US2470604A - Circuit breaker - Google Patents

Description

May 17, 1949. J. M. cUMMlNG CIRCUIT BREAKER Filed Nov. 21, 1944 INVENTOR /mas//mw/)y Ww ATTORN WITNESSES:

Patented May 17, 1949 UNITED vSTATES PATENT 'OFFICE 'CIRCUIT BREAKER James M. Cumming, Turtle Creek, Pa., assigner to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application November 21, 1944, Serial No. 564,447

(Cl. ZOO-450) Claims. 1

This Jinvention 'relates ygenerally to electric circuit linterrupters, and more particularly to `circuit breakers.

lIn relatively large circuit breakers, -Vsuch as those having one or more sets of contacts suspended in 'an insulating medium within a tank or similar enclosure, 'a common mode of actuating su'ch--contacts'is by vmeans of 'a ylift rod lextending downwardly from 'the `top of the enclosure. Where the'breaker 'contacts are biased open, the lift rod is required to withstand the load imposed by vthis bias, and the vcontact pressure load as Well. With modern operating practice requiring high speed breaker operation, the impact load on closing the breaker is extremely high, thus requiring a relatively heavy lift rod structure to withstand the forces encountered. Oi course, any increase in mass of the operating mechanism requires a greater operating force in'order to rapidly'accelerate the mechanism to the desired high speed operation.

One object of this invention, therefore, is to provide means for closing circuit breaker contactsat high speed independently 4oi? the breaker li'it rod.

Another lobject of this invention is to provide a novel karrangement of hydraulic motion transmitting mechainsm for actuating circuit breaker contacts.

Another object 'of this invention is to provide in a circuit breaker of the type having its conta'cts Asuspended from the top of an enclosure, novel means of a compression type for transmitting motion from a point adjacent the top of the enclosure `to the breaker contacts.

Another 'object of this invention is to provide in a circuit breaker oi the type described having 'contacts 'biased to an open circuit position, a lii't 'rod for controlling llatching of the contacts in the closed circuit position, `and motion transmitting means independent of the lift rod for closing said contacts at a high speed.

A further object of this invention is to reduce the mass of the parts required to be moved 'in actuating circuit breaker contacts so that high speed operation is possible by the use of 'a minimum 'actuating force.

These and other objects of this invention will become more apparent upon consideration of cthe 'following detailed description of a preferred 'embodiment thereof, when taken in consideration with the attached drawing, in Which the singie iigure illustrates a circuit breaker with the breaker yiafrlll:ish'ovi/i'rin substantially central longitixdnai section, and with the mechanism in the 2 tank being Yshown partially in section and partially in elevation.

The circuit breaker illustrated is adapted yto be suspended in a tank 2 from the tank v`cover 6. For this purpose, a 'pair of insulating bushings 4 are provided with supporting collars intermediate their ends, and the rbushings are adapted to -extend through Vopenings in tank 'cover 6 the supporting collars secured to the cover, Efor "'example, as by ybolts I0. The cove-r y(i is ffadapted to be removably secured to the tank -in any de'- sired manner, such, for example, as by bolts '8. Bushings 4 are each provided with a longitudinally extending bore for receiving a 4comiiuctznr rod 'l2 adapted to support Aa contact 'assembly H inside casing 2.

Each contact 'assembly le includes 4a supporte ing bracket I6 at the upper 'end thereof, having athre'aded socket for threaded engagement with the lower end of bushing'conductor rod l2. `Each supporting bracket l5 is of a goodfelectricalcom ducting material, and is adapted to have 'si'ipport ing plates 18 Vsecured lto opposite sides thereof, for example, as by bolts I9. Supporting plates 11 should be off'insulating material, 'andare 'adapted to support therebetween a plurality oi hollow arc-extinguishing casi-rigs '20 in spaced relation. The arc-extinguishing casing's 20 may Ib'e "of 4solid construction, but preferably v'are composed of a plurality of superposed plates of insulating inalierial, as shown more particularly in the patent to J. B. MacNeill No. 2,192,772, on a `('il'clli', in'- terrupter, issued March 5, 1940, to the same-fassignee as this invention. The `arc-ezuin-gi'iisnin'g casings 20 are adaptedto be secured between 'supaporting plates I'S by through bolts -2'I, 4and each casing is provided with an opening l'at the t'o'p for receiving fa contact proj ecticn 'Z2 fo'ne'd integral with a contact support 2'4. Each 'cntact support 24 is mounted for limited pivotal moyement at one end by a pivot pin l26 extending between supporting plates IS, and 'each Acontar/t support is biased for movement of its contact projection 2'2 in a direction to in 'ove 'into its associated arc-extinguishing 'casing 20, by a coll compression spring 28 reacting between the -chtact support '-24 and a stationary 'support fs'uch the arc-extinguishing casing immediately there above, or in the case olf the uppermost arc- 'extinguishing 'casii1g,'its spring '28 reacts 'against supporting bracket i6. Movement of each contact projection `22 into its vassociated armemguishin'g 'casing 20 is limited by engagement fof contactsupport 24 with 'the upper side xof the nrceX-tinguishing casing. 'The 'Contact support M associated with the uppermost arc-extinguishing housing 28 is adapted to be electrically connected with supporting bracket I6 by a flexible shunt 36.

