Straightening Machines in Flat Rolling Mills

Straightening Machines in Flat Rolling Mills

Metal straightening or levelling process is used for straightening different type of steel materials like rounds, sections, pipes and flat products after their rolling. The rolled steel material gets deformed during cooling after rolling due to the residual stresses and often has the surface defects in cold condition such as buckles, wavy edges, camber, crossbow, coil set (in case of coils), and many more. Hence, the rolled steel material is flattened in the multi-roller straightening machines.  The straightening process and the straightening machines for flat rolling mills are discussed in this article.

Flat products are plates and sheets which are produced by hot rolling and cold rolling. The rolling process provides the products a specific thickness and mechanical properties. However, during the process of rolling, the rolled products adopt flatness defects and residual stresses appear inside the material. High level of residual stresses inside the flat products at the end of the process can promote the effect of spring-back and this can cause the deformation of the products during cutting and can affect the forming processes where the flat products are used. Also, the flatness tolerances and materials specifications of the flat products needed to meet the requirements of the standards cannot be generally met by the rolling process itself, and for this reason, an additional step is necessary. Hence, the flat products after their rolling are subjected to the roll levelling process.

Roller straightening is a forming process which aims at correcting the flatness defects and minimizing the residual stresses. During the process, the steel material is bent in alternate directions by a certain number of rolls with adjustable overlapping. Straightening is accomplished by bending the material around sets of rollers to alternately stretch and compress the upper and lower surfaces, exceeding its yield point so that both surfaces end up the same length after spring back which results in flat material. During this process, material is subjected to elastic-plastic deformations leading to elongation in the material in order to reduce internal stresses and homogenize those which cannot be removed. Due to the tilt adjustment of the upper rolls, the bends and, accordingly, the deflections are to decrease in the forward direction of the straightening process.

Straightening machines today fall into two basic categories.  The first category of straightening machines is more common and is known as ‘straighteners’ or ‘flatteners’. This arrangement is generally available in machines with between 5 work rollers and 11 work rollers. The roller diameters and centre distances vary depending on material thickness and width but straighteners and flatteners are generally distinguished by fairly large diameter, widely spaced rollers, usually not backed up. A schematic view of a nine rollers straightener is shown in Fig 1.

Fig 1 Schematic view of a nine rollers straightener

The second category of straightening machines is known as a ‘precision leveller’. Precision levellers are distinguished by small diameter closely spaced rollers with backups and the ability to flex those rollers. These machines normally have a far greater number of work rollers than do conventional straighteners. Since these machines work on the material much harder and their rollers can be flexed, precision levellers are used to remove camber, wavy edges, centre buckles, as well as trapped stresses within the material so that it stays flat. Levellers require drives with higher power than straighteners because of the greater amount of work being done to the material.

The straighteners are normally powered with lower capacity drives or they are even non-powered. Non-powered straighteners are known as ‘pull through’ straighteners. As the name suggests, the feed provides the power to pull the strip through the straightener. The advantage of this type is low cost.  With pull through straighteners the power to straighten is to be drawn from the feeder. This can either reduce its speed capability or greatly increase its cost. Additional disadvantages to pull through straighteners include marking due to the non-powered straightening rolls slipping on the material during the starting and stopping and inaccuracy as a result of feed slippage because of the additional load. Power straighteners are normally free standing.

An understanding of the basic principles of straightening is necessary for deciding the specification of the straightening equipment and to obtain effective and consistent results from the straightening machine with high production efficiencies and improved product quality.

In simple theory, three staggered rollers are to be sufficient to straighten most materials. This basic approach can be applied to an application if the quality of the rolled material remains constant throughout the length. However, there is the possible potential difference in the quality at the head end, middle, and the tail end of the material. Hence, with only three staggered rollers, it becomes necessary to constantly adjust the machine to obtain an acceptable level of flatness. Hence, the straighteners are usually built with multiple work rolls to effectively address this issue. As more work rolls are employed in a straightener, the straightener capability of effectively removing the levelling defects becomes greater.

Another basic principle of straightening is that thicker materials require rollers of relatively higher diameter. The centre distance spacing (roller pitch) of these rollers can be relatively larger and they still can do an effective job of back bending the material. Thicker materials typically require fewer straightener rollers. As the material thickness specification increases, the roller diameters and support journal diameters are to increase. The work rollers are to be capable of withstanding the forces required to back bend the material without excessive deflection across their width.

