Colder is often better when it comes to tight-fitting metal machine parts. Decreasing the temperature causes the metal to contract. The extent to which the cooled metal shrinks is given by a material property called the coefficient of linear thermal expansion. Industry uses this property to accurately shrink metals in a process called shrink-fitting. Brass, a soft metal alloy used in a variety of different machining applications, has a coefficient of linear thermal expansion of 18.5x10^-6 m/m/C. Cooling brass with liquid nitrogen effectively shrinks the alloy to 99.6 percent of its original size.
Things You'll Need
- Protective eyewear
- Laboratory gloves
- Styrofoam cup
- Liquid nitrogen
Put on protective eyewear and laboratory gloves suitable for working with liquid nitrogen. Liquid nitrogen boils at minus 320 F or minus 196 C, which is cold enough to cause severe burns if any of the liquid makes contact with skin.
Fill two-thirds of a Styrofoam cup with liquid nitrogen. Pour the liquid nitrogen slowly to minimize loss of the liquid due to excessive boiling and vaporization. You'll experience less waste of liquid nitrogen if you initially add no more than 1/3 cup liquid nitrogen and wait until the boiling subsides before adding the remaining liquid.
Lower the brass object into the Styrofoam cup using a pair of laboratory tongs. Expect to see a cloud of white vapor and some vigorous boiling as the brass object enters the liquid.
Release the brass object from the tongs and leave it to reach thermal equilibrium with the liquid nitrogen. Equilibrium occurs when the object cools to the same temperature as the liquid nitrogen. This point in the process is identified by watching the liquid. The brass object is at the temperature of the liquid when the liquid stops boiling vigorously.
Remove the brass object from the liquid nitrogen using the tongs. Don't handle the object without gloves; touching it will cause severe burns. The brass will have shrunk by 0.4 percent in the liquid nitrogen but will start expanding once removed from the liquid nitrogen. The brass object will return to its initial volume when it reaches room temperature.
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