In addition to the cross-sectional size and shape of the product, factors which affect roll design are steel strip thickness and width tolerance, steel grade, strip edge condition, required finish and accuracy, and the anticipated production volume. All these items must be finalised before detail work is attempted. Basic design begins with an assessment of the required number of forming stages (driven rolls and idlers) and individual configurations.
In order to visualise the progress from flat strip to final shape, intermediate shapes can be drawn superimposed, whereby one obtains flower diagrams such as shown in Figure 1
Detailed consideration must be given to the inward flow of material between the forming stages. Too sudden a transition from one shaped stage to the next can cause longitudinal stretching of the material. This can introduce residual stresses and give rise to a bow or twist in the final product.
The material must be guided laterally throughout the roll-former. Initial location and subsequent guiding are provided by the entry guide, but additional guides may be required between some forming stages. Further guiding is generally carried out by the forming rolls themselves, as the portion of section already formed can provide an adequate lateral location in subsequent stages. Guiding
can also be achieved by provision of locks on the periphery of rolls as shown as Figure 2.
Rolls with locks are not popular among manufacturers as they are more costly to make and require strip feed slit to close tolerances. Mill edge hot-rolled strip with a very wide width tolerance for instance, is quite unsuitable for guiding by roll locks. Where a feed with a wide width tolerance must be accommodated, the best solution is a self-centring entry guide and rolls designed to provide adequate guiding properties. Ideally the peripheral speed of rolls closely equals the speed of the material advancing through the roll-former (roll-former speed).
However, as the roll diameters vary across their width to suit the contour of each forming stage, the peripheral speed of rolls is not constant and thus cannot be made equal to the roll-forming speed at each point. Rather, an average or nominal pitch roll diameter only can be matched with the material speed, and the designer’s task is to determine such a diameter for each pair of rolls. As a guide, the pitch diameter can generally be taken as that of the centroid of profile. Rolls for deep profiles have a great discrepancy between pitch diameter and maximum or minimum working roll diameter. This results in a substantial speed differential between working surfaces, associated with heavy rubbing which can give rise to product scuffing and/or excessive roll wear. One remedy is good lubrication at the critical spots (pump propelled jets of coolant/lubricant are commonly used). Other methods include felt wicks, and drip lubrication. Where possible lubricants should be used sparingly. A number of idler rolls on suitably angled spindles or individual rolls mounted on bearings can also be used to reduce scuffing. Any idler roll adjusts itself to the optimum speed automatically and its mean diameter is not critical. Sometimes rubbing can be minimised by suitable orientation of the product shape to reduce maximum roll diameter, and consequently, differential speeds between surfaces.