Innovative Stainless Steel Applications in transport ... - Euro Inox

A “three-roll” (pyramid-type) bender was chosen for bending tubes with a cross-section
of 50 × 20 × 1 mm and 3400-mm bending radius – which is close to the roof radius of
rail carriages, in which curved sandwich panels can be probably used (Figure 56). The
BUS project also studied the draw-bending method and, especially, springback.
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Figure 56. The three-roll tube-bending
machine used in the DOLTRAC experiments.
The actual bending characteristics of a material will be affected by the size and crosssection
of the tube being bent, the physical properties of the material, the tooling used
and the machine set-up used to make the bend. The many parameters involved make it
very difficult to predict the springback that will occur in a bending operation. A model
based on experimental data is therefore usually required.
4.2.2 Springback model
The common model used for the bending characteristics of tubes assumes that there are
two components to springback: a constant value, which is independent of the bend angle
and is expressed in degrees, and a proportional component, which is expressed as a
percentage of the bend angle. Expressed mathematically, this becomes:
Target angle = ((1+PS) × Desired angle) + CS, (2)
where PS is the proportional factor and CS the constant factor.
All bending tests were carried out using a 3-roll (pyramid-type) bender, where the tube
is fed between three rollers – the central one of which is usually a drive roller and the
other two either powered or idlers (Figure 57).