How to Select the Best Rotor-Type Sprinkler

In order to make good decisions about which sprinkler products will work best for you it is necessary to have some basic knowledge of how sprinklers work and what the various options available to you are. Therefore the following is intended to give you the background necessary to understand the terminology used in my reviews.

Sprinklers fall into one of two types based on the method they use to apply water to the ground; spray-type (sprays) or rotor-type (rotors.) Spray-type sprinklers are the type of sprinkler that spray a fixed water pattern similar to how a shower head works. Rotor-type sprinklers use a rotating stream (or multiple streams) of water to apply the water to the ground. Spray-type sprinklers (also called “spray heads” and “sprays”) are typically used for smaller areas. Rotor-type sprinklers (most often simply called “rotors”) are used for larger areas (generally more than 18' in width.)

Rotor is the term used to describe the various sprinklers which operate by rotating streams of water over the landscape. The example which most people are familiar with is the "impact" rotor sprinkler (often improperly called a "rainbird", Rainbird is the actually the trade name of a sprinkler manufacturer.) The impact sprinkler is mounted on a bearing that allows the entire sprinkler body to spin in circles. It is rotated by the impact of a swinging arm which repeatedly strikes the body of the sprinkler, causing it to rotate slightly each time. You probably know the impact sprinkler best for the distinct sound it makes when operating-- tooka, tooka, tooka, tic, tic, tic, tic, tic, tooka, tooka, tooka, etc... You may run into rotor-type sprinklers called “cam drives” or “ball drives”. These are also impact sprinklers, however the impact is caused by either a cam or a ball bearing inside the body of the sprinkler. With ball and cam drive rotors only the nozzle moves. Most of these ball and cam drive sprinklers are no longer sold, but a few, like the Rainbird R-50 (they don't call it a ball drive, but it is) are still available. While impacts do an acceptable job of irrigating, the jerky motion of the impact drive tends to make them rotate in a less than uniform manner. Thus the impact rotors have now been almost completely replaced by gear-driven rotors, which are very quiet, rotate smoothly, are lower maintenance, and much smaller in size. As with cam and ball drives, only the nozzle on a gear-drive moves. The water moving through the sprinkler spins a turbine, this turns a set of gears, which turn the nozzle. These gear-drive rotors have one or more streams of water which rotate silently across the landscape. The prettiest of these are the multi-stream rotors (or “stream-rotor”) where multiple streams of water rotate over the landscape one after the other. Multi-stream rotors are fascinating to watch but tend to be higher maintenance than the other types of gear-drive sprinkler heads.

Most rotors now come with a set of nozzles rather than a single nozzle. Often these nozzles come attached to what is called a nozzle tree. Just remove the nozzle you want from the tree and install it in the rotor. Save the rest of the nozzles, you may need them later. Multiple nozzles are a big advantage, as you can change to a different size nozzle if needed to help balance your system. Say you have a dry spot between rotors. You might be able to green it up by installing a larger nozzle in the rotors surrounding the dry spot. Some rotors, however, still come with a single, pre-installed nozzle, and some come with stripped down nozzle trees (with only a few of the available nozzle sizes on them.) If you want the whole set of nozzles with these less expensive models, you must purchase them separately. A preinstalled nozzle or limited nozzle tree is common with many of the prepackaged rotors sold at discount stores.

I need to mention a new type of sprinkler, which I am classifying as a rotor since it utilizes a moving stream of water. However, it operates a little different from a traditional rotor. Sometimes called robot sprinklers, these sprinklers direct a stream of water to fall on specific spots. This works something like when you use a hose to water the yard, and you move the nozzle of the hose around to make the stream of water fall on different spots. The size and shape of the area to be watered is programmed into a computer, the computer then controls the sprinklers, telling them where to move the stream and how much water to apply in each spot. Typically they use an electric motor to move the nozzle.

Typically rotors are used for sprinkler spacings from 18 feet to 55 feet apart. There are rotors available that can be spaced closer than 18 feet but they are generally not cost effective, as a spray sprinkler will work well at this spacing and cost less. There are also rotors available for spacings farther apart than 55 feet but they are typically only used for golf courses. Most rotors require a lot more water pressure to operate than spray-type sprinkler heads. When selecting a rotor keep Stryker's Rule in mind; "the water pressure at the rotor head (in PSI) must exceed the distance (in feet) between the heads.") Thus if you want to space rotors 35 feet apart you will need 35 PSI of pressure at the rotor. (Caution: you will actually need more pressure than 35 PSI to run the system due to pressure loss in the pipes and valves between the water source and the sprinkler. Figure a minimum of 15 PSI more will be required for the pipes and valves.) The small rotors sold for residential use work best at 25 to 35 foot spacings, although with careful design some models can be spaced up to 50 feet apart. As a general rule you should hire a professional designer if you need sprinkler spacings greater than 45 feet. Designing with larger sprinklers requires specialized knowledge as many unique factors must be considered in the design process.

