How A Water Pumping Windmill Works


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Categories: Hydro Power

The most common practical uses for a windmill are to irrigate pastures and gardens, water livestock, and supply and aerate ponds. Anything more than that requires a holding tank on "stilts," or a water tower, to provide enough pressure to be "on tap" for household use.

The windmill, barbed wire, and the six-shooter were the "big three" technological advances in those days.

The great windmill boom lasted for over 50 years. Between 1880 and 1935 more than 6 million windmills were sold by about 20 manufacturers. But, sadly, the big spinning wheels came to a screeching halt in the early '30s with the advent of federally subsidized power to remote farms and homesteads. The REA (Rural Electrification Administration) made it possible for people to use electric pumps delivering 20 to 30 gallons of water a minute, and that drowned out the windmill era. By 1970, only three companies in the U.S. produced water pumping windmills: Aermotor, Dempster, and Baker Monitor, and they are still around today.

How a windmill works

Boom truck stood the windmill up. I'm disconnecting the chain and hook 25 feet up.


The windmill's wheel (fan) has 15 to 40 galvanized steel blades which spin around on a shaft. The shaft drives a geared mechanism that converts rotary motion to an up-and-down motion like a piston in a car engine. That motion drives a long pump rod (aka sucker rod) going up and down inside of a pipe in the well. Attached to the end of the pipe is a cylinder with a sealed plunger going up and down in it that forces the water up the pipe. The seals (cupped seals that ride up and down in the pump-cylinder) are called "leathers." (Neoprene instead of leather is used in most cylinders today.) Each up-stroke pulls a certain amount of water into the cylinder, but on the down-stroke a check valve (aka foot valve) in the bottom won't let it be pushed out, so the water has nowhere to go but up (with the next upstroke). It's a simple efficient design that has remained virtually unchanged for more than 100 years.

An average windmill (6 to 8 foot-diameter wheel) spinning in a brisk breeze (15 to 20 mph) will pump about three gallons a minute whenever the wind blows (about 35 percent of the time in many areas). That adds up to about 1500 gallons a day. Another example of output could be calculated by using a 10 to 12-foot wheel pumping against a theoretical 100-foot head (the column of water lifted from the static water level to the tank). This larger windmill will pump an annual average of 4500 gallons per day, or 1.63 million gallons of water a year. This figure is based on moderate winds (8-12mph) blowing part of the time, running the mill at half its rated capacity, and brisk winds (15 to 25 mph) blowing about 30 percent of the year and running the pump at maximum capacity.

Wind speed has an important effect on pumping capacity. Below certain speeds the mill can't "get going." Above 25-35 mph (depending on the model) the windmill's overspeed controls limit the output by turning (furling) the direct face of the wheel out of the main wind direction. This design feature protects the windmill mechanism, but it also limits the pumping rate no matter how fast the wind blows.


There is an optimum wind speed for every windmill size and model. The number of vanes (sails) in the wheel increases its sensitivity to low wind speeds (to get started), but other factors such as fan diameter, the depth of the static water level in the well, and cylinder size all play a part in output capacity. ("Static" water level is the measurement from the top of the well casing to the water surface down in the well—not the depth the pump is set.)

From top to bottom: Pump-rod assembly, cylinder, and plunger with leathers

Uses for a windmill

The most common practical uses for a windmill are to irrigate pastures and gardens, water livestock, and supply, and aerate ponds. Anything more than that requires a holding tank on "stilts," or a water tower, to provide enough pressure to be "on tap" for household use. I placed my water system on top of a hill so the water is gravity-fed for all my needs on the property. (More about my system later). I love the idea that if the power fails, I'll still have fresh water—lots of fresh water. To me, that's self-sufficiency and a nice sense of security. Evidently a lot of backwoods people feel that way because windmills are making a comeback. Aermotor claims that sales of windmills, both for generating electricity and for pumping water, are increasing worldwide, and more windmills are pumping water today than at the turn of the century.

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