Historical Development of the Windmill - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 18

The top of the main post is shaped to make a stepped bearing or pintle, perhaps 20 cm
in diameter and depth, itting into the great crowntree , which takes the entire weight of the
rotatable assembly, including the sails, drive shaft, gearing, and millstones, all contained in
the housing called the buck. A steady (not shown in the igure) is provided to brace the
crowntree against the main post just above the quarterbars to prevent excessive swaying in
high winds. This was formed by transverse and longitudinal joists, making a frame that its
easily around a circular cross section of the main post at this point. In later models, the
wooden steady could be replaced by centering wheels.
In spite of the weight and the absence of antifriction bearings (until later years), the
turning maneuver known as winding the mill was apparently carried out without great
trouble. The rotation is made slowly and sometimes infrequently, so that the wear in the
crowntree bearing is relatively small. As conidence grew in the structural design, the post
mill would have developed in size and longer sails would have been used, so that the height
of the working loor increased. Eventually the ladder required to climb into the mill grew
to a considerable length, and in the full development of the post mill it could contain some
three dozen or more steps and weigh half a ton.
Let us turn now from the basic structure of the foundation and the rotatable body to
that of the moving parts that the structure carried, namely, the sails, the transmission, and
the auxiliaries such as the brakes and hoists. These represent a more straightforward design
exercise, although each had its problems and probably negligible analytic theory behind it
for guidance.
Figure 1-9 is again a simpliied diagram of the essentials. The sails, which are not
shown in detail because this will come later, are carried on the windshaft , which is
supported at its forward or breast end by the large breast beam or weather beam and its
rear or tail end by the tail beam. The weather beam supports the neck bearing and thus the
weight of the sail assembly, which is considerable. The tail bearing takes the axial thrust
which again is considerable, because in this kind of wind turbine, operating on the lift
principle, the axial component of force generated by the wind is much greater than the
tangential component of force that provides the torque. The weather beam and tail beam
transmit their loads by a variety of methods to the buck and the ends of the crowntree, and
thence to the main post.
Each pair of opposing sails had a single longitudinal spar or stock , which was mortised
through the breast end of the windshaft. In later years, the stocks were itted into a cast-
iron poll end or canister , thus preventing a weakening of the windshaft by mortising and
by exposure to the weather. The neck of the shaft was clad with iron, and the neck bearing
was of stone or hardwood, which was later replaced with brass or bronze. Judging by the
crude sketches of the irst post mills, the windshaft was initially placed horizontally, but
quite early on it was inclined some 10° or so upward from tail to breast. The reasons put
forward for this are that it shifted some of the weight of the sail assembly to the tail end
and so improved stability, that it allowed longer sails to be used that could still clear the
base structure or possibly permit a larger buck, and the sails “could catch more wind.”
Based on modern experience, inclining the rotor axis presents design dificulties without any
improvement in performance, and so it is done only to provide clearance between the blades
and the tower. Thus, the second explanation above is the most reasonable one.
The power take-off from the windshaft is made by a large brake wheel , so called
because it also carries the brake on its rim. It was itted originally with hardwood pegs that
transmitted the torque to the wallower , or lantern pinion , the vertical shaft of which either
directly or via intermediate gears powered the millstones or other devices. As time went
on, the wooden pegs or staves become shaped cogs. Iron parts replaced some of the wood,
and eventually the brake wheel and wallower developed into iron cross-helical gears. The
wooden pegs were lubricated and lasted a surprisingly long time; some are still in use.