How Much Loop Depth Does an NC Servo Feeder Need?
Most loop problems on a coil line are sized wrong at the quotation stage and diagnosed as control problems for years afterwards.
The free loop between the coil and the NC servo feeder is a buffer, and like any buffer it has a capacity measured in millimetres of strip.
Get the capacity right and the loop sits quietly at a steady depth all shift. Get it wrong and the strip either drags on the pit floor at high speed or the control hunts until the brake wears out.
This is the sizing arithmetic, worked through on a 2.0 mm strip at 120 strokes a minute, followed by what happens when the press speeds up.
The wider question of where feed error comes from between the coil and the die is covered in the coil line guide library, and the feeding equipment itself is listed in the product range.
Three Inputs Set the Loop Depth
Loop sizing has three inputs, and only one of them is the press.
Strip consumption rate. Feed pitch multiplied by strokes per minute gives the millimetres per second the feeder pulls out of the loop. A 180 mm pitch at 120 strokes a minute consumes 360 mm per second.
Control response time. This is the delay between the loop sensor detecting a change and the brake or the pay-off drive actually correcting.
Sensor, PLC scan and brake reaction add up, typically 0.3 to 0.8 seconds on a standard line.
Stored length per unit of sag. A free loop hanging below a common pass line stores roughly twice its sag. A 120 mm sag holds about 240 mm of strip.
Strip thickness and width sit behind the third input. Heavy strip will not form a tight loop without yielding, so the sag buys less stored length, and light gauge whips and gives the sensor a noisy signal.
Sizing a Loop for a 2.0 mm Strip: A Worked Example
Take a 2.0 mm by 400 mm SPCC strip, a 180 mm feed pitch and a press running 120 strokes a minute, with a measured control response of 0.5 seconds.
Step one. Consumption rate is 180 × 120 ÷ 60, which is 360 mm per second.
Step two. Strip consumed during one control response is 360 × 0.5, which is 180 mm. That is the amount the loop has to give up before the brake reacts.
Step three. Add a margin for sensor deadband and brake wear. Thirty per cent is a reasonable figure on a mechanically braked pay-off, giving 234 mm of stored strip.
Step four. Convert stored length to sag: 234 ÷ 2 is 117 mm, so set the loop to run at about 120 mm of sag.
Step five. Check the geometry. The pit needs enough depth for the loop to swing 50% above and below the set point without touching the floor or the sensor losing the strip.
A 200 mm pit under a 120 mm set point is a workable minimum.
On a line with ±0.05 mm feed accuracy at the die, none of that accuracy depends on the loop. The loop only has to keep the strip supplied without disturbing the feeder's grip.
What Doubling Press Speed Does to the Loop
Run the same line at 240 strokes a minute and the arithmetic changes out of proportion to the speed increase.
Consumption doubles to 720 mm per second, so the strip gives up 360 mm during the same 0.5 second response. With the same 30% margin the loop has to store 468 mm, which needs a 234 mm sag.
The pit designed for 120 mm of sag cannot take it. The loop touches the floor, the sensor loses the strip, and the pay-off runs open until the coil unspools.
| Press speed | Sag needed at 0.2 s response | Sag needed at 0.4 s response | Sag needed at 0.6 s response |
|---|---|---|---|
| 120 strokes per minute | 47 mm | 94 mm | 140 mm |
| 180 strokes per minute | 70 mm | 140 mm | 211 mm |
| 240 strokes per minute | 94 mm | 187 mm | 281 mm |
Read the table down the columns and the cheapest fix becomes obvious.
Moving from a 0.6 second response to a 0.2 second response cuts the required sag by two thirds, which is usually less expensive than deepening a pit and rebuilding the entry guide.
Response time is bought with a faster loop sensor, a servo-controlled pay-off drive and a brake that modulates instead of switching.
Measure it rather than trust it. Run the line at production speed, block the loop sensor by hand, and time how long the pay-off takes to react. That single number is the input the whole calculation rests on.
