Understanding the Importance of Crowning on Plate Rolls
By Kyle Jorgenson
President of JMT
Adapted from an article Kyle wrote for The Fabricator magazine, October 2013 issue.
Most metal fabricators will require a plate roll to work with many materials, which will affect the machine’s rolls in different ways. To accommodate this variety, most manufacturers’ rolls are machined with optimal crowning (see Figure 1).
Typically, optimal crowning is 75 percent of nominal capacity, with all things—yield, width, diameter, pinch pressure, and speed—being equal. For instance, a machine with 1-in. nominal rolling capacity will form very nice 0.75-in.-thick cylinders (under the right pressure), with all surfaces parallel to each other and producing the same radius or diameter throughout the part. However, on this same machine, 0.25-in. material may yield long before enough pressure is applied to deflect crowning out of rolls that is needed for the thicker end of the machine’s range. The result is an hourglass-shaped part—a part tighter in the middle than at the ends.
The range of a roll (thickness/width/diameter) really depends on the tolerance—with accepted variance—of your parts. Options can be installed on the machine to adjust for different tolerances. Specific crowning can be applied to any machine to reach the desired range of material characteristics and part tolerances. Adjusting the pinch pressure can affect or change the parallelism of the roll contact surfaces.
What can happen with insufficient crowning? You don’t get a satisfactory roll form.
A barrel effect is one result of insufficient crowning (see Figure 2). The part has a greater diameter in the middle than at the ends.
An hourglass effect is another possible deformation (see Figure 3). In this case, the material is tighter in the middle than on the ends.
Some suggestions follow that may help you combat these plate rolling deformation problems.
Combating Crowning Deformations
If the part is outside of tolerance because of a barrel crowning deformation, try the following:
• Increase initial/starting crowning in the center
• Reduce pinch pressure
• Add shims in the middle to offset too much deflection
• Acquire a top roll with greater crowning
• Install an overhead support to help limit roll deflection
• Slow down the rolling speed
Also, if the part falls outside of the roll’s capacity range, being too thick or too heavy, additional passes in the machine can offset the deformation—as long as the material does not harden during the multiple passes.
If the part is outside of tolerance because of an hourglass crowning deformation, you can try these remedies:
• Reduce initial start crowning in the center of rolls
• Increase pinch pressure, although some soft materials may not be good candidates for this approach because of possible damage
• Shim outer surfaces of the roll
• Roll the part in fewer passes
• Acquire a top roll with less crowning
• Apply more dynamic crowning pressure, if that option is available on the machine
You also can equip the plate roll machine with oversized rolls. In this case, typically very little, if any, crowning is machined into rolls. The oversized rolls do not yield before the material does. There is very little or no roll deflection to compensate for. The drawback is that as the top roll gets bigger, it may not allow you to roll tight diameters.
In fact, the machine also can be equipped with additional top rolls and different crowning. This allows you to reach a desired tolerance range.
Another alternative approach is dynamic crowning on the bottom rolls or an over-the-top roll deflection compensation system. These may be required to meet part tolerances and diameters.
As always, the plate roll experts at JMT stand ready to assist you with questions you might have about crowning or other machine operation issues. Please feel free to contact us by email at [email protected].
