Many domestic, commercial, and also industrial processes rely ~ above thermostatic bimetals for electric or mechanical actuation applications. They can be uncovered in many regulate switches such as beforehand thermostats, clocks, circuit breakers, and electrical appliances.

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What is a bimetal?

A bimetal or thermostatic metal is a sheet or strip of 2 or an ext composite products having different coefficients of straight thermal growth bonded through riveting, brazing or welding. The product with the larger coefficient that thermal growth (CTE) is considered as the active component, and also that of smaller sized CTE is the passive component. The energetic component usually has alloys containing iron, manganese, nickel, or chrome in differing amounts. Conversely, on the passive side, invar is frequently chosen, an iron-nickel alloy include 36% nickel. Part bimetals encompass a 3rd layer that copper or nickel in in between the active and passive political parties in stimulate to increase thermal conductivity and also decrease the material’s electrical resistivity <1, 2>.

Properties that bimetal

Bimetals work-related with the propensity of metals to broaden when heated, and contract as soon as cooled. The resulting change of curvature, or bending, in solution to temperature change, is a an essential property of every thermostatic bimetals. In effect, the temperature change is converted into mechanical displacement. The action of bimetals is predictable and repeatable. The contents used for bimetals space selected for their temperature characteristics and also their heat conductivity, stability, strength, workability, and also electrical properties.


The basic characteristic that a bimetal is flexivity, additionally known as specific curvature. That is vital property the bimetals the is characterized by the curvature adjust along that is longitudinal axis. This is expressed together <3>:



F = flexivity (°F-1) (SI: °C-1)*

R2, R1 = radii of curvature that the active and passive sides, dong (in) (SI: mm)

T = thickness that the piece (in) (SI: mm)

T2, T1 = temperatures (°F) (SI: °C)

* This simply represents the SI unit once mathematically converted from the conventional system. It is no a conventional unit. That follows an in similar way throughout the message unless declared otherwise. See ASTM Designation B106.

And for a basic beam:



L = distance between support clues (in) (SI: mm)

B = movement (in) (SI: mm)


Image 1: Schematic chart of Test for Flexivity. Retrieved from Ref. 3

Radius of curvature

Moreover, the equation listed below shows the flexing or radius the curvature the a bimetal strip. Here, we have the right to see the factors that affect the bimetal bending and its partnership <3>:



ρ = radius that curvature that the piece (in) (SI: mm)

α1 = very first strip coefficient of growth (°F-1) (SI: °C-1)

α2 = second strip coefficient of development (°F-1) (SI: °C-1)

T0, T1 = temperature (°F) (SI: °C)

E1 = modulus of elasticity for first strip (psi) (SI: Pa)

E2 = modulus that elasticity for 2nd strip (psi) (SI: Pa)

t1, t2 = thickness of every component (in) (SI: mm)

t = thickness of the bonded strip (in) (SI: mm)

n = E1/E2

m = t1/t2

This equation shows that the thermostatic bimetal bending is directly proportional come temperature adjust of the ingredient strips and also the difference in the CTE, and also inversely proportional to the thickness the the an unified strips. The radius of curvature is also affected by the thickness ratio and also the proportion of the moduli the elasticity the the two strips.

Electrical resistivity and also thermal conductivity


For applications where warmth is created by passing an electric existing through a bimetal, knowledge of the electric resistivity and also thermal conductivity as parameters because that temperature adjust is important. This is true for numerous bimetals that are used as circuit breakers. For a resistor, the equation listed below shows exactly how temperature rise deserve to be acquired <3>:



ΔT = temperature increase (°C)

I = present (A)

ε = electrical resistivity (µΩ)

θ = time (s)

c = particular heat capacity (J/g°C), estimated at 0.502 for all Bimetals

d = thickness (g/cm3)

w = broad (mm)

t = total thickness (mm)

By making use of the practically conversion factors, a comparable formula can be derived in English units.

Bimetal applications

Temperature indication

Bimetals are provided for temperature indication as in the spiral or helix actuated reminder thermometers. Together thermometers aid measure temperatures in offices, refrigerators, and even on aircraft wings. The bimetals the this kind are usually 0.005 in (0.127 mm) come 0.015 in (0.381 mm) thick, and also the coil is fitted to the guideline scale as it generates adequate torque in bespeak to relocate the reminder freely. The temperature range covered by bimetals extends around between -50°F and also 1000°F (-46°C and 538°C). Angular deflection price is commonly at 2.5-3° per degree Fahrenheit <4>.

Temperature control

Bimetals are utilised as a way to control the temperature, as in room temperature thermostats. In such devices, a bimetal blade holds a current-carrying contact allude that is connected to a combination static contact point. This allows for the automatically switching that circuits to manage heating and cooling of electrical devices as the blade arches when a certain temperature is reached <4>.

Tube and pipe couplings

For cryogenic, non-magnetic, and nuclear applications where metal properties should be switched reliably, bimetallic couplings are provided to enable direct connection and change for pipes and tubes with various CTEs. This fittings may also be provided in other thermal move applications <4>.

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Function Control

By introducing warmth to a bimetal – or what is well-known as auxiliary heating – the duty of a machine containing the bimetal have the right to be controlled. Circuit breakers and time delay devices are instances of these devices. Relative functions such as current and time can be regulated by installing the bimetal piece as the energetic element in the device <4>.