| Animal Powered Systems |
|7. Profiles .|
Aside from water-raising applications, the potentially most important use of animal powers is, doubtlessly, the grinding of grain. Due to the prime importance of product quality, the situation is, by comparison, much more complex. This is particularly true in view of the fact that in practically any culture, food and food processing have always represented definite domains of the irrational. Thus, it is hardly possible to categorize the quality of comparatively simple food products such as flour in a generally valid manner. Compared to scientifically quantifiable factors like nutritional value, color or keeping qualities, questions of taste, social status and religious or other fundamental convictions can play a decisive role in such matters.
The simplest type of animal-powered mill is the gearless stone mill, known in antiquity as the "hourglass mill" (figure below). The name derives from the characteristic shape of the runner (upper millstone), which rests on a perforated metal plate and a metal pin mounted atop the conical bedder (lower stone). In most cases, the runner was turned by donkey power acting on drawbars anchored in the grooves of the runner. The grain was poured into the hollow upper cone and gradually found its way through the perforated plate and into the grinding furrow due to the stone's vibrations. No information is available on the achievable quality of the product from such mills.
The capacity can be enlarged by increasing the speed of the runner, e.g. by installing a pair of simple gears. Figure below depicts such a modified mill from Tunesia.
While many different types of stone are suitable for use as millstones, the best ones (but' the most difficult to cut) are made from lava. No matter what kind of stone is used, though, its grinding surface has to be "sharpened", i.e. provided with cutting edges, the shape of which is decisive for good grinding quality and smooth running.
A more modern type of animal-powered mill is the combination of a universal power gear and a commercial-type mill' whereby the latter must be of the disk type, since hammer mills operate at speeds that are too high for an animal-power drive.
As indicated above, no definite statement can be offered with regard to the throughput capacities of animal-powered mills. In approximate terms, a donkey-powered mill can probably be expected to handle 10-30 kg of flour per hour, while oxen are capable of increasing the output to something on the order of 100 kg per hour.