A pneumatic brake on a large coil typically lands between 0.5 and 0.8 seconds.
A servo pay-off drive reading the loop sensor directly can be under 0.2 seconds, which is why it changes the pit design rather than just the controls.
On lines we build for export to more than 60 countries, that is the specification line buyers most often leave undecided.
How much strip does a 120 mm sag actually store?
About 240 mm, using the two-to-one relationship for a free loop hanging below a common pass line. That is roughly 0.7 seconds of strip at 120 strokes a minute and a 180 mm pitch.
How fast must the loop control react at 240 strokes per minute?
Under 0.25 seconds if you want to keep the sag below 120 mm on a 180 mm pitch. At 0.5 seconds you need 234 mm of sag, which most standard pits cannot take.
Does a deeper loop improve feed accuracy at the die?
No. The loop decouples the pay-off from the feeder, and that is all it does. Feed accuracy is set by the feed rolls, the back tension and the die's pilot release, not by loop depth.
How much sag variation is acceptable over 200 strokes?
Keep the swing inside 10% of the set point, which is 12 mm on a 120 mm loop. Beyond 15% the control is hunting, and the brake is cycling more than it should.
Can a loop bridge a coil change?
No. A 240 mm loop is a fraction of a second of production. Bridging a two-minute coil change needs an accumulator with several metres of stored strip, not a deeper pit.
Where a Loop Cannot Help You
A loop is a buffer, and buffers get asked to solve problems they were never sized for.
It does not create back tension. If the pay-off brake cannot hold, the loop collapses to the floor and the strip drags. A deeper loop makes that worse, not better, because there is more strip to fall.
It hides slipping feed rolls. If the feeder grip is marginal, a stable loop keeps the line running while the parts drift out of tolerance. The loop is doing its job and the process is still failing.
It struggles with heavy strip. Above about 4 mm, the natural loop radius is large, so each millimetre of sag stores less length. The pit gets deep before the loop gets useful.
It cannot absorb a coil end. The coil end and any weld passing through will whip the loop and disturb the sensor for several strokes.
It reports position, not tension. A loop sitting at exactly 120 mm tells you nothing about whether the strip is being pulled at the tension the process needs.
What to Measure on the First Run
Loop sizing is a calculation, and the first production run is where you confirm it. Log these four things on a new or re-set line.
- Sag range over 200 strokes. Record the minimum and maximum. A 120 mm set point should hold between 108 and 132 mm; wider than that and the control needs retuning.
- Recovery after a stop. Stop the press mid-coil and restart. The loop should return to its set depth within three strokes, without overshooting into the pit floor.
- Brake behaviour. Watch the brake pressure or current trace. A brake that switches on and off is being asked for more than the loop can buffer; a modulating brake is doing the job correctly.
- Behaviour at coil end. Note the loop position in the last two metres. If the loop is collapsing before the coil runs out, the brake is set for a diameter that no longer exists.
- Strip condition entering the feeder. Check for scuffing where the strip runs over the entry guide. Marks there mean the loop is pulling the strip against the guide rather than feeding it.
Loop or Accumulator: Which Fits Your Line
A free loop is the default because it is cheap and it never touches the strip. An accumulator or a dancer roll is the answer when the loop runs out of room.
- Stay with a free loop up to roughly 200 strokes a minute on strip from 0.5 mm to 4 mm, where the pit can hold the sag the arithmetic calls for.
- Move to a dancer roll when light gauge whips and the sensor signal is noisy. A dancer holds the strip in a fixed path and gives the control a clean position signal.
- Move to an accumulator above roughly 250 strokes a minute, or where the line must keep running through a coil change. Stored length is measured in metres, not millimetres.
- Budget for the strip contact. Both the dancer and the accumulator add rolls that touch the strip. On pre-painted or polished material, count that as a marking risk before you count it as a control improvement.
Where to go next
Loop depth is one line in the feed error budget. The rest — roll grip, back tension, lateral position and pilot release timing — is worked through in the guide library.
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