A correct straightening process set-up depends on an analysis of the straightened material production process as well as on an analysis of the product to be straightened and an analysis of the final product.

A product's material characteristics are constantly affected from the production of the original material, e.g. its melting, to the final processing operation. The history of these effects adds up to the material's memory.

Each individual process creates stresses in the product. Incorrect handling of the product to be straightened has a negative impact on the constantly changing state of internal stress, whereas its correct handling helps to prevent additional stress factors, keeping the internal stress conditions practically constant throughout the processing.

The stress factors active during the production of a straightened material can be divided into “avoidable” and “unavoidable” stresses. Needless to say, it is far easier to design a constant straightening process and thus achieve continuously better straightening results and hence final products of reproducible quality if steps are taken to prevent the avoidable stresses.

Processes with unavoidable stress factors should be followed as directly as possible by corrective measures in order to reduce or neutralise the stresses. If not, the stresses will be increased by a multiple.

The only way to obtain constant conditions of internal stress is with selective corrective measures. Uniform directions of rotation and defined deflections with unchanged curvature courses ultimately lead to constant input parameters for the resulting controlled straightening process.

A correct straightening process set-up depends on an analysis of the product to be straightened as well as on an analysis of the straightened material production process and an analysis of the final product.

In an analysis of the product to be straightened conducted in connection with setting up the correct straightening process, efforts are first concentrated on recording the product's geometrical parameters, regardless of the material parameters which need to be determined.

Basically speaking, the product remains in constant directions and zones of curvature during the most diverse process transitions in the course of its production. It is important to scan and measure these parameters:

1. Constant curvature on one plane
2. Curvature zone on only one curvature plane
3. Curvature zone with a pronounced increasing or decreasing radial offset on only one curvature plane
4. Constant curvature and or curvature zone on the same or different curvature planes (helicities)

Case 1 provides good conditions for a constant follow-up process. All the other cases require a more elaborate follow-up process with what are sometimes less constant results. Each non-constant process input will give rise to different outputs, no matter how elaborate the applied corrective measures are.

The use of so-called helix straighteners or killing-straighteners can exert only a limited effect on the quality of the final product.

An analysis of the product to be straightened should include a definition of the straightened material parameter. 

We understand the automation of straightening to be the use of offline and online data, a basic automation system, at least one straightener, a geometry operator, a stretching limit operator and an adjustment operator. Information flow takes place in a loop using all the previously mentioned elements and is aimed at the automatic positioning of the straightening rolls with due consideration of the production, product and process data identified online and of the technology, equipment, product and quality data determined offline. Automatic straightening permits a straightened product to be manufactured in the quality required, regardless of any fluctuations in diameter and stretching limit over the length of the material to be straightened. 

A bend is the strain produced in long and endless bodies by external forces, resulting in internal bending moments over their cross sections. The original straightness or curvature of the bodies is changed either elastically or plastically as the result. The internal bending moments arising in every cross section as a reaction to these external bending moments lead to tensile and compressive forces, resulting in an equilibrium of forces and moments.

Bodies subjected to bending are shortened on the one side by compressive stresses while on the other side they are lengthened by tensile forces. There is a (practically Iinear) continuous transition from tensile stress to compressive stress over the cross section, with a neutral fiber that is neither stretched nor compressed and therefore stress-free.

The diagram opposite shows a wire cross section with an applied external bending moment and the resultant strain and stress characteristic.

A caliber is a measuring instrument used as a gauge. In steel mills, gauges are used to determine the rolling gap. On straightening units, calibers are used to fix the distance between the straightening rolls.

When a caliber is placed between the open straightening rolls of a straightener, all the adjustable straightening rolls can be aligned on one line. The straightening rolls of the one row are thus parallel to the straightening rolls of the second row. It is then possible to set the zero line for the particular product to be straightened. The adjustable rolls are adjusted by an amount equal to the difference between the thickness of the caliber and the thickness of the product to be straightened. It should be note that the adjusting travel is also affected by the geometry of the product to be straightened and the geometry of the groove. Adjustment positions are reproducible at any time.

