US2810808A - Cable type operating mechanism for an electric circuit breaker - Google Patents

Description

Oct. 22, 1957 11R. coast-:SHALL 2,810,808

CABLE TYPE OPERATING MECEANISM FCE AN ELECTRIC CIRCUIT BREAKER Filed Jan. 25, 1955 v3 ya b ma M Hmorneg.

United States Patent O CABLE TYPE OPERATING MECHANISM FOR AN ELECTRIC CIRCUIT BREAKER Thellwell R. Coggeshall, Cynwyd, Pa., assigner to General Electric Company, a corporation of New York Application January 25, 1955, Serial No. 483,945

14 Claims. (Cl. Zim-161) This invention relates to an operating mechanism for transmitting circuit-controlling movement between a source of motive power and the movable switch members of a circuit breaker.

Where it is necessary to transmit such circuit-controlling movement over a relatively great distance, it has been customary to utilize an operating mechanism which consists of a series of interconnected, substantially rigid links and levers. Because of the precise motion requirements frequently involved, it has been necessary to resort to complex mechanisms which utilize a large number of such rigid links and levers. As an example, reference may be had to the straight-line mechanism disclosed in Rankin Patent No. 1,754,179, assigned to the assignee of the present invention. Frequently too, each of the links and levers of such mechanisms must be relatively massive in construction in order to withstand the high forces involved. As a result, the composite mechanism has a high effective mass which tends to retard rapid acceleration of the switch parts. Frequently, as in the case of extreme high speed breakers, such retardation is a serious disadvantage.

Accordingly, a primary object of my invention is to provide a circuit breaker operating mechanism which is capable of providing precisely-controlled movement of the breaker parts and yet has an exceptionally low ef- `fective mass.

Another disadvantage which is inherent in the conventional type of circuit breaker operating mechanism is that because of the large number of links and levers, there is also a large number of interconnecting joints. These joints invariably have an appreciable amount of play and deflection which must be taken up before motion of all of the parts can be initiated. Obviously, if such play and deilection is held to a minimum, the time required to initiate operation of the circuit breaker may be materially shortened.

Accordingly, another object of my invention is to construct a circuit breaker operating mechanism in such a manner that an exceptionally low number of interconnecting joints is required, as a result of which play and deflection is held to a minimum.

In accordance with one form of my invention, a movable circuit breaker switch member is interconnected to a source of motive power by means of a low-mass operating mechanism which includes a flexible tension member, e. g., a steel cable, stationarily anchored at one end and connected at its other end to the switch member. A rotatable pulley is arranged to engage the tension member and is bodily movable to transmit circuit controlling movement to said switch member through said tension member. This pulley is coupled to the source of motive power by a linkage which is actuable by the motive power source to bodily move the pulley whereby to eiect circuit-controlling movement of said switch member.

My invention is especially applicable to multi-pole circuit breakers wherein a plurality of simultaneously-,operable switch members are to be operated from a com- Cil mon source of motive power. For such applications, I provide a plurality of tension members each of which is stationarily anchored at one end and at its other end is connected to a different one of said switch members. A plurality of coaxially-mounted rotatable pulley structures are arranged to engage the tension members and are bodilymovable in unison to transmit simultaneous circuit-controlling movement to the switch members through said tension members.

A practical problem involved in utilizing a cable-type operating mechanism of the above type is that there is a tendency for the cable to whip during periods of abrupt mechanism deceleration. This imposes an undue strain on certain of the parts and, if excessive, can also cause the cable to become disengaged from its pulley or pulleys.

Accordingly, another object of my invention is to minimize the possibility that whip will develop in the cable during operation of the mechanism.

Other objects and advantages will appear more clearly from the following description of an embodiment of my invention when taken in connection with the accompanying drawing wherein:

Fig. 1 shows a single pole circuit breaker having an operating mechanism constructed in accordance with my invention; Fig. 2 is a reduced scale cross-sectional View taken along the line 2-2 of Fig. l; Fig. 3 is a plan view showing my invention applied to a multi-pole breaker; Fig. 4 is a side elevational view of the multi-pole breaker of Fig. 3; whereas Fig. 5 is an enlarged perspective view of a portion of the mechanism of Figs. 3 and 4.

