Higher Woodhill Mill - Page 3

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After the water had left the wheel it flowed through a tunnel beneath the mill grounds and out into the river through this archway.
It can be reached with care via an ill-defined path leading off to the right from the bottom of the cobbled road which descends to the site.

The other design feature of the waterwheel which can be seen in the drawing on the previous page, is that it uses a rim drive.
Waterwheels are limited to a slow speed of rotation, a few revolutions per minute, by the time it takes the water to fall the height of the wheel under gravity. Large waterwheels such as the one at Higher Woodhill Mill could have power outputs of several tens of horsepower, and this caused a problem for the designers.

From basic mechanics, power, or the rate of doing work, can be defined as force multiplied by the velocity in the direction of the force:

Power = Force x Velocity

If a drive shaft is only rotating slowly the velocity at its circumference is low. If it is also transmitting a high power then the forces on the shaft must be very large. These torsional (twisting) forces would tend to break the joint between the central shaft and the spokes of a waterwheel, or between the shaft and any attached gear wheel. Fractures of shafts and attached flanges were common as high power waterwheels began to be built.

The solution was to take the drive from the periphery of the waterwheel using a segmented ring gear, usually of cast-iron sections, attached to the rim or spokes of the wheel. This then drove a much smaller pinion wheel which rotated at higher speed, resulting in correspondingly lower forces on its shaft.

This high r.p.m. could be used more easily to drive machinery, and as a bonus the spokes of the main waterwheel could be made lighter since they no longer transmitted the rotation. The pinion wheel was usually positioned close to where power was developed, as on this drawing:

This photograph shows part of such a rim-drive waterwheel, though with an internal rather than external ring gear.
(From The BP Book of Industrial Archaeology.)

It may seem surprising that a waterwheel was still in use in the 1850s, even though steam engines were becoming common by 1800. However there were several reasons why waterwheels continued to be built and operated well into the age of steam:

• The earliest steam engines (beam engines) only produced reciprocating (up and down) rather than rotary motion, and were not ideal for driving most machinery.
• Large waterwheels could generate over 100 horsepower and it was not until the 1840s that steam engines of this capacity became available.
• A waterwheel was mechanically quite simple and thus cheaper to buy, run and maintain than a steam engine.
• There were no fuel costs involved with a waterwheel whereas a steam engine consumed large quantities of coal, especially early designs which had a thermal efficiency of only a few percent.

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