# Bimetallic Strips

## Heat and bending of bimetallic strips

Bi-metallic strips are subject to bending due to heat - and can be used to convert change in temperature to mechanical displacement.

Bending occurs towards the side of the metal with the lowest temperature coefficient of expansion.

Bi-metal strips

- consists of two strips of different metals with different temperature expansion coefficients
- are usually made with steel (
*12 x 10*) and copper (^{-6}K^{-1}*16.6 x 10*) (or brass^{-6}K^{-1}*18.7 x 10*) joined together (often welded) throughout the length^{-6}K^{-1} - are typically used in thermostats

Assuming two metals with the same shape joined together - bending of a bi-metal strip due to change in the temperature can be calculated as

*s = α L ^{2} dt / t (1)*

*where *

*s = bending (m) *

*α = 13-19 x 10 ^{-6} K^{-1 }(typical aprox. 14 x 10^{-6} K^{-1} - check manufacturers catalogs and specifications)*

*L = length of bimetallic strip (m)*

*dt = temperature difference ( ^{o}C)*

*t = thickness of bimetallic strip (m)*

### Example - Bending of Bi-metal strip

A bi-metal strip with *α = 14 x 10 ^{-6} K^{-1}*, length

*50 mm*and thickness

*2 mm*is heated

*100*. The bending can be calculated as

^{o}C*s = ( 14 x 10^{-6} K^{-1}) ((50 mm) 1/1000 m/mm)^{2} (100 ^{o}C) / ((2 mm) (1/1000 m/mm)) *

* = 0.00175 m*

* = 1.75 mm*