WICAS® - WITELS CASSETTE STRAIGHTENING SYSTEM

The rolling elements mainly used in straightening processes with roll-type straighteners suffer extreme wear as the result of the high roll adjusting forces and ever higher production speeds. At the same time, metal particles contaminate the bearings and contribute likewise to a drastic shortening of rolling element life. The WITELS Cassette Straightening System called WICAS(r) was developed specifically to improve this situation.

Notable features of WICAS®:

A) Expert installation of the rolling elements in a closed housing with an additional labyrinth seal prevents practically all soiling of the bearing system, resulting in a useful life up to 30 times longer than that of a conventional straightening roll.

B) The straightening sheave can now be produced in any form, type, quality and strength irrespective of the material used for the bearing system. Oval pressing of the rolling element outer ring, against which the product to be straightened runs, is ruled out with this design.

C) The use of smaller rolling bearings at high speeds of rotation increases the peripheral speed of the larger straightening sheave. Far higher production speeds are thus possible than with conventional straightening rolls.

D) In the case of miniature straightening rolls, cutting the groove in the relatively thin outer rings weakens the straightening roll to an unacceptable degree. This does not happen with WICAS(r) because the bearing shaft as well as the straightening sheave can be made with annular grooves of any shape and size.

E) Flexible use of needle and ball bearings gives the designer considerable scope with regards to the static and dynamic bearing values.

F) It is possible to use bearings with life-time lubrication as well as systems designed for relubrication.

The above-mentioned 30-fold longer useful life of WICAS(r) compared to conventional straightening rolls greatly minimizes the cost-intensive stoppage of individual machines and complete production lines - a benefit that should not be overlooked. 

For logistical reasons, products to be straightened (wire, strip, rope, etc.) are wound into coils. This entails laying the product, winding for winding, in the direction of the coil axis. If the coil is not rotated simultaneously during the laying, helicities will be produced with each winding. The product will then behave accordingly when it is pulled off. Pulling off the product statically in the direction of the coil axis will cause it to twist and the quality of the straightened product will be negatively affected by additional helicities. These curvature fluctuations and helicities are difficult to eliminate and are therefore best avoided in the first place.

The beginning and end of a coil should always be marked in order to ensure its careful and correct further processing. Constriction-free securing of the ends prevents snap-back and avoids kinks.

Curvature is understood to be a deviation from the straight.

When an endless material is wound onto a spool, the inner layers have a smaller radius of curvature and the outer layers a larger radius of curvature.

Whether or not a winding is elastic or plastic depends on the spool geometry, the properties of the endless material, and the degree of deformation.

Curvature is the reciprocal of the radius of curvature and has the dimension [1/mm].

The decoil-straightener is a combination of a decoiling/unspooling unit and a straightening unit used in conjunction with a material store. The endless material to be straightened is decoiled/unspooled from a coil/spool and directed to the straightening unit via several deflections (store) and a bending roll, always maintaining constant directions of deflection and curvature. Apart from the bending of the material around the bending roll, all the deflections around the pairs of pulleys are performed only elastically.

The coil/spool is rotatably mounted to enable the material to be paid off dynamically (preventing additional torsional stresses and helicities such as occur with static pay-off set-ups). The coil/spool is driven by an electric motor whose speed is controlled in accordance with the dancer position. A sensor scans the dancer position (height), producing an electric signal which is evaluated by a controller and relayed to the motor electronics.

The pull-off force acting on the material to be straightened is constant, consisting only of the straightening force and a variable dancer weight. Additional tensile forces caused by acceleration and deceleration, particularly in discontinuous processes, are minimized by the store function.

