How to Check a Rotary Spreader Pattern With Collection Pans
Why this matters
A rotary spreader can be calibrated to exactly the right pounds per 1,000 square feet and still stripe a lawn. Calibration proves the average; it says nothing about where the granules land across the throw. A spreader that throws heavy to one side, run back and forth across a lawn, lays down alternating heavy and light bands, and three weeks later the customer is looking at dark green and pale green stripes 20 feet apart. The pan test shows the shape of the throw, finds the width you can actually space passes at, and catches the lopsided pattern before it reaches a lawn.
Run this test before calibrating by weight. The effective swath it produces is the strip width on 'How to Calibrate a Rotary Spreader by Weight Over a Measured Area', and the pass spacing every tech uses on the route.
Two terms, kept apart
Edge of throw is the farthest out any granule lands. It is the width a tech sees, and it is the wrong width to space passes at.
Effective swath is the width over which the spreader delivers at least half of what it delivers at its heaviest point. Spacing passes at the effective swath lets the thin tails of each pass overlap the thin tails of the next, so the two halves add up to roughly the center rate. Spacing passes at the edge of throw leaves a light band between every pair of passes, because the tails were never meant to cover ground alone.
So the rule this card enforces: overlap is set from the effective swath, never from the edge of the throw.
Setting up the pans
- Pans: shallow, flat-bottomed, all the same size, with a grid insert (an egg-crate light diffuser cut to fit works) so granules that hit the pan do not bounce out. Eleven pans covers most push rotaries.
- Layout: a straight row across the travel path, one pan on the center line and five each side at 2 ft spacing, so the row spans 20 ft. Number them from the operator's left as the spreader moves forward: L10, L8, L6, L4, L2, C, R2, R4, R6, R8, R10.
- Product: the product you are about to apply, at about its working setting. Pattern changes with the granule as well as with the spreader, so a pattern found with one product is not proof for another.
- Site: calm air, on turf or a tarp, not on pavement. Wind moves fine particles enough to skew the result, and granules left on a drive end up in the storm drain.
Make three passes over the row, all in the same direction, at working pace, with the gate opened well before the row and closed well after it. Why the same direction: you are measuring the shape of one pass. Alternating directions would average a lopsided pattern into a symmetrical one and hide exactly what the test exists to find.
Pour each pan into its own clear tube or vial of the same diameter, lined up in pan order, and read the height. A gram scale works too. The numbers only need to be comparable with each other.
Reading the profile
Find the heaviest pan. Half of that value is the line. Walk outward on each side to where the profile falls through the half line, interpolating between pans. The distance between those two points is the effective swath, and the midpoint between them is where the pattern is centered.
That one reading answers two questions: how wide to space passes, and whether the pattern sits on the wheel path or off to one side. The worked test below is the same spreader before and after a correction, and the same gate gives opposite verdicts.
The test, before and after
Material collected in each pan, in milliliters after three passes, drawn as a profile. Pan spacing is 2 ft.
BEFORE - heavy to the operator's left
# #
# # #
. # # # #
# # # # #
. # # # # #
# # # # # # #
# # # # # # # # # .
L10 L8 L6 L4 L2 C R2 R4 R6 R8 R10
1 2.5 4.5 6 7 7 5 2 1 0.5 0
AFTER - adjuster moved, impeller checked
. # .
# # #
. # # # .
# # # # #
. # # # # # .
# # # # # # #
# # # # # # # # #
L10 L8 L6 L4 L2 C R2 R4 R6 R8 R10
0 1 2.5 4.5 6.5 7 6.5 4.5 2.5 1 0
each # = 1 mL, . = half mL; pans 2 ft apart
travel direction: into the page
Before. The peak is 7 mL (L2 and C), so the half line is 3.5. On the left the profile falls through 3.5 between L6 (4.5) and L8 (2.5), at L7. On the right it falls through between R2 (5) and R4 (2), at R3. The effective swath is 7 + 3 = 10 ft, but its midpoint is 2 ft left of the wheel path.
That looks harmless because the width is right. It is not. On a lawn the tech walks back and forth at 10 ft spacing, so on the outbound pass the heavy side points west and on the return pass it points east. Pattern centers land 2 ft left of each pass line: pass one centered at 2 ft west of its line, pass two centered 2 ft east of its own line 10 ft over. Their centers are 14 ft apart, so a light band opens between them. Pass three, heading out again, has its center 2 ft west of a line 10 ft further on, only 6 ft from pass two's center, and a heavy band forms. The lawn stripes heavy, light, heavy, light on a 20 ft repeat, at a calibrated average rate.