Each arc-extinguishing casing 28 has a movable contact 82 associated therewith, which is mounted for pivotal movement about a pivot pin 34 extending between supporting plates I8, so as to be movable toward and away from its associated contact projection 22 through a suitable opening provided in the hollow arc-extinguishing casing 20. The arc-extinguishing casing 2l) may also be provided with side vent openings to provide for the relief of uid pressure built up within the casing when the contacts separate, and to provide for a flow of fluid laterally through the arc upon separation of the contacts, in the manner shown in the aforesaid MacNeill patent. Each movable contact 32 is electrically connected with the contact support 24 immediately below it by a flexible shunt 38, and the lowermost movable contact 32 is electrically connected with a pair of stationary contact jaws 42, by means of a flexible shunt 40. The contact jaws 42 of the pair of contact assemblies i4 are adapted to be bridged by a bridging contact bar 44 mounted on a movable lift rod 46 of an insulating material, preferably a molded insulating material.

The movable contacts 32 of each contact assembly I4 are connected for movement together by a connecting link 48 pivotally connected with each movable Contact by means of pivot pins 58. Connecting link 48 is provided at its lower end with a latch groove 52 for cooperation with a ball latch 54, to releasably latch the movable contacts in engagement with relatively stationary Contact projections 22, with pressure springs 28 stressed a predetermined amount. A movable latch retainer 56 stressed a predetermined amount. A movable latch retainer 56 is adapted to be held in a position to retain ball latches 54 in engagement with latch grooves 52 in the closed circuit position of the breaker, by engagement of a head 58 provided on the lower end of retainer 56, with the bridging Contact bar 44. A release spring 6D reacts between head 58 of latch retainer 56 and the lower end of each contact assembly I4, so that when bridging contact bar 44 is moved downwardly to disengage contact jaws 42, latch retainer 56 will be moved downwardly by release spring 6Fl to permit ball latch 54 to escape from latch groove 52, and permit movable contacts 32 to rapidly disengage contact projections 22, under the influence of contact opening spring 62, which reacts between a stationary pin 59 anchored in supporting plates I8, and a pin 6I provided on connecting link 48.

In order to reclose movable contacts 32 against opening spring 62, there is provided hydraulic motion transmitting means supported on each contact assembly by means of a bracket 64 having a supporting vsleeve 66, adapted to be supported above and substantially in alignment with connecting link 48. Supporting sleeve 66 is adapted to threadedly receive the lower end of a cylindrical conduit 68 of insulating material, such for example as a molded insulating material. A cylindrical pump sleeve I8 is adapted to be threaded on the upper end of conduit 68, and a bracket 'Il'y connects pump sleeve 'I0 to cover 6, to rigidly support the pump sleeve and upper end of conduit 68. A piston 'I2 is provided in the lower "end of conduit 68, and this piston has a piston .rod I4 secured to the upper end of connecting 'links 48. Piston 'I2 may have its own biasing spring 'I6 which reacts between the piston and the upper end of its associated contact assembly I4, to bias the piston 'l2 in a direction to cause opening of contacts 32. A pump piston 'i8 is provided in pump cylinder and it has a piston rod 8U connected to a retrieving spring 82 reacting between piston rod 86 and cover 6, for a purpose to be described.

