Lexmark T52 Series, Service Manual

Page: 157 / 334

Diagnostic Aids
3-25
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.