The Stone Nut Gear Wheel

The stone nut is the last gear transferring the power from the waterwheel to the millstones. Many visitors to the Mill are surprised to learn that some of the gears, including the stone nut, are fitted with wooden teeth.

The picture below shows three wooden teeth: a rough cut blank tooth as sawn from a block of wood, a shaped but unused tooth ready for inserting into a gear wheel, and a well-worn tooth removed from a gear wheel.

Millers found that using wooden and metal toothed gears together produced less noise when the mill operated and reduced the wear on the metal toothed gears. It also acted as an additional safety factor as the wooden teeth will break under excessive force.

The teeth, or cogs, were usually made from apple, cherry or hornbeam wood, cut to shape before being inserted into the cast iron gear wheel frame. If a tooth became worn or broken, it could be removed and replaced by a new one.

The stone nut is keyed onto the stone spindle but is able to move up and down. In the picture below, blocks of wood are holding the stone nut up and out of gear, away from the driving bevel gear. This prevents the stone spindle and the runner millstone from rotating.

What happens when a rotating cast iron bevel gear meets a stationary stone nut with wooden teeth? A horrible 'crunching' sound and a row of broken teeth! This happened in May 2014, on the first open day of the year.

The teeth are in two parts, an upper and a lower, and as the two gearwheels came into contact, many lower parts were broken. The remains of one upper and two lower are shown below after removal from the stone nut.

As it was impossible to remove the stone nut from the stone spindle without lifting the bed stone, repairs had to carried out while it remained on the stone spindle.

After raising it to a workable position, the damaged teeth were removed either by driving them out from the iron casting or by drilling out the remains to clear the socket. It was fortunate that some suitable teeth were available in a spare stone nut that could be transferred as replacement teeth.

The picture below shows the repaired stone nut with the replacement teeth awaiting final trimming before being carefully lowered down to engage the teeth with the stationary bevel gear for testing.