Steel plate rolls–whether they have two or three bottom rolls–all have a top roll. The top roll can be sized to roll plate into cylinders or cylinder segments to radii close to the diameter of the top roll. However, relatively small top rolls can deflect in the center under the pressure of curving steel plate. Alternately, larger-diameter top rolls deflect less but limit the machine to rolling only larger diameters.
Benders and rollers, those who specialize in curving steel plate among other steel products, are asked to roll a variety of plate widths, lengths and thicknesses. Some of the possibilities will cause deflection of the top roll in the rolling process. The result will be the plate cylinder with a barrel form and ends that are not parallel.
With plate rolls that have a long top roll with a small diameter, you will see more deflection. Imagine standing on the middle of a 6 x 4 wood beam supported at two ends. As the length on the wood beam gets longer, the deflection where you are standing would sag downwards more. The same concept is applied to steel plate rolling.
To compensate for this deflection in a plate roll, the top roll is “crowned.” A “crown” is the barrel shape of the top roll that is needed to obtain a uniform distribution of the pressure required for the rolling of steel plate. A plate roll has supports at each end and the top roll deflects in the middle when the plate is in motion. This affects the parallelism on the edges of the rolled part.
The process of crowning is achieved through grinding or cutting on large, modern CNC lathes. A program takes the deflection formula or tabulated values to produce the correct shape to equalize the pinch pressure.
Top rolls are recommended to be crowned for the material that is run the majority of the time. If, for example, you are always running ½” thick, 3ft wide plate to make 30in diameter cylinders, you can have your top rolled crowned to those specifications. But then if you run 2in plate with the same width through the machine to the same radius, you will see a barrel-shaped end product. If you roll a 1/8” thick plate through that same plate roll with the crowned top roll you will see an hour-glass shape end product.
Benders and rollers do not run one-size-thickness, width and diameter plate only, so they cannot crown their top roll to be a one-size-fits-all. Plate rolling companies often have a number of different plate rolls that work their way up the plate thickness, width and diameter scale.
If you have a plate that you need to roll using a plate roll that has the wrong crown amount, you may be able to use a shim to artificially add thickness to correct for crowning. The most common shim is a cardboard, but you can also use metal and wood shims.
Depending on your requirements, then, you should consider crowning your plate roll to produce quality steel cylinders and cylinder segments.