DRIVES AND CONTROLS 



247 



Permanent 

 magnef 



'S+eel Blade 



^Auxr/j'ary 

 confacis 



Silver conlacis 



Silver 

 conlacfs 



Foggle spring^ 



'Bellows spring 



0- 



'' Ternperalure 

 ad/usfmenl- 



Ternperalure 

 adjuslmeni 



Ad/usling screw 



Dlfferenh'al ' 

 adjuslmeni 



Fig. 520. Fig. 521. Fig. 622. 



Fig. 520. — For heavy-duty room-temperature controls, the Perm thermostat uses a bellows mechanism that 

 develops a high force with small changes in temperature. The bellows is partly filled with liquid butane, which at 

 room temperatures is a gas having a large change in vapor pressure for small temperature differentials. Snap action 

 of the electrical contact is obtained from a small permanent magnet that pulls the steel contact blade into firm contact 

 when the bellows cools. Because of the firm contact, the device is rated at 20 amp. for noninductive loads. To avoid 

 chattering or bounce under tlie impact delivered by the rapid magnetic closing action, small auxiliary contacts are 

 carried on light spring blades. With the large force developed by the bellows, a temperature differential of only 2°F. 

 is obtained. 



Fig. 521. — Snap action in the Tagliabue refrigerator control is obtained from a bowed flat spring. The silver 

 contacts carried on an extended end of the spring open or close rapidly when movement of the bellows actuates the 

 spring. With this snap action, the contacts can control an alternating-current motor as large as IK hp. without the 

 use of auxiliary relays. Temperature differential is adjusted by changing the spacing between two collars on the bel- 

 lows shaft passing through the contact spring. For temperatures used in freezing ice, the bellows system is partly 

 filled with butane. 



Fig. 522. — In the General Electric refrigerator control, the necessary snap action is obtained from a toggle spring 

 supported from a long arm moved by the bellows. With this type of toggle action, the contact pressure is a maximum 

 at the instant the contacts start to open. Thermostatic action is obtained from a vapor-filled system using sulphur 

 dioxide for usual refrigerating service or methyl chloride where lower temperatures are required. To reduce friction, 

 the bellows makes point contact with the bellows cup. Operating temperature is adjusted by changing the initial 

 compression in the bellows spring. For resistance to corrosion, levers and blades are stainless steel with bronze pin 

 bearings. 



Flexible bellows ' Thermosl-a-fTc 

 seal, \ bellows spring 



Capillary 

 lube. 



Needle 

 valve 



Bellows 

 spring 

 adj'usrmen/ 



Fig. 523. — Two bellows units are used in the Fodders thermostatic expansion valve for controlling large refrigera- 

 tion systems. A removable power bellows unit is operated bj' vapor pressure in a bulb attached to the evaporator 

 output line. The second bellows serves as a flexible, gastight seal for the gas valve. A stainless steel spring holds 

 the valve, closed until opened by pressure transmitted from the thermostatic bellows through a molded push pin. 



