Straightening machine is a type of rollforming equipment that straightens pre-coiled material, such as welded or drawn steel or tube, to produce a flat product. This is accomplished by passing the material between a set of work rollers, with each pair of rollers having different heights depending on the size and shape of the tube or bar being processed. The amount of force applied to the work rollers can also be varied depending on the desired end result.
During the straightening process, the material passes through a series of back bending moments (roller pitch). The amount of force required to achieve an acceptable level of straightness depends on the combination of the material thickness, width and yield strength. Consequently, the machine’s power requirements can vary significantly.
In basic theory, a straightener with three staggered work rolls should be capable of handling most materials. However, due to possible differences in the quality of the material at the head, middle and tail ends of a coil, this is not always the case. To compensate for these variations in quality, a more advanced machine is often necessary.
A corrective leveller is generally classified as a straightener that is also able to flex the work rollers. As a result, it typically has far more work rollers than a standard straightener and is able to remove a wide range of defects that can occur in a coil. These defects include centre buckles, wavy edges and trapped stresses, among others.
When it comes to setting up a straightening machine, the correct work roller depth settings are critical for an effective result. A common practice is to calibrate the first upper work roller depth to an optimum value. Then, a second and perhaps third work roller is set to a lower amount of penetration, allowing for a controlled and consistent straightening result.
The work rollers in a straightener are usually supported on a set of support journals with a diameter that is slightly larger than the actual roller. This provides the space needed to allow for a significant amount of back bending of the material without excessive deflection of the work rollers. The exact diameters and centre distance spacing of the support journal and work roller are determined by the required level of straightness and the maximum material thickness.
Generally, the higher the quality of the material to be straightened, the greater the diameter and centre distance spacing of the work rollers should be. This is to ensure that the maximum possible level of straightness can be achieved without causing a loss of the material’s yield strength.
Pinch roll pressures are another important factor in determining the effectiveness of a straightening machine. This can be established by using a combination of scale and pointer or dial height indicators. This allows the operator to establish the optimum pinch roll pressures for any given material. It is essential that these settings are consistently returned to their optimum position each time the machine is used.