Equipped with the right mounting arrangement, users can change over quickly between various straightening devices, helix straighteners and killing-straighteners to suit the different types of material to be straightened. An additional stretch-bend-straightening effect can be produced by using a bending roll with brake, around which the material can wrap itself at least once. To produce a higher tensile stress in the material, on the other hand, a bending roll with brake should be placed in front of the straightening devices. The straightening unit normally takes the form of a straightening system with two straighteners.

The deflection h of a product to be straightened is the maximal distance between the chord a and the arc of the product. The deflection is expressed as mm/reference length.

The permissible deflection values for bars and wires are specified by DIN EN 10 21 8-2.

Endless material must not only be straightened, it must also be transported, pulled or pushed.
Generally this is the job of the actual processing machine. Where a process is divided into various steps, drive units may be integrated upstream and downstream from the process. Dividing the process into several steps also helps to reduce process stress.

Drive units are feeding devices which clamp the material to be pulled or pushed between two or more pairs of rolls or caterpillars, using motor power to apply a tensile force. Any type of controlled or uncontrolled motor may be used according to the application. The contact force of the drive rolls or caterpillars is applied pneumatically, hydraulically, by electric motor, or manually. The level of contact force depends on the tensile force needed to move the material by frictional locking. On the one hand it must be selected high enough to move the material without slippage. On the other hand it must not be too high in order to prevent plastic deformations and surface damage.

Drive rolls are generally made of hardened steel, although other materials such as plastic, Vulkollan(r) or the like are possible. Roll profiling depends on the geometrical shapes of the materials to be transported.

Users are also free to specify any V-belt coating from the range on offer.

Different drive units have been developed for various areas of application: 

• one driven pair of rolls with unilateral bearing  NA
• one driven pair of rolls with bilateral bearing NAB
• two driven pairs of rolls with unilateral bearing NAD,NADV
• two driven pairs of rolls with bilateral bearing  NADB
• two drive belts with unilateral bearing NAK

Drive units are easily combined with other components, e.g. straighteners, roll-type guides, hydraulic units and pneumatic units. The resulting machine modules support existing production lines.  

External friction is the inhibition of relative movement between touching bodies. In the case of solid bodies, friction is mainly based on microscopic unevenness. The frictional values of different materials relative to each requires special attention in this connection. Internal friction is the friction between parts of the same body. In liquids this friction is called viscosity.

During the straightening process, friction occurs mainly in the straightening rolls. It consists of the following components, which together add up to the total running resistance: rolling and sliding friction of the rolling elements and cages relative to each other, lubricant friction and seal friction. The aim must be to keep the friction as low as possible by ensuring an acceptable bearing load and optimal lubrication.

Friction leads to “losses”, i.e. kinetic energy is converted into frictional energy. This energy fraction is therefore no longer available for use in the production process.

A wire wound into a coil or onto a spool should have only one curvature - that of the spool or coil - in one plane if the follow-up process is to proceed without difficulty. Changing preliminary processes with sometimes inconstant process transitions create helicities, i.e. twisting, in the wire.

Helicities need to be avoided or, where constant helicities are concerned, eliminated by means of a helix straightener. A helix straightener can produce as well as remove helicities.

Like a standard straightener, a helix straightener has several straightening rolls which are partly or fully arranged in familiar manner so that they can be adjusted relative to each other.

In addition to this, one or more straightening rolls can be adjusted in height. The groove of the height-adjusted straightening roll thus forces the wire unilaterally from its zero line, bending it with unequal flank pressure into a second curvature plane. Helicities are thus removed or created by force-feeding the wire through these adjustable positions.

In production processes such as casting, rolling and drawing, the outer shape of a workpiece is preserved only if it is adjusted and clamped before the process is started so that any twisting of the workpiece is prevented. Alternatively, the workpiece has to be straightened again at the end of the process. When the external loading is discontinued and the temperature compensation completed, the workpiece adopts an intrinsic shape which continues to exist without the application of any external forces and moments. The only explanation for this condition is that the sum total of internal forces and moments, i.e. internal stresses, is in equilibrium.