After. The peak is 7 mL at C, half line 3.5. Left: between L4 (4.5) and L6 (2.5), at L5. Right: between R4 (4.5) and R6 (2.5), at R5. Effective swath 10 ft, centered on the wheel path. Passes at 10 ft now overlap tail to tail on both sides.
Same rule, same width, opposite verdict: the first pattern fails on its center and the second passes. A test that only measured width would have passed both.
Correcting a lopsided throw
Work these in order, cheapest and most common first, and re-run the pans after each change. Why one change at a time: two changes between tests leave you not knowing which fixed it, and the next spreader with the same symptom gets both changes whether it needs them or not.
- Side deflector or edge guard. Many rotaries have a deflector that blocks throw on one side for edging along beds and drives. Left half-engaged, it produces exactly a one-sided pattern. Confirm it is fully off for open-turf work.
- The impeller. Spin it by hand with the gate shut. It should turn freely and run true. Look at the fins: a bent, broken or worn fin throws granules at a different angle on part of every revolution. A cracked or bent impeller gets replaced, not straightened, because a straightened fin rarely matches the others. See related: 'The Green Stripes From a Spreader With a Bent Impeller'.
- The pattern adjustment. Where a spreader has an adjustment that moves where product drops onto the impeller, that is the tool for a skewed pattern. Granules that land earlier or later in the impeller's rotation leave toward a different side, so moving the drop point moves the pattern sideways. Which way to move it for a heavy left depends on the spreader's design and its direction of rotation, so take the direction from the maker's manual, move it one step and re-test. On a spreader without an adjustment, the drop holes or shutoff plate may be worn or bent out of position.
- The walk. Impeller speed comes from the wheels on a push rotary. A tech who walks noticeably slower than the calibration pace spins the impeller slower and shortens the throw, which narrows the pattern on both sides rather than skewing it. That is a calibration problem as much as a pattern one; re-time the pace.
Confirming the spacing, not just the shape
After the single-direction profile passes, confirm the spacing with a second test: set the row again and make passes the way the route runs them, back and forth at the effective swath spacing, with the center pan on the line between two passes. The pans across the overlap should read close to each other. A common shop target is no pan in the overlap zone more than about 15 percent off the average of the row. Tune it to your products.
Why this second test: the half-max rule assumes the tails are roughly mirror images that sum to a flat line. Most rotary patterns are close enough. A pattern with a hump in the middle and long thin tails can pass the half-max test and still leave a soft band in the overlap, and only the back-and-forth run shows that.
Readings that lie, and how to spot them
- A center pan that reads low on every test. On a push rotary the operator and the frame shadow the area right behind the impeller, and some designs throw a slight hole in the middle. If C reads below L2 and R2 by more than a little, run the back-and-forth test before acting on it: a small center dip usually fills in from the neighboring pass, and a large one means the shutoff plate or drop holes are worn.
- Pans that read near zero all the way across. Too few passes or a gate that was not fully open at the row. Three passes is a minimum for most granular products; a light product at a low rate may need five before the tubes are readable.
- A profile that changes shape between runs. Granules bouncing out of pans with no grid insert, a gusty day, or a tech who sped up over the row. Fix the setup before reading anything into the shape.
- Fines and dust. A product with a lot of fine dust lands short and heavy near the spreader, so the profile shows a taller center than the granules alone would. That is real material reaching the turf, so read it as found, but note on the log that the bag was dusty, since the next bag may not be.
When to run the pans again
- When the shop changes to a product with a visibly different granule (size, weight or shape), since the throw changes with the granule.
- After any repair or adjustment to the impeller, gate, drop holes or deflector.
- When a lawn shows stripes at a regular spacing that matches the pass width or twice the pass width. Stripes at twice the pass width point to a lopsided pattern; stripes at the pass width point to spacing at the edge of throw.
- On a set schedule for each spreader, logged with the calibration. See related: 'The Spreader and Sprayer Calibration Log SOP'.
References
- The spreader maker's operator manual, for the pattern adjustment and its direction.
- See related: 'How to Calibrate a Rotary Spreader by Weight Over a Measured Area'
- See related: 'Rotary Spreader Parts and What Each Setting Changes'
- See related: 'The Green Stripes From a Spreader With a Bent Impeller'