Straightening machine is a specialized machine used to eliminate the size inconsistencies that occur in metal bar after it comes out of the rolling mill. The size inconsistencies may result from the lack of adequate control means and procedures during steel rolling and cooling, unavoidable impact and extrusion in handling and transportation, as well as a number of other factors. These inconsistencies can lead to a variety of problems, including residual stress, up and down bending, side bending, gnawing, correction cracks and correction markings. The purpose of straightening is to eliminate these inconsistencies, which will improve both the quality and tensile strength of the coil.
A wide range of straightening machines are available for a variety of materials and thicknesses. The two major categories of straightening machines are roller-type and stretching machines.
Roller-type straightening machines use sets of work rollers to alternately stretch and compress the upper and lower surfaces of the material, often exceeding the material’s yield point. This stretches the material, and the elastic-plastic deformation reduces internal stresses and homogenizes those that cannot be eliminated. The resulting flat material is suitable for a variety of applications.
These types of machines tend to be larger than other straighteners, and have relatively large diameter and wide centre distance (roller pitch) work rollers. They can be backed up with additional rollers, which allows them to handle a wider range of material thicknesses and widths. The larger diameter of the work rollers also allows them to withhold back bending forces without excessively deflecting.
The rolling machine is driven by one or more drive motors. Some machines have a separate drive for each work roller, while others have an electronic load-sharing system that distributes the power between the top and bottom sets of straightening rollers. The load sharing system helps avoid overstressing the straightener work rollers, and is particularly important for larger machines with higher roller penetration.
Depending on the material type, thickness and width, a straightener will have a recommended minimum level of roller penetration, called the amount of ‘work penetration’. The amount of penetration is determined by the number of the upper set of work rollers and the ‘zero’ position of the work roller depth setting. The ‘zero’ position refers to the point at which the upper work roller is tangent to the lower fixed work roller and there is no daylight between them.
Excessive roll penetration results in poor straightener efficiency, slippage of the material across the straighteners and unnecessary strain on the work rolls. This can result in damaged work surfaces and excessive wear on the drive motors. This is why it is important to understand the requirements for the minimum amount of penetration for your specific application.