US2841662A

US2841662A - Load tap changing transformers

Info

Publication number: US2841662A
Application number: US687013A
Authority: US
Inventors: William C Sealey
Original assignee: Allis Chalmers Corp
Current assignee: Allis Chalmers Corp
Priority date: 1957-09-30
Filing date: 1957-09-30
Publication date: 1958-07-01
Anticipated expiration: 1975-07-01

Description

July 1, 1958 w. c. SEALEY LOAD TAP CHANGING TRANSFORMERS 2 Sh ets-Sheet 1 Filed Sept. 30, 1957 July 1, 1958 Filed Sept; 30, 1957 v W jig. 50L

W. C. SEALEY LOAD TAP CHANGING TRANSFORMERS 2 Sheets-Sheet 2 United States Patent O LOAD TAP CHANGING TRANSFORMERS William C. Sealey, Wauwatosa, Wis., assignor to Allis- Chalrners Manufacturing Company, Milwaukee, Wis.

Application September 30, 1957, Serial No. 687,013 9 Claims. (Cl. 200-17) This invention relates in general to load tap changing transformers. More specifically this invention relates to a load tap changing transformer having a tap changing mechanism which is very quiet in operation.

Load tap changing transformers usually employ a dial type switch for varying the turns ratio of the transformer. The dial switch is provided with spaced stationary contacts which are connected to the transformer taps and movable contacts that move from one dialswitch position to the next dial switch position in a step by step motion. A motor and suitable controls are provided for automatically varying the dial switch position in response to line conditions. When the stationary and movable contacts of the dial switch separate during a tap change an arc is drawn between the breaking contacts that persists until the voltage and current are low enough and the contacts are separated sufficiently to extinguish the arc. The contacts should open rapidly in order to minimize the arcing. On the other hand the contacts should not close in the next operating position before a sufficient delay to assure that the arc is extinguished. The tap changing motor may advantageously operate the dial switch through a spring mechanism to produce a quick break opening of the contacts. A disadvantage of most prior art quick break mechanisms of this type is that noise and shock are produced when the movable contacts and the other quick moving parts are stopped suddenly upon entering an operating position. In large load tap changing transformers a further problem incident to the high speed of the movable contacts exists in providing sutficient time for extinguishing the heavier are drawn in the larger transformers.

In the tap changing mechanism of this invention the movable contacts of the dial switch are snapped rapidly to an open position by a spring operated quick break mechanism. The tap change is then completed at a slow speed by means of the tap changing motor. This arrangement provides a quick break and a delayed closing of the contacts which results in a quiet tap changing operation and sufficient time between operating positions for extinguishing the are drawn between the contacts due to the tap changing operation. In the preferred embodiment the motor operates the dial switch movable contacts through a releasable coupling.

It is an object of this invention to provide a new and improved load tap changing transformer in which the tap changer combines the advantages of a spring operated quick break opening action with a slow direct drive motor closing action.

Another object of this invention is to provide a new and improved load tap changing transformer having a tap changer which is quiet in operation.

Another object of this invention is to provide a load tap changing transformer having a quick break tap changer which has a longer are quenching period.

Other objects and advantages will be apparent from the following description when read in connection with the following drawings in which:

2,841,662 Patented July 1, 1958 Fig. 1 is a side view of a tap changer for a load tap changing transformer embodying this invention;

Fig. 2 is a front view of the tap changer shown in Fig. l with parts removed;

Fig. 3 is an isometric view of a portion of the tap changer shown in Figs. 1 and 2;

Figs. 4a through 4d show a portion of the preferred tap changer in successive stages of a tap change operation;

Figs. 5a through 5d show a second embodiment of this invention in successive stages of a tap change operation; and

Figs. 6a through 612' show a third embodiment of this invention in successive stages of a tap change operation.

The load tap changer illustrated comprises a dial switch 19 and a motor driven quick break mechanism 11. The dial switch It) comprises a plurality of arcuately spaced stationary contacts 13 which are mounted on a nonconducting panel 14 and two spaced contacts 16 mounted on a movable switching arm 17. The stationary contacts 13 are adapted to be connected in sequence to taps on a transformer so that the output voltage of the transformer increases or decreases in steps as the movable contacts 16 change position. The contacts are spaced so that at least one of the movable contacts 16 is always contacting a stationary contact 13. In an operating position the two movable contacts 16 either contact the same stationary contact or bridge adjacent stationary contacts. One of the movable contacts is connected to the other movable contacts by means of slip rings 18 through a reactor or other impedance which limits the circulating current when the movable contacts are in the bridging position. A connection to the external circuit is made at a tap on the reactor.

