US-based Processing Technologies International (PTi) has been granted a US patent for its new roll skewing mechanism which compensates for roll deflection that occurs in sheet roll stands.
High nipping loads across the entire width of the chrome roll face can cause the roll to deflect, resulting in a thick band at the center of the sheet, according to PTi.
“This new invention gives processors the ability to produce thin-gauge sheet that is uniform in thickness and enables them to reach thicknesses that were previously unattainable,” said Sushant Jain, Senior Scientist for PTi.
For example, if the sheet gauge target is 250 microns, even a 50 micron roll deflection will show up as a 20% variation in sheet thickness. The target variation is typically ±3%. Roll skewing compensates for the deflection and enables the operator to obtain the ±3% variation target at the 250 micron sheet thickness.
PTi’s roll skewing mechanism employs a cross-axis positioning of the roll such that the roll deflection is wrapped about the mating roll, thus creating a uniform gap over a wide operating range.
As said, the roll skewing system is versatile and recommended for processing thin-gauge materials such as polypropylene (PP) because of its adjustability over a wide range of deflection patterns and sheet gauge targets.
Within the PTi device, a bearing assembly mounted to the chrome roll journals is used within, and keyed to, an outer housing. The machine direction position of this bearing assembly within the outer housing is controlled by a threaded push/pull device mounted on the front cap of the outer housing.
Using this device to adjust one bearing assembly forward and the other backward by the same amount causes the center of the top roll to remain centered over the center of the middle roll while the gap of the top roll will “wrap” around the middle roll, explained the company, adding that PP has been successfully run down to 0.25mm using its roll skewing device.