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:
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.