What Drive Power Does an NC Straightener Feeder Need?

Power Follows Plastic Work, Not Tonnage

A straightener drive does one job: it drags the strip through a series of reverse bends, each of which pushes the material past its yield point in bending. The energy that goes into that is not recovered — it becomes heat in the strip and in the rolls. Drive power is simply that energy per second, divided by the efficiency of getting it from the motor to the strip.

Two things follow from that. First, the press behind the line is irrelevant; a 400-tonne press and a 160-tonne press require the same drive on the same strip. Second, the material matters more than the geometry, and it matters squared rather than linearly — which is why HSLA surprises people who sized the drive on mild steel.

The useful result of the derivation is a single relationship: the plastic work per metre of strip is proportional to the square of the yield strength, multiplied by the strip width and the thickness. Power is that work multiplied by strip speed. Everything else — roll diameter, roll count, roll spacing — changes the details but not the order of magnitude.

The Five Inputs You Actually Need

Five numbers, all of which should be on the material certificate or the job card.

Two cautions on the inputs. Use the upper end of the yield range from the certificate rather than the nominal grade figure — HSLA 420 routinely tests between 440 and 520 MPa, and because the calculation squares it, a 15% error in yield becomes a 32% error in power. And derive strip speed from the press rather than from the machine's maximum: 200 strokes per minute at a 200 mm feed length is 40 m/min, and that is the number the drive has to deliver continuously, not the 90 m/min on the nameplate.

A Worked Example on 3.0 mm HSLA 420

Three steps, all arithmetic you can check on a calculator.

Step one: the fully plastic bending moment

The moment needed to bend the full strip section past yield is:

Mp = σy × b × t² ÷ 4

With σy = 420 MPa, b = 1.2 m and t = 0.003 m, that gives 420 × 10⁶ × 1.2 × 9 × 10⁻⁶ ÷ 4, or 1,134 N·m. This is the moment carried by the strip itself, independent of roll size.

Step two: the curvature the strip must be taken through

The curvature at which bending becomes plastic rather than elastic is:

κp = 2σy ÷ (E × t)

That is 2 × 420 × 10⁶ ÷ (210 × 10⁹ × 0.003), or 1.33 m⁻¹ — a bend radius of about 750 mm. A working straightener takes the strip well past this, typically two to three times κp in each direction, because the point of the machine is to reverse the incoming coil curvature rather than merely to touch the yield point.

With five working rolls there are four reverse bends. Allowing a curvature swing of 2κp at each gives a total curvature change of 4 × 2 × 1.33, or 10.7 m⁻¹.

Step three: work per metre, then power

Plastic work per metre of strip is the bending moment multiplied by the total curvature change: 1,134 × 10.7, or about 12.1 kJ per metre.

At 40 m/min, which is 0.667 m/s, the mechanical power is 12.1 × 0.667, or 8.1 kW.

Now apply the losses. A gearbox and motor combination delivering power to the rolls runs at roughly 0.75 overall efficiency, which lifts the required motor output to 10.8 kW. A service factor of 1.3 — reasonable when the line will run continuously and the yield strength may drift upward — gives 14.0 kW. The practical selection is a 15 kW motor.

Note what happened there: the machine people usually describe as "a 22 kW head" is, on these numbers, a 15 kW job. The extra 7 kW buys acceleration margin and thermal headroom rather than straightening capability, and on a line with a stable supply it is money spent on insurance rather than on capability. That is a defensible choice — just make it deliberately rather than by default.

Sensitivity: Which Input Moves the Answer

Because the work term reduces to a function of σy squared times width times thickness, the sensitivities are not equal. This table changes one input at a time from the worked example and shows the effect on required power.

Two rows in that table are worth internalising. The yield strength row is the one that catches out plants moving from SPCC to HSLA on the same machine: a 24% increase in yield demands 54% more drive power, and if the motor was sized with no margin the line will simply run slower or stall under load. The roll diameter row is the one that surprises people who expect bigger rolls to need more power — they do not, because the work done on the strip is the same. What bigger rolls change is the torque at the roll shaft and the strip speed for a given motor rpm, which is a gearbox question rather than a power question.

If you are buying a machine that will eventually run higher-strength material, size the drive for the material you intend to run in three years rather than the material you run today. A larger motor costs a few hundred dollars more at the factory and cannot be economically retrofitted later.

Accepting the Result: Motor, Gearbox and Thermal Margin

• Round up to the next standard motor size. The calculation gives a required output; standard motors come in 11, 15, 18.5 and 22 kW. Never round down, because the material certificate is a minimum and the real strip will sometimes be harder.
• Check continuous duty, not peak. A straightener drive runs loaded for the whole shift. Confirm the motor's continuous rating at the ambient temperature of your plant, and derate if the cabinet is in a hot corner of the shop rather than in a conditioned room.
• Size the gearbox on output torque, not on motor power. A 15 kW motor at 1,450 rpm delivers about 99 N·m; the gearbox multiplies that to the roll shaft. Check the gearbox rating against the shaft torque at the slowest line speed, which is where the torque is highest.
• Allow for strip tension if the line pulls. If the straightener head is also pulling the strip off the decoiler rather than taking it from a slack loop, add the back tension force times strip speed to the power requirement. On a 1,200 mm line running 8 MPa of back tension that can add 3–5 kW, which is the difference between a 15 kW and an 18.5 kW selection.
• Check the cooling path on the rolls, not just the motor. The plastic work ends up as heat in the strip and the rolls. On a heavy-gauge line running continuously, roll surface temperature can climb 30–40 °C above ambient, and that changes the friction and the strip finish long before the motor becomes the limit.

Three questions come up on almost every sizing exercise, and the answers decide the final selection.

Where the Calculation Stops Being Useful

The method holds for strip between roughly 0.4 mm and 6 mm on conventional roll diameters. Outside that band, two things change.

At the thin end, below about 0.4 mm, the elastic curvature limit rises sharply because κp is inversely proportional to thickness. Very thin strip bends elastically over the rolls and the plastic work falls away, so the drive power calculation over-predicts. At that point the limiting factor is not power at all — it is the roll bearing friction, the strip tension control and the surface finish, and sizing the motor generously simply wastes energy.

At the thick end, above roughly 6 mm on a 1,200 mm width, the calculation still works but the machine around it changes character. Roll diameters grow past 150 mm, the frame stiffness requirement grows faster than the power requirement, and the limiting factor becomes deflection of the rolls under load rather than drive capability. A 6 mm line that is short of drive power is usually short of roll stiffness instead.

The other boundary is material. The calculation assumes the strip can be taken plastically through the head without cracking. On high-strength steels above about 600 MPa yield, and on some coated and pre-painted products, the curvature needed to reverse the incoming coil set can exceed what the coating or the material will tolerate. In those cases the answer is not more power — it is a different straightening strategy, usually more rolls at smaller curvature per pass rather than fewer rolls at larger curvature.

FANTY has built coil-processing lines for more than 200 installations across 60 countries, and the drive-sizing conversation is one we have on nearly every heavy-gauge enquiry. The pattern is consistent: plants that bring the material certificate and the press speed get a machine that holds its accuracy for a decade. Plants that describe the job as "about 3 mm steel" usually end up buying a bigger motor than they need, or a smaller one than they need, and the difference shows up on the first coil of HSLA.