Lexmark T522 Service Manual - Page 157

4520-xxx Autocompensator Operation The autocompensator is a paper pick device that generates its own normal force. This force generation is inherent in the fundamental design of the pick arm. If light media is used, it picks very gently. If a heavy media is used, it picks very aggressively. No customer adjustments are necessary, therefore no special trays are needed for card stock or labels. The gearing in the arm is designed so the input torque from the motor produces a movement about the pivot of the arm. This movement produces a downward force at the pick rolls. The friction between the pick roll and the paper produces a frictional locking condition. If the paper is physically held and not allowed to feed, then the motor stalls. Slippage between the roll and the paper is theoretically impossible. When the motor is energized the pick rolls are driven down into the stack increasing the normal force and drive force until the bending strength of the paper is overcome and the paper bends and moves up the dam. Once this critical threshold is achieved the normal force remains at a level just high enough to reliably feed the paper. Rather than having a fixed spring force for feeding all weights of paper like the D-roll, this device has its own mechanical logic for producing only enough pick energy to feed a single sheet of paper regardless of its stiffness. High normal force is one of the most significant contributors to double feeding paper. The pick arm is counter balanced by an extension spring located on the pick arm to reduce weight in the rest state. This spring is factory set to exert no more than ten to fifteen grams on the stack. This is as light as can be realistically set and always guarantee there is some force to start the autocompensating phenomena. This spring is not to be considered an adjustment for feeding problems unless it is obvious that the pick arm cannot fall all the way down to the bottom of the tray or has come loose. Poor gear efficiency can cause the arm to generate higher normal forces. If the pick assembly is noisy, replacement may be required. The arm must pivot freely through its full range of motion. On 500 sheet trays there are wrap springs located on the pivot arbors of the arm. These springs aid to prevent the arm from bouncing. If the arm appears to be binding or sticky near the bottom of the tray these springs may be the problem. Reducing the tension on the counterbalance spring may be used as a temporary fix to get additional weight at bottom, until the pick assembly can be replaced. However, the counterbalance spring is not to be considered an adjustment for feeding problems. Diagnostic Aids 3-25