News Letter Extracts

Technical data on Mills

      by James Tallon, Miller of Martry Mill, Kells, Co. Meath.

In the wheelhouse, the ‘dark round of the dripping wheel’ generates the motion that is carried into the interior of the mill by the axle. The type of wheel outside, either overshot or undershot, does not affect the internal operations. At the end of the axle is placed the pit wheel. Half of this component is permanently within the narrow pit that gives it its common name. In ancient mills you may expect to find the pit wheel fashioned in timber. At a later stage of development the wooden wheels were replaced with iron. Iron wheels frequently had wooden teeth and this was almost always the case when the replacement of the wooden pit wheel did not coincide with that of the wooden shaft. Pit wheels vary in size and often contain the iron founder’s name. A close examination of an iron pit wheel will reveal that it is in fact two pieces bolted together.This made it much easier from the millwrights point of view, in that if a pit wheel was to be installed whole, this could only be done when the axle was removed. When you consider how large some pit wheels were, you realize how wise old millwrights were. Even still, it could have been no small task to have persuaded several hundredweight of cast iron to assume a position in a narrow pit, in order for its complimentary half to be bolted in. A good deal of space was allowed between the axle and wheel center so that wedges could be driven and the wheel correctly centered. To convert the horizontal line of the axle into a vertical plane, the pit wheel meshed into the smaller wallower wheel. This wallower wheel, frequently made of iron, caused the heavy vertical shaft to rotate. It had fewer teeth than the pit wheel and the speed of the shaft was increased by a ratio of more than 4 to 1. Mounted on the wallower is the spur wheel and this carries the motion to the millstones via the stone nut. In a mill with a wooden shaft, the spur wheel is more often made of wood.

A wooden spur wheel will frequently be found to have its spokes morticed into the shaft. Wheels of this kind are called compass arm wheels as the spokes – usually four – suggest the cardinal compass points. The spur wheel rotates at about four times the speed of the axle and is larger and has more teeth than the wallower. When it drives the small stone nut, the ratio is increased by 7 or 8 to 1. A stone nut therefore revolves about 28 times faster than the mill wheel.The wooden teeth of the spur wheel mesh with the teeth of the stone nut. Although the gears in the o old mills were wooden, many mills have wooden cogwheels replace with iron ones. It is a relatively simple task to replace a stone nut, as the other machinery does not have to be dismantled. It is not unusual in a mill to find wooden teeth meshing with iron teeth and this was supposedly for smoother running.

Another advantage arose when iron gears were introduced – technical reasons limited the minimum size of wooden wheels – and iron wheels could be cast in smaller dimensions than their wooden counterparts. For example, iron stone nuts allowed the ratio between them and the spur wheel to be increased, thus allowing a higher rate of revolutions to be produced by the millstone. When a smaller iron wheel replaced a wooden stone nut, the vertical axis of the millstones was moved closer to the main shaft. When this occurred, the beams supporting the stones were not always realigned and incidentally, many beams in mills contain timbers from old ships.

The iron stone nut is normally the only adjustable gear in the mill and is moved up its square shaft by the jack ring. A stone nut is lifted into or out of gear when the mill is not working. At the end of a day’s milling, the mill is always left out of gear, particularly if heavy rain is expected as the increased water flow could spill over the sluice and start the wheel. A mill left in gear could run hot, that is, run the stones and possibly catch fire