Which Coil Eye Direction Suits an NC Straightener Feeder?

What Coil Eye Direction Actually Means

The eye is the inner bore of the coil. On a mandrel-mounted decoiler the coil axis is horizontal, so the eye faces the operator, and there are two ways to seat the coil.

In the first, the strip pays off over the top of the coil and travels down and forward into the straightener. In the second, the strip pays off from the bottom and travels up and forward. The coil itself looks nearly identical in both cases. What changes is which side of the strip has been in compression on the inside of the coil wrap, and therefore which direction the strip wants to curl when the constraint of the wrap is removed.

Most operators call these the overwind and underwind positions, or simply "payoff over" and "payoff under". Both descriptions point at the same variable: the entry angle of the strip into the first pair of rolls.

Two Orientations, Two Different Strip Paths

The table below compares the two arrangements as they behave on a real line. Neither column is universally better; the right answer depends on the coil and the machine.

The row that decides most real cases is the third one. A downward bow arriving at the entry rolls is easier to remove than an upward bow, because the straightener's roll arrangement is typically designed with the entry rolls positioned to accept a descending strip. You can see how the decoiler, straightener and feeder sections sit relative to each other in our 3-in-1 decoiler straightener feeder range.

How Eye Direction Changes Strip Curvature

A coil is a spring that has been wound and restrained. The outer wraps are in tension and the inner wraps in compression, and when the strip is unwound the residual stress reveals itself as longitudinal curvature. Which way it curves depends on the winding direction of the coil and on which side the strip leaves from.

Here is the part that surprises people. Two coils of identical material, identical gauge and identical outer diameter can arrive with opposite winding directions depending on the mill and the slitting operation. A line set up for one will produce a persistent bow on the other, and the operator will spend an afternoon adjusting roll penetration to chase a problem that a different coil orientation would have solved in ten minutes.

The practical consequence is that eye direction should be a documented setting, not an operator's habit. Write it on the setup sheet, next to feed length and roll pressure, and note the winding direction of the coil it was validated on.

When to Flip the Coil Instead of Re-Rigging the Line

Flipping a coil means rotating it about its own axis so the opposite face becomes the top. It is a ten-minute job with a coil turner and a bad idea with a forklift and a chain.

There are four situations where flipping is the right answer rather than changing the machine.

• The visible face is on the wrong side. If the part requires a specific face upward or downward and the coil is wound the other way, no feeder setting fixes it. Flip the coil.
• The bow is upward and persistent. A steady upward bow that survives full roll penetration usually means the coil is seated for the wrong payoff direction relative to its winding.
• The line is set for one direction only. Some decoiler drives and loop pit layouts are built for a single payoff direction. If yours is, the coil must adapt rather than the machine.
• Coil set is extreme. On heavy gauge with pronounced set, orienting the coil so the set works with gravity rather than against it removes most of the problem before the straightener sees it.

What flipping cannot fix is a coil with a true crossbow or a twist. Those are conditions of the material itself and they follow the strip regardless of which face is up.

Threading, Scrap and the First Ten Metres

The first ten metres of every coil tell you whether the orientation is right, and they also produce most of the scrap on a changeover.

With payoff over the top, the strip drops toward the straightener under its own weight. Threading is a two-person job at most, and the entry guide usually catches the strip without help. With payoff from the bottom, the strip has to be lifted into the entry guide, and if the guide is set for a descending strip the lead end can ride over the top of the first roll. That single event produces a kinked lead end, which either goes into the scrap bin or, worse, into the die.

Count the scrap from the first ten metres of each coil and you have the real cost difference between the two orientations on your line. On a 900 kg coil with a 40-metre strip length, the first ten metres are a quarter of the coil. Recovering even half of that on every changeover is worth more than most people expect.

A Rule You Can Apply at the Loading Station

After enough changeovers, the decision reduces to one question: which face of the strip must be up when it reaches the die?

Start there, then work backwards through the strip path. If the answer requires a specific face upward, the coil orientation is fixed by the winding direction and there is nothing to choose. If either face will do, pay off over the top, because it puts the strip into the straightener in the direction the rolls were designed to accept and it makes threading easier.

Only when the coil arrives with an obvious upward set do you depart from that rule, and then only after confirming on a short run that the straightener can handle it. Our factory covers 45,000 m² and employs 370 people, and the loading bay is where a surprising number of quality disputes are settled before they start, simply because somebody wrote the payoff direction on the setup sheet.