An analysis of the longitudinal internal stresses in a drawn wire reveals tensile stresses in the core and compressive stresses at the edge. Any change of a wire's parameters, such as curvature or helicity, over its length will result in changes of its internal stresses. Straightening has been shown to change the internal stresses so that the internal stress conditions existing in the material in the unstraightened state are eliminated. The magnitude and distribution of internal stresses existing after the straightening process depends on the design of the straightening system and particularly on the adjustment of the straightening rolls. With technology from WITELS-ALBERT it is possible to minimize the internal stresses arising during the straightening process while leaving the straightened material in optimal shape.

The more precisely the operating conditions of a straightening roll are known or can be calculated, the more exactly and reliably its useful life can be determined.

Three different equations are available: for nominal life-time according to ISO, for modified nominal life according to ISO, and for modified nominal life-time based on a new life theory.

The simplest approach is to determine the nominal life-time according to ISO using the relevant equation.

Calculation of the nominal life-time using the equation is sufficient for conventional bearing applications.

In other applications it may be wise to take detailed account of other life-affecting factors as well. This led in 1977 to the introduction of the modified life-time equations.

In a third equation - modified nominal life-time according to the new SKF life theory - a life-time coefficient is added to take account of a fatigue load limit and various other factors affecting the lubricating conditions and the degree of soiling.

The life-time value calculated with the third equation is regarded as the period of use actually covered by a straightening roll in its life-time. As a rule this value deviates slightly from the calculated nominal life-time.

Mechanical material characteristics describe a process material and largely determine its forming behavior. The modulus of elasticity, elongation or stretching limit, modulus of hardening, carbon content and the parameters of alternating forming used to derive the Bauschinger effect are the most important characteristics in connection with elastic-plastic cold forming.

For considerations and calculations concerning the product to be straightened it is an advantage to set up a mechanical model (a level substitute model) of the real straightening process. It is then possible to make (simplified) calculations and predictions.

The definition of straightening refers to three straightening rolls as a straightening triangle. The product to be straightened can be idealized as an end-fixed beam. In addition to the forces applied, bending moments (clamping moments M) are transmitted via both supports. As these forces and moments are not equal to zero, the result is a three-component statically indeterminate system. Mechanical variables are a central force (corresponding to the straightening force), two vertical supporting forces (FV), two horizontal longitudinal forces (FH) and two clamping moments. The product to be straightened bends as the result of the applied straightening force (FR).

Three main factors have an influence on the number of straightening rolls: the initial curvature range, the pemissible final curvature (i.e. the tolerance of the final product), and material parameters such as the stretching limit, the modulus of elasticity, internal stresses and hardening.

Four simple and intelligible rules of thumb can be derived accordingly:

• The bigger the initial curvature range, the higher the number of rolls.
• A constant initial curvature requires few rolls.
• The higher the strength, the higher the number of rolls.
• The softer the material, the lower the number of rolls.

A further two rules can be derived from these four rules of thumb:

• The higher the number of rolls, the flatter the angle of straightening conicity and the less aggressive the curvature course during straightening. This applies in particular to material with rising straightened material parameter.
• The lower the number of rolls, the steeper the angle of straightening conicity and the more aggressive the curvature course during straightening. This applies to material with low material parameters.

As a general rule, the final quality of a straightened product (decrease of the final curvature range) increases with the number of rolls (alternate bends).

The right strategy - not only in terms of the environment and energy conservation - is “as little as
necessary” rather than “as much as possible”.

A postforming apparatus can be compared to a straightener in that its design is largely similar. Unlike a straightener, however, it is used solely to form process multi-wire type materials such as strands or ropes. The forming is called postforming because the apparatuses, which can be grouped together in postforming systems, are usually positioned downstream from the stranding point on the stranding line. Upstream from the stranding point the material is usually subjected to preforming by preforming heads. Both the preforming and postforming operations are performed for the express purpose of minimizing the recovery efforts of individual elements in the strand or rope structure as well as the internal stresses of the final product, which correlate with the forming conditions.

