Historical Development of the Windmill - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 13

in use are not known for these early times: only very approximate igures can be given for
later eras. From the igures of Jannis Notebaart we can obtain order-of-magnitude estimates
of possible maximum numbers at any time in the eight centuries from their introduction to
the heyday of the multibladed windmill in the United States: 3,000 in Belgium, 10,000 in
England, 650 in Anjou in France, and 9,000 in Holland [1972].

Changing the Axis

So we have what seems to have been the sudden eruption and very rapid spread of
these windmills, which requires some explanation. But there is also an even more important
fact to contend with: the complete change from the vertical-axis mill to the horizontal-axis
mill. The European windmill's four sails, possible lat boards in the earliest instances, were
mounted on a horizontal shaft, with each sail set at a small angle with respect to the plane
of rotation of the whole wheel. This presented several engineering problems. Three major
ones were (1) transmission of power from a horizontal rotor shaft to a vertical shaft, on
which the grindstones were set; (2) turning the mill into the wind; and (3) stopping the rotor
when necessary, because the wind could not be diverted or blocked.
The irst problem was solved by adopting the cog-and-ring gear shown in Figure 1-5,
designed long before by Vitruvius for his horizontal-axis water wheel. To solve the second
problem, the bold step was taken of rotating the whole system on a central spindle
composed of a stout post supported by heavy beams. This is suggested in Figure 1-6,
which is the earliest know representation in a topic of this type of windmill and appears
as part of an illuminated letter in an English psalter of 1270 [Wailes 1956]. The third
problem, stopping the mill, could be solved by turning it out of the wind and applying a
frictional braking action at the outer edge of the large gear wheel shown in Figure 1-5.

Figure 1-6. The earliest known illustra-
tion in a topic of horizontal-axis wind-
mill, from the “Windmill Psalter” of 1270.
The mill is supported on a post and hence is
called a post mill [Wailes 1956]. (Reprinted
by permission of Oxford University Press)

Figure 1-5. Vitruvius' cog-and-ring
gear mechanism. This gear made it
possible for early horizontal-axis
windmills to drive grindtones.