Rotary Spreader Parts and What Each Setting Changes
Why this matters
A rotary spreader looks simple: a hopper, a hole and a spinning plate. But two things decide whether product lands where it should. Rate is how much product goes down per 1,000 square feet. Pattern is whether that product lands evenly across the swath. Different parts control the two, and a tech who blames the dial for a striped lawn, or the impeller for a lawn that got twice its nitrogen, fixes the wrong part. This card names each part, what it changes, and what it cannot change.
Measuring output belongs to 'How to Calibrate a Rotary Spreader by Weight Over a Measured Area', and checking the pattern belongs to 'How to Check a Rotary Spreader Pattern With Collection Pans'. This card is what the two procedures are adjusting.
The machine and its pattern
A walk-behind rotary (broadcast) spreader, the kind on most lawn trucks, seen from above with the hopper cut away. The operator walks toward the top of the page, and the pattern lands ahead of and beside the machine.
TRAVEL ^
THROWN PATTERN (lands ahead of and beside)
. . . .. ::::::::::: .. . . .
light heavy center light
|<------- total throw width ------->|
|<---- lane spacing ---->|
SPREADER, hopper cut away
+------------------------+
| HOPPER |
| agitator ===== |
| drop holes o o o |
| rate gate (slides |
| over the holes) |
| shut-off plate |
+-----------+------------+
|
side [====] ( IMPELLER )
deflector fins on a
(lowered, spinning disk
blocks left)
O wheel wheel O
(drive wheel turns gearbox
that spins the impeller)
Parts that set the RATE
Rate gate. A plate that slides over the drop holes in the hopper bottom. The dial or lever sets how far it opens. A wider opening passes more product per minute. It is rarely linear: doubling the dial number does not reliably double output, because the opening's shape and the way product flows through a small hole versus a large one vary by machine and by product. That is the reason a dial number is never a rate and the shelf standard is to check output by weight over a measured area.
Shut-off plate. A separate plate that opens and closes the holes without disturbing the rate gate. Its job is repeatability: the operator sets the rate once and flips the shut-off at every turn. A machine whose shut-off has worn or bent so it no longer opens fully gives a lower rate than the gate says, and one whose gate stop has slipped drifts between visits.
Agitator. A wire or finger over the holes that keeps product moving so it does not bridge over the opening. A missing or broken agitator gives a rate that falls off through the run, especially with fine or damp product, and then surges when the bridge collapses.
Walking speed. On a ground-driven spreader the gate meters product by time: product falls through the opening at roughly the same rate per minute whether you walk fast or slow. Area covered per minute goes up with speed. So walking faster puts down less product per 1,000 square feet, and walking slower puts down more. Calibration is only true at the pace it was done at, which is why the shop sets one pace (a brisk, steady walk, commonly around 3 mph) and holds it.
Product. Granule size, density and how freely it flows all change how much passes through the same opening. A setting from one product does not transfer to another, even from the same maker. Damp, caked or dusty product flows less and less evenly.
Parts that set the PATTERN
Impeller. The spinning disk with fins (sometimes called vanes or a spinner) that throws product outward. On a push spreader it is driven from the wheel through a small gearbox, so it spins faster when you walk faster. The impeller decides where product goes, not how much. A bent fin, a cracked disk, or a fin worn short throws more product to one side and less to the other, which shows up on the lawn as stripes: alternating bands of darker green where lanes overlapped heavy and lighter green where they left a gap. See 'The Green Stripes From a Spreader With a Bent Impeller'.
Drop-hole position. Where product lands on the impeller relative to its spin sets which way the pattern leans. Many machines have an adjustment for this (a pattern adjuster or an offset plate). A machine that throws heavy to the right with a perfect impeller usually has product landing on the wrong part of the disk.
Side deflector. A shield lowered on one side to block the throw, used along beds, sidewalks, driveways and water. With the deflector down, the throw on that side is cut short and product drops closer to the machine. It changes the pattern on that side, and on most machines it also changes the rate on the deflected pass, because product that would have been spread wide now lands in a narrower strip. Treat a deflected border pass as its own setting, not as a normal pass with a shield on.
Particle size in a blend. Large, heavy granules travel farther off the impeller than small, light ones. In a blended product the larger particles tend to land toward the edges and the fines toward the middle. If those particles carry different nutrients, the pattern can be even by weight and uneven by nutrient. Homogeneous products, where each granule carries the full analysis, avoid this.