Straightening forces and tensile forces are the process forces of relevance during straightening. Straightening forces are understood to be the forces of reaction arising at the interface between the straightening roll and the product to be straightened and owed to the bending moments existing in the process material during forming. Straightening forces act in various directions and magnitudes according to the geometrical boundary conditions. Of the many proposals for calculating the straightening forces (Zelikow, Geleji and others), the approach proposed by Guericke has proven expedient in practice. As his central instrument Guericke uses the bending moment/curvature hysteresis, which incorporates all the relevant factors affecting the straightening process. The straightening forces correlate with the tensile forces which, affected by the boundary conditions, arise as forward tensile force or pull-off force and reverse tensile force.

The work of plastic deformation performed in the specific case has to be determined in order to calculate the forward tensile force or pull-off force needed to transport a process material relative to the straightening system.

If further technological operations are positioned upstream and/or downstream from the straightening process, the tensile force fractions during the individual operations have to be taken into account when determining the total pull-off force. It is an advantage to ensure as constant a tensile force as possible in the direct vicinity of the processing in order to achieve a final product of high quality on a processing line. This can be promoted by using a decoil-straightener.

All straightening results, including the final curvatures of the material to be straightened, ultimately depend on the correct positions of the straightening rolls. It is vital, therefore, for the straightening process to display constant reproducibility.

In addition to being a precondition for constant results, reproducibility also minimizes unnecessary line stoppage times and it wastes less material during repeat set-ups. High speed and continuous availability also play a major role in minimizing costs.

Existing possibilities of achieving reproducibility differ in their use of reference points on the adjustable elements of a straightener and in the methods used to determine actual positions.

The most commonly used possibilities are:

             1)      Engraved veneer scales
             2)      Dial gauges
             3)      Scales
             4)      Screws; hex screws, slotted screws etc.
             5)      Knurled screws
             6)      Micrometer screws
             7)      Veneer scales on buttons
             8)      Collar screws with veneer scales
             9)      Machine knobs/tapered machine handles
           10)      Handwheels
           11)      Geared motors / stepping motors
           12)      Hydraulic / pneumatic cylinders
           13)      Automatic roll positioners

A solution is chosen which best meets the demands of local conditions and technical handling criteria. A combination of two of the above options is also possible, in which case the functions are divided into adjustment and position measurement.

Whatever option is selected it should not be forgotten that continuous documentation of the adjustment values is essential if reproducibility is to be achieved as planned.

Roller crosses/guides are used in the processing of endless material between the spool or coil, straightener, pull-off unit or processing machine. Their function is to guide and support the product to be processed.

The number and the arrangement of the roller crosses/guides in the material flow system are selected so that

• sagging of the material is prevented,
• sharp edges and deflections with small radii do not damage the surface of the material,
• the endless material enters the next apparatus or machine in central position.

The diversity of roll guides results in a wide range of different applications. Which roll guide is selected will depend on the particular requirements. Roll guides are available as rigid and adjustable types. Adjustable roll guides are used when dimensions change and when exact positioning of the material is essential.

Centrally adjustable roll guides are suitable for a range of dimensions. The center of the material is retained even when dimensions change. The material itself does not have to be adjusted for it to enter the line centrally.

Some roll guides are designed to completely enclose the material while they guide it. Others are open on one side for better insertion of the material.

Roll diameter, roll material and clear widths depend on the dimensions and geometry of the material.
Burnished and hardened steel rolls are used e.g. for firm materials. Softer rolls made of PETP can be used for sensitive surfaces. The following basic types of roll are available:

• with chromium-plating
• hardened (up to 64 HRC)
• with rubber coating
• with ceramic coating
• rolls made of PETP, PVC or polyamide

Semi-automatic straightening is straightening with the use of offline and online data, basic automation equipment and at least one straightener. The offline and online data is relayed via the basic automation equipment to the straightener in order to achieve a defined and highly exact adjustment of the straightening rolls. A semi-automatic straightening system enables the reproducible adjustment of roller positions at any time.

