Expansion and Contraction of Materials

Expansion and Contraction of Materials

When most materials are heated they expand and this increase their volume. One example of expansion is the fitting of the starter ring gear to the flywheel. The gear is heated until it expands sufficiently to pass over the rim of the flywheel, and when it is cool the gear tries to return to its original size, this gripping the flywheel with considerable force.

All metals do not expand equally when heated through the same range of temperature, e.g. aluminium alloy expands more than cast iron; copper and brass expand more than mild steel. Gudgeon pins (hardened steel) are removed and replaced y dipping aluminium-alloy pistons in boiling or very hot water' the difference in expansion -- the piston expands more than the gudgeon pin -- makes the pin an easy push fit.

Coefficient of Expansion

A number which denotes the degree of expansion of a substance is called the coefficient of expansion of the material. There are three types of expansion namely, linear, superficial and cubical, and each has its own coefficient of expansion:

1. The coefficient of linear expansion is the increase in unit length of a material when its temperature is raised by 1 degree C. The coefficient fo linear expansion for various metals have been found by experiment and they are given in the list below.

2. The coefficient of superficial expansion is the increase in unit area of a material when its temperature id raised by 1 degree C. Its value is double that of the coefficient of linear expansion for the same material.

3. The coefficient of cubical expansion is the increase in unit volume of a material when its temperature is raised by 1 degree C. Its value in three times that of the coefficient of linear expansion for the same material.

Coefficient of Linear Expansion

Materials Per 1 deg C