If you have a lawn, then at some point during the summer, when it gets really hot and dry, you end up pulling out the sprinkler...
An oscillating sprinkler like the one shown above is one of the most common sprinklers around. It sprays out a fan-shaped curtain of water, and the metal arm oscillates back and forth to cover a rectangular area perhaps 20 feet by 30 feet (6 meters by 9 meters) in size.
If you've thought about it for more than five seconds, you've probably wondered about the "oscillating" part of an oscillating sprinkler: What makes the arm go back and forth? In this edition of HowStuffWorks, you'll learn exactly how it happens!
Here is a typical oscillating sprinkler that you might buy at any discount or home improvement store for a few dollars:
You attach the hose to a connector on the right-hand side (not shown). There is often a knob that lets you adjust the spray pattern (full, center, left or right):
Connected to the knob there is small arm that gets pushed and pulled by a heart-shaped cam. When the sprinkler is running, the cam rotates slowly -- at roughly 1 revolution per minute (rpm) -- this is what causes the arm to oscillate back and forth.
There is also the spray arm itself, which is a hollow aluminum tube with holes in it (on some sprinklers there are nozzles on this tube):
The spray arm has a ferrule and an O-ring seal on it, and it screws into the body of the sprinkler.
You can see that this is an incredibly simple device at its core. Water flows into the spray arm -- the aluminum tube -- and it sprays out through the holes or nozzles. The cam pushes the spray arm back and forth. Why does it have a heart-shaped cam instead of a simple little crankshaft? It's because a circular crankshaft would cause both ends of the spray pattern to get a lot more water than the center of the spray pattern. Think about a piston going up and down in an internal-combustion engine -- it is traveling slowest at each end of its travel, and fastest when it is in the center of the cylinder. The spray arm would have that same problem. The heart-shaped cam evens things out.
Spinning the Sprinkler Cam
So the big question comes down to rotating the cam. One way to do it would be to use a small electric motor. But if you did that, you would need to replace the batteries all the time, and that's no fun. Besides, you have a built-in power source: the running water that flows right through the sprinkler. You might as well use it in the same way that a hydro-electric dam uses water to spin its turbines.
If you cut open the body of the sprinkler, you can see that this is exactly how the sprinkler works. Inside there is a small turbine, or water wheel:
If you look down the tube where you attach the hose, you can see a small restrictor that allows some water to flow straight ahead and forces some water to shoot to the left. It is the water shooting to the left that spins the turbine:
You can see in the video that the turbine spins at several hundred rpm. We need a gear train that can reduce the speed down to 1 rpm. This sprinkler uses a three-stage planetary gear system!
This gear train provides a 512-to-1 reduction in speed, so the cam spins at about 1 rpm.
Many different types of sprinklers use water power in the same way. Some are jet-powered rather than turbine-powered, but they are all using the force of moving water to create linear or rotational motion. In theory, you could create a little water-turbine electrical generator and attach it to your hose to create several watts of power -- you could power a flashlight bulb while you water your lawn at night!
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