The quick break mechanism 11 is designed for converting the slow movement of a motor 19 into a quick movement of the contacts 16 and comprises a stationary index plate 20, an actuating disk 21, a sprocket wheel 23, a pair of spring assemblies 24 and a latch 26. The index plate 20 is mounted on supporting member 28 and the actuating disk 21 is rotatably mounted on a shaft 31 which is coaxial with the index plate 28. The actuating disk 21 is connected to the movable contacts 16 of the dial switch by a suitable linking means 32. The sprocket wheel 23 is rotatably mounted on the shaft 31 intermediate the index plate 20 and the actuating disk 21. The two spring assemblies 24 are positioned symmetrically about the shaft 31 intermediate the actuating disk 21 and the sprocket wheel 23. One end of each spring is attached to the actuating disk and the other end of each spring is attached to the sprocket wheel such that rotation of the sprocket wheel relative to the actuating disk biases one of the spring assemblies and unbiases the other of the spring assemblies. The actuating disk 21 is normally held in an operating position by the cooperation of the latch 26 and spaced notches 35 in the index plate 20. The engaging element shown as latch 26 is pivotally attached to the actuating disk and the free end of the latch extends through a circumferentially elongated aperture 36 in the sprocket wheel 23 and engages the notch corresponding to the position of the dial switch.

At the beginning of a tap change the sprocket wheel 23 is in a balanced position relative to the actuating disk 21 and the latch 26 is positioned in a notch 35 of the index plate 20 by means of a spring 37. As the sprocket wheel 23 is rotated slowly by the motor 19, one of the spring assemblies 24 is biased and the other of the spring assemblies is unbiased. After a predetermined rotation of the sprocket wheel 23, the latch 26 is released by a cam 39 provided on the edge of the aperture 36 of the sprocket wheel. In some quick break mechanisms of the known prior art the latch is timed to release when the sprocket wheel has rotated substantially to the next 59 (J notch position. When the latch is released by the cam the biased spring opens the contacts rapidly. The speed of the movable contacts helps to provide'a rapidnextinguishing of the arc and in the prior art it provides themomentum needed to overcome the resistance between the fixed and movable contacts and propel the movable contacts into the next operating position. Since a high speed closing produces noise and shock i is common practice to slow the movabie contacts somewhat before they reach the next position This slowing action may be provided by the idle drive spring which becomes rebiased as the actuating disk approaches a balanced position with respect to the sprocket. This snubbing action by the idle drive spring depends, on the preloading of the drive springs in the balanced position and can be adjusted easily. Usually a provision is made for relative motion between the ends of the idle drive spring and the actuating disk and sprocket wheel so that snubbing does not occur during the initial part of the cycle. However, in prior art tap changers which depend on the momentum of the quick moving parts for closing the contacts it is not possible to slow the switch suiiiciently to eliminate noise.

In the tap changer of this invention the latch 26 is released after the sprocket wheel 23 has rotated to a position intermediate notch position on the index plate 29. The contacts 13, 16 then separate rapidly and as the actuating disk 21 approaches the sprocket wheel the idle drive spring 24 becomes biased and the actuating disk is snubbed to a stop before reaching the next operating position. The movable contacts 16 are coupled to the sprocket wheel 23 and are rotated to the next operating position at the relatively slow speed of the sprocket wheel;

Coupling means between the sprocket wheel and the actuating disk can be operated conveniently by means of the latch 26 since the latch moves radially outward and inward with respect to the sprocket wheel 23 in response to motion of the actuating disk 21. The movable contacts 16 can be connected to the sprocket wheel 23 through the actuating disk 21. As shown in Figs. 1 through 4d the preferred tap changer of this invention has a dog 40 movably attached to the sprocket wheel 23, springs 41 for centering the dog, and a receiving element 43 attached to the latch 26. The dog 49 can be of any suitable. design and is positioned to engage receiving surfaces on the latch to move the ac-.

