HP LaserJet 4250 Service Manual - Page 135

Multiple-feed prevention The printer uses the separation roller in tray 2 to prevent multiple-feeding. Normally, the separation roller rotates in the same direction as the feed roller. The separation roller is equipped with a torque limiter, but because the force of the feed roller exceeds that of the torque limiter, the separation roller is actually driven by the feed roller. If multiple sheets of media are picked up, however, the low friction force between the sheets weakens the rotational force from the feed roller to the separation roller. Consequently, the torque limiter takes control of the separation roller and rotates the separation roller in the reverse direction, which removes the extra sheets. Figure 5-8 Multiple-feed prevention Media-skew prevention The printer uses a registration shutter on the registration assembly to prevent media from entering the printer skewed, without decreasing the throughput speed. When the leading edge of the media contacts the registration shutter, the shutter does not open. The feed roller continues to rotate and the media begins to sag. When the entire leading edge comes in full contact with the shutter, the media skew is corrected and registration shutter opens. When the shutter opens, the media can continue into the printer paper path. Paper-pickup-and-feed system 121