Referring more particularly to Figs. 1 and 2, l have shown a high voltage oil circuit breaker comprising a single pole unit 7. This pole unit '7 comprises an oilfilled metallic tank 8, a pair of spaced-apart high voltage bushings 9 extending through the top of the tank, and a pair of spaced-apart interrupting devices 10 mounted at the lower ends of the bushings. These interrupting devices 10, which may be of any conventional form, such as, for example, shown in Coggeshall Patent No 2,522,994, are electrically interconnected by means of a conventional contact blade 13 which is vertically reciprocable to eilect opening and closing or" the circuit through the pole unit.

The solid lines of Fig. l show the Contact bla-:le 13 in its upper or closed-circuit position wherein it is suitably latched by means which will soon be described. The contact blade 13 is biased toward its open circuit position by means including a long-travel compression spring 14 acting through an insulated lift rod 1S which is suitably secured to the contact blade 13. The lift rod is guided for straight line motion by suitable slide bearing structure such as 16 mounted within a vertically-extending Vstationary sleeve 18. The compression opening spring 14, which is shown mounted within this sleeve 18, bears at its upper end against a suitable fixed abutment and at its lower end against a suitable shoulder member 17 secured to the movable lift rod 1S. Thus, when the contact blade 13 is unlatched, as during circuit breaker tripping, it is driven rapidly downward toward its dottedline position to effect opening of the circuit through the breaker. Initial downward movement is accelerated by the usual short-travel contact-opening springs such as 19 within the interrupting devices 10. as seen in Fig. 2.

For lifting the contact blade 13 to close the breaker, I provide a source of motive power such as a fluid motor 20. This iluid motor comprises a cylinder 26, which is stationarily mounted on the tank 8 and a piston 27 which is vertically reciprocable within the cylinder 26. With the circuit breaker in open position, the piston 27 occupies a dotted-line position at the upper end of the cylinder 26. Closing of the breaker is effected by driving tne piston 27 downwardly into its lower or closed-circuit position, where it is shown by solid lines in Fig. l. This driving action is produced by supplying pressurized fluid to the upper side of the piston through a suitable electromagnetically-operated closing control valve at 28. Preferably, the valve 28 is of the type which, upon energization, opens to permit pressurized fluid to flow therethrough and into cylinder 26, and upon deenergization, closes and vents the cylinder to atmosphere. The piston 27 is releasably held in its lower or closed-circuit position by means of a trippable hold-closed latch 29 cooperating with a suitable latch roller 29h. For tripping the latch 29, there is provided a suitable electromagnetic trip device 29a which may be arranged in a well known manner to operate in response to any desired condition.

For transmitting the downward or circuit-closing movement of piston 27 to the movable switch member 13, I provide a novel operating mechanism comprising a flexible tension member, such as a multi-strand cable 30, which is stationarily anchored at one of its ends X and is fixed to the switch member 13 at its other end Y. Preferably, such cable should be of the prestressed type thereby insuring that no further distortion of its strands will occur due to stress of the cable under load conditions. Engaging the cable and having outer grooved peripheries about which the cable is looped are a pair of rotatable pulleys 31 and 32. The pulley 31, which is rotatably mounted on stationary brackets 44 fixed atop the tank 8, acts to guide the cable vertically downward in axial alignment with the lift rod 15. The pulley 32, which is rotatable on a shaft 33, is mounted for swinging movement upon a bell crank 34. This bell crank 34 is pivotally mounted upon a stationary pin 35 which extends between suitable transversely-spaced brackets 37 mounted adjacent an edge of the tank 8.

The bell crank 34 has a long arm 38 upon which the pulley 32 is mounted and a short arm 39 which is coupled to the piston 27 by means of a suitable connecting rod 40. This connecting rod 40 is pivotally connected at one of its ends to the short bell crank arm 39 and at its other end to the piston rod 41.

The anchoring means for the end X of the cable 30 preferably comprises a pin 43 about which the end of the cable is looped in such a manner that the looped portion is free to rotate with respect to pin 43.

Assume now that the breaker is in open-circuit position, i. e., with the switch member 13, the bell crank 34 and the piston 27 in their respective dotted line positions. If it is then desired to close the breaker, the closing control valve 28 would be opened, thereby applying pressurized fluid to the top of the piston 27 and driving it rapidly downward toward its solid line closed-circuit position, This piston motion is transmitted through the connecting rod 40 to the bell crank 34 and rapidly drives the bell crank and its associated pulley 32 counterclockwise about their common pivot pin 3S. In so moving, the pulley 32 quickly lengthens the effective distance between pulleys 31 and 32, and thereby pulls the driven end of cable 30 and the interconnected switch member 13 rapidly upward toward the closed-circuit position.

