Understanding spreaders for large-area applications

Fertilizers and pesticides are often applied in granular form. Understanding the differences in spreader performance and calibration procedures can improve the accuracy and uniformity of granular applications. Investing time to adjust application equipment will result in accurate and uniform granular applications.

There are primarily two types of spreaders available for large-area applications: spinner and pendulum spreaders. The most conventional of these is a spinner or broadcast spreader. This spreader style uses a rotating impellar positioned below a hopper to distribute the material. Depending on the spreader make and model, the impellar can be powered by a PTO or the spreader's wheels.

PTO-powered spreaders offer a little more flexibility for calibration than ground-driven spreaders. Two primary factors that affect application rate are impellar speed (influences distribution width) and ground speed. For ground-driven spreaders, impellar speed and ground speed are linked together. Therefore, one cannot be changed without affecting the other.

Most spinner spreaders use one impellar for material distribution. Some manufacturers provide different disks or adjustable impellar veins so that variations can be made to improve the application of particular materials.

Because most spinner spreaders have only one impellar, the uniformity of the spreader's pattern is often slightly skewed to one side. Adjustments to impellars or other spreader components can eliminate or at least reduce the amount of skewing.

The other popular type of large-area spreaders is the pendulum style. It is PTO driven and uses a horizontally mounted chute that shakes back and forth (parallel with the ground) to distribute the material. The distribution patterns of the pendulum spreaders are typically less skewed than those of spinner spreaders. However, material bulk density and particle size can affect and distort the pattern uniformity.

Determining spreader pattern uniformity The first step in setting up a spreader for application is to determine the uniformity of the spreader's pattern for the material being applied. This procedure will indicate if the pattern is skewed or if there are any other problems with the spread pattern.

To determine the distribution pattern, use cake pans or other similarly styled containers and place them in a straight line across the width of the spreader's distribution path. The pans can be spaced a few feet apart. Keep all the spaces between pans and pan dimensions equal.

Ideally, this procedure should be done on a turf area similar in size to the area receiving the application. However, repeated passes will have to be made over the pans in order to collect enough material to weigh, which could result in significant turf damage from excessive rates of fertilizer or pesticides.

A parking lot or other large flat surface can be used for this procedure, but efforts should be made to minimize the bouncing into and out of the collection pans. Lining the pans with thin carpet and/or placing them on a 6 to 10 foot strip of carpet can significantly minimize particle bounce.

Once the pans have been positioned, the spreader should be driven over the pans until sufficient material has been collected in the pans at the edge of the pattern. The direction of travel for each pass should always be in the same direction. Otherwise, distortions in the pattern could be covered up by reversing the travel direction.

After enough material has been captured in all of the pans, collect and weigh the material from each pan. The weights can then be entered into a computer graphing package or drawn on graph paper. The graphs will provide a visual estimate of the spreader's distribution pattern and determine if the pattern is skewed or if there are any other deviations in it.

If there are problems with the spread pattern, adjustments should be made to the spreader to try to improve the pattern. The types of adjustments that can be made depend on the make and model of the spreader. The pattern should be re-evaluated after the adjustments.

If, after all adjustments have been made, the pattern is still distorted, the application should be done in two directions. This can be done by calibrating the spreader to deliver half the desired rate, then sending the application in two directions using a criss-cross pattern. This procedure is the best to reduce striping and turf damage.

The graphs of pattern distribution will also help determine the appropriate overlap to use for the application. Both types of large-area spreaders usually recommend that a 100-percent overlap be used. The 100-percent overlap means material is being thrown back to the center (i.e., wheel marks) of the previous pass, which means that any individual pass is overlapped on the left and right sides once the application is complete. Hence, 100 percent of a single pass is overlapped (50 percent from the passes on each side). You must know what overlap you're using and plug that into the calibration procedures.

Calibrating large-area spreaders Once the spread pattern has been determined and optimized, the spreader must be calibrated in order to provide the desired application rate of material. The first thing to do for calibration is determine the calibration area. This is the effective width of coverage (determined from the collection pans) multiplied by the distance the spreader is to be driven during the calibration procedure.

For example, if the effective width of coverage is 30 feet wide and the spreader is going to be driven 50 feet during calibration, the calibration area would be 1,500 square feet. The next step is to determine how much material should be delivered to the calibration area. This can be done using a ratio and the known product application rate.

For our same example, suppose the application rate of the desired product is 2 pounds of material per 1,000 square feet. The ratio could be set up as follows, with the unknown amount of product (X) in the upper right hand position:

by cross-multiplying, we end up with:

After multiplication and division, we end up with 3 pounds of material needed for 1,500 square feet. Therefore, 3 pounds is the amount of material that we need to collect from the calibration area.

Before we begin to spread material on the calibration area, we need to take the amount of overlap into consideration. Calibration is done with a single pass, and the actual application is completed with several overlapping passes. So the actual rate coming out of the spreader must be corrected for overlap.

For example, if you have100-percent overlap for three passes, the middle pass must only deliver 50 percent of the intended application rate because 25 percent more material is being applied by each of the adjacent passes. For our example, the spreader should be calibrated to deliver 1.5 pounds of material per 1,000 square feet. That will provide 1.5 pounds per pass, plus 0.75 pound for the overlaps on each side of that pass, for a total of 3.0 pounds of material per 1,000 square feet after the application has been completed.

The final step is getting the spreader to deliver the calculated amount to the calibration area. To make it easy for the spinner-type spreaders, a plastic tarp can be draped around the hopper to enclose the impellar. This will cause the distributed material to fall to the ground in an area the width of the hopper. It will greatly reduce the amount of time it takes to sweep up and collect the material for weighing. It will also improve the accuracy of the procedure.

After the material has been dispersed, it should be collected and weighed. Take care to minimize the collection of contaminants and to maximize the recovery of all of the material. The correct spreader setting is achieved when the calculated weight (plus or minus 10 percent) is recovered.

The pendulum spreaders require one additional step for calibration, but are easier overall to calibrate than spinner spreaders. The chute of a pendulum spreader is detachable so that a bucket can be placed under the hopper to collect material. This makes it easier and more precise to capture the material coming out of the hopper, and it can be done with the tractor sitting still. However, the extra step is needed to relate time to distance.

Rather than driving the tractor a certain distance during calibration, you need to relate time and distance by measuring tractor speed. Select the gear that you want the tractor to be in for the application, and set the throttle to 540 rpm PTO speed. Time how long it takes the tractor to travel 50 feet or 100 feet (or any other length).

This factor is used to determine your calibration area. For example, if the effective width of coverage is 40 feet, and the tractor travels 100 feet per minute, the spreader covers 4,000 square feet per minute.

Following the speed calculation, remove the spreader's chute, set the throttle to 540 rpm PTO speed, and collect material from the hopper for one minute. The amount of material collected is what would be distributed to 4,000 square feet. Likewise, material that would be collected in 30 seconds would be applied to 2,000 square feet and so forth. Remember to take the overlap amounts into consideration when determining how much material should be collected per unit time.

Calibration is an essential component of material application. If calibration is not done for either liquid or granular applications, then it is impossible to determine if the materials are being applied at the correct rates. Not knowing what the application rate is could result in product failure, damage to the turf, unexpected response or a combination of any of these problems.

Research conducted at Penn State has shown that material bulk density and application rate can significantly affect spreader distribution patterns and delivery.

Calibration of spreaders should be done any time there is a change in products or application rates of the same product. Considering the costs of material and labor, the calibration of equipment is a very insignificant expenditure of the entire process. It's a man-hour or two of investment to insure that you get the return of an expensive application.

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