tuating disk to the next operating position. Preferably as shown the dog is generally rectangular and pivotably mounted on the sprocket 23. The receiving element 43 is provided with grooves for receiving the dog. When the dial switch 16 is in an operating position the latch 26 is positioned in a notch 35' of the index plate 29 and the dog 40 is centered over the latch 26 by the centering springs 41. When the motor has rotated the sprocket 23 to the intermediate position the latch is released by the cam 39, the actuating disk 21 moves rapidly toward the balanced position relative to the sprocket wheel and the latch 26 strikes against the dog 40 causing the dog to pivot out of the way of the latch. Since the momentum of the quick moving parts carries the latch somewhat beyond the balanced position with respect to the sprocket wheel before the actuating disk is stopped by the snubbing spring, the receiving element is provided with outer surfaces 45, to retain the dog 40 in its raised position. As shown in Fig. 40, when the latch 26 stops, one side of the dog is forced against the retaining outer surface 65 of one'of the grooves 54 by means of the centering spring 41. As the sprocket wheel 23 continues to rotate, the lower end of the dog engages the inner surface 4-6 of the groove i i. The latch 26, the actuating disk 21, and the movable contacts 16 ar then driven slowly to the next switch position by th sprocket wheel. When the latch 26 enters the notch 3 at the completion of the tap change, the dog 46 is released and is returned to its centered position by means of the centering springs as shown in Fig. 4d. Preferably two grooves 44 are provided in the receiving element 43 in order to cause the line of force between the sprocket wheel and the actuating disk to be more in line with the path of the disk to prevent binding between the latch and the index plate.

A second embodiment of the invention shown in Figs. 5a through 5d is generally similar to the preferred embodiment except that the positions of the receiving element and the engaging element are reversed. The latch 47 is preferably rectangular in cross section and a pair of wedges 48 movably attached to the sprocket wheel 23 form a rectangular recess 49 for receiving the latch. When the dial switch is in an operating position the wedges 48 are disposed on either side of the recess 49 over the latch. When the sprocket wheel 23. and the wedges 48 have rotated to the intermediate position the latch 47 is released from the notch 35 and the actuating disk 21 and the latch travel rapidy toward the balanced position relative to the sprocket wheel. The wedges 48 are shaped so that as the latch 47 strikes against the first wedge 48 this wedge is raised out of the path of the latch and the latch enters the recess. The raised wedge is thenv repositioned by means of a spring 50 and it bears against the latch causing the latch 47, the actuating disk 21 and the movable contacts 16 to rotate slowly to the next position in response to the movement of the sprocket 23'.

A third embodiment of this invention shown in Figs. 6a through 6:1 is generally similar to the second embodiment except that a means is provided for raising the second of the wedges out of the path of the latch to allow additional snubbing action by the spring 24. As illustrated a T-shaped cam 52 comprising a lower arm 53 and two outwardly extended upper arms 55, 55a is pivotally attached by a pin 56 to the sprocket wheel 23 intermediate the

wedges

58, 58a. Lugs 59, 5911 are provided on the two wedges. Thus the inner faces 61, 61a of the

wedges

58, 58a form a recess 62 which is divided into two sections 64', 64a by the lower arm 53. Lugs 59, 59:: are provided on the wedges and cooperate with corresponding arms 55, 55a and the latch 65 to raise the second of the wedges out of the path of the latch after the latch has entered the first section of the recess formed by the wedges. When referring to first and second wedges it is understood that first indicates the first wedge to be engaged by the latch and therefore the first wedge is determined by the direction of movement of the latch. Raising of the second wedge 58a as shown in Fig. 61) allows the actuating disk 21 to travel somewhat beyond the balanced position with respect to the sprocket wheel 23 and to encounter additional snubbing spring action. The inner'faces 61, 61a of the

wedges

58, 58a are shaped to allow the wedges to pivot past the pin 56 of the cam. The upper portion of the latch 65 is preferably provided with inclined surfaces 67.

When the latch 65 is released from the notch 35 and is snapped toward the wedges it passes the first wedge 58 as in the second embodiment of this invention and enters the first section 64 of the divided recess 62. The latch 65 then strikes the lower arm 53 of the cam 52 to pivot the cam. As the cam 52 pivots, the upper arm 55a of the cam moves the lug 59a to lift the second wedge 58a out of the path of the latch 65. The lower arm 53 of the cam 52 is made sufiiciently long that the latch does not pass from the first section 64 to the second section 64a of the recess 62. After the actuating disk 21 has been stopped by the snubbing of the springs, the first wedge 58 rotates with the sprocket wheel to engage and drive the latch 65, the actuating disk 21, and the movable contacts 16 to the next operating position.

