There is only one proven windmill design for pumping water.
This statement requires justification.
Water pumping windmills were pioneered in the United States of
America. Originally they were made of wood and consisted of
wooden sails mounted on a rotating shaft which had a crank
attached to it. The rotation of the crank would impart a
reciprocating action to a pump. These windmills introduced
windmill technology to an un-mechanical rural population however
they were unreliable and improvements were made. There is a move
to re-introduce this type of technology today with a view to
making windmill construction accessible to rural third world
farmers. The writer of this manual has no problem with this
provided that it is appreciated that this type of windmill has a
restricted life and should be seen as a stepping stone in the
development of a more sound design.
The modern water pumping windmill.
The term modern is a misnomer strictly speaking as the current
windmill design which ensures long service life was perfected in
the case of most manufactures about 30 years ago.
The windmill head or engine.
It is a fact that small diameter windmills rotate at a faster
speed than large diameter windmills for any given speed. This is
because the ratio of windmill fan peripheral speed to the axial
speed through the fan is a constant. Smaller diameter windmills
have therefor to rotate faster than large windmills to achieve
the constant ratio. This fact caused the downfall of the original
windmill design described above. The high speed of the crank
driving the pump results in stress reversals a simple
construction windmill just cannot handle. The modern windmill is
always geared so that several rotations of the windmill wheel
results in one pumping stroke of the windmill. The correct design
is such that the windmill pump never cycles faster than 40 cycles
per minute. Practical experience shows that exceeding this
somewhat empirical limitation results in much reduced windmill
life. Obviously however for windmills over 5 metres in diameter
the gearing system falls away as the windmill wheel has a lower
rotational speed.
SOUTHERN CROSS WINDMILL GEARING RATIOS.
SIZE WINDMILL (M)
2.5 3,0 3.7 4.3 6.3 7.5
GEAR RATIO OF MILL 3 2.61 2.31 2.31 1 1
STROKE OF MILL (mm) 146 165 184 184 210 241
254 305
* Seneschal direct acting windmills are available with two stroke
lengths.
Windmills must obviously have the capacity to rotate so that they
will always face towards the wind except when the wind speed
exceeds the speed which will over stress the tower and windmill
engine and wind wheel. Many mechanisms have been developed to
achieve this speed governing action on windmills, some using
springs and levers or counterweights. The protection of a
windmill in high winds is vital and this will be gone into in
some depth below.
The center of the wind wheel is offset from the mast pipe by a
small amount which causes the wind to exert a moment on the wheel
tending to turn it about the mast pipe. The member carrying the
tail vane is hinged on the mill gearbox at an axis inclined to
the vertical so that the vane normally rests at it's lowest point
when parallel to the wheel axis. In this position the wheel
presents it's full area to the wind. As the vane is turned about
the hinge axis it rises and it's weight gives rise to a moment
tending to turn it again parallel to the wheel axis. The
governing systems is designed so that these moments oppose one
another.
In operation when a wind of sufficient velocity is encountered,
wind forces on the wheel tend to turn it about the mast pipe and
since the vane will remain parallel to the wind direction it will
rise as the mill gearbox pivots with the wheel and exert a
restoring torque on the gearbox and wheel, tending to turn the
wheel back to face the wind. A condition of equilibrium will
finally be reached in a steady wind where the two opposing
torques will balance, the wheel assuming a position at an angle
to the wind. As the wind speed rises, the equilibrium angle of
the wheel will increase until, when governing is complete, the
wheel axis will be at right angles to the wind. Also, as the
process of governing proceeds, the wheel R.P.M. will decrease as
less of it's area faces the wind until it has practically ceased
at the fully governed position.
Southern Cross Mills are designed to commence governing in a wind
of 33 - 41 km.p.h. and to be completely governed when speed
reaches 50 km.p.h.
The windmill wheel itself consists of fans radially mounted. They
should be dished in the direction from which the wind will strike
the wheel. The fans of smaller windmills should be constructed
from carbon steel sheeting while on larger (over 5 metre
diameter) windmills mild steel sheeting may be used provided the
edges of the fan are beaded or folded over. Water pumping
windmills tend to fail due to metal fatigue in their fans and
accordingly correct material specification is essential.
The mill engine itself should be of robust construction and be
self oiling. This is normally achieved by submerging the working
parts in oil which should be replaced once per year. The engine
bearings and shaft to the wind wheel hub should be strong enough
to withstand the reciprocating load of the pump and connecting
pump rods as well as the gyroscopic forces experienced during
wind direction changes particularly at high wind speeds. Many
windmill manufacturers make the mistake of incorporating ball or
roller bearings in their designs. This is fatal to the long life
of the windmills as ball bearings are inappropriate in
applications with intermittent loads of varying degree. The
bearings should be of the babbitt (or white metal) type. Further
the windmill should obviously be designed so that all wearing
bearings and moving parts are replaceable.
The tower supporting the windmill engine is of prime importance.
In a tower the main load carrying members are the legs,
reinforced against buckling by horizontal stiffening members
called girts and diagonal stiffening members called braces.
There has long been a controversy concerning the relative merits
of three of four post towers, many observers claiming that they
prefer four post towers "because they are stronger". However,
unqualified statements such as this are misleading because, even
supposing a four post design possessed some advantage, a three
post could easily be made stronger by making each member much
heavier. An investigation into the stresses occurring in each
member of three and four post towers shows that for towers of
equal strength, the three post tower will weigh the least, or to
restate it, a three post tower makes more efficient use of it's
material than a four post.
A three post design also possesses other advantages for
windmills, and to sum up, the following factors favor the three
post tower :-
1. It makes most economical use of material.
2. there are fewer parts, which in turn mean mower cost of
material, lower manufacturing costs, and less assembly time.
3. In the unlikely event of sinking of one of the foundations,
all members are sill carrying the load, which is not the
case with a four post design.
However, Southern Cross has four as well as three post towers for
those who prefer this design.
Ultimately the overturning moment developed by the applied load
must be resisted by the weight of the foundations, and if these
are not of sufficient mass, the tower may fail. Note, however,
that no load carrying tower member will fail initially if the
foundations tear out.
It is therefore important to provide good, solid foundations for
any windmill tower in order to protect the entire investment and
the best of all foundations is sound, reinforced concrete. All
Southern Cross towers are unconditionally guaranteed provided the
anchorage holds, and it is indeed false economy to overlook the
provision of a sound foundation because it is normally out of
sight.
All Southern Cross towers are entirely made of steel, each
component being hot dip galvanized after punching to ensure an
unbroken protective covering, which means that the strength of
the tower will remain unaffected throughout it's life. Painted
steel parts, although cheaper, lose their protection through
weathering, leaving the parts open to corrosion, which reduces
their sectional area and weakens the tower.
All tower components are rolled steel sections except the braces
on tower for geared windmills, which are of high tensile steel
rod.
Southern Cross towers are easily erected, being assembled on the
ground and raised into position afterwards. It is very important
that the mast pipe be plumb to ensure that the mill will turn
easily to face any direction.
The above may appear to be confusing and unintelligible to the
average reader. In selecting windmills for rural water supply the
easiest way of selecting prospective suppliers is to ask how long
they have been in business and further to see some old windmills
in operation.