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→5.3 Triple-Layer Carrier Materials
The thickness ratios of the components can be selected according to the application requirements. The two outer layers usually have the same thickness.
===5.4 Thermostatic Bimetals===
Thermostatic bimetals are composite materials consisting of two or three layers of materials with different coefficients of thermal expansion. They are usually bonded together by cladding. If such a material part is heated either directly through current flow or indirectly through heat conduction or radiation, the different expansion between the active (strong expansion) and passive (low expansion) layer causes bending of the component part.
Directional or force effects on the free end of the thermostatic bimetal part is then used as a trigger or control mechanism in thermostats, protective switches, or in control circuits. Depending on the required function of the thermostatic bimetal component different design shapes are used:
*'''Straight or U-shaped strips''' for nearly linear motion
*'''Circular discs''' for small linear motions with high force
*'''Spirals and filament spring shapes''' for circular motion
*'''Stamped and formed parts''' for special designs and applications
The wide variety of thermostatic bimetal types is specified mostly through DIN 1715 and/or applicable ASTM standards (Table 5.23). The different types have varying material compositions for the active and passive side of the materials. The mostly used alloys are iron-nickel and manganese-copper-nickel. Mainly used in circuit protection switches (i.e. circuit breakers) some thermo-bimetals include an intermediate layer of copper or nickel which allows to design parts with a closely controlled electrical resistance.