Brief description

A strip composed of two different metals bonded together, which bends predictably when heated or cooled due to differing thermal expansion coefficients.

Use / Function

• Primary use: Mechanical thermal actuation and temperature control (thermostats, circuit breakers, dial thermometers).
• Secondary uses: Thermal compensation in precision clocks, steam traps, and over-current protection devices.
• Scale: Small component / Instrument level.

Operating principle

The bimetallic strip relies on differential thermal expansion:

1. Dual Layer Bonding: Two metals with significantly different coefficients of thermal expansion (e.g., brass expands rapidly, steel expands slowly) are mechanically joined or welded along their entire length.
2. Thermal Response: When heated, the high-expansion metal expands more than the low-expansion metal.
3. Curvature: Because the two layers are rigidly bonded, the length difference forces the strip to bend into an arc toward the side of the metal with the lower expansion rate.
4. Reversibility: Upon cooling, the strip contracts back to its original flat shape (or bends in the opposite direction if cooled below room temperature).

How to create it

Minimum functional version

1. Material Selection: Obtain two thin, flat strips of equal dimensions: one of high thermal expansion (Brass) and one of low expansion (Steel).
2. Surface Cleaning: Thoroughly clean and degrease the joining surfaces with solvent and light abrasives.
3. Joining: Rivet the two strips tightly together along their length at regular intervals, or solder/braze them into a solid laminate.
4. Calibration: Heat the strip gently over a flame to verify uniform bending toward the steel side upon heating.

Technical level

Basic to Intermediate. Requires clean metal surfaces and tight mechanical bonding or brazing.

Materials needed

• Essential materials:
  • High-Expansion Metal: Brass or copper sheet/strip.
  • Low-Expansion Metal: High-carbon or stainless Steel sheet/strip (or Invar alloy).
  • Fasteners/Solder: Copper/iron rivets or silver brazing alloy.
• Tools:
  • Metal shears, drill or hole punch, hammer, torch or soldering iron.

Variants and improvements

• Helical / Spiral Bimetal Coil: Coiling the long strip into a spiral amplifies angular displacement, ideal for dial thermometers and clock balance wheels.
• Snap-Action Disc: Pre-stressed disc shape that flips abruptly at a threshold temperature, creating a crisp make-or-break electrical contact.
• Invar Alloy Combination: Pairing brass with Invar (nickel-iron alloy with near-zero expansion) maximizes deflection sensitivity per degree.

Limits and risks

• Delamination: Repeated heating cycles or excessive bending force can separate the bonded metals.
• Plastic Deformation: Overheating beyond the elastic limit permanently deforms the strip, ruining calibration.
• Slow Response: Thicker strips have high thermal mass and respond slowly to temperature changes.