The advantages of a semi-automatic straightener over a straightener with conventional roll adjustment are:

• defined fine adjustment of the individual straightening rolls,
• reproducibility of straightening roller positions within close tolerances,
• application of high adjustment forces,
• operation of the roll adjustment system from any location,
• use of process data (data bases),
• integration in a central control environment.

In the forming processes employed in the wire producing and wire processing industry it is normal for the process material or workpiece to move relative to the tooling. Forming speed is one of the variables which characterize a forming process. It is derived from the degree of forming as a function of time.

Exemplary straightening tests conducted at different speeds of the process have proven that the final curvature and other parameters of the straightening process do not change significantly in the speed range up to approximately 10 m/s. At speeds above this limit, an identical adjustment of the straightening rolls results in a different deformation.

Latin for "the existing state".
To be able to design a straightening process in a defined manner it is first vital to identify the status quo as the starting point for producing the necessary change of state with the bending operations and tools available. For a reproducible straightening process it is essential, for example, to document the positions of the system's straightening rolls relative to the process material. This data can then be used at any time to produce specific straightening results under identical framework conditions.

Straightened material parameters describe the characteristics of the straightened material. They can be classified in three groups:

• Parameters of the material
• Parameters of the cross sectional geometry
• Parameters of the initial curvature

The straightened material parameters have an influence on the type, design and size of:

• Straighteners and straightening systems
• Drive units
• Guides
• Preforming heads
• Postforming apparatus

Rather than describe the straightener as a tool and its function, we want to draw attention here to its precise and modular design.

The “A” parts of a straightener's design, i.e. its basic load-bearing elements, are adapted in size to the number of rolls to be accommodated and serve as mounts for the “B” parts such as rails, grooved blocks, threaded bolts, pins and other standard parts.

The final block of this modular design are the straightening rolls. They are also the parts exposed to the greatest wear.

An approach based on the ideas of environmental management gives rise to a product and production engineering of outstanding accuracy, impressive ruggedness and durability, and
unique operability and results.

A correct straightening process set-up depends on an analysis of the product to be straightened, an analysis of the straightened material production process, and an analysis of the final product. A specific procedure has to be followed in order to choose the correct straightening system, the correct number of straightening rolls, and the correct location of the one or more straighteners.

The purpose in setting up a straightening process is to determine the status quo from the essential analyses and to define the production steps and corresponding equipment needed to obtain the final product. Using our knowledge of the process we can create constant process transitions, i.e. output and input parameters, in order to produce constant final results.

Constant and reproducible straightening roll settings based on the constant input parameters known to us at the beginning of the straightening process result in a constant final curvature.

If all these aspects are duly considered, the straightener will provide optimal processing conditions for obtaining a final product of the required quality.

The profile cut into the outer ring of a straightening roll is called the straightening roll groove. Its purpose is to accommodate and guide the product to be straightened.

Angular grooves of 90°, 100° or 110° are normal for round, solid material. They are particularly advantageous for straightening ranges, i.e. wire of different diameters.

Profiled, soft or tubular material, on the other hand, is straightened in grooves which are adapted to the material to be straightened to produce a non-slip fit. Smooth, unprofiled rolls are used to flatten strip material.

A straightening system is understood to be a combination of straighteners which, duly adapted to the product to be straightened, transform difficult and sometimes alternating initial material parameters into constant values.

The constant process transition thus obtained is essential for designing controlled follow-up processes.

The simplest straightening system generally consists of two individual straighteners arranged in different planes and mounted using a connecting bracket. Acceptable, constant results are achieved in many applications using a simple straightening system of this type.

