An Investigation of Switching Behaviors of Bimetallic-Disk Thermostats
Final rept. Apr 1977-Apr 1978
AEROSPACE CORP EL SEGUNDO CA VEHICLE ENGINEERING DIV
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Thermostatic switches that use a bimetallic, curved disk as the control element have been known to exhibit, on occasion, anomalous switching behaviors, such as a gradual drift of or sudden deviation from the original switching temperatures. A detailed analysis is presented in this report to show that a class of these switching anomalies can be attributed to an improperly designed interaction between the disk and the elastic switching armature. The analysis is based on Wittricks theory of thermoelastic stability of the bimetallic disk, extended to include the effects of a central force. The resulting nonlinear problem was solved numerically on a digital computer with the aid of an existing solution routine for two-point boundary-value problems. Numerical results indicate that under certain combinations of geometrical, mechanical and thermal conditions, the thermostat may fail to respond to temperature changes with positive snap actions of the disk, thus resulting in the so-called temperature creepage or dithering phenomenon. Such events are usually characterized by a series of openings and closings of the contacts with reduced travel but greatly increased frequency, which can induce excessive arcing or fatigue damage that would cause the thermostat to malfunction.
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