after the 12 volt circuit through finger switch 136a has been broken by insulating tape 156a, the insulating tape 15611 moves into contact with finger switch 136!) and breaks this particular 12 volt circuit. Breaking of this 12 volt circuit effects a closing of this 120 volt circuit through its relay 170, and energization of coil 174 of solenoid valve 120 (Fig. 1), thus placing the lamp 40 in communication with a supply of flushing gas. Thus, at this point, an inert gas, such as argon for example, is being introduced into the lamp 40 at one end thereof while the lamp is being evacuated through valve 128 connected to the other end thereof. The supply of inert gas may, for example, be at a pressure of about 6 lbs/sq. in. gauge and one or more restrictions (not shown) are disposed in the line 118 to cut down the rate of flow of the inert gas to the lamp 40 and thereby reduce the pressure thereof to a value substantially below atmospheric.

As the program plate 132 continues its travel, the insulating tape 156c thereon moves into engagement with finger switch 136c, thereby breaking this 12 volt circuit and effecting a closing of this 120 volt circuit to heat the electrodes 50 of the lamp 40. As was described above, the circuit for heating the electrodes 50, to break down the coating thereon with which fluorescent lamp electrodes are normally supplied, is a combination of the circuit of Figure 6 with the circuit of Figure 5, the Figure 6 circuit being substituted for the coil 174 of Figure 5. Thus at this point in the lamp exhaust operation the electrodes are being heated to break down the coating thereon, a flushing gas is being introduced into one end of the lamp through valve 120 and the lamp is being evacuated from the other end thereof through valve 128.

After the lamp has been pumped down fairly well and most of the contaminants therein flushed therefrom, the solenoid valve 128 is closed by the movement of insulating tape 156a out of contact with finger switch 136a and the consequent closing of this 12 volt circuit through the program plate 132 and opening of this 120 volt circuit at 176-180 in relay 170. After the solenoid valve 128 has been closed to disconnect the lamp 40 from the rough pump, the solenoid valve 130 is opened to connect the lamp 40 to a fine pump. Opening of the valve 130 is effected when insulating tape 156d moves into engage- 6 ment with finger switch 136d, thereby opening this 12 volt circuit, closing this volt circuit through its relay 170, and energizing coil 174 of the valve.

Heating of the electrodes 50 is terminated by the movement of insulating tape 156c out of engagement with finger switch 1360, and results in the closing of this 12 volt circuit and opening of this 120 volt circuit. Pumping of the lamp 40 through solenoid valve is continued after heating of the electrodes 50 has been discontinued. The oven 10 is then retracted from encompassing relationship with respect to the lamp by manipulation of the crank handle 34.

Mercury is then introduced into the lamp when insulating tape 156a moves into engagement with finger switch 136a and thereby opens this particular .12 volt circuit, closes this 120 volt circuit and energizes 'coil 174 of solenoid 98 (Fig. 3). The energized coil 174 draws the solenoid plunger 104 downwardly and, through the pin 106 thereof, effects clockwise rotation of spring finger lever extension 102a and lever 102 about its fulcrum 110. Clockwise rotation of lever 102 effects an upward movement of mercury dispensing pin 78, the displacement of which permits mercury, which is at atmospheric pressure in reservoir 74, to drop from the tapered bottom of mercury dispensing chamber 76, through bore 84 and into bore 66 where the pressure is substantially less than atmospheric. Since flushing of the lamp with argon is taking place at the same time that a small quantity of mercury is being released by the mercury dispensing pin 78, the mercury in the bore 66 is swept into the lamp thereby.

Since the amount of mercury introduced into the lamp is relatively small, between about 30 to 60 milligrams for example, the making and breaking of the 120 volt circuit to effect energization and de-energization of the solenoid coil is effected very quickly, as the length of the insulating tape 156s indicates. If desired, a trip switch may be incorporated in the mercury dispensing circuit to limit the effective time this circuit is closed and thereby make sure that only the desired small quantity of mercury is dispensed. After the lamp has been filled with the desired amount of mercury, the insulating tape 15Gb moves out of engagement with the finger switch 136b, thereby closing the 12 volt circuit therethrough, breaking the 120 volt circuit and closing the solenoid valve 120 through which the argon was introduced to the lamp. The lamp is then pumped down through solenoid valve 130 until the pre-determined final argon pressure, such as about 3 mm. of mercury for example, is reached, and then this valve is closed by the passage of insulating tape 156d out of engagement with finger switch 136d. This completes the exhausting and filling of the lamp 40, and the tubulations 46 are then sealed by some suitable means, such as by a conventional hand torch for example. The motor 148 (Fig. 4) is de-energized to terminate the travel of the program plate 132, the program plate is tilted to effect disengagement of the rack 142 with its pinion 144, and then the program plate is pulled back to starting position.