It will be apparent to those skilled in the art that various changes and modifications may be made in the ap paratus illustrated and described without departing from the spirit of the invention and the scope of the appended claims.

What is claimed is:

1. A mechanism for moving an element from one operating position to another with a quick break, slow make movement comprising an index plate, a sprocket member, means for rotating said sprocket member about an axis at a relatively slow uniform speed, an actuating member, means connecting said actuating member to said element, means for rotating said actuating member about said axis with a rapid snap action to move said element to an intermediate position, said means comprising a pair of spring assemblies disposed between said members, means connecting the ends of said assemblies to said members, and latch means engageable with said index plate to prevent rotation of said actuating member until said one of said assemblies is biased to a predetermined extent by rotation of said first member, the other of said springs acting to stop the movement of said actuating member intermediate operating positions, and means connected to said sprocket member for moving said member from its intermediate position to the next operating position at a slow speed.

2. A mechanism for moving an element from one operating position to another with a quick break, slow make movement comprising an index plate, a sprocket member, means for rotating said sprocket member about an axis at a relatively slow uniform speed, an actuating member,

means connecting said actuating member to said element, means for rotating said actuating member about said axis with a rapid snap action to move said element to an intermediate position, said means comprising a pair of spring assemblies disposed between said members, means connecting the ends of said assemblies to said members, and latch means engageable with said index plate to prevent rotation of said actuating member until said one of said assemblies is biased to a predetermined extent by rotation of said first member, the other of said springs acting to stop the movement of said actuating member intermediate operating positions, and means connected to said sprocket member for engaging said actuating member in its intermediate position and moving it at a slow speed to the next operating position.

3. A tap changing underload transformer including a dial type switch having a plurality of arcuately spaced stationary contact positions and a pair of movable contacts, a mechanism for moving said movable contacts from one of said stationary contact positions to another of said stationary contact positions, said mechanism comprising a stationary index plate provided with a plurality of arcuately spaced notches, a shaft mounted in the center of said plate, a sprocket member rotatably mounted on said shaft, an actuating member rotatably mounted on said shaft in axial spaced relationship with said sprocket member, means connecting said actuating member to said movable contacts, means for rotating said sprocket member at a relatively slow uniform speed, and means for converting the relatively slow uniform speed of said sprocket to a rapid snap action of said actuating member to cause said movable contacts to move with a quick step movement, said converting means comprising a pair of spring assemblies spaced symmetrically on opposite sides of said shaft and disposed between said members to act along force lines parallel to the face of said members, each of said assemblies having means connecting the ends of said assembly to said members to provide relative movement between said assembly and said members, and a latch device mounted on said actuating member and engaging one of said notches of said index plate to prevent rotation of said actuating member until one of said spring assemblies is biased to a predetermined extent, the other of said spring assemblies being positioned to oppose the movement of said latch after it has moved a predetermined distance and stop said latch before it reaches the next operating position, and means 6 connected to said sprocket for engaging said latch and moving it to the next operating position.

4. A load tap changing transformer comprising a tap changing switch having a plurality of spaced stationary contacts and a pair of correspondingly spaced movable contacts cooperating with said stationary contact members to define operating positions for said transformer, means for switching one of said movable contacts from one of said stationary contacts with a snap action to cause said one movable contact to be rapidly separated from of said one stationary contacts, said means including a stationary index plate defining a plurality of circumferentially spaced notches, a latch device for moving said movable contacts and engageable with said notches to accurately position said movable contacts with respect to said stationary contacts at said operating positions, a resilient element for moving said latch with a quick movement, resilient means adapted to momentarily oppose and stop the movement of said movable contacts at a point between said operating positions and means for engaging said latch intermediate said operating positions and moving said latch and stationary contacts to the next operating position.