Thinner materials require relatively smaller diameter rollers. The centre distance spacing of these rollers is to be relatively shorter to effectively stretch and compress the material. Thinner materials normally require a greater number of rollers to effectively remove the varying amount of levelling defects present in the material. Consideration is still to be given to the support journal diameters of the work rollers on such applications. As the material and machine widths increase, the tendency for the smaller diameter rollers to flex and deflect also increases.

When determining the level of flexibility and range of materials which a straightener can process, the maximum width of the material and machine is to be considered in parallel with the range of material thickness. As the width of a given model of straightener increases, the ability of the machine to process a material with a defined thickness and width is compromised. The tendency of work rollers and end journals to deflect becomes greater as the machine width increases. Excessive deflection of the roller results in a loss of contact surface area, decreased straightening efficiency, slippage of the material through the straightener, and in the worst case, broken work rollers.

When an application requires that the straightener is to work effectively across a wide range of material thickness and widths, the machine is generally equipped with ‘back-up’ rollers for the work rollers. Depending on the maximum width of the material and machine, the back-up rollers can be positioned in one, two, or three places across the width of the work roller. The back-up roller assembly usually consists of precision cam followers mounted on a heavy duty weldment and supported on a precision adjustment mechanism like a jack or screw. The proper placement of back-up rollers minimizes the stress and potential deflection of the work roller.

A common mistake in the specification of straighteners is to ask for a straightening machine which is capable of processing wide material without giving consideration to the effect which the narrower material is going to have on the machine. The cross section and strength of the narrower material is substantially less than the wider material, but the straightener rollers are most likely to experience a greater amount of deflection when running the narrower material. The forces and stresses are now concentrated at the centre of the rollers. This area is furthest from the end journals and bearings which support the rollers. Placement of a single row of back-up rollers gives the machine the capacity to efficiently straighten the narrower material.

The power needed to operate a straightener is often a misunderstood part of the straightening machine. Obviously the maximum material thickness and width of material are fundamental in determining the power requirement. There are many other factors, some of them not as obvious. The maximum yield strength of the materials is to be defined. Most straighteners are rated by their capacity to process mild steel, less than 35 tons/sq cm yield strength. Materials with higher yield strengths need greater power to straighten to an acceptable level of flatness. The combination of work roller diameter and centre distance spacing can drastically affect the power demands. If two straighteners both have same diameter work rollers, with the first machine having lower centre distance spacing than the second machine, then the first machine requires more power to process material with the same thickness and width.

The process requirements for throughput in metre per minute are necessary to accurately calculate the power requirements for the straightener. This is obtained by multiplying the maximum speed of the press by the maximum progression length. Care is to be taken to not be short sighted when determining this variable. Most often, the throughput parameter is established based on past or current production limitations, rather than on the potential of the equipment and tooling in the manufacturing process.

With a wide potential variation in material types, thickness, and widths, no single straightener can effectively meet the demands of every application. There is no such thing as a ‘universal’ straightener. The manufacturer of the straightener is required to address the potential variations in application requirements. Hence, at the equipment specification process, careful consideration is to be given to all variables associated with the straightening process. The variables related to the material include the ranges for thickness, width, yield strength, and surface finish. For all straightener applications, the maximum line speed is to be defined. Attention to detail in defining all of the variables for a given application, gives the straightener manufacturer a solid understanding of the process requirements, and assures that the correct machine is selected for the job.

Once a machine is properly specified and built for an application. Effective results are contingent on correct and consistent set-up. The combinations of pinch roll pressures, drag brake strength, and work roll depth settings, determine the level of effectiveness for the straightening operation. Pinch roll pressures are typically established by an air pressure regulator or screw down and gauge combination. All straighteners have a set of entrance side pinch rolls as the primary means of gripping. Some machine models are also provided with exit side pinch rolls to further improve the gripping and pulling capability of the machine. The amount of pinch roll force required for a specific material is based on the material width, thickness, and surface condition. High thickness materials normally require greater pinch roll forces. Thin materials have a tendency to wrinkle under excessive pinch roll forces. Too much pinch roll force not only damages the material, but it can also result in pinch roll deflection. Any deflection of the pinch rolls results in a loss of effective contact surface area on the material and promotes slippage.

