How to Adjust Arc and Radius on a Gear-Drive Rotor

Why this matters

A rotor watering the driveway is the complaint a customer sees from the kitchen window, and the dry strip beside it is the one they see in August. Both usually come from the same two settings on one head: an arc set past the lawn edge and a radius cranked in or out to cover for it. Adjusting a gear-drive rotor takes a key and a few minutes, but a tech who sets arc by the number on the cap, or who trims radius as far as the screw will go, fixes the wet drive and creates a soggy corner or a dry ring. This card sets the fixed stop, the adjustable stop and the radius in the order that keeps the zone's water where it was designed to land. Which nozzle belongs in each head is the subject of Matched Precipitation on a Rotor Zone.

Before you start

• Traffic and spray. Heads along a street or a shared drive are adjusted with the truck parked to shield the work area and, on a street edge, a high-visibility vest on. A head that pops while you are bent over it throws grit and water at face height, so keep your face and hands out from over the cap when the zone starts.
• Tools: the adjustment key for that head family, a small flat screwdriver, the maker's adjustment sheet and nozzle chart, flags, a measuring tape long enough for the head spacing, and a bucket of clean water for flushing.
• Information: the head spacing on this zone, the nozzle number in each head, and whether this model adjusts arc wet, dry or both (the sheet says).
• Control: run the zone manually from the controller or with a remote, and note the program before you touch anything so it goes back exactly as it was.

The adjustment, in order

 ROTOR AT A LAWN CORNER, seen from above
 (stand behind the head, facing into the lawn)

  fixed LEFT stop
  aims along this edge
        |
        |    . . . . .   stream lands at
        |  .           .  the set radius
        | .    ARC       .
        |.    90 deg      .
  (HEAD)+------------------- adjustable RIGHT
   cap: arc key socket,      stop aims along
   radius screw over         this edge (drive)
   the nozzle

1. Make the head worth adjusting. Run the zone and watch this head before touching a setting. The riser should pop fully, the turret should rotate smoothly in both directions, and the stream should be solid, not split or fanned. If it is weak or ragged, shut the zone, pull the nozzle, flush it and the screen, and refit. Reason: arc and radius set on a clogged nozzle or a sticking riser are set wrong, and they go wrong again the day the debris clears.

2. Find the fixed left stop. On most gear-drive rotors, the left edge of the arc is fixed relative to the case and the right edge is the one the key moves; the maker's sheet confirms which on your model. Standing behind the head facing the lawn, turn the turret by hand (or watch it run) to find where it reverses on the left. Reason: the left stop is the reference. Setting the arc first and the left edge second rotates a correct arc onto the wrong ground.

3. Aim the left stop at its edge. If the left stop does not point along the lawn edge, turn the whole head body in the ground, or on models that allow it, the internal assembly in its case, until it does. With the body turned, check the riser is still vertical and the cap still sits level with the finished grade. Reason: a head tilted while being turned throws high on one side and short on the other, and a cap left above grade is a mower strike.

4. Set the arc with the key while the head runs. Put the key in the arc socket and turn toward the plus or minus marked on the cap, a little at a time, watching where the turret reverses on the right. Stop when it reverses along the right-hand edge of the lawn, then let it make two full sweeps to confirm both stops hold. Reason: the degree marks on a cap are approximate and the lawn edge is what matters. Setting while it runs lets you watch the result rather than guess it; on a model the sheet says to set dry, set it dry and then confirm wet.

5. Check the nozzle matches the arc. A head moved from roughly a third of a circle to a quarter needs the nozzle a quarter-circle head carries, or the zone's precipitation goes out of balance. Look at the nozzle number against the zone layout. Reason: the nozzle sets flow and the arc sets area; change one and not the other and that head puts down too much or too little water per hour.

6. Trim radius, a little, with the screw. With the zone running, watch where the stream lands. If it overthrows the edge, turn the radius screw in a quarter turn at a time, letting the head pass through a full sweep between turns. The maker's sheet gives the maximum reduction, commonly around a quarter of the nozzle's rated radius; this shop's default is to trim no more than about 10 percent with the screw and to change to a shorter-radius nozzle past that. Reason: the screw breaks up the stream but does little to the flow, so the same water lands on a smaller circle. The worked example shows how fast that raises the head's application rate.