Because the bell crank has arms of unequal length, it will be clear that the horizontal displacement D of pulley 32 will be proportionately greater than the vertical stroke A of the driving piston 27. Furthermore, because the cable 30 is anchored at a fixed point and is looped over approximately 180 degrees of the periphery of pulley 32., it will also be clear that the displacement of the cable 30 at the guide pulley 31 will be approximately double that of D. These two features produce effective multiplication of the displacement of the piston 27 and enable the switch member 13 to be moved through a relatively long stroke B in response to a relatively short stroke A of piston 27.

- Another feature of my bell crank 34 is that it is designed to provide a progressively increasing mechanical fil 4 advantage as it is driven from its dotted-line open position to its solid-line closed position. More specifically, during this movement, the effective lever arm which extends normal to the cable 30 from the crank pivot axis at 35 becomes progressively shorter, whereas the effective lever arm between the pivot axis and the connecting rod 40 becomes progressively longer. This progressively increasing mechanical advantage enables the liuid motor 20 to more effectively overcome the progressively increasing retarding forces which are encountered as the switch member 13 is driven toward its closed-circuit position.

It will also be apparent that this cable-type of operating mechanism is capable of moving the lift rod 15 through a straight line path without the need for complex vstraight-line linkages, such as shown in the aforementioned Rankin patent. Obviating the need for such massive and complex linkages permits a substantial reduction in the overall mass of the operating mechanism. Still further reductions in mass are made possible by virtue of the well-recognized fact that a multi-strand cable, such as my cable 30, has an appreciably greater strength-toweight ratio than most solid rigid links, thus permitting the use of a cable of comparatively low mass. These reductions in rnass permit the parts of the mechanism to be more rapidly accelerated and thus permit appreciable reductions in the time required for tripping the breaker. A description of a tripping operation will soon follow.

lt will also be apparent that, because the cable permits the omission of a large number of the links and levers heretofore required for conventional operating mechanisms, the number of interconnecting joints in the mechanism is correspondingly reduced. This minimizes the deection or give characteristics of the mechanism, and thereby further decreases the tendency of the mechanism to retard initial acceleration. The give of the cable itself is held to a minimum by the aforementioned pre-stressing operation.

To trip the breaker from the solid line closed-circuit position 4to the dotted line open circuit position, a tripping impulse would be applied to the latch 29 thereby freeing the latch roller 2gb for upward movement. The spring 14 aided by springs 19, would then immediately drive the contact blade 13y downwardly toward its open circuit position. The cable 30, the bell crank 34, and the interconnected piston 27 would, of course, be pulled along behind the switch member 13, but because my invention permits these parts to have a relatively low mass, they would otter little retardation to the desired rapid acceleration. Since the space above the piston 27 would have been previously dumped to atmosphere by the thendeenergized control valve 28, at the end of the closing stroke, the piston would be capable of moving upwardly as described, substantially unimpeded by static fluid within the cylinder 26.

For controlling the speed of this opening movement once the desired initial acceleration has been obtained,

there is provided an opening dashpot 45 which comprises a cylinder 46 and a piston 47 coupled to the fluid motor piston 27. This dashpot 45 is constructed in such a manner that it offers very little opposition to initial acceleration. Only after this initial acceleration has been achieved does it act to retard the then-opening parts. This retardation permits etective circuit interrupting action and also acts to smoothly terminate the opening stroke. Such speed control characteristics may be obtained in any well known manner, as by suitably grooving the internal wall of the dashpot cylinder 45. A suitable by-pass duct 48 containing a check valve 49 permits substantiaily unimpeded movement of the piston 47 in a downward or circuit-closing direction.

The location of the opening dashpot 45 at that eud of the cable 30 which constitutes the driven end during circuit-opening movement is another important feature of my invention. This location is referred to hereinafter as the motive-means end of the cable. By so locating the dashpot, I am able to miniinize any 'tendencyl of cable to whip or become alternately slack and tight during relatively abrupt changes in the velocity -of the opening movement. This is the case because the dashpot 45, during opening movement, maintains the entire operating mechanism including the cable 30 in tension and, as a result, effectively minimizes the tendency of any of the parts to overrun each other.

