DRIVES AND CONTROLS 



243 



/Intake manifold 



Insulaled healer 

 winding , 



I winding , | 



/load 



Conlacis close when bimelal is healed 

 Fig. 505 

 iHeol barriers/ 

 .1^. ^.^ 



Aclive bimelal sirip 

 healed by eie clric curreni, 



( EC-d -| r' c-r-3 S 



"•lemperolure compensalingV—^ 

 strip &3i 



Fig. 506. 



Fig. 504. 



Fig. 504. — On the Dodge carburetor, when the engine is cold, a vane in the e.xhaust passage to the "hot spot " is held 

 open by a bimetal spring against the force of a small counterweight. When the thermostatic spiral is heated by the out- 

 side air or by the warm air stream from the radiator, the spring coils up and allows the weight to close the vane. Since 

 high accuracy is not needed, a thin, flexible cross section is used with a long length to give the desired sensitivity. 



Fig. 505. — In the Friez relay, a constant current through an electrical heating coil around a straight bimetal strip 

 gives a time-delay action. Since the temperature range is relatively large, high sensitivity is not necessary, hence a 

 short straight strip of bimetal is suitable. Because of the relatively heavy thickness used, the strip is sufficiently stiff 

 to close the contact firmly without chattering. 



Fig. 506. — A similar type of bimetal element is used in the Ward Leonard time-delay relay for mercury-vapor 

 rectifiers. This relay closes the potential circuit to the mercury tube only after the filament has had time to reach its 

 normal operating temperature. To eliminate the effect of changes in room temperature on the length of the contact 

 gap, and therefore the time interval, the stationary contact is carried by a second bimetal strip similar to the heated 

 element. Barriers of laminated plastic on both sides of the active bimetal strip shield the compensating strip and 

 prevent air currents from affecting the heating rate. The relatively high temperature range allows the use of a 

 straight thick strip, whereas the addition of the compensating strip makes accurate timing possible with a short travel. 



'Oil pressure diaphragm 



Grounded conlaci 

 pioved by diaphragm 



I Bimelal 



Receiver on 

 dash 



^^^^^ 

 Sender 



Fig. 507. Fig. 508. 



Fig. 507. — Oil pressure, engine temperature, or gasoline level are indicated electrically on automobile dashboard 

 instruments built by King-Seeley in which a bimetal element is used in both the sender and receiver. A grounded con- 

 tact at the sender completes an electric circuit through heaters around two similar bimetal strips. Since the same cur- 

 rent flows around the two bimetal elements, their deflections are the same. But the sender element when heated will 

 bend away from the grounded contact until the circuit is broken. Upon cooling, the bimetal again makes contact and 

 the cycle continues, allowing the bimetal to follow the movement of the grounded contact. For the oil-pressure gage, 

 the grounded contact is attached to a diaphragm; for the temperature indicator, the contact is carried by another 

 thermostatic bimetal strip; in the gasoline-level device, the contact is shifted by a cam on a shaft rotated by a float. 

 Deflections of the receiving bimetal are amplified through a linkage that operates a pointer over the scale of the 

 receiving instrument. Since only small deflections are needed, the bimetal element is in the form of a short stiff strip. 



Fig. 508. — Oil dashpots used in heavy-capacity Toledo scales have a thermostatic control to compensate for changes 

 in oil viscosity with temperature. A rectangular orifice in the plunger is covered by a swaged projection on the bimetal 

 element. With a decrease in oil temperature, the oil viscosity increases, tending to increase the damping effect ; but the 

 bimetal deflects upward, enlarging the orifice enough to keep the damping force constant. A wide bimetal strip is used 

 for stiffness so that the orifice will not be altered by the force of the flowing oil. 