Pump piston 'I8 is adapted to be actuated by an operating rod 34 secured to the upper end of lift rod 46, and having a shoulder 86 adapted to engage one end of an actuating lever 88, which is pivotally mounted intermediate its ends on a bracket 90 supported from cover 6, and lever 88 is connected with piston rod 8llat its other end.

The operation of the circuit breaker described above will now be apparent. If it is assumed that the breaker is in its closed-circuit position illustrated in the drawing, the circuit may be opened by moving operating rod 84 downwardly. This releases ball latches 54 in the manner previously described to permit opening springs 62 to rapidly separate movable contacts 32 from contact projection 22. This downward movement of operating rod 84 and lift rod 46 may be very small, for example about one-half inch, and as a matter of fact the contacts may be opened so rapidly by opening spring 62 that they will be fully opened before bridging contact bar 44 is disengaged from contact jaws 42. Pump piston 'I8, being larger than piston 12, needs to be retrieved only a short distance by its retrieving spring 82, to permit suicient flow of liquid through conduit 68 that pi-ston 'l2 is not checked in the circuit opening movement of connecting link 48, but this movement is actually assisted by piston spring 76. Opening movement of operating rod E4 will continue of course, until bridging contact bar 44 reaches substantially the position shown in dotted lines on the drawing, to assure a safe gap in the circuit, and thu-s prevent restriking of the arc after circuit interruption. The rapid opening movement of movable contacts 32 and the action of arc-extinguishing casings 26 contribute to obtain rapid extinguishment of the series arcs drawn during circuit opening in the several arcextinguishing casings 28. Since tank 2 is adapted to be lled substantially to the level L by an arcextiguishing liquid, this liquid will fill the arcextinguishing casings 28 so that when the contacts separate therein, and an arc is drawn, gases will be formed to create pressure within each arcextinguislring casing 26 and 'cause a blast of gas substantially transversely through the arc and out the vent openings. This flow of fluid through the arc, together with the rapid opening movement of the contacts and confinement of the arc within the arc-extinguishing casings, contributes to rapid and eiiicient arc extinction, as pointed out in the above-mentioned MacNeill patent.

In closing the circuit breaker. operating rod 84 is moved upwardly from the dotted line position shown on the drawings, and this results in bridging bar 44 moving into engagement with contact jaws 12, and thereafter shoulder 86 on the operating rod engages actuating levers 88 to move pump pistons 'i8 downwardly, thereby exerting pressure on the liquid entrapped between pump piston 'i8 and piston 'i2 in conduit 68, to cause downward movement of piston 'I2 and consequent closing movement of movable contacts 32. This reclosing movement of contacts 32 must overcome opening spring 62 and stress the same suiiiciently to cause rapid opening of the contacts, and, in addition, suicient force must be employed to stress each contact pressure spring 2B. Also, it is highly desirable that the closing movement of the breaker beas` rapid ass' possible, which means that relatively large forces are required toaccomplish closing of the breaker contactsbecause of' the high force required to prevent rebounding and consequent momentary separation of the contacts, at the moment of initial contact or impact of the contacts.

Itwill be observed that in the breaker construction described above, the operating force is transn'ii'ttedy from operating rod 84 which may be of steel or similar material having a relatively high strength, and that the lift rod 4S- may be relatively light because it is relieved of the force required to actually close the breaker contacts, and the only force it is required to withstand is that necessary to engage and disengage bridging barv 44 with contact jaws 42, and to compress relatively light latch release springs Bil. This is of considerable importance because lift rod 4S must be of an insulating material, and if' it were required to carryy the contact closing load, it would havey to be excessively large and massive construction. Accordingly, it is not only possible; to obtain faster contact movement with the operating mechanism disclosed herein due to the lighter constructionoi the parts, but faster opening' movement of the contacts is possible dine to the small movement of lift rod 45 needed to open the contacts. In other words, it is possible withy this `construction to obtain opening of thev contacts in a shorter time after initiation of movement of the contact operating mechanism.

Operating rod- 84 may be operated in any desired manner. and there is illustrated for this purpose abell cran-l: 92 pivotally mounted on casing cover 6 and having a slot Sill in one arm for receiving a pin 96'r on the operating rod, with thel other arm of the bell crank pivoted to a connecting rod 98, which in turn is operably connected to a push rod lllZ mounted alongside elisir-'rgy 2` by a second` bell crank intl. Push rod F02' may be actuated in any desired manner, such for example as by actuating means IM mounted on the breaker casing adjacent the lower end thereof. Operating means w4- may comprise any well known operating means, such as a solenoid or the like.

