Spreader rolls are critical to most web handling processes, especially in paper, board and tissue making. They are normally located before nipped rollers, before and after coating stations, after size presses, before drive points, and in long web runs.
There are many types of spreader rolls - bowed roll, concave roller, D-bar, dual bowed roller, Pos-Z, etc, but bowed roll spreaders are the most common type of spreader on all paper machines.
Bowed spreader rolls are constructed of a central beam that has a bow in it, with bearings mounted across the entire roll, and a metal or rubber cover over the bearings. The amount of bow is measured by arc height divided by roll width x 100. Typical bowed rollers are set up for 0.03 - 0.05%. This includes roll deflection due to gravity and web tension.
Web spreading follows the Normal Entry Law under traction conditions - the web going into a downstream roller will orient itself at 90 degrees to the roller surface. For a properly set up spreader, this causes the sheet to track toward the edges. It should be noted that only the ingoing web is being spread by the bowed roller, not the downstream web. This determines where the spreader rolls are located in the web path. The greater the upstream web span going into a bowed spreader, the greater the spreading capability will be.
Bowed roll spreader cover materials affect spreading forces. Under traction conditions, the cover type’s order of increasing spreading capability are:
The degree of wrap impacts spreading forces as well, with higher wrap angles having higher spreading capabilities. Wrap angles vary from 15 to 35° but can be higher. Bows are normally set in the direction of the bisector of the ingoing and outgoing web path angles. This is also known as the zero point, where the web path distance is the same at the center and ends. Adjustments from this point are made depending on how the sheet looks. If there are wrinkles in the center, turn the bow into the sheet more. If the edges are slack, turn the bow out of the sheet path.
When changing or adjusting spreader rolls, it is good to know where the bow should be set to minimize setup time. Most spreader rolls have a keyway in the journal that is either on the same side as the direction of the bow or 180 degrees from it. One way to guarantee that a new spreader roll is set properly after a roll change is to measure the angle of the spreader before removal. This is easily done by placing a piece of keystock in the keyway and using an inclinometer to measure the angle of the bow. Maintenance personnel can then return the bow angle to the original setting without having to set it with a sheet in it at startup.
A good practice would be to develop a reference sheet with all of your bowed roll angle positions on it.
Finally, spreading ability is maximized under traction conditions where the roll and the web are traveling at the same speed.
For more information on spreader rolls and web handling, contact your Valmet representative.