Leveler and Straightener in Roll Forming

1. Definition and Function

A leveler (also called a straightener or flattening machine) is a roller-based machine installed upstream of the roll forming mill that bends strip back and forth through a series of offset rolls to remove residual curvature and improve flatness before forming. Coiled strip arrives at the roll forming line with coil set — a longitudinal curvature induced by winding tension — and often with crossbow (transverse curvature), edge wave, or center buckle from prior rolling and slitting operations. The leveler applies controlled plastic bending cycles that exceed the material yield point locally, resetting the strip to near-flat condition.

Flat strip entering the forming stands produces more consistent flange angles, reduces twist in the finished profile, and allows tighter roll gap settings without strip wrinkling or buckling in the web. For pre-punched profiles, flat strip ensures hole patterns remain in the intended plane relative to the forming direction. For coated strip (galvanized, aluminized, pre-painted), leveler roll diameter and surface condition must avoid coating damage while delivering sufficient bending moment.

The leveler sits between the decoiler and the roll forming mill entry guide, sometimes combined with a pinch roll unit that provides traction for the strip through the leveler and into the mill. Leveler capacity must match the line speed, strip width range, and thickness range of all profiles the line produces.

2. Coil Set and Strip Defects

Coil set is the most common flatness defect in roll forming feedstock. When strip is wound under tension onto a coil, the outer layers stretch and the inner layers compress. Upon unwinding, the elastic recovery creates a tendency for the strip to curl upward (top-side-out unwinding) or downward (bottom-side-out). The curvature radius depends on coil diameter, strip thickness, yield strength, and time in storage.

2.1 Flatness Defect Types

2.2 Residual Curvature vs Material Properties

Higher yield strength and thicker gauge require deeper roll intermesh (greater offset between upper and lower roll rows) and more bending cycles to fully reverse coil set plastic deformation. Under-leveled strip enters the forming mill with residual curvature that manifests as camber in the finished profile, often exceeding EN 10162 limits.

3. Leveler and Straightener Types

Levelers are classified by roller count, adjustability, and drive configuration. The roller count determines the number of bending cycles; adjustability determines whether intermesh depth and roll angle can be set for different thicknesses and materials.

3.1 Drive Configuration

4. Roller Configuration and Settings

Leveler effectiveness depends on roll diameter, roll spacing (center distance), intermesh depth, and backup roll support. Roll diameter must be large enough relative to strip thickness to avoid marking and coating damage while small enough to generate sufficient bending moment.

4.1 Roll Geometry Guidelines

4.2 Setting Procedure by Material Change

5. Line Placement and Integration

The leveler occupies the entry zone of the roll forming line, after the decoiler and before the entry guide rolls of the forming mill. Placement relative to other entry equipment affects leveling performance and line layout.

5.1 Entry Line Equipment Sequence

Distance between leveler exit and mill entry should be minimized to prevent strip sag under gravity, which reintroduces curvature on thin gauge. Support rollers or a strip table bridge this gap on lines running strip below 0.8 mm at speeds above 30 m/min.

6. Flatness Specification and Measurement

Flatness is quantified using wave height (peak-to-valley deviation from a reference plane) measured over defined gauge lengths. Roll forming feedstock flatness targets are typically tighter than coil exit flatness from the steel mill because forming amplifies residual stress non-uniformity into profile defects.

6.1 Flatness Measurement Methods

6.2 Target Flatness Before Roll Forming

I-units (international flatness units) express wave height as mm/m of wave length. A strip with 4 I-units has a wave amplitude of 4 mm over a 1 m wavelength. Lower I-unit values indicate flatter strip. Levelers on roll forming lines typically achieve 2–5 I-units on incoming coil set material that may measure 10–20 I-units directly off the decoiler.

7. Roll Forming Applications

Leveler selection and setting vary by product type, material, and speed. The following table summarizes leveler configurations used across common roll forming segments.

Leveler rolls require periodic inspection for wear, particularly on lines running abrasive-coated steels or high-strength grades. Roll surface hardness of 58–62 HRC on work rolls extends service life. Coated strip applications use polyurethane-covered or polished chrome rolls to prevent zinc pickup and surface marking. Leveler settings are documented per material grade and thickness on setup sheets stored at the line, enabling consistent reproduction after coil change or profile changeover.

Strip tension between the leveler and the forming mill affects flatness retention over the entry span. Pinch roll tension typically ranges from 50 to 200 N/mm of strip width depending on gauge and line speed. Excessive tension reintroduces longitudinal stress that can appear as camber in the finished profile; insufficient tension allows strip sag that causes inconsistent entry into the first forming stand.