How to Set Loop Height on a 3-in-1 Straightener Feeder

Why Loop Height Reaches All the Way to the Die

A free loop exists to decouple two machines that run at different instantaneous speeds. The decoiler turns in steps; the feed rolls index in strokes.

When the loop is deep, the strip enters the straightener with almost no back tension. When it is shallow, the strip is being pulled and the straightener has to drag it through.

That drag changes the strip path by a fraction of a millimetre, and a fraction at the roll gap becomes a few hundredths at the die.

At 200 SPM with a 310 mm feed, the line consumes 62 m of strip a minute. The loop absorbs every mismatch between that average and the decoiler's actual delivery.

Most of what follows applies to any NC straightener feeder with a free loop, not only to the integrated layout. The numbers change with pit depth; the logic does not.

Step One: Find the Neutral Height

Neutral height is the point where the strip hangs in a clean curve and touches neither the pit floor nor the straightener entry under tension.

Find it with the line stopped. Lower the loop until the strip is just taut, mark the pit wall, then raise it until the strip rests on the floor and mark again.

The midpoint between those two marks is neutral. On most 3-in-1 machines it lands somewhere between 30% and 50% of the available pit depth.

Write the number on the machine. Operators change shifts, and a marked height survives a handover better than a paragraph in a manual.

Step Two: Set the Sensor Window, Not the Height

The height is a target; the sensor window is what the control actually uses. Set the window too tight and the decoiler hunts all shift.

A window of ±30 mm around neutral suits most 3-in-1 lines running between 100 and 250 SPM. Below ±15 mm the drive starts chasing its own corrections.

Above ±60 mm the loop becomes a reservoir, and the strip spends part of each cycle taut and part slack. Feed length then varies with the coil diameter.

Set the window with the line running, not stopped. The sensor reads a moving strip, and a static setting will sit slightly off centre.

Step Three: Match Decoiler Speed to the Feed Profile

The decoiler should fill the loop and pause, not run continuously at the average speed. A drive that runs flat out keeps the loop shallow for the whole coil.

Set the reference speed 5% to 10% above average demand, so the loop fills during the feed stroke and holds during the dwell.

On a 62 m/min average that means a reference of 65 to 68 m/min. The exact figure depends on how much of the cycle the feed rolls are actually moving.

Check it by watching the loop over one full coil. A loop that slowly sinks across 300 strokes is a speed reference set too low, not a sensor fault.

Heavy coils need a little more margin. A 3.7-tonne coil accelerates slowly, so the reference has to allow for the first few seconds after a stop.

Light coils need less. Setting 10% margin on a 600 kg coil makes the decoiler overrun and cycle its brake, which shows up as noise rather than as a part defect.

Step Four: Prove It at the Die Face

Feed length on the control screen is a count of roll rotation. It is not a measurement of where the strip ended up.

Run fifty parts, measure the pitch on the strip with a calliper, and compare the first ten against the last ten. Drift between them is loop behaviour, not encoder error.

Then change the coil and repeat. The second coil runs at a different diameter, so the decoiler speed profile changes even though nothing was touched.

If the second coil drifts and the first did not, the loop is doing the compensation that the drive should be doing.

Measure with the press running, not in single stroke. The loop behaves differently when the flywheel is carrying the stroke.

Five Settings That Look Right and Are Not

Most loop problems I am called about are not faults. They are settings that pass a visual check and fail a part measurement.

Each row is a compromise that works until the material or the speed changes. None of them is a defect in the machine.

There is a fifth trap that is not in the table: copying a setting from another line. Two machines with the same stroke and the same material can need different windows.

Pit depth, sensor type and drive response all differ between builds, and a copied number usually lands inside the range where it half works.

Where Loop Control Cannot Help

Loop height is a tension tool, not a cure. Four conditions defeat it no matter how carefully it is set.

Badly slit strip with camber will walk sideways regardless of loop depth. The loop controls tension along the strip, not the strip's direction.

A coil wound with varying tension produces a loop that changes character as it unwinds. The setting that suits the outside of the coil will not suit the core.

Very thin strip below about 0.4 mm has too little stiffness to form a stable loop. It folds rather than hangs, and a dancer roll is the better answer.

And a die that pulls the strip, through a jammed pilot or a tight stripper, will drag material out of the loop faster than any control can refill it.

The loop also cannot fix a decoiler brake that drags. If the coil resists turning, the loop goes shallow on every stroke and the strip arrives pre-loaded.

One more boundary: on lines running above roughly 350 SPM the loop becomes short enough that sensor response time matters more than height. That is a control specification question, not a setup one.

What to Write Down for the Next Coil

Four numbers turn loop setting from an art into a procedure. They take two minutes to record and save the next setup a shift.

• Neutral height. The marked midpoint in millimetres from the pit floor.
• Sensor window. Upper and lower limits, written as a band rather than a single figure.
• Speed reference. The percentage above average demand, with the feed length and SPM it was set for.
• Pitch drift. The difference between the first ten and last ten parts on a full coil.

That last number is the one that catches problems early. A drift of 0.03 mm is normal; 0.08 mm means the loop is compensating for something else.

Keep the sheet with the die, not with the machine. Loop settings belong to the job, and the next operator needs them beside the tooling.

Sequence matters more than the individual numbers

Neutral height first, then window, then speed reference, then measurement. Changing them in any other order means re-checking the earlier ones.

Allow half a shift for the first setup on an unfamiliar line. After that, the same job takes fifteen minutes because the four numbers already exist.

If the drift persists after all four are set, stop adjusting the loop. The cause is upstream in the straightener or downstream at the die, and the error budget shows where.

That budget, and the share each contributor takes, is laid out in 3-in-1 decoiler straightener feeder.