7. Confirm head-to-head reach. In a head-to-head layout every head should throw to the heads next to it. Watch each stream land at, or just past, the neighboring heads on both sides of the arc, with only light mist on the hardscape. Reason: radius short of the next head leaves a dry band between heads; that is the August strip, and it is a spacing or pressure problem if the radius screw is already backed fully out.

8. Put the zone back. Return the controller to its automatic program with the run times exactly as found, unless the visit was booked to change them. Note on the ticket each head adjusted, the before and after arc and radius, and any nozzle changed. Reason: a controller left in manual or off is a dead lawn in a week, and an adjustment with no record cannot be undone by the next tech.

Worked example: the corner head that watered the drive

A back-lawn rotor zone laid out head to head at 33 ft spacing. The customer reports water running down the driveway. The corner head at the drive carries a nozzle rated at 35 ft radius at the zone's pressure.

Step 1. The riser pops fully and the stream is solid. No flush needed.

Step 2. The turret reverses on the left pointing along the lawn's left edge. The fixed stop is right.

Steps 3 and 4. No body rotation needed. Running the zone, the turret reverses on the right well past the drive edge, about a third of a circle, roughly 120 degrees. Working the key toward minus a little at a time brings the reversal to the drive edge. The arc is now the quarter circle the corner needs, 90 degrees.

Step 5. The nozzle is the quarter-circle size on the zone layout. At 120 degrees it had been spreading a quarter-circle flow over a third of a circle: the same water on 120 / 90 = 4/3 of the intended area, so the corner was getting 3/4 of its design rate on the lawn, with the rest on the concrete. At 90 degrees it is matched again.

Step 6. At 35 ft the stream lands 2 ft past the next head along the drive and puts a band of water on the concrete. The radius needs to come in from 35 to 33 ft, a trim of 2 / 35 = 5.7 percent, inside the 10 percent default. The screw goes in a quarter turn at a time until the stream lands at the neighboring head.

What the trim did to this head's rate: the watered area scales with the radius squared, so (33 / 35) squared = 1,089 / 1,225 = 0.889 of the original area takes the same flow. 1 / 0.889 = 1.125, so the head now applies about 12 percent more per hour on the area it covers. Checking backward, 0.889 x 1.125 = 1.0. Twelve percent on one corner head is within what the zone's overlap absorbs.

Had the nozzle thrown 41 ft, the trim to 33 ft would be 8 / 41 = 19.5 percent. (33 / 41) squared = 0.648, and 1 / 0.648 = 1.54: more than half again the rate on the corner, enough to puddle on clay. That is why it would get a shorter-radius nozzle instead, even though the maker's ceiling might allow the trim.

Step 7. The corner stream now lands at the next head along the drive and at the next head along the left edge. Light mist reaches the drive edge, no sheet of water.

Step 8. Controller back to automatic, program unchanged. Ticket notes: corner head, arc about 120 to 90 degrees, radius 35 to 33 ft, nozzle unchanged.

What changes the job

The head will not reach even with the screw backed out. That is pressure or flow, not adjustment: a partly closed valve, a kinked or broken lateral, too many heads on the zone, or low supply pressure. Pressure at the far head is measured as part of Irrigation Startup and Audit, and turning other heads' radius screws to compensate makes the zone worse.

Full-circle, non-reversing heads. Some rotors are set full circle with no stops. They have no arc to adjust; changing one to a part circle means a head or setting change per the maker's sheet.

Wind. Adjust in still air, typically early morning. A stream set to reach in a breeze overthrows on a calm day, and one set on a calm day may fall short in the afternoon wind it usually runs in, which is a reason to schedule the zone for calm hours rather than to chase the radius.

Slopes. A head at the top of a slope throws farther downhill than up. Set arc and radius with the head running, by where water lands, not by the chart radius.

How to tell it is right

• Both stops reverse on the lawn edges through two full sweeps.
• The nozzle in the head is the size the zone layout calls for at that arc.
• Radius trimmed no more than about 10 percent below rated, and within the maker's limit.
• Every stream reaches the neighboring heads, with no sheet of water on hardscape.
• Controller back on its original program, adjustments on the ticket.

References

• See related: Matched Precipitation on a Rotor Zone, for nozzle choice by arc.
• See related: Irrigation Startup and Audit, for pressure at the head and catch-can measurement.
• See related: Irrigation Controller Programming.
• The rotor manufacturer's adjustment sheet and nozzle performance chart for that model.