bimetal

A bimetal is a device composed of two different metals bonded together that have different coefficients of thermal expansion. When temperature changes, the two metals expand or contract by different amounts, causing the composite strip or strip-like shape to bend or curl. This mechanical movement forms the basis of many heat-responsive actuators and switches.

Construction and principle: Typically made as a strip, coil, or band, the two metals are bonded in a laminate, clad, or foil arrangement. The bonding can be achieved by rolling, diffusion bonding, or other metallurgical processes. Upon heating, the metal with the greater expansion grows more, bending the strip toward the metal with the smaller expansion. The exact curvature depends on the metal pair, thickness, and geometry. Common pairings include steel with brass or copper, though many other combinations exist, including nickel-iron and copper-nickel.

Applications: The primary use is as a temperature-sensitive actuator or switch. In thermostats and thermal switches, a bimetal strip bends to make or break electrical contacts as temperature varies. They also serve in some overload protection devices, where self-actuation helps disconnect circuits when heat is generated by current. Bimetal elements are found in certain temperature gauges, indicators, and other simple sensing devices where passive, energy-free response is advantageous.

Advantages and limitations: Bimetal devices are simple, rugged, reliable, and require no external power. They are inexpensive and can provide hysteresis useful for stable switching. Limitations include limited reading accuracy, slower response relative to electronic sensors, and aging or creep that can shift calibration over time. They are most suitable for relatively coarse temperature control and protection rather than precise measurements.