The possibility that slack or whip will develop in the cable during movement in a reverse or circuit-closing direction is materially lessened by the action of a metering pin 52, which I have incorporated into the fluid motor by xing it to the piston rod 41.V This metering pin tits into an inlet oriiice 53 and is shaped to provide, in cooperh ation with the oriiice, a gradually increasing ilow to the top side of piston 27 during the first portion of closing stroke. Thus, the metering pin permits the piston 27 and the operating mechanism to be brought up to speed at a relatively gradual rate without any tendency toward abrupt retardation. The opening .spring 14 further aids in reducing any tendency for the cable 30 to whip during closing movement. This is the case because the spring 14 is located at the driven end of the cable during closing movement and hence maintains the cable under tension during such movement.

My invention is especially applicable to multi-pole circuit breakers wherein a plurality of simultaneously operable switch members are to be operated from a common source of motive power. For example, referring to Figs. 3 and 4, I have shown, in schematic form, a multi-pole breaker comprising three pole units 60, 61 and 62 each having a movable switch member 13 corresponding to the similarly designated switch member 13 of Fig. l. All of these switch members 13 are to be simultaneously operated from a common source of motive power, such as a fluid motor 20 corresponding to the motor 2t) of Fig. 1. This motor is preferably mounted on the end pole unit 60. For providing the desired operation of the switch members, I provide three cables 30a, 3011, and 30C, each of which has one of its ends stationarily anchored at 65 and its other end connected to a different one of the lift rods of switch members 13. A plurality of coaxially disposedl pulleys 32a, 32h, and 32e are mounted on a bell crank 34 corresponding to the similarly designated single-pulley bell crank of Fig. l. Each pulley is arranged to engage one of the cables in the same manner as described in connection with Fig. l. The three pulleys are mounted for movement in unison by suitable means such as a common centrally disposed shaft 66. The cable 30a, which is looped about the grooved periphery of pulley 32a extends to pole unit 6i) where it is looped about a guide pulley 31a which acts to guide it vertically downwardly into the pole unit, where it is connected to its corresponding switch member 13. Similarly, the cable 36h, which is looped about the periphery of pulley 32h, extends to the center pole unit 61 where it is looped about guide pulley 31b which acts to guide it vertically downward into the pole unit 61, where it is connected to its corresponding switch member 13. Similarly, the cable 30e, which is looped about periphery of pulley 32C extends to the remote end pole 62 where it is looped about a guide pulley 31C which acts to guide the cable vertically downwardly into the pole unit 62, where it is connected to its corresponding switch member 13. All of the guide pulleys 31a, 31b, and 31e are rotatably mounted about a fixed axis in the same manner as described with respect to the guide pulley 31 ofthe single-pole arrangement shown by Fig. l.

For assuring precise guidance of the two longer cables 36h and 39e I prefer to provide idler pulleys which are arranged to engage intermediate portions of the cables 30b and 30C. For example, I provide on the pole unit 60 an idler pulley 67 which engages and guides cable 30h. On pole units 60 and 61 I provide similar idler pulleys 68 and 69 which engage and guide the cable 31C. All of these idler pulleys 67, 68, and 69 are suitably mounted for rotation about stationary axes.

Each of the cables 30a, 3011, and 30C is provided with a long-travel opening spring (not shown) adjacent a switch-member-connected end which corresponds to the opening spring 14 of Fig. l. Each of these springs tends to maintain its corresponding cable in tension in the same manner as the long-travel spring 14 of Fig. l. Additionally, the iuid motor 20 is provided with an opening dashpot 45 which corresponds to the similarly designed dashpot 4d of Fig. l. This dashpot 45 of Fig. 4 acts to maintain all of the cables under tension during circuit breaker opening movement, thereby suppressing cable whip action in the same manner as the dashpot 45 in Fig. 1.

A feature which contributes to the ability of the cables to produce equal and simultaneous movement of the switch members is that the stationarily anchored ends of the cables are in alignment. For example, as shown in detail in Fig. 5, I have provided a common horizontally extending stationary pivot pin 65 about which all three of the cables Aare looped and anchored.

Although I prefer to utilize three separate and distinct cables for operating the three switch members, it will be apparent that certain features of my invention readily lend themselves to other types of cable arrangements. For example, I may provide a single cable comprising a trunk terminating in three parallel branches, each of which is looped about one of the guide pulleys and is connected to a corresponding switch member. The trunk portion of the cable would be looped about a single grooved pulley mounted on the bell crank 34 and would have its end stationarily anchored as at 65.