Water enters the base of the sprinkler. From there it passes through a filter screen and then through a turbine. The water turns the turbine, which powers a set of gears, which rotate the sprinkler nozzle. The water then passes up through the body and exits through the nozzle. Generally the nozzles can be removed and there are numerous nozzle sizes available. At the nozzle there is a radius reduction screw. With most rotors this screw also doubles as the set screw that holds the nozzle in place.

As the radius reduction screw is turned clockwise the end of the screw comes into contact with the stream of water exiting the nozzle. This distorts the water stream and results in a reduction of the radius. Many people in the irrigation industry refer to this screw as the “break-up screw” because it “breaks up” the water stream. There is a temptation to play around with this screw setting in an attempt to create a better water pattern. However, most modern rotor-type sprinklers are designed to give optimum uniformity with the radius reduction screw not in contact with the water stream. Therefore you should only turn the screw so that it contacts the water stream if you need to reduce the sprinkler radius. Be warned- most people look at the stream of water exiting the nozzle and feel it is not giving a uniform pattern. So they mess with the radius reduction screw trying to get a “better pattern”. Unfortunately, this most often hurts rather than helps. The fact is that without training the average person can't tell when the water pattern is best by simply looking at it. The moral is if you don't need to reduce the radius of the stream, then leave the screw in a position where it does not contact the water stream! With that said, there is a situation where the screw does need to be used to break up the water stream to improve the pattern. It occurs when the water pressure (PSI) at the rotor is below the optimum level (indicating in most cases that the system wasn't designed correctly). You can't tell if this is a problem by looking at the stream, so look at the ground around the sprinkler. If you have a low pressure problem the sprinkler will create a donut-shaped water pattern on the ground. There will be a very small green area right around the sprinkler head where water leaks from the head. Then there is a dry area for several feet (the donut hole), with a wet area farther out. This creates a pattern on grass shaped like a huge green donut, with a small bit of green in the very center. If this is happening you will need to use the radius adjustment screw to break up the stream so that more water falls closer to the rotor. If there are dry patches located midway between two rotors, but no donut pattern, that usually means the rotors are spaced too far apart, or the radius adjustment screw is too far into the stream. Try backing off the radius adjustment screw until the screw does not contact the water stream. If that doesn't fix the dry spot try using a larger nozzle size in the rotors adjacent the dry spot. Unfortunately in most cases these simple efforts to fix the problem just result in creating a new problem. For example, using a larger nozzle may cause the pressure to drop and create the previously mentioned donut pattern problem. If new problems are created you will either need to live with dry spots, or spend some serious money making major repairs. Major repairs that are often needed include adding a booster pump to create more water pressure, relocating the rotors so that they are closer together, replacing the pipes with larger size pipes, or a combination of these.

Rotor bodies come in two basic styles, “pop-up” and “shrub” style. Pop-ups do just what the name implies, the sprinkler body is installed below ground and the nozzle is lifted up above ground on a riser when the sprinkler is operating. After the irrigation is complete, the riser and nozzle are pulled by a spring back down into the sprinkler body. Since the body is typically installed below ground, the sprinkler becomes more or less invisible when it is in the “down” position. This has two advantages. The first is that the sprinkler does not detract as much from the appearance of the landscape. The second advantage is that the sprinkler is not as likely to trip someone, or be damaged by yard care equipment such as lawn mowers.

• Bermuda grass, dethatched yearly - use 3” or 4” pop-up.
• Bermuda grass, dethatched less often - use 4” pop-up.
• Fescue, bluegrass, rye grass - use 4”, 5” or 6” pop-up.
• St. Augustine or any grass planted near blowing sand - use 5” or 6” pop-up.
• Low ground cover - use 5” or 6” pop-up.
• Medium height ground cover - use 12” pop-up.
• Shrubs- see discussion below.

The 12 inch bodies work good for taller ground covers and low shrubs. Generally in a landscape design shorter shrubs are used at the edges of planters and larger, taller shrubs are used farther back in the area. So 6 or 12 inch pop-ups at the perimeter will often provide adequate coverage.

