Patent GB-644520-A (GB)
Title: Improvements in or relating to snap-action electric switches
Inventor: JACKSON HAROLD ERNEST
IPC: H01H5/18
Filed: 1948-05-10
Abstract: 644,520. Snap-action switches; thermal switches. KIEFT & CO., Ltd., C. May 10, 1948, Nos. 12746/48 and 1031/49. [38(v)] A switch comprises a member 3 held in tension by compression members 4, 5, the free ends of which engage in notches 9, 10 in stationary pillars 2 fixed to a base 1. The movable ends 6 may carry contacts or stops 7, 8, 12 on both their upper and lower faces which co-operate with contacts or stops 11, 15 on the base and with stops or contacts 13, 14 on an upper fixed part of the switch as shown in Fig. 2e. The members 5 are of bowed form, the members 4 being stiffened by being bent at right angles to the end 6. The notches 9, 10 may be at different heights in the pillars 2 so that when pressure is applied to the member 3 the ends 6 move downward in succession from the position shown in Fig. 2e. When the pressure is again reduced one end 6 moves upwards and is followed by the other end when the pressure is further reduced. The positions of the notches 9, 10 may however be arranged so that when the pressure on the member 3 is removed only one pair of contacts 7, 11 will be opened, the contacts 8, 15 remaining closed till a positive force is applied to the member 3. Fig. 3 shows the switch fitted for application to a cooking apparatus in which the required pressure on the member 3 is applied by a pin 37 controlled by a thermal device consisting of a tube 17 made of a metal having a large coefficient of expansion and a rod 18 of low coefficient of expansion fixed to the lower end of the tube. The upper end of the rod engages the forked end of a lever 47 pivoted at 48 and bearing at its other end on the head of the pin 37. The switch in this form controls only a single heater and the notches 9, 10 are arranged so that when the pressure on the member 3 is removed only one of the movable contacts 7 moves, the contact 8 remaining against the stop 15. The circuit to the heater is then completed through the contacts 7, 14. The switch is normally rendered inoperative by pressure on the pin 37 applied through the lever 47 and a lever 44 on which the pivot 48 is formed. The lever 44 is pressed upwards by a spring 49. In order to set the switch for operation by the rod 18 a hand operated screw 39 which abuts against the end 43 of the lever 44 is turned to lower the pivot 48 and remove the pressure of the lever 47 on the pin 37. An adjusting screw 40 is provided in the screw 39. In a modification, Fig. 7 (not shown), the position of the notches 9, 10 and the lengths of the members 4. 5 are arranged so that both contacts 7, 8 snap over from their positions against the base 1 shown in Fig. 3, an additional fixed contact being provided for engagement by the contact 8. This construction enables two heating circuits to be controlled. Fig. 10 shows a further modification in which the position of the notches 9, 10 and the relative dimensions of the moving parts of the switch are such that when the pressure on the pin 37 is removed only the contact 7 is actuated, the contact 12 remaining against the stop 15 until pressure is applied to the member 3 by a movable pin 63 on the other side of the member. Contact 12 then snaps into engagement with contact 13 and the contacts 7, 14 open. Either of two heaters can be energized separately but not both together. Pin 63 is actuated by a spring arm 62 secured to one arm 60 of a lever 58 pivoted at 59. One end 62a of the arm 62 is bent as shown and can be engaged by a pin 69 which abuts against a collar 71 carrying a projection 70. When the screw 39 is rotated it rotates the collar and brings the projection 70 against the pin 69 which is pressed inwards and bears against the arm 62a and tilts the lever 58 and causes the arm 62 to depress the pin 63 whereupon the contact 12 snaps into engagement with contact 13. The lever 58 is forked and carries a bar 65 which withdraws the contact 14 from the contact 7 when lever 58 is rotated in a clockwise direction about its pivot 59 so that the contacts 14, 7 are opened when the contacts 12, 13 close. In a further modification the switch shown in Fig. 1 is mounted in a casing through which the pin 37 projects. The pin is engaged by a continuously rotating cam and the casing is pivotally mounted so that by adjusting its position the pin may be engaged by the cam during a part or the whole of a revolution so that the contacts 7, 14 are brought into engagement for periods depending on the position of the casing. The cam may be driven electrically.