By: Colin Campbell
Table of Contents
Project Background
An oscillating sprinkler is a mechanism common to many yards in America. It provides a simple and inexpensive means to water a large area of turf in a controlled and uniform fashion. Its appeal lies in the fact that it is completely powered by the incoming flow of water and the output range is adjustable with a simple dial. The secondary usage of oscillating sprinklers is providing hours of entertainment to young children going stir crazy from multiple months of shelter-in-place orders.
The goal of this project is to reverse engineer the mechanism that enables the sprinkler’s automated motion and perform a position analysis to verify that the desired output sweep ranges are achieved with the recommended presets on the adjustment dial.
Mechanism Analysis
The sprinkler mechanism consists of two subsystems. First, a gear train is responsible for transforming the system input (a jet of water supplied by the connected garden hose) to a low speed rotational output. The output from the gear train drives the input of the second subsystem, an adjustable four-bar mechanism, which enables the oscillating motion of the water spray nozzles.
Gear Train
Disassembling the gear train housing reveals the gear train assembly, as illustrated below. A small turbine blade spun by the water flowing into the sprinkler drives a two-stage compound worm gear assembly.
Each worm has a single start, and both worm gears have 24 teeth. The resulting velocity ratio is
By observation, the output angular velocity is approximately 2 RPM, so the input speed from the water turbine is approximately 1150 RPM. The two compound worm gear stages provide a substantial speed reduction and torque amplification in a very compact package.
Four-Bar Linkage
The output from the gear train is coupled directly to the input link of a four-bar linkage in a crank-rocker configuration. The water outlet nozzles are connected to the output link which allows the sprinkler to create a rocking motion from the rotational output of the gear train.
The flexibility of the sprinkler comes from the adjustment dial on the output link of the mechanism. By turning the dial, the user can change the effective link 4 length, and its angular offset from the water outlet.
The effective length of link 4 (c) and the angular offset (delta) between link 4 and the output vector P can be determined from a geometric analysis of the lengths c1, c2, and the relative angle beta, as defined in the figure.
This has the effect of modifying the output sweep between left side only, right side only, full range, narrow centered, and various intermediate combinations.
MATLAB Simulation
With the link lengths established, a four bar position analysis was performed to determine Θ4 over a full rotation of the input crank. The vector representing the position of the outlet nozzles, P, is then defined as
The analysis repeated for the prescribed dial settings for left, right, center, and full sweep ranges (120°, 300°, 215°, and 20°, respectively). The XY position of the resulting nozzle location, P, with respect to the global XY coordinate frame is shown below.
A velocity analysis was also performed to determine the angular velocity of the output nozzle with respect to the rotation angle of the nozzle.
Simulation Conclusions
As seen in the figures above, the sprinkler successfully achieves the four distinct sweep ranges. It is noted that there are some slight discrepancies between the left and right side sweeps. The full range sweep is likewise biased towards the right.
Also, the velocity analysis suggests there are further nonuniformities in the water distribution. It is particularly noticeable in the full sweep, where the water nozzle travels much quicker through one phase of the right side portion, possibly resulting in less dense water distribution to that portion of the yard and in a slightly larger area compared to the left (due to the position sweep right bias).
Overall, the sprinkler is effective in controlling its output range as designed, in spite of slight inconsistencies.
SolidWorks Modeling
The following animations demonstrate the motion of the sprinkler with the four dial settings addressed in the MATLAB simulation to verify the resulting sweeps.
Full Sweep
Left Sweep
Right Sweep
Center Sweep
*In the above videos the turbine blade sometimes appears to spin in different directions and different speeds. This is due to frame rate issues when exporting the animations.