Snap Action Disc

The snap acting thermostat metal disc is becoming increasing popular as a cost effective protection of small motors and appliances.


A disc of thermostat metal is pretensioned by dishing or doming. With a rise in temperature, the thermal stresses produced due to the bimetal action reach a point where they equal the pretensioning stresses. At this point the disc snaps to the opposite concave. Upon cooling the reverse action takes place but at a lower temperature.

The upper and lower snapping temperatures depend upon the disc geometry. No exact mathematical formulas are possible to determine these temperatures but certain guidelines will help to establish the size and geometry of trial discs.