5. A tap changing underload transformer including a dial type switch having a plurality of circumferentially spaced stationary contact positions and a pair of movable contacts, a mechanism for moving said movable contacts from one of said stationary contact positions to another of said stationary contact positions, said mechanism comprising a stationary index plate provided with a plurality of spaced notches, a shaft rotatably mounted in the center of said plate, a sprocket member rotatably mounted on said shaft, an actuating member fixedly mounted on said shaft in axial spaced relationship with said sprocket member, means connecting said actuating member to said movable contacts, means for rotating said sprocket member at a relatively slow uniform speed in response to variation in the load demand in the transformer, and means for converting the relatively slow uniform speed of said sprocket to a rapid snap action of said actuating member to cause said movable contacts to move with a quick step movement, said converting means comprising a pair of spring assemblies spaced symmetrically on opposite sides of said shaft and disposed between said members to act along force lines parallel to the face of said members, each of said assemblies having means connecting the ends of said assembly to said members, and a latch device mounted on said actuating member and extending through a circumferentially elongated aperture in said sprocket memher to engage one of said notches of said index plate to prevent rotation of said actuating member, said rotation of said sprocket member relative to said actuating member biasing one of said springs and unbiasing the other of said springs, said biased spring urging said actuating member to the position of said sprocket memher, said unbiased spring yieldably resisting movement of said actuating member induced by said biased spring, said biased spring and said unbiased spring cooperating with said sprocket to snap said latch out of said notch and move said actuating member with a quick motion toward the next operating position and to stop said actuating member at a position intermediate operating positions and means connected to said sprocket for engaging said latch and moving it from said intermediate position to the next operating position.

6. A tap changing underload transformer including a dial type switch having a plurality of circumferentially spaced stationary contact positions and a pair of movable contacts, a mechanism for moving said movable contacts from one of said stationary contact positions to another of said stationary contact positions, said mechanism comprising a stationary index plate provided with a plurality of spaced notches, a shaft rotatably mounted in the center of said plate, a sprocket member rotatably mounted on said; shaft, an actuating, member fixedly mounted onsaid shaft in axial spaced relationship with said sprocket member, means connecting said actuating member to said movable contacts, means for rotating said sprocket member at a relatively slow uniform speed in response to variation in the load demand in the transformer, and means for converting the relatively slow uniform speed of said sprocket to a rapid snap action of said'actuating member to cause said movable contacts to move with a quick step movement, said converting means comprising 'a pair of spring assemblies spaced symmetrically on opposite sides of said shaft and disposed between said members to act along force lines parallel to the face of said members, each of said assemblies having means connecting the ends of said assembly to said members, and a latch device mounted on said actuating member and extending through a circumferentially elongated aperture in said sprocket member to engage one of said notches of said index plate to prevent rotation of said actuating member until one of said spring assemblies is biased to a predetermined extent, the other of said spring assemblies being positioned to oppose the movement of said latch after it has moved a predetermined distance and stop said latch in an intermediate position before it reaches the next operating position, and means for coupling said relatively slow moving sprocket member to said actuating member, said coupling means comprising a latch engaging element movably attached to said sprocket member, said latch being in the operating path of said engaging element when said latch is intermediate said notch positions on said index plate and outside the operating path of said engaging element when said latch is in a notch position, said element engaging said latch in the intermediate position for moving said latch to the next operating position in response to the movement of said sprocket.

7. A tap changing underload transformer including a dial type switch having a plurality of circumferentially spaced stationary contact positions and a pair of movable contacts, a mechanism for moving said movable contacts from one of said stationary contact positions to another of said stationary contact positions, said mechanism comprising a stationary index plate provided with a plurality of spaced notches, a shaft rotatably mounted in the center of said plate, a sprocket member rotatably mounted on said shaft, an actuating member fixedly mounted on said shaft in axial spaced relationship with said sprocket member, means connecting said actuating member to said movable contacts, means for rotating said sprocket member at a relatively slow uniform speed in response to variation in the load demand in the transformer, and means for converting the relatively slow uniform speed of said sprocket to a rapid snap action of said actuating member to cause said movable contacts to move with a quick step movement, said converting means comprising a pair of spring assemblies spaced symmetrically on opposite sides of said shaft and disposed between said members to act along force lines parallel to the face of said members, each of said assemblies having means connecting the ends of said assembly to said members, and a latch device mounted on said actuating member and extending through a circumferentially elongated aperture in said sprocket member to engage one of said notches of said index plate to prevent rotation of said actuating member until one of said spring assemblies is biased to a predetermined extent, the other of said spring assemblies being positioned to oppose the movement of said latch after it has moved a predetermined distance and stop said latch in an intermediate position before it reaches the next operating position, and means for transmitting the relatively slow motion of said sprocket member to said actuating member, said transmitting means comprising a dog movably attached to said sprocket member and a receiving element attached to said latch, said receiving element being in the path of said dog when said latch is intermediate said notch positions and being outside the 'path of said dog when said. latch is in a notch position, said dog being movable out of v termediate'said retaining surfaces, said dog moving into.