Shrub-style sprinkler bodies are mounted above ground level on a vertical pipe. This allows the sprinkler to be elevated above the level of the plants, where it can spray water over them. Shrub-type sprinklers are most often used in areas with tall, dense shrubs. Shrub-type bodies should not be used next to sidewalks, paths, driveways, lawn edges, or anywhere near places people walk or play. This is because the shrub sprinklers may trip people, or worse, someone may fall on one and be impaled (more likely just badly bruised, but the injury is still significant.) Not a pretty thought. For this reason I recommend, as do almost all professionals, that shrub-type spray heads only be used when there is no other option. If you have tall shrubs adjacent to a sidewalk you should consider replacing them with ground cover or lower growing shrubs. I use shrub-style sprinklers mostly on inaccessible hillsides where we are trying to grow plants for erosion control. I typically install them 36" above ground to reduce problems with people tripping over them or falling onto them. Often I attach them to a large post to make them more visible. One of the arguments often given for using shrub style sprinklers is cost savings. Cost wise, I have found that shrub-style sprinklers are no less expensive than pop-up style bodies once you include the cost of the riser pipe and a stake to keep the sprinkler from wobbling. In order to make a shrub-style sprinkler cost less than a pop-up you must install it within 6" of ground level. That is the height that makes it most dangerous.

Nothing essential to read in this section, it's provided for those who are curious. The earliest rotors were impact types, and Rainbird was one of the earliest sprinkler companies. Although several companies made similar products, the name Rainbird came to be almost synonymous with impact sprinklers. But impacts had problems in turf areas where pop-up style sprinklers are needed. The drive arms were large and to make a pop-up style of rotor required a huge case (called a bucket) for the sprinkler to retract into. They required a lot of maintenance because grass would grow into the cases and interfere with the arms. These large impact pop-ups are commonly are called “rat traps” in the industry. It was realized that a better solution was needed. In an attempt to create a smaller, closed-case (no exposed moving parts) rotor, cam and ball drive rotors were invented, primarily made by Buckner and a company named Safe-T-Rain. The “Safe-T-Rain” name resulted from the idea that their ball drive rotors were smaller than the impact rotors and presented less of a hazard. Both cam and ball drives also use an impact to turn the sprinkler, but now at least all the moving parts were smaller and internal. But they also wore out quickly and were never really widely accepted by the turf grass industry.

In the mid 70's the first turbine powered gear-drive sprinklers came along. The Toro and K-Rain companies pioneered the gear drive industry. Toro used a sealed gear train filled with oil and their Toro S700 was the dominant residential size gear-drive for a number of years. They worked very well, but they were really difficult to adjust and the oil-filled gear boxes leaked oil and were expensive to manufacture. About the same time Rainbird introduced the very inexpensive, all-plastic Mini-Paw impact sprinkler. The Mini-Paw was smaller than other impact rotors due to a unique design of the impact mechanism. The Mini-Paw quickly became the biggest seller in the residential rotor market. Unfortunately the Mini-Paws didn't hold up very well and had the standard impact sprinkler problem of high maintenance costs.

The next innovation in rotor design was water-lubricated gear-drives. These were inexpensive, reliable, and easier to adjust than the Toro models. Hunter, Nelson, and K-Rain quickly came to dominate the entire rotor market. Rainbird desperately needed a sprinkler to compete with these gear-drive rotors, and introduced the R-50, a variation of a ball-drive. The R-50 was a moderate success, but it was hard to adjust and the early models had a lot of maintenance problems. Rainbird then introduced the T-Bird, their first gear-drive rotor. It was essentially a first try and had problems. Finally, on the third try, Rainbird came out with the 3500 and 5000 series gear-drive rotors and belatedly joined the others. Now, finally, all the major irrigation manufacturers have a good gear-drive rotor available.

So what's next for rotors? The latest innovation is the micro-rotor. This tiny rotor is the size of a spray sprinkler nozzle and fits on a standard spray sprinkler body. This makes it less than half the size of the other rotors, and much less expensive. Developed by the Walla Walla Sprinkler Company and sold under the name MP Rotator, this micro-rotor is a stream rotor type (multiple rotating streams of water.) It is driven by a turbine, but does not have a gear box. To keep the sprinkler rotating at a slow, steady speed it utilizes a sealed gel-filled clutch. The thick gel slows the speed of the nozzle. Rainbird recently introduced a mini-rotor also, and other manufacturer's will follow. Only time will tell if this will be the next big thing in rotors.

