ROSENFELD IMAGES LTD / SCIENCE PHOTO LIBRARY ROSENFELD IMAGES LTD / SCIENCE PHOTO LIBRARY
Component testing. Overhead view of engineers testing the surface hardness of a steel cog or gear wheel which has been case- hardened. Case-hardening involves heating the cog so that carbon or nitrogen are dissolved into its surface layers. This makes the cog hard-wearing on the outside to resist erosion but soft on the inside to resist the chance of fracturing. The cog shown here has been designed for use in the gears of rolling mill machinery. Cogs are used to transfer rotary motion from one shaft to another. Photographed in Germany.
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