engagement with one of said engaging surfaces in response to movement of said sprocket for moving said actuating member and said movable contacts to the next operating position. 7

8. A'tap changing underload transformer including a dial type switch having a plurality of circumferentially spaced stationary contact positions and a pair of movable contacts, a mechanism for moving said movable contacts from one of said stationary contact positions to another of said stationary contact positions, said mechanism comprising a stationary index plate provided with a plurality of spaced notches, a shaft rotatably mounted in the center of said plate, a sprocket member rotatably mounted on said shaft, an actuating member fixedly mounted on said shaft in axial spaced relationship with said sprocket member, means connecting said actuating member to said movable contacts, means for rotating said sprocket member at a relatively slow uniform speed in response to variation in the load demand in the transformer, and means for converting the relatively slow uniform speed of said, sprocket to a rapid snap action of said actuating member to cause said movable contacts to move with a quick step movement, said converting means comprising a pair of spring assemblies, spaced symmetrically onopposite sides of said shaft and.

disposed between said members to act along force lines parallel to the face of said members, each of said assemblies having means connecting the ends of said assembly to said members, and a 'latch device mounted on said actuating member and extending through a circumferentially elongated aperture in said sprocket member to engage one of said notches of said index plate to prevent rotation of said actuating member until one of said spring assemblies is biased to a predetermined extent, the other of said spring assemblies being posi-.

tioned to oppose the movement of said latch after it has moved a predetermined distance and stop said latch in an intermediate position before it reaches the next operating position, and means for transmitting the relatively slow motion of said sprocket member to said actuating member, said transmitting means comprising said latch and a pair of spaced wedges movably attached to said sprocket member, said latch being in the operating path of said wedges when said latch is intermediate said notch positions on said index plate and being outside the operating path of said wedges when said latch is in a notch position, said wedges forming a recess for receiving said latch, said latch positioned to engage one wedge and raise it out of the path of said latch to allow said latch to enter said recess, and resilient means for repositioning said raised wedge in the path of said latch, whereby the motion of said sprocket member is transmitted to said movable contacts through one of said Wedges and said latch.

9. A tap changing underload transformer including a dial type switch having a plurality of circumferentially spaced stationary contact positions and a pair of movable contacts, a mechanism for moving said movable contacts from one of said static-nary contact positions to another of said stationary contact positions, said mechanism comprising a stationary index plate provided with a plurality of spaced notches, a shaft rotatably mounted in the center of said plate, a sprocket member rotatably mounted on said shaft, an actuating member fixedly mounted on said shaft in axial spaced relationship with said sprocket mem-. ber, means connecting said actuating member to said movable contacts, means for rotating said sprocket membet at a relatively slow uniform speed in response to variation in the load demand in the transformer, and means for converting the relatively slow uniform speed of said sprocket to a rapid snap action of said actuating member to cause said movable contacts to move with a quick step movement, said converting means comprising a pair of spring assemblies spaced symmetrically on opposite sides of said shaft and disposed between said members to act along force lines parallel to the face of said members, each of said assemblies having means connecting the ends of said assembly to said members, and a latch device mounted on said actuating member and extending through a circumferentially elongated aperture in said sprocket member to engage one of said notches of said index plate to prevent rotation of said actuating member until one of said spring assemblies is biased to a predetermined extent, the other of said spring assemblies being positioned to oppose the movement of said latch after it has moved a predetermined distance and stop said latch in an intermediate position before it reaches the next operating position, and means for coupling said relatively slow moving sprocket member to said actuating member, said coupling means comprising said latch and a pair of spaced wedges movably attached to said sprocket member, said wedges having inner surfaces forming a recess for receiving said latch, the outer surface of said wedges being inclined and cooperating with said latch to raise said wedge out of the path of said latch, means interposed between said wedges for cooperating with said latch to raise the other of said wedges out of the path of said latch, said means comprising a T-shaped cam having a lower arm and a pair of laterally extending upper arms pivotally attached to said sprocket member and lugs for attaching the laterally extending arm of said cam to corresponding said wedges, said latch positioned to engage said lower arm when intermediate said notch positions to pivot said cam and to lift the other of said wedges out of the path of said latch, and resilient means for repositioning said wedges in said recess forming position around said latch, whereby the motion of said sprocket member is transmitted to said movable contacts through said wedges and said latch.

No references cited.