Having described preferred embodiments of the invention in accordance with the Patent Statutes, it is desired that this invention be not limited to the particular structure disclosed herein inasmuch as it will be obvious particularly to persons skilled in this art, that many changes and modifications may be made in this particular structure without departing from the broad spirit and scope of this invention, as deiined irrthe following claims.

I claim as my invention:

l. In a circuit breaker, a liquid container having side, bottom and top walls, separable contacts 'supported from said top Wall so as to be positioned in said container below the liquid level thereof, said contacts being biased apart to an open circuit. position, means located adjacent said contacts for lat'ching said contacts closed, actuating mechanism supported on said top Wall, liquid motion transmitting means also supported from said top wall and connected between said mechanismA and contacts. for closing said contacts against .said bias when said mechanism is operated in one direction, and said mechanism ,having a relatively ylight movable rod which is separate from said motion transmitting means and contacts and extends downwardly in said container into proximity with said latchingA means for releasing said latching means by movement independent of.` said contacts in response to operation of said mechanism in thev opposite direction.

2. In a circuit breaker, a container, a, pair of spaced bushings of insulating material extending into said container, a conductor in each of saidl bushings, an. interrupting structure mountedy on each of said bushings, each of said structures having at least one set of separable contacts which are biased apart and are adapted to control the circuit through said: conductors, a contact on the outer end of each interrupting; structure, a movable bridging contact for bridging the outer contacts of said interrupting structures, actuating mechanism for ope-rating said contacts, said mechanism including a movable rod carrying said bridging contact at its outer end, means to releasably latch said separable conta-cts closed, and mean-s controlled by initial opening movement. of said rod prior to separation of said bridging contact from said outer contacts for releasing said latch means and permitting said separable contacts to separate under thev iniiuence of their bias to open the circuit, after which said bridging contact is moved by said lift rod to insert a relatively large gap in the circuit.

3'. In a circuit breaker, a container, a pair of spaced bushings of insulating material extending into said container, a conductor in each of said bushings, an interrupting structure mounted on each of said bushings, each of said structuresv having at least one set of separable contacts which are biased. apart and are adapted to control the circuit through said conductors, a contact on the outer end of each interrupting structine,v a movable bridging contact for bridging the outer contacts of said interrupting structures, actuating mechanism for operating said contacts, said mechanism including a movable rod carrying said bridging contact at its outer end, meansv to releasably latch said separable contacts closed, means controlled by initial opening movement of said rod prior to separation of said bridging contact from said outer contacts for releasing said latch mean-,s and permit said separable contacts to separate under the influence of their bias to open the circuit, after which said bridging contact is moved by said rod to insert a relatively large gap in the circuit, and means cooperative with said rod at a point adjacent the other end thereof for reclosing said separable contacts in responsey to reverse movement of said rod.

4. In a circuit breaker, a container, a pair of spaced bushings of insulating material extending into said container, a conducto-r in each of lsaid bushings, an interrupting structure mounted on each of saidbushing-s, each of said structures having at least one set of separable contacts which are biased apart and are adapted to control the circuit through said conductors, a contact on the outer end oi each interrupting structure, a movable bridging contact for bridging the outer contacts oi said interrupting structures, actuating mechanism for operating said contacts, said mechanism including a movable rod carrying. said bridging Contact at its, outer` end, means to releasably vlatch said separable contacts closed, means controlled by initial opening movement of said rod prior to separation of said bridging contact from said outer contacts for releasing said latch means and permitting said separable contacts to separate under the influence of their bias to open the circuit, after which said bridging Contact is moved by said rod to insert a relatively large gap in the circuit, and means cooperative with a portion of said mechanism for reclosing said separable contacts in response to reverse movement of said rod.

