Abstract:
A double pole single throw electric switch has normally closed contacts of one pole connected in a dynamic braking circuit with a motor load and normally open contacts connecting the motor to a source of power. A partial movable contact conductively connected to movable contacts of the normally open contacts is disposed in normally open position relative to one contact of the normally closed contacts to provide a parallel path through the switch for making and breaking the circuit to the power source.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to electric switches of the type including a dynamic braking circuit wherein the motor controlled by the switch is connected in a closed loop circuit upon opening a power circuit by the switch. More particularly, the invention relates to an electric switch of the aforementioned type which has a trigger operator for particular application in the handles of portable electric tools and the like. 
     A common practice for incorporating a dynamic braking circuit with an electric switch is to utilize a two-pole electric switch, using one pole for connection between the power source and load and the other pole for connection in the dynamic braking circuit. However, such embodiments place all of the arcing and erosion associated with the making and breaking of a power circuit on the contacts of one pole, while the contacts of the other pole remain relatively clean and unaffected because they only function to close or open a dynamic braking circuit to the motor, and therefore do not open or close on a significant current. Accordingly, switch life of the aforedescribed switch is substantially dependent upon the condition of the contacts of the pole utilized for making and breaking the power circuit. 
     SUMMARY OF THE INVENTION 
     This invention provides a two-pole electric switch having a normally closed pole of the switch connected in a loop with the motor to provide dynamic braking and a normally open pole connected in circuit with the power source to selectively connect or disconnect the motor load to the power source, wherein an additional movable contact conductively connected to the normally open movable bridging contact of the normally open pole, is arranged in normally open relationship with one stationary contact of the normally closed pole to provide an additional contact for sharing the circuit making and breaking load with the contacts of the normally open pole upon operation of the switch between open and closed conditions. That stationary contact of the normally closed pole which is engaged by the additional normally open movable contact is electrically jumpered to the load terminal of the normally open pole to connect the additional movable contact and that stationary contact in parallel with the contacts of the normally open pole. The invention reduces erosion of the contacts from switch operation and increases the operating life of the switch. 
     The invention, its features and advantages will become more readily apparent when reading the following description and claims in conjunction with the appended drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a longitudinal cross sectional view of the electric switch of this invention taken generally along the line 1--1 in FIG. 2 showing the switch in an OFF position; 
     FIG. 2 is a cross sectional view taken generally along the line 2--2 in FIG. 1 viewing the switch contact structure from the bottom (as oriented in FIG. 1 ) of the switch showing the switch in an OFF position; 
     FIG. 3 is an exploded isometric view of the movable contact carrier and movable contacts constructed in accordance with this invention; 
     FIG. 4 is a transverse cross sectional view taken substantially along the line 4--4 in FIG. 1 showing an electrical jumper connecting load-end terminals of the switch together; 
     FIG. 5 is a view similar to FIG. 4 but showing an alternate embodiment of an electrical jumper for the load-end terminals of the switch; and 
     FIG. 6 is a schematic diagram of the switch of this invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The invention is illustrated in an embodiment of a double-pole single-throw electric tool handle switch of well known construction, but is equally useful and applicable in other types of switches such as rocker switches, toggle switches, slide switches and the like. Switch 2 comprises a molded insulating base 4 having a pair of longitudinally extending cavities 4a and 4b open to the upper side of the base as oriented in FIG. 1. A molded insulating cover 6 is attached over the open upper side of base 4 by a plurality of windows in resilient depending side walls of cover 6 which snap over a corresponding plurality of laterally extending projections 4c on the exterior of sidewalls of base 4. Only one of the projections 4c and corresponding windows are shown in dotted line in FIG. 1. Cover 6 further comprises a superstructure 6a which provides a pivot for a trigger operator 8, also molded of insulating material. Superstructure 6a is preferably provided with a pair of oppositely directed laterally extending trunions 6b (only one shown, FIG. 1 ) over which holes 8a in the side walls of trigger 8 are disposed to provide a snap-on attachment of the trigger 8 to the superstructure 6a. The cover and superstructure also provide a guide channel 6c for a switch actuator stem 10 which is pivotally attached to the trigger 8 by a rivet 12 which extends through the actuator stem and the side walls of the trigger. Rivet 12 is remotely located from pivot 6b such that clockwise pivotal movement of trigger 8 around 6b effects leftward linear motion of actuator stem 10 as viewed in FIG. 1. The end of actuator stem 10 within the switch housing comprising base 4 and cover 6 comprises a recess 10a in which a helical compression spring 14 is disposed. Spring 14 bears between a depending projection 6d on the interior of cover 6 and the right-hand end of recess 10a to bias the actuator stem 10 rightward as viewed in FIG. 1, thereby providing a counter-clockwise bias to trigger 8. 
     Base 4 is provided with four identical stationary contacts 16, 18, 20 and 22 arranged in oppositely disposed cooperating pairs in each of the cavities 4a and 4b. As viewed in FIG. 1, the stationary contacts 16, 18, 20, 22 each comprise an inverted U-shaped terminal end 16a, 18a, 20a, 22a which is positioned in a respective pocket provided at the opposite ends of each of the cavities 4a and 4b. The inner leg of the inverted U-shaped terminal end has a contact arm 16b, 18b, 20b, 22b extending to the center of the respective cavity. The respective opposed pairs of stationary contacts 16, 18 and 20, 22 are reversely arranged such that the distal ends of the contact arms 16b, 18b, and 20b, 22b are generally longitudinally overlapping, but laterally spaced apart (FIG. 2). A generally flat rectangular terminal plate 24 is disposed against the inside surface of the outer leg of each inverted U-shaped portion of the stationary contacts 16 and 20. Terminal plate 24 is provided with an extruded tapped hole to receive a terminal screw such as 26 which extends through a correspondingly aligned hole in the outer leg of the respective stationary contact. A generally U-shaped wire retention clip 28 is disposed against the terminal plate 24, the distal ends of the legs of clip 28 abutting the inner leg of U-shaped terminal portion 16a and 20a to resiliently grip and retain a wire inserted through appropriate holes 4d in the base 4 to provide for push-in terminations as well as screw terminations. 