Straighteners are provided with a method of calibration for the upper work roller depth setting. The amount of work roller penetration required to back bend the material to an acceptable level of flatness depends on the combination of material thickness, material type, roller diameter, and roller centre distance spacing. Once the optimum depth setting is established for a specific material, it is critical that the work rollers are consistently returned to this position each time the material is run. As standard, most straighteners are provided with a simple calibrated scale and pointer combination to establish the roller position. When more accurate positioning is required, alternative methods of positioning are utilized. These methods include mechanical indicators, dial height indicators, and LED (light emitting diode) readouts. The upper work rollers of most straighteners are contained in precision guiding slide block assemblies. The alternative methods for raising and lowering the rollers within the slide block assemblies include fine threaded screw and nut combinations, worm gear and screw mechanisms, and precision screw jacks.

Straighteners are normally equipped with an odd number of work rollers. The extra work roller is in the lower ‘fixed’ bank of rollers. Further, straighteners have a ‘zero’ or ‘home’ position for the work roller depth setting. This is the point at which the upper work roller is tangent to the corresponding lower work roller. It is also referred to as the point at which there is zero daylight between the upper and lower rollers. Simply put, if all the upper work rollers are placed in the ‘zero’, a line with a zero millimeter thickness can be run through the straightener without bending that line.

The guidelines for establishing proper work roller depth settings tend to vary as much as the potential variations in material types, thickness, and width. The recommended roller depth setting is also referred to as amount of penetration relative to the nominal material thickness. As a guideline, the first work roller is to carry out the most straightening work, with each successive roller set to a declining amount of penetration. Regardless of the number of upper work rollers, the operator is to be able to draw a straight line through the centre of each upper work roller when the machine is initially set-up. This guideline is demonstrated in Fig 1. Some trial and error can be required to obtain an acceptable level of flatness for a specific material. Once again, the variables of material type, thickness, and yield strength combined with the work roller diameter and centre distance spacing create a wide range of potential settings.

Fig 2 Typical work roller depth setting in a seven roller straightener

It is important to use the minimum roll penetration which produces an acceptable level of flatness. Excessive penetration of the roller causes poor straightener efficiency, cause material to slip across the straightener, and place unnecessary strain on the machine drive components. A quick visual check of the flatness can be done before the material is fed to the straightener.

Capacity of a straightener 

A powerful straightener owns strong ability to  level  the  extensive range  of material, but the capacity of a specific straightener with the definite structure and power is limited. The capacity of a straightener is influenced by the following factors.

Plastic ratio – The straightening process is very complicated with the elastic-plastic deformation in the material.  The percentage the thickness of the plastic deformation which accounts for the whole thickness is called the plastic ratio. The plastic ratio ‘Rp’ is given by the equation Rp = (2Tp/T) x 100, where Tp is the thickness of the plastic region and T is the total thickness of the material. Overstretch (S) normally reflect the plastic ratio in the engineering. It is defined as S = 1/(1-Rp). The plastic ratio is an important parameter in the process of straightening. The quality of the material can be improved well if Rp increases to around 70 % to 80 % (S = 3.3 to 5). The material cannot be straightened well if the plastic ratio is less than the expected value even though it is bent around  the  rollers (r = D/2) where ‘r’ is the bending radius of the material and ‘D’ is the diameter of the roller (Fig 3).

Fig 3 Representation of plastic ratio and overstretch

Maximum straightening force – The force acting during the process of straightening follows the principle of beam bending. The maximum force is a constant value and it is dependent on the design of the straightener. Hence, the straightening capacity of a straightener is constrained by its maximum force and its structure.  The straightening forces, as shown in the Fig 4, are generated by the rollers with the the action of the roller gap.  In the figure, number of rollers in the straightening machine has been considered as ’n’. The forces can be calculated by the the moment equation of a multi-supported beam.

Fig 4 Principle of beam bending and the forces acting during straightening

Total motor power – Similar to the maximum straightening force, the total motor power is also a constant for the straightener. The torque which the transmission system is required to overcome includes the friction resisting moment at the roller journal, the friction resisting moment between the roller and the material, and the plastic deformation resisting moment.

Important aspects of a straightener

For the capacity of a straightener, the important aspects are (i) the greater is the plastic ratio is, the narrower is the straightening range, (ii) the larger is the straightening speed, the narrower is the range in the same overstretch condition, (iii) the straightening capacity decreases to some degree when the strength of the material increases, (iv) the larger is the elastic module , the larger is the straightening capacity, and (v) the higher is the material width, the smaller is the straightening capacity.

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