How Pivots Work
Early center pivot irrigation systems showed a lot of potential, but they also broke down. The broke down too often, even according to Robert Daugherty (right) the principal owner of Valley Irrigation. "A farmer, when [he] bought one of [our early systems] bought a headache," Daugherty admits now. "And some didn't want the headache. But after we got it so it was a reliable functioning system, why then the sales part of it became infinitely easier."
Center pivot systems were complex engineering systems that relied on both old and new technologies. They relied heavily on technologies developed for other uses for key components --
- Turbine pumps. In the 1940s, irrigators adapted oil pumps to raise underground water to the surface. Combined with more powerful internal combustion and electric motors, these pumps could deliver water under pressure to the new pivot systems.
- Pipes. After World War II, more and more irrigators started using steel or aluminum pipes either with sprinklers or with gates to flood plant rows to water their fields. Pivots raised the same pipes off the ground and moved the pipes around automatically.
- Sprinklers. In the early 1900s, urban areas had pressurized water systems and used new sprinkler heads to water lawns in town. Gradually, plant nurseries adapted the sprinklers to their use and then farmers combined sprinklers with high pressure pumps and pipes. In 1946, sprinklers irrigated less then 250,000 acres of farmland. By 1959, 3.4 million acres were under sprinklers.
Frank Zybach took new pumps, pipes and sprinklers, put them together and raised the system off the ground. He supported the system with a series of towers and figured out how to rotate the entire apparatus around a central pivot point. Zybach, Daugherty and all the engineers and inventors who followed their footsteps had to devise and refine solutions to a whole series of problems.
- How to move the pivot around. Zybach's first system siphoned some of the pressurized water off from the main pipe and sent it into a piston that pushed a linkage that moved the steel wheels forward. Other inventors used oil hydraulic pistons to provide power, and still others installed electric motors on their towers. Rubber wheels replaced steel fairly quickly. Now, the majority of systems use electric motors to drive rubber wheels. They can go both backwards and forwards at controlled speeds.
- Keeping the towers in line. All center pivot systems will set the overall speed of the system by governing the tower on the outside of a center pivot circle that's the tower that has to travel farther and faster than the towers toward the center. Then, each manufacturer has developed ways for the inner towers to sense when they are behind or ahead of the tower farther out. When one tower gets behind, its motor turns on or runs faster to catch up. Over the years these control sensors have evolved from simple mechanical linkages to electronic sensors under computer control.
- Distributing water evenly. Again, because the pipe travels faster the farther out it gets, more water has to be distributed at the ends of a pivot system than in the middle. The manufacturers have adapted more and more sophisticated sprinkler heads and placed them in precise patterns along the length of the pipe to provide even coverage. Some of today's systems even use GPS and wireless communication technology to control how much water gets to specific portions of the field and to control the system from a cell phone or remote laptop computer.
- Supporting the pipe. Water is heavy 62.5 pounds per cubic foot. The largest pivots use 10-inch diameter pipe and extend out 2,600 feet. So, the water in that system can weigh over 88,000 pounds. (Do the math here.) When you raise those pipes off the ground, you have to support them. Early systems used wires running from the top of tall towers supporting arched or bowed pipe. Later, manufacturers developed a truss system that bowed the pipe up and transferred the weight back to the towers. This "undertruss" system is most often used today and allows the pipe to be raised higher than a wire support system does.
- Raising the pipe high enough. Zybach's first system was only a few feet off the ground. Corn, one of the most popular crops across America, grows six to eight feet high (depending on the variety). Raising the pipe high enough to travel around over the corn made undertruss systems more attractive.
- Getting electricity down the system. The pivot point of a center pivot system is stationary. Everything else moves around in a circle. So if you simply ran a cable up the pivot point and down to the towers, the cable would wrap around the pivot point and eventually break. So the engineers at the pivot companies figured out how to bring the power up into a device called a collector ring. The power from the pivot point is distributed to a series of stationary circular plates and then connections are made to the wires that travel down the pivot's pipe spans.
- Keeping the pipes from rusting. Early pivot systems were built out of steel to support the weight and pressures of the systems. The steel was painted to protect it from rust. But paint wears off. So, by the late 1960s, Valley irrigation systems were being built out of hot-dipped galvanized steel pipes and components. This protected the steel much better. Later systems were built from aluminum which doesn't rust and even from stainless steel or plastic coated pipes that can resist highly corrosive water or chemicals.
Each of these technical innovations borrowed state-of-the-art engineering and scientific knowledge of the time and was a major step in making the systems work reliability.
Written by Bill Ganzel, the Ganzel Group. First published in 2006. A partial bibliography of sources is here.