The specific lamp exhausting and filling schedule described above and illustrated by the sequence layout 134 on the program plate 132 in Figure 4 is but one of a number of schedules which may be employed satisfactorily. The length and the relative disposition of the several strips of insulating tape 156 may be varied to effect such changes as may be desired. Not only may the timing of these operations relative to one another be changed but additional operations may also be performed without departing from the spirit of our invention. For example, a single valve may be employed to connect the left hand end of lamp 40 in Figure 1 to a single pump instead of using two valves to connect this end of the lamp to two separate pumps. In some cases it may be found desirable to have at least one pump connected to each end of the lamp and exhaust alternately from one end then the other or from both ends at the same time. Lamps may be pre-heated to the desired temperature before the lamp exhaust schedule cycle is initiated or they may be brought up to the desired temperature in the oven 10 during the cycle. If the former alternative is employed, the lamp exhaust schedule may be completed more quickly than if the latter alternative is used. We have also found that satisfactory results may be ob tained by pre-heating the lamps to the desired bulb temperature and then carrying out the exhausting and filling operations at room temperature, i. e., with pre-heated lamps an oven 10 is not absolutely necessary.

What we claim is:

1. The method of exhausting an electric gaseous discharge device which comprises: heating said device; and introducing a flushing gas into said device at one end thereof and simultaneously evacuating said device from another end thereof.

2. The method of exhausting an electric gaseous discharge device which comprises: heating said device; evacuating said device to a pressure substantially below atmospheric pressure; and then introducing a flushing gas into said device at one end thereof and simultaneously evacuating said device from another end thereof.

3. The method of exhausting a fluorescent lamp envelope having a mount sealed thereto at each end thereof and a tubulation projecting from each end and in communication with the interior thereof, said method comprising: heating said envelope; and introducing a flushing gas into said envelope through the tubulation at one end thereof and simultaneously evacuating said envelope through the tubulation at the other end thereof.

4. The method of exhausting a fluorescent lamp envelope having a mount sealed thereto at each end thereof and a tubulation projecting from each end and in communication with the interior thereof, said method comprising: heating said envelope; evacuating said envelope to a pressure substantially below atmospheric pressure through the tubulation at one end thereof; and then introducing a flushing gas into said envelope through the tubulation at one end thereof and simultaneously evacuating said envelope through the tubulation at the other end thereof.

5. The method of exhausting and filling an electric gaseous discharge device, said method comprising: heating said device; introducing a flushing gas into said device at one end thereof and simultaneously evacuating said device from another end thereof; introducing an inert gas into said device; and sealing the ends of said device.

6. The method of exhausting and filling a fluorescent lamp envelope having a mount sealed thereto at each end thereof and a tubulation projecting from each end and in communication with the interior thereof, said method comprising: heating said envelope; introducing a flushing gas into said envelope through the tubulation at one end thereof and simultaneously evacuating said envelope through the tubulation at the other end thereof; introducing a small quantity of mercury and an inert filling gas into said envelope through the tubulation at an end thereof; and sealing the tabulations at each end of said envelope.

7. The method of exhausting an electric gaseous discharge device which comprises introducing a flushing gas into said device at one end thereof and simultaneously evacuating said device from another end thereof.

8. The method of exhausting a fluorescent lamp envelope having a mount sealed thereto at each end thereof and a tubulation projecting from each end and in communication with the interior thereof, said method comprising introducing a flushing gas into said envelope through the tubulation at one end thereof and simultaneously evacuating said envelope through the tubulation at the other end thereof.

9. The method of exhausting and filling an electric gaseous discharge device, said method comprising: introducing a flushing gas into said device at one end there of and simultaneously evacuating said device from another end thereof; introducing an inert gas into said device; and sealing the ends of said device.

10. The method of exhausting and filling a fluorescent lamp envelope having a mount sealed thereto at each end thereof and a tubulation projecting from each end and in communication with the interior thereof, said method comprising: introducing a flushing gas into said envelope through the tubulation at one end thereof and simultaneously evacuating said envelope through the tubulation at the other end thereof; introducing a small quantity of mercury and an inert filling gas into said envelope through the tubulation at an end thereof; and sealing the tubulations at each end of said envelope.

References Cited in the file of this patent UNITED STATES PATENTS 1,551,527 Pirani Aug. 25, 1925 1,601,902 B01 Oct. 5, 1926 1,786,858 Merrell et al. Dec. 30, 1930 2,154,542 Swanson Apr. 18, 1939 2,313,788 Van Dyke Mar. 16, 1943 2,417,361 Herzog Mar. 11, 1947 2,449,676 Seitz Sept. 21, 1948 2,457,651 Eaves et al Dec. 28, 1948 2,477,372 Herzog July 26, 1949 2,565,298 De Groat Aug. 21, 1951 2,570,103 De Groat Oct. 2, 1951 wit-4