Although I have shown my invention applied to a multipole 'breaker of the type wherein simultaneous tripping and closing of all the poles is desired, it will be apparent that certain features of my invention are also applicable to multi-pole breakers wherein independent tripping of the individual pole units is desired. To this end, I would provide separate bell-cranks and trip latches for each of the driving pulleys 32a, 3217, and 32e. Closing devices for effecting the `desired closing operation of such individ ually trippable parts are well-known.

Various other changes and modifications may be made without departing from my invention in its broader aspects. I, therefore, intend in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

I claim as my invention:

l. In an electric circuit breaker comprising a tank and a switch member vertically movable therein, motive means for effecting circuit-controlling movement of said switch member, a guide pulley mounted atop said tank, a flexible cable-type tension member stationarily anchored at one end and having a portion adjacent its other end which is looped about said guide pulley and is connected to said switch member, a second pulley acting against said tension member and bodily movable to transmit circuit-controlling movement to said switch member through said tension member, a pivotally mounted crank having a long arm on which said second pulley is mounted and a short arm which is coupled to and is actuable by said motive means to bodily move said second pulley whereby to effect circuit-controlling movement of said switch member.

2. In an electric circuit breaker, a switch member, motive means for effecting circuit-controlling movement thereof, a flexible cable-type tension member stationarily anchored at one end and connected at its other end to said switch member, a rotatable pulley operatively engaging said tension member and bodily movable to transmit circuit-controlling movement to said switch member through said tension member, a pivotally mounted crank having a long arm and a relatively short arm, means for mounting said pulley on said long arm, and means coupling said motive means to said short arm and actuable by said motive means to bodily move said pulley whereby to effect circuit-controlling movement of said switch member.

3. In an electric circuit breaker, a switch member, motive means for electing circuit-closing movement thereof, a exible cable-type tension member stationarily anchored at one end and connected at its other end to said switch member, a rotatable pulley acting against said tension member and bodily Amovable to transmit circuit-closing movement to said switch member through said tension member, a pivotally mounted crank having a long arm and a relatively short arm, means tor mounting said pulley on said long arm, coupling means connected between said short arm and said motive means for transmitting circuitclosing movement to said crank, the pivot axis of said crank being so located that the effective lever arm between said pivot axis and said cable progressively decreases during circuit-closing movement and the eiective lever arm between said axis and said coupling means progressively increases during said circuit-closing movement.

4. In an electric circuit breaker, a switch member, motive means for electing circuit-closing movement thereof, a flexible cable-type tension member stationarily anchored at one end and connected at its other end to said switch member, a rotatable pulley operativelyy engaging said tension member and bodily movable to transmit circuitclosing movement to said switch member through said tension member, a pivotally mounted crank having a long arm on which said pulley is mounted and a short arm which is coupled to and is actuable by said motive means to bodily move said pulley thereby to edect circuit-closing movement of said switch member, the pivot axis of said crank being so located that the crank provides a progressively increasing mechanical advantage as said switch member moves toward its closed position.

5. in an electric circuit breaker, a switch member, a flexible cable-type tension member stationarily anchored at one end and operatively connected at its other end to said switch member, spring means at said other end biasing said switch member toward open-circuit position, a pulley acting against said flexible member and bodily movable to transmit circuit-closing movement to said switch member through said flexible member, motive means coupled to said pulley and operable to bodily move the pulley whereby to eiiect circuit-closing movement of said switch member, and an opening dashpot coupled to said flexible member at a location adjacent said one end, said dashpot acting to maintain said flexible member in tension during circuit-opening movement.

6. In an electric circuit breaker, a biased-open switch member, a fluid motor comprising a piston which is movable to effect circuit-closing movement of said switch member, a liexible cable-type tension member operatively interconnecting said piston and said switch member for transmitting forces therebetween, fluid control means for applying a progressively increasing dow of lluid to said piston during initial circuit-closing movement whereby to minimize abrupt variations in piston-speed during circuitclosing movement, and means for mantaining said exible member in tension during circuit-opening movement comprising an opening dash-pot coupled to said piston at the motor end of said liexible member.

7. In a multi-pole electric circuit breaker, a switch member individual to each pole of the breaker, motive means for effecting simultaneous circuit-controlling movement of said switch members, cable means connected to each of said switch members and having a portion which is stationarily anchored, pulley structure acting against said cable means and bodily moveable to transmit circuitcontrolling movement to said switch members through said cable means, a pivotally-mounted crank having a long arm on which said pulley structure is mounted and a relatively short arm which is coupled to and is actuable 8 Y by said motive means to bodily move said pulley structure whereby to elect circuit-controlling movementof said switch members.

