What is Thermal Expansion?

bridge thermal expansion

Everything Changes Size When the Temperature Changes.

One of the more noticeable examples is the expansion joint in bridges that allow for seasonal variations in the length of the bridge span. In diamond turning and other precision engineering applications, small differences in size are critical.

When a material is heated the distance between individual atoms will change. For most materials the atoms get further apart and the total length change depends on how many atoms are in the length. This makes the temperature change proportional to length. For example, if a one-meter piece of metal changes length by some small amount, a two meter piece would be expected to change by twice the amount.

Changes in the length change is proportional to the temperature change. The constant of proportionality is called the coefficient of thermal expansion, denoted by the Greek letter alpha (a).

coefficient of thermal expansion
effect of thermal expansion

Predicting Thermal Expansion

Precision instrumentation is built from components of a variety of materials—which, with different CTE’s, expand and contract by different amounts with temperature changes. To further complicate the problem, different materials have different rates of thermal conductivity, making the individual instrument parts and the work piece fluctuate at different rates.

Because of the complexity of the system and time to execute the activity, the combined thermal changes in a precision manufacturing process are impossible to predict.

Fortunately, there is an easy and inexpensive solution to the problem: Stablize the temperature of the instrument and the part being measured or cut with precision temperature control.

  Drinking glass with fracture due to uneven thermal expansion after pouring of hot liquid into a cool glass.

 

Drinking glass with fracture due to uneven thermal expansion after pouring of hot liquid into a cool glass.

  Three days of temperature fluctuations recorded at four different points on a diamond turning machine.

 

Three days of temperature fluctuations recorded at four different points on a diamond turning machine.