WITELS-ALBERT GmbH

Simulation is the representation of specific interesting properties of a system by the actions of a different system. By copying the behavior of a system in a simulation program it is possible to study behavioral characteristics or to examine variants. Simulation can be used at low cost and with little risk to identify behavioral patterns in time-consuming, costly, uncertain and hazardous processes.

With a simulation of the wire straightening process it is possible to pre-calculate the positions of the straightening rolls needed to prcduce a defined final curvature while taking due account of the parameters of the straightener (number of rolls, external roll diameter, roll pitch, groove width, groove angle) and of the wire (cross sectional geometry, elongation limit, modulus of elasticity, modulus of hardening, initial curvature, etc.). The simulation is based on a model of the elastic-plastic material characteristic under alternate bending and on the relationship between bending moment and curvature. Simulation of the wire straightening process also supplies data which can be used to determine the process forces.

Spools are used for the safe transportation of endless material. The material is wound onto the spool layer by layer so that it is difficult for individual windings to slip. Spools have a minimal (inner) and a maximal (outer) diameter. This has two important consequences: a difference in speed and a difference in curvatures.

Straightened material is a workpiece whose properties have been changed by the straightening process. Its most important property is straightness or final curvature. Other properties, apart from straightness, changed by the straightening process include the material's internal stress condition.

A straightened material can be endless or finite. Rails and sheets are examples of finite straightened material. Wire, cable and rope are endless straightened material.

The range of accessories available for a straightener or straightenrng system can be classified according to

• application, size and surroundings,
• safety and operating reliability
• handling and use,
• design,
• maintenance.

For each of these categories WITELS-ALBERT offers a large selection of elements and devices which are well described in the company's documentation.

The handling of a straightener, for example, largely depends on the technical elements provided to position the straightening rolls. In addition to the conventional roller positioning method using an adjusting screw it is possible to use a micrometer screw, an adjusting spindle with knurled edge, or an actuator (hydraulic cylinder, pneumatic cylinder, stepping motor or servo motor). 

The commonly held idea that the position of a straightener's body corresponds to the straightening plane is wrong.

Basically, the straightening plane is defined by the axial position of the straightening rolls. If this is horizontal, then the straightening plane is horizontal. The same applies to the curvature plane in which the product to be straightened is fed in or bent.

With a vertical axis we speak of a vertical straightening and curvature plane.

The most frequently used arrangement is the double straightener, which eliminates the initial curvatures in two planes (horizontal/vertical).

You may well ask what happens with the other curvature planes lying outside the two straightening planes? Their processing is possible at best on a random and sporadic basis or not at all.

Hence it is important when producing wire to maintain a constant curvature course and not to leave the curvature plane.

A constant straightening plane can only produce constant final curvatures from constant curvatures in the same plane.

Where this is impossible for whatever reason, the use of killing-straightener, and helix straighteners in straightening systems can prove very useful.

A straightening roll is the last link in a straightener's modular design chain and the component exposed to the greatest wear. Its design, production and use thus deserves special attention.

The following criteria are particularly important:

A) Materials
The raceway of a straightening roll is subjected during roll-over to local loads which find
expression in high Hertzian surface pressures. The raceway material should be selected so that
ideally through-hardening but at least the necessary depth of hardness and a surface hardness
of 670-840 HV is achieved. The material normally used is through-hardening steel according to
DIN 17230, e.g. 100 Cr 6.

B) Bearing arrangement design
The bearing arrangement for a straightening roll is that of a locating bearing. The shaft or pin end provides radial support as well as axial guidance. Radial bearings able to absorb combined loads, e.g. radial ball bearings, angular contact ball bearings or track rollers, are suitable for use as straightening rolls. To enable the bearing's load capacity to be used to the full, the cylindrical contact faces need to be firmly and uniformly supported over their entire circumference and across the full width of the raceway. Perfect radial mounting is also vital.

C) Dynamic/static load rating
A straightening roll's dynamic load capacity depends on the fatigue behavior of its material. Fatigue time is an expression of useful life and depends on the straightening roll's loading and speed as well as on the statistical probability and random timing of the first incident of damage. Static load capacity, on the other hand, is limited by the plastic deformations produced in the raceways and rolling elements by a high resting load.

D) Bearing capacity and life-time
The size of bearing required for a specific straightening process depends firstly on the straightening range of the straightening roll. A further factor, however, is the magnitude of loading. This is expressed by the dynamic load rating C and the static load rating Co. A straightening roll's useful life is defined as the number of rotations made by its bearing before the first signs of material fatigue are noted on a raceway or rolling element.

E) Friction, speeds, and temperature
A straightening roll's frictional moment depends on many factors such as load, speed, condition of lubrication and seal friction. The maximum possible speed of a straightening roll is mainly defined by the permissible operating temperature of the rolling bearing. In other words, speed depends on the type of load, the conditions of lubrication and the conditions of cooling. A straightening roll temperature of 70°C is considered normal.

F) Dimensional, geometrical and bearing tolerances
A straightening roll's dimensional, geometrical and bearing tolerances comply with tolerance class PN conforming to DIN 620.

G) Installation and dismantling
The smooth and troublefree operation of Witels straightening rolls largely depends on the care with which they are installed and replaced. Inner rings have to be mounted on the shaft or pin so that the mounting force is evenly distributed over the front face of the inner ring. The mounting force must not be transferred to the rolling elements.

H) Lubrication, maintenance and corrosion protection
For straightening rolls to perform reliably they require sufficient lubrication to prevent direct metallic contact between the rolling elements, raceways and cage while at the same time reducing wear and protecting surfaces from corrosion. All straightening rolls are supplied with grease lubrication as standard.

J) Storage of straightening rolls
Although Witels straightening rolls are always delivered with life-time lubrication, they cannot be stored indefinitely. Under certain circumstances a straightening roll kept in storage for a long period may initially display higher frictional moments than one fresh from the factory. Nor is it possible to rule out deterioration or even gumming of the grease charge in the course of prolonged storage. Attention is drawn in this connection to the WlCAS cassette system, which finds use mainly in problematic applications for straightening rolls.

The dynamic load capacity of a straightening roll depends on its material fatigue characteristics and wear. Useful life depends on the straightening roll's loading, speed, conditions of lubrication, soiling, installation and temperature, the statistical probability of the first incidence of damage, and diverse other factors. All too often the impact of dirt is underestimated.

The load capacity and useful life of straightening rolls from Witels are determined by the methods customary in rolling bearing practice. At the same time allowance is made for the fact that the outer ring undergoes elastic deformation through contact with the product to be straightened. Compared to a rolling bearing supported in a housing bore, there is a different distribution of load in the bearing and bending stress in the outer ring. We differentiate between internal and external factors with an influence on wear.

Contact between the product to be straightened and the outer ring results in the following external factors:

• material of the outer ring
• material and surface consistency of the product to be straightened
• speed
• slip
• soiling and lubrication

Internal factors with an influence on straightening roll wear are:

• lubrication of the bearing
• type of bearing
• dynamic and static bearing load
• speed
• seal arrangement
• elastic deformation

The term “straightening” covers the actions and measures needed to eliminate process-induced curvatures in the process material in order to produce a condition of straightness, levelness or defined curvature

In the stretch-straightening process the product to be straightened is subjected to a tensile force. This results in a tensile stress inside the workpiece, which superimposes on the existing internal stresses.

The tensile force is applied in sections and at a level of magnitude designed to produce plastic deformations in the material to be straightened.