The Green Stripes From a Spreader With a Bent Impeller

Why this matters

Striping is the callback every customer can see from the street, and it lands on a crew that did the arithmetic right. This case is a spreader that was calibrated by weight, on target to within 2 percent, and still painted a new route in dark and pale bands. The lesson is that a weight check measures how much product leaves the hopper on average and says nothing about where it lands. Finding the real cause took one observation most crews never make, the repeat distance of the stripes, and one test most crews skip, the collection pans.

The signal

A crew picked up a new neighborhood of 22 lawns and gave them all their first granular feeding in one day with one walk-behind rotary spreader. The work order called for 0.75 lb of nitrogen per 1,000 sq ft from a 30-0-4 homogeneous granule, so 0.75 / 0.30 = 2.5 lb of product per 1,000 sq ft. Passes were spaced at 12 ft, the figure written on the card for the shop's previous spreader of the same size.

Eight days later, three customers called about stripes. The crew lead drove the route and found the banding on most of the lawns, plainest on the large, open front lawns and hard to see on the small ones cut up by beds and walks. The bands ran straight, parallel to the long side of each lawn, the direction the tech had walked.

What it was not

The rate. The calibration log for that spreader, run the morning of the route, showed a 200 ft course at the 12 ft spacing, 2,400 sq ft, using 6.1 lb. 6.1 / 2.4 = 2.54 lb per 1,000, which is 1.7 percent over the 2.5 target and inside the shop's 5 percent band. The bag count closed too: the route's measured turf called for 412 lb and the crew used 420, about 2 percent over. A spreader putting down the wrong amount does not stripe; it greens the whole lawn too much or too little. Rate was off the list.

Irrigation. Most of the lawns had rotor systems, and a double-watered band can look like a fertilizer stripe. But sprinkler bands follow arcs around heads, and these were straight lines parallel to the walking direction, on irrigated and unirrigated lawns alike.

Passes spaced too wide. This was the crew lead's first guess, and the obvious one: the old card's 12 ft might be wider than this spreader throws. If spacing were simply too wide for a symmetric pattern, every gap between passes would go light and every pass center would stay green, so the pale bands would repeat at the pass spacing, every 12 ft.

The crew lead paced the biggest front lawn. The bands were not light-dark-light-dark at 12 ft. Dark band to dark band was about 24 ft, with a pale band halfway between. A pattern that repeats at twice the pass spacing means alternate gaps are behaving differently, and the only thing that differs between alternate passes is the direction the tech was walking. That points at a pattern that leans to one side.

The side deflector. A half-engaged edge guard throws one-sided. On this spreader it was fully up and latched.

The test that found it

The spreader went back to the shop for a pan test, run per How to Check a Rotary Spreader Pattern With Collection Pans: a row of pans 2 ft apart across the path, three passes in one direction, then the catch in each pan in milliliters. Nobody stands in front of the discharge while the impeller throws, and the tech wears safety glasses.

 PAN CATCH, mL after three passes, pans 2 ft apart
 travel into the page, operator's left on the left

 pan     L8  L6  L4  L2  C   R2  R4  R6  R8  R10
 bent    0   0.5 1.5 4   6   8   7   4.5 2.5 1
 new     1   3   5   6.5 7   6.5 5   3   1   0

 bent impeller: half line 4, edges at L2 and R6.5,
   swath 8.5 ft, center 2.25 ft right of wheel path
 new impeller:  half line 3.5, edges at L5.5, R5.5,
   swath 11 ft, centered on the wheel path

The bent profile. The heaviest pan is R2 at 8 mL, so half of peak is 4. On the left, the catch reaches 4 exactly at L2. On the right, it falls through 4 between R6 (4.5) and R8 (2.5), a quarter of the way across, at R6.5. The effective swath, the width over which the spreader delivers at least half its peak, runs from 2 ft left to 6.5 ft right: 8.5 ft wide, with its center 2.25 ft right of the wheel path. The right side throws heavy and long; the left side barely throws at all.

With the hopper emptied and the spreader on its side, gloves on against the fertilizer dust, the tech spun the impeller by hand with the gate shut. It ran with a visible wobble, and one fin was bent back toward the disk. Nobody on the crew could say when it had been hit; a curb or a trailer tie-down are the usual suspects. A bent fin throws granules at a different angle on part of every revolution, and on this disk it was robbing the left side.

