Historical Development of the Windmill - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 17

seem that the irst we know of are those of Ramelli in 1588, and excellent drawings they
are. It is reasonable to suppose that the irst horizontal-axis mills were small, with a ixed
orientation, having their foundation members sunk in the ground, because the thrust of the
wind on the sails gave rise to large overturning moments that presented a problem.
However, the directional variability of the European winds would severely limit the energy
output of a ixed-direction mill, so once a suficiently strong structural design that resisted
the large moments had been achieved, a method of turning the mill into the wind must have
been sought at an early stage of development. The mounting of the sails, transmission, and
grinding stones on a frame that could rotate on a heavy, central post securely fastened to
a ixed bottom structure was a bold step; one could expect that many mills were damaged
or destroyed by the wind before a suitable design was found.
Before looking at the complete engineering design of a post mill, which is quite
complex in its full development, let us look at a diagrammatical sketch of a very common
type of post with supporting members, which is the heart of the structural design and gave
the mill its name. This is shown in Figure 1-8. The main post , usually a great block of
oak perhaps 80 cm square, is supported by quarterbars mortised in at the top end and
anchored to the crosstrees at the bottom with bird's mouth joints. Secured by the iron
straps , these joints take the longitudinal outward thrust of the quarterbars. The ends of the
crosstrees are supported on masonry piers but are not fastened to them. Sometimes the
bottom of the main post is cut out to provide side tongues itting loosely over the crosstrees
as a steady. It is interesting to note that the best oak for windmills was considered to be
from trees about 100 years old!
Figure 1-8. Structural components in the base of a post mill. The great main post was
usually oak and might be 70 to 80 cm square and cut from a 100-year old tree.