Environmental Engineering Reference
In-Depth Information
The Tower-Mill Design
In order to be able to make larger mills, builders had to take another major inventive
step: changing the design from one in which the whole body of the mill had to move to
face the wind to one in which only the sails, windshaft, and brake wheel had to move. This
was accomplished by mounting the windshaft assembly in the cap of the mill. which turned
in a curb or track mounted on the top of a ixed tower. The fact that the mill tower was
ixed allowed it to be larger in cross section and higher than the post mill, it could now
be made of brick with a circular cross section or of wood in an octagonal shape. The mills
made of timber were covered with clapboarding in England and often painted white, so that
they came to be called
smock mills
,
from their supposed likeness to the rural smock or
frock. Many Dutch tower mills had a brick base and a rush-thatched body. There was little
difference in the machinery and sails of either type, except for those engaged in speciic
applications, such as sawmills, which did require some special design considerations.
The tower mill seems to have been introduced in the fourteenth century. The earliest
representation is given as 1390; a traveler's sketch dated 1420 shows several located in the
Byzantine town of Gallipoli. Many such tower mills are still extant in Holland. Some now
inside the modern towns were originally on walls bounding the old towns and were more
easily defended than post mills. Many observers might agree with Wailes [1957] that
“the best-constructed tower- and smock-mills are to be found in The Nether-
lands; these cannot be rivalled elsewhere, and the Dutch always led in the design
of cloth-spread mills”
Figure 1-12 shows an elevation section through a large Dutch tower mill [Stokhuyzen
1965]. Its essential difference from the post mill is the
cap
,
which contains only the
windshaft and the brake wheel. Its size was kept small, and its external design was varied
according to the degree that the effect of its shape on the wind low behind the sails was
recognized, and perhaps according to the aesthetic sense of the miller or the builder.
The top of the tower had to be of stout construction and have two essential features.
The irst was the provision of a ixed
curb
or
rail
on which the cap could turn with a
minimum of friction between the horizontal surfaces through which the gravity load was
transmitted. The second feature as a means of keeping the cap truly centered, again with
a minimum of friction between vertical ixed and moving surfaces through which thrust
loads were carried. The horizontal bearing was initially wood blocks sliding on a curb, well
greased, or with iron plates ixed below the cap frame. Later, iron trolley wheels were
mounted on the cap ring, and inally iron rollers were placed between special iron tracks
attached to both tower curb and cap ring, so that a
roller-bearing
was effectively formed.
Figure 1-12 shows the greater room available with twin drives to a pair of stones, an
economical way of increasing output without increasing the tooth loads on the
great spur
wheel
or the size of the stones. Many tower mills were built with a
staging
or
deck
around
the tower at the level of the sail tips, to reduce the amount of effort that had to be
expended to climb up and down the steep ladders to make changes in the sails. There was
also room for living quarters for the miller and his family, which made the mill quite as
commodious as the small, “two-up, two-down' cottages of the workers. A tower
constructed of bricks could be very sturdy and resistant to weather, but it was not easy to
repair if splits appeared as a result of a shift in the foundation or because of the constant
vibrations. Thus, the usual practice of placing windows in a linearly symmetrical pattern
(as in Figure 1-12) was sometimes changed to a spiral pattern to avoid lines of structural
weakness. Wooden smock-mill towers, on the other hand, were subject to joints opening
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