Patent GB-744683-A (GB)
Title: Improvements in and relating to snap-action devices particularly for electric switches operating in response to temperature or pressure changes
IPC: H01H37/36
Filed: 1953-04-22
Abstract: 744,683. Thermal switches; actuation of switches. GENERAL ELECTRIC CO. April 22, 1953 [April 29, 1952], No. 11083/53. Class 38(5) A switch which may function as a thermostat or a pressurestat and described in relation to the control of the compressor of a refrigerator system and responsive to the temperature of the evaporator, comprises, Figs. 1, 3, a metal casing 1 carrying on the bottom wall 5 a liquid-filled bellows 22 having a capillary tube 25 and a movable head 23. A lever 40 pivoted at 47 between the casing side walls 2, 9 has a dimple 49 engaged by the bellows head 23, a, contact-actuating blade 41 being fixed to the left-hand end of the lever. The free end of the blade 41 insulatingly carries a rivet 43 and has knife edge engagement with an over-centre spring 44 adjustably tensioned by a screw 45. Downward movement of the free end of the blade 41 is limited by a fixed stop 66 whilst upward movement is limited by the foot 76 of a slide 74. When the blade moves downwards the rivet 43 engages an upstanding finger 72 on a contact-carrying spring 68 to separate a contact 70 from a fixed contact 63 before the blade is arrested by the stop 66. The lever 40 is biased clockwise by a spring 34 acting between a knife edge 51 on a vertical arm 46 of the lever and a knife edge 88 formed on a lever 83 pivoted at 92 between the side walls 2, 9. The lever 83 is adjustable about its pivot 92 to vary the operating range of the thermostat by a screw 108 locked within a tubular screw 100 rotatable in a fixed bushing 101 by a knob 107. On expansion of the bellows 22 the lever 40 is pivoted counter-clockwise against the bias of the spring 84 and the blade 41 is bowed and eventually becomes sufficiently strained to move the spring 44 over-centre whereupon the rivet 43 moves upwards to release the finger 72 and permit the contacts 63, 70 to close. On decrease of pressure in the bellows the spring 84 moves the parts downwardly to their original positions and the contacts 63, 70 are opened, Rotation of the knob 107 to increase the tension of the spring 84 increases the upper and lower temperatures of the operating range, the highest range corresponding to the dotted line position of the spring in Fig. 1. The amplitude of the range is determined by the tension of spring 44. Manual operation of the contacts to the "off" position is effected by a plate 111 rotatable with the knob 107 and having a cam surface 114 which engages the head 82 of slide 74 biased upwardly by a spring 78 against a stop screw 81. On rotation of the knob 107 to the "off position the cam 114 depresses the slide so that the foot 76 engages the rivet 43 to move the finger 72 and hold the contacts 63, 70 separated. A closed chamber having a flexible end wall may replace the bellows. In a modification, Fig. 11, in which the contacts 63, 70 always close at the same.temperature (38‹F.) but different ranges are obtained by opening the contacts at different temperatures, the spring 120 exerts a non-variable thrust on the lever 40 and the variable-amplitude ranges are obtained by adjustment of the upper stop 76 which permits variation of the degree of movement of the blade 41 above the over-centre position of the spring 44, the greater this movement the greater the force required to return the lever 40 clockwise to open the contacts due to the increased angle of the over-centre slope x. The stop 76 is moved by the engagement of an extension 126 on the slide 123 with the forked end 125 of a pivoted arm 124 controlled similarly to the lever 83 of Fig. 1. In another modification, Fig. 12, having similar operating characteristics to Fig. 11. a bell crank lever 130 has an arm 131 overlying the rivet 43 and constituting the upper adjustable stop, the lever being biased by a spring 133 against the adjustment screw 108. The cam 114 operates a push-rod 134 to hold the contacts in the "open" position.