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# 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 10-6 K-1) and copper (16.6 x 10-6 K-1) (or brass 18.7 x 10-6 K-1) joined together (often welded) throughout the length
• 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 L2 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 (oC)

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 oC. The bending can be calculated as

s = (14 x 10-6 K-1) ((50 mm) 1/1000 m/mm)2 (100 oC) / ((2 mm) (1/1000 m/mm))

= 0.00175 m

= 1.75 mm

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Temperature

oC
oF

Length

m
km
in
ft
yards
miles
naut miles

Area

m2
km2
in2
ft2
miles2
acres

Volume

m3
liters
in3
ft3
us gal

Weight

kgf
N
lbf

Velocity

m/s
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ft/min
ft/s
mph
knots

Pressure

Pa (N/m2)
bar
mm H2O
kg/cm2
psi
inches H2O

Flow

m3/s
m3/h
US gpm
cfm

3 10