DRIVES AND CONTROLS 



245 



Double-ihrow confac^ b/ads/ 



Confac-/-, 

 spring ' 



Concave 

 bi'mei-al disk. 



Terminak 

 Coniacfs 



" - Nickel silver 

 vacuum check 



-Nickel silver 

 valve pin 



~ Valve closed 

 by floaf or 

 expansion 

 of bellows 



lead 

 washer 



Bimefal 

 strip 



^ VenI regulaling screw 

 Fig. 514. 



Temperolure / 

 adjuslmenl 



Fig. 512. Fig. 513. 



Fig. 612. — Toggle action, without separate springs, is obtained in the Spencer disk thermostat. The disk is a 

 saucer-shaped piece of bimetal sheet which snaps itself from a concave to a convex shape at a predetermined tempera- 

 ture. Both the amount of movement and the temperature diiferential between opening and closing temperature 

 depend on the design of the disk. For greater sensitivity, smaller differential and a larger movement than can be 

 obtained with the plain disk, the bimetal disk is corrugated. Since the disk is small and stores but little heat, it warms 

 or cools rapidly. 



When used as an electrical control device, insulated silver contacts are mounted on the bimetal disk. In the cold 

 position shown, each of the contacts bridges a gap in insulated plates connected to the heavy terminals. When 

 heated, the disk snaps to a convex shape, the circuit opening through the ^evice at three points. 



Fig. 513. — In the Westinghouse thermostat for electric hot-water heaters, a small range of temperature difference 

 between on and off is needed, and to eliminate the necessity for an intermediate relay, the contacts must break a rela- 

 tively heavy current. These conflicting requirements are met by using a double-toggle mechanism. A light toggle 

 spring on the contact blade keeps the contacts firmly seated until the stronger toggle on the bimetal strip comes into 

 operation. 



The bimetal blade is free to move nearly to the dead-center position, thereby storing energy in its toggle spring 

 before any pressure is applied to the contact blade. Energy released by the toggle spring, when the bimetal blade 

 passes dead center, delivers an impact to the contact blade, breaking loose any slight welding that may have occurred 

 during the previous operation. This thermostat is used as a current-limiting switch, disconnecting one heater as 

 another is connected. Because of the double-toggle design, the thermostat contacts will safely interrupt 5 kw. at 

 220 volts alternating current with a temperature differential of 5°F. or less. 



Fig. 514. — In radiator air valves made by the Anderson Manufacturing Company, air forced into the valve 

 passes around a small bellows partly filled with a liquid. When steam reaches the valve, the heat increases the vapor 

 pressure within the bellows, and the resultant expansion raises the float, thereby closing the air-vent orifice. 



I Adjusfing screw 



W////yyyyy////A 



Fig. 515. — Electric irons require a convenient adjustment for the temperature at which the bimetal element opens 

 the circuit. In the mechanism designed by Proctor & Schwartz, a double lever not only permits adjustment of the 

 operating temperature, but also relieves the bimetal strip of anj' restriction when it cools to room temperature. Since 

 the operating temperature range is high, a heat-resisting bimetal material is used in the form of a short stiff strip. Cur- 

 rent is conducted to the bimetal contact through a flexible silver ribbon eliminating the effect of heat caused by current 

 passing through the bimetal strip. 



