244 



HANDBOOK OF MECHANICAL DESIGN 



Helical 

 expansion 

 iube - . 



/Bimelol 

 compen- 

 sallng 

 lever 



Fig. 509. — In mercury-filled indicating thermometers, 

 expansion of the mercury in a bulb at the end of a 

 capillary line causes the spiral tube in the gage to uncoil, 

 the dial pointer being moved by means of a linkage. 

 However, changes va the temperature of the mercury 

 in the capillary and spiral also affect the movement of the 

 linkage introducing an error in the reading. In the 

 Taylor indicating thermometer, compensation for changes 

 in gage temperature is obtained by a flat bimetal strip 

 that forms a part of the pointer linkage. The strip is 

 designed so that its deflections are equal but opposite 

 to the effect caused by changes in gage temperature. 

 Since little load is imposed on the thermostatic strip, the 

 compensating action can be obtained with high accuracy. 



isss\ssy 



Valve closed 

 of l40deg.F 

 Full open of 

 ISOdeg.F 



Fig. 510. — In cooling-water thermostats for auto- 

 mobile engines, the water flow imposes a load on the 

 bimetal spiral, and in addition the overtravel caused 

 by continued cooling after the valve is closed sets up 

 stresses that increase as the temperature decreases. 

 Sufficient strength and cross section to safely with- 

 stand these stresses without permanent deformation 

 requires a long flexible element. High accuracy is 

 not obtainable, but in this application a relatively 

 large variation in operating temperature is permis- 

 sible. In the Chase thermostat, the bimetal element 

 is in the form of a tapered spiral spring v.'hich is con- 

 nected to a rotating valve by a simple linkage. To 

 stabilize the bimetal element, it is subjected to a 

 series of hot and cold treatments at temperatures 

 beyond the normal temperature range. 



Fig. 511. — When the bimetal element in a gas pilot-light control is placed near the pilot flame, the bimetal is 

 subjected to a temperature near its maximum operating range, and in service over long periods of time the valve may 

 become corroded and fail to function when an emergency arises. In the pilot control made by the Patrol Valve 

 Company, operating temperature of the bimetal is reduced and distortion from overheating is prevented by a dual 

 pilot construction. The constant-burning pilot ignites a second pilot which heats the bimetal strip when the thermo- 

 static control calls for heat. The bimetal strip upon heating opens the toggle-operated main burner valve, which, hy 

 means of a double-seat construction, reduces the supply of gas to the second pilot, just enough flame being left to keep 

 the bimetal from closing the valve. Since relatively wide limits for temperature of operation are permissible, the 

 bimetal element is designed to develop sufficient force to operate the toggle spring without the use of high working 

 stresses. 