If the initial parameters of the product to be straightened change during the process and if, for example, curvature fluctuations or helicities arise, then it will be necessary to use problem-related tools and jigs such as helix straighteners, killing-straighteners, etc.

A straightening system set-up consists accordingly of a clearly structured straightening process sequence. Fluctuating initial parameters require greater structuring of the straightening process to achieve a defined and constant final curvature. Simply increasing the number of straightening rolls would have little effect in this case.

The first priority is to transform changing curvatures in different planes into a constant curvature in a single plane. This is the essence of a successful straightening process with constant final curvatures.

The term “stretch-bend-straightening” is understood to be the bending of a material around rolls of small diameter with the simultaneous application of tensile stresses. Local plastic elongation occurs in those areas where bending tensile stresses are added to tensile stresses. Upsetting can occur, on the other hand, where the bending compressive stresses superimpose on the tensile stresses. However, the biggest part of the material's cross section is subjected to a tensile load. If such a process is performed by alternate bending, the product is stretched in steps, producing a final curvature in wire material and a flatness profile in strip material.

A stretch-bend-straightening system generally consists of an infeed unit for producing a reverse tension, a straightening unit for the bending or alternate bending, and an outfeed unit for applying a forward tension. For the purpose of straightening wire, the infeed unit can be e.g. a killing-straightener and the outfeed unit can be a drive unit. The straightening unit is either a conventional straightening system, a semi-automatic straightening system or a straightening system with an automatic roll positioner.

The stretching limit is a characteristic material parameter which can be determined in a tensile test. It is the stress defined as the internal force of resistance per unit of area perpendicular to the direction of load application with which a material reacts to an external tensile load of specific magnitude. At the stretching limit the deformation of the workpiece or the elongation of the tensile specimen due to the external tensile load continues to increase although the inner resistance remains constant or even decreases.

For process materials in the wire industry, where the materials used are preferable drawn wires, it is typical for the resistance to the load not to decrease at the stretching limit. Hence the stretching limit cannot be identified in the stress/strain diagram. DIN EN 10 002, the tensile test guideline for metallic materials, thus recommends determining an elongation limit at a non-proportional strain. Accordingly it is no longer the stretching limit, which is determined for drawn materials in the wire industry, but the elongation limit at a non-proportional strain of 0.2%. This is also referred to as the technical elongation limit.

The stretching limit or elongation limit at a non-proportional strain is a value used in the simulation of the WITELS-ALBERT wire straightening process for calculating the adjustments required to achieve a defined final curvature.

Tensile strength is a characteristic material parameter which can be derived form a tensile test. It is the stress defined as the internal force of resistance per unit of area perpendicular to the direction of load application with which a material reacts to an external tensile load of specific magnitude. At a material's tensile strength the deformation of the workpiece or the elongation of the tensile specimen due to the external tensile load continues to increase while only the inner resistance decreases in relation to the cross sectional area prior to the loading. The visual result of the tensile test is that the tensile specimen constricts locally at the material's tensile strength, whereas up to this point it stretched over its entire length. This constriction is the reason for the specimen's failure by breakage if subjected to further loading.

Tensile strength is derived from the quotient of the maximal load and the cross sectional area prior to loading. A material's tensile strength thus denotes its maximal endurable stress.

Basically the methods used to unspool or unwind a material which is to be straightened break down into two categories: static or dynamic.

With the static method the spool or reel is not moved. Material is paid off in axial direction so that a helix or twist is created in each winding as it is removed, with negative consequences for the follow-up process.

With the dynamic method of pay-off the spool or reel is rotated around its axis, which may be arranged vertically or horizontally, enabling the material to be paid off without suffering any torsion.

Windings are at risk of slipping only in vertical unspooling or unreeling set-ups, particularly in intermittent processes. A horizontal pay-off may be more elaborate technically, but it is the better solution with a view to follow-up processing operations.