Straightening machine have been used for over a century to level and smooth the surface of sheet steel before rolling or forming. They are commonly divided into two main categories, roll-type machines and stretching machines. The former are distinguished by having a set of work rollers which rotate to produce a series of cyclic deflections which straighten the metal. The latter use a combination of mechanical and electrical power to generate stresses in the metal close to the yield point. The resulting elastic-plastic deformation allows the metal to recover its previous shape after the bending stress is removed, producing a straighter, smoother surface.
Straighteners are available in a wide range of sizes and configurations with differing maximum material widths, roller diameters and centre distance spacings and bending forces generated during the straightening process. Regardless of the type or size of machine, effective straightening results are contingent on proper and consistent set-up and calibration of the machine. The combination of pinch roll pressures, drag brake strength and work roller depth settings will determine the level of straightening effectiveness for a specific application.
Pinch roll pressures are generally established by air pressure regulators or screw down and gauge combinations. Depending on the application and material conditions, they can vary from as low as zero up to the maximum machine capacity for the particular materials being processed. The amount of gripping force applied by the first pair of pinch rolls determines the maximum permissible work roll penetration which can be achieved. The pressures of the second and subsequent pairs of pinch rollers will depend on the application requirements and material conditions.
The method of driving the work rollers is another important variable that influences straightening capacity. Most modern machines have two drive motors which are used to control the speed of both sets of rollers independently. This system is referred to as load sharing and is necessary in order to ensure the correct balance of power between the two sets. This is not possible if each set of rollers has its own drive motor as in older machines.
In order to achieve optimum straightening results, the work rollers should be set to a maximum of just under half of the material thickness. This will allow the maximum amount of back bending to occur during the straightening process. This bending force is proportional to the square of the material thickness and is known as the plastic ratio – Rp (see Fig. 1).
A good quality straightening machine is provided with a means of calibrating the upper work roller depth setting to the required value for a given material type, thickness and width. Once a recommended depth setting is established, it must be consistently returned to the same position each time the material is run through the machine in order to obtain consistent and effective results. Typically, this is accomplished by using a calibrated scale and pointer or dial height indicators.