Roller for conveying a web or sheet of paper in paper converting machines and conveying method thus obtained

A method of folding a sheet, said method comprising:

• conveying a sheet of paper from a conveying roller to a folding roller of a couple of folding rollers, each of the folding rollers comprising first rows of holes connected to a first suction system by first suction channels and a first vacuum distributor, said first suction system having a first suction force,
• wherein said first rows of holes are arranged to capture from said conveying rollers a sheet of paper and to bring the sheet up to a point of contact between said folding rollers, and each of the folding rollers comprising second rows of holes connected to a second suction system having a second suction force, wherein the second suction force of the second suction system is greater than the first suction force of the first suction system,
• said second rows of holes being arranged to cause said sheet to pass from one folding roller to the other folding roller and to follow the other folding roller up to a folding point, each of the conveying rollers comprising a conveying system arranged for conveying said sheet to and to release it to one of said folding rollers at one row of holes of said first rows of holes,
• said folding rollers further comprising:
• a first tubular body capable of rotating about an axis of rotation, said first tubular body having a plurality of couples of neighboring parallel rows of radial holes of said second rows of holes,
• a second tubular body within said first tubular body that extends for all length of said first tubular body and is always connected to a vacuum source, whereby said second tubular body is always under vacuum,
• an interposition means integral to said second tubular body and arranged between said second tubular body and said first tubular body for defining a suction chamber between said second tubular body and said first tubular body,
• wherein said interposition means is arranged between said second tubular body and said first tubular body in such a way that said angular positions are located between said point of contact and said folding point;
• wherein said interposition means arranged in such a way that by the rotation of said first tubular body said interposition means selectively causes said suction chamber to connect a row of holes at a time of said couples of neighboring parallel rows of radial holes of said second rows of holes at determined angular positions of said first tubular body and to provide the second suction force on a sheet captured by one row of holes of said first rows of holes and to cause said sheet to pass from one folding roller to the other folding roller and to follow the other folding roller between said positions up to said folding point.