The Straightening machine is a mechanical process that corrects flatness defects in rolled and drawn metal products. The process involves bending the material around sets of rollers that alternately stretch and compress the upper and lower surfaces, thereby exceeding the yield point of the material and relieving internal stresses. The resulting flat material is easier to work, and it is less likely to experience fatigue from subsequent forming operations.
The maximum force that can be applied during a straightening operation is determined by the geometry of the straightener and the work-roll diameters. The bending radius and the distance between the work-roll centres also play an important role. The straightening machine must be designed to take into account the wide range of possible variations in metal type, thickness and width.
Generally, the higher the number of work-rolls in a straightener, the more effective it is. However, this does not necessarily mean that the straightening process will be faster. In fact, the opposite may be true: because of the greater bending radius and the greater distance between the work-roll centres, a high number of work-rolls may increase the straightening time while at the same time decreasing the level of flatness achieved.
Straighteners come in two basic flavors-pull-through (unpowered) and powered. Unlike unpowered machines, powered straighteners are equipped with one or more drive motors to turn the work-rolls. This system overcomes the issues associated with unpowered straighteners, such as excessive deflection of the work-rolls and the potential for damage to the cylinders.
As with all equipment, determining the best Straightening machine is a matter of matching the specification to the application. Different types of metal have different yield points, which makes it difficult to find a single machine that will be effective for all applications. For example, aluminum is more elastic than steel and requires more power to bend it into an acceptable level of flatness. The same applies to the work-roll diameters-smaller-diameter rollers are better for thinner materials, while larger-diameter rollers are needed for thicker metals.
A properly adjusted Straightening machine can produce a very high degree of flatness in a relatively short period of time. To achieve this, it is necessary to start with the correct working position of the work-rolls. This is typically accomplished by setting the first work-roll to a height that is significantly below the maximum material thickness and then gradually adjusting the other work-rolls until they are all at a nominal level of straightening.
For more accurate positioning of the work-rolls, most straighteners are supplied with a method of calibration for the upper work-roll depth setting. Typical methods for establishing this include the use of simple calibrated scales and pointers or, in more sophisticated applications, dial height indicators or LED (light-emitting diode) readouts. Once the optimum work-roll penetration has been established, it is essential that this position is consistently returned to each time the straightener is operated. Failure to do this will result in uneven levels of flatness and unnecessary strain on the work-rolls and their support bearings.