5. In a circuit breaker, a container, a pair of spaced bushings of insulating material extending into said container, a conductor in each of said bushings, an interrupting structure mounted on each of said bushings, each of said structures having at least one set of separable contacts which are biased apart and are adapted to control the circuit through said conductors, a contact on the outer end of each interrupting structure, a movable bridging contact for bridging the outer contacts of said interrupting structures, actuating mechanism for operating said contacts, said mechanism including a movable rod carrying said bridging contact at its outer end, means to releasably latch said separable contacts closed, means controlled by initial opening movement of said rod prior to separation of said bridging contact from said outer contacts for releasing said latch means and permitting said separable contacts to separate under the inuence of their bias to open the circuit, after which said bridging contact is moved by said rod to insert a relatively large gap in the circuit, means cooperative with said rod at a point adjacent the other end thereof for reclosing said separable contacts in response to reverse movement of said rod, including liquid motion transmitting means located adjacent said bushings.

6. In a circuit breaker, a container, a pair of spaced bushings of insulating material extending into said container, a conductor in each of said bushings, an interrupting structure mounted on each of said bushings, each of said structures having at least one set of separable contacts which are biased apart and are adapted to control the circuit through said conductors, Va contact on the outer end of each interrupting structure, a movable bridging contact for bridging the outer contacts of said interrupting structures, actuating mechanism for operating said contacts, said mechanism including a movable rod carrying said bridging contact at its outer end, means to releasable latch said separable contacts closed, means controlled by initial opening movement of said rod prior to separation of said bridging contact from said outer contacts `for releasing said ltch means and permitting said separable contacts to separate under the influence of their bias to open the circuit, after which said bridging contact is moved by said rod to insert a relatively large gap in the circuit, and means for transmitting motion from a portion of said mechanism located adjacent the other end of said rod to said separable contacts when said mechanism is operated to move said rod to close the circuit, said motion transmitting means being ineiective to transmit motion when said mechanism is operated to open the circuit.

7. In a circuit breaker, a container, a pair of spaced bushings of insulating material extending into said container, a conductor in each of said bushings, an interrupting structure mounted on each of said bushings, each of said structures having a plurality of sets of separable contacts which are biased apart and are adapted to control the circuit through said conductors, said sets of contacts being arranged in longitudinally spaced relation, a contact on the outer end of each interrupting structure, a movable bridging contact for bridging the outer contacts of said interrupting structures, actuating mechanism for operating said contacts, said mechanism including a movable rod carrying said bridging contact at its outer end, means to releasably latch said separable contacts closed, and means controlled by initial opening movement of said rod prior to separation of said bridging contact from said outer contacts for releasing said latch means and permitting said separable contacts to separate under the influence of their bias to open the circuit, after which said bridging contact is moved by said rod to insert a relatively large gap in the circuit.

8. In a circuit breaker, a container, a bushing of insulating material extending into said container and having an electrical conductor therein, an interrupting structure mounted on the inner end of said bushing and including at least one set of separable .contacts which are biased apart and are adapted to control the circuit through said conductor, a contact on the outer end of said interrupting structure, actuating mechanism including a movable rod extending down into said container, a contact carried by said rod at its outer end for movement with said rod into and out of engagement with said outer end contact, means for releasably latching said separable contacts closed, and means controlled by initial movement of said rod in a direction to open the circuit to release said latching means prior to separation of said rod contac-t from said lower end Contact.

9. In a circuit breaker, a container, a bushing of insulating material extending into `said container and having an electrical conductor therein, an interrupting structure mounted on the inner end of said bushing and including at least one set ci separable contacts which are biased apart and are adapted to control the circuit through said conductor, a contact on the outer end of said interrupting structure, actuating mechanism in.. cluding a movable rod extending down into said container, a contact carried by said rod at its outer end for movement with said rod into and out of engagement with said outer end contact, means for releasably latching said separable contacts closed, means controlled by initial movement of said rod in a direction to open the circuit to release said latching means prior to separation of said rod contact from said lower end contact, and means .cooperative with said rod at a point adjacent the other end thereof for reclosing said separable contacts in response to reverse movement of said rod.

10. In a circuit breaker, a container, a bushing of insulating material extending into said container and having an electrical conductor therein, an interrupting structure mounted on the inner end of said bushing and including at least one set of separable contacts which are biased apart and are adapted to control the circuit through said conductor, a contact on the outer end of said interrupting structure, actuating mechanism including a movable rod extending into said container, a contact carried by said rod at its outer end for movement with said rod into and out of engagement with said outer end contact, means for releasably latching said separable contacts closed, means controlled by initial movement of said rod in a direction to open the circuit to release said latching means prior to separation of said rod Contact from said lower end contact,

and means for transmitting motion from a portion of said mechanism located adjacent the other end of said rod to said separable contacts when said mechanism in operated to move said rod to close the circuit, said motion transmitting means being ineiective to transmit motion when said mechanism is operated to open the circuit.