     The stationary contacts 18 and 22 of double-pole single throw switch 2 are interconnected to be electrically common in dynamic braking applications of the switch. One method of such interconnection is shown in FIG. 5 wherein a terminal plate 24 and wire retention clip 28 are placed in cavity 4a within the inverted U-shaped terminal portion 18a of stationary contact 18. Although not shown in FIG. 5, a terminal screw 26 is threadably engaged in the extruded tapped hole of terminal plate 24. No terminal plate or wire retention clip are provided for stationary contact 22. Cover 6 has a rectangular boss 6e depending from the top wall thereof to which an electrical bus 30 is attached by a press fit, heat staking or other suitable method. Bus 30 is made from good electrically conductive spring material and has a preformed downward bow at the outer ends to conductively engage the top surfaces of inverted U-shaped terminal portions 18a and 22a of stationary contacts 18 and 22, respectively. Attachment of cover 6 to base 4 compresses the bus 30 against stationary contacts 18 and 22 to establish a good electrical connection. 
     Another method of interconnecting stationary contacts 18 and 22 is particularly shown in FIG. 4. A double terminal plate 32 having lateral portions 32a and 32b joined along their upper edges by a bridge portion 32c is disposed within the U-shaped terminal portions 18a and 22a of stationary contacts 18 and 22. Lateral portion 32a is disposed in cavity 4a adjacent stationary contact 18. A wire retention clip 28 is also disposed in the stationary contact 18. Lateral portion 32a has a tapped extruded hole for receiving a terminal screw 26. Lateral portion 32b has no tapped hole for a terminal screw, but does have a wire retention clip 28 associated therewith to maintain good connection between terminal plate lateral portion 32b and stationary contact 22. A groove 4e is provided in the upper central wall of base 4 between cavities 4a and 4b for receiving bridging portion 32c. Although not specifically shown, it is further contemplated to affix stationary contact 22 directly to terminal plate lateral portion 32b such as by welding and to eliminate the wire retention clip 28 associated with stationary contact 22. 
     A U-shaped movable contact carrier 34 is pivotally mounted by trunnions 34a within appropriate openings (not shown) in cover 6. An overcenter compression spring assembly 35 is mounted between a depending boss 10b on actuator stem 10 and an upstanding boss 34b on movable contact carrier 34 to provide a snap action overcenter driving connection between actuator stem 10 and movable contact carrier 34. The legs of U-shaped movable contact carrier 34 depend into each of the cavities 4a and 4b. One end of the leg disposed in cavity 4a is radially serrated on the outer surface (see FIG. 2) and has a single serration (not numbered) on its inner surface. A slot 34c separates the serrated portion from a contact mounting surface 34d on which a U-shaped movable contact 36 is mounted with the closed end of the U of the movable contact 36 in the slot 34c and directed toward the serrated portion. A rivet 38 is provided to secure the movable contact 36 to the contact carrier 34. The leg of the contact carrier that extends into cavity 4b has the serrated portion located opposite that on the first leg and has a plurality of serrations along both the inner and outer surfaces. A slot 34e separates the serrated portion from contact mounting surface 34f. The corresponding U-shaped movable contact 40 (seen best in FIG. 3 ) is reversely disposed relative to the first movable contact 36. Movable contact has a transversely projecting arm 40a extending from one leg thereof. The distal end of arm 40a has a contact portion 40b extending generally parallel with the legs of U-shaped movable contact 40. One edge 40c of contact portion 40b is curved downward, whereby contact portion 40b has the configuration of one-half a U-shaped contact such as 36 or 40. A slot 34g is provided in the inner surface of one contact carrier leg adjacent the single serration to receive the downwardly curved edge 40c of contact portion 40b. A groove 34h is provided in the underside of contact carrier 34 between the legs thereof to receive arm 40a of movable contact 40. 
     With the contact carrier 34 installed as shown in FIG. 1, the overcenter drive connection 35 causes the carrier to assume a left-hand position within the base 4. In that position, movable contact 36 bridges the contact arms 16b and 18b of the respective stationary contacts 16 and 18 to complete a dynamic braking circuit for the motor M when properly wired as shown in FIG. 6. Depression of trigger 8 pulls actuator stem 10 to the left as viewed in FIG. 1, moving boss 10b overcenter of the drive connection 35 to effect rotation of movable contact carrier 34 to the right-hand position whereupon the dynamic braking circuit is interrupted by movement of movable contact 36 out from between contact arms 16b and 18b. In the right-hand position of contact carrier 34, movable contact 40 moves into bridging engagement between stationary contact arms 20b and 22b and contact portion 40b moves into engagement with stationary contact arm 18b, completing a parallel circuit between contacts 40b and 18b with that circuit between contacts 20b and 22b through contact 40. Thus load current from the source 42 (FIG. 6) to the load M is shared through the primary path of contacts 20, 22 and through the additional path of contacts 20-40b-18 to reduce the amount of current carried by contacts 20, 22. Reduction in current at the contacts reduces arcing and erosion of the contacts to increase the life of the switch. Although a preferred embodiment of this invention has been disclosed, it is to be understood tat this invention is susceptible of various modifications without departing from the scope of the claims.