Like a bowed shaft spreader roll, straight shaft rollers can also be used as spreader rolls. They are sometimes called "Bowtie" rollers when used as spreaders, because a concave crown is machined into the roller.
The sheet, when under traction conditions, will move outwards and remove wrinkles in the center of the sheet. This is opposite of what one might think. If slipping occurs between the sheet and this roller, the web will move to the center and create wrinkles there.
Also, since the outer diameters are larger than the center diameter, a small speed difference exists across the sheet. This creates a force that wants to pull the sheet toward the center. The two forces are opposite each other, with the outward moving force dominating.
The concave crown ideally is cut as a smooth curve. The diameter difference is 10-25% of the machine direction strain of the sheet at running sheet tensions.
Another simple approach to remove center wrinkles is to create a pseudo-bowtie roll by using electrical or similar tape wound on the ends of the straight cut roller. This type of roller is normally seen just before the reel drum or in front of any nips.
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.
For assistance improving your sheet spreading, contact your Valmet representative.