11. In a circuit breaker, a container, a bushing of insulating material extending into said container and having an electrical conductor therein, an interrupting structure mounted on the inner end of said bushing and including a plurality of longitudinally spaced sets of separable contacts which are biased apart and are adapted to control the circuit through said conductor, a -contact on the outer end of said interrupting structure, actuating mechanism including a movable rod extending into said container, a contact carried by said rod at its outer end for movement with said rod into and out of engagement with said outer end contact, means for releasably latching said separable contacts closed, and means controlled by initial movement of said rod in a direction to open the circuit to release said latching means prior to separation of vsaid rod contact from said lower end contact.

12. In a circuit breaker, a container, a bushing of insulating material extending into said container and having an electrical conductor therein, an interrupting structure mounted on the inner end of said bushing and including at least one set of separable contacts which are adapted to control the circuit through said conductor, actuating mechanism mounted on said container, means for actuating said contacts including tubular means of insulating material extending between said mechanism and said contacts, said tubular means adapted to be filled with liquid, and displaceable means at each end of said tubular means associated with said `contacts and mechanism, respectively, to at least actuate said contacts to closed circuit position in response to actuating of said mechanism in one direction.

13. In a circuit breaker, a container, a bushing of insulating material extending into said casing and having an electrical conductor therein, an interrupting structure mounted on the inner end of said bushing and including lat least one set of separable contacts which are adapted to control the circuit through said conductor, actuating mechanism mounted on said container, means for `actuating said contacts ,including tubular means of insulating material extending between said mechanism and said contacts, said tubular means adapted to be lled with liquid, displaceable means at each end of said tubular means associated with said contacts and mechanism, respectively, to at least actuate said contacts to closed circuit position in response to actuation of said mechanism in one direction, said contacts being biased apart, releasable latch means for maintaining said contacts in engagement, means responsive to actuation of said mechanism in the opposite direction to release said latch means, Iand said mechanism being movable in said opposite direction independently of the displaceable means associated therewith.

14. In a circuit breaker, a liquid container having side, bottom and top walls, separable interrupting contacts positioned in said container below the liquid level thereof, said contacts being biased apart -to an open circuit position, means located adjacent said contacts latching said contacts closed, actuating mechanism suported on said top Wall, liquid motion transmitting means connected between said mechanism and contacts for closing said contacts against said bias when said mechanism is operated in one direction, separable disconnect contacts in said container and connected in series circuit relation with said interrupting contacts, said mechanism including a relatively light movable rod which is separate from said motion transmitting means and said interrupting contacts and carries one of said disconnect contacts adjacent its lower end, and means controlled by initial movement of said rod by said mechanism in opening the circuit to release said latching means by movement independent of said interrupting contacts to permit separation of said interrupting contacts under the influence of their bias to open the circuit, after which said one disconnect contact is separated from the other of said disconnect contacts.

15. In a circuit breaker, a liquid container having side, bottom and top walls, separable interrupting contacts positioned in said container below the liquid level thereof, said contacts being .biased apart to an open circuit position, means located adjacent said contacts latching said contacts closed, actuating mechanism supported on said top wall, liquid motion transmitting means connected between said mechanism and contacts for closing said contacts against said bias when said mechanism is operated in one direction, separable disconnect contacts in said container and connected in series circuit relation with said interrupting contacts, said mechanism including a relatively light movable lift rod which is separate from said motion transmitting means and said interrupting contacts for carrying one of said disconnect contacts adjacent its lower end, means controlled by initial movement of said rod by said mechanism in opening the circuit to release said latching means by movement independent of said interrupting contacts and permit separa-tion of said interrupting contacts under the influence of their bias to open the circuit, after which said one disconnect contact is separated from the other of said disconnect contacts, and an operative connection between said motion transmitting means and the upper end of said lift rod for closing said interrupting contacts upon movement of said rod in the opposite direction.

JAMES M. CUMMING.

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

UNITED STATES PATENTS Number Name Date 2,086,752 Thornburg 1 July 1.3, 1937 2,192,772 MacNeill Mar. 5, 1940 2,249,006 Kesselring et al. July 15, 1941

Images