The composite photo above shows the "great spur wheel" on the Prairie Mill.  The great spur wheel is the large wheel in the center.  The smaller wheel at right is one of three "stone nuts".  The smaller wheel at left is a drive wheel used in the 1920s and 1930s when a gasoline engine replaced wind power to turn the great spur wheel.  These components are located above the sack hoist floor.

The great spur wheel is the largest geared wheel in the windmill and is at the bottom of the main spindle.  It drives one of three millstones through a stone nut and stone spindle.  When engaged, the great spur wheel turns a stone nut which turns a millstone.

This great spur wheel is 12 feet in diameter and is made of wood.  There are 101 straight cogs or "teeth" in the great spur wheel.  Each stone nut is approximately 3 feet in diameter and each contains 25 wooden staves.  The number of staves and cogs are not proportional to promote even wear.

Each stone nut is attached to a "runner stone" through the "stone spindle".  The runner stone is the top millstone that turns above the bottom millstone, or "bedstone."  Each millstone had its own stone spindle and stone nut.  This feature enables millers to quickly change to another millstone to mill different grains.

All stone nuts are normally in a "neutral" position, meaning that they aren't in contact with the great spur wheel.  When ready to mill, the miller pulls the stone nut toward the great spur wheel so the staves contact the straight cogs.

The mill is capable of turning only one millstone at a time.

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