What is pre-bending in a plate rolling machine?

To bend the plate, the rolls have to grip it; the material beyond the grip point receives no curvature and comes out straight. In conventional rolling that flat edge is on the order of 8 to 10 times the plate thickness: on a thick plate, several centimeters per side. Without pre-bending, that section has to be cropped or corrected in a press afterwards, with loss of material and time.

By pre-bending the ends before rolling, the flat section is drastically reduced. With pre-bending the flat edge drops to just 2 to 2.5 times the thickness, against the 8 to 10 times of rolling without pre-bending. The result is a cylinder that closes evenly, with a clean weld seam and no plate waste at the ends.

Pre-bending demands more force than rolling itself, because it bends the material over a short, unsupported section. That is why a machine's pre-bending capacity is always lower than its rolling capacity: they are two different figures, and you must look at the pre-bending one if the flat edge matters. How this combines with material, thickness and diameter is developed in the article on how to specify a plate rolling machine.

Pre-bending is not a luxury: it determines how much work is left after rolling. Good pre-bending means less cropping, less press correction and a cleaner closing weld. That is why, on parts where the edge matters —vessels, precision cylinders—, pre-bending capacity is as relevant as rolling capacity.