Why Does Coil Weight Set Straightener Feeder Run Time?
Two plants run the same part on the same press at the same speed. One changes coil every nine minutes and the other every eighteen.
The difference is coil weight, and it shows up directly in the scrap rate and the labour cost per part.
This article works out how long a coil actually runs on an NC straightener feeder, using four numbers you already have.
It also covers the sensitivity that catches people out and the point where a heavier coil stops paying for itself.
Run time per coil is one of the few line numbers you can calculate on paper before buying anything, and it drives more of your operating cost than press speed does.
It is worth ten minutes with a calculator.
Pick the Four Numbers That Set Run Time
You do not need the machine specification to start. You need the coil and the part.
- Coil weight in kilograms. The nameplate figure. If your supplier ships "about 5 tonnes", measure three coils before you trust it.
- Strip width and thickness. These set the mass per metre, which is the number that converts weight into length. Use nominal thickness; mills hold within about 2%.
- Feed length per stroke. The progression of the die, in millimetres. Multiply by strokes per minute and you have the strip consumption rate.
- Coil changeover time. From last good part to first good part of the next coil, including loading, threading and the first-off check. Stopwatch it rather than estimating.
Steel density of 7,850 kg per cubic metre is close enough for mild and most coated grades. Stainless is around 7,900 and aluminium around 2,700.
Worked Example: A 5-Tonne Coil at 200 Strokes per Minute
Take a 5,000 kg coil of 1.2 mm SPCC, 1,200 mm wide, feeding a die with a 250 mm progression at 200 strokes per minute.
Step one: mass per metre. Cross-section is 1.2 m by 0.0012 m, or 0.00144 m². Multiply by 7,850 kg per cubic metre and you get 11.3 kg per metre of strip.
Step two: strip length. Divide 5,000 kg by 11.3 kg per metre. The coil holds roughly 442 metres of strip.
Step three: parts per coil. Divide 442 metres by the 250 mm progression and you get about 1,770 strokes, before any coil-end allowance.
Step four: run time. At 200 strokes per minute, 1,770 strokes take 8.9 minutes.
Now add the changeover. Loading, threading, strip end handling and a first-off check take about four minutes on a well-organised line.
The full cycle is 12.9 minutes, of which 8.9 minutes produce parts. That is 69% utilisation.
Double the coil to 10 tonnes and the run time doubles to 17.7 minutes. The same four-minute changeover now takes the utilisation to 82%.
The Sensitivity That Surprises People
Coil weight is not the only lever, and the others move in directions that are easy to misread.
| Change | Effect on Run Time | Where It Costs You |
|---|---|---|
| Coil weight doubled, 5 t to 10 t | Run time roughly doubles, 8.9 to 17.7 minutes | A heavier uncoiler class, a bigger mandrel, a stronger foundation slab and a coil car rated for 10 t |
| Feed length up 50%, 250 to 375 mm | Parts per coil fall by a third, run time falls with them | More changeovers per shift, plus a heavier part to handle at the press |
| Thickness up 25%, 1.2 to 1.5 mm | Run time drops about 20% because each metre weighs more | Thicker material often needs more straightening force, which changes the machine class |
| Press speed up 25%, 200 to 250 SPM | Run time drops 20%, and the changeover share rises | Higher speed demands more loop capacity and tighter pilot release timing |
| Changeover cut in half, 4 to 2 minutes | Run time unchanged, utilisation rises from 69% to 82% | Quick-change mandrels, a second coil car and pre-staged coils all carry a capital cost |
Note that the last row reaches the same 82% as doubling coil weight, for a fraction of the capital.
That comparison is the one most plants never run.
Where the Calculation Misleads You
The arithmetic above is correct and still overstates your production time. Four corrections matter.
Rated speed is not average speed. Most lines average 70-85% of the nameplate figure once acceleration, deceleration and short stops are included. Use 0.8 as a multiplier before you believe any run time.
The last few metres of a coil are not usable. Depending on your die and strip end handling, the final 3-8 metres may be too curled or too short to feed reliably.