I recommend using a gear-drive rotor, or for smaller areas a turbine-driven micro-rotor. Pretty much all of the gear-drive rotors hold up well and perform well. The biggest problem with the gear-drive rotors is adjusting the arcs. Nelson makes the easiest to adjust but has a problem with the cap that covers the adjustment collars coming off. The Toro S700 has the hardest arc to adjust, and everyone else falls someplace in between. The bottom line is to be prepared to get wet when adjusting any rotor! The good news is that every one of the gear-drive rotors currently on the market has essentially the same basic features, so any of them will meet what I would consider the minimum requirements; a 3" pop-up height, a good wiper seal, stainless steel retraction spring, sturdy plastic body and cap.

The radius of a rotor-type sprinkler is determined by the water pressure at the rotor inlet and the nozzle used. Most rotors come with several nozzles. More nozzles gives you better control and helps avoid over-watered areas. For a standard rotor you need to use a different nozzle for each arc, so you need one size nozzle for a 1/4 circle, another for 1/2 circle, another for 3/4, and another for full circle. So for even a simple sprinkler system you need at least 3-4 different nozzle sizes! If you also need different radius distances you will likely need even more nozzles. With stream rotors you don't to use different nozzles for each different arc, so they don't come with multiple nozzles.

The sprinkler manufacturer will list the radius for each sprinkler nozzle they make on the packaging, or on a separate reference chart (but please read the next paragraph before relying on that data.) Most also have performance data charts on their websites. The radius will vary based on the water pressure and flow, more pressure and flow will result in a larger radius. So for a typical nozzle you will see a table that gives the various radius, pressure and flow combinations for the nozzle (see example below.) If you can't find the desired radius, get the nozzle with the next size larger radius than you need. The radius can be reduced by means of the radius adjustment screw on the top of the sprinkler nozzle. Some nozzles are designed to spray the water at a very low angle. Be careful of these nozzles, if you have mounded areas or hillsides they may spray water into the ground. It is best only to use low angle nozzles in relatively flat areas.

A big problem with all of the rotors is that they don't perform as well in actual use as the manufacturer's performance data tables suggest. The problem is that the radius the manufacturer measures on their indoor test range is not accurate out in the real world where the wind blows. Competition forces them to aim for the largest radius possible at the lowest possible pressure and flow. But the gallonage and pressure have a huge effect on how effectively the water is distributed. For example, a low gallonage, low pressure nozzle is going to be much more effected by wind (even wind so slight you don't even notice it) than a higher flow, higher pressure nozzle. So you need to be careful. Unfortunately it takes lots of experience to know what works and what does not. Because of competition, the advice you get from the manufacturer's literature and help lines has, in my opinion, stretched the limits considerably. So, based on my experience I am providing you with nozzle GPM/pressures combinations I recommend you use for various situations. I very strongly suggest that you follow these recommendations. In some cases I have provided more specific recommendations in my reviews. If you don't follow them you may very well regret it! Many people before you have made this mistake, please don't join them. Fixing the problem if you use the wrong nozzles can be very expensive! People are really unhappy when I tell them what is needed to fix the problem.

As a general rule I advise against spacing rotors more than 45 feet apart unless you have a professional irrigation consultant design the system. Most contractor's and "free design services" don't have the design knowledge to handle a design of this complexity, you need to find a designer with lots of experience. I'm not trying to make anyone look bad, I just have a lot of experience fixing problems on these large radius systems, and have noted that the problems almost always resulted from poor original design. Wide spacings simply bring many difficult design problems with them.

Important! Rotors require what is know in the industry as “head-to-head spacing”. Head-to-head spacing means that the water from one sprinkler must spray all the way to the next sprinkler in all directions- to the right, left and also to the head(s) across from it. This is very important, spacing the sprinkler heads too far apart is one of the most common design errors, and it is almost impossible to go back and correct the problem later without spending tons of money. Simply stated if the radius is 30 feet you need to space the rotors no more than 30 feet apart, less in many cases. The sprinkler design tutorial gives a more in-depth description of the importance of head-to-head spacing. Head-to-head spacing is critical for avoiding dry spots and disease problems. With spacings wider than 45 feet it is often necessary to use spacings even closer than head-to-head.

