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HANDBOOK OF MECHANICAL DESIGN 



THERMOSTATIC MECHANISMS 



Sensitivity or change in deflection for a given temperature change depends upon 

 the combination of metals selected as well as the dimensions of the bimetal element. 

 Sensitivity increases with the square of the length and inversely with the thickness. 

 The force developed for a given temperature change also depends on the type of 

 bimetal, whereas the allowable working load for the thermostatic strip increases with 

 the width and the square of the thickness. Thus, the design of bimetal elements 

 depends upon the relative importance of sensitivity and working load. 



Bi'mefa/ helix ^ 



I Pen arm 



Fig. 501. — In the Taylor recording thermometer, a pen is moved vertically 

 across a revolving chart by a brass-invar bimetal element. To obtain sensi- 

 tivity, the long movement of the pen requires a long strip of bimetal, which is 

 coiled into a helix to save space. For accuracy, a relatively large cross section 

 gives stiffness, although the large thickness requires increased length to obtain 

 the desired sensitivity. 



Heating coil ^ 



Heal barrier 



Coniacf 

 r of a led 

 by bi- 

 metal 

 spring 



Fig. 503. — In this Westinghouse overload relay for large motors, a portion 

 of the motor current is passed through a heating coil within the relay. Heat 

 from the coil raises the temperature of a bimetal spiral which rotates a shaft 

 carrying an electrical contact. To withstand the operating temperature, a 

 heat-resistant bimetal is used, coiled into the spiral form for compactness. 

 Because of the large deflection needed, the spiral is long and thin, whereas the 

 width is made large to provide the required contact pressure. 



By the use of heat barriers between the bimetal spiral and the heating coil, 

 temperature rise of the bimetal can be made to follow closely the increase in 

 temperature within the motor. Thus, momentary overloads do not cause 

 sufficient heating to close the contacts, whereas a continued overload will in 

 time cause the bimetal to rotate the contact arm around to the adjustable 

 stationary contact, causing a relay to shut down the motor. 



Bimetal 



Fig. 502. — Room tempera- 

 tures in summer as well ' as 

 winter are controlled over a 

 wide range by a single large- 

 diameter coil of brass-invar in 

 the Friez thermometer. To 

 prevent chattering, a small 

 permanent magnet is mounted 

 on each side of the steel contact 

 blade. The magnetic attrac- 

 tion on the blade, increasing 

 inversely with the square of the 

 distance from the magnet, gives 

 a snap action to the contacts. 



