Booklet “A Description of the Mill and its Workings” by Bob Steele. [extract]
The machinery at Michaelchurch Mill
The waterwheel at Michaelchurch Mill is carried on an iron axle tree, running in bronze bearings and braced in an oak bearer frame.On the same axle, but inside the mill, is the pit wheel - the main driver for the machinery. In total these weigh in excess of five tons, but when properly set up and balanced they can easily be turned by hand.
The pit wheel meshes with a bevelled pinion called the wallower (or waller wheel). This is secured on the wooden upright shaft with wood wedges, and translates the motion of the pit wheel ninety degrees from the vertical to the horizontal plane. The gearing effect of 64 teeth on the pit wheel linking with 24 teeth on the wallower means that the upright shaft rotates at about three times the speed of the waterwheel.
Above the wallower on the upright shaft is the great spur wheel. Approximately 7 feet in diameter, this transmits the drive to the millstones by engaging with two iron gears known as the stone nuts. These were located either side of the great spur wheel, underneath the millstones which are on the floor above. From each stone nut a stone spindle rose vertically through a bronze bearing, known as the neck box, in the centre of the bedstone above (the stationary stone of the pair).The runner stone was mounted on the spindle, and rotated over the bedstone to give the grinding action required.The base of the stone spindle ran in a bronze bearing - the bridging box - mounted on a heavy oak beam known as the bridging tree. This supported the weight of the runner stone.
The eighteen inch diameter stone nuts had 25 teeth, and the great spur wheel with its 126 teeth drove the stone spindles at about five times the speed of the upright shaft. Overall, therefore, the 10 rpm of the waterwheel is stepped up to drive the 1½ ton runner stones at about 150 rpm, the optimum grinding speed for a four foot diameter stone - an overall gearing ratio of fifteen to one.
Above the great spur wheel is the crown wheel. This engaged a bevel pinion to turn a horizontal lay shaft. Belt drives from the lay shaft operated the sack hoist, a smutter for cleaning the grain before grinding, and a flour dresser, or bolter.
Both the pit wheel and the great spur wheel feature replaceable wooden teeth in a cast iron wheel structure; there is no metal-to-metal contact anywhere in the drive mechanism. Dovetail wedges (spur wheel) or nails (pit wheel) were used to secure the teeth in place. This meant that damaged or worn teeth were easily replaceable as part of day to day maintenance. A supply of spare teeth and wedges made by local carpenters was an important part of the miller’s toolkit.