A piece of paper through a printer, 12 rolls around in more than one circle. The photosensitive drum turned the largest 2 to 3 times, the paper feeding roller can be turned 10 laps. When the interval is equal to the dirty marks on the paper, may be caused by the roll dirty or damaged. If a roll on the stained dirty, such as being near, may be a small roll forming; far away, you should check the large number of rolls. Measure the distance between the dirty mark, and then calculate the roll diameter formula caused by dirty: roll diameter = dirty track distance / pi. For the HP laser printer, drum diameter of 3.75 inches (91mm), a developing roller diameter is 2 inches (51mm), a fixing roller diameter is 3 inches (76mm).6BYservice manual download site