Pressure, Torque and Gears
The basic idea of a traveling sprinkler is to power a simple transmission with water pressure. In other words, the force of the water from the hose turns a series of gears connected to an axle, which slowly turns the sprinkler's wheels.
A tractor traveling sprinkler like this one has only seven essential parts:
- A sprinkler arm assembly
- A worm gear
- A transmission
- Three wheels
- A plunger on the bottom of the sprinkler
- A ramp
- A hose connection
All water entering your house from the water company has pressure behind it -- typically 40 to 60 psi (pounds per square inch). When you open the spigot, water pressure forces the water down the hose to the sprinkler. The water flows through a pipe in the sprinkler and out through two connected sprinkler arms.
As the water pushes out, it exerts an equal and opposite force back on the arms (this is the same principle behind rockets and jet engines). The two arms are bent at the end so that the force pushes on each arm at an angle. The arms are bent to point in opposite directions, so the force acting on both arms combines to generate torque –- a rotational force. It's like a merry-go-round with water rockets on each side.
The arms are connected to a plastic worm gear inside the tractor body. The spinning arms rotate the worm gear, which engages a gear in the transmission. The transmission is a simple gear train that transmits the rotational force of the worm gear to the drive axle that turns the tractor’s back wheels.
In a standard design, the transmission has three settings (although it could have more or fewer, and some designs don’t have different speed options). The settings include a high and a low speed, as well as neutral for stationary watering. Adjusting the setting engages and disengages gears in the transmission. Totally disengaging the gear train from the drive axle puts the sprinkler in neutral, and switching between the different-sized gears changes the rotation speed of the axle (see How Gear Ratios Work for more information).
The gear drive mechanism is something like the gear train in a car, but everything happens in slow motion. Most only go approximately 60 feet per hour! The back wheels push the front wheel, which guides the tractor along the hose.
When the sprinkler reaches the end of its path, it activates its automatic shut-off. In the simplest design, the automatic shut-off trigger is a small ramp at the end of the tractor’s path. When the tractor sprinkler reaches this ramp, the front wheel travels over it, just as if it were another hill in the yard. But the back wheels go around the ramp, moving on either side. The ramp hits a small spring-loaded plunger on the bottom of the tractor. The plunger stops the water from reaching the sprinkler arms, shutting down the tractor.
That's really all there is to it. It's a simple conversion of the force of water pressure to a rotational force on the wheels.
For more information about traveling sprinklers, check out the links on the next page.