The first-off check is not optional. Any plant that skips it is trading a four-minute stop for a scrap bin, and the arithmetic should carry the cost either way.
Coil weight may be capped by your building, not your machine. Crane capacity, floor loading and aisle width set a hard ceiling that no calculation can move.
The risk runs the other way as well. Heavier coils raise the severity of any handling incident, and the safety case has to be made on its own terms.
Cheaper per part is not the same as better.
Turning Run Time Into a Changeover Cost
Utilisation is an engineering number. Cost per part is what the plant manager asks about.
In the example above, each coil cycle loses four minutes of press time. If the line costs USD 60 per hour to run, including labour and overhead, that is USD 4 per coil.
Divide by 1,770 parts and the changeover adds about 0.23 US cents per part. On a small part that can exceed the material cost saving that motivated the light coil.
Change the coil to 10 tonnes and the changeover cost per part halves, because the same four minutes is spread over twice as many strokes.
The figure to watch is the ratio of run time to changeover time, not either one alone.
Setting a Changeover Frequency You Can Live With
There is a simple threshold that works across a wide range of lines, and it is easier to apply than a full cost model.
Aim for run time at least six times the changeover time. With a four-minute changeover, that means a minimum run of about 24 minutes per coil.
At 250 mm feed and 200 strokes per minute, 24 minutes is roughly 4,800 strokes, or about 13.5 tonnes of 1.2 mm material at 1,200 mm wide.
That is a big coil. If your crane or your supplier cannot deliver it, the honest answer is to attack the changeover time instead.
| Changeover Time | Minimum Run Time at 6:1 | Where It Costs You |
|---|---|---|
| 2 minutes | 12 minutes, about 6.8 t of 1.2 mm strip | Needs a quick-change mandrel and a second coil car, both of which are capital items |
| 4 minutes | 24 minutes, about 13.5 t | Beyond most crane and floor limits, so the rule cannot be met by coil weight alone |
| 6 minutes | 36 minutes, about 20 t | Not deliverable in most plants; the changeover has to be engineered down instead |
| 8 minutes | 48 minutes, about 27 t | Every extra minute of changeover doubles the coil weight needed to compensate |
The pattern in the last column is the point. Changeover time compounds, and coil weight runs out of room quickly.
Cutting changeover from four minutes to two is usually cheaper than doubling coil weight. A quick-change mandrel and a second coil car cost far less than a heavier uncoiler class.
Add pre-staged coils and the reinforced floor disappears from the budget entirely.
FANTY has built coil handling equipment for twelve years and supplies more than 60 countries. The lines that run leanest are consistently the ones that invested in changeover time first and coil weight second.
The coil line guide library sets out how roll count, pitch, gap and penetration turn that strip into flat parts once it is on the line.
Machine ranges are listed on our products index.
How many parts does a 5-tonne coil yield at 250 mm feed?
On 1.2 mm strip at 1,200 mm wide, about 1,770 strokes. Allow 1-2% less for the unusable strip at the coil end.
At what run time does a changeover stop being worth it?
Below about six times the changeover time, the stop starts dominating the cycle. With a four-minute changeover that means anything under 24 minutes of run time deserves attention.
How much does 1.2 mm versus 2.0 mm thickness change run time?
At the same coil weight, the 2.0 mm coil holds about 40% less strip length, so run time falls by roughly 40% before any speed change.
Does a heavier coil always reduce cost per part?
No. It reduces the changeover share, but it also raises the machine class, the foundation requirement and the handling risk. Past a point the extra capital outweighs the labour saved.
What coil weight suits a 1,200 mm wide line?
Five to seven tonnes is the common range for a standard line, and 10-15 tonnes where the crane, floor and coil car allow it. The binding constraint is usually the building, not the machine.
Where to go next
This article sits in the flatness branch of the coil line guide library, one layer below the roll geometry pillar. It works out how long the strip lasts rather than how to flatten it.
Read the flatness specification guide