Lane spacing. This is the operator's choice, not a part, but it ties the two halves together. A rotary pattern is heavy in the middle and tapers toward the edges, so passes have to overlap for the edges of one pass to fill in the edges of the next. The lane spacing (center of one pass to center of the next) is set from the pattern check, and it is narrower than the total throw. Wheel-to-wheel or "edge of the throw" spacing is a common way to get it wrong.
The rate equation behind all of it
Strip away the parts and the average rate on the lawn comes down to three numbers:
rate per 1,000 sq ft = product flow per minute / (speed x lane spacing), converted to 1,000 sq ft units.
Notice what is not in it: throw width and impeller speed. Those change how evenly the product lands, not how much lands on average, as long as you keep the same lane spacing. That is the split this card has been drawing: gate, product and speed move the rate; impeller, drop position, deflector and particle size move the pattern; lane spacing joins them.
Worked example: one machine, two changes
A tech calibrated a spreader last week on a granular product (call it Product A) at setting 14, walking 3 mph with a 10 ft lane spacing. Calibration found the gate passing 8.0 lb of product per minute (illustrative).
The rate at calibration. 3 mph is 3 x 5,280 / 60, or 264 ft per minute. At a 10 ft lane spacing that is 2,640 sq ft per minute. 8.0 lb divided by 2.64 thousand sq ft is about 3.03 lb of product per 1,000 sq ft. Check backward: 3.03 lb times 2.64 is 8.0 lb.
Change one: the tech walks faster on a big open lawn, 3.5 mph. Speed becomes 3.5 x 5,280 / 60, or 308 ft per minute, and area becomes 3,080 sq ft per minute. The gate still passes about 8.0 lb per minute. 8.0 divided by 3.08 is about 2.60 lb per 1,000 sq ft. That is 2.60 against 3.03, about 14% light. The impeller also spun faster and the throw got wider, but at the same 10 ft lane spacing that only changed the overlap, not the average.
Change two: back at 3 mph, but the truck is loaded with Product B at the same setting 14 because "it's the same kind of fertilizer." Product B is a denser, rounder granule and flows faster; say the gate now passes 10.4 lb per minute at setting 14 (illustrative; you only learn this number by calibrating). At 2,640 sq ft per minute, 10.4 divided by 2.64 is about 3.94 lb per 1,000 sq ft. That is 3.94 against 3.03, about 30% heavy, before anyone looks at whether B's nitrogen percentage differs from A's. Product B's larger granules also throw farther, so the pattern needs its own check before the 10 ft lane spacing is trusted.
What the tech should conclude. Neither error is visible from the handle. Both come from parts on the rate side of the split: speed in the first case, product in the second. Neither would have been fixed by touching the impeller. The fix is to recalibrate for Product B at the shop pace, and to hold the pace.
What a bent impeller would have done instead. If the tech had walked 3 mph with Product A and a bent fin, the average rate would still be about 3.03 lb per 1,000 sq ft. The lawn would show stripes, because one side of every pass was heavy and the other light. Rate fine, pattern broken.
What to check before every season, and why
- Gate travel. Close the gate fully and confirm it seals; open to a mid setting and confirm the dial and the actual opening still agree with the last calibration. A gate that drifts open is the cause, not the operator.
- Shut-off. Opens fully and snaps closed. Product weeping past a closed shut-off at turns double-feeds every headland.
- Impeller. Fins straight, none cracked or worn short, disk spins freely without wobble. A wobbling impeller means a bent shaft or a failing bearing.
- Gearbox. Engages when the wheel turns, no slipping. A slipping gearbox makes the impeller slow down unpredictably, which narrows the throw.
- Tires. Pneumatic tires at the same pressure as when calibrated. Lower pressure shrinks the rolling circumference, so the wheel turns more times per foot, the impeller speeds up and the throw widens.
- Agitator. Present and not bent off the holes.
Wash and dry the hopper and impeller after fertilizer, because the salts corrode metal and seize gates. Respirable dust from granular product can come off the impeller, so stand upwind when running the machine on a stand or tarp, and wear eye protection and whatever else the product's label or SDS calls for when emptying or cleaning it.
References
- Spreader manufacturer's operator manual for your model (gate, pattern adjustment, deflector behavior and maintenance)
- Product label for every granular product applied, which governs the product's use
- See related: How to Calibrate a Rotary Spreader by Weight Over a Measured Area; How to Check a Rotary Spreader Pattern With Collection Pans
- See related: The Green Stripes From a Spreader With a Bent Impeller; The Spreader and Sprayer Calibration Log SOP
- See related: The Fertilizer Bag Label, Read Line by Line