Standard rotors (the water stream moves back and forth) require a little more design effort. To make this easier I have suggested which nozzles you should use in many of my reviews of specific rotors. In those cases you should simply use the nozzles I recommend. You should also use the free on-line Sprinkler System Design Tutorial when you actually design your system! This is just background information, the tutorial will guide you through the design process step-by-step. However, this information will help you understand why it is important that the rotor you choose to use should come with a large selection of nozzles.

The precipitation rates for standard rotors are not matched, so you must select the right nozzle size for each rotor based on the radius, pattern, and flow of the various nozzles available. Here's an example of a performance chart for a typical standard rotor. (Don't actually use this chart, it is just a sample! Each brand and model has it's own chart, you need to use the correct chart.)

The chart above shows four nozzles (for many rotors there will be more nozzles available.) For each nozzle it gives the radius and GPM combinations that result at different water pressures. Remember this is the water pressure at the sprinkler, which will be less than the water available water pressure due to pressure lost in the pipe and valves.

Let's consider a 60 foot x 60 foot square lawn area (see image below.) Nine rotors would be required to water this area as follows: In each of the four corners there would be a 1/4 circle rotor (4 total). At the mid-point of each side there would be a half circle rotor (4 total). In the exact center of the lawn there would be a single full circle rotor. Each rotor would need a radius of 30 feet. This would provide the desired head-to-head spacing.

Rotor Spacing Example for 60' x 60' Lawn
(Using 30' Radius Rotors)

Yes, this is how many rotors you need to water a 60 foot square lawn! No need to write and ask me if I made a mistake. Nope, I don't get kick-backs from the sprinkler companies for helping them sell more sprinkler heads. There are a number of good reasons for all the overlap, if you really are interested you can take a couple of classes at your local university in physics and hydraulics. OK, I'm guessing that doesn't sound very interesting to you, so let's just move on.

As you recall from the description of a standard rotor, the rotor shoots a stream of water that rotates one way around the rotor, then reverses back the other direction, covering the same area again (except for a full circle rotor, which just moves continuously in a circle.) This back and forth motion creates a problem- since all the rotors turn at the same speed. So in the time it takes the full circle rotor to make one complete rotation, the 1/4 circle rotor will have gone back and forth over the same area 4 times. So if you use the same size nozzle in all the rotors the area watered by the quarter circle rotor would have 4 times more water applied to it than the area watered by the full circle rotor. This will not work! The area around the corners would turn to mud while the area in the center of our lawn would be dry! Therefore, to compensate we use a smaller nozzle in the corners and a larger one in the center. The full circle nozzle will need to have a GPM four times higher than that of the quarter circle. Using our 60 foot square lawn and the Sample Rotor Nozzle Performance Chart above we would select the following nozzles:

One thing you will notice is that these nozzles are not a perfect solution. To be perfect, all the radii would be identical and the GPM values would be exact multiples of each other (ie; all radii would be 30 feet and GPM values would be 1.1, 2.2, 3.3, and 4.4 GPM.) Unfortunately perfection is not attainable, so we must settle for the next best thing. In this case the GPM values are close enough to being multiples of each other. Because the radii are all larger than we need, it will be necessary to reduce the radius of each rotor after it is installed by using the radius adjustment screw. Now you can understand why a rotor that includes a lot of nozzles is better than one that only includes one nozzle. If you go to a typical discount retail store you will find a lot of rotors that only come with one nozzle. Now you know why that is not such a good deal for most yards. No doubt a lot of you are thinking to yourself, “so that's why the middle of my yard is as dry as an old bone and the corners are swamps!”

Stream rotors have matched precipitation rates, so you don't have to deal with the different nozzles for different arcs problem. If you are designing with them you will use the same nozzle for all the sprinklers that have the same radius. A chart provided by the manufacturer will tell you the GPM for each rotor based on the pattern used (ie; quarter circle, half circle, full circle.) This makes designing with stream rotors a bit easier than with standard type single stream rotors. Select stream rotor nozzles based on the radius desired. Most stream rotors do not have a radius adjustment feature, so the radius can't be changed except by changing the nozzle. Here's an example of a nozzle chart for a stream rotor:

From the nozzle chart you start by selecting the radius you want. The table tells you the pressure you will need and the GPM that the sprinkler will require. So if you need a 21 foot radius you would select nozzle #02. The chart then tells you that you will require 35 PSI at the rotor and for a half circle pattern (180°) the flow used by the rotor will be 1.44 GPM.

Most rotor-type sprinklers are available with a number of optional features. Here are some situations where you should consider using a rotor sprinkler with optional features.