Why it striped at 24 ft and not 12

On a lawn the tech walks out and back. Going out, the heavy, long side points to the operator's right, say east. Coming back on the next pass, 12 ft east, the operator's right is now west, so the heavy side points back toward the first pass. Both heavy sides land in the same 12 ft gap: a dark band. In the next gap, between the return pass and the third pass, both short sides face each other, and with only 2 ft of useful throw on the short side, neither reaches the middle: a pale band. Dark gap, pale gap, dark gap. The pattern repeats every two passes, 24 ft, which is what the crew lead paced.

That is also why weight calibration passed. The gate meters the rate and the impeller only distributes it, as Rotary Spreader Parts and What Each Setting Changes sets out, so the spreader dropped the right weight per foot walked and put it in the wrong places.

The fix

Replace, do not straighten. The impeller was replaced with a new one. A straightened fin rarely matches its neighbors, and it fails the same way again.

Retest the pattern. The second row in the block above is the same spreader with the new impeller. Peak 7 mL at C, half line 3.5. Left, between L4 (5) and L6 (3), three quarters of the way across, at L5.5. Right, the same at R5.5. Effective swath 11 ft, centered on the wheel path.

Reset the spacing from the swath. The old card's 12 ft had never been measured on this spreader. Pass spacing is set from the effective swath, so the new card reads 11 ft.

Recalibrate at the new spacing. Narrowing the spacing at the same gate puts the same product per foot walked onto less area. The original run used 6.1 lb over a 200 ft course. At 11 ft that course is 2,200 sq ft, so the same gate would put down 6.1 / 2.2 = 2.77 lb per 1,000, about 11 percent over target and outside the 5 percent band. Checking it the other way: 12 / 11 = 1.09 more product per area from the spacing change, times the original 1.017, is 1.11. The tech closed the gate one step and ran the 2,200 sq ft course: 5.6 lb used, 5.6 / 2.2 = 2.55 lb per 1,000, 2 percent over and inside the band. The log line was written per The Spreader and Sprayer Calibration Log SOP, with the new effective swath beside it.

Run the back-and-forth check. A final row of pans with passes back and forth at 11 ft, the center pan on the line between two passes, read even across the overlap, within the pattern card's tolerance.

What to do about the lawns already striped

The dark bands got more than the work-order rate and the pale bands got less, and nothing takes nitrogen back out. The crew lead's first instinct was to send the tech back with the spreader at a light setting over the pale bands. That was stopped for two reasons. First, the pale bands are hard to hit exactly on a lawn already mown over twice, so a touch-up risks making new dark bands. Second, any extra nitrogen counts against the program's annual total and against whatever the local fertilizer ordinance caps or prohibits, and the touch-up would need to clear The Fertilizer Ordinance Check Before Application SOP on each lawn like any other application.

The decision was to leave the lawns to even out, which uniform growth and mowing do over a few weeks on most lawns, and to make sure the next round goes down from the corrected spreader at the 11 ft spacing. The office called each of the three customers with the cause in plain terms: a damaged part on the spreader, replaced, tested and logged, and the next feeding would go down evenly. One customer with a heavily striped lawn asked for more, and the crew lead walked it with them the following week to show the bands already fading.

What the crew changed

The pan test is now part of the first calibration of any spreader on a new route and after any strike, not only when a new product comes in.

Spacing comes from the machine, never from a card for a different machine. The old card had carried a 12 ft spacing across from a different spreader for two seasons. Every spreader now has its own card with its own measured swath.

The impeller spin is on the daily check. Ten seconds with the gate shut, the spreader tipped, watching for wobble. A wobble means the spreader stays at the shop.

Stripe spacing is the first thing measured on a striping call. Pale bands at the pass spacing point to spacing too wide for a symmetric pattern. Alternating dark and pale bands at twice the pass spacing point to a one-sided pattern: impeller, deflector or drop-hole position. A grid points to perimeter passes laid twice. The general reading of overlap patterns is in Green-Up Uneven Only Where Spreader Overlapped Decision Tree.

References

• See related: How to Check a Rotary Spreader Pattern With Collection Pans.
• 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: Green-Up Uneven Only Where Spreader Overlapped Decision Tree.
• See related: The Fertilizer Ordinance Check Before Application SOP.