8. The combination of claim 7 in which the pivot axis of said crank is so located that the crank provides a progressively increasing mechanical advantage between said motive means and said switch members as the motive means moves the switch members in a circuit-closing direction.

9. in a meriti-pole electric circuit breaker, a switch member individual to each po-le of the breaker, common motive means for electing simultaneous circuit-controlling movement of said switch members, a plurality of flexible cable-type tension members each of which is stationarily anchored at one end and is connected at its other end to a dilerent one of said switch members, a plurality of pulley structures each of which engagesV a different one of said tension members and is bodily movable to transmit circuit-controlling movement to its corresponding switch member, support stmcture mounting said pulley structures in coaxial relationship for bodily movement in unison and actuable to transmit circuitcontrolling movement to each of said switch members through its corresponding tension member, and means coupling said motive means to said support structure and operable by said motive means to actuate said support structure whereby to elect simultaneous circuit-controlling movement of said switch members.

il). The structure of claim 9 in combination with means for maintaining said flexible members in tension during circuit-opening movement comprising an opening dashpot coupled to said piston at the motive-means-end `of said flexible members.

11. In a multi-pole circuit breaker, a plurality of horizontally spaced pole units, a vertically movable switch member individual to each of said units, common motive means mounted on one of said units and operable to elect simultaneous circuit-controlling movement of said switch members, a plurality of guide pulleys each of which is mounted on a dilerent one of said pole units, a plurality of flexible cable-type tension members each having an end portion which is looped about one of said guide pulleys and extends downwardly into coupled relationship with one of said switch members, means for stationarily anchoring the opposite end of each of said tension members, a plurality of rotatable pulley structures each of which operatively engages a dilierent one of said tension members and is bodily movable to transmit circuit-controlling movement to its corresponding switch member, support structure mounting said pulley structures in coaxial relationship for bodily movement in unison and actuable to transmit circuit-controlling movement to each of said switch members through its corresponding tension member, and means coupling said motive means to said support structure and operable by said motive means to actuate said support structure whereby to eli'ect simultaneous circuit-controlling movement of said switch members.

l2. The combination of claim l1 in which said opposite ends of said tension members are anchored in horizontally aligned relationship.

13. ln a multi-pole circuit breaker, a switch member individual to each pole of the breaker, common motive means for eecting simultaneous circuit-controlling movement of said switch members, a plurality of flexible cabletype tension members each of which is stationarily anchored at one end and is connected at its other end to a different one of said switch members, a plurality of rotatable pulley structures each of which operatively engages a different one of said tension members and is bodily moveable to transmit circuit-controlling movement to its corresponding switch member, a pivotally mounted bell crank having a long arm and a short arm, means for mounting said pulley structures in coaxially disposed relationship on said long arm, and means coupling said short arm to said motive means and actuable by said motive means to bodily move said pulley structures in unison whereby to effect simultaneous circuit-controlling movement of said switch members.

14. In a multi-pole circuit breaker, a plurality of horizontally spaced pole units, a switch member individual to each of said pole units, common motive means mount ed on one of said pole units and operable to effect simultaneous circuit-controlling movement of said switch members, a plurality of eXible tension members each of which is stationarily anchored at one end on said one pole unit and is connected at its other end to a different one of said switch members, a plurality of rotatable pulley structures each of which operatively engages a different one of said tension members and is bodily movable to transmit circuit controlling movement to its corresponding switch member, support structure mounting said pulley structures in coaxial relationship for bodily movement in unison and actuable to transmit circuit controlling movement to each of said switch members through its corresponding tension member, and means coupling said motive means to said support structure and operable by said motive means to actuate said support structure whereby to effect simultaneous circuit controlling movement of said switch members.

References Cited in the le of this patent UNITED STATES PATENTS 977,079 Dunbar Nov. 29, 1910 1,357,847 Clothier Nov. 2, 1920 1,506,409 Classen Aug. 26, 1924 1,532,081 Rankin Mar. 31, 1925 2,319,645 Thumim May 18, 1943 2,394,036 Boden et al. Feb. 5, 1946 2,456,971 MacNeill Dec. 21, 1948 2,597,985 Grossu May 27, 1952

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